FcRn ANTAGONISTS FOR VETERINARY USE

Abstract

The present application relates to canine and feline FcRn antagonists comprising canine or feline Fc fragment polypeptides that reduce or block binding of canine or feline FcRn to canine or feline IgG autoantibodies, and the use of these FcRn antagonists in treating canine or feline subjects suffering from autoantibody associated autoimmune disorders, such as pemphigus, bullous pemphigoid, lupus erythematosus, and stomatitis.

Description

FcRn ANTAGONISTS FOR VETERINARY USE CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Provisional Patent Application No. 63 / 711,570, filed on October 24, 2024, and the priority benefit of U.S. Provisional Patent Application No.63 / 615,130, filed on December 27, 2023, the entirety of each of which is hereby incorporated herein by reference. FIELD

[0002] This disclosure relates to modified canine and feline FcRn antagonists that comprise canine and feline Fc fragment polypeptides that reduce or block binding of FcRn to canine and feline IgG autoantibodies, and the use of these antagonists in treating canine or feline subjects suffering from autoantibody associated autoimmune disorders, such as pemphigus, bullous pemphigoid, lupus erythematosus, stomatitis, immune-mediate hemolytic anemia (IMHA), immune-mediated thrombocytopenia (ITP), Evans syndrome, Sjögren-like syndrome, keratoconjunctivitis sicca (KCS), and immune-mediated neuropathies and central nervous system (CNS) diseases such as myasthenia gravis (MG) or Guillain-Barré syndrome (acute canine polyradiculoneuritis (ACP) or immune-mediated polyneuropathy (IMPN)). REFERENCE TO SEQUENCE LISTING

[0003] The official copy of the Sequence Listing is submitted concurrently with the specification via USPTO Patent Center as an WIPO Standard ST.26 formatted XML file with file name “20136- 003WO1.xml”, a creation date of December 23, 2024, and a size of 29,428 bytes. This Sequence Listing filed via USPTO Patent Center is part of the specification and is incorporated in its entirety by reference herein. BACKGROUND

[0004] The neonatal fragment, crystallizable receptor (FcRn) is an MHC class 1 molecule present in many tissues. FcRn is necessary to maintain immunoglobulin G (IgG) and albumin levels. IgG is one of the most abundant autoantibodies and high levels are associated with many autoimmune diseases in humans, as well as in canines and felines. IgG and albumin bind to distinct regions of FcRn, thereby making FcRn a key determinant of IgG levels circulating or deposited in tissues (Pyzik et al.2019). Therefore, antagonists of FcRn can shorten the half-life of autoantibodies, and reduce levels of IgG which are responsible in autoimmune diseases.

[0005] Feline and canine companion animals suffer from many autoimmune disorders. However, there has been no demonstration to date that antagonists (or blockers) of FcRn can be used to treat autoimmune disorders in companion animals.

[0006] Stomatitis affects both canines and felines, and when left untreated has high morbidity. Stomatitis is dogs is commonly referred to as canine chronic ulcerative stomatitis (CCUS) whilestomatitis in cats is referred to as feline chronic gingivostomatitis (FCGS) (Anderson et al.2017; Lee, Verstraete, and Arzi 2020). For both canines and felines, stomatitis presents as inflammation and ulcers in the mouth of the pet that often causes pets to stop eating, drinking or even swallowing. In dogs, stomatitis is almost exclusively seen with ulcers around the teeth while in cats, there is ulcers and inflammation throughout the mouth (Lommer 2013). Cats with stomatitis had higher IgG levels in their saliva compared to cats without stomatitis (Harley, Gruffydd-Jones, and Day 2003, 2010). When evaluating tissue of dogs with stomatitis, there was high tissue-bound IgG antibodies and IgG antibodies were the most prevalent antibodies seen via immunofluorescence (Anderson et al.2020). These data suggests that IgG levels may be indicative of disease state and / or severity of stomatitis and lowering IgG levels may be one way to treat the disease.

[0007] Pemphigus in dogs and cats encompasses a group of rare, autoimmune skin blistering diseases that are characterized by blisters, ulcers, pustules and crusting of the skin as a result of autoantibodies attacking the skin. Currently the only treatment with limited success is corticosteroids and immunosuppressive treatments (Almela n.d.). Although all four clinical types of pemphigus have slightly different clinical presentations, they all have a similar relationship with IgG levels and disease severity. Pemphigus foliaceus (PF) is the most common of the four clinical types of pemphigus immune mediated skin diseases diagnosed (Rosenkrantz 2004; Scott, Miller, and Erb 2013). Similar to PF in humans, IgG interferes with the cell adhesion molecule leading to disruption of the cellular attachment of keratinocytes resulting in blister and pustule formation. High levels of IgG have been correlated in dogs and cats with PF, with higher titers associated with more severe cases of PF (Levy, Mamo, and Bizikova 2020; Nishifuji, Yoshida-Yamakita, and Iwasaki 2005; Olivry 2006). Although less prevalent, pemphigus vulgarius, pemphigus vegetans and pemphigus erythematosus also have a similar relationship with IgG levels and disease severity (Olivry et al.2003). Due to this direct relationship of disease severity and IgG levels, an FcRn antagonist could be one way to lower IgG levels in cats or dogs with pemphigus.

[0008] A few other autoimmune skin diseases of note that are driven by IgG autoantibodies but are rare in pets include bullous pemphigoid and lupus erythematosus. Due to the rarity of these diseases in pets, the literature is limited, however literature in humans is much more abundant and we can use this literature to make assumptions about the benefits of treatment with pet-specific anti-FcRn monoclonal antibodies. Bullous pemphigoid presents in dogs and cats in haired areas such as the groin and arm pits with ulcers, pustules or erosions (Iwasaki et al.1995; Olivry and Jackson 2001). Mucosal lesions may be present in bullous pemphigoid (Olivry and Jackson 2001). Similar to pemphigus, IgG autoantibodies target Type XVII collagen causing subepidermal lesions (Olivry et al.1999). In humans the severity of bullous pemphigoid diseases is associated with IgG levels (Iwata et al.2008, 2008). Although it has not been evaluated in dogs and cats, it would be unsurprising if IgG levels was also a driver of disease severity. Lupus is an autoimmune mediated disease that affects multiple organs. There are two types of lupus found in our pets that are similar to that seen in humans: systematic lupus erythematosus and discoid lupus erythematosus. Similar to initial symptoms seen in humans, skin lesions and rash are also common in dogs and cats. The presence of IgG autoantibodies have been detected in dogs with lupus(Jackson et al.2004). Although the literature is lacking for cats, like most autoimmune diseases, the presence of IgG autoantibodies is likely the underlying mechanism of action for the disease. For both bullous pemphigoid and lupus erythematosus, an FcRn antagonist could be one way to lower IgG levels in cats or dogs and minimize the disease state.

