Antibody-drug conjugates comprising an FLT3 binding antibody

Abstract

The present invention relates to novel antibody-drug conjugates comprising a monoclonal antibody or an antigen-binding fragment thereof wherein the monoclonal antibody or the antigen-binding fragment thereof binds to FLT3.

Description

[0001] Antibody-Drug conjugates comprising an FLT3 binding antibodyDESCRIPTION The present invention relates to novel antibody-drug conjugates comprising a monoclonal antibody or an antigen-binding fragment thereof wherein the monoclonalantibody or the antigen-binding fragment thereof binds specifically to FLT3. Thepayload of this novel antibody-drug conjugate is preferably a topoisomerase Iinhibitor, a tubulin polymerization / depolymerization inhibitor or a transcriptioninhibitor. BACKGROUND The protein known as Cluster of Differentiation antigen 135 (CD135), also referred to as fms-like tyrosine kinase 3 (FLT-3), receptor-type tyrosine-protein kinase FLT3, or fetal liver kinase-2 (Flk2), is encoded by the FLT3 gene in humans. This gene is part of the class III receptor tyrosine kinase family and CD135 acts as the receptor for the cytokine FLT3 ligand (FLT3L). FLT3 mutations, especially internal tandem duplication mutations (ITDs) within the juxtamembrane domain, are significant prognostic factors for overall survival in patients with AML under the age of 60. These mutations lead to constitutive activation of the receptor, which is phosphorylated independently of its ligand, promoting uncontrolled cell proliferation. (Abu-Duhier et al., 2000). Patients with AML who have FLT3 internal tandem duplication mutations have a markedly poorer prognosis compared to those without these mutations. The presence of these mutations is associated with a shorter survival from diagnosis (mean survival of 12.8 months for patients with ITD mutations vs. 29.1 months for those without) (Abu-Duhier et al., 2000). FLT3-ITD mutations not only enhance factor-independent growth in hematopoieticcell lines but are also believed to contribute to the initiation of myeloproliferativediseases such as AML in mice. These mutations are also present in approximately interventions. The efficacy of FLT3-ITD inhibition has been demonstrated through the development of corresponding inhibitors, which is of great relevance for the treatmentof this patient group. (Kelly et al., 2002; Reindl et al., 2006).It is further known that FLT3 is primarily expressed on the surface of hematopoieticprogenitor cells and plays a crucial role in the development of stem cells. Both elevated levels of the wild-type FLT3 protein and mutations of this gene, appear to drive disease progression and are associated with a poorer prognosis, with FLT3 ligand dependent signaling further enhancing proliferation and survival of leukemia cells. Antibody-Drug Conjugates (ADCs) represent a novel class of therapeutic agents that combine the specificity of monoclonal antibodies with the potency of cytotoxic drugs. ADCs are designed to selectively deliver cytotoxic agents to cancer cells, thereby minimizing the systemic toxicity often associated with conventional chemotherapy.However, there still is a need for ADCs specifically targeting FLT3.One aspect of the invention therefore relates to antibody drug conjugates capable ofdelivering a cytotoxic payload to malignant cells expressing FLT3.SUMMARY OF THE INVENTION The present invention relates to an antibody-drug conjugate according to formula (I), AB-M-D (I),whereinAB is an antibody or antigen-binding fragment thereof that binds to FLT 3,M is a linker conjugating AB and D, andD is an active agent or drug, in particular a topoisomerase inhibitor I, a tubulinpolymerization / depolymerization inhibitor or a transcription inhibitor. ANTIBODIES AB The antibodies used in the ADCs according to the present invention are antibodiesthat bind to FLT3 or antigen-binding fragment thereof.The antibodies of the invention are monoclonal antibodies (mAb) or monoclonal antibody fragments. If not indicated differently, the term “monoclonal” refers to a single species, i.e., single amino acid composition of antibodies or antibody fragments. The antibodies provided herein show preferably specific binding to FLT3 and no essential or no cross-reactivity to other proteins. The antigen-binding site of an inventive antibody comprises heavy chain variable domains / regions (VH) and / or antibody light chain variable domains / regions (VL), or pairs of VH / VL. In specific embodiments, the antibodies used in the ADCs according to the present invention are antagonistic antibodies that bind to FLT3.In a further aspect of the present invention the antibody is a FLT3 binding antibodypreferably comprisinga CDRH1 having the sequence of SEQ ID NO:1 or SEQ ID NO:2, a CDRH2 havingthe sequence of SEQ ID NO:3, a CDRH3 having the sequence of SEQ ID NO:4 orSEQ ID NO:5,a CDRL1 having the sequence of SEQ ID NO:6 or SEQ ID NO:7, a CDRL2 havingthe sequence of SEQ ID NO:8, and a CDRL3 having the sequence of SEQ ID NO:9or SEQ ID NO:10, and / or antibodies or antigen-binding fragments thereof having atleast 90 % amino acid sequence identity, preferably 95%, most preferable 99%, orbind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga CDRH1 having the sequence of SEQ ID NO:1 or SEQ ID NO:2, a CDRH2 havingthe sequence of SEQ ID NO:3, a CDRH3 having the sequence of SEQ ID NO:4,a CDRL1 having the sequence of SEQ ID NO:6, a CDRL2 having the sequence ofSEQ ID NO:8, and a CDRL3 having the sequence of SEQ ID NO:9, and / or antibodiesor antigen-binding fragments thereof having at least 90 % amino acid sequenceidentity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga CDRH1 having the sequence of SEQ ID NO:1 or SEQ ID:2, a CDRH2 having thesequence of SEQ ID NO:3, a CDRH3 having the sequence of SEQ ID NO:5,a CDRL1 having the sequence of SEQ ID NO:7, a CDRL2 having the sequence ofSEQ ID NO:8, and a CDRL3 having the sequence of SEQ ID NO:11, and / or antibodiesor antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga CDRH1 having the sequence of SEQ ID NO:12 or of SEQ ID NO:13, a CDRH2having the sequence of SEQ ID NO:14, a CDRH3 having the sequence of SEQ IDNO:15,a CDRL1 having the sequence of SEQ ID NO:16, a CDRL2 having the sequence ofSEQ ID NO:17, and a CDRL3 having the sequence of SEQ ID NO:18, and / orantibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga VL having the sequence of SEQ ID NO:22, and a VH sequence of SEQ ID NO:19,and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the sameepitope as the respective antibody. According to this invention, a FLT3 bindingantibody comprising a VL having the sequence of SEQ ID NO:22 and a VH sequenceof SEQ ID NO:19 and / or at least an antigen-binding fragment thereof is in particularpreferred. Even more preferred, it is a Fc silenced antibody comprising a VL havingthe sequence of SEQ ID NO:22 and a VH sequence of SEQ ID NO:19 and / or at leastan antigen-an binding fragment thereof.In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga VL having the sequence of SEQ ID NO:24, and a VH sequence of SEQ ID NO:21,and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga VL having the sequence: of SEQ ID NO:23, and a VH sequence of SEQ ID NO:20,and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. In a further aspect of the present invention the antibody is a FLT3 binding antibody preferably comprisinga light chain of SEQ ID NO:28 and a heavy chain of SEQ ID NO:25, ora light chain of SEQ ID NO:29 and a heavy chain of SEQ ID NO:26, ora light chain of SEQ ID NO:30 and a heavy chain of SEQ ID NO:27, and / or antibodiesor antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody. Further preferred FLT3 targeting antibodies are disclosed in WO95 / 07348 and WO98 / 25457 as well as Zheng R. et al. Blood 103(1):267-274 (2004), Li Y, et al„ Blood104(4):1137-44 (2004), Piloto, 0., et al., Cancer Res.65(4): 1514-22 (2005), Williams B„ et al., Leukemia 19(8):1432-8 (2005), Piloto O, et al., Cancer Res.66(9):4843-51(2006) Piloto O, et al. Blood 109(4): 1643- 1652 (2007) or Brent R, et al, AACR AnnualMeeting 2007, Los .Angeles (2007). In preferred embodiments, the antibody or antigen binding fragment are sequence- modified to prevent their interaction with Fc-receptors, in particular Fc-gamma receptors. The term “antigen-binding site” denotes the region(s) of an antibody molecule to whicha ligand (e.g., the antigen, i.e., FLT3 or antigen fragment of it) actually binds andwhich is derived from an antibody. The variable domains / regions denote each of the pair of light and heavy chains, which is involved directly in binding the antibody to the antigen. The variable domain of a heavy chain is abbreviated as “VH” and the variable domain of a light chain is abbreviated as “VL”. An antigen-binding site of an antibody according to the invention can contain six complementarity determining regions (CDRs) which contribute in varying degrees to the affinity of the binding site for the antigen. There are three heavy chain variable domain CDRs (CDR-H1, CDR-H2 and CDR-H3) and three light chain variable domain CDRs (CDR-L1, CDR-L2 and CDR-L3). Also included within the scope of the invention are functional antigen binding sites comprised of fewer CDRs (i.e., where binding specificity is determined by three, four or five CDRs). For example, less than a complete set of 6 CDRs may be sufficient for binding. In some cases, a VH or a VL domain will be sufficient. According to the present invention, a VH region or the CDRs thereof alone may constitute a complete antigen-binding site. In certain embodiments, the antibody comprises a VH region alone. In other embodiments, the antibody comprises a VH region together with a VL region. The position of CDRs within a VH or VL region may be defined according to Kabat, et al., Sequences of Proteins of Immunological Interest, 5th Ed. Public Health Service, National Institutes of Health, Bethesda, MD (1991) or the IMGT numbering system, both of which are known to the person skilled in the art. The IMGT numbering has been defined to compare the variable domains whatever the antigen receptor, thechain type, or the species (Lefranc M.-P., "Unique database numbering system forimmunogenetic analysis" Immunology Today, 18, 509 (1997); Lefranc M.-P., "The IMGT unique numbering for Immunoglobulins, T cell receptors and Ig-like domains" The Immunologist, 7, 132-136 (1999)). If not stated otherwise, the Kabat system is used herein. As used herein, the terms “binding” and “specific binding” refer to the binding of theinventive antibody or fragment thereof to an epitope of the antibodies describedherein. The measure of the binding strength of an antibody is referred to as affinity. Methods for determining such a binding and / or affinity using in vitro assays are known to the person skilled in the art. According to the present invention, detection with flow cytometry by fluorescence, immuno-histochemistry and / or surface plasmon resonance are described and particularly preferred herein. The affinity of the binding of an antibody to an antigen is defined by the terms Ka (rate constant for the association of the antibody from the antibody / antigen complex), KD (dissociation constant), and Kdis (KD / Ka). In certain embodiments, the antibody of the invention may be a chimeric antibody, a multispecific antibody, in particular a bispecific antibody, a human antibody, a humanized antibody, or an antigen-binding fragment thereof. According to the present invention, a “chimeric” antibody refers to an antibody in which a portion of the heavy and / or light chain is derived from a particular source or species, while the remainder of the heavy and / or light chain is derived from a different source or species.“Multispecific antibodies” bind two or more different epitopes. The epitopes may beon the same or different antigens. A preferred example of a multispecific antibody is a “bispecific antibody” or an antigen-binding fragment thereof which binds twodifferent epitopes. A special subset of bispecific antibodies is a “biparatopic” antibodyor antigen-binding fragment, in which each antigen-binding domain recognizes unique, non-overlapping epitopes on the same target antigen.In especially preferred embodiments, the antibody of the invention is a humanizedantibody. The term "humanized antibody" or "humanized version of an antibody" refers to antibodies for which both heavy and light chains are humanized as a result of antibody engineering. A humanized chain is typically a chain in which the V-region amino acid sequence has been changed so that, analyzed as a whole, is closer in homology to a human germline sequence than to the germline sequence of the species of origin. For example, a murine CDR may be grafted into the framework region of a humanantibody to prepare the “humanized antibody.” See, e.g., Riechmann, L., et al., Nature332 (1988) 323-327; and Neuberger, M. S., et al., Nature 314 (1985) 268-270. Otherforms of humanized antibodies encompassed by the present invention are those in which the constant region has been additionally modified or changed from that of the original antibody to generate the properties according to the invention. Humanization assessment is based on the resulting amino acid sequence and not on the methodology per se. Another preferred embodiment refers to human antibodies. The term “human antibody”, as used herein, is intended to include antibodies having variable and constant regions derived from human germ line immunoglobulin sequences. Human antibodies are well-known in the state of the art (van Dijk, M. A., and van de Winkel, J. G., Curr. Opin. Chem. Biol. 5 (2001) 368-374). Human antibodies can also be produced in transgenic animals (e.g., mice) that are capable, upon immunization, of producing a full repertoire or a selection of human antibodies in the absence of endogenous immunoglobulin production. The antibody of the present invention may be of any suitable class. The term “class” refers to the type of constant domain or constant region possessed by its heavy chain. As used herein, “constant domain” or “constant region” denotes the sum of the domains of an antibody other than the variable region. The constant region is not directly involved in binding of an antigen but exhibits various effector functions. The antibody may be of any of the five major classes of antibodies, particularly of the five major classes of human antibodies: lgA, lgD, lgE, lgG, and IgM, or any subclass thereof (isotype), e.g., lgG1, lgG2, lgG3, lgG4, lgA1, and lgA2. The heavy chain constant domains that correspond to the different classes of immunoglobulins are called a, δ, ε, γ and µ, respectively. According to the present invention, an antibody of the class, particularly of the human class IgG, IgA or IgM or a fragment thereof is particularly suitable. According to a preferred embodiment, an antibody of the invention is selected from class IgG, particularly from the class of human IgG, e.g., of subclass IgG1, IgG2, IgG3 of IgG4, of class IgM, of class IgA or an antigen-binding fragment thereof. In certain embodiments, the antibody comprises a constant domain, particularly a heavy chain constant domain, more particularly a heavy chain constant domain of the class IgG, particularly of the class human IgG, e.g., of subclass IgG1, IgG2, IgG3 of IgG4, of class IgM, of class IgA, which has a reduced effector function compared to a wild-type sequence of the same subclass, e.g., which has a reduced binding to the Fc receptor. Examples of heavy chain constant domains with reduced effector functions may contain at least one of the mutations D265C / A, L234A / F, L235A / E,P329G, P331S, and / or N297A / S / G of human IgG, e.g., IgG1 or IgG4 sequences.In some embodiments, the payload-linker construct is preferably coupled to an accessible (reactive) thiol side group of a cysteine on the antibody. The ratio of payload to antibody, known as drug antibody ratio (DAR), might in these embodiments be influenced by changing the number of cysteines in the antibody. In preferred embodiments, non-cysteine amino acids of the antibody are replaced with cysteines to increase the drug antibody ratio. In preferred embodiments, naturally occurring cysteine amino acids of the antibody are deleted, or altered or their thiol side groups are masked to decrease the drug antibody ratio. Such modifications of the antibody are commonly referred to as cysteine engineering or thio-engineering and are suitable for all linker-payload constructs that include an IgG antibody component. Respective methods are described, for example, in WO 2009 / 012268 and allow for instance for the production of antibody-drug conjugates with a constant DAR, facilitating the development of antibody-drug conjugates with controlled cytotoxicity and improved safety. In preferred embodiments, the cysteine engineering involves replacing alanine with cysteine at position 118 of the heavy chain (EU numbering) of an IgG antibody; this exchange is referred to as A118C. In another preferred embodiment, valine is replaced with cysteine at position 205 (Kabat numbering) of the light chain of an IgG antibody; this exchange is referred to as V205C. An "antigen-binding fragment" of an antibody refers to a molecule comprising a portion of an intact antibody that binds the antigen to which the intact antibody binds. Examples of antibody fragments include but are not limited to Fv, Fab, Fab´, Fab´-SH, F(ab’)2; diabodies; linear antibodies; single domain antibodies (e.g. VHH), single-chain antibody molecules (e.g., scFv); and multi-specific antibodies formed from antibody fragments. The term also encompasses a fusion protein, e.g., a fusion protein with a non-immunoglobulin peptide or polypeptide, and a conjugate with a non-proteinaceous structure, e.g., a label or a toxin. The terms “antigen-binding fragment of an antibody (thereof)” and “fragment of an antibody (thereof)” may be used interchangeably herein.The antibody or the antigen-binding fragment may be mono- or multivalent, i.e., it maycomprise a single antigen-binding site or multiple antigen-binding sites. For example,Fab fragments have single antigen-binding site, antibodies of the IgG class or Fv orscFv fragments have two antigen-binding sites and antibodies of the IgM class have 5 antigen-binding sites. The term “antibody” also encompasses hetero-specific antibodies, e.g., hetero-bispecific antibodies, which have different antigen-binding sites, particularly antibodies, which are directed to two different epitopes on the antigen. As used herein, “epitope” is a region of an antigen that is bound by an antibody. The term “epitope” includes any polypeptide determinant capable of specific binding to an antibody. "Percent (%) amino acid sequence identity" with respect to a peptide or polypeptide sequence is defined as the percentage of amino acid residues in a candidate sequence that are identical with the amino acid residues in the specific peptide or polypeptide sequence, after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent sequence identity, Alignment for purposes of determining percent amino acid sequence identity can be achieved in various ways that are within the skill in the art, for instance, using publicly available computer software such as BLAST. Another aspect of the present invention is a combination of at least 2 different monoclonal antibodies or fragments as described herein.According to a preferred embodiment, the antibodies or antibody fragments usedherein are Fc silenced antibodies or antibody fragments. As understood herein, Fcsilencing of an antibody refers to the intentional modification of the Fc region of an antibody to eliminate or reduce its interaction with immune effector functions, such as antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity(CDC), or Fc receptor (FcγR) binding. Examples of heavy chain constant domainswith reduced effector functions, i.e. Fc silenced antibodies, may contain at least oneof the mutations D265C / A, L234A / F, L235A / E, P329G, P331S, and / or N297A / S / G ofhuman IgG, e.g., IgG1 or IgG4 sequences as already described above.

