Slitrk6 binding agents, conjugates thereof and methods of using the same

Abstract

The present invention provides SLITRK6 antibodies, antigen binding portions thereof, other binding agents and SLITRK6 conjugates thereof, as well as methods and uses of such antibodies and conjugates the treatment of cancer and autoimmune disease.The present invention provides SLITRK6 antibodies, antigen binding portions thereof, other binding agents and SLITRK6 conjugates thereof, as well as methods and uses of such antibodies and conjugates the treatment of cancer and autoimmune disease.

Description

SLITRK6 BINDING AGENTS, CONJUGATES THEREOF AND METHODS OF USING THE SAMECROSS REFERENCEThis patent application claims the benefit of U.S, Provisional Application No. 63 / 619,728, filed January 10, 2024, and of International Application No. PCT / CN 2024 / 086162, filed April 04, 2024, each of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0001] The present disclosure generally relates to antibodies and antibody-drug conjugates, as well as methods of using the antibodies and antibody-drug conjugates, and in particular, to such antibodies, antibody-drug conjugates, and methods related to SLITRK6-expressing diseases and disorders.BACKGROUND

[0002] A great deal of interest has surrounded the use of monoclonal antibodies (mAbs) for the targeted delivery of cytotoxic agents to cells associated with disease, such as cancer cells and other cells, in the form of antibody drug conjugates (or ADCs). The design of antibody drug conjugates, by attaching a cytotoxic agent, immune modulatory agent or other agent (collectively a “drug”) to an antibody, typically via a linker, involves consideration of a variety of factors. These factors include the identity and location of the chemical group for attachment of the drug, the mechanism of drug release, the structural element(s) (if any) providing release of the drug, and structural modification of the released free drug, if any. If the drug is released in the extracellular environment, the released form of the drug must be able to reach its target. If the drug is to be released after antibody drug conjugate internalization, the structural elements and mechanism of drug release must be consonant with the intracellular trafficking of the conjugate.

[0003] Another important factor in the design of antibody drug conjugates is the amount of drug that can be delivered per targeting agent (i.e. , the number of drugs attached to each targeting agent (e.g., an antibody), referred to as the drug load or drug loading). Historically, assumptions were that higher drugs loads were superior to lower drug loads (e.g., 8-loads vs 4- loads). The rationale was that higher loaded conjugates would deliver more drug (e.g., cytotoxic agent) to the target cells. This rationale was supported by the observations that conjugates with higher drug loadings were more active against cell lines in vitro. Certain later studies revealed, however, that this assumption was not confirmed in animal models. Conjugates having drug loads of 4 or 8 of certain auristatins were observed to have similar activities in mouse models. See, e.g., Hamblett et al., Clinical Cancer Res. 10:7063-70 (2004). Hamblett et al. further reported that the higher loaded ADCs were cleared more quickly from circulation in animalmodels. This faster clearance suggested a PK liability for higher loaded species as compared to lower loaded species. See Hamblett et al. In addition, higher loaded conjugates had lower maximum tolerated doses (MTDs) in mice, and as a result had narrower reported therapeutic indices. Id. In contrast, ADCs with a drug loading of 2 at engineered sites in a monoclonal antibody were reported to have the same or better PK and therapeutic indices as compared to certain 4-loaded ADCs. For example, see Junutula et al., Clinical Cancer Res. 16:4769 (2010). Thus, recent trends are to develop ADCs with low drug loadings.

[0004] An attractive target for cancer therapies employing ADCs is SLIT and NTRK-like protein 6 (SLITRK6). SLITRK6 protein is engaged in tight control of developmental processes, such as neurite outgrowth and modulation, cellular differentiation, and hormonal regulation. While SLITRK6 is found to be highly expressed in various cancers, such as BLCA, BRCA, HNSC, LUCA and GBMLGG, it has limited expression in normal tissues. The different expression between the cancer tissues and normal tissues makes it a promising tumor associated antigen. (Mol Cell Neurosci 2003; 24:117-129. Gene 2003; 315:87-94. Uniprot.) However, the pace for constructing effective SLITRK6 antibodies and related conjugates has been slow and the clinical trials with SLITRK6 antibodies and SLITRK6 ADCs have met with limited success thus far.

[0005] There is a need, therefore, for SLITRK6 antibodies generally, and for SLITRK6 ADCs in particular that allow for higher drug loading, but that maintain other characteristics of lower loaded conjugates, such as favorable PK properties. Embodiments of the present invention address these and related needs.SUMMARY

[0006] Provided herein are SLITRK6 binding agents, antibody drug conjugates (ADCs), and methods of using the binding agents and ADC to treatment diseases such as but not limited to cancers and autoimmune diseases.

[0007] In some embodiments, provided is a binding agent that includes a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having amino acids sequences selected from the sets of amino acid sequences set forth in the group consisting of: SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively; SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively; SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively; SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively.

[0008] In some embodiments, provided is a binding agent (e.g., an antibody or antigen-bindingportion thereof) comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively.

[0009] In some embodiments, the binding agent (e.g., an antibody or antigen-binding portion thereof) comprises a heavy chain constant region comprising the amino acid sequence of SEQ ID NO: 49, 51 , or 52. In some embodiments, the binding agent (e.g., an antibody or antigenbinding portion thereof) comprises a light chain constant region comprising the amino acid sequence of SEQ ID NO: 50. In some embodiments, the binding agent (e.g., an antibody or antigen-binding portion thereof) comprises a heavy chain constant region comprising the amino acid sequence of SEQ ID NO: 49, 51, or 52, and a light chain constant region comprising the amino acid sequence of SEQ ID NO: 50.

[0010] In some embodiments, provided is a binding agent (e.g., an antibody or antigen-binding portion thereof) comprising a heavy chain and a light chain comprising amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively. In some embodiments, provided is a binding agent (e.g., an antibody or antigen-binding portion thereof) comprising a heavy chain and a light chain comprising amino acid sequences set forth in SEQ ID NOs: 54 and 55, respectively.

[0011] In some embodiments, provided herein is a pharmaceutical composition comprising the binding agent of the present disclosure and a pharmaceutically acceptable carrier.

[0012] In some embodiments, provided herein is a nucleic acid encoding the binding agent of the present disclosure.

[0013] In some embodiments, provided herein is a vector comprising the nucleic acid of the present disclosure.

[0014] In some embodiments, provided herein is a cell line comprising the binding agent, the vector, or the nucleic acid of the present disclosure.

[0015] In some embodiments, provided herein is a conjugate that comprises the binding agent, at least one linker attached to the binding agent; at least one drug unit, wherein each drug unit is attached to a linker, wherein the linker optionally comprises at least one polar group.

[0016] In some embodiments, for the conjugate of the present disclosure, the linker is derived from a linker compound, or a stereoisomer or salt thereof, and the linker compound comprises: a linker unit; a stretcher group connected to the linker unit; an optional amino acid unit; and the at least one polar group; wherein: the stretcher group has an attachment site to the binding agent and an attachment site to the amino acid unit (when present) or the linker subunit; the amino acid unit (when present) has an attachment site to the stretcher group and an attachment site to the linker unit; and the linker unit has an attachment site to the amino acid unit (when present) or to the stretcher group and to the at least one drug unit.

[0017] In some embodiments, for the conjugate of the present disclosure, the linker compoundcomprises:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein the polymer unit comprises the formula:~R°-(R3-R1-[O-CH2-CH2]n0-R6-([O-CH2-CH2]n0-R2-R3-(NR4R5)nl)n2)n3 (la) or a stereoisomer or salt thereof, wherein:R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene; each R3is independently selected from a bond, C1-C12 alkylene, -C(O)-, -NRa-C1- C12 alkylene, -C1-C12 alkylene-NRa-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -C1-C12 alkylene-NRa-C(O)-, -C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C1-C12 alkylene-C(O)-, - C(O)-C1-C12 alkylene-NRa-, -NRa-C(O)-NRa-, -NRa-C(O)-, -NRa-C(O)-C1-C12 alkylene, -C(O)- NRa-C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C12 alkylene, heteroaryl-C1-C12 alkylene- C(O)-, -NRa-C(O)-C1-C12 alkylene-C(O)-, -C(O)-NRa-C1-C12 alkylene-(CH(OH))i.8-C1-C12 alkylene-, -O-CH2-CH2, -O-C(O)-NRa-C1-C12 alkylene, -O-CH2-CH(OH)-C(O)-, -O-CH2-CH(OH)- C(O)-NRa- C1-C12 alkylene-, -CH(OH)-, -CH(OH)-C1-C12 alkylene-, C1-C12 alkylene-CH(OH)-, - CH(OH)-C(O)-, -CH(OH)-C(O)-NRa-C1-C12 alkylene-, -CH(OH)-C1-C12 alkylene-NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, - CH(OH)-NRa-C1-C12 alkylene-, -[C(O)-(CH2)i-8-NRa]i-8-, triazolyl, -C1-C12 alkylene-triazolyl-, - N(polyhydroxyl group)-, and -C(O)NR7R8, wherein one of R7and R8is H or C1-C12 alkylene and the other is C1-C12 alkylene, each Rais independently selected from H, C1-6 alkyl, and wherein any of the above alkylene groups may be substituted with -SO3H; each R4and R5are independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)- polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each R6is selected from:each n3and n4are independently 0-1 , each Rbis independently H or C1-6 alkyl,each R9is independently H, acetyl, -P(=0)(0H)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, m is 1-4, each v is independently 1-6, and n2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, n6is 1-10, each p is independently 0-6, and n2is 1 ;each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, q is 1-8, each v is independently 1-6, and n2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, each p is independently 0-6, and n2is 1 ;(v) -R10-[O-CH2-CH2]I-8-R10-, wherein: each Rbis independently H or C1-6 alkyl, dneach R10is independentlyeach p is independently 1-6, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)V-O-S(=O)2(OH), and q is 1-8; n2is 1 ; and(vi) -N-(R1-X-R2-)2, wherein: each X is independently -NRa-C(O)- or -C(O)NRa-, and n2is 2; and the wavy line (~) indicates the attachment site of the amino acid unit to R°; each n° is independently 2-26; each n1is independently 1-6; and n3is 1-6.

[0018] In some embodiments, for the conjugate of the present disclosure, the linker compound comprises:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises the formula:~R°-(R3-R1-[0-CH2-CH2]no-R2-(NR4R5)ni)n3(la’) or a stereoisomer or salt thereof, wherein:R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene; each R3is independently -N(polyhydroxyl group)-, triazolyl, -C1-C12 alkylene- triazolyl-,each R4and R5are independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)- polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each Rais independently H or C1-6 alkyl;indicates the attachment site of R3to R° the wavy lineindicates the attachment site of the R3to R1; each p is 1-6; each n° is independently 2-8; each n1is independently 1-6; and n3is 1-6.

[0019] In some embodiments, for the conjugate of the present disclosure, the linker compound comprises:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises the formula:~R0-(R1-[0-CH2-CH2]no-R2-R3-(NR4R5)ni)n3(la”)or a stereoisomer or salt thereof, wherein:(i) R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene;R3is -C(O)-;R4is H;R5is independently a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate; the wavy line (~) indicates the attachment site of the amino acid unit to R°; n° is independently 2-26; n1is 1-6; and n3is 1-6;(ii) R° is -C(O)-;R1, R2, and R3are each a bond;R4and R5are each independently H, a polyhydroxyl group, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; the wavy line (~) indicates the attachment site of the amino acid unit to R°; n° is 6; n1is 1-6; and n3is 1 ;(iii) R° is a functional group for attachment to a subunit of the amino acid unit;R1and R2are each, independently, a bond or C1-C6 alkylene;R3is-NRa-C(O)-C1-C12 alkylene-C(O)-, wherein the alkylene is substituted with -SO3H;Rais H or C1-6 alkyl;R4and R5are each independently H, a carboxyl-containing moiety, a polyhydroxyl group, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; the wavy line (~) indicates the attachment site of the amino acid unit to R°; each n° is independently 1-26; n1is 1-6; and n3is 1-6; oreach R1is independently a bond or C1-C6 alkylene;R2and R3are each a bond;R4and R5are each independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each Rais independently H or C1-6 alkyl; the wavy line (indicates the attachment site of R° to the remainder of the polymer unit; the wavy line (-*) indicates the attachment site of the amino acid unit to R°; n° is 1-8; n1is 1-6; and n3is 2.

[0020] In some embodiments, for the conjugate of the present disclosure, the linker compound comprises:(a) a linker unit having from 1 to 4 attachment sites for a drug unit, said linker unit comprising a moiety of formula:or a stereoisomer or salt thereof, wherein: a — represents a direct or indirect attachment site to an amino acid unit;8 — represents an attachment site to at least one of the drug units or for a linking group attached to the at least one of the drug units; andRais H or C1-6 alkyl;(b) the amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit.

[0021] In some embodiments, for the conjugate of the present disclosure, the linker compound comprises:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises:(i) an optionally substituted polyamide comprising the formulastereoisomer thereof, wherein each Rais independently H or C1-6 alkyl and each Rbis independently H or C1-6 alkyl, and n° is independently 2-26;(ii) a substituted polyether comprising the formulastereoisomer thereof, wherein each Rbis independently H or C1-6 alkyl, and n° is independently2-26; or(iii) combinations thereof.

[0022] In some embodiments, for the conjugate of the present disclosure, the linker compound comprises:(a) the linker unit, which has from 1 to 4 attachment sites for the drug units and having one of the following structures (i) or (ii):(b) the at least one polar group, each comprises a polymer unit, and(c) the stretcher group, which has an attachment site for the binding agent; wherein: a — is an attachment site to an enzyme-cleavable group;P — is an attachment site to the at least one polar group;8 — is H, an attachment site to at least one of the drug units, or an attachment site to alinking group attached to the at least one of the drug units; the polymer unit comprises a polyamide, a polyether, or a combination thereof, wherein the polyether comprises a hydroxyl group, a polyhydroxyl group, a sugar group, a carboxyl group, or combinations thereof; each Raindependently is H or C1-C6 alkyl; each Rbindependently is halo, C1-6 alkyl, an attachment site to at least one of the drug units, or an attachment site to at least one of the polar groups; x is 0, 1 , 2, 3 or 4; y is 0, 1 , 2 or 3;Rcis a bond, -C(O)-, -S(O)-, -SO2-, C1-6 alkylene, C1-6 alkynylene, triazolyl or combinations thereof; andY is a bond, -O-, -S-, -N(Ra)-, -C(O)-, -S(O)-, -SO2-C1-C6 alkylene, C1-C6 alkenylene, C1- Ce alkynylene, triazolyl or combinations thereof.

[0023] In some embodiments, provided herein is a pharmaceutical composition comprising the conjugate of the present disclosure and a pharmaceutically acceptable carrier.

[0024] In some embodiments, provided herein is a method of treating a SLITRK6+ cancer, comprising administering to a subject in need thereof a therapeutically effective amount of the binding agent, the pharmaceutical composition, the conjugate, or the pharmaceutical composition of the present disclosure.

[0025] In some embodiments, provided herein is a use of the conjugate or the pharmaceutical composition of the present disclosure for the treatment of SLITRK6+ cancer in a subject.

[0026] Additional features will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by production or operation of the examples. The features of the present disclosure may be realized and attained by practice or use of various aspects of the methodologies, instrumentalities, and combinations set forth in the detailed examples discussed below.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present disclosure is further described in terms of exemplary embodiments.These exemplary embodiments are described in detail with reference to the drawings. It should be noted that the drawings are not to scale. These embodiments are non-limiting exemplary embodiments, in which like reference numerals represent similar structures throughout the several views of the drawings, and wherein:

[0028] FIG. 1 is a graph illustrating a method of epitope binning;

[0029] FIG. 2A is a graph illustrating ELISA binding results of antibodies phA290, phA108, hu1 H2-03, hu1 H2-13, hu14G1-03, and Sirtratumab on human SLITRK6;

[0030] FIG. 2B is a graph illustrating ELISA binding results of antibodies phA290, phA108, hu1 H2-03, hu1 H2-13, hu14G1-03, and Sirtratumab on cynomolgus monkey SLITRK6;

[0031] FIG. 3 is a graph illustrating copy number of SLITRK6 on different SLITRK6-expressing cell lines;

[0032] FIG. 4A is a graph illustrating binding activity of antibodies to 293F-huSLITRK6-C9 cells;

[0033] FIG. 4B is a graph illustrating binding activity of antibodies to 293F cells (negative cells);

[0034] FIG. 5A is a graph illustrating binding activity of antibodies to SLITRK6-expressing tumor cell lines SW780, RT4, RT112 / 84, CHP-212, and SCC-4;

[0035] FIG. 5B is a graph illustrating binding activity of antibodies to SLITRK6-expressing tumor cell line SW780;

[0036] FIG. 50 is a graph illustrating binding activity of antibodies to SLITRK6-expressing tumor cell line RT4;

[0037] FIG. 5D is a graph illustrating binding activity of antibodies to SLITRK6-expressing tumor cell line RT112 / 84;

[0038] FIG. 5E is a graph illustrating binding activity of antibodies to SLITRK6-expressing tumor cell line CHP-212;

[0039] FIG. 6A is a graph illustrating binding activity of antibodies (phA290, phA108, hu1 H2- 03, hu1 H2-13, hu14G1-03, and Sirtratumab) and ADCs (phA290-LD038 (8), phA108-LD038 (8), hu1 H2-03-LD038 (8), hu1 H2-13-LD038 (8), hu14G1-03-LD038 (8), and Sirtratumab-LD038 (8)) to SLITRK6-expressing cell line SW780;

[0040] FIG. 6B is a graph illustrating binding activity of antibodies (phA290, phA108, hu1H2- 03, hu1 H2-13, hu14G1-03, and Sirtratumab) and ADCs (phA290-LD038 (8), phA108-LD038 (8), hu1 H2-03-LD038 (8), hu1 H2-13-LD038 (8), hu14G1-03-LD038 (8), and Sirtratumab-LD038 (8)) to SLITRK6-expressing cell line RT4;

[0041] FIG. 6C is a graph illustrating binding activity of antibodies (phA290, phA108, hu1 H2- 03, hu1 H2-13, hu14G1-03, and Sirtratumab) and ADCs (phA290-LD038 (8), phA108-LD038 (8), hu1 H2-03-LD038 (8), hu1 H2-13-LD038 (8), hu14G1-03-LD038 (8), and Sirtratumab-LD038 (8)) to SLITRK6-expressing cell line 293F-huSLITRK6-C9;

[0042] FIG. 6D is a graph illustrating binding activity of antibodies (phA290, phA108, hu1 H2- 03, hu1 H2-13, hu14G1-03, and Sirtratumab) and ADCs (phA290-LD038 (8), phA108-LD038 (8), hu1 H2-03-LD038 (8), hu1 H2-13-LD038 (8), hu14G1-03-LD038 (8), and Sirtratumab-LD038 (8)) to SLITRK6-expressing cell line CHP-212;

[0043] FIG. 7A is a graph illustrating internalization rates of antibodies (phA290, phA108, and Sirtratumab) and ADCs (phA290-LD038 (8), phA108-LD038 (8), and Sirtratumab-LD038 (8)) in 293F-SLITRK6-C9 cells;

[0044] FIG. 7B is a graph illustrating internalization rates of antibodies (hu1 H2-03, hu1 H2-13, and hu14G1-03) and ADCs (hu1H2-03-LD038 (8), hu1H2-13-LD038 (8), and hu14G1-03-LD038 (8)) in 293F-SLITRK6-C9 cells;

[0045] FIG. 70 is a graph illustrating internalization rates of antibodies (hu1H2-03, hu1H2-13, and hu14G1-03) and ADCs (hu1H2-03-LD038 (8), hu1H2-13-LD038 (8), and hu14G1-03-LD038 (8)) in RT4 cells;

[0046] FIG. 7D is a graph illustrating internalization rates of antibodies (hu1H2-03, hu1H2-13, and hu14G1-03) and ADCs (hu1H2-03-LD038 (8), hu1H2-13-LD038 (8), and hu14G1-03-LD038 (8)) in RT4 cells;

[0047] FIG. 8A is a graph illustrating cytotoxicity of ADCs with linker-drug LD038 on 293F- SLITRK6-C9 cells;

[0048] FIG. 8B is a graph illustrating cytotoxicity of ADCs with linker-drug LD038 on 293F cells (negative cells);

[0049] FIG. 8C is a graph illustrating cytotoxicity of ADCs with linker-drug LD038 on RT4 cells;

[0050] FIG. 8D is a graph illustrating cytotoxicity of ADCs with linker-drug LD038 on CHP-212 cells;

[0051] FIG. 9A is a graph illustrating in vivo efficacy of single dose of SLITRK6-targeting ADCs phA290-LALA-LD038 (8), AGS15E, b12-WT-LD038 on CHP-212;

[0052] FIG. 9B is a graph illustrating in vivo efficacy of single dose of SLITRK6-targeting ADCs phA108-WT-LD038 (8), hu14G1-03-WT-LD038 (8), hu1H2-03-WT-LD038 (8), and AGS15E on CHP-212;

[0053] FIG. 9C is a graph illustrating in vivo efficacy of multiple dose of SLITRK6-targeting ADCs phA108-WT-LD038 (8), hu14G1-03-WT-LD038 (8), hu1H2-03-WT-LD038 (8), and AGS15E on CHP-212;

[0054] FIG. 9D is a graph illustrating in vivo efficacy of single dose and multiple dose of SLITRK6-targeting ADCs phA108-WT-LD038 (8), hu14G1-03-WT-LD038 (8), hu1 H2-03-WT- LD038 (8), and AGS15E on SW780; and

[0055] FIG. 9E is a graph illustrating in vivo efficacy of multiple dose of SLITRK6-targeting ADCs phA290-WT-LD038 (8), phA108-WT-LD038 (8), hu14G1-03-WT-LD038 (8), hu1 H2-03- WT-LD038 (8), hu1 H2-13-WT-LD038 (8), AGS15E, and b12-WT-LD038 (8) on RT4.

[0056] FIG. 10A is a graph illustrating binding activity of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to SLITRK6- expressing tumor cell line RT4;

[0057] FIG. 10B is a graph illustrating binding activity of antibodies (hu1 H2-03 andSirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to SW780 cells;

[0058] FIG. 10C is a graph illustrating binding activity of antibodies (hu1H2-03 andSirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to RT112 / 84 cells;

[0059] FIG. 10D is a graph illustrating binding activity of antibodies (hu1H2-03 andSirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to CHP-212 cells;

[0060] FIG. 10E is a graph illustrating binding activity of antibodies (hu1 H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to KYSE-30 cells;

[0061] FIG. 10F is a graph illustrating binding activity of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to KYSE-410 cells;

[0062] FIG. 10G is a graph illustrating binding activity of antibodies (hu1 H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to 293F-huSLITRK6 cells;

[0063] FIG. 10H is a graph illustrating binding activity of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) to negative cells.

[0064] FIG. 11 A is a graph illustrating plasma PK of hu1 H2-03-LD038 (8) or hu1H2-03 in rats (n=3 / group);

[0065] FIG. 11 B is a graph illustrating plasma PK of hu1H2-03-LD038 (8) in monkeys (n=2);

[0066] FIG. 12 is a graph illustrating hydrophobicity interaction chromatograph of hu1 H2-03- LD038 (8), hu1 H2-03-LD343 (8), AGS15E, and their parent mAbs (hu1 H2-03, sirtratumab);

[0067] FIG. 13A is a graph illustrating binding activity of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD343 and AGS15E) to SW780 cells;

[0068] FIG. 13B is a graph illustrating binding activity of antibodies (hu1 H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD343 and AGS15E) to RT4 cells;

[0069] FIG. 14A is a graph illustrating internalization rates of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) in SW780 cells;

[0070] FIG. 14B is a graph illustrating internalization rates of antibodies (hu1 H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD038 and AGS15E) in RT4 cells;

[0071] FIG. 14C is a graph illustrating internalization rates of antibodies (hu1H2-03 and Sirtratumab) and their respective ADCs (hu1H2-03-LD343 and AGS15E) in RT4 cells;

[0072] FIG. 15A is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on 293F- SLITRK6 cells;

[0073] FIG. 15B is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on 293F cells;

[0074] FIG. 15C is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E onSW780 cells;

[0075] FIG. 15D is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on RT4 cells;

[0076] FIG. 15E is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on RT112 / 84 cells;

[0077] FIG. 15F is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on CHP- 212 cells;

[0078] FIG. 15G is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on KYSE- 410 cells;

[0079] FIG. 15H is a graph illustrating cytotoxicity of hu1H2-03-LD038 and AGS15E on KYSE- 30 cells;

[0080] FIG. 151 is a graph illustrating cytotoxicity of hu1 H2-03-LD343 and AGS15E on SW780 cells;

[0081] FIG. 15J is a graph illustrating cytotoxicity of hu1 H2-03-LD343 and AGS15E on RT4 cells;

[0082] FIG. 16A is a graph illustrating in vivo efficacy of single dose of hu1H2-03-LD038 analog, hu1H2-03-LD038, and AGS15E on SW780 cells;

[0083] FIG. 16B is a graph illustrating in vivo efficacy of single dose of hu1 H2-03-l_AI_A-LD038 and AGS15E on RT 4 cells;

[0084] FIG. 16C is a graph illustrating in vivo efficacy of single dose of hu1H2-03-WT-LD038 and AGS15E on CHP-212 cells;

[0085] FIG. 16D is a graph illustrating in vivo efficacy of single dose of hu1H2-03-LD038 and AGS15E on KYSE-410 cells;

[0086] FIG. 16E is a graph illustrating in vivo efficacy of single dose of hu1 H2-03-LD038 and AGS15E on KYSE-30 cells;

[0087] FIG. 16F is a graph illustrating in vivo efficacy of single dose of hu1H2-03-LD343 and AGS15E on SW780 cells;

[0088] FIG. 16G is a graph illustrating in vivo efficacy of single dose of hu1 H2-03-LD343 andAGS15E on RT4 cells; and

[0089] FIG. 17 is a graph illustrating in vivo efficacy of single dose of hu1H2-03-LD038, hu1 H2-03-LD343, and AGS15E on bladder cancer cells in PDX models.DETAILED DESCRIPTION

[0090] The following description is presented to enable any person skilled in the art to make and use the present disclosure and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present disclosure is not limited to the embodiments shown but is to be accorded the widest scope consistent with the claims.

[0091] The terminology used herein is to describe particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and / or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0092] These and other features, and characteristics of the present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, may become more apparent upon consideration of the following description with reference to the accompanying drawing(s), all of which form a part of this specification. It is to be expressly understood, however, that the drawing(s) is for the purpose of illustration and description only and are not intended to limit the scope of the present disclosure. It is understood that the drawings are not to scale.DEFINITIONS

[0093] For convenience, certain terms in the specification, examples and claims are defined here. Unless stated otherwise, or implicit from context, the following terms and phrases have the meanings provided below. The definitions are provided to aid in describing particular embodiments, and are not intended to limit the claimed invention, because the scope of the invention is limited only by the claims. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0094] As used herein and unless otherwise indicated, the terms "a" and "an" are taken to mean "one", "at least one" or "one or more". Unless otherwise required by context, singular terms used herein shall include pluralities and plural terms shall include the singular.

[0095] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusivesense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

[0096] The terms "decreased," "reduce," "reduced", "reduction", "decrease," and "inhibit" are all used herein generally to mean a decrease by a statistically significant amount relative to a reference.

[0097] The terms "increased", "increase" or "enhance" or "activate" are all used herein to generally mean an increase by a statically significant amount relative to a reference.

[0098] As used herein, the terms "protein" and "polypeptide" are used interchangeably herein to designate a series of amino acid residues each connected to each other by peptide bonds between the alpha-amino and carboxyl groups of adjacent residues. The terms "protein" and "polypeptide" also refer to a polymer of amino acids, including modified amino acids (e.g., phosphorylated, glycated, glycosylated, etc.) and amino acid analogs, regardless of its size or function. "Protein" and "polypeptide" are often used in reference to relatively large polypeptides, whereas the term "peptide" is often used in reference to small polypeptides, but usage of these terms in the art overlaps. The terms "protein" and "polypeptide" are used interchangeably herein when referring to an encoded gene product and fragments thereof. Thus, exemplary polypeptides or proteins include gene products, naturally occurring proteins, homologs, orthologs, paralogs, fragments and other equivalents, variants, fragments, and analogs of the foregoing.

[0099] SLITRK6 (protein tyrosine kinase-like 7) is a member of the receptor protein tyrosine kinase family and is also known as colon carcinoma kinase 4 (CCK4). In humans, this protein is encoded by the SLITRK6 gene. SLITRK6 is overexpressed on the surface of many types of cancer, including but not limited to solid tumors. Such solid tumors include but are not limited to breast cancer (BC), lung cancer (LC), esophageal cancer (EsC), gastric cancer (GC), bladder cancer (BLC), endometrial cancer (EC), ovarian cancer (OVC), head and neck cancer (HNC) as well as hematological malignancies. Human SLITRK6 polypeptides include, but are not limited to, those having the amino acid sequences set forth in UniProt identifiers Q13308, and those having the amino acid sequences set forth in Genbank such as but not limited to GenBank accession numbers NP_002812.2, NP_690619.1, NP_690620.1, NP_690621.1 , NP_001257327.1, XP_011513067.1 , XP_011513068.1, XP_047275113.1 , XP_054212039.1 , XP_054212040.1, and XP_054212041.1 , which are incorporated by reference herein.Although lacking detectable catalytic tyrosine kinase activity, SLITRK6 overexpression is intimately involved in Wnt signaling and promotes cancer cell sternness, survival and tumor progression (See e.g., Atasaven et al., 2013; Cui et al., 2021; Gartner, 2014; Chen et al., 2014; Jiang et al., 2020; Shin et al., 2013; Liu et al., 2017; Lin et al., 2012; Ozqelik et al., 2020; Xiang et al., 2022; Wang et al., 2014; Prebet et al., 2010; Jiang et al., 2012; Damelin et al., 2017). SLITRK6 expression in normal tissues is generally low, although some levels of protein expression have been reported in the digestive tract, such as urinary bladder, kidney, mammarygland, lung, ovary, uterus, and dendritic cells (Damelin et al., 2017). SLITRK6 is thus a promising target for novel anti-cancer therapy.