[0009] Immune-mediated hemolytic anemia (IMHA) and immune-mediated thrombocytopenia (ITP) are both IgG antibody mediated hematologic diseases that occur in dogs and more rarely in cats (Balch and Mackin 2007; Ellis et al.2018; Oksenberg et al.2006). IMHA is more prevalent in dogs and more rare in cats; however, cats may develop IMHA secondary to other diseases like FeLV (Hartmann 2012; Swann et al.2019). Dogs with IMHA may present at the clinic with various clinical symptoms including lethargy, vomiting, loss of appetite, diarrhea, and discoloration of urine and / or feces. Dogs often have additional clinical signs of anemia such as tachycardia, tachypnoea, fever, thrombocytopenia, etc. (Balch and Mackin 2007; Goggs et al.2015; Oksenberg et al.2006; Piek 2011). Presentation of mechanism of action of IMHA and ITP between humans and pets is similar with treatments currently limited to glucocorticoids and other immunomodulatory therapies (Swann et al.2019). Evans syndrome occurs when a dog presents with both IMHA and ITM and is more serious than either disease alone. There are several Phase 2 studies in humans evaluating FcRn blockers treatment for IMHA and ITP indications (Murakhovskaya et al.2022; Newland et al.2020; Robak et al.2017) suggesting that an animal-specific FcRn blocker warrants evaluation for IMHA and ITP treatment.

[0010] In humans, Sjogren’s syndrome results immune-mediated secretory dysfunction leading to sicca syndrome causing dryness of the eyes, oral cavities, mouth, throat, and or vagina (Mariette and Criswell 2018; Rischmueller, Tieu, and Lester 2016). KCS in dogs has a similar immunopathology and clinical presentation to Sjogren’s syndrome in humans and has been considered Sjogren-like syndrome (Hermida-Prieto, García-Castro, and Mariñas-Pardo 2021). Although less common, there has been evidence of cats also developing Sjogren-like syndrome (Canapp Jr et al.2001). As measuring saliva in the dog or cat is not common, it is unknown if other symptoms like dry mouth, throat of vagina are experienced in the dog or cat. The primary cause of KCS in dogs is immune-mediated inflammation and destruction of the lacrimal glands (Dodi 2015). Clinical signs of KCS vary as the disease progresses. Dogs may present initially with clinical signs of conjunctivitis and can ultimately progress to ocular pain and corneal ulceration (Dodi 2015).

[0011] Current therapies for canine KCS consist of immunosuppressive eye drops such as cyclosporine A, tacrolimus, or pimecrolimus. These treatments typically involve owners / caregivers giving eye drops multiple times a day, which often results in poor compliance and, thus, sub-optimal clinical outcomes. Accordingly, there is a need for a longer-lived treatment that could be dosed monthly or weekly subcutaneously and thereby provide a more effective, safe and convenient treatment for KCS in dogs.

[0012] Immune-mediated neuropathies and central nervous system (CNS) diseases such as myasthenia gravis (MG) or Guillain-Barré syndrome, also known as acute canine polyradiculoneuritis (ACP) in the dog or immune-mediated polyneuropathy (IMPN) in the cat are all immune mediated diseases that appear to be IgG mediated based on literature in humans (Alfaidi et al.2024; Jaffry et al.2023; Nelke etal.2022). Although details of the mechanism of action in cat or dog is less elucidated, similarities in many immune-mediated disease across humans, dogs and cats, suggests that pets with these immune- mediated neuropathies and CNS disease may benefit from treatment with a canine or feline specific FcRn antagonist compound. Accordingly, there is a need for FcRn antagonists with long pharmacokinetic lifetimes that can be used to treat canines and felines suffering from autoantibody associated autoimmune disorders, immune-mediated neuropathies, and central nervous system (CNS) diseases. SUMMARY

[0013] The present disclosure relates generally to FcRn antagonists that comprise a canine or feline Fc region fragment that specifically binds to the canine or feline FcRn protein, and thereby blocks its activity. The FcRn antagonists are polypeptides derived from canine or feline Fc region fragments, and act as FcRn antagonists in compositions and methods for treating autoantibody associated conditions and disorders in canines and felines. This summary is intended to introduce the subject matter of the present disclosure, but does not cover each and every embodiment, combination, or variation that is contemplated and described within the present disclosure. Further embodiments are contemplated and described by the disclosure of the detailed description, drawings, and claims.

[0014] The present disclosure provides canine FcRn antagonists that comprise a canine Fc fragment that has enhanced affinity for binding to canine FcRn, and / or enhanced stability. The present disclosure also provides feline FcRn antagonists that comprise a feline Fc fragment that has enhanced affinity for binding to feline FcRn, and / or enhanced stability. As described further herein, the canine and feline FcRn antagonists can further comprise single or multiple amino acid substitutions in their polypeptide sequence relative to the “wild-type” canine Fc region or feline Fc region sequences, that provide enhanced affinity and / or stability. The FcRn antagonist activity of the Fc fragments further provides for the uses of the molecules in methods of treating FcRn associated diseases or disorder in canines or felines, for example, treatment of autoimmune conditions.