[0002] ACTIVE AGENTS / DRUGS DAn antibody-drug conjugate of the present invention comprises a monoclonal antibodyor antigen-binding fragment thereof as defined above, and a drug conjugated to areactive amino acid residue on the antibody or antigen-binding fragment, e.g., an amino acid residue having a side chain comprising an amino, hydroxy or thiol group, or a reactive group in the antibody glycan structure.The drug of the antibody-drug conjugate of the invention is preferably selected fromtopoisomerase I inhibitors, transcription inhibitors (in particular RNA polymerase IIinhibitors), MM monomethyl auristatin E (MMAE) or monomethyl auristatin F (MMAF),antimitotic agents, cytotoxic agents, and / or corticosteroids. MMAF is in particularpreferred. A topoisomerase I inhibitor is a compound, which is capable of forming a ternary complex with topoisomerase I and DNA, thereby preventing DNA re-ligation and introducing DNA strand breaks in the cellular genome. The topoisomerase I inhibitor may be e.g., selected from camptothecin or analogs thereof, indenoisoquinolines and indolocarbazoles. In a particular embodiment, the topoisomerase-I-inhibitor is camptothecin or an analog thereof, i.e. a compound comprising the pentacyclic basic structure of camptothecin and modified substituents optionally resulting in the presence of a further ring. Specific examples are camptothecin, topotecan, irinotecan, SN-38,belotecan, and exatecan including derivatives thereof such as deruxtecan, lurtotecanor atiratecan. Exatecan and / or deruxtecan are in particular preferred.Transcription is the process by which the information stored in DNA is converted intoa translatable RNA. Inhibitors of transcription can either act directly on RNApolymerase II, the main RNA-synthesizing enzyme in eukaryotic cells, or on one ormore of its gene- or cell-specific co-factors. Without being bound by theory, eventhough the here proposed compounds (e.g. trabectedin, lurbinectedin or α-amanitin)influence the transcription process in different ways, the effect they induce leads to a direct or indirect inhibition of RNA polymerase II and results in a significant reductionin transcription. According to the invention ecteinascidins and derivatives thereof suchas trabectedin or lurbinectedin as well as α-amanitin are in particular preferred.Further preferred ecteinascidin derivatives are described in WO2024 / 155565A.This reduction in transcription is particularly effective in the context of tumor cells,where high levels of transcription are often necessary for rapid cell proliferation.In preferred embodiments the transcription inhibitor used in the ADCs of the inventionis lurbinectedin. Its cell and co-factor independent depression of transcriptionalactivity promotes tumor cell death and contributes to the normalization of the tumormicroenvironment, specifically the latter making it a promising therapeutic agent, forthe treatment of solid tumors. Lurbinectedin affects the tumor microenvironment by modulating the activity of macrophages, which play a role in tumor progression and immune system evasion. By dampening macrophage-mediated activation of the immune system and inhibiting the production of tumor growth factors, lurbinectedin not only halts the proliferation of tumor cells but also alters the supportive tumor environment which otherwise facilitates cancer growth.Lurbinectedin was recently approved as second line treatment for the treatment ofsmall cell lung cancer (SCLC). However, given an initial response rate of 35,2 % (S.Patel et al. 2021) treatment success in SCLC and probably other neuroendocrinecancers seems to depend on the genetic background of the cancer cells. In SCLC,specifically subjects with low expression of the biomarker SLFN11 seem to benefit from treatment with lurbinectedin. In addition, the therapeutic potential of lurbinectedin which also acts by covalently binding to specific sites within the minor groove of DNA, thereby hindering DNA replication and repair mechanisms might be increased by combining it with agents that support this mode of action. For example, ATR (ataxia telangiectasia and Rad3- related protein) plays a crucial role in the cellular response to DNA damage by controlling cell cycle progression and repair mechanisms. Combining lurbinectedin with ATR inhibitors such as AZD6738, which specifically inhibits ATR kinase activity, DNA damage can be exacerbated in cancer cells, especially those with inherent DNA repair deficiencies, making them potentially amenable to therapeutic treatment with lurbinectedin. And without being bound by theory, the same applies to closely related trabectedin and similar acting compounds.In a preferred embodiment the transcription inhibiting ADC of the invention is thereforecombined with an ATR inhibition.The drug D of the antibody conjugate of the invention may be a camptothecinderivative represented by the formula (II) , wherein R1 is -F, -CH3, or -CF3, preferably -F,R2 is -H, -F, -OR3, -SR3, -S(O)R4, -S(O)2R4, C1-C6 alkyl, or C1-C6fluoroalkyl, preferably -F or methyl; or R1 and R2 taken together withthe carbon atoms to which they are attached form a methylenedioxy or a difluoromethylenedioxy ring; R3is H or C1-C6alkyl; and R4is C1-C6alkyl. The position where M or L is attached to the camptothecin derivative is indicated by the waved line.R1 is preferably -H or -F, in particular –F.R2 may be preferably -H, -F, -OCF3, -CF3, -OMe, -OEt, -SMe, -S(O)Me, -S(O)2Me, -SEt, -S(O)Et, -S(O2)Et, methyl, or ethyl, in particular –F. R2 may be also selected fromthe group consisting of -OMe, -SMe, -S(O)Me, or methyl, in particular methyl.According to preferred embodiments R1 is –F and R2 is –F.According to preferred other embodiments R1 is methyl and R2 is –F, while in otherpreferred embodiments R1 is –F and R2 is methyl.It is further preferred that the drug D of the antibody conjugate of the invention is a camptothecin derivative represented by the formula (IIa) or (IIb): Formula (IIa) wherein Yis -H, hydroxyl, C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C1-C6alkoxy, C1-C6aminoalkoxy, C1-C6alkylthio, C1-C6hydroxyalkyl, C1-C6halogenalkyl, C1-C6cyanoalkyl, C1-C6nitroalkyl, halogen, nitro, cyano, or mercapto, preferably -H or –F; (Formula (IIb) wherein R1 is -CH3, optionally substituted by halogen,preferably -CF2H, or H, andR2 is -H, or C1-C6 alkyl, or –(CH2)2NR4CH(CH3)2, andR3is H, amino, or C1-C6alkyl. R4 is H, or –SO2CH3The position where M or L is attached to the camptothecin derivative is indicated by the waved line.Preferred examples the transcription inhibitors are trabectedin (ecteinascidin 743) offormula (III) and lurbinectedin of formula (IV), or α-amanitin. Tubulin inhibitors, also known as mitotic or microtubule inhibitors, are a diverse class of substances primarily derived from plant alkaloids. These inhibitors interfere with the assembly and disassembly of tubulin into microtubule polymers, thereby halting the cell cycle during the M-phase (mitosis), when two sets of fully formed chromosomes are supposed to separate into daughter cells. This disruption hinders the proliferation of cancer cells, which, due to their rapid growth rates, rely oncontinuous cell division cycles, making them particularly vulnerable to disturbances incell division.Tubulin inhibiting substances are classified based on their mode of action as (i) tubulindepolymerization inhibitors such as paclitaxel, epothilone, docetaxel, anddiscodermolide; and (ii) tubulin polymerization inhibitors such as colchicine,combretastatin, 2-methoxyestradiol, methoxy benzenesulfonamides (E7010), or those belonging to the group of Vinca alkaloids including vinblastine, vincristine, vinorelbine, vinflunine, dolastatins, halichondrins, hemiasterlins, and cryptophysin 52.All of which are examples of preferred tubulin inhibitors within the meaning of theinvention.Further preferred examples of antimitotic agents are auristatins, a class of cytostaticagents derived from Dolastatin 10, which was extracted from the sea spongeDolabella auricularia. Auristatins bind to tubulin and prevent the polymerization ofmicrotubules, which disrupts the mitotic cell cycle and leads to cell death. Thesecompounds exhibit high cytotoxicity towards tumor cells while having a manageableimpact on normal cells. Particularly preferred embodiments are, for example, the monomethyl auristatins consisting of five amino acids such as MMAE, MMAD, and MMAF, with MMAE and MMAF exhibiting significantly increased cytotoxicity compared to Dolastatin 10. MMAF differs from MMAE in that it has a charged (carboxyl) phenylalanine at the C- terminus, while MMAE is uncharged. The negatively charged carboxylic acid group hinders the diffusion of MMAF in the cell, making MMAF slightly less toxic than MMAE. It will of course be obvious to a skilled person that further auristatins can be selected or the aforementioned ones can be modified based on common considerations foruse in ADCs of the invention.A further preferred cytotoxic agent is maytansine and / or derivatives thereof, the so-called maytansinoids.Maytansine is represented by formula (V) Formula (V)Preferred examples of maytansinoids are Ansamitocin, Mertansine / emtansine(DM1) or Ravtansine / soravtansine (DM4).According to the invention, the drug may be also a corticosteroid. Corticosteroids referto a class of steroid hormones that are produced in the adrenal cortex of vertebrates,as well as the synthetic analogues of these hormones. Two preferred main classes ofcorticosteroids are glucocorticoids and mineralocorticoids.According to the invention in particular preferred corticosteroids are11-Dehydrocorticosterone (11-oxocorticosterone, 17-deoxycortisone), 11-Deoxycorticosterone (deoxycortone, desoxycortone; 21-hydroxyprogesterone), 11-Deoxycortisol (cortodoxone, cortexolone), 11-Ketoprogesterone (11-oxo-progesterone; Ketogestin), 11β-Hydroxyprogesterone, 11β-Hydroxypregnenolone(21-deoxycorticosterone), 11β,17α,21-Trihydroxypregnenolone, 17α,21-Dihydroxypregnenolone, 17α-Hydroxypregnenolone, 17α-Hydroxyprogesterone, 18-Hydroxy-11-deoxycorticosterone, 18-Hydroxycorticosterone, 18-Hydroxy-progesterone, 21-Deoxycortisol, 21-Deoxycortisone, 21-Hydroxypregnenolone(prebediolone), Aldosterone, Corticosterone (17-deoxycortisol),Cortisol (hydrocortisone), Cortisone, Pregnenolone, Progesterone and syntheticanalogues thereof.In particular preferred corticosteroids are Progesteron Cortisol, Corticosterone,Cortisone and Aldosterone and synthetic analogues thereof.Preferred synthetic analogues of the Progesteron-type are Flugestone (flurogestone),Fluorometholone, Medrysone (hydroxymethylprogesterone) and Prebedioloneacetate (21-acetoxypregnenolone).Preferred synthetic analogues of the Hydrocortison-type are Chloroprednisone,Cloprednol, Difluprednate, Fludrocortisone, Fluocinolone, Fluperolone,Fluprednisolone, Loteprednol, Methylprednisolone, Prednicarbate, Prednisolone,Prednisone, Tixocortol and Triamcinolone.In certain embodiments, the antibody drug conjugate has a molar drug- antibody / antibody fragment ratio (DAR) of greater than 1, i.e., more than one drug molecule is attached to an antibody / antibody fragment. Typically, the conjugate has a DAR of about 2:1 to about 16:1, particularly of about3:1 to 12, 3,5:1 to about 10:1. According to a preferred embodiment a DAR of 3,5:1to about 4,5:1 and in particular 