[0100] As used herein, an "epitope" refers to the amino acids conventionally bound by an immunoglobulin VH / VL pair, such as the antibodies, antigen binding portions thereof and other binding agents described herein. An epitope can be formed on a polypeptide from contiguous amino acids or noncontiguous amino acids juxtaposed by tertiary folding of a protein. Epitopes formed from contiguous amino acids are typically retained on exposure to denaturing solvents, whereas epitopes formed by tertiary folding are typically lost on treatment with denaturing solvents. An epitope typically includes at least 3, and more usually, at least 5, about 9, or about 8-10 amino acids in a unique spatial conformation. An epitope defines the minimum binding site for an antibody, antigen binding portions thereof and other binding agent, and thus represents the target of specificity of an antibody, antigen binding portion thereof or other immunoglobulin- based binding agent. In the case of a single domain antibody, an epitope represents the unit of structure bound by a variable domain in isolation.

[0101] As used herein, "specifically binds" refers to the ability of a binding agent (e.g., an antibody or antigen binding portion thereof) described herein to bind to a target, such as human SLITRK6, with a KD of 10-5 M (10000 nM) or less, e.g., 10-6 M, 10-7 M, 10-8 M, 10-9 M, 10-10 M, 10-11 M, 10-12 M, or less. Specific binding can be influenced by, for example, the affinity and avidity of the antibody, antigen binding portion or other binding agent and the concentration of target polypeptide. The person of ordinary skill in the art can determine appropriate conditions under which the antibodies, antigen binding portions and other binding agents described herein selectively bind to SLITRK6 using any suitable methods, such as titration of an antibody or other binding agent in a suitable cell binding assay. A binding agent specifically bound to SLITRK6 is not displaced by a non-similar competitor. In certain embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent is said to specifically bind to SLITRK6 when it preferentially recognizes its target antigen, SLITRK6, in a complex mixture of proteins and / or macromolecules.

[0102] In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of 10'5M (10000 nM) or less, e.g., 10'6M, 10'7M, 10'8M, 10'9M, 10’10M, 10-11M, 10-12M, or less. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'5M to 10'6M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'7M to 10'8M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'8M to 10'9M. In someembodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'9M to 10'10M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'10M to 10'11M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'11M to 10'12M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of from about 10'12M to 10'13M. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent as described herein specifically binds to a SLITRK6 polypeptide with a dissociation constant (KD) of less than 10'9M.

[0103] Unless otherwise indicated, the term "alkyl" by itself or as part of another term refers to a substituted or unsubstituted straight chain or branched, saturated hydrocarbon having the indicated number of carbon atoms (e.g., "-C1-C5 alkyl", "-C1-C8 alkyl" or "-C1-C10" alkyl refer to an alkyl group having from 1 to 5, 1 to 8, or 1 to 10 carbon atoms, respectively). Examples include methyl (Me, -CH3), ethyl (Et, -CH2CH3), 1-propyl (n-Pr, n-propyl, -CH2CH2CH3), 2-propyl (i-Pr, i-propyl, -CH(CH3)2), 1-butyl (n-Bu, n-butyl, -CH2CH2CH2CH3), 2-methyl-1 -propyl (i-Bu, i- butyl, -CH2CH(CH3)2), 2-butyl (s-Bu, s-butyl, -CH(CH3)CH2CH3), 2-methyl-2-propyl (t-Bu, t-butyl, -C(CH3)3), 1-pentyl (n-pentyl, -CH2CH2CH2CH2CH3), 2-pentyl (-CH(CH3)CH2CH2CH3), 3-pentyl (- -CH(CH2CH3)2), 2-methyl-2-butyl (-C(CH3)2CH2CH3), 3-methyl-2-butyl (-CH(CH3)CH(CH3)2), 3- methyl- 1-butyl (-CH2CH2CH(CH3)2), 2-methyl- 1-butyl (-CH2CH(CH3)CH2CH3), 1-hexyl (- CH2CH2CH2CH2CH2CH3), 2-hexyl (-CH(CH3)CH2CH2CH2CH3), 3-hexyl (- CH(CH2CH3)(CH2CH2CH3)), 2-methyl-2-pentyl (-C(CH3)2CH2CH2CH3), 3-methyl-2-pentyl (- CH(CH3)CH(CH3)CH2CH3), 4-methyl-2-pentyl (-CH(CH3)CH2CH(CH3)2), 3-methyl-3-pentyl (- C(CH3)(CH2CH3)2), 2-methyl-3-pentyl (-CH(CH2CH3)CH(CH3)2), 2,3-dimethyl-2-butyl (- C(CH3)2CH(CH3)2), and 3,3-dimethyl-2-butyl (-CH(CH3)C(CH3)3.

[0104] Unless otherwise indicated, "alkenyl" by itself or as part of another term refers to a C2- C8 substituted or unsubstituted straight chain or branched, hydrocarbon with at least one site of unsaturation (i.e. , a carbon-carbon, sp2 double bond). Examples include, but are not limited to: ethylene or vinyl (-CH=CH2), allyl (-CH2CH=CH2), cyclopentenyl (-C5H7), and 5-hexenyl (- CH2CH2CH2CH2CH=CH2).

[0105] Unless otherwise indicated, "alkynyl" by itself or as part of another term refers to a refers to C2-C8, substituted or unsubstituted straight chain or branched, hydrocarbon with at least one site of unsaturation (i.e., a carbon-carbon, sp triple bond. Examples include, but are not limited to: acetylenic and propargyl.

[0106] Unless other indicated, "alkylene" refers to a saturated, branched or straight chain or hydrocarbon radical of 1-8 carbon atoms, and having two monovalent radical centers derived by the removal of two hydrogen atoms from the same or two different carbon atoms of a parent alkane. Typical alkylene radicals include, but are not limited to: methylene (-CH2-), 1 ,2-ethyl (- CH2CH2-), 1 ,3-propyl (-CH2CH2CH2-), 1 ,4-butyl (-CH2CH2CH2CH2-), and the like.

[0107] Unless otherwise indicated, "alkenylene" refers to an unsaturated, branched or straight chain hydrocarbon radical of 2-8 carbon atoms, and having two monovalent radical centers derived by the removal of two hydrogen atoms from the same or two different carbon atoms of a parent alkene. Typical alkenylene radicals include, but are not limited to: 1 ,2-ethylene (- CH=CH-).

[0108] Unless otherwise indicated, "alkynylene" refers to an unsaturated, branched or straight chain or cyclic hydrocarbon radical of 2-8 carbon atoms, and having two monovalent radical centers derived by the removal of two hydrogen atoms from the same or two different carbon atoms of a parent alkyne. Typical alkynylene radicals include, but are not limited to: acetylene, propargyl, and 4-pentynyl.

[0109] Unless otherwise indicated, the term "heteroalkyl," by itself or in combination with another term, refers to a substituted or unsubstituted stable straight or branched chain hydrocarbon, or combinations thereof, saturated and from one to ten, preferably one to three, heteroatoms selected from the group consisting of O, N, Si and S, and wherein the nitrogen and sulfur atoms may optionally be oxidized and the nitrogen heteroatom may optionally be quaternized. The heteroatom(s) O, N and S may be placed at any interior position of the heteroalkyl group (i.e., as part of the main chain) or at the position at which the alkyl group is attached to the remainder of the molecule. The heteroatom Si may be placed at any position of the heteroalkyl group, including the position at which the alkyl group is attached to the remainder of the molecule. Examples of heteroalkyl include the following: -CH2CH2OCH3, - CH2CH2NHCH3, -CH2CH2N(CH3)CH3, -CH2SCH2CH3, CH2CH2S(O)CH3, -CH2CH2S(O)2CH3, and -Si(CH3)3, -. Up to two heteroatoms may be consecutive, such as, for example, -CH2NHOCH3 and CH2OSi(CH3)3. In some embodiments, a C1 to C4 heteroalkyl has 1 to 4 carbon atoms and 1 or 2 heteroatoms and a C1 to C3 heteroalkyl has 1 to 3 carbon atoms and 1 or 2 heteroatoms.

[0110] Unless otherwise indicated, the terms "heteroalkenyl" and “heteroalkynyl” by themselves or in combination with another term, refers to a substituted or unsubstituted stable straight or branched chain alkenyl or alkynyl having from one to ten, preferably one to three, heteroatoms selected from the group consisting of O, N, Si and S, and wherein the nitrogen and sulfur atoms may optionally be oxidized and the nitrogen heteroatom may optionally be quaternized. The heteroatom(s) O, N and S may be placed at any interior position of a heteroalkenyl or heteroalkynyl group (i.e., as part of the main chain) or at the position at which the alkyl group is attached to the remainder of the molecule. The heteroatom Si may beplaced at any position of a heteroalkenyl or heteroalkynyl group, including the position at which the alkyl group is attached to the remainder of the molecule.

[0111] Unless otherwise indicated, the term "heteroalkylene" by itself or as part of another substituent refers to a substituted or unsubstituted divalent group derived from a heteroalkyl (as discussed above), as exemplified by -CH2CH2SCH2CH2- and -CH2SCH2CH2NHCH2-. In some embodiments, a C1 to C4 heteroalkylene has 1 to 4 carbon atoms and 1 or 2 heteroatoms and a C1 to C3 heteroalkylene has 1 to 3 carbon atoms and 1 or 2 heteroatoms. For heteroalkylene groups, heteroatoms can also occupy either or both of the chain termini. Still further, for alkylene and heteroalkylene linking groups, no orientation of the linking group is implied.

[0112] Unless otherwise indicated, the terms "heteroalkenylene" and “heteroalkynylene" by themselves or as part of another substituent refers to a substituted or unsubstituted divalent group derived from an heteroalkenyl or heteroalkynyl (as discussed above). In some embodiments, a C2 to C4 heteroalkenylene or heteroalkynylene has 1 to 4 carbon atoms. For heteroalkenylene and heteroalkynylene groups, heteroatoms can also occupy either or both of the chain termini. Still further, for alkylene and heteroalkenylene and heteroalkynylene linking groups, no orientation of the linking group is implied.

[0113] Unless otherwise indicated, a "C3-C8 carbocycle," by itself or as part of another term, refers to a substituted or unsubstituted 3-, 4-, 5-, 6-, 7- or 8-membered monovalent, substituted or unsubstituted, saturated or unsaturated non-aromatic monocyclic or bicyclic carbocyclic ring derived by the removal of one hydrogen atom from a ring atom of a parent ring system. Representative -C3-C8 carbocycles include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentadienyl, cyclohexyl, cyclohexenyl, 1,3-cyclohexadienyl, 1,4- cyclohexadienyl, cycloheptyl, 1,3-cycloheptadienyl, 1 ,3,5-cycloheptatrienyl, cyclooctyl, and cyclooctadienyl.

[0114] Unless otherwise indicated, a "C3-C8 carbocyclo", by itself or as part of another term, refers to a substituted or unsubstituted C3-C8 carbocycle group defined above wherein another of the carbocycle groups' hydrogen atoms is replaced with a bond (i.e. , it is divalent).

[0115] Unless otherwise indicated, a "C3-C10 carbocycle," by itself or as part of another term, refers to a substituted or unsubstituted 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-membered monovalent, substituted or unsubstituted, saturated or unsaturated non-aromatic monocyclic, bicyclic or tricyclic carbocyclic ring derived by the removal of one hydrogen atom from a ring atom of a parent ring system. Representative -C3-C10 carbocycles include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclopentadienyl, cyclohexyl, cyclohexenyl, 1 ,3- cyclohexadienyl, 1 ,4-cyclohexadienyl, cycloheptyl, 1,3-cycloheptadienyl, 1,3,5-cycloheptatrienyl, cyclooctyl, and cyclooctadienyl. -C3-C10 carbocycles can further include fused cyclooctyne carbocycles, such as the fused cyclooctyne compounds disclosed in International PublicationNumber WO2011 / 136645 (the disclosure of which is incorporated by reference herein), including BCN (bicyclo[6.1.0]nonyne) and DBCO (Dibenzocyclooctyne).

[0116] Unless otherwise indicated, a "C3-C8 heterocycle," by itself or as part of another term, refers to a substituted or unsubstituted monovalent substituted or unsubstituted aromatic or non-aromatic monocyclic or bicyclic ring system having from 3 to 8 carbon atoms (also referred to as ring members) and one to four heteroatom ring members independently selected from N, O, P or S, and derived by removal of one hydrogen atom from a ring atom of a parent ring system. One or more N, C or S atoms in the heterocycle can be oxidized. The ring that includes the heteroatom can be aromatic or nonaromatic. Unless otherwise noted, the heterocycle is attached to its pendant group at any heteroatom or carbon atom that results in a stable structure. Representative examples of a C3-C8 heterocycle include, but are not limited to, pyrrolidinyl, azetidinyl, piperidinyl, morpholinyl, tetrahydrofuranyl, tetrahydropyranyl, benzofuranyl, benzothiophene, indolyl, benzopyrazolyl, pyrrolyl, thiophenyl (thiophene), furanyl, thiazolyl, imidazolyl, pyrazolyl, pyrimidinyl, pyridinyl, pyrazinyl, pyridazinyl, isothiazolyl, and isoxazolyl. Unless otherwise indicate, the term “heterocarbocycle” is synonymous with the terms “heterocycle” or “heterocyclo” as described herein.

[0117] Unless otherwise indicated, "C3-C8 heterocyclo", by itself or as part of another term, refers to a substituted or unsubstituted C3-C8 heterocycle group defined above wherein one of the heterocycle group's hydrogen atoms is replaced with a bond (i.e. , it is divalent).

[0118] Unless otherwise indicated, "aryl" by itself or as part of another term, means a substituted or unsubstituted monovalent carbocyclic aromatic hydrocarbon radical of 6-20 carbon (preferably 6-14 carbon) atoms derived by the removal of one hydrogen atom from a single carbon atom of a parent aromatic ring system. Some aryl groups are represented in the exemplary structures as "Ar". Typical aryl groups include, but are not limited to, radicals derived from benzene, substituted benzene, naphthalene, anthracene, biphenyl, and the like. An exemplary aryl group is a phenyl group.

[0119] Unless otherwise indicated, an "arylene" by itself or as part of another term, is an unsubstituted or substituted aryl group as defined above wherein one of the aryl group's hydrogen atoms is replaced with a bond (i.e., it is divalent) and can be in the ortho, meta, or para orientations.

[0120] Unless otherwise indicated, “heteroaryl" and "heterocycle" refer to a ring system in which one or more ring atoms is a heteroatom, e.g., nitrogen, oxygen, and sulfur. A heterocycle radical comprises 1 to 20 carbon atoms and 1 to 3 heteroatoms selected from N, O, P, and S. A heterocycle may be a monocycle having 3 to 7 ring members (2 to 6 carbon atoms and 1 to 3 heteroatoms selected from N, O, P, and S) or a bicycle having 7 to 10 ring members (4 to 9 carbon atoms and 1 to 3 heteroatoms selected from N, O, P, and S), for example: a bicyclo [4,5], [5,5], [5,6], or [6,6] system.

[0121] Unless otherwise indicated, an "heteroarylene" by itself or as part of another term, is an unsubstituted or substituted heteroaryl group as defined above wherein one of the heteroaryl group's hydrogen atoms is replaced with a bond (i.e. , it is divalent).

[0122] Unless otherwise indicated, “carboxyl” refers to COOH or COO-M+, where M+ is a cation.

[0123] Unless otherwise indicated, “oxo” refers to (C=O).

[0124] Unless otherwise indicated, "substituted alkyl" and "substituted aryl" mean alkyl and aryl, respectively, in which one or more hydrogen atoms are each independently replaced with a substituent. Typical substituents include, but are not limited to, -X, -R10, -O-, -OR10, -SR10, -S-, -NR102, -NR103, =NR10, -CX3, -CN, -OCN, -SCN, -N=C=O, -NCS, -NO, -NO2, =N2, -N3, - NR10C(=O)R10, -C(=O)R10, -C(=O)NR102, -SO3-, -SO3H, -S(=O)2R10, -OS(=O)2OR10, - S(=O)2NR10, -S(=O)R10, -OP(=O)(OR10)2, -P(=O)(OR10)2, -PO-3, -PO3H2, -AsO2H2, - C(=O)R10, -C(=O)X, -C(=S)R10, -CO2R10, -CO2-, -C(=S)OR10, C(=O)SR10, C(=S)SR10, C(=O)NR102, C(=S)NR102, or C(=NR10)NR102, where each X is independently a halogen: -F, -Cl, -Br, or -I; and each R10 is independently -H, -C1-C20 alkyl, -C6-C20 aryl, -C3-C14 heterocycle, a protecting group or a prodrug moiety. Typical substitutents also include (=0). Alkylene, carbocycle, carbocyclo, arylene, heteroalkyl, heteroalkylene, heterocycle, and heterocyclo groups as described above may also be similarly substituted.

[0125] Unless otherwise indicated, “polyhydroxyl group” refers to an alkyl, alkylene, carbocycle or carbocyclo group including two or more, or three or more, substitutions of hydroxyl groups for hydrogen on carbon atoms of the carbon chain. In some embodiments, a polyhydroxyl group comprises at least three hydroxyl groups. In some embodiments, a polyhydroxyl group comprises carbon atoms containing only one hydroxyl group per carbon atom. A polyhydroxyl group may contain one or more carbon atoms that are not substituted with hydroxyl. A polyhydroxyl group may have each carbon atom substituted with a hydroxyl group. Examples of polyhydroxyl group includes linear (acyclic) or cyclic forms of monosaccharides such as C6 or C5 sugars, such as glucose, ribose, galactose, mannose, arabinose, 2-deoxyglucose, glyceraldehyde, erythrose, threose, xylose, lyxose, allose, altrose, gulose, idose, talose, aldose, and ketose, sugar acids such as gluconic acid, aldonic acid, uronic acid or ulosonic acid, and an amino sugars, such as glucosamine, N-acetyl glucosamine, galactosamine, and N-acetyl galactosamine. In some embodiments, polyhydroxyl group includes linear or cyclic forms of disaccharides and polysaccharides.

[0126] Unless otherwise indicated by context, "optionally substituted" refers to an alkyl, alkenyl, alkynyl, alkylaryl, arylalkyl heterocycle, aryl, heteroaryl, alkyl heteroaryl, heteroarylalkyl, or other substituent, moiety or group as defined or disclosed herein wherein hydrogen atom(s) of that substituent, moiety or group has been optionally replaced with different moiety(ies) or group(s), or wherein an alicyclic carbon chain that comprise one of those substituents, moiety or group is interrupted by replacing carbon atom(s) of that chain with different moiety(ies) orgroup(s). In some aspects an alkene function group replaces two contiguous sp3 carbon atoms of an alkyl substituent, provided that the radical carbon of the alkyl moiety is not replaced, so that the optionally substituted alkyl is an unsaturated alkyl substituent.

[0127] Optional substituent replacing hydrogen(s) in any of the foregoing substituents, moieties or groups is independently selected from the group consisting of aryl, heteroaryl, hydroxyl, alkoxy, aryloxy, cyano, halogen, nitro, fluoroalkoxy, and amino, including mono-, di- and tri-substituted amino groups, and the protected derivatives thereof, or is selected from the group consisting of -X, -OR', -SR' , -NH2, -N(R')(R”), -N(R”)3, =NR, -CX3, -CN, -NO2, - NR'C(=O)H, -NR'C(=O)R, -NR'C(=O)R”, -C(=O)R', -C(=O)NH2, -C(=O)N(R')R”, -S(=O)2R”, - S(=O)2NH2, -S(=O)2N(R')R”, -S(=O)2NH2, -S(=O)2N(R')R”, -S(=O)2OR', -S(=O)R”, - OP(=O)(OR')(OR”), -OP(OH)3, -P(=O)(OR')(OR”), -PO3H2, -C(=O)R', -C(=S)R”, -CO2R', - C(=S)OR”, -C(=O)SR', -C(=S)SR', -C(=S)NH2, -C(=S)N(R')(R”)2, -C(=NR')NH2, - C(=NR')N(R')R”, and salts thereof, wherein each X is independently selected from the group consisting of a halogen: -F, -Cl, -Br, and -I; and wherein each R” is independently selected from the group consisting of C1-C20 alkyl, C2-C20 alkenyl, C2-C20 alkynyl, C6-C24 aryl, C3- C24 heterocyclyl (including C5-C24 heteroaryl), a protecting group, and a prodrug moiety or two of R” together with the heteroatom to which they are attached defines a heterocyclyl; and R' is hydrogen or R”, wherein R” is selected from the group consisting of C1-C20 alkyl, C6-C24 aryl, C3-C24 heterocyclyl (including C5-C24 heteroaryl), and a protecting group.

[0128] Typically, optional substituents are selected from the group consisting of -X, -OH, -OR", -SH, -SR", -NH2, -NH(R"), -NR'(R")2, -N(R")3, =NH, =NR", -CX3, -CN, -NO2, -NR'C(=O)H, NR'C(=O)R", -CO2H, -C(=O)H, -C(=O)R", -C(=O)NH2, -C(=O)NR'R"- -S(=O)2R", -S(=O)2NH2, - S(=O)2N(R')R", -S(=O)2NH2, - S(=O)2N(R')(R"), -S(=O)2OR', -S(=O)R", -C(=S)R", -C(=S)NH2, -C(=S)N(R')R", -C(=NR')N(R")2, and salts thereof, wherein each X is independently selected from the group consisting of -F and -Cl, R" is typically selected from the group consisting of CI- 06 alkyl, C6-C10 aryl, C3-C10 heterocyclyl (including C5-C10 heteroaryl), and a protecting group; and R' independently is hydrogen, C1-C6 alkyl, C6-C10 aryl, C3-C10 heterocyclyl (including C5-C10 heteroaryl), and a protecting group, independently selected from R". More typically, substituents are selected from the group consisting of -X, -R", -OH, -OR", -NH2, - NH(R"), -N(R")2, -N(R")3, -CX3, -NO2, -NHC(=O)H, -NHC(=O)R", -C(=O)NH2, -C(=O)NHR", - C(=O)N(R")2, -CO2H, -CO2R", -C(=O)H, -C(=O)R", -C(=O)NH2, -C(=O)NH(R"), -C(=O)N(R")2, -C(=NR')NH2, -C(=NR')NH(R"), -C(=NR')N(R")2, a protecting group and salts thereof, wherein each X is -F, R" is independently selected from the group consisting of C1-C6 alkyl, C6- 010 aryl, 05-010 heteroaryl and a protecting group; and R' is selected from the group consisting of hydrogen, C1-C6 alkyl and a protecting group, independently selected from R".

[0129] The compounds of the invention, or their pharmaceutically acceptable salts may contain one or more asymmetric centers and may thus give rise to enantiomers, diastereomers, and other stereoisomeric forms that are defined, in terms of absolute stereochemistry, as (R)or (S) or, as (D) or (L) for amino acids. The present invention is meant to include all such possible isomers, as well as their racemic and optically pure forms. Optically active (+) and ( ), (R) and (S), or (D) and (L) isomers may be prepared using chiral synthons or chiral reagents, or resolved using conventional techniques, for example, chromatography and fractional crystallization. Conventional techniques for the preparation / isolation of individual enantiomers include chiral synthesis from a suitable optically pure precursor or resolution of the racemate (or the racemate of a salt or derivative) using, for example, chiral high pressure liquid chromatography (HPLC). When the compounds described herein contain olefinic double bonds or other centres of geometric asymmetry, and unless specified otherwise, it is intended that the compounds include both E and Z geometric isomers. Likewise, all tautomeric forms are also intended to be included.

[0130] A “stereoisomer” refers to a compound made up of the same atoms bonded by the same bonds but having different three-dimensional structures, which are not interchangeable. The present invention contemplates various stereoisomers and mixtures thereof and includes “enantiomers”, which refers to two stereoisomers whose molecules are nonsuperimposeable mirror images of one another. The present invention also includes “diastereomers”, which refers to two or more stereoisomers of a compound that have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.

[0131] Although structures shown throughout the specification are depicted with specific stereocenters, the specification should be read to include variations in those stereocenters. For example, the structure of exatecan may be shown in the (S,S) configuration, but the (R,S) diastereomer of exatecan is also envisioned as being found in a separate embodiment of a conjugate as described herein.

[0132] Unless otherwise indicated, the term “drug unit” or drug refers to cytotoxic agents (such as chemotherapeutic agents or drugs), immunomodulatory agents, nucleic acids (including siRNAs), growth inhibitory agents, toxins (e.g., protein toxins, enzymatically active toxins of bacterial, fungal, plant, or animal origin, or fragments thereof), radioactive isotopes, PROTACs and other compounds that are active against target cells when delivered to those cells.

[0133] Unless otherwise indicated, the term “polymer unit” refers to a polymeric moiety composed of repeating subunits. Examples of polymer units include polyamides and polyethers. In some embodiments, the polymer unit is selected from an optionally substituted polyamide, a substituted polyether, or combinations thereof. In further embodiments, the polymer unit is selected from0 RaRb\ O-M-Pl \Rb /

[0134] (i) an optionally substituted polyamide comprising the formulax'no , or a stereoisomer thereof, wherein each Ra is independently H or C1-6 alkyl and each Rb is independently H or C1-6 alkyl, and nO is independently 2-26;(ii) a substituted polyether comprising the formula, or a stereoisomer thereof, wherein each Rb is independently H or C1-6 alkyl, and nO is independently 2-26; or(iii) combinations thereof.

[0135] Unless otherwise indicated, the term “sugar unit” or “sugar group” refers to a carbohydrate group. Examples of sugar units include glycosides.

[0136] Unless otherwise indicated, the term “carboxyl unit” or “carboxyl group” refers to a group including a carbonyl group [-C(O)-], a carboxyl group [-CO2H], and / or a carboxylate group [-CO2M, M refers to a cationic counterion],

[0137] Unless otherwise indicated, the term “stretcher group” refers to a linking moiety that connects the SLITRK6 binding agent to the enzyme-cleavable group.

[0138] Unless otherwise indicated, the term “polyamide” refers to polymeric groups composed of repeating subunits containing amide bonds.

[0139] Unless otherwise indicated, the term “polyether” refers to polymeric groups composed to repeating subunits containing ether bonds.

[0140] Unless otherwise indicated, the term “enzyme-cleavable group” refers to a group that is cleavable by the action of a metabolic process or reaction inside a cell or in the extracellular milieu, whereby the covalent attachment between a drug unit (e.g., a cytotoxic agent) and the linker unit or portion thereof is broken, resulting in the free drug unit, or a metabolite of the linker unit-drug, which is dissociated from the remainder of the linker unit.

[0141] The phrase "pharmaceutically acceptable salt," as used herein, refers to pharmaceutically acceptable organic or inorganic salts of a compound (e.g., a linker, drug linker, or a conjugate). The compound typically contains at least one amino group, and accordingly acid addition salts can be formed with this amino group. Exemplary salts include, but are not limited to, sulfate, citrate, acetate, oxalate, chloride, bromide, iodide, nitrate, bisulfate, phosphate, acid phosphate, isonicotinate, lactate, salicylate, acid citrate, tartrate, oleate, tannate, pantothenate, bitartrate, ascorbate, succinate, linleate, gentisinate, fumarate, gluconate, glucuronate, saccharate, formate, benzoate, glutamate, methanesulfonate, ethanesulfonate, benzenesulfonate, toluenesulfonate, and pamoate (i.e. , 1,1'-methylene-bis -(2- hydroxy-3- naphthoate)) salts. A pharmaceutically acceptable salt may involve the inclusion ofanother molecule such as an acetate ion, a succinate ion or other counterion. The counterion may be any organic or inorganic moiety that stabilizes the charge on the parent compound. Furthermore, a pharmaceutically acceptable salt may have more than one charged atom in its structure. Instances where multiple charged atoms are part of the pharmaceutically acceptable salt can have multiple counter ions. Hence, a pharmaceutically acceptable salt can have one or more charged atoms and / or one or more counterion.

[0142] As used herein, the term "consisting essentially of" refers to those elements required for a given embodiment. The term permits the presence of elements that do not materially affect the basic and novel or functional characteristic(s) of that embodiment.

[0143] As used herein, the term "consisting of" refers to compositions, methods, and respective components thereof as described herein, which are exclusive of any element not recited in that description of the embodiment.

[0144] Other than in the examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein should be understood as modified in all instances by the term "about." The term "about" when used in connection with percentages can mean + / -1%.

[0145] The terms "statistically significant" or "significantly" refer to statistical significance and generally mean a two standard deviation (2SD) difference, above or below a reference value.

[0146] Other terms are defined herein within the description of the various aspects of the invention.ANTI BODIES AND BINDING AGENTS

[0147] Provided herein are SLITRK6 binding antibodies (also referred to as SLITRK6 antibodies) and antigen binding portions thereof and other binding agents that specifically bind to human SLITRK6. Also provided herein are conjugates of the SLITRK6 antibodies and antigen binding portions and other binding agents bound to drugs, such as cytotoxic agents or immune modulatory agents (also referred to as SLITRK6 conjugates). In some embodiments, the SLITRK6 antibodies, antigen binding portions, other binding agents and / or SLITRK6 conjugates specifically bind to and reduce the number of SLITRK6+ cells in a subject. In some embodiments, the SLITRK6 antibodies, antigen binding portions, other binding agents and / or SLITRK6 conjugates specifically bind to and reduce the number of SLITRK6+ cancer cells in a subject. In some embodiments, the SLITRK6 antibodies, antigen binding portions, other binding agents and / or SLITRK6 conjugates specifically bind to and reduce the number of SLITRK6+ cells associated with a disease or condition in a subject, such as a cancer or an autoimmune disease. In some embodiments, the SLITRK6 antibodies, antigen binding portions, other binding agents and / or SLITRK6 conjugates specifically bind to and reduce the number of SLITRK6+ cells associated with a disease or condition in a subject, such as a human or an animal.

[0148] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO :10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO:1 and SEQ ID NO:2, respectively. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34, respectively.

[0149] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variableregions are not modified. The phrase “wherein the CDRs of the heavy or light chain variable regions are not modified” refers to the VH and VL CDRs that do not have amino acid substitutions, deletions or insertions.

[0150] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences, which have a similarity of at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% with the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34. The levels of similarities result from amino acid substitutions, deletions or insertions to the VH and VL sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively.