[0015] Various embodiments provided by the present disclosure include, but are not limited to: Embodiment 1. A canine FcRn antagonist comprising a canine Fc fragment having an amino acid substitution L252Y (Eu numbering; or L37Y relative to SEQ ID NO: 1); optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 2. Embodiment 2. The canine FcRn antagonist of Embodiment 1, wherein the canine Fc fragment further comprises an amino acid substitution A254T (Eu numbering; or A39T relative to SEQ ID NO: 1) and / or T256E (Eu numbering; or T41E relative to SEQ ID NO: 1); optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 3. Embodiment 3. The canine FcRn antagonist of any one of Embodiments 1-2, wherein the canine Fc fragment further comprises an amino acid substitution at position N434 (Eu numbering; or N221 relative to SEQ ID NO: 1) selected from N434Y, N434F, N434H, N434R, and N434W (or N221Y, N221F, N221H, N221R, and N221W, relative to SEQ ID NO: 1); optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 13.Embodiment 4. The canine FcRn antagonist of any one of Embodiments 1-3, wherein the canine Fc fragment further comprises an amino acid substitution of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule. Embodiment 5. The canine FcRn antagonist of Embodiment 4, wherein the amino acid substitution of a C in the canine Fc fragment is T396C (Eu numbering; or T181C relative to SEQ ID NO: 1); optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 7. Embodiment 6. The canine FcRn antagonist of any one of Embodiments 1-5, wherein the canine Fc fragment further comprises at least two amino acid substitutions of a C at positions capable of forming an intradomain disulfide bridge. Embodiment 7. The canine FcRn antagonist of Embodiment 6, wherein the two amino acid substitutions of a C in the canine Fc fragment are A343C (Eu numbering; or A128C relative to SEQ ID NO: 1) and A433C (Eu numbering; or A218C relative to SEQ ID NO: 1); optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 8. Embodiment 8. The canine FcRn antagonist of any one of Embodiments 1-7, wherein the canine Fc fragment further comprises at least two amino acid substitutions of a C at positions capable of forming an intradomain disulfide bridge, and an amino acid substitution of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule; optionally, wherein the canine Fc fragment comprises an amino acid sequence of SEQ ID NO: 9. Embodiment 9. The canine FcRn antagonist of any one of Embodiments 1-8, wherein the canine Fc fragment comprises at least a portion of a canine heavy chain constant region selected from IgG-A, IgG-B, IgG C, and IgG-D Embodiment 10. The canine FcRn antagonist of any one of Embodiments 1-9, wherein the canine Fc fragment has an amino acid sequence of 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 greater identity to a sequence selected from SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17. Embodiment 11. A feline FcRn antagonist comprising a feline Fc fragment having an amino acid substitution S252Y (Eu numbering; or S39Y relative to SEQ ID NO: 4); optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 5. Embodiment 12. The feline FcRn antagonist of Embodiment 11, wherein the feline Fc fragment further comprises an amino acid substitution S254T (Eu numbering; or S41T relative to SEQ ID NO: 4) and / or T256E (Eu numbering; or T43E relative to SEQ ID NO: 4); optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 6. Embodiment 13. The feline FcRn antagonist of any one of Embodiments 11-12, wherein the feline Fc fragment further comprises an amino acid substitution at position S434 (Eu numbering; or S223 relative to SEQ ID NO: 4) selected from S434Y, S434F, S434H, S434R, and S434W (or S223Y, S223F, S223H, S223R, and S223W, relative to SEQ ID NO: 4); optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 18.Embodiment 14. The feline FcRn antagonist of any one of Embodiments 11-13, wherein the feline Fc fragment further comprises an amino acid substitution of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule. Embodiment 15. The feline FcRn antagonist of Embodiment 14, wherein the amino acid substitution of a C in the feline Fc fragment is T396C (Eu numbering; or T183C relative to SEQ ID NO: 4); optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 10. Embodiment 16. The feline FcRn antagonist of any one of Embodiments 11-15, wherein the feline Fc fragment further comprises at least two amino acid substitutions of a C at positions capable of forming an intradomain disulfide bridge. Embodiment 17. The feline FcRn antagonist of Embodiment 16, wherein the two amino acid substitutions of a C in the feline Fc fragment are P343C (Eu numbering; or P130C relative to SEQ ID NO: 4) and A433C (Eu numbering; or A220C relative to SEQ ID NO: 4); optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 11. Embodiment 18. The feline FcRn antagonist of any one of Embodiments 11-17, wherein the feline Fc fragment further comprises at least two amino acid substitutions of a C at positions capable of forming an intradomain disulfide bridge, and an amino acid substitution of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule; optionally, wherein the feline Fc fragment comprises an amino acid sequence of SEQ ID NO: 12. Embodiment 19. The feline FcRn antagonist of any one of Embodiments 11-18, wherein the feline Fc fragment further comprises at least a portion of a feline heavy chain constant region selected from IgG-A(IgG1), and IgG-B(IgG2). Embodiment 20. The feline FcRn antagonist of any one of Embodiments 11-19, wherein the feline Fc fragment has an amino acid sequence of 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 greater identity to a sequence selected from SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22. Embodiment 21. An isolated nucleic acid or vector encoding the FcRn antagonist of any one of Embodiments 1-20. Embodiment 22. An isolated host cell comprising the nucleic acid or vector of Embodiment 21. Embodiment 23. A method of producing an FcRn antagonist comprising culturing the host cell of Embodiment 22 and isolating the FcRn antagonist. Embodiment 24. A pharmaceutical composition comprising the FcRn antagonist of any one of Embodiments 1-20 and a pharmaceutically acceptable carrier. Embodiment 25. A method of treating a canine or a feline having an autoantibody associated disease or condition, the method comprising administering to the canine or feline a therapeutically effective amount of the FcRn antagonist of any one of Embodiments 1-20 or the pharmaceutical composition of Embodiment 24. Embodiment 26. The method of Embodiment 25, wherein the autoantibody associated disease or condition is an autoimmune disease.Embodiment 27. The method of Embodiment 25, wherein the autoantibody associated disease or condition is pemphigus. Embodiment 28. The method of Embodiment 25, wherein the autoantibody associated disease or condition is bullous pemphigoid. Embodiment 29. The method of Embodiment 25, wherein the autoantibody associated disease or condition is lupus erythematosus. Embodiment 30. The method of Embodiment 25, wherein the autoantibody associated disease or condition is stomatitis. Embodiment 31. The method of Embodiment 25, wherein the autoantibody associated disease or condition is myasthenia gravis (gMG). Embodiment 32. The method of any one of Embodiments 25-31, wherein the FcRn antagonist or the pharmaceutical composition is administered parenterally. Embodiment 33. The method of any one of Embodiments 25-31, wherein the FcRn antagonist or the pharmaceutical composition is administered by an intramuscular route, an intraperitoneal route, an intracerebrospinal route, a subcutaneous route, an intra-arterial route, an intrasynovial route, an intrathecal route, or an inhalation route. Embodiment 34. The method of any one of Embodiments 25-33, wherein the FcRn antagonist is administered in an amount in the range of 0.01 mg / kg body weight to 100 mg / kg body weight per dose. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A better understanding of the novel features and advantages of the present disclosure will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the disclosure are utilized, and the accompanying drawings (also “Figure” and “FIG.” herein), of which:

[0017] FIG.1 depicts plots showing the IgG concentration in two dogs on Days 0-14 following a single treatment with the canine FcRn antagonist Fc fragment polypeptide of SEQ ID NO: 13 on Day 0, as described in Example 8.

[0018] FIG.2 depicts plots showing the percent change in IgG concentration in two dogs on Days 0-14 following a single treatment with the canine FcRn antagonist Fc fragment polypeptide of SEQ ID NO: 13 on Day 0, as described in Example 8. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] The present disclosure provides a detailed description, including examples, of antagonists that specifically bind to canine FcRn or feline FcRn. The present disclosure provides various exemplary forms of these canine or feline modified Fc, and uses of these FcRn antagonists, including method of treatment of various diseases and disorders that are mediated or associated with the autoantibodies. Methods of designing, producing, or purifying antagonists to canine FcRn or feline FcRn are also provided.