3:9 is preferred. The DAR may be calculated from astatistical distribution according to known methods. LINKER M The drug can be conjugated to the antibody or antigen-binding fragment thereof via alinker M.In certain embodiments, the linker is a cleavable linker, i.e., a linker cleavable under physiological conditions, e.g., by physiological enzymes. Specific examples of cleavable linkers are peptide-based linkers, which may be subject to cleavage by a protease, or glycoside-based linkers, which may be subject to cleavage by aglycosidase. β-glucuronide comprising linkers are particularly preferred.Another specific example are branched tandem-cleavage linkers that combine moieties that can be cleaved under different conditions or by different enzymes, e.g. protease-cleavable dipeptide and glycosidase-cleavable sugar moieties. In this way, several identical or different payloads can be connected to the antibody. In certain embodiments, the linker is a hydrophilic polysarcosine linker e.g., as described by Conilh et at. “Exatecan antibody drug conjugates based on a hydrophilic polysarcosine drug-linker platform” (Pharmaceuticals 14 (2021), 247). Further preferred linkers include linkers comprising at least one ethylene glycol unit, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more ethylene glycol units, e.g., the linker of the antibody-drug conjugate MEDI7247, an mcc-triazole spacer-PEG7-x-Lys-PABC glycol linker from Trodelvy®.Further preferred linkers include oligopeptides, particularly di- to decapeptide, e.g.,tetrapeptide sequences such as glycine-glycine-phenylalanine-glycine, glycine-glycine-glycine-serine as well as the peptides A described below in detail. A glycine-glycine-glycine-serine peptide is in particular preferred.Further preferred linkers include highly polar spacers such as an acyl group, carbamoyl group and / or sulfamide group added to at least at least one ethylene glycol unit, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more ethylene glycol units, e.g., the linkertechnology called HydraspaceTM. A particularly preferred linker is a ^^-glucuronidelinker. In certain embodiments, the linker is a hydrophilic polysarcosine linker comprising, e.g., up to 15 sarcosine units, and at least one ethylene glycol unit, e.g., 1, 2, 3, 4, 5,6, 7, 8, 9, or 10 or more ethylene glycol units, wherein the linker is subject to cleavageby a glycosidase, and particularly subject to cleavage by a glucuronidase. In certain embodiments, the linker is a hydrophilic polysarcosine linker comprising, e.g., about 8-12 sarcosine units, and at least one ethylene glycol unit, e.g., up to 10 ethylene glycol units, and wherein the linker is subject to cleavage by a glycosidase, and particularly subject to cleavage by a glucuronidase.In certain embodiments, the linker is a hydrophilic polysarcosine linker comprising 8-12, preferably 10 sarcosine units, and 2 or 4 ethylene glycol units, and wherein thelinker is subject to cleavage by a glucuronidase. In particular preferred is apolysarcosine linker comprising 10 sarcosine units, 2 ethylene glycol units and βglucuronide. A further aspect of the invention relates to a linker-drug conjugate comprising: (i) a hydrophilic polysarcosine linker comprising, e.g., about 8-12 sarcosine units, and at least one ethylene glycol unit, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more ethylene glycol units, wherein the linker is subject to cleavage by a glycosidase, and particularly subject to cleavage by a glucuronidase, and wherein the linker comprises a thiol- reactive group, e.g., a maleimide group capable of reacting with cysteine residues on an antibody or antigen-binding fragment thereof, and(ii) the drug or a derivative thereof covalently attached to the linker.In particular embodiments, the linker-drug conjugate comprises: (i) a hydrophilic polysarcosine linker comprising 10 sarcosine units, and 2 ethyleneglycol units, wherein the linker is subject to cleavage by a glycosidase, and particularlysubject to cleavage by a glucuronidase, preferably comprising β glucuronide residue,and wherein the linker comprises a maleimide group capable of reacting with cysteine residues on an antibody or antigen-binding fragment thereof, and (ii) the drug or a derivative thereof covalently attached to the linker.A further aspect of the invention relates to a linker-drug conjugate comprising a p-aminobenzyl (PAB) spacer and in particular a para-aminobenzyl carbamate (PABC)spacer In particular preferred is a linker-drug conjugate comprising (i) a PAB residue, in particular a PABC residue,(ii) a polyethylene glycol (PEG) spacer, preferably comprising 2-10 ethyleneglycol units, in particular 4 ethylene glycol units,or peptide A as herein defined below, preferably GGGS, and(iii) β-glucuronide residue and(iv) a maleimide group capable of reacting with cysteine residues on anantibody or antigen-binding fragment thereof as herein described.In particular preferred is a maleimide-propionyl-Gly-Gly-Gly-Ser-β-glucuronide-PABClinker.According to other embodiments, the linker M of the inventive antibody drug conjugatemay be characterized by formula (VI): -L-A-E- (VI),whereinL is -(C1-C6-alkylene)-(O-CH2)m-NR1-,-(C1-C6-alkylene)-NR1-, or -(C1-C5-alkylene)-NR2C(C=O)-(C1-C5-alyklene)-(O-CH2)m-NR1-, wherein R1 and R2 are each independently –H, C1-C3-alkyl, in particularmethyl, or benzyl, m is an integer from 1 to 6, in particular 1,A is a peptide comprising 2 to 8 amino acids, preferably 2 to 4 amino acids,wherein A may be substituted with at least one polyol, wherein polyol ispreferably -(C1-C6 alkylene)-Xa-Yb, wherein:Xa is -NRcC(=O)- or -C(=O)NRc- or absent;Yb is –CH2-(C*1-C*5-alkyl)-, each C* substituted with a OH group,Rc is -H, C1-C6-alkyl, C1-C6 fluoroalkyl, C3-C6 cycloalkyl, aryl, heteroaryl, orbenzyl, andE is –C(=O)-(C1-C10-alkylene)-Q-, wherein Q is an antibody coupling group.L is -(C1-C6-alkylene)-(O-CH2)m-NR1-, in particular -(C1-C4-alkylene)-O-CH2-NR1-, -(C1-C6-alkylene)-NR1-, in particular -(C1-C4-alkylene)-NR1-, or-(C1-C5-alkylene)-NR2C(C=O)-(C1-C5-alyklene)-(O-CH2)m-NR1-, in particular -(C1-C5-alkylene)-NR2C(C=O)-(C1-C5-alyklene)-O-CH2-NR1-, wherein R1and R2are each independently –H, C1-C3-alkyl, in particular methyl, or benzyl, and m isan integer from 1 to 6, in particular 1.According to a preferred embodiment L is -(C1-C4-alkylene)-O-CH2-NR1-, -(C1-C4-alkylene)-NR1-, or -(C1-C5-alkylene)-NR2C(C=O)-(C1-C5-alyklene)-O-CH2-NR1-,wherein R1 and R2 are each independently –H, methyl, or benzyl.As used herein “alkylene” refers to a saturated linear or branched divalent hydrocarbon radical. As used herein “cycloalkyl” refers to the radical of a saturated carbocyclic ring. Suitable cycloalkyls include cyclohexyl, cyclopentyl, cyclobutyl and cyclopropyl. The term “aryl” as used herein, includes substituted or unsubstituted single-ringaromatic groups in which each atom of the ring is carbon. “Aryl” groups include, butare not limited to, phenyl, phenol, aniline, etc. The terms "aryl" also includes "polycyclyl", "polycycle", and "polycyclic" ring systems having two or more rings in which two or more atoms are common to two adjoining rings, e.g., the rings are "fused rings," wherein at least one of the rings is aromatic, e.g., the other cyclic rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, or aromatic rings.Preferably, the ring is a 5- to 7-membered ring, more preferably a 6-membered ring.Aryl groups include, but are not limited to, phenyl, phenol, aniline, and the like. “Heteroaryl” as used herein refers to substituted or unsubstituted aromatic single ringstructures, preferably 6- to 18-member rings, preferably 5- to 7-membered rings, morepreferably 5- to 6-membered rings, whose ring structures include at least oneheteroatom (e.g., O, N, or S). The term “heteroaryl” as used herein, refers to substituted or unsubstituted aromaticsingle ring structures, preferably 6- to 18-member rings, preferably 5- to 7-memberedrings, more preferably 5- to 6-membered rings, whose ring structures include at leastone heteroatom (e.g., O, N, or S), preferably one to four or one to three heteroatoms, more preferably one or two heteroatoms. When two or more heteroatoms are present in a heteroaryl ring, they may be the same or different. For examples, “heteroaryl” moieties include, but are not limited to, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, pyrazole, pyridine, pyrazine, pyridazine, quinoline, pyrimidine, indolizine, indole, indazole, benzimidazole, benzothiazole, benzofuran, benzothiophene, cinnoline, phthalazine, quinazoline, carbazole, phenoxazine, quinoline, purine and the like. Unless specifically stated as "unsubstituted," references to chemical moieties herein are understood to also include substituted variants, i.e. can be substituted with one or more (e.g., 2, 3, 4, 5, 6 or more) substituents. For example, reference to an "alkyl" group or moiety implicitly includes both substituted and unsubstituted variants. Examples of substituents on chemical moieties includes but is not limited to, halogen, hydroxyl, carbonyl (such as carboxyl, alkoxycarbonyl, formyl, or acyl), thiocarbonyl (such as thioester, thioacetate, or thioformate), alkoxyl, alkylthio, acyloxy, phosphoryl, phosphate, phosphonate, amino, amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, aralkyl, or aryl or heteroaryl moiety. The amino acids of peptide A may be selected independently. The amino acids may be independently L or D amino acids. According to one preferred embodiment at least one amino acid is an L amino acid. According to other preferred embodiments allamino acids are D or L amino acids. The amino acids alanine (Ala), valine (Val), orglycine (Gly) are in particular preferred.Suitable examples for a peptide A comprise inter alia -Ala-Ala-, -Gly-Gly-, -Val-Val-, -Val-Ala-, -Ala-Ala-Ala-, -Gly-Ala-Gly-Gly-, -Gly-Gly-Ala-Gly-, -Gly-Val-Gly-Gly-, -Gly-Gly-Val-Gly-, -Gly-Gly-Phe-Gly-, or -Gly-Phe-Gly-Gly- but also flexible linkers likeglycine polymers (G)n, glycine-serine polymers (including, for example, (GS)n, (GSGGS)nand (GGGS)n, where n is an integer of at least one, glycine-alanine polymers, alanine-serine polymers, and other flexible linkers known in the art. Suchflexible linkers include, but are not limited to -Gly-Gly-Ser-Gly-, -Gly-Gly-Gly-Ser-, -Gly-Gly-Ser-Gly-Gly-, -GIy-Ser-Gly-Ser-Gly-, -Gly-Ser-Gly-Gly-Gly-, -Gly-Gly-Gly-Ser-Gly-, -Gly-Ser-Ser-Ser-Gly-, and the like. A _-Gly-Gly-Gly-Ser- peptide may be inparticular preferred.E is –C(=O)-(C1-C10-alkylene)-Q-, wherein Q is an antibody coupling group.Such antibody coupling groups are well known to the person skilled in the art. Beforebeing coupled, the antibody coupling group is preferably a maleimide group. For alllinkers described herein maleimide is the most preferred antibody coupling group. After being coupled Q is preferablywherein # is the site covalently attached to the remainder of E,i.e. –C(=O)-(C1-C10-alkylene)-#, andV is an antibody or antigen binding fragment thereof as hereindefined. Corresponding coupling reactions are well known to the person skilled in the art. According to a preferred embodiment, Ybis C5-alkyl substituted with 5 OH groups. According to a further preferred embodiment the at least one polyol is , wherein R12is H or methyl.According to another embodiment linker M is as shown in formula (VII)