[0151] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0152] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical with the sequences set forth in SEQ ID NO:1 and SEQ ID NO:2, respectively.

[0153] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are notmodified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable region (VH) and a light chain variable region (VL), the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0154] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical with the sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively.

[0155] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0156] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical with the sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively.

[0157] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH andVL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0158] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical with the sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively.

[0159] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in and SEQ ID NO: 33 and SEQ ID NO: 34; respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34; respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0160] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical with the sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34, respectively.

[0161]

[0162] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable region (VH) and a light chain variable region (VL), the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to human SLITRK6 with a higher or a similar binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable region (VH) and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18; respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. As described herein, a binding agent includes a SLITRK6 antibody or antigen binding portion(s) thereof and can optionally include other peptides or polypeptides covalently attached to the SLITRK6 antibody or antigen binding portion thereof. In any of these embodiments, the binding agent specifically binds to human SLITRK6.

[0163] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the binding agent specifically binds to human SLITRK6 with a higher or a similar binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or lightchain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0164] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the binding agent specifically binds to SLITRK6 with a higher binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0165] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the binding agent specifically binds to SLITRK6 with a higher binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservativeamino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0166] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the binding agent specifically binds to SLITRK6 with a higher binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0167] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to SLITRK6 with a higher binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0168] In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to human SLITRK6. In some embodiments, the binding agent comprises a heavy chain variable region (VH) and a light chain variable region (VL), the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the binding agent specifically binds to human SLITRK6 with a higher or a similar binding affinity (lower Kd) than that of antibody sirtratumab. In some embodiments, provided herein is a binding agent comprising a heavy chain variable region (VH) and a light chain variable (VL) region, the VH and VL regions having the amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. As described herein, a binding agent includes a SLITRK6 antibody or antigen binding portion(s) thereof and can optionally include other peptides or polypeptides covalently attached to the SLITRK6 antibody or antigen binding portion thereof. In any of these embodiments, the binding agent specifically binds to human SLITRK6.

[0169] In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain comprise the amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain comprise the amino acid sequences set forth in SEQ ID NOs: 54 and 55, respectively. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain comprise the amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain comprise amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical to the sequences set forth in SEQ ID NO: 53 and SEQ ID NO: 55, respectively. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain comprise amino acid sequences that are at least 80%, at least 85%, at least 90%, at least 95%, at least 98%, or at least 99% identical to the sequences set forth in SEQ ID NO: 54 and SEQ ID NO: 55, respectively. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain differ in amino acid sequence from SEQ ID NOs: 53 and 55, respectively, by at most 1-20, 1-10, 1-5, 1- 3, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 amino acid residues. In some embodiments, provided herein is a binding agent (e.g., an antibody) comprising a heavy chain and a light chain, wherein the heavy chain and light chain differ in amino acid sequence from SEQ ID NOs: 54 and 55, respectively, by at most 1-20, 1-10, 1-5, 1-3, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 amino acid residues.

[0170] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in the sets of amino acid sequences selected from SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively; SEQ ID NO: 11 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO:15 and SEQ ID NO: 16, respectively; SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively; SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0171] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0172] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, and SEQ ID NO: 16, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0173] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, and SEQ ID NO: 24, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0174] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO:27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, and SEQ ID NO: 32, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0175] In some embodiments, provided is an antibody or antigen binding portion comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0176] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in the sets of amino acid sequences selected from SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively; SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively; SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively; SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0177] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0178] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15, and SEQ ID NO: 16, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0179] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, and SEQ ID NO: 24, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0180] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31, and SEQ ID NO: 32, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0181] In some embodiments, provided is a binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having the amino acids sequences set forth in SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively. In some embodiments, each VH and VL region comprises a humanized framework region. In some embodiments, each VH and VL region comprises a human framework region.

[0182] In some embodiments, the compositions and methods described herein relate to reduction of SLITRK6+ cells in a subject (e.g., reducing the number of SLITRK6+ cells in acancer or tumor, or SLITRK6+ cells associated with an autoimmune disease or disorder) by a SLITRK6 antibody, antigen binding portion thereof, other binding agent or conjugate thereof in vivo. In some embodiments, the compositions and methods described herein relate to the treatment of SLITRK6+ cancer in a subject by administering a SLITRK6 antibody, antigen binding portion thereof, other binding agent or conjugate thereof. In some embodiments, the compositions and methods described herein relate to the treatment of an autoimmune disorder in a subject by administering a SLITRK6 antibody, antigen binding portion thereof, other binding agent or conjugate thereof. In some embodiments, the compositions and methods described herein relate to the treatment of disease or disorder associated with SLITRK6+ cells in a subject by administering a SLITRK6 antibody, antigen binding portion thereof, other binding agent or conjugate thereof. In any of these embodiments, the methods further include a reduction in the number of SLITRK6+ cells in the subject that are associated with the disease, condition or cancer.

[0183] As used herein, the term "antibody" refers to immunoglobulin molecules and immunologically active portions of immunoglobulin molecules, i.e. , molecules that contain an antigen binding site(s) that specifically binds to an antigen, e.g., human SLITRK6. The term generally refers to antibodies comprised of two immunoglobulin heavy chain variable regions and two immunoglobulin light chain variable regions including full length antibodies (having heavy and light chain constant regions).

[0184] Each heavy chain is composed of a variable region (abbreviated as VH) and a constant region. The heavy chain constant region may include three domains CH1, CH2 and CH3 and optionally a fourth domain, CH4. Each light chain is composed of a variable region (abbreviated as VL) and a constant region. The light chain constant region is a CL domain. The VH and VL regions may be further divided into hypervariable regions referred to as complementaritydetermining regions (CDRs) and interspersed with conserved regions referred to as framework regions (FR). Each VH and VL region thus consists of three CDRs and four FRs that are arranged from the N terminus to the C terminus in the following order: FR1 , CDR1, FR2, CDR2, FR3, CDR3, and FR4. This structure is well known to those skilled in the art.

[0185] As used herein, an "antigen-binding portion" of a SLITRK6 antibody refers to the portions of a SLITRK6 antibody as described herein having the VH and VL sequences of the SLITRK6 antibody or the CDRs of a SLITRK6 antibody and that specifically binds to SLITRK6. Examples of antigen binding portions include a Fab, a Fab', a F(ab')2, a Fv, a scFv, a disulfide linked Fv, a single domain antibody (also referred to as a VHH, VNAR, sdAb, or nanobody) or a diabody (see, e.g., Huston et al., Proc. Natl. Acad. Sci. U.S.A., 85, 5879-5883 (1988) and Bird et al., Science 242, 423-426 (1988), which are incorporated herein by reference). As used herein, the terms Fab, F(ab’)2 and Fv refer to the following: (i) a Fab fragment, i.e. a monovalent fragment composed of the VL, VH, CL and CH1 domains; (ii) an F(ab')2 fragment, i.e. a bivalent fragment comprising two Fab fragments linked to one another in the hinge regionvia a disulfide bridge; and (iii) an Fv fragment composed of the VL and VH domains, in each case of a SLITRK6 antibody. Although the two domains of the Fv fragment, namely VL and VH, are encoded by separate coding regions, they may further be linked to one another using a synthetic linker, e.g., a poly-G4S amino acid sequence ('(G4S)n' disclosed as SEQ ID NO: 56, wherein n =1 to 5), making it possible to prepare them as a single protein chain in which the VL and VH regions combine in order to form monovalent molecules (known as single chain Fv or scFv). The term "antigen-binding portion" of an antibody is also intended to include such single chain antibodies. Other forms of single chain antibodies such as "diabodies" are likewise included here. Diabodies are bivalent, bispecific antibodies in which VH and VL domains are expressed on a single polypeptide chain, but using a linker connecting the VH and VL domains that is too short for the two domains to be able to combine on the same chain, thereby forcing the VH and VL domains to pair with complementary domains of a different chain (VL and VH, respectively), and to form two antigen-binding sites (see, for example, Holliger, R, et al. (1993) Proc. Natl. Acad. Sci. USA 90:64446448; Poljak, R. J, et al. (1994) Structure 2:1121-1123).

[0186] A single-domain antibody is an antibody portion consisting of a single monomeric variable antibody domain. Single domains antibodies can be derived from the variable domain of the antibody heavy chain from camelids (e.g., nanobodies or VHH portions). Furthermore, the term single-domain antibody includes an autonomous human heavy chain variable domain (aVH) or VNAR portions derived from sharks (see, e.g., Hasler et al., Mol. Immunol. 75:28-37, 2016).

[0187] Techniques for producing single domain antibodies (e.g., DABs or VHH) are known in the art, as disclosed for example in Cossins et al. (2006, Prot Express Purif 51:253-259) and Li et al. (Immunol. Lett. 188:89-95, 2017). Single domain antibodies may be obtained, for example, from camels, alpacas or llamas by standard immunization techniques. (See, e.g., Muyldermans et al., TIBS 26:230-235, 2001; Yau et al., J Immunol Methods 281:161-75, 2003; and Maass et al., J Immunol Methods 324:13-25, 2007.) A VHH may have potent antigen-binding capacity and can interact with novel epitopes that are inaccessible to conventional VH-VL pairs (see, e.g., Muyldermans et al., 2001). Alpaca serum IgG contains about 50% camelid heavy chain only IgG antibodies (HCAbs) (see, e.g., Maass et al., 2007). Alpacas may be immunized with antigens and VHHs can be isolated that bind to and neutralize a target antigen (see, e.g., Maass et al., 2007). PCR primers that amplify alpaca VHH coding sequences have been identified and may be used to construct alpaca VHH phage display libraries, which can be used for antibody fragment isolation by standard biopanning techniques well known in the art (see, e.g., Maass et al., 2007).

[0188] In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof are part of a bispecific or multispecific binding agent. Bispecific and multi-specific antibodies include the following: an scFv1-ScFv2, an ScFv12-Fc-scFv22, an IgG-scFv, a DVD-lg, a triomab / quadroma, a two-in-one IgG, a scFv2-Fc, a TandAb, and an scFv-HSA-scFv. In someembodiments, an IgG-scFv is an lgG(H)-scFv, scFv-(H)lgG, lgG(L)-scFv, svFc-(L)lgG, 2scFV- IgG or lgG-2scFv. See, e.g., Brinkmann and Kontermann, MAbs 9(2):182-212 (2017); Wang et al., Antibodies, 2019, 8, 43; Dong et al., 2011, MAbs 3:273-88; Natsume et al., J. Biochem. 140(3):359-368, 2006; Cheal et al., Mol. Cancer Ther. 13(7): 1803-1812, 2014; and Bates and Power, Antibodies, 2019, 8, 28.Modification of VH and VL Regions

[0189] As to the VH and VL amino acid sequences, one of skill will recognize that individual substitutions, deletions or additions (insertions) to a nucleic acid encoding the VH or VL, or amino acids in a polypeptide that alter a single amino acid or a small percentage of amino acids in the encoded sequence is a "conservatively modified variant", where the alteration results in the substitution of an amino acid with a chemically similar amino acid (a conservative amino acid substitution) and the altered polypeptide retains the ability to specifically bind to SLITRK6.

[0190] In some embodiments, a conservatively modified variant of a SLITRK6 antibody or antigen binding portion thereof can have an alteration(s) in the framework regions (i.e. , other than in the CDRs), e.g. a conservatively modified variant of a SLITRK6 antibody has the amino acid sequences of the VH and VL CDRs (set forth in sets of amino acid sequences (i) SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively; (ii) SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively; (iii) SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively; (iv) SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and (v) SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively) and has at least one conservative amino acid substitution in a framework region (FR). In some embodiments, the VH and VL amino acid sequences collectively have no more than 8 or 6 or 4 or 2 or 1 conservative amino acid substitutions in the FR, as compared to the amino acid sequences of the unmodified VH and VL regions. In some embodiments, the VH and VL amino acid sequences have 8 to 1, 6 to 1, 4 to 1 or 2 to 1 conservative amino acid substitutions in the FR, as compared to the amino acid sequences of the unmodified VH and VL regions. In further aspects of any of these embodiments, a conservatively modified variant of the SLITRK6 antibody, antigen binding portion thereof or other binding agent exhibits specific binding to SLITRK6.

[0191] For conservative amino acid substitutions, a given amino acid can be replaced by a residue having similar physiochemical characteristics, e.g., substituting one aliphatic residue for another (such as lie, Vai, Leu, or Ala for one another), or substitution of one polar residue for another (such as between Lys and Arg; Glu and Asp; or Gin and Asn). Other such conservative amino acid substitutions, e.g., substitutions of entire regions having similar hydrophobicity characteristics, are well known. Polypeptides comprising conservative amino acid substitutionscan be tested in any one of the assays described herein to confirm that a desired activity, e.g. antigen-binding activity and specificity of a native or reference polypeptide is retained, i.e. , to SLITRK6.

[0192] In some embodiments, a SLITRK6 antibody or antigen binding portion thereof or other binding agent can be further optimized to, for example, decrease potential immunogenicity or optimize other functional property, while maintaining functional activity, for therapy in humans. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof or other binding agents comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, the SLITRK6 antibodies or antigen binding portions thereof or other binding agents comprise a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in the pairs of amino acid sequences selected from SEQ ID NO: 1 and SEQ ID NO: 2, respectively; SEQ ID NO: 9 and SEQ ID NO: 10, respectively; SEQ ID NO: 17 and SEQ ID NO: 18, respectively; SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions, wherein the CDRs of the heavy or light chain variable regions are not modified.

[0193] In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 1 and SEQ ID NO: 2, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0194] In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 9 and SEQ ID NO: 10, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0195] In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 17 and SEQ ID NO: 18, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0196] In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 25 and SEQ ID NO: 26, respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6,1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0197] In some embodiments, provided herein is a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprising a heavy chain variable region (VH) and a light chain variable region (VL), the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34; respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 conservative amino acid substitutions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified. In some embodiments, provided herein is a binding agent comprising a heavy chain variable region (VH) and a light chain variable region (VL), the VH and VL regions having amino acid sequences set forth in SEQ ID NO: 33 and SEQ ID NO: 34; respectively; wherein the heavy and light chain variable framework regions are optionally modified with from 1 to 8, 1 to 6, 1 to 4 or 1 to 2 amino acid substitutions, deletions or insertions in the framework regions and wherein the CDRs of the heavy or light chain variable regions are not modified.

[0198]

[0199] In any of these embodiments, the functional activity of the SLITRK6 binding antibody or antigen binding portion thereof or other binding agent includes specifically binding to SLITRK6. Additional functional activities include depletion of SLITRK6+ cells (e.g., cancer cells or autoimmune cells). In the case where dose dependency does exist, it needs not be identical to that of the reference antibody or antigen-binding portion thereof, but rather substantially similar to or better than the dose-dependence in a given activity as compared to the reference antibody or antigen-binding portion thereof as described herein (i.e. , the candidate polypeptide will exhibit greater activity relative to the reference antibody).

[0200] For conservative substitutions, amino acids can be grouped according to similarities in the properties of their side chains (in A. L. Lehninger, in Biochemistry, second ed., pp. 73-75, Worth Publishers, New York (1975)): (1) non-polar: Ala (A), Vai (V), Leu (L), lie (I), Pro (P), Phe (F), Trp (W), Met (M); (2) uncharged polar: Gly (G), Ser (S), Thr (T), Cys (C), Tyr (Y), Asn (N), Gin (Q); (3) acidic: Asp (D), Glu (E); and (4) basic: Lys (K), Arg (R), His (H).

[0201] Alternatively, for conservative substitutions naturally occurring residues can be divided into groups based on common side-chain properties: (1) hydrophobic: Norleucine, Met, Ala, Vai, Leu, lie; (2) neutral hydrophilic: Cys, Ser, Thr, Asn, Gin; (3) acidic: Asp, Glu; (4) basic: His, Lys, Arg; (5) residues that influence chain orientation: Gly, Pro; and (6) aromatic: Trp, Tyr, Phe. Nonconservative substitutions will entail exchanging a member of one of these classes or another class.

[0202] Particular conservative substitutions include, for example; Ala to Gly or to Ser; Arg to Lys; Asn to Gin or to His; Asp to Glu; Cys to Ser; Gin to Asn; Glu to Asp; Gly to Ala or to Pro; His to Asn or to Gin; lie to Leu or to Vai; Leu to lie or to Vai; Lys to Arg, to Gin or to Glu; Met to Leu,to Tyr or to lie; Phe to Met, to Leu or to Tyr; Ser to Thr; Thr to Ser; Trp to Tyr; Tyr to Trp; and / or Phe to Vai, to lie or to Leu.

[0203] In some embodiments, a conservatively modified variant of a SLITRK6 antibody or antigen binding portion thereof preferably is at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or more, identical to the reference VH or VL sequence, wherein the VH and VL CDRs are not modified. The degree of homology (percent identity) between the reference and modified sequence can be determined, for example, by comparing the two sequences using freely available computer programs commonly employed for this purpose on the world wide web (e.g., BLASTp or BLASTn with default settings).

[0204] In some embodiments, the VH and VL amino acid sequences collectively have no more than 8 or 6 or 4 or 2 or 1 conservative amino acid substitutions in the framework regions, as compared to the amino acid sequences of the unmodified VH and VL regions. In some embodiments, the VH and VL amino acid sequences collectively have 8 to 1 , or 6 to 1 , or 4 to 1 , or 2 to 1 conservative amino acid substitutions in the framework regions, as compared to the amino acid sequences of the unmodified VH and VL regions. In some embodiments, the VH and VL amino acid sequences collectively have no more than 8 or 6 or 4 or 2 or 1 amino acid substitutions, deletions or insertions in the framework regions, as compared to the amino acid sequences of the unmodified VH and VL regions. In some embodiments, the VH and VL amino acid sequences have 8 to 1, 6 to 1, 4 to 1 , or 2 to 1 conservative amino acid substitutions in the framework regions, as compared to the amino acid sequences of the unmodified VH and VL regions. In some embodiments, the VH and VL amino acid sequences collectively have no more than 8 or 6 or 4 or 2 or 1 amino acid substitutions, deletions or insertions, as compared to the amino acid sequences of the unmodified VH and VL regions.

[0205] Modification of a native (or reference) amino acid sequence can be accomplished by any of a number of techniques known to one of skill in the art. Mutations can be introduced, for example, at particular loci by synthesizing oligonucleotides containing the desired mutant sequence, flanked by restriction sites enabling ligation to fragments of the native sequence. Following ligation, the resulting reconstructed sequence encodes a variant having the desired amino acid insertion, substitution, or deletion. Alternatively, oligonucleotide-directed site-specific mutagenesis procedures can be employed to provide an altered nucleotide sequence having particular codons altered according to the substitution, deletion, or insertion desired. Techniques for making such alterations are very well established and include, for example, those disclosed by Walder et al. (Gene 42:133, 1986); Bauer et al. (Gene 37:73, 1985); Craik (BioTechniques, January 1985, 12-19); Smith et al. (Genetic Engineering: Principles and Methods, Plenum Press, 1981); and U.S. Pat. Nos. 4,518,584 and 4,737,462, which are herein incorporated by reference in their entireties.Constant Regions

[0206] In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent has fully human constant regions. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent has humanized constant regions. In some embodiments, a SLITRK6 antibody or antigen-binding portion thereof or other binding agent has non-human constant regions. An immunoglobulin constant region refers to a heavy or light chain constant region. Human heavy chain and light chain constant region amino acid sequences are known in the art. A constant region can be of any suitable type, which can be selected from the classes of immunoglobulins, IgA, IgD, IgE, IgG, and IgM. Several immunoglobulin classes can be further divided into isotypes, e.g., lgG1 , lgG2, lgG3, lgG4, or IgAI, and lgA2. The heavychain constant regions (Fc) that correspond to the different classes of immunoglobulins can be a, 5, E, y, and p, respectively. The light chains can be one of either kappa (or K) and lambda (or A).

[0207] In some embodiments, a constant region can have an IgG 1 isotype. In some embodiments, a constant region can have an lgG2 isotype. In some embodiments, a constant region can have an lgG3 isotype. In some embodiments, a constant region can have an lgG4 isotype. In some embodiments, an Fc domain can have a hybrid isotype comprising constant regions from two or more isotypes. In some embodiments, an immunoglobulin constant region can be an lgG1 or lgG4 constant region. In some embodiments, a SLITRK6 antibody heavy chain is of the IgG 1 isotype and has the amino acid sequence set forth in SEQ ID NO: 49. In some embodiments, a SLITRK6 antibody heavy chain is of the I gG 1 isotype and has the amino acid sequence set forth in SEQ ID NO: 51. In some embodiments, a SLITRK6 antibody heavy chain is of the IgG 1 isotype and has the amino acid sequence set forth in SEQ ID NO: 52. In some embodiments, a SLITRK6 antibody light chain is of the kappa isotype and has the amino acid sequence set forth in SEQ ID NO: 50.

[0208] Furthermore, a SLITRK6 antibody or an antigen-binding portion thereof or other binding agent may be part of a larger binding agent formed by covalent or noncovalent association of the antibody or antigen binding portion with one or more other proteins or peptides. Relevant to such binding agents are the use, for example, of the streptavidin core region in order to prepare a tetrameric scFv molecule (Kipriyanov, S. M., et al. (1995), Human Antibodies and Hybridomas 6:93-101) and the use of a cysteine residue, a marker peptide and a C-terminal polyhistidinyl peptide, e.g. hexahistidinyl tag in order to produce bivalent and biotinylated scFv molecules (Kipriyanov, S. M., et al. (1994) Mol. Immunol. 31:10471058).

[0209] Fc regions may have at their C-terminus a lysine. The origin of this lysine is a naturally occurring sequence found in humans from which these Fc regions are derived. During cell culture production of recombinant antibodies, this terminal lysine can be cleaved off by proteolysis by endogenous carboxypeptidase(s), resulting in a constant region having the same sequence but lacking the C-terminal lysine. For manufacturing purposes of antibodies, the DNAencoding this terminal lysine can be omitted from the sequence such that antibodies are produced without the lysine. Antibodies produced from nucleic acid sequences that either do, or do not encode a terminal lysine are substantially identical in sequence and in function since the degree of processing of the terminal lysine is typically high when e.g. using antibodies produced in CHO-based production systems (Dick, L.W. et al. Biotechnol. Bioeng. 2008;100: 1132-1143). Hence, it is understood that proteins in accordance with the invention, such as antibodies, can be generated with or without encoding or having a terminal lysine. It is also understood in accordance with the invention that, sequences with a terminal lysine, such as a constant region sequence having a terminal lysine, can be understood as the corresponding sequences without a terminal lysine, and that sequences without a terminal lysine can also be understood as the corresponding sequences with a terminal lysine.Fc Domain Modifications to Alter Effector Function

[0210] In some embodiments, an Fc region or Fc domain of a SLITRK6 antibody or antigen binding portion thereof or other binding agent has substantially no binding to at least one Fc receptor selected from FcyRI (CD64), FcyRIIA (CD32a), FcyRIIB (CD32b), FcyRIIIA (CD16a), and FcyRI 11 B (CD16b). In some embodiments, an Fc region or domain exhibits substantially no binding to any of the Fc receptors selected from FcyRI (CD64), FcyRIIA (CD32a), FcyRIIB (CD32b), FcyRIIIA (CD16a), and FcyRI 11 B (CD16b). As used herein, “substantially no binding” refers to weak to no binding to a selected Fcgamma receptor or receptors. In some embodiments, “substantially no binding” refers to a reduction in binding affinity (i.e., increase in Kd) to a Fc gamma receptor of at least 1000-fold. In some embodiments, an Fc domain or region is an Fc null. As used herein, an “Fc null” refers to an Fc region or Fc domain that exhibits weak to no binding to any of the Fcgamma receptors. In some embodiments, an Fc null domain or region exhibits a reduction in binding affinity (i.e., increase in Kd) to Fc gamma receptors of at least 1000-fold.

[0211] In some embodiments, an Fc domain has reduced or substantially no effector function activity. As used herein, “effector function activity” refers to antibody dependent cellular cytotoxicity (ADCC), antibody dependent cellular phagocytosis (ADCP) and / or complement dependent cytotoxicity (CDC). In some embodiments, an Fc domain exhibits reduced ADCC, ADCP or CDC activity, as compared to a wildtype Fc domain. In some embodiments, an Fc domain exhibits a reduction in ADCC, ADCP and CDC, as compared to a wildtype Fc domain. In some embodiments, an Fc domain exhibits substantially no effector function (i.e., the ability to stimulate or effect ADCC, ADCP or CDC). As used herein, “substantially no effector function” refers to a reduction in effector function activity of at least 1000-fold, as compared to a wildtype or reference Fc domain.

[0212] In some embodiments, an Fc domain has reduced or no ADCC activity. As used herein reduced or no ADCC activity refers to a decrease in ADCC activity of an Fc domain by a factor of at least 10, at least 20, at least 30, at least 50, at least 100 or at least 500.

[0213] In some embodiments, an Fc domain has reduced or no CDC activity. As used herein reduced or no CDC activity refers to a decrease in CDC activity of an Fc domain by of a factor of at least 10, at least 20, at least 30, at least 50, at least 100 or at least 500.

[0214] In vitro and / or in vivo cytotoxicity assays can be conducted to confirm the reduction / depletion of ADCC and / or CDC activity. For example, Fc receptor (FcR) binding assays can be conducted to ensure that the antibody lacks Fcgamma receptor binding (hence likely lacking ADCC activity). The primary cells for mediating ADCC, NK cells, express FcgammaRIII only, whereas monocytes express FcgammaRI, FcgammaRII and FcgammaRIII. FcR expression on hematopoietic cells is summarized in Table 3 on page 464 of Ravetch and Kinet, Annu. Rev. Immunol. 9:457-492 (1991). Non-limiting examples of in vitro assays to assess ADCC activity of a molecule of interest are described in U.S. Pat. No. 5,500,362 (see, e.g. Hellstrom, I. et al. Proc. Nat'l Acad. Sci. USA 83:7059-7063 (1986)) and Hellstrom, I et al., Proc. Nat'l Acad. Sci. USA 82:1499-1502 (1985); U.S. Pat. No. 5,821,337 (see Bruggemann, M. et al., J. Exp. Med. 166:1351-1361 (1987)). Alternatively, non-radioactive assay methods may be employed (see, for example, ACTITM non-radioactive cytotoxicity assay for flow cytometry (CellTechnology, Inc. Mountain View, Calif.; and CytoTox 96TM non-radioactive cytotoxicity assay (Promega, Madison, Wis.). Useful effector cells for such assays include peripheral blood mononuclear cells (PBMC) and Natural Killer (NK) cells. Alternatively, or additionally, ADCC activity of the molecule of interest may be assessed in vivo, e.g., in an animal model such as that disclosed in Clynes et al., Proc. Nat'l Acad. Sci. USA 95:652-656 (1998).

[0215] C1q binding assays may also be carried out to confirm that an antibody or Fc domain or region is unable to bind C1q and hence lacks CDC activity or has reduced CDC activity. See, e.g., C1q and C3c binding ELISA in WO 2006 / 029879 and WO 2005 / 100402. To assess complement activation, a CDC assay may be performed (see, for example, Gazzano-Santoro et al., J. Immunol. Methods 202:163 (1996); Cragg, M. S. et al., Blood 101:1045-1052 (2003); and Cragg, M. S. and M. J. Glennie, Blood 103:2738-2743 (2004)).

[0216] In some embodiments, an Fc domain has reduced or no ADCP activity. As used herein reduced or no ADCP activity refers to a decrease in ADCP activity of an Fc domain by a factor of at least 10, at least 20, at least 30, at least 50, at least 100 or at least 500.

[0217] ADCP binding assays may also be carried out to confirm that an antibody or Fc domain or region lacks ADCP activity or has reduced ADCP activity. See, e.g., US20190079077 and US20190048078 and the references disclosed therein.

[0218] A SLITRK6 antibody or antigen binding portion thereof or other binding agent with reduced effector function activity includes those with substitution of one or more of Fc region residues, such as, for example, 238, 265, 269, 270, 297, 327 and 329, according to the EUnumber of Kabat (see, e.g., U.S. Pat. No. 6,737,056). Such Fc mutants include Fc mutants with substitutions at two or more of amino acid positions 265, 269, 270, 297 and 327, including the so-called "DANA" Fc mutant with substitution of residues 265 and 297 to alanine, according to the EU numbering of Kabat (see U.S. Pat. No. 7,332,581). Certain antibody variants with diminished binding to FcRs are also known. (See, e.g., U.S. Pat. No. 6,737,056; WO 2004 / 056312, and Shields et al., J. Biol. Chem. 9(2): 6591-6604 (2001).) A SLITRK6 antibody or antigen binding portion thereof or other binding agent with diminished binding to FcRs can be prepared containing such amino acid modifications.

[0219] In some embodiments, a SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises an Fc domain or region with one or more amino acid substitutions which diminish FcgammaR binding, e.g., substitutions at positions 234 and 235 of the Fc region (EU numbering of residues). In some embodiments, the substitutions are L234A and L235A (LALA), according to the EU numbering of Kabat. In some embodiments, the Fc domain comprises D265A and / or P329G in an Fc region derived from a human IgG 1 Fc region, according to the EU numbering of Kabat. In some embodiments, the substitutions are L234A, L235A and P329G (LAI_A-PG), according to the EU numbering of Kabat, in an Fc region derived from a human lgG1 Fc region. (See, e.g., WO 2012 / 130831). In some embodiments, the substitutions are L234A, L235A and D265A (l_AI_A-DA) in an Fc region derived from a human IgG 1 Fc region, according to the EU numbering of Kabat. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a human IgG 1 Fc region comprising a L234A mutation and an L235A mutation (LALA), wherein the amino acid positions are numbered according to Eu numbering. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain constant region comprising the sequence set forth in SEQ ID NO: 51. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain constant region comprising the sequence set forth in SEQ ID NO: 51 and a light chain constant region comprising the sequence set forth in SEQ ID NO: 50. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain constant region comprising the sequence set forth in SEQ ID NO: 52. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain constant region comprising the sequence set forth in SEQ ID NO: 52 and a light chain constant region comprising the sequence set forth in SEQ ID NO: 50. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain comprising the sequence set forth in SEQ ID NO: 53. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain comprising the sequence set forth in SEQ ID NO: 53 and a light chain comprising the sequence set forth in SEQ ID NO: 55. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chaincomprising the sequence set forth in SEQ ID NO: 54. In some embodiments, the SLITRK6 antibody or antigen binding portion thereof or other binding agent comprises a heavy chain comprising the sequence set forth in SEQ ID NO: 54 and a light chain comprising the sequence set forth in SEQ ID NO: 55.