[0020] For the descriptions herein and the appended claims, the singular forms “a”, and “an” include plural referents unless the context clearly indicates otherwise. Thus, for example, reference to “a protein” includes more than one protein, and reference to “a compound” refers to more than one compound. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. The use of “comprise,” “comprises,” “comprising” “include,” “includes,” and “including” are interchangeable and not intended to be limiting. It is to be further understood that where descriptions of various embodiments use the term “comprising,” those skilled in the art would understand that in some specific instances, an embodiment can be alternatively described using language “consisting essentially of” or “consisting of.”

[0021] Where a range of values is provided, unless the context clearly dictates otherwise, it is understood that each intervening integer of the value, and each tenth of each intervening integer of the value, unless the context clearly dictates otherwise, between the upper and lower limit of that range, and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges, and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of these limits, ranges excluding (i) either or (ii) both of those included limits are also included in the invention. For example, “1 to 50,” includes “2 to 25,” “5 to 20,” “25 to 50,” “1 to 10,” etc.

[0022] Generally, the nomenclature used herein and the techniques and procedures described herein include those that are well understood and commonly employed by those of ordinary skill in the art, such as the common techniques and methodologies described in e.g., Green and Sambrook, Molecular Cloning: A Laboratory Manual (Fourth Edition), Vols.1-3, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 2012 (hereinafter “Sambrook”); and Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., originally published in 1987 in book form by Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., and regularly supplemented through 2011, and now available in journal format online as Current Protocols in Molecular Biology, Vols.00 - 130, (1987-2020), published by Wiley & Sons, Inc. in the Wiley Online Library (hereinafter “Ausubel”).

[0023] All publications, patents, patent applications, and other documents referenced in this disclosure are hereby incorporated by reference in their entireties for all purposes to the same extent as if each individual publication, patent, patent application or other document were individually indicated to be incorporated by reference herein for all purposes.

[0024] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention pertains. It is to be understood that the terminology used herein is for describing particular embodiments only and is not intended to be limiting. For purposes of interpreting this disclosure, the following description ofterms will apply and, where appropriate, a term used in the singular form will also include the plural form and vice versa.

[0025] “FcRn antagonist” refers to a molecule that binds to a target FcRn molecule with sufficient affinity such that it is useful as an inhibitor of FcRn activity, and / or diagnostic for detecting FcRn. FcRn antagonist may be further specified to refer to a molecule that binds to a specific type of FcRn, such as canine FcRn (e.g., “canine FcRn antagonist” or “antagonist that binds canine FcRn”). In some embodiments, the extent of binding of an FcRn antagonist to an unrelated, non-FcRn antigen is less than about 20%, less than about 15%, less than about 10%, or less than about 5% of the binding of the antagonist to FcRn as measured, e.g., by a radioimmunoassay (RIA) or surface plasmon resonance (SPR). In some embodiments, an FcRn antagonist that binds to FcRn has a dissociation constant (KD) of < 1 μΜ, < 100 nM, < 10 nM, < 1 nM, < 0.1 nM, < 0.01 nM, or < 1 pM (e.g., 10-8M or less, e.g., from 10-8M to 10-13M, e.g., from 10-9M to 10-13M).

[0026] “Class” of an antibody molecule or fragment thereof (e.g., Fc fragment) refers to the type of constant domain or constant region possessed by its heavy chain. There are five major classes of human antibodies: IgA, IgD, IgE, IgG, and IgM, and several of these are further divided into subclasses (isotypes), e.g., IgG1, IgG2, IgG3, IgG4, IgA1, and IgA2. The heavy chain constant domains that correspond to the different classes of immunoglobulins are called α, δ, ε, γ, and μ, respectively. Canine, feline, and equine species have different classes of antibodies that are shared by many other mammalian species. For example, canine species have antibody classes, IgGA, IgGB, IgGC, IgGD, and feline species have antibody classes, IgGA1, IgGA2, IgGB.

[0027] “Antibody,” as used herein, refers to a molecule comprising one or more polypeptide chains that specifically bind to, or are immunologically reactive with a particular antigen. “Native antibody” refers to a naturally occurring immunoglobulin molecule. For example, native IgG antibodies are heterotetrameric glycoproteins of about 150,000 Daltons, composed of two identical light chains and two identical heavy chains that are disulfide- bonded. From N- to C-terminus, each heavy chain has a variable region (VH), also called a variable heavy domain or a heavy chain variable domain, followed by three constant domains (CH1, CH2, and CH3). Similarly, from N- to C-terminus, each light chain has a variable region (VL), also called a variable light domain or a light chain variable domain, followed by a constant light (CL) domain. The light chain of an antibody may be assigned to one of two types, called kappa (κ) and lambda (λ), based on the amino acid sequence of its constant domain.

[0028] The term “constant region” or “constant domain” as used herein refers to a region comprising at least three constant domains of an antibody.

[0029] The terms “heavy chain constant region” or “constant heavy chain” are used interchangeably to refer to a region comprising at least three heavy chain constant domains, CH1, CH2, and CH3. Nonlimiting exemplary heavy chain constant regions include γ, δ, α, ε, and μ. Each heavy chain constant region corresponds to an antibody isotype. For example, an antibody comprising a γ constant region is an IgG antibody, an antibody comprising a δ constant region is an IgD antibody, an antibody comprising an α constant region is an IgA antibody, an antibody comprising a μ constant region is an IgM antibody,and an antibody comprising an ε constant region is an IgE antibody. Certain isotypes can be further subdivided into subclasses. For example, IgG antibodies include, but are not limited to, IgG1 (comprising a γ1constant region), IgG2 (comprising a γ2constant region), IgG3 (comprising a γ3constant region), and IgG4 (comprising a γ4constant region) antibodies; IgA antibodies include, but are not limited to, IgA1 (comprising an α1constant region) and IgA2 (comprising an α2constant region) antibodies; and IgM antibodies include but are not limited to IgM1 and IgM2.

[0030] The terms “light chain constant region” or “constant light chain” are used interchangeably to refer to a region comprising a light chain constant domain, CL. Nonlimiting exemplary light chain constant regions include λ and κ. Non-function-altering deletions and alterations within the domains are encompassed within the scope of the term “constant region” unless designated otherwise. Canine, feline, and equine have antibody classes such as IgG, IgA, IgD, IgE, and IgM. Within the canine IgG antibody class are IgG-A, IgG-B, IgG-C, and IgG-D.