[0003] wherein AB is the antibody or functional fragment thereof as shown informula (I) and herein defined, Fuc is fucose;GlcNAc is N-acetylglucosamine;S is a sugar or sugar derivative;M is -N(H)C(O)CH2-, -N(H)C(O)CF2-, -CH2-, -CF2- or a 1,4-phenylene containing 0-4 fluorine substituents, preferably 2 fluorine substituents which are preferably positioned on C2 and C6 or on C3 and C5 of the phenylene; R1is independently selected from the group consisting ofhydrogen, halogen, -OR8, -NO2, -CN, S(O)2R8, C1-C24 alkyl groups, C6-C24 (hetero)arylgroups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen, and wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are optionally substituted, wherein two substituents R8may be linked together to form an annelated cycloalkyl or an annelated (hetero)arene substituent, and wherein R8is independently selectedfrom the group consisting of hydrogen, halogen, C1-C24 alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups; R2and are R3are independently selected from the group consisting of hydrogen, halogen, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen; R4is selected from the group consisting of hydrogen, halogen, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen, the alkyl groups optionally beinginterrupted by one of more hetero-atoms selected from the group consisting of O, N and S, wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are independently optionally substituted; Yis O, S or NR7, wherein R7 is independently selected from the groupconsisting of hydrogen, halogen, -OR8, -NO2, -CN, S(O)2R8, C1-C24 alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups and wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are optionally substituted, wherein two substituents R8may be linked together to form an annelated cycloalkyl or an annelated (hetero)arene substituent and wherein R8is independently selected from the group consisting of hydrogen, halogen, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24 (hetero)arylalkyl groups; L is a linking group, preferably a sulfamide based linking group, ais 0 or 1;x is 1 or 2, preferably 1;y is 1, 2, 3 or 4, preferably 1,r is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19or 20, preferably 1 or 2; nn is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, preferably 1 or 2, in particular 1;pp is 0 or 1, preferably 1;D is is the drug or active agent as herein defined.The general term "sugar" is herein used to indicate a monosaccharide, for example glucose (Glc), galactose (Gal), mannose (Man) and fucose (Fuc). The term "sugar derivative" is herein used to indicate a derivative of a monosaccharide sugar, i.e. a monosaccharide sugar comprising substituents and / or functional groups. Examples of a sugar derivative include amino sugars and sugar acids, e.g. glucosamine (GlcNH2), galactosamine (GalNH2) N-acetylglucosamine (GlcNAc), N- acetylgalactosamine (GalNAc), sialic acid (Sia) which is also referred to as N- acetylneuraminic acid (NeuNAc), and N-acetylmuramic acid (MurNAc), glucuronic acid (GlcA) and iduronic acid (IdoA). GalNAc is in particular preferred. GlcNAc moieties as shown in Formula (I) are preferably present at a native N-glycosylation site in the Fc-fragment anti-FLT3 antibody or functional fragmentthereof. Preferably said GlcNAc moieties are attached to an asparagine amino acid in the region 290-305 of the antibody. In a further preferred embodiment, the antibody is an lgG type antibody, and depending on the particular IgG type antibody, said GlcNAc moieties are present on amino acid asparagine 297 (Asn297 or N297) of the antibody. Of course, other attachment sites, i.e. “attachment amino acids” are also possible. According to a preferred embodiment b is 1. According to a preferred embodiment x is 1. According to a preferred embodiment R1, R2, R3and R4are hydrogen. According to a preferred embodiment nn is 1 or 2, in particular 1. According to a preferred embodiment Y is O. If Y is O, b is preferably 0.According to a preferred embodiment M is –CH2-, particular pp being 1.According a preferred embodiment x is 1, and R1, R2, R3and R4are hydrogen.According a preferred embodiment x is 1, and R1, R2, R3 and R4 are hydrogen and nnis 1. According a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen and nn is 1 and Y is O. According a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen, Y is O, nnis 1 and M is –CH2- with pp being 1. The compounds disclosed in this description and in the claims may comprise one ormore asymmetric centers, and different diastereomers and / or enantiomers may existof the compounds. The description of any compound in this description and in the claims is meant to include all diastereomers, and mixtures thereof. In addition, the description of any compound in this description and in the claims is meant to include both the individual enantiomers, as well as any mixture of the enantiomers and / or diastereomers. When the structure of a compound is depicted as a specificenantiomer, it is to be understood that the invention of the present application is notlimited to that specific enantiomer.According to a preferred embodiment L comprises a group according to formula (VIII) wherein b is 0 or 1; R5is selected from the group consisting of hydrogen, C1-C24alkyl groups, C3-C24cycloalkyl groups, C2-C24(hetero)aryl groups, C3-C24alkyl(hetero)aryl groups and C3-C24(hetero)arylalkyl groups, preferably hydrogen, the C1-C24alkyl groups, C3-C24cycloalkyl groups, C2-C24(hetero)aryl groups, C3-C24alkyl(hetero)aryl groups and C3-C24(hetero)arylalkyl groups optionally substituted and / or optionally interrupted by one or more heteroatoms selected from O, S or NR6, wherein R6is hydrogen or C1-C4alkyl, preferably hydrogen; wherein D is optionally connected to N via a spacer moiety. A “spacer moiety” as used herein refers to a moiety that spaces (i.e. provides distance between) and covalently links together two (or more) parts of a linker. The linker may be part of e.g. a linker-construct, the linker-conjugate or a bioconjugate as herein defined. If Y is O, b is preferably 0. R5is preferably hydrogen. According a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen, Y is O, nnis 1 and M is –CH2- with pp being 1, and L comprises Formula (VIII), wherein inparticular preferred R1, R2, R3, R4and R5are hydrogen.According to a preferred embodiment the spacer moiety is comprises the Formula (IX)Q-(L1)n-(CO)-(L2)o-(L3)p-(L4)q- ,wherein Qis –(W)k-(A)d-(B)e-(A1)f- , whereinW is –OC(O)-, -C(O)O-, -C(O)NH-, -NHC(O)-, -OC(O)NH-, -NHC(O)O-, -C(O)(CH2)mC(O)-, -C(O)(CH2)mC(O)NH-, or –(4-Ph)CH2NHC(O)(CH2)mC(O)NH-, wherein m is an integer in the range 0-10; Ais of Formula (VIII) as defined above;A1is of Formula (VIII) as defined above with R5being preferably (L1)n-(CO)-(L2)o-(L3)p-(L4)q-, Bis a (-CH2-CH2-O-)g or a -O-CH2-CH2- moiety, or (B)g is a -(CH2-CH2-O)g-CH2CH2- moiety, wherein g is an integer in the range1 – 10, preferably being 1 or 2;k is 0 or 1, with the proviso that if k is 1 then d is 0;d is 0 or 1;e is an integer in the range 1 – 10;f is 0 or 1;L1 is a –CH2-CH2-O-, –CH2-CH2-O-C(O)-, -O-CH2-CH2-, or –(CH2-CH2-O)n1-CH2-CH2- moiety, wherein n1 is an independently selected integerin the range of 1-10, preferably –CH2-CH2-O- or –CH2-CH2-O-C(O)-,L2is a peptide spacer, preferably a dipeptide wherein L2is represented by general structure (X):wherein R9 is hydrogen, CH3 or CH2CH2CH2NHC(O)NH2, preferably R9is hydrogen; L3is self-immolative spacer, preferably a para-aminobenzyloxycarbonyl (PABC) derivative according to structure (XI) wherein R10is hydrogen, R11or C(O)R11, wherein R11is selected from C1-C24(hetero)alkyl groups, C3-C10(hetero)cycloalkyl groups, C2-C10(hetero)aryl groups, C3-C10alkyl(hetero)aryl groups and C3-C10(hetero)arylalkyl groups, which are optionally substituted and optionally interrupted by one or more heteroatoms selected from O, S and NR12wherein R12is independently selected from the group consisting of hydrogen and C1-C4alkyl groups, R10preferably being hydrogen or C(O)R11wherein R11is preferably 4- methyl-piperazine or morpholine, most preferably wherein R10being hydrogen; L4 is an aminoalkanoic acid spacer according to the structure -N-(Cx-alkylene)-C(O)-, wherein x is an integer in the range 1-10, or L4is an ethyleneglycol spacer according to the structure –N-(CH2-CH2-O)x1-(CH2)x2-C(O)-, wherein x1 is an integer in the range 1-10 and x2 is an integer in the range 1-3; and n, o, p, q are independently selected from 0 or 1. Preferably at least one of d and f is 1.According to a preferred embodiment R5 of A1 is of -(L1)n-(CO)-(L2)o-(L3)p-(L4)q- asdefined herein, i.e. the corresponding N atom is substituted by two residues of Formula -(L1)n-(CO)-(L2)o-(L3)p-(L4)q- as defined above. The individual variables may be selected independently. Accordingto a preferred embodiment the residues of Formula -(L1)n-(CO)-(L2)o-(L3)p-(L4)q- areidentical. If Y is O, k is preferably 0, in particular b and k are 0. “Peptide spacers” as used herein are known by the skilled person preferably comprising 2-5 amino acids, more preferably a dipeptide or tripeptide spacer, mostpreferably a dipeptide spacer. L2 may be selected from Val-C1t, Val-Ala, Val-Lys, Val-Arg, Phe-C1t, Phe-Ala, Phe-Lys, Phe-Arg, Ala-Lys, Leu-C1t, Ile-C1t, Trp-C1t, Ala-Ala-Asn, Ala-Asn, more preferably Val-Cit, Val-Ala, Val-Lys, Phe-Cit, Phe-Ala, Phe-Lys, Ala-Ala-Asn, more preferably Val-Cit, Val-Ala, Ala-Ala-Asn. In the particular preferred embodiment the peptide spacer L2 is represented by general formula (IX). According to a preferred embodiment R9is hydrogen. According to a preferred embodiment R10is hydrogen. According to a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen and L2is represented by general structure (X). According to a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen and L2is represented by general structure (X) and R9is hydrogen. According to a preferred embodiment x is 1, R1, R2, R3and R4are hydrogen and L2is represented by general structure (X), R9is hydrogen and Y is O. According to a preferred embodiment x is 1, R1, R2, R3and R4and are hydrogen and L3is a PABC derivative as herein defined. According to a preferred embodiment x is 1, R1, R2, R3and R4and are hydrogen, Y is O, L2is represented by general structure (X), and L3is a PABC derivative as herein defined. According to a preferred embodiment x is 1, R1, R2, R3, R4and R9are hydrogen. According to a preferred embodiment x is 1, R1, R2, R3, R4and R10are hydrogen. According to a preferred embodiment x is 1, R1, R2, R3, R4, R9and R10are hydrogen. According to a preferred embodiment x is 1, R1, R2, R3, R4and R10are hydrogen and Y is O. According to a preferred embodiment x is 1, R1, R2, R3, R4and R10are hydrogen, Y is O and L3is a PABC derivative as herein defined. According to a preferred embodiment x is 1, R1, R2, R3, R4and R10are hydrogen, Y isO, L3 is a PABC derivative as herein defined and M is –CH2- with pp being 1.According to a preferred embodiment x is 1, R1, R2, R3, R4, R9and R10are hydrogen and Y is O.According to an especially preferred embodiment x is 1, R1, R2, R3, R4, R9 and R10 arehydrogen, L2 is represented by general structure (X), Y is O and L3 is a PABCderivative as herein defined. According to an especially preferred embodiment x is 1, R1, R2, R3, R4, R9and R10are hydrogen, L2is represented by general structure (X), Y is O, L3is a PABC derivativeas herein defined and M is –CH2- with pp being 1.A "self-immolative group" as used herein is a part of a linker in an antibody-drug conjugate with a function to conditionally release a free drug at the site targeted by the ligand unit. The activatable self-immolative moiety may comprise an activatable group (AG) and a self-immolative spacer unit. Upon activation of the activatable group, for example by enzymatic conversion of an amide group to an amino group or by reduction of a disulfide to a free thiol group, a self-immolative reaction sequence is initiated that leads to release of free drug by one or more of various mechanisms. Alternatively, the self-immolative group is not an inherent part of the chemical spacer, but branches off from the chemical spacer connecting the antibody and the payload. The linker described herein is, for example, described in WO 2021 / 144313,WO 2016 / 053107 as well as WO 2017 / 137456. Methods for attaching the describedlinkers to antibodies are, for example, described in WO 2011 / 136645,WO 2022 / 2022049211, WO 2014 / 065661, WO 2016 / 270186, WO 2017 / 137459 andWO 2021 / 015622.The linker-drug conjugates described herein are particularly suitable for attachmentto an anti-FLT3 antibody as described herein. It should be noted, however, that thelinker-drug conjugates are also suitable for attachment to any antibody or antigen- binding fragment thereof, e.g., an antibody binding to a tumor antigen or an antigen- binding fragment thereof. The antibody-drug conjugate of the present invention may be prepared by known methods. The antibody or the antigen-binding fragment thereof may be produced in a suitable host cell comprising a nucleic acid molecule, e.g., a DNA molecule, encoding an antibody VH region, or an antibody VL region, or encoding a complete antibody or an antibody fragment, or a vector or vector system, i.e. a plurality of vectors, comprising said nucleic acid molecule(s), preferably in operative linkage with an expression control sequence, particularly with a heterologous expression control sequence. The host cell may be any known host cell for producing antibodies or antibody fragments, e.g., a prokaryotic cell such as an E. coli cell, a yeast cell, an insect cell, or a mammalian cell, e.g., a CHO cell or a hybridoma cell. The antibody or antigen-binding fragment thereof may be reacted with a linker-drug conjugate to obtain the antibody-drug conjugate. The linker-drug conjugate comprises a drug molecule, e.g., having attached thereto a suitable linker, wherein the linker comprises a reactive group capable of reacting with desired attachment positions on the antibody or antigen-binding fragment thereof. For attachment to cysteine residues, the linker-drug conjugate comprises a thiol-reactive group, e.g., a maleimide group. In another specific embodiment, binding of the conjugating linker to the antibody can take place, for example, via aldehyde groups that may have been previously introduced into the antibody. For this purpose, the ligand may include a Hydrazine- iso-Picted-Spengler (HIPS) chemistry, the binding of which to the aldehyde group leads to the formation of a stable C-C bond. This HIPS chemistry can be combined with any of the linker types described above.