[0220] In some embodiments, alterations are made in the Fc region that result in altered (i.e. , either diminished) C1q binding and / or Complement Dependent Cytotoxicity (CDC), e.g., as described in U.S. Pat. No. 6,194,551, WO 99 / 51642, and Idusogie et al. J. Immunol. 164: 4178- 4184 (2000).Methods of Making Antibodies, Antigen Binding Portions and Other Binding Agents

[0221] In various embodiments, SLITRK6 antibodies, antigen binding portions thereof and other binding agents can be produced in human, murine or other animal-derived cells lines. Recombinant DNA expression can be used to produce SLITRK6 antibodies, antigen binding portions thereof and other binding agents. This allows the production of SLITRK6 antibodies as well as a spectrum of SLITRK6 antigen binding portions and other binding agents (including fusion proteins) in a host species of choice. The production of SLITRK6 antibodies, antigen binding portions thereof and other binding agents in bacteria, yeast, transgenic animals and chicken eggs are also alternatives for cell-based production systems. The main advantages of transgenic animals are potential high yields from renewable sources.

[0222] In some embodiments, a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NOs:1, 9, 17, 25, and 33 is encoded by a nucleic acid. In some embodiments, a SLITRK6 VL polypeptide having the amino acid sequence set forth in SEQ ID NOs: 2, 10, 18, 26, and 34 is encoded by a nucleic acid. In some embodiments, a nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NOs: 1, 9, 17, 25, and 33 . In some embodiments, a nucleic acid encodes a SLITRK6 VL polypeptide having the amino acid sequence set forth in SEQ ID NOs: 2, 10, 18, 26, and 34 . In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 1. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 9. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 17. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 25. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 33. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 2. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 10. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 18. In some embodiments, the nucleic acidencodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 26. In some embodiments, the nucleic acid encodes a SLITRK6 VH polypeptide having the amino acid sequence set forth in SEQ ID NO: 34.

[0223] In some embodiments, the nucleic acid encodes VH and VL polypeptides having the amino acid sequences set forth in SEQ ID NOs: 1 and 2. In some embodiments, the nucleic acid encodes VH and VL polypeptides having the amino acid sequences set forth in SEQ ID NOs: 9 and 10. In some embodiments, the nucleic acid encodes VH and VL polypeptides having the amino acid sequences set forth in SEQ ID NOs: 17 and 18. In some embodiments, the nucleic acid encodes VH and VL polypeptides having the amino acid sequences set forth in SEQ ID NOs: 25 and 26. In some embodiments, the nucleic acid encodes VH and VL polypeptides having the amino acid sequences set forth in SEQ ID NOs: 33 and 34.

[0224] In some embodiments, the nucleic acid encodes heavy chain polypeptide having the amino acid sequence set forth in SEQ ID NO: 53. In some embodiments, the nucleic acid encodes heavy chain polypeptide having the amino acid sequence set forth in SEQ ID NO: 54. In some embodiments, the nucleic acid encodes light chain polypeptide having the amino acid sequence set forth in SEQ ID NO: 55. In some embodiments, the nucleic acid encodes heavy and light chain polypeptides having the amino acid sequences set forth in SEQ ID NOs: 53 and 55. In some embodiments, the nucleic acid encodes heavy and light chain polypeptides having the amino acid sequences set forth in SEQ ID NOs: 54 and 55.

[0225] As used herein, the term "nucleic acid" or "nucleic acid sequence" or “polynucleotide sequence” or “nucleotide” refers to a polymeric molecule incorporating units of ribonucleic acid, deoxyribonucleic acid or an analog thereof. The nucleic acid can be either single-stranded or double-stranded. A single-stranded nucleic acid can be one strand nucleic acid of a denatured double-stranded DNA. In some embodiments, the nucleic acid can be a cDNA, e.g., a nucleic acid lacking introns.

[0226] Nucleic acid molecules encoding the amino acid sequence of a SLITRK6 antibody, antigen binding portion thereof as well as other binding agents can be prepared by a variety of methods known in the art. These methods include, but are not limited to, preparation of synthetic nucleotide sequences encoding of a SLITRK6 antibody, antigen binding portion or other binding agent(s). In addition, oligonucleotide-mediated (or site-directed) mutagenesis, PCR-mediated mutagenesis, and cassette mutagenesis can be used to prepare nucleotide sequences encoding a SLITRK6 antibody or antigen binding portion as well as other binding agents. A nucleic acid sequence encoding at least a SLITRK6 antibody, antigen binding portion thereof, binding agent, or a polypeptide thereof, as described herein, can be recombined with vector DNA in accordance with conventional techniques, such as, for example, blunt-ended or staggered-ended termini for ligation, restriction enzyme digestion to provide appropriate termini, filling in of cohesive ends as appropriate, alkaline phosphatase treatment to avoid undesirable joining, and ligation with appropriate ligases or other techniques known in the art.Techniques for such manipulations are disclosed, e.g., by Maniatis et al., Molecular Cloning, Lab. Manual (Cold Spring Harbor Lab. Press, NY, 1982 and 1989), and Ausubel et al., Current Protocols in Molecular Biology (John Wiley & Sons), 1987-1993, and can be used to construct nucleic acid sequences and vectors that encode a SLITRK6 antibody or antigen binding portion thereof or a VH or VL polypeptide thereof or other binding agent.

[0227] A nucleic acid molecule, such as DNA, is said to be "capable of expressing" a polypeptide if it contains nucleotide sequences that contain transcriptional and translational regulatory information and such sequences are "operably linked" to nucleotide sequences that encode the polypeptide. An operable linkage is a linkage in which the regulatory DNA sequences and the DNA sequence sought to be expressed (e.g., a SLITRK6 antibody or antigen binding portion thereof or other binding agent) are connected in such a way as to permit gene expression of a polypeptide(s) or antigen binding portions in recoverable amounts. The precise nature of the regulatory regions needed for gene expression may vary from organism to organism, as is well known in the analogous art. See, e.g., Sambrook et al., 1989; Ausubel et al., 1987-1993.

[0228] Accordingly, the expression of a SLITRK6 antibody or antigen-binding portion thereof as described herein can occur in either prokaryotic or eukaryotic cells. Suitable hosts include bacterial or eukaryotic hosts, including yeast, insects, fungi, bird and mammalian cells either in vivo or in situ, or host cells of mammalian, insect, bird or yeast origin. The mammalian cell or tissue can be of human, primate, hamster, rabbit, rodent, cow, pig, sheep, horse, goat, dog or cat origin, but any other mammalian cell may be used. Further, by use of, for example, the yeast ubiquitin hydrolase system, in vivo synthesis of ubiquitin-transmembrane polypeptide fusion proteins can be accomplished. The fusion proteins so produced can be processed in vivo or purified and processed in vitro, allowing synthesis of a SLITRK6 antibody or antigen binding portion thereof or other binding agent as described herein with a specified amino terminus sequence. Moreover, problems associated with retention of initiation codon-derived methionine residues in direct yeast (or bacterial) expression maybe avoided. (See, e.g., Sabin et al., 7 Bio / Technol. 705 (1989); Miller et al., 7 Bio / Technol. 698 (1989).) Any of a series of yeast gene expression systems incorporating promoter and termination elements from the actively expressed genes coding for glycolytic enzymes produced in large quantities when yeast are grown in medium rich in glucose can be utilized to obtain recombinant SLITRK6 antibodies or antigen-binding portions thereof or other binding agents. Known glycolytic genes can also provide very efficient transcriptional control signals. For example, the promoter and terminator signals of the phosphoglycerate kinase gene can be utilized.

[0229] Production of SLITRK6 antibodies or antigen-binding portions thereof or other binding agents in insects can be achieved, for example, by infecting an insect host with a baculovirus engineered to express a polypeptide by methods known to those of ordinary skill in the art. See Ausubel et al., 1987-1993.

[0230] In some embodiments, the introduced nucleic acid sequence(s) (encoding a SLITRK6 antibody or antigen binding portion thereof or other binding agent or a polypeptide thereof) is incorporated into a plasmid or viral vector capable of autonomous replication in a recipient host cell. Any of a wide variety of vectors can be employed for this purpose and are known and available to those of ordinary skill in the art. See, e.g., Ausubel et al., 1987-1993. Factors of importance in selecting a particular plasmid or viral vector include: the ease with which recipient cells that contain the vector may be recognized and selected from those recipient cells which do not contain the vector; the number of copies of the vector which are desired in a particular host; and whether it is desirable to be able to "shuttle" the vector between host cells of different species.

[0231] Exemplary prokaryotic vectors known in the art include plasmids such as those capable of replication in E. coli. Other gene expression elements useful for the expression of DNA encoding SLITRK6 antibodies or antigen-binding portions thereof or other binding agents include, but are not limited to (a) viral transcription promoters and their enhancer elements, such as the SV40 early promoter. (Okayama et al., 3 Mol. Cell. Biol. 280 (1983)), Rous sarcoma virus LTR (Gorman et al., 79 PNAS 6777 (1982)), and Moloney murine leukemia virus LTR (Grosschedl et al., 41 Cell 885 (1985)); (b) splice regions and polyadenylation sites such as those derived from the SV40 late region (Okayarea et al., 1983), and (c) polyadenylation sites such as in SV40 (Okayama et al., 1983). Immunoglobulin-encoding DNA genes can be expressed as described by Liu et al., infra, and Weidle et al., 51 Gene 21 (1987), using as expression elements the SV40 early promoter and its enhancer, the mouse immunoglobulin H chain promoter enhancers, SV40 late region mRNA splicing, rabbit S-globin intervening sequence, immunoglobulin and rabbit S-globin polyadenylation sites, and SV40 polyadenylation elements.

[0232] For immunoglobulin encoding nucleotide sequences, the transcriptional promoter can be, for example, human cytomegalovirus, the promoter enhancers can be cytomegalovirus and mouse / human immunoglobulin.

[0233] In some embodiments, for expression of DNA coding regions in rodent cells, the transcriptional promoter can be a viral LTR sequence, the transcriptional promoter enhancers can be either or both the mouse immunoglobulin heavy chain enhancer and the viral LTR enhancer, and the polyadenylation and transcription termination regions. In other embodiments, DNA sequences encoding other proteins are combined with the above-recited expression elements to achieve expression of the proteins in mammalian cells.

[0234] Each coding region or gene fusion is assembled in, or inserted into, an expression vector. Recipient cells capable of expressing the SLITRK6 variable region(s) or antigen binding portions thereof or other binding agents are then transfected singly with nucleotides encoding a SLITRK6 antibody or an antibody polypeptide or antigen-binding portion thereof or other binding agent, or are co-transfected with a polynucleotide(s) encoding VH and VL chain coding regionsor other binding agents. The transfected recipient cells are cultured under conditions that permit expression of the incorporated coding regions and the expressed antibody chains or intact antibodies or antigen binding portions or other binding agents are recovered from the culture.

[0235] In some embodiments, the nucleic acids containing the coding regions encoding a SLITRK6 antibody or antigen-binding portion thereof or other binding agent are assembled in separate expression vectors that are then used to co-transfect a recipient host cell. Each vector can contain one or more selectable genes. For example, in some embodiments, two selectable genes are used, a first selectable gene designed for selection in a bacterial system and a second selectable gene designed for selection in a eukaryotic system, wherein each vector has a set of coding regions. This strategy results in vectors which first direct the production, and permit amplification, of the nucleotide sequences in a bacterial system. The DNA vectors so produced and amplified in a bacterial host are subsequently used to cotransfect a eukaryotic cell, and allow selection of a co-transfected cell carrying the desired transfected nucleic acids (e.g., containing SLITRK6 antibody heavy and light chains). Nonlimiting examples of selectable genes for use in a bacterial system are the gene that confers resistance to ampicillin and the gene that confers resistance to chloramphenicol. Selectable genes for use in eukaryotic transfectants include the xanthine guanine phosphoribosyl transferase gene (designated gpt) and the phosphotransferase gene from Tn5 (designated neo). Alternatively the fused nucleotide sequences encoding VH and VL chains can be assembled on the same expression vector.

[0236] For transfection of the expression vectors and production of the SLITRK6 antibodies or antigen binding portions thereof or other binding agents, the recipient cell line can be a Chinese Hamster ovary cell line (e.g., DG44) or a myeloma cell. Myeloma cells can synthesize, assemble and secrete immunoglobulins encoded by transfected immunoglobulin genes and possess the mechanism for glycosylation of the immunoglobulin. For example, in some embodiments, the recipient cell is the recombinant Ig-producing myeloma cell SP2 / 0. SP2 / 0 cells only produce immunoglobulins encoded by the transfected genes. Myeloma cells can be grown in culture or in the peritoneal cavity of a mouse, where secreted immunoglobulin can be obtained from ascites fluid.

[0237] An expression vector encoding a SLITRK6 antibody or antigen-binding portion thereof or other binding agent can be introduced into an appropriate host cell by any of a variety of suitable means, including such biochemical means as transformation, transfection, protoplast fusion, calcium phosphate-precipitation, and application with polycations such as diethylaminoethyl (DEAE) dextran, and such mechanical means as electroporation, direct microinjection and microprojectile bombardment. Johnston et al., 240 Science 1538 (1988), as known to one of ordinary skill in the art.

[0238] Yeast provides certain advantages over bacteria for the production of immunoglobulin heavy and light chains. Yeasts carry out post-translational peptide modifications includingglycosylation. A number of recombinant DNA strategies exist that utilize strong promoter sequences and high copy number plasmids which can be used for production of the desired proteins in yeast. Yeast recognizes leader sequences of cloned mammalian gene products and secretes polypeptides bearing leader sequences (i.e., pre-polypeptides). See, e.g., Hitzman et al., 11th Inti. Conf. Yeast, Genetics & Molec. Biol. (Montpelier, France, 1982).

[0239] Yeast gene expression systems can be routinely evaluated for the levels of production, secretion and the stability of antibodies, and assembled SLITRK6 antibodies and antigen binding portions thereof and other binding agents. Various yeast gene expression systems incorporating promoter and termination elements from the actively expressed genes coding for glycolytic enzymes produced in large quantities when yeasts are grown in media rich in glucose can be utilized. Known glycolytic genes can also provide very efficient transcription control signals. For example, the promoter and terminator signals of the phosphoglycerate kinase (PGK) gene can be utilized. Another example is the translational elongation factor lalpha promoter, such as that from Chinese hamster cells. A number of approaches can be taken for evaluating optimal expression plasmids for the expression of immunoglobulins in yeast. See II DNA Cloning 45, (Glover, ed., IRL Press, 1985) and e.g., U.S. Publication No. US 2006 / 0270045 A1.

[0240] Bacterial strains can also be utilized as hosts for the production of the antibody molecules or antigen binding portions thereof or other binding agents as described herein. E. coli K12 strains such as E. coli W3110, Bacillus species, enterobacteria such as Salmonella typhimurium or Serratia marcescens, and various Pseudomonas species can be used. Plasmid vectors containing replicon and control sequences that are derived from species compatible with a host cell are used in connection with these bacterial hosts. The vector carries a replication site, as well as specific genes which are capable of providing phenotypic selection in transformed cells. A number of approaches can be taken for evaluating the expression plasmids for the production of SLITRK6 antibodies and antigen binding portions thereof and other binding agents in bacteria (see Glover, 1985; Ausubel, 1987, 1993; Sambrook, 1989; Colligan, 1992- 1996).

[0241] Host mammalian cells can be grown in vitro or in vivo. Mammalian cells provide post- translational modifications to immunoglobulin molecules including leader peptide removal, folding and assembly of VH and VL chains, glycosylation of the antibody molecules, and secretion of functional antibody and / or antigen binding portions thereof or other binding agents.

[0242] Mammalian cells which can be useful as hosts for the production of antibody proteins, in addition to the cells of lymphoid origin described above, include cells of fibroblast origin, such as Vero or CHO-K1 cells. Exemplary eukaryotic cells that can be used to express immunoglobulin polypeptides include, but are not limited to, COS cells, including COS 7 cells; 293 cells, including 293-6E cells; CHO cells, including CHO--S and DG44 cells; PERC6TM cells (Crucell); and NSO cells. In some embodiments, a particular eukaryotic host cell is selectedbased on its ability to make desired post-translational modifications to the heavy chains and / or light chains. For example, in some embodiments, CHO cells produce polypeptides that have a higher level of sialylation than the same polypeptide produced in 293 cells.

[0243] In some embodiments, one or more SLITRK6 antibodies or antigen-binding portions thereof or other binding agents can be produced in vivo in an animal that has been engineered or transfected with one or more nucleic acid molecules encoding the polypeptides, according to any suitable method.

[0244] In some embodiments, an antibody or antigen-binding portion thereof is produced in a cell-free system. Non-limiting exemplary cell-free systems are described, e.g., in Sitaraman et al., Methods Mol. Biol. 498: 229-44 (2009); Spirin, Trends Biotechnol. 22: 538-45 (2004); and Endo et al., Biotechnol. Adv. 21 : 695-713 (2003).

[0245] Many vector systems are available for the expression of the VH and VL chains in mammalian cells (see Glover, 1985). Various approaches can be followed to obtain intact antibodies. As discussed above, it is possible to co-express VH and VL chains and optionally the associated constant regions in the same cells to achieve intracellular association and linkage of VH and VL chains into complete tetrameric H2L2 antibodies or antigen-binding portions thereof. The co-expression can occur by using either the same or different plasmids in the same host. Nucleic acids encoding the VH and VL chains or antigen binding portions thereof can be placed into the same plasmid, which is then transfected into cells, thereby selecting directly for cells that express both chains. Alternatively, cells can be transfected first with a plasmid encoding one chain, for example the VL chain, followed by transfection of the resulting cell line with a VH chain plasmid containing a second selectable marker. Cell lines producing antibodies, antigen-binding portions thereof via either route could be transfected with plasmids encoding additional copies of peptides, VH, VL, or VH plus VL chains in conjunction with additional selectable markers to generate cell lines with enhanced properties, such as higher production of assembled SLITRK6 antibodies or antigen binding portions thereof or other binding agents or enhanced stability of the transfected cell lines.

[0246] Additionally, plants have emerged as a convenient, safe and economical alternative expression system for recombinant antibody production, which are based on large scale culture of microbes or animal cells. SLITRK6 binding antibodies or antigen binding portions thereof or other binding agents can be expressed in plant cell culture, or plants grown conventionally. The expression in plants may be systemic, limited to sub-cellular plastids, or limited to seeds (endosperms). See, e.g., U.S. Patent Pub. No. 2003 / 0167531; U.S. Pat. No. 6,080,560; U.S. Pat. No. 6,512,162; and WO 0129242. Several plant-derived antibodies have reached advanced stages of development, including clinical trials (see, e.g., Biolex, N.C.).

[0247] For intact antibodies, the variable regions (VH and VL regions) of the SLITRK6 antibodies are typically linked to at least a portion of an immunoglobulin constant region (Fc) or domain, typically that of a human immunoglobulin. Human constant region DNA sequences canbe isolated in accordance with well-known procedures from a variety of human cells, such as immortalized B-cells (WO 87 / 02671). A SLITRK6 binding antibody can contain both light chain and heavy chain constant regions. The heavy chain constant region can include CH1, hinge, CH2, CH3, and, optionally, CH4 regions. In some embodiments, the CH2 domain can be deleted or omitted.

[0248] Techniques described for the production of single chain antibodies (see, e.g. U.S. Pat. No. 4,946,778; Bird, Science 242:423-42 (1988); Huston et al., Proc. Natl. Acad. Sci. USA 85:5879-5883 (1988); and Ward et al., Nature 334:544-54 (1989); which are incorporated by reference herein in their entireties) can be adapted to produce single chain antibodies that specifically bind to SLITRK6. Single chain antibodies are formed by linking the heavy and light chain variable regions of the Fv region via an amino acid bridge, resulting in a single chain polypeptide. Techniques for the assembly of functional Fv portions in E. coli can also be used (see, e.g. Skerra et al., Science 242:1038-1041 (1988); which is incorporated by reference herein in its entirety).

[0249] In some embodiments, an antigen binding portion or other binding agent comprises one or more scFvs. An scFv can be, for example, a fusion protein of the variable regions of the heavy (VH) and light chain (VL) variable regions of an antibody, connected with a short linker peptide of ten to about 25 amino acids. The linker is usually rich in glycine for flexibility, as well as serine or threonine for solubility, and can either connect the N-terminus of the VH with the C- terminus of the VL, or vice versa. This protein retains the specificity of the original antibody, despite removal of the constant regions and the introduction of the linker. scFv antibodies are, e.g. described in Houston, J. S., Methods in Enzymol. 203 (1991) 46-96. Methods for making scFv molecules and designing suitable peptide linkers are described in, for example, U.S. Pat. No. 4,704,692; U.S. Pat. No. 4,946,778; Raag and Whitlow, FASEB 9:73-80 (1995) and Bird and Walker, TIBTECH, 9: 132-137 (1991). scFv-Fcs have been described by Sokolowska- Wedzina et al., Mol. Cancer Res. 15(8): 1040- 1050, 2017.

[0250] In some embodiments, an antigen binding portion or other binding agent is a singledomain antibody is an antibody portion consisting of a single monomeric variable antibody domain. Single domains antibodies can be derived from the variable domain of the antibody heavy chain from camelids (e.g., nanobodies or VHH portions). Furthermore, a single-domain antibody can be an autonomous human heavy chain variable domain (aVH) or VNAR portions derived from sharks (see, e.g., Hasler et al., Mol. Immunol. 75:28-37, 2016).

[0251] Techniques for producing single domain antibodies (DABs or VHH) are known in the art, as disclosed for example in Cossins et al. (2006, Prot Express Purif 51:253-259) and Li et al. (Immunol. Lett. 188:89-95, 2017). Single domain antibodies may be obtained, for example, from camels, alpacas or llamas by standard immunization techniques. (See, e.g., Muyldermans et al., TIBS 26:230-235, 2001; Yau et al., J Immunol Methods 281 :161-75, 2003; and Maass et al., J Immunol Methods 324:13-25, 2007.) A VHH may have potent antigen-binding capacityand can interact with epitopes that are inacessible to conventional VH-VL pairs (see, e.g., Muyldermans et al., 2001). Alpaca serum IgG contains about 50% camelid heavy chain only IgG antibodies (HCAbs) (see, e.g., Maass et al., 2007). Alpacas may be immunized with antigens and VHHs can be isolated that bind to and neutralize the target antigen (see, e.g., Maass et al., 2007). PGR primers that amplify alpaca VHH coding sequences have been identified and can be used to construct alpaca VHH phage display libraries, which can be used for antibody fragment isolation by standard biopanning techniques well known in the art (see, e.g., Maass et al., 2007).

[0252] Techniques for making multispecific antibodies include, but are not limited to, recombinant co-expression of two immunoglobulin heavy chain-light chain pairs having different specificities (see, e.g., Milstein and Cuello, Nature 305: 537 (1983)), WO 93 / 08829, and Traunecker et al., EMBO J. 10: 3655 (1991)), and "knob-in-hole" engineering (see, e.g., U.S.Pat. No. 5,731 ,168; Carter (2001), J Immunol Methods 248, 7-15). Multi-specific antibodies may also be made by engineering electrostatic steering effects for making antibody Fc-heterodimeric molecules (see, e.g., WO 2009 / 089004A1); cross-linking of two or more antibodies or antigen binding portions thereof (see, e.g., U.S. Pat. No. 4,676,980, and Brennan et al., Science, 229: 81 (1985)); using leucine zippers to produce bi-specific antibodies (see, e.g., Kostelny et al., J. Immunol., 148(5):1547-1553 (1992)); using "diabody" technology for making bispecific antibody portions (see, e.g., Hollinger et al., Proc. Natl. Acad. Sci. USA, 90:6444-6448 (1993)); and using single-chain Fv (scFv) dimers (see, e.g. Gruber et al., J. Immunol., 152:5368 (1994)); and preparing trispecific antibodies as described, e.g., in Tutt et al. J. Immunol. 147: 60 (1991).

[0253] Engineered antibodies with three or more functional antigen binding sites, including "Octopus antibodies," also can be binding agents (see, e.g. US 2006 / 0025576A1).

[0254] The binding agents (e.g., antibodies or antigen binding portions) herein also include a "Dual Acting FAb" or "DAF" comprising an antigen binding site that binds to two different antigens (see, e.g., US 2008 / 0069820 and Bostrom et al., 2009, Science 323:1610-14)."Crossmab" antibodies are also included herein (see e.g. WO 2009 / 080251 , WO 2009 / 080252, W02009 / 080253, W02009 / 080254, and WO2013 / 026833).

[0255] In some embodiments, the binding agents comprise different antigen-binding sites, fused to one or the other of the two subunits of the Fc domain; thus, the two subunits of the Fc domain may be comprised in two non-identical polypeptide chains. Recombinant co-expression of these polypeptides and subsequent dimerization leads to several possible combinations of the two polypeptides. To improve the yield and purity of the bispecific molecules in recombinant production, it will thus be advantageous to introduce in the Fc domain of the binding agent a modification promoting the association of the desired polypeptides.

[0256] Generally, this method involves replacement of one or more amino acid residues at the interface of the two Fc domains by charged amino acid residues so that homodimer formation becomes electrostatically unfavorable but heterodimerization electrostatically favorable.

[0257] In some embodiments, a binding agent is a "bispecific T cell engager" or BiTE (see, e.g., W02004 / 106381, W02005 / 061547, W02007 / 042261, and W02008 / 119567). This approach utilizes two antibody variable domains arranged on a single polypeptide. For example, a single polypeptide chain can include two single chain Fv (scFv) portions, each having a variable heavy chain (VH) and a variable light chain (VL) domain separated by a polypeptide linker of a length sufficient to allow intramolecular association between the two domains. This single polypeptide further includes a polypeptide spacer sequence between the two scFvs. Each scFv recognizes a different epitope, and these epitopes may be specific for different proteins, such that both proteins are bound by the BiTE.

[0258] As it is a single polypeptide, the bispecific T cell engager may be expressed using any prokaryotic or eukaryotic cell expression system known in the art, e.g., a CHO cell line.However, specific purification techniques (see, e.g., EP1691833) may be necessary to separate monomeric bispecific T cell engagers from other multimeric species, which may have biological activities other than the intended activity of the monomer. In one exemplary purification scheme, a solution containing secreted polypeptides is first subjected to a metal affinity chromatography, and polypeptides are eluted with a gradient of imidazole concentrations. This eluate is further purified using anion exchange chromatography, and polypeptides are eluted using with a gradient of sodium chloride concentrations. Finally, this eluate is subjected to size exclusion chromatography to separate monomers from multimeric species. In some embodiments, a binding agent that is a bispecific antibody is composed of a single polypeptide chain comprising two single chain FV portions (scFV) fused to each other by a peptide linker.

[0259] In some embodiments, a binding agent is multispecific, such as an IgG-scFV. IgG- scFv formats include lgG(H)-scFv, scFv-(H)lgG, lgG(L)-scFv, svFc-(L)lgG, 2scFV-lgG and IgG- 2scFv. These and other bispecific antibody formats and methods of making them have been described in for example, Brinkmann and Kontermann, MAbs 9(2):182-212 (2017); Wang et al., Antibodies, 2019, 8, 43; Dong et al., 2011 , MAbs 3:273-88; Natsume et al., J. Biochem.140(3):359-368, 2006; Cheal et al., Mol. Cancer Ther. 13(7): 1803-1812, 2014; and Bates and Power, Antibodies, 2019, 8, 28.

[0260] IgG-like dual-variable domain antibodies (DVD-lg) have been described by Wu et al., 2007, Nat Biotechnol 25:1290-97; Hasler et al., Mol. Immunol. 75:28-37, 2016 and in WO 08 / 024188 and WO 07 / 024715. Triomabs have been described by Chelius et al., MAbs 2(3):309-319, 2010. 2-in-1-lgGs have been described by Kontermann et al., Drug Discovery Today 20(7): 838-847, 2015. Tanden antibody or TandAb have been described by Kontermann et al., id. ScFv-HSA-scFv antibodies have also been described by Kontermann et al. (id.).

[0261] Intact (e.g., whole) antibodies, their dimers, individual light and heavy chains, or antigen binding portions thereof and other binding agents can be recovered and purified by known techniques, e.g., immunoadsorption or immunoaffinity chromatography, chromatographic methods such as HPLC (high performance liquid chromatography), ammonium sulfateprecipitation, gel electrophoresis, or any combination of these. See generally, Scopes, Protein Purification (Springer- Verlag, N.Y., 1982). Substantially pure SLITRK6 binding antibodies or antigen binding portions thereof or other binding agents of at least about 90% to 95% homogeneity are advantageous, as are those with 98% to 99% or more homogeneity, particularly for pharmaceutical uses. Once purified, partially or to homogeneity as desired, an intact SLITRK6 antibody or antigen binding portions thereof or other binding agent can then be used therapeutically or in developing and performing assay procedures, immunofluorescent staining, and the like. See generally, Vols. I & II Immunol. Meth. (Lefkovits & Pernis, eds., Acad. Press, NY, 1979 and 1981).ANTIBODY DRUG CONJUGATES

[0262] In some embodiments, a SLITRK6 antibody, antigen binding portion or other binding agent as described herein is part of a SLITRK6 antibody drug conjugate (also referred to as a SLITRK6 conjugate or SLITRK6 ADC). In some embodiments, the SLITRK6 antibody, antigen binding portion or other binding agent is attached to at least one linker, and at least one drug is attached to each linker. As used herein, in the context of a conjugate, the term “drug” refers to cytotoxic agents (such as chemotherapeutic agents or drugs), immunomodulatory agents, nucleic acids (including siRNAs), growth inhibitory agents, toxins (e.g., protein toxins, enzymatically active toxins of bacterial, fungal, plant, or animal origin, or fragments thereof), radioactive isotopes, PROTACs and other compounds that are active against target cells when delivered to those cells.Cytotoxic Agents

[0263] In some embodiments, a SLITRK6 conjugate includes at least one drug (or termed as “drug unit”) that is cytotoxic agent. A "cytotoxic agent" refers to an agent that has a cytotoxic effect on a cell. A "cytotoxic effect" refers to the depletion, elimination and / or the killing of a target cell(s). Cytotoxic agents include, for example, tubulin disrupting agents, topoisomerase inhibitors, DNA minor groove binders, and DNA alkylating agents.