[0031] “Fc region” refers to a dimer complex that includes a C-terminal polypeptide sequence of an immunoglobulin heavy chain. An Fc region can refer to a native Fc region sequence or a variant or engineered version of a native Fc region sequence. The boundaries of the Fc region of an immunoglobulin heavy chain may vary depending on immunoglobulin class and species. The Fc region sequence typically includes an immunoglobulin heavy chain CH2 and CH3 constant domains, and can optionally include a CH4 domain. Binding of the Fc region to various biological molecules mediates a range of biological activities referred to as “effector functions.” Examples of Fc region mediated effector functions include: Fc receptor (FcRn) binding; Clq binding and complement dependent cytotoxicity (CDC); antibody- dependent cell-mediated cytotoxicity (ADCC); phagocytosis; down regulation of cell surface receptors (e.g., B cell receptor); and B cell activation.

[0032] “Fc fragment” refers to a polypeptide that contains all or part of an Fc region, e.g., the C-terminal polypeptide obtained by papain digestion of an intact antibody. Exemplary canine or feline Fc fragments provided by the present disclosure specifically bind to canine or feline FcRn molecules with sufficient affinity to act as FcRn antagonists useful as a therapeutic agents.

[0033] The term “IgX Fc” refers to an Fc region derived from a particular antibody class or isotype (e.g., IgG, IgA, IgD, IgE, IgM, etc.), where “X” denotes the antibody isotype. Thus, “IgG Fc” denotes the Fc region of a γ chain, “IgA Fc” denotes the Fc region of an α chain, “IgD Fc” denotes the Fc region of a δ chain, “IgE Fc” denotes the Fc region of an ε chain, “IgM Fc” denotes the Fc region of a μ chain, etc. In some embodiments, the IgG Fc region comprises CH1, hinge, CH2, CH3, and CL1. “IgX-N-Fc” denotes that the Fc region is derived from a particular subclass of antibody isotype (such as canine IgG subclass A, B, C, or D; or feline IgG subclass 1, 2a, or 2b), where “N” denotes the subclass. In some embodiments, IgX Fc or IgX-N-Fc regions are derived from a companion animal, such as a dog. In some embodiments, IgG Fc regions are isolated from canine γ heavy chains, such as IgG-A, IgG-B, IgG-C, or IgG-D. Antibodies comprising an Fc region of IgG-A, IgG-B, IgG-C, or IgG-D may provide for higher expression levels in recombination production systems. “IgX Fc” and “IgX Fc polypeptide” are intended to include wild-type IgX Fc polypeptides and variant IgX Fc polypeptides.

[0034] “Monoclonal antibody” as used herein refers to an antibody obtained from a substantially homogeneous population of antibodies, i.e., the individual antibodies comprising the population are identical and / or bind the same epitope, except for possible variant antibodies (e.g., variant antibodies contain mutations that occur naturally or arise during production of a monoclonal antibody, and generally are present in minor amounts). In contrast to polyclonal antibody preparations, which typically include different antibodies directed against different determinants (epitopes), each monoclonal antibody of a monoclonal antibody preparation is directed against a single determinant on an antigen.

[0035] “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. A “canine chimeric antibody” refers to a chimeric antibody having at least a portion of a heavy chain or a portion of a light chain are derived from a dog. In some embodiments, a canine chimeric antibody can include mouse VH and / or VL sequences and canine heavy and light chain constant domains.

[0036] “Caninized antibody” refers to a chimeric antibody comprising amino acid sequences from non- canine HVRs and amino acid sequences from canine FRs. In certain embodiments, a caninized antibody will comprise substantially all of at least one, and typically two, variable domains, in which all or substantially all of the HVRs correspond to those of a non-canine antibody, and all or substantially all of the FRs correspond to those of a canine antibody. A caninized antibody optionally may comprise at least a portion of an antibody constant region derived from a canine antibody. A “caninized form” of an antibody, e.g., a non-canine antibody, refers to an antibody that has undergone caninization.

[0037] “Canine antibody” refers to an antibody which possesses an amino acid sequence corresponding to that of an antibody produced by a canine or a canine cell or derived from a non-canine source that utilizes canine antibody repertoires or other canine antibody-encoding sequences. This definition of a canine antibody specifically excludes a caninized antibody comprising non-canine antigen-binding residues.

[0038] “Felinized antibody” refers to a chimeric antibody comprising amino acid sequences from non- feline HVRs and amino acid sequences from feline FRs. In certain embodiments, a felinized antibody will comprise substantially all of at least one, and typically two, variable domains, in which all or substantially all of the HVRs correspond to those of a non-feline antibody, and all or substantially all of the FRs correspond to those of a feline antibody. A felinized antibody optionally may comprise at least a portion of an antibody constant region derived from a feline antibody. A “felinized form” of an antibody, e.g., a non-feline antibody, refers to an antibody that has undergone felinization.

[0039] “Feline antibody” refers to an antibody which possesses an amino acid sequence corresponding to that of an antibody produced by a feline or a feline cell or derived from a non-feline source that utilizes feline antibody repertoires or other feline antibody-encoding sequences. This definition of a feline antibody specifically excludes a felinized antibody comprising non-feline antigen-binding residues.

[0040] “Affinity” refers to the strength of the sum total of noncovalent interactions between a single binding site of a molecule (e.g., an antibody or Fc fragment) and its target molecule or binding partner(e.g., an antigen). “Binding affinity” refers to intrinsic binding affinity which reflects a 1:1 interaction between members of a binding pair (e.g., antibody and antigen). The affinity of a molecule X for its partner Y can generally be represented by the equilibrium dissociation constant (KD). Affinity can be measured by common methods known in the art, including those described herein. Specific illustrative and exemplary embodiments for measuring binding affinity are described in the following.

[0041] “Binds specifically” or “specific binding” refers to binding of a molecule (e.g., an antibody or Fc fragment) to a target molecule (e.g., an antigen) with an affinity value of no more than about 1 x 10-7M.

[0042] “Isolated” refers to a molecule (e.g., an antibody or Fc fragment) which has been separated from a component of its natural environment. In some embodiments, a molecule is purified to greater than 95% or 99% purity as determined by, for example, electrophoretic (e.g., SDS-PAGE, isoelectric focusing (IEF), capillary electrophoresis) or chromatographic methods (e.g., ion exchange or reverse phase HPLC). For review of methods for assessment of immunoglobulin purity, see, e.g., Flatman et al., J. Chromatogr. B 848:79-87.

[0043] As used herein, “percent (%) amino acid sequence identity” and “homology” with respect to a polypeptide, or antibody sequence are 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, and not considering any conservative substitutions as part of the 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, BLAST-2, ALIGN, or MEGALINE™ (DNASTAR) software. Those skilled in the art can determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the full length of sequences being compared.