[0004] PHARMACEUTICAL COMPOSITIONS A further aspect of the present invention is a pharmaceutical composition comprising an active agent, which is an antibody-drug conjugate as herein described and a pharmaceutically acceptable carrier and / or excipient. Examples of suitable carriers and excipients for formulating antibodies and antibody drug conjugates include saline and aqueous buffer solutions and are well known in the art. Typically, the pharmaceutical composition is adapted for parenteral administration, e.g., for subcutaneous, intramuscular, or intravenous injection or by infusion. In further embodiments, the pharmaceutical composition is adapted for local administration, e.g., for intravesical instillation into the bladder. Depending on the stage and the severity of the disorder, the pharmaceutical composition may be administered once or several times in a therapeutically effective dose to a subject in need thereof, particularly to a human subject. For example, it may be administered once or several times daily, each second day, two times weekly oronce weekly for a suitable period, e.g., of at least one week, or at least one month.The aforementioned administration regimens are not to be understood as limiting thescope of the application. Rather, the attending physician will be able to choosealternative administration regimens based on the medical needs of the patient, suchas administration once every two weeks or once every three weeks. Adjusting the dosage to meet the specific requirements of a patient generally falls within thecompetence of the attending physician, who will be able to establish differentadministration regimens based on common medical considerations, withoutexercising inventive activity.MEDICAL APPLICATIONSAccording to a further aspect of the invention the antibody-drug conjugate orpharmaceutical composition as described above is used in medicine, including human and veterinary medicine, particularly in human medicine. In a particular embodiment, the antibody-drug conjugate targeting FLT3 or the pharmaceutical composition comprising it is used in a method for the prevention and / or treatment of a FLT3 associated disorders such as cancer, in particular hematopoietic cancers, e.g. Acute Myeloid Leukemia (AML), Mixed Phenotype Acute Leukemia (MPAL), Acute Lymphoblastic Leukemia (ALL) and subtypes thereof such as MLL-rearranged (KMT2A-r) infant B-ALL or Ph-like ALL with FLT3 lesions and Early T-cell Precursor ALL (ETP-ALL), Chronic Myeloid Leukemia (CML) and B-cell lymphoma biphenotypic acute leukemia (BAL). The skilled person understands that the aforementioned disorders merely illustrate the treatment options of FLT3 directed therapy and is certainly aware of furtherdiseases that might benefit from FLT3 directed therapy in treatment and / orprevention. In therapeutic applications, the active agent is administered in an effective amount to a subject in need thereof, particularly to a human subject. The dose will depend on the specific type of agent, e.g., type of antibody or antibody fragment, the type of disease, and the mode of administration, e.g., locally or systemically. In the prevention and / or treatment of cancer, a therapeutic dose of an antibody or antibody drug conjugate is typically from about 0.3 mg / kg to about 10 mg / kg. The antibody-drug conjugate may be administered alone or together with a further active agent, which may be selected from chemotherapeutic agents, e.g., anti- metabolites, alkylating agents, intercalating agents, or anti-mitotic agents), inhibitors of specific kinases e.g., tyrosine kinase inhibitors, serine / threonine kinase inhibitors or phosphoinositide kinase inhibitors, immunotherapeutic compounds, e.g., immunecheckpoint inhibitors, CAR-T cells, bi- or multi-specific immune cell engager such asNK- or T-cell engagers, or therapeutic vaccines or oncolytic viruses.A preferred aspect of the present invention refers to an antibody-drug conjugate or apharmaceutical composition as herein described for use in the treatment of an FLT3associated disorder of a patient who has relapsed or is refractory to prior FLT3directed treatment.Another aspect relates to a method of treating an FLT3 associated disorder of apatient who has relapsed or is refractory to prior treatment of an FLT3 associateddisorder including prior treatment with a FLT3 directed drug, comprising administeringto the patient a therapeutically effective amount of the antibody-drug conjugate or apharmaceutical composition as herein described.