[0264] Tubulin disrupting agents include, for example, auristatins, dolastatins, tubulysins, colchicines, vinca alkaloids, taxanes, cryptophycins, maytansinoids, hemiasterlins, as well as other tubulin disrupting agents. Auristatins are derivatives of the natural product dolastatin 10. Exemplary auristatins include MMAE (N-methylvaline-valine-dolaisoleuine-dolaproine- norephedrine), MMAF (N-methylvaline-valine-dolaisoleuine-dolaproine-phenylalanine) and AFP (see W02004 / 010957 and W02007 / 008603). Other auristatin like compounds are disclosed in, for example, Published US Application Nos. US2021 / 0008099, US2017 / 0121282, US2013 / 0309192 and US2013 / 0157960. Dolastatins include, for example, dolastatin 10 and dolastatin 15 (see, e.g., Pettit et al., J. Am. Chem. Soc., 1987, 109, 6883-6885; Pettit et al., AntiCancer Drug Des., 1998, 13, 243-277; and Published US Application US2001 / 0018422).Additional dolastatin derivatives contemplated for use herein are disclosed in U.S. Patent 9,345,785, incorporated herein by reference. In some embodiments, the tubulin disrupting agent is MMAE.

[0265] Tubulysins include, but are not limited to, tubulysin D, tubulysin M, tubuphenylalanine and tubutyrosine. WO2017 / 096311 and WO / 2016-040684 describe tubulysin analogs including tubulysin M.

[0266] Colchicines include, but are not limited to, colchicine and CA-4.

[0267] Vinca alkaloids include, but are not limited to, vinblastine (VBL), vinorelbine (VRL), vincristine (VCR) and vindesine (VOS).

[0268] Taxanes include, but are not limited to, paclitaxel and docetaxel.

[0269] Cryptophycins include but are not limited to cryptophycin-1 and cryptophycin-52.

[0270] Maytansinoids include, but are not limited to, maytansine, maytansinol, maytansine analogs in DM1, DM3 and DM4, and ansamatocin-2. Exemplary maytansinoid drug moieties include those having a modified aromatic ring, such as: C-19-dechloro (U.S. Pat. No. 4,256,746) (prepared by lithium aluminum hydride reduction of ansamitocin P2); C-20-hydroxy (or C-20- demethyl) + / -C-19-dechloro (U.S. Pat. Nos. 4,361,650 and 4,307,016) (prepared by demethylation using Streptomyces or Actinomyces or dechlorination using LAH); and C-20- demethoxy, C-20-acyloxy (-OCOR), + / -dechloro (U.S. Pat. No. 4,294,757) (prepared by acylation using acyl chlorides), and those having modifications at other positions.

[0271] Maytansinoid drug moieties also include those having modifications such as: C-9-SH (U.S. Pat. No. 4,424,219) (prepared by the reaction of maytansinol with H2S or P2S5); C-14- alkoxymethyl(demethoxy / CH2OR) (U.S. Pat. No. 4,331,598); C-14- hydroxymethyl or acyloxymethyl (CH2OH or CH2OAc) (U.S. Pat. No. 4,450,254) (prepared from Nocardia); C-15- hydroxy / acyloxy (U.S. Pat. No. 4,364,866) (prepared by the conversion of maytansinol by Streptomyces); C-15-methoxy (U.S. Pat. Nos. 4,313,946 and 4,315,929) (isolated from Trewia nudiflora); C-18-N-demethyl (U.S. Pat. Nos. 4,362,663 and 4,322,348) (prepared by the demethylation of maytansinol by Streptomyces); and 4,5-deoxy (U.S. Pat. No. 4,371,533) (prepared by the titanium trichloride / LAH reduction of maytansinol).

[0272] Hemiasterlins include but are not limited to, hemiasterlin and HTI-286.

[0273] Other tubulin disrupting agents include taccalonolide A, taccalonolide B, taccalonolide AF, taccalonolide AJ, taccalonolide Al-epoxide, discodermolide, epothilone A, epothilone B, and laulimalide.

[0274] In some embodiments, a cytotoxic agent can be a topoisomerase inhibitor, such as a camptothecin. Exemplary camptothecins include, for example, camptothecin, irinotecan (also referred to as CPT-11), belotecan, (7-(2-(N-isopropylamino)ethyl)camptothecin), topotecan, 10- hydroxy-CPT, SN-38, exatecan and the exatecan analog DXd (see US20150297748). Other camptothecins are disclosed in W01996 / 021666, WO00 / 08033, US2016 / 0229862 and WO2020 / 156189.

[0275] In some embodiments, a cytotoxic agent is a duocarmcycin, including the synthetic analogues, KW-2189 and CBI-TMI.Immune Modulatory Agents

[0276] In some embodiments, a drug is an immune modulatory agent. An immune modulatory agent can be, for example, a TLR7 and / or TLR8 agonist, a STING agonist, a RIG-1 agonist or other immune modulatory agent.

[0277] In some embodiments, a drug is an immune modulatory agent, such as a TLR7 and / or TLR8 agonist. In some embodiments, a TLR7 agonist is selected from an imidazoquinoline, an imidazoquinoline amine, a thiazoquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2-d]pyrimidine-2,4-diamine, pyrimidine-2,4-diamine, 2-aminoimidazole, 1-alkyl-1H- benzimidazol-2-amine, tetrahydropyridopyrimidine, heteroarothiadiazide-2,2-dioxide, a benzonaphthyridine, a guanosine analog, an adenosine analog, a thymidine homopolymer, ssRNA, CpG-A, PolyGIO, and PolyG3. In some embodiments, the TLR7 agonist is selected from an imidazoquinoline, an imidazoquinoline amine, a thiazoquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2-d]pyrimidine-2,4-diamine, pyrimidine-2,4-diamine, 2- aminoimidazole, 1-alkyl-1H-benzimidazol-2-amine, tetrahydropyridopyrimidine, heteroarothiadiazide-2,2-dioxide or a benzonaphthyridine. In some embodiments, a TLR7 agonist is a non-naturally occurring compound. Examples of TLR7 modulators include GS- 9620, GSK-2245035, imiquimod, resiquimod, DSR-6434, DSP-3025, I MO-4200, MCT-465, MEDI-9197, 3M-051, SB-9922, 3M-052, Limtop, TMX-30X, TMX-202, RG- 7863, RG-7795, and the compounds disclosed in US20160168164 (Janssen), US 20150299194 (Roche), US20110098248 (Gilead Sciences), US20100143301 (Gilead Sciences), and US20090047249 (Gilead Sciences).

[0278] In some embodiments, a TLR8 agonist is selected from a benzazepine, an imidazoquinoline, a thiazoloquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2- d]pyrimidine-2,4-diamine, pyrimidine-2,4-diamine, 2-aminoimidazole, 1-alkyl-1 H-benzimidazol-2- amine, tetrahydropyridopyrimidine or a ssRNA. In some embodiments, a TLR8 agonist is selected from a benzazepine, an imidazoquinoline, a thiazoloquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2-d]pyrimidine-2,4-diamine, pyrimidine-2,4-diamine, 2- aminoimidazole, 1-alkyl-1H-benzimidazol-2-amine, and a tetrahydropyridopyrimidine. In some embodiments, a TLR8 agonist is a non-naturally occurring compound. Examples of TLR8 agonists include motolimod, resiquimod, 3M-051 , 3M-052, MCT-465, IMO-4200, VTX-763, VTX- 1463.

[0279] In some embodiments, a TLR8 agonist can be any of the compounds described WG2018 / 170179, WG2020 / 056198 and WG2020056194.

[0280] Other TLR7 and TLR8 agonists are disclosed in, for example, WO2016142250, W02017046112, W02007024612, W02011022508, W02011022509, W02012045090,WO2012097173, WO2012097177, WO2017079283, US20160008374, US20160194350, US20160289229, US Patent No. 6043238, US20180086755 (Gilead), WO2017216054 (Roche), WO2017190669 (Shanghai De Novo Pharmatech), W02017202704 (Roche), W02017202703 (Roche), W020170071944 (Gilead), US20140045849 (Janssen), US20140073642 (Janssen), WO2014056953 (Janssen), WO2014076221 (Janssen), WO2014128189 (Janssen), US20140350031 (Janssen), WO2014023813 (Janssen), US20080234251 (Array Biopharma), US20080306050 (Array Biopharma), US20100029585 (Ventirx Pharma), US20110092485 (Ventirx Pharma), US20110118235 (Ventirx Pharma), US20120082658 (Ventirx Pharma), US20120219615 (Ventirx Pharma), US20140066432 (Ventirx Pharma), US20140088085 (Ventirx Pharma), US20140275167 (Novira Therapeutics), and US20130251673 (Novira Therapeutics), WO2018198091 (Novartis AG), and US20170131421 (Novartis AG).

[0281] In some embodiments, an immune modulatory agent is a STING agonist. Examples of STING agonists include, for example, those disclosed in W02020059895, WO2015077354, WO2020227159, WG2020075790, WG2018200812, and WG2020074004.

[0282] In some embodiments, an immune modulatory agent is a RIG-I agonist. Examples of RIG-I agonists include KIN1148, SB-9200, KIN700, KIN600, KIN500, KIN100, KIN101 , KIN400 and KIN2000.Toxins

[0283] ] In some embodiments, a drug is an enzymatically active toxin or fragment thereof, including but not limited to diphtheria A chain, nonbinding active fragments of diphtheria toxin, exotoxin A chain (from Pseudomonas aeruginosa), ricin A chain, abrin A chain, modeccin A chain, alpha-sarcin, Aleurites fordii proteins, dianthin proteins, Phytolaca americana proteins (PAPI, PAPII, and PAP-S), momordica charantia inhibitor, curcin, crotin, sapaonaria officinalis inhibitor, gelonin, mitogellin, restrictocin, phenomycin, enomycin, and the tricothecenes.Radioisotopes

[0284] In some embodiments, a drug is a radioactive atom. A variety of radioactive isotopes are available for the production of radioconjugates. Examples include 1131 , 1125, Y90, Re186 , Re188 , Sm153, Bi213, P32, Pb212 and radioactive isotopes of Lutetium (e.g., Lu177).PROTACs

[0285] In some embodiments, a drug is a proteolysis targeted chimera (PROTAC). PROTACs are described in, for example, Published US Application Nos. 20210015942, 20210015929, 20200392131 , 20200216507, US20200199247 and US20190175612; the disclosures of which are incorporated by reference herein.Linkers

[0286] The SLITRK6 conjugates typically comprise at least one linker, each linker having at least one drug attached to it. Typically, a conjugate includes a linker between a SLITRK6 antibody (or antigen binding portion thereof or other binding agent) and the drug (in some cases termed “drug unit”). In various embodiments, a linker may be a protease cleavable linker, an acid-cleavable linker, a disulfide linker, a disulfide-containing linker, or a disulfide-containing linker having a dimethyl group adjacent the disulfide bond (e.g., an SPDB linker) (see, e.g., Jain et al., Pharm. Res. 32:3526-3540 (2015); Chari et al., Cancer Res. 52:127-131 (1992); U.S. Patent No. 5,208,020), a self-stabilizing linker (see, e.g., WO2018 / 031690 and WO2015 / 095755 and Jain et al., Pharm. Res. 32:3526-3540 (2015)), a non-cleavable linker (see, e.g., W02007 / 008603), a photolabile linker, and / or a hydrophilic linker (see, e.g., W02015 / 123679).

[0287] In some embodiments, a linker is a cleavable linker that is cleavable under intracellular conditions, such that cleavage of the linker releases the drug from the antibody (or antigen binding portion thereof or other binding agent) and / or linker in the intracellular environment. For example, in some embodiments, a linker is cleavable by a cleaving agent that is present in the intracellular environment (e.g., within a lysosome or endosome or caveolae). A linker can be, for example, a peptidyl linker that is cleaved by an intracellular peptidase or protease enzyme, including, but not limited to, a lysosomal or endosomal protease (see, e.g., W02004 / 010957, US20150297748, US2008 / 0166363, US20120328564 and US20200347075). Typically, a peptidyl linker is at least one amino acid long or at least two amino acids long. Intracellular cleaving agents can include cathepsins B and D and plasmin, all of which are known to hydrolyze dipeptide drug derivatives resulting in the release of active drug inside target cells (see, e.g., Dubowchik and Walker, 1999, Pharm. Therapeutics 83:67-123). Most typical are peptidyl linkers that are cleavable by enzymes that are present in target antigenexpressing cells. For example, a peptidyl linker that is cleavable by the thiol-dependent protease cathepsin-B, which is highly expressed in cancerous tissue, can be used (e.g., a Phe- Leu or a Gly-Phe-Leu-Gly linker). Other such linkers are described, for example, in U.S. Pat. No. 6,214,345. In specific embodiments, the peptidyl linker cleavable by an intracellular protease is a Val-Cit linker or a Phe-Lys linker (see, e.g., U.S. Pat. No. 6,214,345, which describes the synthesis of doxorubicin with the val-cit linker) or Gly-Gly-Phe-Gly (SEQ ID NO: 57) linker (see, e.g., US2015 / 0297748). One advantage of using intracellular proteolytic release of the drug is that the drug is typically attenuated when conjugated and the serum stabilities of the conjugates are typically high. See also US Patent No. 9,345,785.

[0288] As used herein, the terms "intracellularly cleaved" and "intracellular cleavage" refer to a metabolic process or reaction inside a cell on an antibody drug conjugate, whereby the covalent attachment, e.g., the linker, between a drug (e.g., a cytotoxic agent) and the antibody is broken, resulting in the free drug, or other metabolite of the conjugate dissociated from the antibody inside the cell. The cleaved moieties of the conjugate are thus intracellular metabolites.

[0289] In some embodiments, a cleavable linker is pH-sensitive, i.e. , sensitive to hydrolysis at certain pH values. Typically, a pH-sensitive linker is hydrolyzable under acidic conditions. For example, an acid-labile linker that is hydrolyzable in the lysosome (e.g., a hydrazone, semicarbazone, thiosemicarbazone, cis-aconitic amide, orthoester, acetal, ketal, or the like) can be used. (See, e.g., U.S. Pat. Nos. 5,122,368; 5,824,805; and 5,622,929; Dubowchik and Walker, 1999, Pharm. Therapeutics 83:67-123; Neville et al., 1989, Biol. Chem. 264:14653- 14661.) Such linkers are relatively stable under neutral pH conditions, such as those in the blood, but are unstable at below pH 5.5 or 5.0, the approximate pH of the lysosome. In certain embodiments, a hydrolyzable linker is a thioether linker (such as, for example, a thioether attached to the drug via an acylhydrazone bond (see, e.g., U.S. Pat. No. 5,622,929)).

[0290] In some embodiments, a linker is cleavable under reducing conditions (e.g., a disulfide linker). A variety of disulfide linkers are known, including, for example, those that can be formed using SATA (N-succinimidyl-5-acetylthioacetate), SPDP (N-succinimidyl-3-(2- pyridyldithio)propionate), SPDB (N-succinimidyl-3-(2-pyridyldithio)butyrate) and SMPT (N- succinimidyl-oxycarbonyl-alpha-methyl-alpha-(2-pyridyl-dithio)toluene)-, SPDB and SMPT (see, e.g., Thorpe et al., 1987, Cancer Res. 47:5924-5931; Wawrzynczak et al., In Immunoconjugates: Antibody Conjugates in Radioimagery and Therapy of Cancer (C. W. Vogel ed., Oxford U. Press, 1987. See also U.S. Pat. No. 4,880,935.)

[0291] In some embodiments, the linker is a malonate linker (Johnson et al., 1995, Anticancer Res. 15:1387-93), a maleimidobenzoyl linker (Lau et al., 1995, Bioorg-Med-Chem. 3(10):1299- 1304), or a 3'-N-amide analog (Lau et al., 1995, Bioorg-Med-Chem. 3(10): 1305-12). In some embodiments, the linker is not cleavable, such as a maleimidocaproyl linker, and the drug is released by antibody degradation. (See U.S. Publication No. 2005 / 0238649).

[0292] In some embodiments, a linker is not substantially sensitive to the extracellular environment. As used herein, "not substantially sensitive to the extracellular environment," in the context of a linker, means that no more than about 20%, typically no more than about 15%, more typically no more than about 10%, and even more typically no more than about 5%, no more than about 3%, or no more than about 1% of the linkers, in a sample of the antibody drug conjugate (ADC), are cleaved when the ADC is present in an extracellular environment (e.g., in plasma). Whether a linker is not substantially sensitive to the extracellular environment can be determined, for example, by incubating independently with plasma both (a) the ADC (the "ADC sample") and (b) an equal molar amount of unconjugated antibody or drug (the "control sample") for a predetermined time period (e.g., 2, 4, 8, 16, or 24 hours) and then comparing the amount of unconjugated antibody or drug present in the ADC sample with that present in control sample, as measured, for example, by high performance liquid chromatography.

[0293] In some embodiments, a linker promotes cellular internalization. In some embodiments, a linker promotes cellular internalization when conjugated to the drug such as a cytotoxic agent (i.e., in the milieu of the linker-drug moiety of the ADC as described herein). In yet otherembodiments, a linker promotes cellular internalization when conjugated to both the drug and the SLITRK6 antibody (i.e., in the milieu of the ADC as described herein).

[0294] A variety of linkers that can be used with the present compositions and methods are described in WO 2004010957. In some embodiments, a protease cleavable linker comprises a thiol-reactive spacer and a dipeptide. In some embodiments, the protease cleavable linker consists of a thiol-reactive maleimidocaproyl spacer, a valine-citrulline dipeptide, and a p- amino-benzyloxycarbonyl spacer.

[0295] In some embodiments, an acid cleavable linker is a hydrazine linker or a quaternary ammonium linker (see WO2017 / 096311 and WO2016 / 040684.)

[0296] In some embodiments, a linker is a self-stabilizing linker comprising a maleimide group as described in U.S. Patent 9,504,756.

[0297] In some embodiments, a linker is a hydrophilic linker, such as, for example, the hydrophilic peptides in W02015 / 123679 and the sugar alcohol polymer-based linkers disclosed in W02013 / 012961 and WO2019 / 213046 (each of which is incorporated by reference in its entirety).

[0298] In some embodiments, the binging agents herein disclosed may be connected to a linker as disclosed in WO2023280227, the contents of which are incorporated by reference in its entirety.

[0299] In other embodiments, conjugates of a SLITRK6 antibody (or antigen binding portion or other binding agent) and a drug may be made using a variety of bifunctional protein coupling agents such as N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP), succinimidyl-4-(N- maleimidomethyl) cyclohexane- 1 -carboxylate (SMCC), iminothiolane (IT), bifunctional derivatives of imidoesters (such as dimethyl adipimidate HCI), active esters (such as disuccinimidyl suberate), aldehydes (such as glutaraldehyde), bis-azido compounds (such as bis (p-azidobenzoyl) hexanediamine), bis-diazonium derivatives (such as bis-(p- diazoniumbenzoyl)-ethylenediamine), diisocyanates (such as toluene 2,6-diisocyanate), and bis-active fluorine compounds (such as 1,5-difluoro-2,4-dinitrobenzene). Chelating agents for conjugation of a radionucleotide(s) to an antibody, antigen binding portion thereof or other binding agent have been described in, for example WO94 / 11026.

[0300] The conjugates of a SLITRK6 antibodies (or antigen binding portion or other binding agent) include, but are not limited to such conjugates prepared with cross-linker reagents including, but not limited to, BMPS, EMCS, GMBS, HBVS, LC-SMCC, MBS, MPBH, SBAP, SIA, SIAB, SMCC, SMPB, SMPH, sulfo-EMCS, sulfo-GMBS, sulfo-KMUS, sulfo-MBS, sulfo-SIAB, sulfo-SMCC, and sulfo-SMPB, and SVSB (succinimidyl-(4-vinylsulfone)benzoate) which are commercially available (e.g., from Pierce Biotechnology, Inc., Rockford, IL., U.S.A).

[0301] In some embodiments, a linker is attached to a terminus of an amino acid sequence of an antibody, antigen binding portion or other binding agent or can be attached to a side chain modification of an antibody, antigen binding portion or other binding agent, such as the sidechain of a lysine, serine, threonine, cysteine, tyrosine, aspartic acid, a non-natural amino acid residue, glutamine, or glutamic acid residue. An attachment between an antibody, antigen binding portion or other binding agent and a linker or drug can be via any of a number of bonds, for example but not limited to, an amide bond, an ester bond, an ether bond, a carbon-nitrogen bond, a carbon-carbon single double or triple bond, a disulfide bond, or a thioether bond. Functional groups that can form such bonds include, for example, amino groups, carboxyl groups, aldehyde groups, azide groups, alkyne and alkene groups, ketones, carbonates, carbonyl functionalities bonded to leaving groups such as cyano and succinimidyl and hydroxyl groups.

[0302] In some embodiments, a linker is attached to an antibody, antigen binding portion or other binding agent at an interchain disulfide. In some embodiments, a linker is connected to an antibody, antigen binding portion or other binding agent at a hinge cysteine residue. In some embodiments, a linker is attached to an antibody, antigen binding portion or other binding agent at an engineered cysteine residue. In some embodiments, a linker is connected to an antibody, antigen binding portion or other binding agent at a lysine residue. In some embodiments, a linker is connected to an antibody, antigen binding portion or other binding agent at an engineered glutamine residue. In some embodiments, a linker is connected to an antibody, antigen binding portion or other binding agent at an unnatural amino acid engineered into the heavy chain.

[0303] In some embodiments, a linker is attached to an antibody, antigen binding portion or other binding agent via a sulfhydryl group. In some embodiments, a linker is attached to an antibody, antigen binding portion or other binding agent via a primary amine. In some embodiments, a linker is attached via a link created between an unnatural amino acid on an antibody, antigen binding portion or other binding agent by reacting with oxime bond that was formed by modifying a ketone group with an alkoxyamine on a drug.

[0304] In some embodiments, a linker is attached to an antibody, antigen binding portion or other binding agent via Sortase A linker. A Sortase A linker can be created by a Sortase A enzyme fusing an LPXTG recognition motif (SEQ ID NO: 58) to an N-terminal GGG motif to regenerate a native amide bond.

[0305] In some embodiments, the conjugate of the present disclosure comprises: one of the SLITRK6 antibodies, antigen binding portions thereof, and other binding agents, at least one linker attached to the binding agent; at least one drug unit, wherein each drug unit is attached to a linker, and wherein the linker optionally comprises at least one polar group.

[0306] In some embodiments, in the conjugate of the present disclosure, the linker is derived from a linker compound, or a stereoisomer or salt thereof, and the linker compound comprises: a linker unit; a stretcher group connected to the linker unit, an optional amino acid unit; and the at least one polar group; wherein: the stretcher group has an attachment site to the binding agent and an attachment site to the amino acid unit (when present) or the linker subunit; theamino acid unit (when present) has an attachment site to the stretcher group and an attachment site to the linker unit; and the linker unit has an attachment site to the amino acid unit (when present) or to the stretcher group and to the at least one drug unit.

[0307] Some of the components and variations of the linker (and the linker compound) are exemplified and demonstrated by the “linker embodiments” herein provided.ENUMERATED EMBODIMENTS

[0308] The linker (and linker compound) of the present disclosure is further illustrated by the following embodiments which should not be construed as limiting.

[0309] Embodiment 1. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein the polymer unit comprises the formula:~R°-(R3-R1-[O-CH2-CH2]n0-R6-([O-CH2-CH2]n0-R2-R3-(NR4R5)nl)n2)n3(la) or a stereoisomer or salt thereof, wherein:R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene; each R3is independently selected from a bond, C1-C12 alkylene, -C(O)-, -NRa-C1- C12 alkylene, -C1-C12 alkylene-NRa-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -C1-C12 alkylene-NRa-C(O)-, -C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C1-C12 alkylene-C(O)-, - C(O)-C1-C12 alkylene-NRa-, -NRa-C(O)-NRa-, -NRa-C(O)-, -NRa-C(O)-C1-C12 alkylene, -C(O)- NRa-C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C12 alkylene, heteroaryl-C1-C12 alkylene- C(O)-, -NRa-C(O)-C1-C12 alkylene-C(O)-, -C(O)-NRa-C1-C12 alkylene-(CH(OH))i.8-C1-C12 alkylene-, -O-CH2-CH2, -O-C(O)-NRa-C1-C12 alkylene, -O-CH2-CH(OH)-C(O)-, -O-CH2-CH(OH)- C(O)-NRa- C1-C12 alkylene-, -CH(OH)-, -CH(OH)-C1-C12 alkylene-, C1-C12 alkylene-CH(OH)-, - CH(OH)-C(O)-, -CH(OH)-C(O)-NRa-C1-C12 alkylene-, -CH(OH)-C1-C12 alkylene-NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, - CH(OH)-NRa-C1-C12 alkylene-, -[C(O)-(CH2)i-8-NRa]i-8-, triazolyl, -C1-C12 alkylene-triazolyl-, - N(polyhydroxyl group)-, and -C(O)NR7R8, wherein one of R7and R8is H or C1-C12 alkylene and the other is C1-C12 alkylene, each Rais independently selected from H, C1-6 alkyl, and wherein any of the above alkylene groups may be substituted with -SO3H; each R4and R5are independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)- polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or achelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each R6is selected from:each n3and n4are independently 0-1 , each Rbis independently H or C1-6 alkyl, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, m is 1-4, each v is independently 1-6, and n2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, n6is 1-10, each p is independently 0-6, and n2is 1 ;each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, q is 1-8, each v is independently 1-6, andn2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, each p is independently 0-6, and n2is 1 ;(v) -R10-[O-CH2-CH2]I-8-R10-, wherein: each Rbis independently H or C1-6 alkyl, w each R10is independentlyeach p is independently 1-6, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)V-O-S(=O)2(OH), and q is 1-8; n2is 1 ; and(vi) -N-(R1-X-R2-)2, wherein: each X is independently -NRa-C(O)- or -C(O)NRa-, and n2is 2; and the wavy line (~) indicates the attachment site of the amino acid unit to R°; each n° is independently 2-26; each n1is independently 1-6; and n3is 1-6.

[0310] Embodiment 2. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises the formula:~R0-(R3-R1-[O-CH2-CH2]n0-R2-(NR4R5)nl)n3(la’) or a stereoisomer or salt thereof, wherein:R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene; each R3is independently -N(polyhydroxyl group)-, triazolyl, -C1-C12 alkylene- triazolyl-,each R4and R5are independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)- polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each Rais independently H or C1-6 alkyl;indicates the attachment site of R3to R° the wavy line ( ^x) indicates the attachment site of the R3to R1; each p is 1-6; each n° is independently 2-8; each n1is independently 1-6; and n3is 1-6.

[0311] Embodiment 3. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises the formula:~R0-(R1-[0-CH2-CH2]no-R2-R3-(NR4R5)ni)n3(la”) or a stereoisomer or salt thereof, wherein:(i) R° is a functional group for attachment to a subunit of the amino acid unit; each R1and R2are independently a bond or C1-C6 alkylene;R3is -C(O)-;R4is H;R5is independently a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate; the wavy line (~) indicates the attachment site of the amino acid unit to R°; n° is independently 2-26; n1is 1-6; and n3is 1-6;(ii) R° is -C(O)-;R1, R2, and R3are each a bond;R4and R5are each independently H, a polyhydroxyl group, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; the wavy line (~) indicates the attachment site of the amino acid unit to R°; n° is 6; n1is 1-6; and n3is 1 ;(iii) R° is a functional group for attachment to a subunit of the amino acid unit;R1and R2are each, independently, a bond or C1-C6 alkylene;R3is-NRa-C(O)-C1-C12 alkylene-C(O)-, wherein the alkylene is substituted with -SO3H;Rais H or C1-6 alkyl;R4and R5are each independently H, a carboxyl-containing moiety, a polyhydroxyl group, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator,wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; the wavy line (~) indicates the attachment site of the amino acid unit to R°; each n° is independently 1-26; n1is 1-6; and n3is 1-6; oreach R1is independently a bond or C1-C6 alkylene;R2and R3are each a bond;R4and R5are each independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)-polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each Rais independently H or C1-6 alkyl; the wavy line (indicates the attachment site of R° to the remainder of the polymer unit; the wavy line (-*) indicates the attachment site of the amino acid unit to R°; n° is 1-8; n1is 1-6; and n3is 2.

[0312] Embodiment 4. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit, said linker unit comprising a moiety of formula:or a stereoisomer or salt thereof, wherein: a — represents a direct or indirect attachment site to an amino acid unit;8 — represents an attachment site to at least one of the drug units or for a linking group attached to the at least one of the drug units; andRais H or C1-6 alkyl;(b) the amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit.

[0313] Embodiment 5. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit;(b) an amino acid unit having from 1 to 12 amino acid subunits; and(c) at least one polar group attached to the amino acid unit, wherein the polar group comprises a polymer unit, optionally a sugar unit, and optionally a carboxyl unit, wherein said polymer unit comprises: / o RaRb\O-M-Pl\Rb / (i) an optionally substituted polyamide comprising the formulax zno , or a stereoisomer thereof, wherein each Rais independently H or C1-6 alkyl and each Rbis independently H or C1-6 alkyl, and n° is independently 2-26;(ii) a substituted polyether comprising the formula, or a stereoisomer thereof, wherein each Rbis independently H or C1-6 alkyl, and n° is independently 2-26; or(iii) combinations thereof.