[0044] “Amino acid sequence,” means a sequence of amino acids residues in a peptide or protein. The terms “polypeptide” and “protein” are used interchangeably to refer to a polymer of amino acid residues and are not limited to a minimum length. Such polymers of amino acid residues may contain natural or non-natural amino acid residues, and include, but are not limited to, peptides, oligopeptides, dimers, trimers, and multimers of amino acid residues. Both full-length proteins and fragments thereof are encompassed by the definition. The terms also include post-expression modifications of the polypeptide, for example, glycosylation, sialylation, acetylation, phosphorylation, and the like. Furthermore, for purposes of the present disclosure, a “polypeptide” refers to a protein which includes modifications, such as deletions, additions, and substitutions (generally conservative in nature), to the native sequence, as long as the protein maintains the desired activity. These modifications may be deliberate, as through site- directed mutagenesis, or may be accidental, such as through mutations of hosts which produce the proteins or errors due to PCR amplification.

[0045] An “amino acid substitution” refers to the replacement of one amino acid in a polypeptide with another amino acid. In some embodiments, an amino acid substitution is a conservative substitution. Amino acid substitutions may be introduced into a molecule of interest and the products screened for adesired activity, for example, retained / improved antigen binding, decreased immunogenicity, or improved ADCC or CDC or enhanced pharmacokinetics.

[0046] The term “vector” is used to describe a polynucleotide that can be engineered to contain a cloned polynucleotide or polynucleotides that can be propagated in a host cell. A vector can include one or more of the following elements: an origin of replication, one or more regulatory sequences (such as, for example, promoters or enhancers) that regulate the expression of the polypeptide of interest, or one or more selectable marker genes (such as, for example, antibiotic resistance genes and genes that can be used in colorimetric assays, for example, β-galactosidase). The term “expression vector” refers to a vector that is used to express a polypeptide of interest in a host cell.

[0047] A “host cell” refers to a cell that may be or has been a recipient of a vector or isolated polynucleotide. Host cells may be prokaryotic cells or eukaryotic cells. Exemplary eukaryotic cells include mammalian cells, such as primate or non-primate animal cells; fungal cells, such as yeast; plant cells; and insect cells. Nonlimiting exemplary mammalian cells include, but are not limited to, NS0 cells, PER.C6® cells (Crucell), 293 cells, and CHO cells, and their derivatives, such as 293-6E, DG44, CHO- S, and CHO-K cells. Host cells include progeny of a single host cell, and the progeny may not necessarily be completely identical (in morphology or in genomic DNA complement) to the original parent cell due to natural, accidental, or deliberate mutation. A host cell includes cells transfected in vivo with a polynucleotide(s) encoding an amino acid sequence(s) provided herein.

[0048] The term “companion animal species” refers to an animal suitable to be a companion to humans. In some embodiments, a companion animal species is a small mammal, such as a canine, feline, dog, cat, horse, rabbit, ferret, guinea pig, rodent, etc. In some embodiments, a companion animal species is a large animal like camel or farm animal, such as a horse, cow, pig, etc.

[0049] To “reduce” or “inhibit” means to decrease, reduce, or arrest an activity, function, or amount as compared to a reference. In some embodiments, by “reduce” or “inhibit” is meant the ability to cause an overall decrease of 20% or greater. In some embodiments, by “reduce” or “inhibit” is meant the ability to cause an overall decrease of 50% or greater. In some embodiments, by “reduce” or “inhibit” is meant the ability to cause an overall decrease of 75%, 85%, 90%, 95%, or greater. In some embodiments, the amount noted above is inhibited or decreased over a period of time, relative to a control dose (such as a placebo) over the same period of time. A “reference” as used herein, refers to any sample, standard, or level that is used for comparison purposes. A reference may be obtained from a healthy or non-diseased sample. In some examples, a reference is obtained from a non-diseased or non-treated sample of a companion animal. In some examples, a reference is obtained from one or more healthy animals of a particular species, which are not the animal being tested or treated.

[0050] “Substantially similar” or “substantially the same,” as used herein, refers to a sufficiently high degree of similarity between two numeric values (for example, one associated with a test antibody and the other associated with a reference antibody), such that one of skill in the art would consider the difference between the two values to be of little or no biological and / or statistical significance within the context of the biological characteristic measured by said values (e.g., KDvalues).

[0051] “Substantially different,” as used herein, refers to a sufficiently high degree of difference between two numeric values (generally one associated with a molecule and the other associated with a reference molecule) such that one of skill in the art would consider the difference between the two values to be of statistical significance within the context of the biological characteristic measured by said values (e.g., KDvalues).

[0052] “Treatment,” “treat” or “treating” refers to intervention in an attempt to alter the natural course of a disorder in the individual being treated and can be performed either for prophylaxis or during the course of clinical pathology. Desired results of treatment can include, but are not limited to, preventing occurrence or recurrence of the disorder, alleviation of symptoms, diminishment of any direct or indirect pathological consequences of the disorder, preventing metastasis, decreasing the rate of progression, amelioration or palliation of a disease state, and remission or improved prognosis. For example, treatment can include administration of a therapeutically effective amount of pharmaceutical formulation comprising an FcRn antagonist to a subject to delay development or slow progression of a disease or condition associated with the FcRn protein such as an autoimmune disease or condition in which autoantibodies may play a role in the pathogenesis and / or progression. Treatment does not require one- hundred percent removal of all aspects of the disorder.

[0053] “Pharmaceutical formulation” refers to a preparation in a form that allows the biological activity of the active ingredient(s) to be effective, and which contain no additional components which are toxic to the subjects to which the formulation is administered. A pharmaceutical formulation may include one or more active agents. For example, a pharmaceutical formulation may include an FcRn antagonist as the sole active agent of the formulation or may include an FcRn antagonist and one or more additional active agents.

[0054] “Pharmaceutically acceptable carrier” refers to an ingredient in a pharmaceutical formulation, other than an active ingredient, which is nontoxic to the subject to whom it is administered. A pharmaceutically acceptable carrier includes, but is not limited to, a buffer, excipient, stabilizer, or preservative.

[0055] “Therapeutically effective amount” refers to the amount of an active ingredient or agent (e.g., a pharmaceutical formulation) to achieve a desired therapeutic or prophylactic result, e.g., to treat or prevent a disease, disorder, or condition in a subject. In the case of autoantibody associated disease or condition, the therapeutically effective amount of the therapeutic agent is an amount that reduces, prevents, inhibits, and / or relieves to some extent one or more of the symptoms associated with the disease, disorder, or condition.

[0056] “Individual” or “subject” refers to a mammal, including but not limited to, domesticated or companion animals (e.g., cows, sheep, cats, dogs, and horses), primates (e.g., humans and non-human primates such as monkeys), rabbits, and rodents (e.g., mice and rats).