[0005] FIGURESFigure 1: Amino acid sequences of antibodies of the inventionFigure 2: Homogeneous DAR 3.9 ADCA) Monomeric purity determination of ADCX-040 ADC after sterile filtration by SEC-UPLC. ADCX-040 is an Exatecan conjugated, IgG1 based, Fc-silenced ADCcomprising the heavy and light chain variable regions according to SEQ ID NOs: 19and 22. HMWS: high molecular weight species; RT: Retention Time; SEC-UPLC: Size exclusion chromatography-ultra-high performance liquid chromatographyB) LC-MS analysis of ADCX-040ADCDeconvolutions of m / z spectra to estimate the corresponding masses of HC. The DAR was estimated from experimental intensities obtained from each chain, HC: heavy chain, H0: unconjugated heavy chain, H1: Heavy chain conjugated to one molecule of linker-payload, H2: Heavy chain conjugated to two molecules of linker-payload, MW: molecular weight, LC-MS: liquid chromatography-mass spectrometry.Figure 3: Stable DAR 3.9 ADCA) Histogram of LC-MS analysis of ADCX-040 ADC under various stress conditions.The DAR was estimated from experimental intensities obtained from each chain. QC:control quality of ADC after production, T0: ADC after thawing, T2W: ADC after two weeks at 40°C, T4W: ADC after four weeks at 40°C, FT: ADC after five freeze / thaw cycles.B) Estimated mean DAR over time and percentage of hydrolyzed thiosuccinimide ringof ADCX-040 ADC incubated over time at 37°C in human plasma.C) Table summarizing estimated mean DAR and percentage of hydrolyzedthiosuccinimide ring over time in human plasma. SD: standard deviation, N=3.D) Estimated mean DAR and percentage of hydrolyzed thiosuccinimide ring over timeof ADCX-040 ADC in C57BL / 6 mouse plasma after IV injection at 10mg / kg.E) Table summarizing estimated mean DAR and percentage of hydrolyzedthiosuccinimide ring over time after IV injection at 10 mg / kg in mice. SD: standarddeviation, N: number of replicates, N=3. LC-MS: liquid chromatography-massspectrometry.Figure 4: ADC binding and EC50 compared to parental antibody on NKM-1 humancell lines. 105NKM-1cells / well expressing FLT3 were incubated with dose range of the indicated antibodies or ADC (100 nM to 0.0004 nM) 1 hour at 4°C, washed and incubated with anti-human secondary antibody (Goat Anti human Fc, Thermo FisherScientific) 30 min à 4°C. After cells fixation (4% PFA for 15min), medians offluorescence intensities (MFI) were obtained by flow cytometry. EC50 was calculatedusing MFI(f)=F(Concentration) data with GraphPad Prism 10 Nonlinear regressionlog(agonist) vs. response -- Variable slope (four parameters).ADCX-040: Anti-FLT3 Exatecan-based ADC; ADCX-040a: Anti-FLT3 antibody alone;ICT: Isotype control; ICT-Exa: Isotype control Exatecan-based ADCFigure 5: ADC tumor-killing activity compared to free linker payload on NKM-1 humancells Shown are the fitting curves of relative luminescence (%) as a function ofconcentration (F(Conc)) in an ATP-based cytotoxicity assay. IC50 was determinedusing the percentage of relative luminescence correlated to concentration withGraphPad Prism 10 Nonlinear regression log [Inhibitor] vs. response -- Variable slope (four parameters)LP: Linker payload MA-G3S-Ex; ADCX-040: Anti-FLT3 Exatecan-based ADC; ICT-Exa: Isotype control Exatecan-based ADCFigure 6: ADC in vivo anti-tumor activity in cancerous cell derived xenograftsData depicts ADC in vivo efficacy in acute myeloid leukemia cell line derivedxenografts. Cell line names are indicated over the graphs. N=6-10 mice per groupA) Graphs represent mean tumor volume over time for each group (+ / - SEM) overtime. When a mouse dies in a group, the mean tumor volume is no longer plotted as the curve could mechanically inflect when larger tumors are removed, independently of the effect of the treatment. Treatment groups and doses are indicated. Injection time are indicated on each graph. Intravenous injections were performed / B) Graphs represent the evolution of body weight over time as percentage of weightat first day of treatment. For each group, the body weight is plotted (+ / - SEM) overtime. When a mouse dies in a group, the body weight is no longer plotted,Table 1: Summary of mean tumor volume and tumor growth inhibition in human tumorcancerous cell derived xenografts. Tumor growth inhibition (TGI) was calculated as follows at the last time point when allmice were still alive: TGI (%) : [100 - (Ti-T0) / (Vi-V0)] × 100%, (Ti: the mean tumorvolume of treatment group on day i after administration, T0: the mean tumor volume of treatment group on day 0 after administration; Vi: the mean tumor volume of negative control group on day i after administration, V0: the mean tumor volume ofnegative control group on day 0). N = 6-10 mice. ADCX-040: Anti-FLT3 Exatecan-based ADC; ICT-Exa: Isotype control Exatecan-based ADC.