[0314] Embodiment 6. The linker compound of Embodiment 4 or 5, wherein the at least one polar group attached to the amino acid unit comprises the formula:~R°-(R3-R1-[O-CH2-CH2]n0-R6-([O-CH2-CH2]n0-R2-R3-(NR4R5)nl)n2)n3(la),~R°-(R3-R1-[0-CH2-CH(OH)-CH2]no-R6-[0-CH2-CH(OH)-CH2]no-R2-R3-(NR4R5)ni)n3 (lb), or~R°-(R3-R1-[O-CH2-GH(OH)-CH2]n0-R6-[O-CH2-CH(OH)-CH2]n0-R2-R3-(NR4R5)n-)-3 I(R3-R’-[0-CH2-CH2]no-R2-R3-(NR4R5) m)n3(Ic) or a stereoisomer or salt thereof, wherein:R° is a functional group for attachment to a subunit of the amino acid unit;each R1and R2are independently a bond or C1-C6 alkylene; each R3is independently selected from a bond, C1-C12 alkylene, -C(O)-, -NRa-C1- C12 alkylene, -C1-C12 alkylene-NRa-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -C1-C12 alkylene-NRa-C(O)-, -C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C1-C12 alkylene-C(O)-, - C(O)-C1-C12 alkylene-NRa-, -NRa-C(O)-NRa-, -NRa-C(O)-, -NRa-C(O)-C1-C12 alkylene, -C(O)- NRa-C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C12 alkylene, heteroaryl-C1-C12 alkylene- C(O)-, -NRa-C(O)-C1-C12 alkylene-C(O)-, -C(O)-NRa-C1-C12 alkylene-(CH(OH))i.8-C1-C12 alkylene-, -O-CH2-CH2, -O-C(O)-NRa-C1-C12 alkylene, -O-CH2-CH(OH)-C(O)-, -O-CH2-CH(OH)- C(O)-NRa-C1-C12 alkylene-, -CH(OH)-, -CH(OH)-C1-C12 alkylene-, C1-C12 alkylene-CH(OH)-, - CH(OH)-C(O)-, -CH(OH)-C(O)-NRa-C1-C12 alkylene-, -CH(OH)-C1-C12 alkylene-NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, - CH(OH)-NRa-C1-C12 alkylene-, -[C(O)-(CH2)i-8-NRa]i-8-, triazolyl, -C1-C12 alkylene-triazolyl-, and - C(O)NR7R8, wherein one of R7and R8is H or C1-C12 alkylene and the other is C1-C12 alkylene, each Rais independently selected from H, C1-6 alkyl, and wherein any of the above alkylene groups may be substituted with -SO3H; each R4and R5are independently H, a polyhydroxyl group, a carboxyl-containing moiety, a substituted polyhydroxyl group, a -C(O)-polyhydroxyl group, a substituted -C(O)- polyhydroxyl group, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group, or a chelator, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, and wherein at least one of R4and R5is not H; each R6is independently a bond or selected from:each n3and n4are independently 0-1 , each Rbis independently H or C1-6 alkyl, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, m is 1-4, and each v is independently 1-6, and n2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, n6is 1-10, and each p is independently 0-6, and n2is 1 ;(iii), wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, each R9is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH), each p is independently 0-6, and q is 1-8, each v is independently 1-6, and n2is 1 ;wherein: each Rais independently H or C1-6 alkyl, each Rbis independently H or C1-6 alkyl, and each p is independently 0-6, and n2is 1 ;(v) -R10-[O-CH2-CH2]I-8-R10-, wherein: each Rbis independently H or C1-6 alkyl, dneach R10is independentlyeach p is independently 1-6, and q is 1-8; and(vi) -N-(R1-X-R2-[0-CH2-CH2]no-R2-R3-(NR4R5)ni)2, wherein: each X is independently -NRa-C(O)- or -C(O)NRa-, and n2is 2; and the wavy line (~) indicates the attachment site of the amino acid unit to R°; each n° is independently 2-26; n1is 0-6, and when n1is 0 then R3is -OH or -C(O)ORb, wherein Rbis independently H or C1-6 alkyl; and n3is 1-6.

[0315] Embodiment 7. The linker compound of any one of Embodiments 1 and 4-6, wherein each R3is independently selected from a bond, -C(O)-, -NRa-C(O)-C1-C12 alkylene-C(O)-, - C(O)-NRa-C1-C12 alkylene-(CH(OH))i-8-C1-C12 alkylene-, -O-CH2-CH(OH)-C(O)-, -O-CH2- CH(OH)-C(O)-NRa- C1-C12 alkylene-, -CH(OH)-, -CH(OH)-C1-C12 alkylene-, C1-C12 alkylene- CH(OH)-, -CH(OH)-C(O)-, -CH(OH)-C(O)-NRa-C1-C12 alkylene-, -CH(OH)-C1-C12 alkylene- NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1-C12 alkylene-, -NRa-C(O)-C1-C12 alkylene-C(O)-NRa-C1- Ci2alkylene-, -CH(OH)-NRa-C1-C12 alkylene-, -[C(O)-(CH2)i-8-NRa]i-8-, triazolyl, and -C1-C12 alkylene-triazolyl-, -N(polyhydroxyl group)-, each Rais independently selected from H, C1-6 alkyl; and wherein any of the above alkylene groups may be substituted with -SO3H.

[0316] Embodiment 8. The linker compound of any one of Embodiments 1-3 and 5-7, wherein the linker unit comprises a moiety selected from:or a stereoisomer or salt thereof, wherein: a — represents a direct or indirect attachment site to the amino acid unit or ;5 — represents an attachment site to at least one of the drug units or an attachment site to a linking group attached to the at least one of the drug units; and Rais H or C1-6 alkyl.

[0317] Embodiment 9. A linker compound, or a stereoisomer or salt thereof, comprising:(a) a linker unit having from 1 to 4 attachment sites for a drug unit and having one of the following structures (i) or (ii):(b) at least one polar group comprising a polymer unit, optionally a sugar unit, optionally a carboxyl unit, and combinations thereof; and(c) optionally a stretcher group having an attachment site for a SLITRK6 binding agent; wherein: a — is an attachment site to an enzyme-cleavable group;P — is an attachment site to the at least one polar group;8 — is H, an attachment site to at least one of the drug units, or an attachment site to a linking group attached to the at least one of the drug units; the polymer unit comprises a polyamide, a polyether, or a combination thereof, wherein the polyether comprises a hydroxyl group, a polyhydroxyl group, a sugar group, a carboxyl group, or combinations thereof; each Raindependently is H or C1-C6 alkyl; each Rbindependently is halo, C1-6 alkyl, an attachment site to at least one of the drug units, or an attachment site to at least one of the polar groups; x is 0, 1 , 2, 3 or 4; y is 0, 1 , 2 or 3;Rcis a bond, -C(O)-, -S(O)-, -SO2-, C1-6 alkylene, C1-6 alkynylene, triazolyl or combinations thereof; andY is a bond, -O-, -S-, -N(Ra)-, -C(O)-, -S(O)-, -SO2-C1-C6 alkylene, C1-C6 alkenylene, C1- Ce alkynylene, triazolyl, a group containing triazolyl, or combinations thereof.

[0318] Embodiment 10. The linker compound of Embodiment 9, wherein the linker unit has one of the following structures (i-a), (ii-a) , or (iii-a):or a stereoisomer or salt thereof.

[0319] Embodiment 11. The linker compound of Embodiment 9 or 10, wherein the linker unit has one of the following structures (i-b), (i-c), (i-d), (i-e) or (i-f):

[0320] Embodiment 12. The linker compound of Embodiments 9, wherein the linker unit has the following structure (ii-b) or (iii-b):or a stereoisomer or salt thereof.

[0321] Embodiment 13. The linker compound of any one of Embodiments 9-12, wherein the polar group comprises at least one sugar unit having the following formula:L3-N(CH2- (CH(XR))k- Xi(X2))2(X) or a stereoisomer or salt thereof, wherein: each X is independently selected from NH and O; each R is independently selected from hydrogen, acetyl, a monosaccharide, a disaccharide, and a polysaccharide; each Xi is independently selected from CH2and C(O); each X2is independently selected from H, OH and OR; k is 1 to 10; andL3 is a point of attachment to a remainder of the polar group.

[0322] Embodiment 14. The linker compound of Embodiment 13, wherein the at least one sugar unit has one of the following structures (XII) or (XIII):or a stereoisomer or salt thereof, wherein: each R is independently selected from hydrogen, a monosaccharide, a disaccharide and a polysaccharide; m is 1 to 8; andn is 0 to 4.

[0323] Embodiment 15. The linker compound of any one of Embodiments 1-14, the polar group has a formula selected from:(a) ~R20-R21-[O-CH2-CH2]n20-R22-NR24R25(XX) or a stereoisomer a salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, a bond or C1-C3 alkylene;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- Cs alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle; and n20 is 2 to 26; or(b) ~R2°-R21-[0-CH2-CH2]n2o-R22-NR24R25(XXI) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, a bond or C1-C3 alkylene; one of R24and R25is selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)-polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3- C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1-C8 alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); and the other of R24and R25is a polyethylene glycol, optionally having 1 to 24 ethylene glycol subunits; and n20 is 2 to 26; or(c) ~R2°-[-R26-[R29-[0-CH2-CH2-]n2oR29]n2i-R27-NR24R25]n27 (XXII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R26and R27are each optional and are, independently, selected from a bond, C1-C12 alkylene, -NH-C1-C12 alkylene, -C1-C12 alkylene-NH-, -C1-C12 alkylene-N(CH3)-, - C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -NH-C1-C12 alkylene-C(O)- and - C(O)-C1-C12 alkylene-NH-; one of R24and R25is selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)-polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-Cio carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1-C8 alkyl; a chelator; - C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); and the other of R24and R25is selected from H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)-polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3- C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1-C8 alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); and polyethylene glycol, optionally having 1 to 24 ethylene glycol subunits; or - NR24R25together from a C3-C8 heterocycle; each R29is optional and independently selected from -C(O)-, -NH-, -C(O)-C1-C6 alkylene-, -NH-C1-C6 alkylene-, -C1-C6 alkylene-NH-, -C1-C6 alkylene-C(O)-, - NH(CO)-C1-C6alkylene-, -N(CH3)-(CO)-C1-C6alkylene-, -NH(CO)NH-, and triazole; n20 is 2 to 26; n21 is 1 to 4; and n27 is 1 to 4, or(d) ~R20-R21-[-C(Ra)H-C(O)-N(RN)-]n20-R22-NR24R25(XXIII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21is a bond, C1-C3 alkylene,-C1-C3alkylene-[0-CH2-CH2-]n2o, -[CH2-CH2-O]n2o-C1-C3alkylene- or -C1-C3alkylene-[0-CH2-CH2-]n2o-C(0)-;R22is C1-C3 alkylene,-C1-C3alkylene-[0-CH2-CH2-]n2o, -[CH2-CH2-O]n2o-C1-C3alkylene- or -C1-C3alkylene-[0-CH2-CH2-]n2o-C(0)-; each Rais independently H or -R22-NR24R25; each RNis independently H, C1-C6 alkyl or -R22-NR24R25;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- C8 alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle; and each n20 is independently 2 to 26, or(e) ~R20-R21-[-C(Ra)H-C(O)-N(RN)-]n20-R22-CO2R26(XXIV)or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; R21and R22are each, independently, a bond, C1-C3 alkylene, or -C1-C3alkylene[0-CH2-CH2-]n2o; each Rais independently H or -R22-NR24R25; each RNis independently H, C1-C6 alkyl or -R22-NR24R25;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- Cs alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle;R26is H or C1-C4 alkyl; and each n20 is independently 2 to 26, with the proviso that at least one Raor RNis -R22-NR24R25; or(f) ~R20-R21-[C(Ra)H-C(O)-N(RN)-]n20-R22-N-(R23-NR24R25)2(XXV) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; R21and R22are each, independently, a bond, C1-C3 alkylene, or -C1-C3alkylene-[0-CH2-CH2-]n2o; each Rais independently H or -R22-NR24R25; each RNis independently H or C1-C6 alkyl; each R23is independently C1-C6 alkylene;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- C8 alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle; and each n20 is independently 2 to 26.

[0324] Embodiment 16. The linker compound of Embodiment 15, wherein R24and R25are each independently selected from H and a polyhydroxyl group, provided that R24and R25are not both H.

[0325] Embodiment 17. The linker compound of Embodiment 15 or 16, wherein the polyhydroxyl group is a linear monosaccharide, optionally selected from a C6 or C5 sugar, asugar acid and an amino sugar.

[0326] Embodiment 18. The linker compound of Embodiment 17, wherein: the C6 or C5 sugar is selected from glucose, ribose, galactose, mannose, arabinose, 2- deoxyglucose, glyceraldehyde, erythrose, threose, xylose, lyxose, allose, altrose, gulose, idose, talose, aldose, and ketose; the sugar acid is selected from gluconic acid, aldonic acid, uronic acid and ulosonic acid; or the amino sugar is selected from glucosamine, N-acetyl glucosamine, galactosamine, and N-acetyl galactosamine.

[0327] Embodiment 19. The linker compound of any one of Embodiments 15 to 18, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein each R is independently H or alkyl; each R39is independently selected from H, a linear monosaccharide and polyethylene glycol, optionally having from 1 to 24 ethylene glycol subunits; each n independently is 1-12; and the wavy line is an attachment to site p or to site Rb,or to the enzyme-cleavable group.

[0328] Embodiment 20. The linker compound of Embodiment 15 or 16, wherein one of R24and R25is a linear monosaccharide and the other is a cyclic monosaccharide.

[0329] Embodiment 21. The linker compound of Embodiment 20, wherein-(NR24R25) is selected from the following, or a stereoisomer or salt thereof:wherein R11is a cyclic monosaccharide.

[0330] Embodiment 22. The linker compound of Embodiment 20, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein R41is a cyclic monosaccharide; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0331] Embodiment 23. The linker compound of Embodiment 15, wherein R24and R25are independently a polyhydroxyl selected from a cyclic monosaccharide, disaccharide, and polysaccharide.

[0332] Embodiment 24. The linker compound of Embodiment 23, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:wherein each R12is selected from H and a monosaccharide, a disaccharide, or a polysaccharide; and R5is selected from a cyclic monosaccharide, disaccharide, or polysaccharide.

[0333] Embodiment 25. The linker compound of Embodiment 23, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein each R45is selected from H and a monosaccharide, a disaccharide, or a polysaccharide; and R46is selected from a cyclic monosaccharide, disaccharide, or polysaccharide; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0334] Embodiment 26. The linker compound of Embodiment 15, wherein R24and R25are independently selected from a linear monosaccharide and a substituted linear monosaccharide, wherein the substituted linear monosaccharide is substituted with a monosaccharide, a disaccharide, or a polysaccharide.

[0335] Embodiment 27. The linker compound of Embodiment 26, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:wherein R13is a linear monosaccharide; and each R14is selected from a monosaccharide, a disaccharide and a polysaccharide.

[0336] Embodiment 28. The linker compound of Embodiment 26, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein R47is a linear monosaccharide; and each R49is selected from a monosaccharide, a disaccharide, and a polysaccharide; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0337] Embodiment 29. The linker compound of Embodiment 15, wherein R24and R25are independently selected from a linear monosaccharide and a substituted monosaccharide, wherein the substituted linear monosaccharide is substituted with one or more substituents selected from carboxyl, ester, and amide, and optionally further substituted with a monosaccharide, disaccharide, or a polysaccharide.

[0338] Embodiment 30. The linker compound of Embodiment 29, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:wherein each R15is independently selected from a linear monosaccharide and a substituted linear monosaccharide; each R16is independently selected from hydroxyl, carboxyl, ester, and amide.

[0339] Embodiment 31. The linker compound of Embodiment 29, the polar group is selectedfrom the following, or a stereoisomer or salt thereof:wherein each R42is independently selected from a linear monosaccharide and a substituted linear monosaccharide; each R43is independently selected from hydroxyl, carboxyl, ester, and amide; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0340] Embodiment 32. The linker compound of Embodiment 15, wherein one of R24and R25is a -C(O)-polyhydroxyl group or substituted -C(O)-polyhydroxyl group, and the other of R24and R25is a H, -C(O)-polyhydroxyl group, substituted -C(O)-polyhydroxyl group, polyhydroxyl group or substituted polyhydroxyl group; wherein the substituted -C(O)-polyhydroxyl group and polyhydroxyl group are substituted with a monosaccharide, a disaccharide, a polysaccharide, carboxyl, ester, or amide.

[0341] Embodiment 33. The linker compound of Embodiment 32, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:

[0342] Embodiment 34. The linker compound of Embodiment 32, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0343] Embodiment 35. The linker compound of Embodiment 15, wherein wherein - (NR24R25) is selected from the following, or a stereoisomer or salt thereof:wherein R18is selected from OH, CH2OH, COOH or -C1-C6 alkyl substituted with hydroxyl or carboxyl.

[0344] Embodiment 36. The linker compound of Embodiment 15, wherein R24and R25are independently H or substituted -C1-C8 alkyl, provided that both R24and R25are not H; wherein substituted -C1-C8 alkyl is substituted with hydroxyl and / or carboxyl.

[0345] Embodiment 37. The linker compound of Embodiment 36, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein R48is selected from H, OH, CH2OH, COOH, or -C1-C6 alkyl substituted with hydroxyl or carboxyl; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0346] Embodiment 38. The linker compound of Embodiment 15, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:

[0347] Embodiment 39. The linker compound of Embodiment 15, wherein one of R24and R25is H or substituted -C(O)-C1-C8 alkyl, and the other of R24and R25is substituted -C(O)-C1-C8 alkyl, or substituted -C1-C8 alkyl, , wherein substituted -C(O)-C1-C8 alkyl and substituted -C1-C8 alkyl, are substituted with hydroxyl and / or carboxyl.

[0348] Embodiment 40. The linker compound of Embodiment 39, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0349] Embodiment 41. The linker compound of Embodiment 15, wherein R24and R25are independently selected from H and a chelator, wherein the chelator is optionally attached to the nitrogen of -NR24R25by an alkylene, arylene, carbocyclo, heteroarylene, or heterocarbocylo; provided that both R24and R25are not H.

[0350] Embodiment 42. The linker compound of Embodiment 41 , wherein the chelator is selected from ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetraminehexaacetic acid (TTHA), benzyl-DTPA, 1,4,7,10- tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA), benzyl-DOTA, 1,4,7- triazacyclononane-N,N',N"-triacetic acid (NOTA), benzyl-NOTA, 1,4,8,11- tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA) and N,N'-dialkyl substituted piperazine.

[0351] Embodiment 43. The linker compound of Embodiment 42, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0352] Embodiment 44. The linker compound of Embodiment 15, wherein R24and R25are independently selected from a H, a polyhydroxyl-ether group, a substituted polyhydroxyl-ether group.

[0353] Embodiment 45. The linker compound of Embodiment 15, wherein -(NR24R25) is selected from the following, or a stereoisomer or salt thereof:

[0354] Embodiment 46. The linker compound of any one of Embodiments 13-32, wherein each monosaccharide is independently selected from: a C5 or C6 sugar selected from glucose, ribose, galactose, mannose, arabinose, 2-deoxyglucose, glyceraldehyde, erythrose, threose, xylose, lyxose, allose, altrose, gulose, idose talose, aldose, and ketose; a sugar acid selected from gluconic acid, aldonic acid, uronic acid and ulosonic acid; or an amino sugar selected from glucosamine, N-acetyl glucosamine, galactosamine, and N-acetyl galactosamine.

[0355] Embodiment 47. The linker compound of any one of Embodiments 1 to 9, wherein the attachment site p is formed from a functional group of a precursor compound of the polar group, said functional group selected from halo, aldehyde, carboxyl, amino, alkynyl, azido, hydroxyl, carbonyl, carbamate, thiol, urea, thiocarbamate, thiourea, sulfonamide, acyl sulfonamide, alkyl sulfonate, triazole, azadibenzocyclooctyne, hydrazine, carbonylalkylheteroaryl, and protected forms thereof.

[0356] Embodiment 48. The linker compound of any one of Embodiments 1 to 14, the polar group has a formula selected from the following:~R20-R21-[O-CH2-CH2]n20-R22-R30(XXX) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; R21and R22are each independently, a bond or C1-C3 alkylene groups;R30is selected from an optionally substituted C3-C10 carbocycle; thiourea; optionally substituted thiourea; urea; optionally substituted urea; sulfamide; alkyl sulfamide; acyl sulfamide, optionally substituted alkyl sulfamide; optionally substituted acyl sulfamide; sulfonamide; optionally substituted sulfonamide; guanidine, including alkyl and aryl guanidine; phosphoramide; or optionally substituted phosphoramide; or R30is selected from azido, alkynyl, substituted alkynyl, -NH- C(O)-alkynyl, -NH-C(O)-alkynyl-R65; cyclooctyne; -NH-cyclooctyne, -NH-C(O)- cyclooctyne, or -NH-(cyclooctyne)2; wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionallysubstituted heterocarbocycle or optionally substituted heteroaryl; and n20 is 2 to 26;(b) ~R20-R21-[O-CH2-CH2]n20-R22-NH-C(O)-R31(XXXI) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, a bond or C1-C3 alkylene groups;R31is a branched polyethylene glycol chain, each branch having 1 to 26 ethylene glycol subunits and each branch having an R35at its terminus;R35is azido, alkynyl, alkynyl-R65, cyclooctyne or cyclooctyne-R65, wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heterocarbocycle or optionally substituted heteroaryl; and n20 is 2 to 26;(c) ~R2°-R21-[0-CH2-CH2]n2o-R22-C(0)NH-R31(XXXII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each , independently, a bond or C1-C3 alkylene groups;R31is a branched polyethylene glycol chain, each branch, independently, having 1 to 26 ethylene glycol subunits and each branch having an R35at its terminus;R35is azido, alkynyl, alkynyl-R65, cyclooctyne or cyclooctyne-R65, wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heterocarbocycle and optionally substituted heteroaryl; and n20 is 2 to 26;(d) ~R20-R21-[O-CH2-CH2]n20-R22-C(O)NR31-R22-NR24R25(XXXIII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R31is H or R22-NR24R25;R21and R22are each, independently, a bond or C1-C3 alkylene groups;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group, provided that R24and R25are not both H; and n20 is 2 to 26;(e) ~R2°-R21-[0-CH2-CH2]n2o-R22-N(R33-R31)2(XXXIV) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, a bond or C1-C3 alkylene groups;R31is a branched polyethylene glycol chain, each branch having 1 to 26 ethyleneglycol subunits and each branch having an R35at its terminus;R33is C1-C3 alkylene, C1-C3 alkylene-C(O), -C(O)-C1-C3 alkylene, or -C(O)-C1-C3 alkylene-C(O);R35is azido, alkynyl, alkynyl-R65, cyclooctyne or cyclooctyne-R65, wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heterocarbocycle or optionally substituted heteroaryl; and n20 is 2 to 26;(f) ~R20-(R21-[CH2-CH(OR34)-CH2-O]n2o-R36)n25 (XXXV) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; each R21is independently a bond, -O- or C1-C3 alkylene group; each R34is independently H, -[CH2-CH(OH)-CH2-O]n2o-R36, -C(O)-NR24R25or -C(O)N(RN)-C1-C6alkylene-NR24R25;RNis H or C1-C4alkyl;R24and R25are each independently selected from a H; polyhydroxyl group; or substituted polyhydroxyl group, provided that both R24and R25are not H; each R36is independently H, C1-C6alkylene-C(OH)H-NR44R45, C1-C6alkylene- C(OH)H-C1-C6alkylene-NR44R45, -C(O)-NR24R25, -C(O)N(RN)-C1-C6alkylene- NR24R25, C1-C6alkylene-C(O)NR24R25or C1-C6alkylene-CO2R37; each R37is independently H or C1-C6 alkyl;R44and R45are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; provided that both R44and R45are not H; each n20 is independently 1 to 26; and n25 is 1 or 2;(g) ~R20-R21-[[CH2-CH2-O]n20-R22-[CH2-[CH(OH)]n23-CH2-O]n2l]n22- R23-NR24-R25(XXXVI) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; R21, R22and R23are each independently a bond or C1-C3 alkylene group;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group, provided that R24and R25are not both H; each n20 is independently 0 to 26, and each n21 is independently 0 to 26, with the proviso that at least one of n20 or n21 is 2 to 26; n22 is 1 to 5; each n23 is independently 1 or 2;(h) ~R20-(R21-[O-CH2-CH2]n20-R22-N(RN)-CO2-[CH2-CH(OR34)-CH2-O]n2i-R36)n25(XXXVII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p orto site Rb, or to the enzyme-cleavable group;R21and R22are each independently a bond or C1-C3 alkylene groups;RNis H or C1-C4alkyl;R24and R25are each independently selected from a H; polyhydroxyl group; or substituted polyhydroxyl group, provided that both R24and R25are not H; each R34is independently H, -[CH2-CH(OH)-CH2-O]n2o-R36or -C(O)N(RN)-C1- Cealkylene-NR24R25; each R36is independently H, C1-C6alkylene-C(OH)H-NR44R45, C1-C6alkylene- C(OH)H-C1-C6alkylene-NR44R45, -C(O)N(RN)-C1-C6alkylene-NR24R25, C1-C6alkylene-C(O)NR24R25or C1-C6alkylene-CO2R37; each R37is independently H or C1-C6 alkyl;R44and R45are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; provided that both R44and R45are not H; n20 is 2 to 26; n21 is 1 to 26; and n25 is 1 or 2;(i) ~R20-(R21-[N(RN)-C(0)-[0-CH2-CH(OH)-CH2]n2o]n2i-R22-NR24R25)n25(XXXVIII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each independently bond or C1-C3 alkylene groups;RNis H or C1-C4alkyl;R24and R25are each independently selected from a H; polyhydroxyl group; or substituted polyhydroxyl group, provided that R24and R25are not both H; n20 is 2 to 26; n21 is 1 to 4; and n25 is 1 , 2 or 3;0) ~R20-(R21-[C(Ra)H-C(O)-N(RN)]n20-R22-[CH2-CH2-O]n20-NR24R25)n25(XXXIX) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, a bond, C1-C3 alkylene, -C1-C3alkylene-[O-CH2- CH2-]n2o, -[CH2-CH2-O]n2o-C1-C3alkylene- or -C1-C3alkylene-[0-CH2-CH2-]n2o- C(O)-;each Rais independently H or -R22-NR24R25; each RNis independently H, C1-C6 alkyl or -R22-NR24R25;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- Cs alkyl; a chelator; -C(O)-R28, wherein R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle), provided that R24and R25are not both H; each n20 is independently 0 to 26, with the proviso that at least one n20 is 2 to 26; and n25 is 1 or 2; or(k) ~R20-R21-[C(Ra)H-C(O)-N(RN)]n20-R22-[CH2-CH2-O]n20-NR24R25I R21-[C(Ra)H-C(O)-N(RN)]n2i-R22-[CH2-CH2-O]n2i-R23-CO2-R26(XXXVX) or a stereoisomer or salt thereof, wherein: R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group; R21, R22and R23are each, independently, a bond, C1-C3 alkylene, -C1-C3alkylene-[O- CH2-CH2-]n2o, -[CH2-CH2-O]n2o-C1-C3alkylene- or -C1-C3alkylene-[0-CH2-CH2-]n2o- C(O)-; each Rais independently H or -R22-NR24R25; each RNis independently H, C1-C6 alkyl or -R22-NR24R25;R24and R25are each independently selected from a H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)-polyhydroxyl group; substituted -C(O)- polyhydroxyl group; optionally substituted C3-C10 carbocycle; optionally substituted C1-C3 alkylene C3-C10 carbocycle; optionally substituted heteroaryl; optionally substituted carbocycle; substituted -C1-C8 alkyl; substituted -C(O)-C1- Cs alkyl; a chelator; -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); or -NR24R25together from a C3-C8 heterocycle), provided that R24and R25are not both H;R26is H or C1-C6 alkyl; each n20 is independently 0 to 26, with the proviso that at least one n20 is 2 to 26; and each n21 is independently 0 to 26, with the proviso that at least one n21 is 2 to 26.

[0357] Embodiment 49. The linker compound of any one of Embodiments 1 to 14, the polar group has a formula selected from the following, or a stereoisomer or salt thereof:~R2°-R21-[0-CH2-CH2]n2o-R22-NH-C(0)-R31(XXXI),~R20-R21-[O-CH2-CH2]n20-R22-C(O)NH-R31(XXXII), and~R2°-R21-[0-CH2-CH2]n2o-R22-N-(R33-R31)2(XXXIII); wherein:R20is an attachment group to site p or to site Rb, or to the enzyme-cleavable group;R21and R22are each, independently, bond or C1-C3 alkylene groups;R31is a branched polyethylene glycol chain, each branch having 1 to 26 ethylene glycol subunits and each branch having an R35at its terminus;R33is C1-C3 alkylene, -C1-C3 alkylene-C(O), -C(O)-C1-C3 alkylene or -C(O)-C1-C3 alkylene-C(O);R35is azido, alkynyl, alkynyl-R65, cyclooctyne or cyclooctyne-R65, wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heterocarbocycle or optionally substituted heteroaryl; the wavy (~) line indicates an attachment site to R20; and n20 is 2 to 26.

[0358] Embodiment 50. The linker compound of Embodiment 48 or 49, the polar group is formed from a precursor group selected from the following:wherein R65is selected from optionally substituted alkyl, optionally substituted alkenyl, optionally substituted alkynyl, optionally substituted carbocycle, optionally substituted aryl, optionally substituted heterocarbocycle or optionally substituted heteroaryl; and the wavy line is an attachment site to Rb, or to the enzyme-cleavable group.

[0359] Embodiment 51. The linker compound of of Embodiment 48 or 49, wherein the attachment to site > to site Rb, or to the enzyme-cleavable group is formed from a functional group of a precursor compound of the polar group, said functional group selected from halo, aldehyde, carboxyl, amino, alkynyl, azido, hydroxyl, carbonyl, carbamate, thiol, urea, thiocarbamate, thiourea, sulfonamide, acyl sulfonamide, alkyl sulfonate, triazole, azadibenzocyclooctyne, hydrazine, carbonylalkylheteroaryl, and protected forms thereof.