[0057] Canine and Feline Fc Fragments Useful as FcRn Antagonists

[0058] The present disclosure provides canine and feline Fc fragments capable of acting as antagonists of canine and feline FcRn, respectively. The canine and feline Fc fragments provided herein include “wild-type” Fc fragments as well as variants of these Fc fragments that are engineered with Fc region mutations that result in enhanced binding affinity to FcRn and / or enhanced stability.

[0059] Table 1 below provides a summary description of the amino acid sequences of the canine and feline Fc fragments referenced in the present disclosure. The sequences also are included in the accompanying Sequence Listing.

[0060] Table 1: Sequences of exemplary canine and feline Fc fragments SEQ ID Q Q C.SVLPIGHQDWLKGKQFTCKVNNKALPSPIERTISKARGQAHQPS (T181C relative to VYVLPPSREELSKNTVSLTCLIKDFFPPDIDVEWQSNGQQEPES SEQ ID NO: 1) KYRTCPPQLDEDGSYFLYSKLSVDKSRWQRGDTFICAVMHEALH Dimer on NR SDS n 8 .VYVLPPSREELSKNTVSLTCLIKDFFPPDIDVEWQSNGQQEPES (N221Y relative to KYRTTPPQLDEDGSYFLYSKLSVDKSRWQRGDTFICAVMHEALH SEQ ID NO: 1) YHYTQESLSHSPGK g)

[0061] As shown elsewhere herein, including the Examples, the “wild-type” canine Fc fragment of SEQ ID NO: 1 and the engineered variant Fc fragments of SEQ ID NO: 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17 are capable of acting as antagonists of canine FcRn due to their ability to bind with high affinity to the canine FcRn. The ability of the canine Fc fragments to specifically bind canine FcRn is demonstrated via Biacore SPR using a commercially available canine FcRn protein, such as the biotinylated heterodimeric “FCGRT&B2M” FcRn protein (Acrobiosystems, Inc.; Newark, DE, USA), as shown in the Examples. This heterodimeric canine FcRn protein is expressed from human 293 cells (HEK293) and comprises a heterodimer of FCGRT and B2M with a FCGRT chain comprising amino acids Ala 23 to Pro 298 of accession #XP_038512242.1 (SEQ ID NO: 23) and a B2M chain comprising amino acids Val 28 - Asn 125 of accession #NP_001271408.1 (SEQ ID NO: 24).

[0062] In some embodiments, the present disclosure also provides a canine FcRn antagonist comprising an engineered variant canine Fc fragment of any one the polypeptide sequences of SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17 further comprising 1, 2, 3, 4, 5, 6, or more amino acid substitutions relative to the Fc fragment polypeptide of SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17. A range of Fc region mutations and amino acid substitutions are known in the art of therapeutic antibody development. Accordingly, it is contemplated that such further mutations can be incorporated in the canine Fc fragments of the present disclosure. Exemplary amino acid substitutions include those provided in the sequences of Table 1 and the Examples. For example, a range of amino acid substitutions at position 434 relative to SEQ ID NO: 2 are illustrated in the Fc fragment sequences of SEQ ID NOs: 13, 14, 15, 16, and 17.

[0063] In some embodiments, the canine Fc fragment can further comprise further polypeptide sequences extending from the N-terminus and / or C-terminus of any one of SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17. In at least one embodiment, the present disclosure contemplates an FcRn antagonist comprising a canine Fc fragment polypeptide of any one of SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17 attached or fused with all or a portion of a canine heavy chain constant region selected from an IgG-A, IgG-B, IgG-C, and IgG-D constant region.

[0064] Similarly, as shown elsewhere herein including the Examples, the “wild-type” feline Fc fragment of SEQ ID NO: 4, and the engineered variant Fc fragments of SEQ ID NO: 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22, are capable of acting as antagonists of feline FcRn due to their ability to bind with high affinity to the feline FcRn. The ability of these feline Fc fragments to specifically bind feline FcRn can be demonstrated via Biacore SPR using the commercially available biotinylated feline heterodimeric “FCGRT&B2M” FcRn protein (Acrobiosystems, Inc.; Newark, DE, USA), as shown in the Examples. This feline FcRn is expressed from human 293 cells (HEK293) and comprises a heterodimer of FCGRT and B2M with a FCGRT chain comprising amino acids Ala 24 to Ser 297 of the amino acid sequence of accession # XP_023100999.2 (SEQ ID NO: 25) and a B2M chain comprising amino acids Val 21 to Met

[0065] In some embodiments, the present disclosure also provides a feline FcRn antagonist comprising an engineered variant feline Fc fragment of any one SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22 further comprising 1, 2, 3, 4, 5, 6, or more amino acid substitutions relative to the Fc fragment polypeptide of any one of SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22. A range of Fc region mutations and amino acid substitutions are known in the art of therapeutic antibody development, and it is contemplated that such mutations can be incorporated in the feline Fc fragments of the present disclosure. Exemplary amino acid substitutions include those provided in the sequences of Table 1 and the Examples.

[0066] In some embodiments, the canine Fc fragment can further comprise further polypeptide sequences extending from the N-terminus and / or C-terminus of any one of SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22. In at least one embodiment, the present disclosure contemplates a feline FcRn antagonist comprising a feline Fc fragment polypeptide of any one of SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22 attached to or fused with all or a portion of a feline heavy chain constant region selected from an IgG-A (IgG1) or IgG-B (IgG2) constant region.

[0067] In some embodiments, the FcRn antagonist comprises (i) a canine Fc variant comprising the amino acid sequence of SEQ ID NO: 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17, or a variant thereof having 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%, or at least 99% sequence identity to the amino acid sequence of SEQ ID NO: 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17; or (ii) a feline Fc variant comprising the amino acid sequence of SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22, or a variant thereof having 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%, or at least 99% sequence identity to the amino acid sequence of SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22.

[0068] The canine or feline Fc fragment polypeptides of the present disclosure retain the biological activity of a polypeptide comprising the full canine or feline Fc region sequence, including, as shown in the Examples, the ability to bind to FcRn with high affinity. In some embodiments, the canine or feline Fc fragments of the present disclosure also retain the ability to bind FcRn, C1q, CD16, and / or protein A.

[0069] In some embodiments, the engineered variant canine or feline Fc fragment polypeptide of the present disclosure can exhibit an in vivo half-life that is shortened relative to other variants but which surprisingly provide for improved therapeutic properties when used in treatment. For example, the engineered variant Fc fragment polypeptide of SEQ ID NO: 13 has a shorter in vivo half-life relative to SEQ ID NO: 2, and because of this can be used in a therapeutic treatment in a higher dose which is desirable for a more potent FcRn antagonist blocking effect.