[0006] EXAMPLES Material and methods Cell lines: NKM-1 and MOLM13 were cultured in Roswell Park Memorial Institute medium and CHO cells were cultured Dulbecco's Modified Eagle medium. Culture media weresupplemented with 10 % fetal Bovine serum, 100 I.U / mL penicillin, 100 μg / mLstreptomycin and 2 mM glutamine. Mice: 9 weeks-old C57BL / 6NRj, NXG or NOD-SCID female mice (Janvier Labs, France) were handled in accordance with national and European laws for laboratory animal welfare and experimentation (EEC Council Directive 2010 / 63 / EU, September 2010). Antibodies generation, production, purification and control: After PCR amplification of the cDNAs of interest, they were cloned into expression vectors using In-fusion® cloning procedure according to manufacturer’s instructions. Final plasmid DNAs were purified by anion exchange chromatography and sequences were verified by Sanger sequencing. Antibodies were produced in CHO expression system, using a modified human IgG1 bearing L234A / L235A / S239C / S442C mutations of the heavy chain and a kappa LC. Cysteine insertions are for the site-specific coupling of linker payloads. Alanine insertions are for limiting the ability of the mAb to interact with Fc receptors. Light and heavy chain expression vectors were co-transfected into suspension cells. Five liters of cells were transiently transfected and grown in animal-component free and serum- free medium. Feeds were added during the production process following a definedfeeding strategy and cells were kept in culture for at least 12 days. Supernatants werethen harvested and clarified before antibody purification. Antibodies were purified from cell supernatants by a double purification strategy. The affinity purification with Protein A resin was followed by a Size Exclusion Chromatography (SEC). Elution was performed at acidic pH and samples were then immediately neutralized with basic Tris buffer before SEC in PBS 1X buffer. Purified fractions were pooled and sterile filtered before quality controls. For binding, the buffer 0.5 M Glycine, 3 M NaCl, pH 8.9 was used, and the buffer 0.1 M Citrate pH 3 wasused for elution. Neutralization was done with 10% (V / V) 1 M Tris-HCI pH 9.Monoclonal antibodies were then dialyzed against PBS 1X pH 7.4 (Mini dialysis devices, 2 mL-10k, Thermo Scientific) followed by filtration on 0.22 µm filter (MilelexGV hydrophilic PVDF, Millipore). Concentration was determined with a Nanodrop2000 Spectro-photometer (Thermo Scientific) considering the specific extinction coefficient (E1%280nm) of each monoclonal antibody. Purity was determined byUPLC-SEC using an Acquity UPLC-H Class Bio (Waters) using a Protein-BEH 200Acolumn equilibrated in 0.2 M NaPO4, 0.3 M NaCl pH 6.9 supplemented with 10% isopropanol. The mass of the antibodies was determined in a Xevo G2-S Q-Tof mass spectrophotometer (Waters) using a reversed-phase column (PLRP-S 4000A, Agilent technologies). All samples were analyzed after deglycosylation with PNGase F glycosidase (New England Biolabs) at 37°C, according to the manufacturer’s instructions. Fragmentation and / or aggregation of the final material was evaluated by SDS-PAGE. Conjugation: The cysteine reactive linker-exatecan compound Maleimide-propionyl-Gly-Gly-Gly- Ser-β-glucuronide-PABC-Exatecan was conjugated to inserted cysteine residues (S239C, S442C) of selected antibodies. In brief, mAbs in PBS 1 mM EDTA were reduced with 50 molar equivalents of TCEP for 4 hours at 37°C, followed by re- oxidation with 20 molar equivalents of DHAA overnight at +4°C. Six molar equivalents of the cysteine reactive linker-exatecan compound were used for conjugation with reactive cysteines for 35 min at room temperature. The final exchange buffer was performed in 10 mM acetate, 250 mM sucrose, pH 5.0 before filtration 0.22 µM filter. The drug-antibody ratio (DAR) according to LC-MS analysis was 4 toxins per conjugated antibody. UPLC-SEC analysis: 20 µL of ADC was prepared in formulation buffer and 10 µL of a 1 mg / mL solution were injected into the SEC analytical column. UPLC-SEC using an Acquity UPLC-H Class Bio (Waters) using a Protein-BEH 200A column equilibrated in 0.2 M NaPO4, 0.3 M NaCl pH 6.9 supplemented with 10% isopropanol. DAR monitoring: ADCs were purified and supplied in solution.For in vivo ADC solutions were prepared on the day of injection in aseptic conditionsby dilution into sterile 1X PBS at 10 mg / kg for administration by intravenous route in C57BL / 6NRj mice. Blood collection was performed on individual mice at: T0, 3 min, 3 h, 6 h, 24 h, 96h, 168h, 240h, 336h and 504 h. Blood samples were taken from the submandibular vein of vigilant mice. Three independent blood samples were collected for each time points, each corresponding to one individual mouse.For ex vivo DAR monitoring, ADCs were incubated in human plasma (Sigma Aldrich)during 0 h, 2 h, 4 h, 8 h, 24 h, 48 h, 96 h and 168 h.To capture ADC from human and mice plasma, magnetic streptavidin beads (Dynabeads M-280 streptavidin, Thermofisher Scientific) were prepared and washed using magnet and mixed with biotinylated Human anti-LC-kappa-biot (CaptureSelect Biotin Anti-LC-kappa, Thermofisher Scientific) or anti-Fc-biot (Mouse Anti-Human IgG Fc-Biot, Southern BioTech) to form ADC-capturing complexes. Each plasma samples from all-time points containing ADC, internal controls: QC plasma (T : 0 min) &QC_ADC (ADC diluted in buffer, not plasma) were incubated with ADC-capturingcomplexes according to manufacturer’s instructions. ADC Capture from plasma samples were performed with a specific magnet, adapted to a 96 deep well plate format at room temperature. Samples were washed twice to eliminate plasma protein and salt. ADC were eluted from ADC-capturing complexes with acid solution: 70:30 H2O / ACN +0.1 % AF (pH ~2.7). DAR was evaluated under reduced (using dithiothreitol) and deglycosylation conditions by LC-MS mass spectrometry (XevoG2-S Q-Tof mass spectrophotometer, Waters) to evaluate payload conjugation stability over time. Flow Cytometry, EC50determination:105 NKM-1cells / well expressing FLT3 were incubated with a dose range of theindicated antibodies or ADC (100 nM to 0,0004 nM) 1 hour at 4°C, washed andincubated with Anti human secondary antibody (Goat Anti human Fc, Thermo FisherScientific) 30min at 4°C. After cells fixation (4% PFA for 15min), medians offluorescence intensities (MFI) were obtained by flow cytometry. EC50 was calculated using MFI(f):F(Conc) data with GraphPad Prism 10 Nonlinear regression log(agonist)vs, response -- Variable slope (four parameters).ATP-based cytotoxicity assay and IC50determination: 14,000 cells were distributed in white flat-bottom 96 wells plate (triplicates) and incubated 7 days at 37°C with compounds (100 to 4.572x10-2µg / mL for ADC and 1 to 4.572x10-4µg / mL for linker payload), prior to revelation with CellTiter-Glo®(Promega). IC50 was determined using the percentage of relative luminescencecorrelated to concentration with GraphPad Prism 10 Nonlinear regression log[Inhibitor] vs, response Variable slope (four parameters).Antitumor efficacy on cell derived xenografts:9 weeks-old NXG or NOD-SCID female mice were subcutaneously inoculated with,0.5 or 1M cells (MOLM-13; NKM-1, respectively) with (NKM-1) or without (MOLM-13)Matrigel. When the average volume of the tumor was ~100-150 mm3, animals were randomly divided into groups according to the tumor volume with 6-10 mice per group. Animals in all groups were administered intravenously. The tumor volume and body weight were measured twice a week. Tumor xenografts volume (TV) was measured in mm3using: (TV) = (length×width2) × (3.14 / 6). For each group, the mean tumorvolume is plotted (+ / - SEM) over time using GraphPad Prism 10 software. When amouse dies or is terminated in a group, the mean tumor volume and weight is nolonger plotted, independently of the effect of the treatment. Tumor growth inhibition (TGI) was calculated as follows at the last time point when all mice were still alive:TGI (%) : [100 - (Ti-T0) / (Vi-V0)] × 100%, (Ti: the mean tumor volume of treatmentgroup on day i after administration, T0: the mean tumor volume of treatment group on day 0 after administration; Vi: the mean tumor volume of negative control group on day i after administration, V0: the mean tumor volume of negative control group on day 0). ResultsFigure 2: ADCX-040 has 2 cysteines inserted at specific position for site-specificconjugation of LP, resulting in a homogenous aggregation profile of ADC with a precise DAR of 3.9Figure 3: The homogeneity and stability of ADCX-040 DAR was evaluated in vitrounder various stress conditions, ex vivo in human plasma and in vivo after IVadministration in mice by mass spectrometry analysis. ADCX-040 has a linkerdesigned to force the self-hydrolysis of the thio-succinimide ring, to prevent itsexchange with serum albumin through the retro-Michael reaction. Further to this, two cysteine sites have been introduced in such specific locations of the antibody heavychain, limiting the retro-Michael reaction even when the thio-succinimide ring is notfully hydrolyzed. Both features ensure minimal systemic payload deconjugation in plasma.After after five freeze / thaw cycles, long term incubation at 40°C or in human plasmaor after IV injection in mice, minimal reduction in DAR of ADCX-040 was observed,thus confirming high stability of ADCX-040 ADC.Figure 4: Determination of half maximal effective concentration (EC50) with flowcytometry on human FLT3 positive cancer cell lines demonstrated sub-nanomolar apparent affinity of ADCX-040a antibody and ADCX-040 ADC to FLT3 and no influence of the LP on the binding profile of the antibody.Figure 5: ADCX-040 tumor- killing activity, measured by half-maximal inhibitoryconcentration (IC50), was determined by luminescent adenosine triphosphate (ATP)- based assay observed in NKM-1 myeloid leukemia cell line. Briefly, 7 days after cells were exposed to increasing concentrations of ACDX-040, the amount of ATP detected by luciferase reaction (luminescent signal) was directly proportional to the number of viable cells present in culture. In the NKM1-1 FLT3 positive cells, ADCX040 demonstrated better tumor cell- killing effect compared to its isotype controlconjugated with exatecan (ICT-Exa), which confirms the importance of FLT3 binding and FLT3-mediated internalization.Figure 6 and Table 1 : In vivo data demonstrate that ADCX-040 has strong in vivoanti-tumor efficacy in two human tumor CDX mouse models positive for FLT3 (with various expression levels, based on flow cytometry fluorescence intensity, data not shown). In every experiment, no weight loss of mice was observed during the study, indicating that the animals tolerated ADCX-040 treatment.