[0360] Embodiment 52. The linker compound of any one of Embodiments 1-14, the polar group has a formula:~R20-(R43-R41-[O-CH2-CH2]n40-R42-R43-(NR44R45)n4i)n42 (XL) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site □, to site Rb, or to the enzyme-cleavable group;R41and R42are each, independently, bond or C1-C6 alkylene; each R43is, independently, a bond or is selected from C1-C12 alkylene, -NH-C1-C12 alkylene, -C1-C12 alkylene-NH-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -NH- C1-C12 alkylene-C(O)-, -C(O)-C1-C12 alkylene-NH-, -NH-C(O)-NH-, -NH-C(O)-, -NH- C(O)-C1-C12 alkylene, -C(O)-NH-C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C12alkylene, heteroaryl-C1-C12 alkylene-C(O)-, or -C(O)NR46R47, wherein one of R46and R47is H or C1-C12 alkylene and the other is C1-C12 alkylene;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate; n40 is 2 to 26; n41 is 1 to 6; and n42 is 1 to 6.

[0361] Embodiment 53. The linker compound of any one of Embodiments 1-14, the polar group has a formula:~R20-(R41-[O-CH2-CH2]n40-R42-R43-(NR44R45)n4i )n42 (XLI) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site □, to site Rb, or to the enzyme-cleavable group;R41and R42are each, independently, bond or C1-C6 alkylene;R43is a bond or is selected from C1-C12 alkylene, -NH-C1-C12 alkylene, -C1-C12 alkylene- NH-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -NH-C1-C12 alkylene-C(O)-, - C(O)-C1-C12 alkylene-NH-, -NH-C(O)-NH-, -NH-C(O)-, -NH-C(O)-C1-C12 alkylene, C(O)-NH-C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C12 alkylene, heteroaryl-C1- C12 alkylene-C(O)-, or -C(O)NR46R47, wherein one of R46and R47is H or C1-C12 alkylene and the other is C1-C12 alkylene;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate; n40 is 1 to 26; n41 is 1 to 6; and n42 is 1 to 6.

[0362] Embodiment 54. The linker compound of any one of Embodiments 1-14, the polar group has a formula:~R20-(R41-[O-CH2-CH2]n40-R42-R43-(NR44R45)n4i )n42 (XLII) or a stereoisomer or salt thereof, wherein:R20is an attachment group to site □, to site Rb, or to the enzyme-cleavable group;R41and R42are each, independently, bond or C1-C3 alkylene;R43is a bond or is selected from C1-C6 alkylene, -NH-C1-C12 alkylene, -C1-C6 alkylene- NH-, -C(O)-C1-C6alkylene, -C1-C6alkylene-C(O)-, -NH-CI-C6alkylene-C(O)-, -C(O)- C1-C6alkylene-NH-, -NH-C(O)-NH-, -NH-C(O)-, -NH-C(O)-CI-C6alkylene, -C(O)-NH- C1-C12 alkylene, -heteroarylene, heteroaryl-C1-C6 alkylene, heteroaryl- C1-C6 alkylene-C(O)-, or -C(O)NR46R47, wherein one of R46and R47is H or C1-C6 alkylene and the other is C1-C12 alkylene;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate; n40 is 1 to 16; n41 is 1 to 4; and n42 is 1 to 4.

[0363] Embodiment 55. The linker compound of any one of Embodiments 15, 48, 49, and 52-54 wherein R20is formed from a functional group of a precursor compound of the polar group, said functional group selected from halo, aldehyde, carboxyl, amino, alkynyl, azido, hydroxyl, carbonyl, carbamate, thiol, urea, thiocarbamate, thiourea, sulfonamide, acyl sulfonamide, alkyl sulfonate, triazole, azadibenzocyclooctyne, hydrazine, carbonylalkylheteroaryl, or protected forms thereof.

[0364] Embodiment 56. The linker compound of any one of Embodiments 15, 48, 19, and 52-54, wherein R20comprises one of the following structures:or a stereoisomer thereof, wherein R is H, C1-C6 alkyl or polyhydroxyl group, n is 0 to 12, the indicates an attachment to site p or to site Rb, or to the enzyme-cleavable group, and the indicates an attachment site to a remainder portion of the polar group.

[0365] Embodiment 57. The compound of any one of Embodiments 15, 48, 19, and 52-54, wherein R20has one of the following structures:or a stereoisomer thereof, wherein n = 0 to 12, the (•“«*) indicates an attachment to site p or to site Rb, or to the enzyme-cleavable group, and the (•«««) indicates an attachment site to a remainder portion of the polar group.

[0366] Embodiment 58. The linker compound of any one of Embodiments 52-57, whereinR43-(NR44R45)n4i has one of the following structures:or a stereoisomer thereof, wherein R = H, C1-C6 alkyl, a polyhydroxyl group, or a substituted polyhydroxyl group; and the (-vw) indicates the attachment site of R43to the remainder of the polar group.

[0367] Embodiment 59. The linker compound of any one of Embodiments 52-57, wherein R43-(NR44R45)n4i has one of the following structures:or a stereoisomer thereof, wherein the ) indicates the attachment site of R43to the remainder of the polar group.

[0368] Embodiment 60. The linker compound of any one of Embodiments 52-59, wherein - NR44R45has one of the following structures:or a stereoisomer thereof, wherein the (>~w) indicates the attachment site of -NR44R45to the remainder of the polar group.

[0369] Embodiment 61. The linker compound of any one of Embodiments 1-60, the polar group has one of the following structures prior to attachment to the linker unit:A32wherein:(*) indicates the attachment site p or to site Rb, or to the enzyme-cleavable group; each R is independently H or C1-C6 alkyl;R’ is H, C1-C6alkyl, -N(R24)(R25) or -CO2H; each n is independently 1 to 12;X is O, NR or -CH2-;V is bond or C1-C6 alkyl; one of R24and R25is selected from a H; polyhydroxyl group; substituted polyhydroxyl group; - C(O)-polyhydroxyl group; substituted -C(O)-polyhydroxyl group; substituted -C(O)-C1-C8 alkyl; a chelator; and -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); and the other of R24and R25is selected from H; polyhydroxyl group; substituted polyhydroxyl group; -C(O)- polyhydroxyl group; substituted -C(O)-polyhydroxyl group; substituted -C(O)-C1-C8 alkyl; a chelator; -C(O)-R28, where R28is the sugar unit of formula (XII) or (XIII); and polyethylene glycol, optionally having 1 to 24 ethylene glycol subunits, provided that R24and R25are not both H.

[0370] Embodiment 62. The linker compound of any one of Embodiments 1-14, the polar group has a formula selected from:(a) ~R40-(R43-R41-[O-CH2-CH2]n40-R46-[O-CH2-CH2]n40-R42-R43-(NR44R45)n4l)n42(XLIII) or a stereoisomer or salt thereof, wherein:R40is an attachment group to site Rb, or to the enzyme-cleavable group;R41and R42are each, independently, a bond or C1-C6 alkylene; each R43is, independently, selected from a bond, C1-C12 alkylene, -OC1-C12 alkylene, - C(=O)-, -NH-C1-C12 alkylene, -C1-C12 alkylene-NH-, -C(O)-C1-C12 alkylene, -C1-C12 alkylene-C(O)-, -NH-C1-C12 alkylene-C(O)-, -C(O)-C1-C12 alkylene-NH-, -NH-C(O)-NH-, - NH-C(O)-, -NH-C(O)-C1-C12 alkylene, -C(O)-NH-C1-C12 alkylene, C1-C12alkylene-NH- C(O)-, -heteroarylene, heteroaryl-C1-C12 alkylene, heteroaryl-C1-C12 alkylene-C(O)-, or - C(O)NR46R47, wherein one of R46and R47is H or C1-C12 alkylene and the other is C1-C12 alkylene;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, provided that R44and R45are not both H; each R46is independently selected from -NR50-, -NR50-C1-C6alkylene-NR50-, -NR50-C(O)- NR50-S(O)2-NR50- or -NR50-C(O)-C1-6alkylene-; each R50is independently selected from H, C1-C6 alkyl, or polyhydroxyl group; each n40 is independently 2 to 26; n41 is 1 to 6; and n42 is 1 to 6;(b) ~R40-(R51-[O-CH2-CH2]n43-R52-Xi-R55-X2-R53-[O-CH2-CH2]n43-R54-[X3-R56]n44-R57)n45(XLIV) or a stereoisomer or salt thereof, wherein:R40is an attachment group to site Rb, or to the enzyme-cleavable group; R51, R52, R53and R54are each, independently, a bond or C1-C6 alkylene; Xi, X2 and X3 are each independently -NRN-C(O)- or -C(O)-NRN-; each RNindependently represent H, C1-C6 alkyl, or polyhydroxyl group;R55and R56each independently represent a bivalent polyhydroxyl group;R57is H, OH or C1-C6alkyl; each n43 is independently 0 to 26, with the proviso that at least one n43 is 1 to 26; n44 is 0 to 10; and n45 is 1 or 2; or(c) ~R40-R51-[O-CH2-CH2]n43-R52-N-(R53-Xi-R54-[O-CH2-CH2]n43-(NR44R45))2(XLV) or a stereoisomer or salt thereof, wherein:R40is an attachment group to site Rb, or to the enzyme-cleavable group;R51, R53and R54are each, independently, a bond or optionally-substituted C1-C6 alkylene;R52is a bond, C1-C6 alkylene, -C(O)- or -O-C(O)-; each Xi is independently -NRN-C(O)- or -C(O)-NRN-; each RNindependently represent H, C1-C6 alkyl, or polyhydroxyl group;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, provided that R44and R45are not both H; and each n43 is independently 2 to 26.

[0371] Embodiment 63. The linker compound of Embodiment 52, the polar group has one of the following structures prior to attachment to the enzyme-cleavable group and / or to the linker unit:wherein:(*) indicates the attachment site to site Rb, or to the enzyme-cleavable group; each R is independently H, alkyl or polyhydroxyl group;R44and R45are each, independently, H, polyhydroxyl group, substituted polyhydroxyl group, -C(O)-polyhydroxyl group, or substituted -C(O)-polyhydroxyl group, wherein optional substituents are selected from sulfate, phosphate, alkyl sulfate, and alkyl phosphate, provided that R44and R45are not both H; and each n is independently 1 to 12.

[0372] Embodiment 64. The linker compound of any one of Embodiments 1-14, the polar group has a formula selected from:or a stereoisomer or salt thereof, wherein: each Y is independentlyeach R76is independently H, acetyl, -P(=O)(OH)2, or -(CH2)v-O-S(=O)2(OH); each Raand Rb is independently H or Raand Rb are taken together with the carbon to which they are attached to form an oxo group; each q is independently 2-26; each m is independently 1 to 4; each n is independently 1 to 4; each v is independently 1 to 6; and each * is an attachment site to Rb, or to the enzyme-cleavable group.

[0373] Embodiment 65. The linker compound of any one of Embodiments 1-14, the polar group has a formula selected from:(XVI 11 a) or a stereoisomer or salt thereof, wherein: each R76is independently H, acetyl, -P(=O)(OH)2, or -(CH2)VS(=O)2(OH); each q is independently 2-26; each m is independently 1 to 4; each n is independently 1 to 4;each v is independently 1 to 6; and each * is an attachment site to Rb, or to the enzyme-cleavable group

[0374] Embodiment 66. The linker compound of any one of Embodiments 1-14, the polar group has a formula selected from:(XVI I lb) or a stereoisomer or salt thereof, wherein:each q is independently 2-26; each m is independently 1 to 4; each n is independently 1 to 4; and each * is an attachment site to Rb, or to the enzyme-cleavable group.

[0375] Embodiment 67. The linker compound of Embodiment 66, wherein Y is R76.

[0376] Embodiment 68. The linker compound of Embodiment 66, wherein

[0377] Embodiment 69. The linker compound of Embodiment 66, wherein each Raand Rb is independently H.

[0378] Embodiment 70. The linker compound of Embodiment 66, wherein Raand Rb are taken together with the carbon to which they are attached to form an oxo group.

[0379] Embodiment 71. The linker compound of any one of Embodiments 64-66, wherein q is 10-20.

[0380] Embodiment 72. The linker compound of any one of Embodiments 64-66, wherein q is 12.

[0381] Embodiment 73. The linker compound of any one of Embodiments 1-72, wherein the polar group has one of the following structures prior to attachment to the amino acid unit:or a stereoisomer thereof, wherein Rais H or C1-6 alkyl and n is 1-20.

[0382] Embodiment 74. The linker compound of any one of Embodiments 1-73, wherein the polar group has one of the following structures prior to attachment to the amino acid unit:or a stereoisomer thereof, wherein Rais H or C1-6 alkyl and n is 1-20.

[0383] Embodiment 75. The linker compound of any one of Embodiments 1-74, wherein the polar group has one of the following structures prior to attachment to the amino acid unit:or a stereoisomer thereof, wherein Rais H or C1-6 alkyl and n is 1-20.

[0384] Embodiment 76. The linker compound of any one of Embodiments 1-75, the polar group is selected from the following, or a stereoisomer or salt thereof:wherein each Z is attached at * and is individually selected from:wherein each 'n™ is an attachment to site p or to site Rb, or to the enzyme-cleavable group.

[0385] Embodiment 77. The linker compound of any one of Embodiments 1 to 75, wherein the polar group is selected from the following:or a stereoisomer thereof, wherein each indicates an attachment site of the amino acid unit.

[0386] Embodiment 78. The linker compound of any one of Embodiments 1 to 77, wherein the polar group comprises at least one carboxyl unit having the following formula:R70(XXXX) or a stereoisomer or salt thereof, wherein:(a)L70is selected from C1-C8 alkylene, C1-C8 alkylene-C(O)-, -C(O)-C1-C8 alkylene-, and -C(O)-C1-C8 alkylene-C(O)-, and * is an attachment site to Rb, to the enzyme- cleavable group, or to a remainder of the polar group;R70is ~NR71(R72-R73), wherein R71is selected from H, C1-C12 alkyl, substituted C1-C12 alkyl, or polyethylene glycol (optionally having 1 to 12 ethylene glycol subunits), R72is a bond or is selected from optionally substituted C1-C3 alkylene, optionally substituted ether, optionally substituted thioether, optionally substituted ketone, optionally substituted amide, polyethylene glycol (optionally having 1 to 12 ethylene glycol subunits), optionally substituted carbocycle, optionally substituted aryl or optionally substituted heteroaryl, and R73is a carboxyl or polycarboxyl, wherein polycarboxyl comprises 1 to 10, or 1 to 6, or 1 to 4 carboxyl groups, wherein the carboxyl groups are interconnected by alkyl, alkylene, substituted alkyl, substituted alkylene, heteroalkyl, heteroalkylene, amino and / or amide; or(b)L70is selected from C1-C8 alkylene, C1-C8 alkylene-C(O)-, -C(O)-C1-C8 alkylene-, and -C(O)-C1-C8 alkylene-C(O)-, and * is an attachment site to Rb, to the enzyme- cleavable group, or to a remainder of the polar group;R70is ~NR71(R75.(R73)2), wherein R71is selected from H, C1-C12 alkyl, substituted C1-C12 alkyl, or polyethylene glycol (optionally having 1 to 12 ethylene glycol subunits), R75is a branched optionally substituted C1-C3 alkylene, optionally substituted ether, optionally substituted thioether, optionally substituted ketone, optionally substituted amide, polyethylene glycol (optionally having 1 to 12 ethylene glycol subunits), optionally substituted carbocycle, optionally substituted aryl or optionally substituted heteroaryl and each R73is independently carboxyl or polycarboxyl, wherein polycarboxyl comprises 1 to 10, or 1 to 6, or 1 to 4 carboxyl groups, wherein the carboxyl groups are interconnected by alkyl, alkylene, substituted alkyl, substituted alkylene, heteroalkyl, heteroalkylene, amino and / or amide; or(c)L70is selected from C1-C8 alkylene, C1-C8 alkylene-C(O)-, -C(O)-C1-C8 alkylene-, and -C(O)-C1-C8 alkylene-C(O)-, and * is an attachment site to Rb, to the enzyme- cleavable group, or to a remainder of the polar group;R70is ~N(R74-R73)(R72.R73), wherein R72and R74are each independently selected from optionally substituted C1-C3 alkylene, optionally substituted ether, optionally substituted thioether, optionally substituted ketone, optionally substituted amide, polyethylene glycol (optionally having 1 to 12 ethylene glycol subunits), optionally substituted carbocycle, optionally substituted aryl or optionally substituted heteroaryl, and each R73is independently carboxyl or polycarboxyl, wherein the polycarboxyl comprises 1 to 10, or 1 to 6, or 1 to 4 carboxyl groups, wherein the carboxyl groups are interconnected by alkyl, alkylene, substituted alkyl, substituted alkylene, heteroalkyl, heteroalkylene, amino and / or amide.

[0387] Embodiment 79. The linker compound of any one of Embodiments 1 to 78, the polar group includes the polymer unit and the sugar unit.

[0388] Embodiment 80. The linker compound of any one of Embodiments 1 to 78, the polar group includes at least two polymer units.

[0389] Embodiment 81. The linker compound of any one of Embodiments 1 to 78, the polar group includes the polymer unit and the carboxyl unit.

[0390] Embodiment 82. The linker compound of any one of Embodiments 1 to 78, comprising at least two polar groups.

[0391] Embodiment 83. The linker compound of any one of Embodiments 1 to 78, the polar group includes the polymer unit, the sugar unit and the carboxyl unit.

[0392] Embodiment 84. The linker compound of any one of Embodiments 1 to 78, the polar group includes at least two polymer units, at least one sugar unit and at least one carboxyl unit.

[0393] Embodiment 85. The linker compound of any one of Embodiments 9 to 78, wherein the enzyme-cleavable group comprises at least two amino acids.

[0394] Embodiment 86. The linker compound of any one of Embodiments 9 to 85, comprising at least one of the polar groups attached to the enzyme-cleavable group.

[0395] Embodiment 87. The linker compound of any one of Embodiments 1-86, having one of the following structures:whereinRcis a bond or C1-6 alkylene; the wavy line on the amino group indicates an attachment site for a stretcher group or, prior to attachment to the stretcher group, indicates H;P — is the attachment site to the at least one polar group; and the benzylic H on the benzylic OH is optionally replaced with a bond to at least one of the the drug unit or to a linking group attached to at least one of the drug units.

[0396] Embodiment 88. The linker compound of any one of Embodiments 1-86, comprising one of the following structures:or a stereoisomer thereof, wherein the polar group is attached to an amino acid subunit of the amino acid unit, the H of a hydroxyl or amino group of the para-aminobenzyl group or the H of a hydroxyl of the glycine residue of a GGFG peptide is optionally replaced with a bond to at least one of the drug units, or to a linking group attached to the at least one of the drug units, the wavy line on the amino group indicates an attachment site for the stretcher group or the amino acid unit or, prior to attachment, indicates H.

[0397] Embodiment 89. The linker compound of any one of Embodiments 1-88, comprising a formula selected from the following:~ [SU - aa] - L2 ~~ [aai(POLY) - aa] - L2 ~ or~ [CU - aa] - L2 ~ wherein the square brackets indicate the amino acid unit, each aa is an optional subunit of the amino acid unit, L2 is the linker unit, each wavy line (~) indicates an attachment site for a stretcher group; aai(POLY) is the polymer unit attached to an amino acid subunit of the amino acid unit, Sil is the sugar unit attached to a subunit of the amino acid unit or to the linker unit, and CU is the carboxyl unit attached to a subunit of the amino acid unit or to the linker unit; and the double wavy («) line indicates an attachment site for at least one of the drug units, wherein aa and aai are independently selected from alpha, beta and gamma amino acids and derivatives thereof.

[0398] Embodiment 90. The linker compound of any one of Embodiments 1-88, comprising a formula selected from the following:~ [SU-aa]~ [aai(POLY)-aa]~ [CU-aa] wherein the square brackets indicate the amino acid unit, each aa is an amino acid subunit of the amino acid unit, L2 is the linker Subunit attached to a side chain of aa, the wavy line (~) indicates an attachment site for a stretcher group; aai(POLY) is the polymer unit attached to aa, Sil is the sugar unit attached to aa, CU is the carboxyl unit attached to aa, and the double wavy («) line indicates an attachment site for at least one of the drug units; wherein aa and aai are independently selected from alpha, beta and gamma amino acids and derivatives thereof.

[0399] Embodiment 91. The linker compound of any one of Embodiments 1-88, comprising a formula selected from the following:~ [Sil - aa - Sil] - L2~ [aai(POLY) - aa - aa2(POLY)] - L2 or~ [CU - aa - CU] - L2 = wherein the square brackets indicate the amino acid unit, aa is an optional subunit of the amino acid unit, L2 is the linker unit, the wavy line (~) indicates an attachment site for a stretcher group; each of aai(POLY) and aa2(POLY) is the polymer unit attached to aa or to the other Polymerunit; each SU is the sugar unit attached to aa or the other sugar unit, each CU is the carboxyl unit attached to aa or to the other carboxyl unit, and the double wavy («) line indicates an attachment site for at least one of the drug units; wherein aa, aai and aa2are independently selected from alpha, beta and gamma amino acids and derivatives thereof.

[0400] Embodiment 92. The linker compound of any one of Embodiments 1-88, comprising a formula selected from the following:~ [SU-aa-SU]IL2~ [aai(POLY)-aa-aa2(POLY)]IL2 or~ [CU-aa-CU]IL2 wherein the square brackets indicate the amino acid unit, aa is an amino acid subunit of the amino acid unit, L2 is the linker unit attached to a side chain of aa, each wavy line (~) indicatesan attachment site for a stretcher group; each of aai(POLY) and aa2(P0LY) is the polymer unit attached to aa, each Sil is the sugar unit attached to aa; each CU is the carboxyl unit attached to aa; and the double wavy («) line indicates an attachment site for at least one of the drug units; wherein each of aa, aai and aa2 is independently selected from alpha, beta and gamma amino acids and derivatives thereof.

[0401] Embodiment 93. The linker compound of any one of Embodiments 1-92, wherein the linker unit is a cleavable linker unit.

[0402] Embodiment 94. The linker compound of any one of Embodiments 1-92, wherein the enzyme-cleavable group comprises a peptide that is cleavable by an intracellular protease.

[0403] Embodiment 95. The linker compound of Embodiment 94, wherein the intracellular protease is Cathepsin B.

[0404] Embodiment 96. The linker compound of Embodiment 94, wherein the enzyme- cleavable group comprises a cleavable peptide including a valine-citrulline peptide, a valinealanine peptide, a valine-lysine peptide, a phenylalanine-lysine peptide, or a glycine-glycine- phenylalanine-glycine peptide.

[0405] Embodiment 97. The linker compound of any one of Embodiments 94-96, wherein the cleavable peptide is attached to a para-aminobenzyl alcohol self immolative group (PABA).

[0406] Embodiment 98. The linker compound of any one of Embodiments 1-14, comprising one of the following structures:whereinRcis a bond or C1-6 alkylene; the wavy line on the amino group indicates an attachment site for the stretcher group or, prior to attachment to the stretcher group, indicates H;P — is an attachment site to a POLY unit; and the H on the benzylic OH is optionally replaced with a bond to at least one of the drug units or to the attachment site to at least one of the drug units.

[0407] Embodiment 99. The linker compound of any one of Embodiments 1-14, having one of the following structures:wherein the wavy line on the amino group indicates an attachment site to the stretcher group; or, prior to attachment to the stretcher group, indicates H, and the H on the benzylic OH is optionally replaced with a bond to at least one of the drug units or a linking group attached to the at least one of the drug units.

[0408] Embodiment 100. The linker compound of any one of Embodiments 1 to 14, comprising one of the following structures:wherein the wavy line on the oxygen group or the *-amino group indicates the attachment site to at least one of the drug units or for a linking group attached to the at least one of the drug units; and the wavy line on the amino group indicates an attachment site for the stretcher group or the amino acid unit or, prior to attachment, indicates H.

[0409] Embodiment 101. The linker compound of any one of Embodiments 1 to 100, wherein the enzyme-cleavable group is joined to the Stretch group by a non-peptidic linking group.

[0410] Embodiment 102. The linker compound of Embodiment 101, wherein the non-peptidic linking group is selected from optionally-substituted C1-C10 alkylene, optionally-substituted C2- C10 alkenylene, optionally-substituted C2-C10 alkynylene, or optionally-substituted polyethylene glycol.

[0411] Embodiment 103. The linker compound of any one of Embodiments 1-102, comprising the stretcher group attached to the enzyme-cleavable group.

[0412] Embodiment 104. The linker compound of Embodiment 103, wherein the stretcher group is selected from the following:wherein R17is -C1-C10 alkylene-, -C1-C10 heteroalkylene-, -C3-C8 carbocyclo-, -O-(C1-C8 alkylene)-, -(CH2-O-CH2)b-C1-C8 alkylene- (where b is 1 to 26), -C1-C8 alkylene-(CH2- O-CH2)b- (where b is 1 to 26), -C1-C8 alkylene-(CH2-O-CH2)b-C1-C8 alkylene- (where b is 1 to 26), -arylene-, -C1-C10 alkylene-arylene-, -arylene-C1-C10 alkylene-, -C1-C10 alkylene-(C3-C8 carbocyclo)-, -(C3-C8 carbocyclo)-C1-C10 alkylene-, -C3-C8 heterocyclo-, -C1-C10 alkylene-(C3-C8 heterocyclo)-, -(C3-C8 heterocyclo)-C1-C10 alkylene-, -C1-C10 alkylene-C(=O)-, - C1-C10alkylene-C(O)NH-C1-C8alkylene-[O-CH2- CH2]n-C(O)- (where n is 1 to 26), C1-C10 heteroalkylene-C(=O)-, -C1-C8 alkylene- (CH2-O-CH2)b-C(=O)- (where b is 1 to 26), -(CH2-O-CH2)b-C1-C8 alkylene-C(=O)- (where b is 1 to 26), -C1-C8 alkylene-(CH2-O-CH2)b-C1-C8 alkylene-C(=O)- (where b is 1 to 26), -C3-C8 carbocyclo-C(=O)-, -O-(C1-C8 alkyl)-C(=O)-, -arylene-C(=O)-, -C1- C10 alkylene-arylene-C(=O)-, -arylene-C1-C10 alkylene-C(=O)-, -C1-C10 alkylene-(C3- Cs carbocyclo)-C(=O)-, -(C3-C8 carbocyclo)-C1-C10 alkylene-C(=O)-, -C3-C8 heterocyclo-C(=O)-, -C1-C10 alkylene-(C3-C8 heterocyclo)-C(=O)-, -(C3-C8 heterocyclo)-C1-C10 alkylene-C(=O)-, -C1-C10 alkylene-NH-, -C1-C10 heteroalkylene- NH-, -C1-C8 alkylene-(CH2-O-CH2)b-NH- (where b is 1 to 26), -(CH2-O-CH2)b-C1-C8alkylene-NH- (where b is 1 to 26), -C1-C8 alkylene-(CH2-O-CH2)b-C1-C8 alkylene-NH- (where b is 1 to 26), -C1-C8 alkylene-(C(=O))-NH-(CH2-O-CH2)b-C(=O)- (where b is 1 to 26), -C1-C8 alkylene-(C(=O))-NH-(CH2-O-CH2)b-C1-C8 alkylene-C(=O)- (where b is 1 to 26), -C1-C8 alkylene-NH-(C(=O))-(CH2-O-CH2)b-NH- (where b is 1 to 26), -C1-C8alkylene-NH-(C(=O))-(CH2-O-CH2)b-C1-C8 alkylene-NH- (where b is 1 to 26), -C3-C8carbocyclo-NH-, -O-(C1-C8 alkyl)-NH-, -arylene-NH-, -C1-C10 alkylene-arylene-NH-, - arylene-C1-C10 alkylene-NH-, -C1-C10 alkylene-(C3-C8 carbocyclo)-NH-, -(C3-C8 carbocyclo)-C1-C10 alkylene-NH-, -C3-C8 heterocyclo-NH-, -C1-C10 alkylene-(C3-Cs heterocyclo)-NH-, -(C3-C8 heterocyclo)-C1-C10 alkylene-NH-, -C1-C10 alkylene-S-, C1- C10 heteroalkylene-S-, -C3-C8 carbocyclo-S-, -O-(C1-C8 alkyl)-S-, -arylene-S-, -C1-C10 alkylene-arylene-S-, -arylene-C1-C10 alkylene-S-, -C1-C10 alkylene-(C3-Cs carbocyclo)-S-, -(C3-C8 carbocyclo)-C1-C10 alkylene-S-, -C3-C8 heterocyclo-S-, -C1- C10 alkylene-(C3-C8 heterocyclo)-S-, or -(C3-C8 heterocyclo)-C1-C10 alkylene-S-; or wherein the stretcher group comprises maleimido(C1-C10alkylene-C(0)-, maleimido(CH20CH2)P2( C1-C10alkyene)C(0)-, maleimido(C1-C10alkyene) (CH2OCH2)p2C(O)-, or a ring open form thereof, wherein p2 is from 1 to 26; and wherein * is an attachment to the SLITRK6 binding agent, and the wavy line is an attachment to the enzyme-cleavable group.

[0413] Embodiment 105. The linker compound of Embodiment 104, wherein the stretcher group is selected from the following:or a stereoisomer thereof, wherein each Rais independently H or C1-6 alkyl, each n is independently 0-12, and the wavy line indicates an attachment site of the stretcher group to the amino acid unit, and the attachment site for the Targeting unit is on a maleimide, primary amine or alkyne functional group.

[0414] Embodiment 106. The linker compound of Embodiment 104, wherein the stretcher group is selected from the following:wherein the wavy line indicates an attachment site of the stretcher group to the enzyme- cleavable group, and the attachment site to the SLITRK6 binding agent is on the maleimide, primary amine or alkyne functional group.

[0415] Embodiment 107. The linker compound of any one of Embodiments 1-9, having one of the following structures:wherein the H on the benzylic OH is optionally replaced with a bond to the at least one drug unit or to a linking group attached to the at least one drug unit.

[0416] Embodiment 108. The linker compound of any one of Embodiments 1-9, having one of the following structures:stereoisomer thereof, wherein the wavy line indicates the attachment site to at least one of the drug units or for a linking group attached to the at least one of the drug units.

[0417] Embodiment 109. A drug-linker compound, comprising the linker compound of any one of Embodiments 1-108 attached to the at least one drug unit, or attached to a linking group attached to the at least one drug unit.

[0418] Embodiment 110. The drug-linker compound of Embodiment 109, wherein the drug unit is selected from a cytotoxic agent, an immune modulatory agent, a nucleic acid, a growth inhibitory agent, a PROTAC, a toxin, a radioactive isotope and a chelating ligand.

[0419] Embodiment 111. The drug-linker compound of Embodiment 111, wherein the drug unit is a cytotoxic agent.

[0420] Embodiment 112. The drug-linker compound of Embodiment 111 , wherein the cytotoxic agent is selected from the group consisting of an auristatin, a maytansinoid, a camptothecin, a duocarmycin, and a calicheamicin.

[0421] Embodiment 113. The drug-linker compound of Embodiment 111 , wherein the cytotoxic agent is an auristatin.

[0422] Embodiment 114. The drug-linker compound of Embodiment 113, wherein the cytotoxic agent is MMAE or MMAF

[0423] Embodiment 115. The drug-linker compound of Embodiment 111 , wherein the cytotoxic agent is a camptothecin.

[0424] Embodiment 116. The drug-linker compound of Embodiment 115, wherein the cytotoxic agent is exatecan, or SN-38, or DxD.

[0425] Embodiment 117. The drug-linker compound of Embodiment 116, wherein the cytotoxic agent is RS-exatecan or SS-exatecan.

[0426] Embodiment 118. The drug-linker compound of Embodiment 111 , wherein the cytotoxic agent is a calicheamicin.

[0427] Embodiment 119. The drug-linker compound of Embodiment 111 , wherein the cytotoxic agent is a maytansinoid.

[0428] Embodiment 120. The drug-linker compound of Embodiment 119, wherein themaytansinoid is maytansine, maytansinol or ansamatocin-2.

[0429] Embodiment 121. The drug-linker of Embodiment 112, wherein the cytotoxic agent is MMAE, MMAF, exatecan, RS-exatecan, SS-exatecan, SN-38, DxD, maytansine, maytansinol or ansamatocin-2.

[0430] Embodiment 122. The drug-linker compound of Embodiment 112, wherein the cytotoxic agent is MMAE.

[0431] Embodiment 123. The drug-linker compound of Embodiment 112, wherein the cytotoxic agent is exatecan.

[0432] Embodiment 124. The drug-linker compound of Embodiment 110, wherein the drug unit is an immune modulatory agent.

[0433] Embodiment 125. The drug-linker compound of Embodiment 124, wherein the immune modulatory agent is selected from a TRL7 agonist, a TLR8 agonist, a STING agonist, or a RIG-I agonist.

[0434] Embodiment 126. The drug-linker compound of Embodiment 125, wherein the immune modulatory agent is an TLR7 agonist.

[0435] Embodiment 127. The drug-linker compound of Embodiment 126, wherein the TLR7 agonist is an imidazoquinoline, an imidazoquinoline amine, a thiazoquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2-d]pyrimidine-2,4-diamine, pyrimidine-2,4- diamine, 2-aminoimidazole, 1-alkyl-1H-benzimidazol-2-amine, tetrahydropyridopyrimidine, heteroarothiadiazide-2,2-dioxide, a benzonaphthyridine, a guanosine analog, an adenosine analog, a thymidine homopolymer, ssRNA, CpG-A, PolyGIO, or PolyG3.

[0436] Embodiment 128. The drug-linker compound of Embodiment 126, wherein the immune modulatory agent is a TLR8 agonist.

[0437] Embodiment 129. The drug-linker compound of Embodiment 128, wherein the TLR8 agonist is selected from an imidazoquinoline, a thiazoloquinoline, an aminoquinoline, an aminoquinazoline, a pyrido [3,2-d]pyrimidine-2,4-diamine, pyrimidine-2,4-diamine, 2- aminoimidazole, 1-alkyl-1H-benzimidazol-2-amine, tetrahydropyridopyrimidine or a ssRNA.

[0438] Embodiment 130. The drug-linker compound of Embodiment 125, wherein the immune modulatory agent is a STING agonist.

[0439] Embodiment 131. The drug-linker compound of Embodiment 125, wherein the immune modulatory agent is a RIG-I agonist.

[0440] Embodiment 132. The drug-linker compound of Embodiment 131, wherein the RIG-I agonist is selected from KIN1148, SB-9200, KIN700, KIN600, KIN500, KIN100, KIN101, KIN400 and KIN2000.

[0441] Embodiment 133. The drug-linker compound of Embodiment 110, wherein the drug unit is a chelating ligand.

[0442] Embodiment 134. The drug-linker compound of Embodiment 133, wherein the chelating ligand is selected from platinum (Pt), ruthenium (Ru), rhodium (Rh), gold (Au), silver(Ag), copper (Cu), molybdenum (Mo), titanium (Ti), or iridum (Ir); a radioisotope such as yittrium-88, yittrium-90, technetium-99, copper-67, rhenium-188, rhenium-186, galium-66, galium-67, indium-111 , indium-114, indium-115, lutetium-177, strontium-89, sararium-153, and lead-212.

[0443] Embodiment 135. The drug-linker compound of Embodiment 109, having one of the

[0444] Embodiment 136. The drug-linker compound of Embodiment 109, the drug-linker compound has one of the following structures:or a stereoisomer thereof.

[0445] Embodiment 137. A conjugate, comprising the drug-linker compound of any one of Embodiments 109-136, wherein the drug-linker compound is attached to a binding agent (e.g., SLITRK6 binding agent).

[0446] Embodiment 138. The conjugate of Embodiment 137, wherein an average drug loading (pload) of the conjugate is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

[0447] Embodiment 139. The conjugate of any one of Embodiments 137-138, selected from the following:or a stereoisomer thereof, wherein Ab is a SLITRK6 binding agent and n is pload.

[0448] Embodiment 140. The conjugate of any one of Embodiments 137-138, selected from the following:or a stereoisomer thereof, wherein Ab is a SLITRK6 binding agent and n is pload.

[0449] Embodiment 141. The conjugate of any one of Embodiments 137-138, selected from the following:LD110 conjugateLD163 conjugateor a stereoisomer thereof, wherein Ab is a SLITRK6 binding agent and n is pload.

[0450] Embodiment 142. The conjugate of any one of Embodiments 137-141 , wherein the Ab is a SLITRK6 binding agent (e.g., an antibody or antigen-binding portion thereof which binds to SLITRK6) comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1 , HCDR2 andHCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having amino acids sequences selected from the sets of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively;(ii) SEQ ID NO: 11 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively;(iii) SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21 , SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively;(iv) SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and(v) SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively.

[0451] Embodiment 143. The conjugate of any one of Embodiments 137-142, wherein the VH and VL regions have amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 1 and SEQ ID NO: 2, respectively;(ii) SEQ ID NO: 9 and SEQ ID NO: 10, respectively;(iii) SEQ ID NO: 17 and SEQ ID NO: 18, respectively;(iv) SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and(v) SEQ ID NO: 33 and SEQ ID NO: 34, respectively.Embodiment 144. The conjugate of any one of Embodiments 137-142, wherein the VH and VL regions have amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 1 and SEQ ID NO: 2, respectively;(ii) SEQ ID NO: 9 and SEQ ID NO: 10, respectively;(iii) SEQ ID NO: 17 and SEQ ID NO: 18, respectively;(iv) SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and(v) SEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain framework regions are optionally modified with from 1 to 8 amino acid substitutions, deletions or insertions in the framework regions.Embodiment 145. The conjugate of any one of Embodiments 137-144, wherein the framework regions are human framework regions.Embodiment 146. The conjugate of any one of Embodiments 137-145, wherein the binding agent is a monoclonal antibody, a Fab, a Fab’, an F(ab’), an Fv, a disulfide linked Fc, a scFv, a single domain antibody, a diabody, a bi-specific antibody, or a multi-specific antibody.Embodiment 147. The conjugate of any one of Embodiments 137-146, wherein the heavy chain variable region further comprises a heavy chain constant region.Embodiment 148. The conjugate of any one of Embodiments 137-147, wherein the heavy chain constant region is of the IgG isotype.Embodiment 149. The conjugate of Embodiment 148, wherein the heavy chain constant region is an IgG 1 constant region.Embodiment 150. The conjugate of Embodiment 148, wherein the heavy chain constant region is an lgG4 constant region.Embodiment 151. The conjugate of Embodiment 147, wherein the heavy chain constant region comprises the amino acid sequence of SEQ ID NO: 49, 51, or 52.Embodiment 152. The conjugate of any one of Embodiments 137-151, wherein the light chain variable region further comprises a light chain constant region.Embodiment 153. The conjugate of Embodiment 152, wherein the light chain constant region is of the kappa isotype.Embodiment 154. The conjugate of Embodiment 153, wherein the light chain constant region has the amino acid sequence set forth in SEQ ID NO: 50.Embodiment 155. The conjugate of any one of Embodiments 147-154, wherein the heavy chain constant region further comprises an amino acid modification that decreases binding affinity to human FcyRIII.Embodiment 156. The conjugate of any one of Embodiments 137-155, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively.Embodiment 157. The conjugate of any one of Embodiments 137-155, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 54 and 55, respectively.Embodiment 158. The conjugate of any one of Embodiments 137-157, wherein the binding agent is mono-specific.Embodiment 159. The conjugate of any one of Embodiments 137-158, wherein the binding agent is bivalent.Embodiment 160. The conjugate of any one of Embodiments 137-157 and 159, wherein the binding agent is bispecific.Embodiment 161. A binding agent which binds to SLITRK6 comprising a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs having amino acids sequences selected from the sets of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, and SEQ ID NO: 8, respectively;(ii) SEQ ID NO: 11 , SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively;(iii) SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21 , SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively;(iv) SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and(v) SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively.Embodiment 162. The binding agent of Embodiment 161, wherein the VH and VL regions have amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 1 and SEQ ID NO: 2, respectively;(ii) SEQ ID NO: 9 and SEQ ID NO: 10, respectively;(iii) SEQ ID NO: 17 and SEQ ID NO: 18, respectively;(iv) SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and(v) SEQ ID NO: 33 and SEQ ID NO: 34, respectively.Embodiment 163. The binding agent of Embodiment 161, wherein the VH and VL regions have amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:SEQ ID NO: 1 and SEQ ID NO: 2, respectively;SEQ ID NO: 9 and SEQ ID NO: 10, respectively;SEQ ID NO: 17 and SEQ ID NO: 18, respectively;SEQ ID NO: 25 and SEQ ID NO: 26, respectively; andSEQ ID NO: 33 and SEQ ID NO: 34, respectively; wherein the heavy and light chain framework regions are optionally modified with from 1 to 8 amino acid substitutions, deletions or insertions in the framework regions.Embodiment 164. The binding agent of Embodiment 161, wherein the framework regions are human framework regions.Embodiment 165. The binding agent of any of Embodiments 161-164, wherein the binding agent is a monoclonal antibody, a Fab, a Fab’, an F(ab’), an Fv, a disulfide linked Fc, a scFv, a single domain antibody, a diabody, a bi-specific antibody, or a multi-specific antibody.Embodiment 166. The binding agent of any of Embodiments 161-165, wherein the VH region further comprises a heavy chain constant region.Embodiment 167. The binding agent of Embodiment 166, wherein the heavy chain constant region is of the IgG isotype.Embodiment 168. The binding agent of Embodiment 167, wherein the heavy chain constant region is an IgG 1 constant region.Embodiment 169. The binding agent of Embodiment 167, wherein the heavy chain constant region is an lgG4 constant region.Embodiment 170. The binding agent of Embodiment 167, wherein the heavy chain constant region comprises the amino acid sequence set forth in SEQ ID NO: 49, 51, or 52.Embodiment 171. The binding agent of any of the Embodiments 161-170, wherein the VL region further comprises a light chain constant region.Embodiment 172. The binding agent of Embodiment 171, wherein the light chain constant region is of the kappa isotype.Embodiment 173. The binding agent of Embodiment 172, wherein the light chain constant region comprises the amino acid sequence set forth in SEQ ID NO: 50.Embodiment 174. The binding agent of any one of Embodiments 166-173, wherein the heavy chain constant region further comprises at least amino acid modification that decreases binding affinity to human FcyRIII.Embodiment 175. The binding agent of any one of Embodiments 161-174, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively.Embodiment 176. The binding agent of any one of Embodiments 161-174, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 54 and 55, respectively.Embodiment 177. The binding agent of any of Embodiments 161-176, wherein the binding agent is mono-specific.Embodiment 178. The binding agent of any one of Embodiments 161-177, wherein the binding agent is bivalent.Embodiment 179. The binding agent of any one of Embodiments 161-176 and 178, wherein the binding agent is bispecific.Embodiment 180. A nucleic acid, or set of nucleic acids, encoding the binding agent of any one of Embodiments 161-179.Embodiment 181. A vector, or set of vectors, comprising the nucleic acid, or set of nucleic acids, of Embodiment 180.Embodiment 182. A cell line (e.g., an isolated cell line) comprising the nucleic acid, or set of nucleic acids, of Embodiment 180.Embodiment 183. A cell line (e.g., an isolated cell line) comprising the vector, or set of vectors, of Embodiment 181.Embodiment 184. A pharmaceutical composition comprising the conjugate of any one of Embodiments 137-160 or binding agent of any one of 161-179, and a pharmaceutically acceptable carrier.Embodiment 185. A method of treating a SLITRK6+ cancer, comprising administering to a subject in need thereof a therapeutically effective amount of the conjugate of any one of Embodiments 137-160, binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184.Embodiment 186. The method of Embodiment 185, wherein the SLITRK6+ cancer is a solid tumor or a hematologic malignancy.Embodiment 187. The method of Embodiment 186, wherein the SLITRK6+ cancer is selected from breast cancer (BC), lung cancer (LC), ovarian cancer (OVCA), esophageal cancer (EsC), gastric cancer (GC), bladder cancer (BLC), endometrial cancer (EC), head and neck cancer (HNC), cervical cancer, pharynx cancer, stomach cancer, myeloma, uterine cancer, colon cancer, hepatocellular cancer, and colorectal cancer.Embodiment 188. The method of Embodiment 186, wherein the SLITRK6+ cancer is a hematologic malignancy.Embodiment 189. The method of Embodiment 186, wherein the SLITRK6+ cancer is a solid tumor.Embodiment 190. The method of Embodiment 186, wherein the SLITRK6+ cancer is triplenegative breast cancer (TNBC).Embodiment 191. The method of Embodiment 186, wherein the SLITRK6+ cancer is non-small- cell lung cancer (NSCLC).Embodiment 192. The method of Embodiment 186, wherein the SLITRK6+ cancer is ovarian cancer.Embodiment 193. The method of Embodiment 186, wherein the SLITRK6+ cancer is pharynx cancer.Embodiment 194. The method of Embodiment 186, wherein the SLITRK6+ cancer is gastric cancer.Embodiment 195. The method of Embodiment 186, wherein the SLITRK6+ cancer is endometrial adenocarcinoma.Embodiment 196. The method of Embodiment 186, wherein the SLITRK6+ cancer is bladder cancer.Embodiment 197. The method of Embodiment 186, wherein the SLITRK6+ cancer is bladder transitional cell papilloma.Embodiment 198. The method of Embodiment 186, wherein the SLITRK6+ cancer is EsCC.Embodiment 199. The method of any one of Embodiments 185-198, further comprising administering an immunotherapy to the subject.Embodiment 200. The method of Embodiment 199, wherein the immunotherapy comprises a checkpoint inhibitor.Embodiment 201. The method of Embodiment 200, wherein the checkpoint inhibitor is selected from an antibody that specifically binds to human PD-1, human PD-L1, or human CTLA4.Embodiment 202. The method of Embodiment 200, wherein the checkpoint inhibitor is pembrolizumab, nivolumab, cemiplimab or ipilimumab.Embodiment 203. The method of any one of Embodiments 185-202, further comprising administering chemotherapy to the subject.Embodiment 204. The method of any one of Embodiments 185-203, wherein the conjugate or pharmaceutical composition is administered intravenously.Embodiment 205. The method of Embodiment 204, wherein the conjugate or pharmaceutical composition is administered in a dose of about 0.1 mg / kg to about 12 mg / kg.Embodiment 206. The method of any one of Embodiments 185-205, wherein a treatment outcome of the subject is improved.Embodiment 207. The method of Embodiment 206, wherein the improved treatment outcome is an objective response selected from stable disease, a partial response or a complete response.Embodiment 208. The method of Embodiment 207, wherein the improved treatment outcome is reduced tumor burden.Embodiment 209. The method of Embodiment 207, wherein the improved treatment outcome is progression-free survival or disease-free survival.Embodiment 210. Use of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for the treatment of SLITRK6+ cancer in a subject.Embodiment 211. A method of treating an autoimmune disease, comprising administering to a subject in need thereof a therapeutically effective amount of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184.Embodiment 212. The method of Embodiment 211 , wherein the autoimmune disease is rheumatoid arthritis, multiple sclerosis, or systemic lupus erythematosus.Embodiment 213. The method of Embodiment 211 or 212, further comprising administering an immunosuppressive therapy to the subject.Embodiment 214. The method of any one of Embodiments 211-213, wherein the conjugate or pharmaceutical composition is administered intravenously.Embodiment 215. The method of Embodiment 214, wherein the conjugate or pharmaceutical composition is administered in a dose of about 0.1 mg / kg to about 12 mg / kg.Embodiment 216. The method of any one of Embodiments 211-215, wherein a treatment outcome of the subject is improved.Embodiment 217. The method of Embodiment 216, wherein the improved treatment outcome is a reduction in disease progression or alleviation of disease severity.Embodiment 218. Use of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for the treatment of an autoimmune disease in a subject.Embodiment 219. A method of producing the binding agent which binds to SLITRK6, the method comprising culturing the cell line of Embodiment 182 or 183 and isolating the antibody from the cell.Embodiment 220. An anti-idiotypic antibody which binds to the binding agent of any one of Embodiments 161-179.Embodiment 221. The conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for use as a medicament.Embodiment 222. The conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for use in the treatment of cancer in a subject, preferably said cancer is SLITRK6+ cancer.Embodiment 223. The conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for use in the treatment of an autoimmune disease in a subject.Embodiment 224. Use of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for the manufacture of a medicament.Embodiment 225. Use of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for the manufacture of a medicament for the treatment of cancer in a subject, preferably said cancer is SLITRK6+ cancer.Embodiment 226. Use of the conjugate of any one of Embodiments 137-160, the binding agent of any one of Embodiments 161-179, or the pharmaceutical composition of Embodiment 184 for the manufacture of a medicament for the treatment of an autoimmune disease in a subject.Drug Loading

[0452] Conjugates can contain one or more drug unit per SLITRK6 binding agent. The number of drug units per SLITRK6 binding agent is referred to as drug loading. The drug loading of a Conjugate is represented by pload, the average number of drug units (drug molecules (e.g., cytotoxic agents)) per SLITRK6 binding agent (e.g., an antibody or antigen binding portion or non-antibody scaffold or non-antibody protein) in a conjugate. For example, if ptoad jSabout 4 the average drug loading taking into account all of the SLI...

Claims

1. A binding agent which binds to SLIT and NTRK-like protein 6 (SLITRK6) comprising:a heavy chain variable (VH) region and a light chain variable (VL) region, the VH region comprising complementarity determining regions HCDR1, HCDR2 and HCDR3 disposed in heavy chain variable region framework regions and the VL region comprising LCDR1, LCDR2 and LCDR3 disposed in light chain variable region framework regions, the VH and VL CDRs comprising amino acids sequences selected from the sets of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23, and SEQ ID NO: 24, respectively; (ii) SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 15 and SEQ ID NO: 16, respectively; (iii) SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7 and SEQ ID NO: 8, respectively;(iv) SEQ ID NO: 27, SEQ ID NO: 28, SEQ ID NO: 29, SEQ ID NO: 30, SEQ ID NO: 31 and SEQ ID NO: 32, respectively; and(v) SEQ ID NO: 35, SEQ ID NO: 36, SEQ ID NO: 37, SEQ ID NO: 38, SEQ ID NO: 39 and SEQ ID NO: 40, respectively.

2. The binding agent of claim 1, wherein the VH and VL regions comprise amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 17 and SEQ ID NO: 18, respectively;(ii) SEQ ID NO: 9 and SEQ ID NO: 10, respectively;(iii) SEQ ID NO: 1 and SEQ ID NO: 2, respectively;(iv) SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and(v) SEQ ID NO: 33 and SEQ ID NO: 34, respectively.

3. The binding agent of claim 1, wherein the VH and VL regions comprise amino acid sequences that are selected from the pairs of amino acid sequences set forth in the group consisting of:(i) SEQ ID NO: 1 and SEQ ID NO: 2, respectively;(ii) SEQ ID NO: 9 and SEQ ID NO: 10, respectively;(iii) SEQ ID NO: 17 and SEQ ID NO: 18, respectively;(iv) SEQ ID NO: 25 and SEQ ID NO: 26, respectively; and(v) SEQ ID NO: 33 and SEQ ID NO: 34, respectively;wherein the heavy and light chain framework regions are optionally modified with from 1 to 8 amino acid substitutions, deletions or insertions in the framework regions.

4. The binding agent of any of the preceding claims, wherein HCDR1, HCDR2 and HCDR3 and LCDR1, LCDR2 and LCDR3 comprise the amino acid sequences set forth in SEQ ID NO: 19, SEQ ID NO: 20, SEQ ID NO: 21, SEQ ID NO: 22, SEQ ID NO: 23 and SEQ ID NO: 24, respectively.

5. The binding agent of any of the preceding claims, wherein the VH and VL regions comprise the amino acid sequences of SEQ ID NOs: 17 and 18, respectively.

6. The binding agent of claim 1 or 4, wherein the framework regions are human framework regions.

7. The binding agent of any one of the preceding claims, wherein the binding agent is an antibody or an antigen-binding portion thereof.

8. The binding agent of any of the preceding claims, wherein the binding agent is a monoclonal antibody, a Fab, a Fab’, an F(ab’), an Fv, a disulfide linked Fc, a scFv, a single domain antibody, a diabody, a bi-specific antibody, or a multi-specific antibody.

9. The binding agent of any of the preceding claims, wherein the heavy chain variable region further comprises a heavy chain constant region.

10. The binding agent of claim 9, wherein the heavy chain constant region is of the IgG isotype.

11. The binding agent of claim 10, wherein the heavy chain constant region is an IgG1 constant region.

12. The binding agent of claim 10, wherein the heavy chain constant region is an IgG4 constant region.

13. The binding agent of claim 10, wherein the heavy chain constant comprises an amino acid sequence set forth in SEQ ID NO: 49, 51, or 52.

14. The binding agent of any of the preceding claims, wherein the light chain variable region further comprises a light chain constant region.

15. The binding agent of claim 14, wherein the light chain constant region is of the kappa isotype.

16. The binding agent of claim 15, wherein the light chain constant region has the amino acid sequence set forth in SEQ ID NO:

50.

17. The binding agent of any one of claims 9-13, wherein the heavy chain constant region further comprises at least amino acid modification that decreases binding affinity to human FcγRⅢ.

18. The binding agent of any one of claims 1-17, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 53 and 55, respectively. 19.The binding agent of any one of claims 1-17, wherein the binding agent comprises a heavy chain and a light chain comprising the amino acid sequences set forth in SEQ ID NOs: 54 and 55, respectively.

20. The binding agent of any of the preceding claims, wherein the binding agent is mono-specific.

21. The binding agent of any one of claims 1 to 20, wherein the binding agent is bivalent.

22. The binding agent of any one of claims 1 to 19, wherein the binding agent is bispecific.

23. A nucleic acid encoding the binding agent of any one of claims 1 to 22.  24. A vector comprising the nucleic acid of claim 23.  25. A cell line comprising the vector of claim 24 or the nucleic acid of claim 23.

26. A conjugate comprising: the binding agent of any one of claims 1 to 22, at least one linker attached to the binding agent; at least one drug unit, wherein each drug unit is attached to a linker, wherein the linker optionally comprises at least one polar group.

27. The conjugate of claim 26, wherein the linker is derived from a linker compound, or a stereoisomer or salt thereof, and the linker compound comprises:the linker unit;a stretcher group connected to the linker unit, an optional amino acid unit; andthe at least one polar group; wherein:the stretcher group has an attachment site to the binding agent and an attachment site to the amino acid unit (when present) or the linker subunit;the amino acid unit (when present) has an attachment site to the stretcher group and an attachment site to the linker unit; andthe linker unit has an attachment site to the amino acid unit (when present) or to the stretcher group and to the at least one drug unit.

28. The conjugate of claim 26 or 27, wherein the drug unit is selected from a cytotoxic agent, an immune modulatory agent, a nucleic acid, a growth inhibitory agent, a PROTAC, a toxin, a radioactive isotope and a chelating ligand.

29. The conjugate of any one of claims 26-28, wherein the average drug loading (pload) of the conjugate is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

30. The conjugate of claim 29, wherein the average pload of the conjugate is about 8.

31. The conjugate of any one of claims 26-30, selected from the following: wherein Ab is the binding agent and n is pload.

32. The conjugate of claim 26, wherein the conjugate has the following structure: LD110 conjugate, LD163 conjugate,LD038 conjugate, or LD343 conjugate, and wherein Ab is the binding agent and n is pload, wherein pload is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

33. A conjugate comprising the following structure: wherein Ab is an antibody, or antigen-binding portion thereof, which binds to SLIT and NTRK-like protein 6 (SLITRK6), wherein the antibody comprises a heavy chain variable (VH) region and a light chain variable (VL) region, wherein the VH region comprises the VHCDR1, VHCDR2, and VHCDR3 sequences set forth in SEQ ID NOs: 19, 20, and 21, respectively, and the VL region comprises the VLCDR1, VLCDR2, and VLCDR3 sequences set forth in SEQ ID NOs: 22, 23, and 24, respectively,and wherein n is pload, wherein pload is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

34. A conjugate comprising the following structure: wherein Ab is an antibody, or antigen-binding portion thereof, which binds to SLIT and NTRK-like protein 6 (SLITRK6), wherein the antibody comprises a heavy chain variable (VH) region and a light chain variable (VL) region, wherein the VH region comprises the VHCDR1, VHCDR2, and VHCDR3 sequences set forth in SEQ ID NOs: 19, 20, and 21, respectively, and the VL region comprises the VLCDR1, VLCDR2, and VLCDR3 sequences set forth in SEQ ID NOs: 22, 23, and 24, respectively,and wherein n is pload, wherein pload is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

35. A conjugate comprising the following structure: wherein Ab is an antibody, or antigen-binding portion thereof, which binds to SLIT and NTRK-like protein 6 (SLITRK6), wherein the antibody comprises a heavy chain variable (VH) region and a light chain variable (VL) region, wherein the VH region comprises the VHCDR1, VHCDR2, and VHCDR3 sequences set forth in SEQ ID NOs: 19, 20, and 21, respectively, and the VL region comprises the VLCDR1, VLCDR2, and VLCDR3 sequences set forth in SEQ ID NOs: 22, 23, and 24, respectively,and wherein n is pload, wherein pload is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

36. A conjugate comprising the following structure:wherein Ab is an antibody, or antigen-binding portion thereof, which binds to SLIT and NTRK-like protein 6 (SLITRK6), wherein the antibody comprises a heavy chain variable (VH) region and a light chain variable (VL) region, wherein the VH region comprises the VHCDR1, VHCDR2, and VHCDR3 sequences set forth in SEQ ID NOs: 19, 20, and 21, respectively, and the VL region comprises the VLCDR1, VLCDR2, and VLCDR3 sequences set forth in SEQ ID NOs: 22, 23, and 24, respectively,and wherein n is pload, wherein pload is from about 1 to about 8, about 2, about 4, about 6, about 8, about 10, about 12, about 14, about 16, about 3 to about 5, about 6 to about 8, or about 8 to about 16.

37. The conjugate of any one of claims 33-36, wherein the VH and VL regions comprise the amino acid sequences set forth in SEQ ID NOs: 17 and 18, respectively.

38. The conjugate of any one of claims 33-37, wherein the antibody comprises a heavy chain and a light chain comprising the amino acid sequences of SEQ ID NOs: 53 and 55, respectively.

39. The conjugate of any one of claims 33-37, wherein the antibody comprises a heavy chain and a light chain comprising the amino acid sequences of SEQ ID NOs: 54 and 55, respectively.

40. A pharmaceutical composition comprising the binding agent of any one of claims 1-22 or conjugate of any one of claims 26-39 and a pharmaceutically acceptable carrier.

41. A method of treating cancer, preferably a SLITRK6+ cancer, comprising administering to a subject in need thereof a therapeutically effective amount of the binding agent of any one of claims 1-22, the conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40.

42. The binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for use as a medicament.

43. The binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for use in the treatment of cancer in a subject, preferably said cancer is SLITRK6+ cancer.

44. Use of the binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for the manufacture of a medicament.

45. Use of the binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for the manufacture of a medicament for the treatment of cancer in a subject, preferably said cancer is SLITRK6+ cancer.

46. The method of claim 41, the binding agent, conjugate or the pharmaceutical composition for use of claim 43, or the use of claim 45, wherein the SLITRK6+ cancer is a solid tumor or a hematologic malignancy.

47. The method of claim 41, the binding agent, conjugate or the pharmaceutical composition for use of claim 43, or the use of claim 45, wherein the SLITRK6+ cancer is selected from breast cancer (BC), lung cancer (LC), ovarian cancer (OVCA), esophageal cancer (EsC), gastric cancer (GC), bladder cancer (BLC), endometrial cancer (EC), head and neck cancer (HNC), cervical cancer, pharynx cancer, stomach cancer, myeloma, uterine cancer, colon cancer, hepatocellular cancer, and colorectal cancer.

48. A method of treating an autoimmune disease, comprising administering to a subject in need thereof a therapeutically effective amount of the binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40.

49. The binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for use in the treatment of an autoimmune disease in a subject.

50. Use of the binding agent of any one of claims 1-22, conjugate of any one of claims 26-39, or the pharmaceutical composition of claim 40 for the manufacture of a medicament for the treatment of an autoimmune disease in a subject.

51. The method of claim 48, the binding agent, conjugate or the pharmaceutical composition for use of claim 49, or the use of claim 50, wherein the autoimmune disease is rheumatoid arthritis, multiple sclerosis, or systemic lupus erythematosus.

52. A method of producing the binding agent which binds to SLITRK6, the method comprising culturing the cell line of claim 25 and isolating the binding agent from the cell.

53. An anti-idiotypic antibody which binds to the binding agent of any one of claims 1-22.  

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