Claims

Claims:

1. An antibody-drug conjugate of Formula I, or pharmaceutically salt thereof,AB-M-D (I),wherein AB is an antibody or antigen-binding fragment thereof that binds to FLT 3,M is a linker conjugating AB and D, andD is an active agent or drug, in particular a topoisomerase inhibitor I, atranscription inhibitor, or a tubulin polymerization / depolymerizationinhibitor.

2. The antibody-drug conjugate according to claim 1, wherein the topoisomeraseinhibitor I is selected from the group comprising camptothecin or analogs andderivatives thereof, indenoisoquinolines and indolocarbazoles, and in particular topotecan, irinotecan, SN-38, belotecan and exatecan includingderivatives thereof such as deruxtecan, lurtotecan or atiratecan.

3. The antibody-drug conjugate according to claim 1 or 2,wherein D is a camptothecin derivative represented by the formula (II), wherein R1 is -F, -CH3, or -CF3, preferably -F,R2 is -H, -F, -OR3, -SR3, -S(O)R4, -S(O)2R4, C1-C6 alkyl, or C1-C6fluoroalkyl, preferably -F or methyl; or R1 and R2 taken together withthe carbon atoms to which they are attached form a methylenedioxy or a difluoromethylenedioxy ring; R3is H or C1-C6alkyl; and R4 is C1-C6 alkyl, orwherein D is a camptothecin derivative represented by the formula (IIa)Formula (IIa) wherein Yis -H, hydroxyl, C1-C6 alkyl, C2-C6 alkenyl, C2-C6 alkynyl, C1-C6alkoxy, C1-C6aminoalkoxy, C1-C6alkylthio, C1-C6hydroxyalkyl, C1-C6halogenalkyl, C1-C6cyanoalkyl, C1-C6nitroalkyl, halogen, nitro, cyano, or mercapto, preferably -H or –F; orwherein D is a camptothecin derivative represented by the formula (IIb)Formula (IIb) wherein R1 is -CH3, optionally substituted by halogen,preferably -CF2H, or H, andR2 is -H, or C1-C6 alkyl, or –(CH2)2NR4CH(CH3)2, andR3is H, amino, or C1-C6alkyl R4 is H, or –SO2CH3,wherein the position where M is attached to the camptothecin derivative is indicated by the waved line.

4. The antibody-drug conjugate according to any of claims 1-3, wherein theinhibitor is selected from transcription inhibitors, in particular trabectedin,lurbinectedin, and α-Amanitin, or tubulin polymerization / depolymerizationinhibitors, in particular MMAE, MMAD, MMAF, or derivatives thereof.

5. The antibody-drug conjugate of any one of claims 1- 4, whereinthe FLT3 binding antibody preferably comprises a CDRH1 having the sequence of SEQ ID NO:1 or SEQ ID NO:2, a CDRH2 having the sequence of SEQ ID NO:3, a CDRH3 having the sequence of SEQ ID NO:4, a CDRL1 having the sequence of SEQ ID NO:6, a CDRL2 having the sequence of SEQ ID NO:8, and a CDRL3 having the sequence of SEQ ID NO:9, and / ora CDRH1 having the sequence of SEQ ID NO:1 or SEQ ID NO:2, a CDRH2 having the sequence of SEQ ID NO:3, a CDRH3 having the sequence of SEQ ID NO:5, a CDRL1 having the sequence of SEQ ID NO:7, a CDRL2 having the sequence of SEQ ID NO:8, and a CDRL3 having the sequence of SEQ ID NO:11, and / ora CDRH1 having the sequence of SEQ ID NO:12 or SEQ ID NO:13, a CDRH2 having the sequence of SEQ ID NO:14, a CDRH3 having the sequence of SEQ ID NO:15, a CDRL1 having the sequence of SEQ ID NO:16, a CDRL2 having the sequence of SEQ ID NO:17, and a CDRL3 having the sequence of SEQ ID NO:18, and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody.

6. The antibody-drug conjugate of any one of claims 1- 5, whereinthe FLT3 binding antibody preferably comprises a a VL having the sequence of SEQ ID NO:22, and a VH sequence of SEQ ID NO:19, and / ora VL having the sequence of SEQ ID NO:24, and a VH sequence of SEQ ID NO:21, and / ora VL having the sequence: of SEQ ID NO:23, and a VH sequence of SEQ ID NO:20, and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody.

7. The antibody-drug conjugate of any one of claims 1-6, wherein the FLT3binding antibody preferably comprises alight chain of SEQ ID NO:28 and a heavy chain of SEQ ID NO:25, and / ora light chain of SEQ ID NO:29 and a heavy chain of SEQ ID NO:26, and / ora light chain of SEQ ID NO:30 and a heavy chain of SEQ ID NO:27, and / or antibodies or antigen-binding fragments thereof having at least 90 % amino acid sequence identity, preferably 95%, most preferable 99%, or bind the same epitope as the respective antibody.

8. The antibody-drug conjugate of any of the preceding claims,wherein the antibody or antigen-binding fragment thereof is selected from achimeric antibody, a multispecific antibody, in particular a bispecific antibody, ahuman antibody, a humanized antibody, or an antigen-binding fragmentthereof.

9. The antibody-drug conjugate of any of the preceding claims,wherein the antibody or antigen-binding fragment thereof is an antibodyselected from an antibody of class IgG, e.g., of subclass IgG1, IgG2, IgG3 of IgG4, of class IgM, of class IgA or an antigen-binding fragment thereof, or a single-chain antibody, or an antibody Fv fragment, wherein the antibody optionally has a heavy chain constant domain having a reduced effectorfunction, e.g., which has a reduced binding to the Fc receptor.

10. The antibody-drug conjugate of any of the preceding claims, wherein theantibody or antibody fragment is thio-engineered.

11. The antibody-drug conjugate of any of the preceding claims,wherein M is a cleavable linker, and / or wherein M is a hydrophilicpolysarcosine linker, a hydrophilic linker comprising at least one ethyleneglycol unit, a linker comprising a highly polar spacer, or an oligopeptide linker.

12. The antibody-drug conjugate of any of the preceding claims,wherein M is a hydrophilic polysarcosine linker comprising, e.g., about 8-12sarcosine units, and at least one ethylene glycol unit, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more ethylene glycol units, or wherein M is a linker comprising Val-Ala, GGGS or GGFG and optionally atleast one ethylene glycol unit, e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 or more ethylene glycol units, in particular a Val-Ala or GGFG linker with 8 ethylene glycol units, or wherein M is a linker comprising(i) a PAB residue, in particular a PABC residue,(ii) a polyethylene glycol (PEG) spacer, preferably comprising 2-10ethylene glycol units, in particular 4 ethylene glycol units,or peptide A, preferably a GGGS peptide, and(iii) a β-glucuronide residue.

13. The antibody-drug conjugate of any of the preceding claims,wherein the linker is a hydrophilic polysarcosine linker comprising 10sarcosine units and 2 ethylene glycol units.

14. The antibody-drug conjugate of any of the preceding claims,wherein the linker is subject to cleavage by a glycosidase, and particularlysubject to cleavage by a glucuronidase.

15. The antibody-drug conjugate of any of the preceding claims,which is of formula (VII)whereinAB is the antibody or functional fragment thereof as definedaccording to any of the preceding claims,Fuc is fucose;GlcNAc is N-acetylglucosamine;S is a sugar or sugar derivative;M is -N(H)C(O)CH2-, -N(H)C(O)CF2-, -CH2-, -CF2- or a 1,4-phenylene containing 0-4 fluorine substituents, preferably 2 fluorine substituents which are preferably positioned on C2 and C6 or on C3 and C5 of the phenylene; R1is independently selected from the group consisting of hydrogen, halogen, -OR8, -NO2, -CN, S(O)2R8, C1-C24 alkylgroups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen, and wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are optionally substituted, wherein two substituents R8may be linked together to form an annelated cycloalkyl or an annelated (hetero)arene substituent, and wherein R8is independently selected from the group consisting of hydrogen, halogen, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups; R2and R3are independently selected from the group consisting ofhydrogen, halogen, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen; R4is selected from the group consisting of hydrogen, halogen, C1- C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups, preferably hydrogen, the alkyl groups optionally beinginterrupted by one of more hetero-atoms selected from the group consisting of O, N and S, wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are independently optionally substituted;Y is O, S or NR7, wherein R7 is independently selected from thegroup consisting of hydrogen, halogen, -OR8, -NO2, -CN,S(O)2R8, C1-C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups and wherein the alkyl groups, (hetero)aryl groups, alkyl(hetero)aryl groups and (hetero)arylalkyl groups are optionally substituted, wherein two substituents R8may be linked together to form an annelated cycloalkyl or an annelated (hetero)arene substituent and wherein R8is independently selected from the group consisting of hydrogen, halogen, C1- C24alkyl groups, C6-C24(hetero)aryl groups, C7-C24alkyl(hetero)aryl groups and C7-C24(hetero)arylalkyl groups;L is a linking group, preferably a sulfamide based linking group,a is 0 or 1;x is 1 or 2, preferably 1;y is 1, 2, 3 or 4, preferably 1,r is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19or 20, preferably 1 or 2;nn is 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, preferably 1 or 2, in particular 1;pp is 0 or 1, preferably 1;D is is a drug.

16. A pharmaceutical composition comprising an active agent, which is anantibody-drug conjugate according to any one of claims 1-15, and apharmaceutically acceptable carrier and / or excipient.

17. An antibody-drug conjugate of any one of claims 1-15 or a pharmaceuticalcomposition of claim 16 for use in medicine, particularly in human medicine.

18. An antibody-drug conjugate according to any one of claims 1 to 15 or apharmaceutical composition according to claim 16 for use in the treatment ofan FLT3 associated disorder of a patient who has relapsed or is refractory toprior FLT3 directed treatment.

19. A method of treating an FLT3 associated disorder of a patient who hasrelapsed or is refractory to prior treatments, preferably to prior FLT3 targetingtreatment, comprising administering to the patient a therapeutically effectiveamount of the antibody-drug conjugate of any one of claims 1-15 or apharmaceutical composition of claim 16.

20. An antibody-drug conjugate of any one of claims 1-15 or a pharmaceuticalcomposition of claim 16 for use in the prevention and / or treatment of FLT3associated disorder, particularly for the prevention and / or the treatment of a hematopoietic malignancy, such as Acute Myeloid Leukemia (AML), Mixed Phenotype Acute Leukemia (MPAL), Acute Lymphoblastic Leukemia (ALL) and subtypes thereof such as MLL-rearranged (KMT2A-r) infant B-ALL or Ph- like ALL with FLT3 lesions and Early T-cell Precursor ALL (ETP-ALL), Chronic Myeloid Leukemia (CML) and B-cell lymphoma biphenotypic acute leukemia (BAL).

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