[0070] In some embodiments, the canine or feline Fc fragments of the present disclosure may be modified to provide a more stable Fc fragment form may be created such as introducing extra disulfide(s) to lock further conformations of intra-chain or inter-chain. Exemplary canine and feline Fc fragments that form intra- and interdomain disulfide linkages are provided herein as SEQ ID NO: 7, 8, 9, 10, 11, and 12.

[0071] In some embodiments, the canine or feline Fc fragments of the present disclosure may be modified to provide a variant with enhanced binding to canine FcRn at low pH or at neutral pH or at both pHs. For example, a variant canine Fc fragment polypeptide (e.g., SEQ ID NO:13) has modified FcRn binding affinity in a lower pH range as compared to a reference polypeptide. In some embodiments, a variant IgG Fc fragment has increased canine FcRn binding affinity at an acidic pH (e.g., at a pH in the range of from about 5.0 to about 6.5, such as at a pH of about 5.0, a pH of about 5.5, a pH of about 6.0, or a pH of about 6.5) compared to a reference polypeptide (e.g., SEQ ID NO: 2).

[0072] In some embodiments of a variant feline Fc fragment (e.g., a variant feline IgG Fc fragment polypeptide) has modified FcRn binding affinity in a lower pH range compared to a reference polypeptide. In some embodiments, a variant IgG Fc fragment has increased feline FcRn binding affinity at an acidic pH (e.g., at a pH in the range of from about 5.0 to about 6.5, such as at a pH of about 5.0, a pH of about 5.5, a pH of about 6.0, or a pH of about 6.5) compared to a reference polypeptide.

[0073] In some embodiments of the canine or feline Fc fragments of the present disclosure, a variant has at least about 50% sequence identity with the reference nucleic acid molecule or polypeptide after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent sequence identity, and not considering any conservative substitutions as part of the sequence identity. Such variants include, for instance, polypeptides wherein one or more amino acid residues are added, deleted, at the N- or C-terminus of the polypeptide. In some embodiments, a variant has at least about 50% sequence identity, at least about 60% sequence identity, at least about 65% sequence identity, at least about 70% sequence identity, at least about 75% sequence identity, at least about 80% sequence identity, at least about 85% sequence identity, at least about 90% sequence identity, at least about 95% sequence identity, at least about 97% sequence identity, at least about 98% sequence identity, or at least about 99% sequence identity with the sequence of the reference nucleic acid or polypeptide.

[0074] In some embodiments of the canine or feline Fc fragments of the present disclosure, an FcRn antagonist may reduce IgG in vivo half-life that can be measured in a companion animal species by at least 2%, at least 5%, at least 10%, at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%,compared to IgG half-life in the absence of the antagonist.

[0075] As described elsewhere herein, including the Examples, the relative FcRn antagonist activity of the Fc fragments of the present disclosure correlates with the affinity (e.g., Kd) of the Fc fragment for binding to a FcRn protein molecule, such as FcRn heterodimer. In at least one embodiment, the canine FcRn antagonist activity can be determined by measuring binding affinity to the canine FcRn protein comprising a heterodimer of a canine FCGRT chain (e.g., SEQ ID NO: 23) and a canine B2M chain (e.g., SEQ ID NO: 24).

[0076] In some embodiments, a canine FcRn antagonist comprising an Fc fragment of the present disclosure binds to canine FcRn with a dissociation constant (Kd) of less than 5 x 10-6M, less than 1 x 10-6M, less than 5 x 10-7M, less than 1 x 10-7M, less than 5 x 10-8M, less than 1 x 10-8M, less than 5 x 10-9M, less than 1 x 10-9M, less than 5 x 10-10M, less than 1 x 10-10M, less than 5 x 10-11M, less than1 x 10-11M, less than 5 x 10-12M, or less than 1 x 10-12M, as measured by SPR or biolayer interferometry (e.g., Biacore analysis).

[0077] In some embodiments, a canine FcRn antagonist comprising an Fc fragment of the present disclosure binds to canine FcRn with a Kd of between 5 x 10-6M and 1 x 10-6M, between 5 x 10-6M and 5 x 10-7M, between 5 x 10-6M and 1 x 10-7M, between 5 x 10-6M and 5 x 10-8M, 5 x 10-6M and 1 x 10-8M, between 5 x 10-6M and 5 x 10-9M, between 5 x 10-6M and 1 x 10-9M, between 5 x 10-6M and 5 x 10-10M, between 5 x 10-6M and 1 x 10-10M, between 5 x 10-6M and 5 x 10-11M, between 5 x 10-6M and 1 x 10-11M, between 5 x 10-6M and 5 x 10-12 M, between 5 x 10-6M and 1 x 10-12M, between 1 x 10-6M and 5 x 10-7M, between 1 x 10-6M and 1 x 10-7M, between 1 x 10-6M and 5 x 10-8M, 1 x 10-6M and 1 x 10-8M, between 1 x 10-6M and 5 x 10-9M, between 1 x 10-6M and 1 x 10-9M, between 1 x 10-6M and 5 x 10-10M, between 1 x 10-6M and 1 x 10-10M, between 1 x 10-6M and 5 x 10-11M, between 1 x 10-6M and 1 x 10-11M, between 1 x 10-6M and 5 x 10-12M, between 1 x 10-6M and 1 x 10-12M, between 5 x 10-7M and 1 x 10-7M, between 5 x 10-7M and 5 x 10-8M, 5 x 10-7M and 1 x 10-8M, between 5 x 10-7M and 5 x 10-9M, between 5 x 10-7M and 1 x 10-9M, between 5 x 10-7M and 5 x 10-10M, between 5 x 10-7M and 1 x 10-10M, between 5 x 10-7M and 5 x 10-11M, between 5 x 10-7M and 1 x 10-11M, between 5 x 10-7M and 5 x 10-12M, between 5 x 10-7M and 1 x 10-12M, between 1 x 10-7M and 5 x 10-8M, 1 x 10-7M and 1 x 10-8M, between 1 x 10-7M and 5 x 10-9M, between 1 x 10-7M and 1 x 10-9M, between 1 x 10-7M and 5 x 10-10M, between 1 x 10-7M and 1 x 10-10M, between 1 x 10-7M and 5 x 10-11M, between 1 x 10-7M and 1 x 10-11M, between 1 x 10-7M and 5 x 10-12M, between 1 x 10-7M and 1 x 10-12M, between 5 x 10-8M and 1 x 10-8M, between 5 x 10-8M and 5 x 10-9M, between 5 x 10-8M and 1 x 10-9M, between 5 x 10-8M and 5 x 10-10M, between 5 x 10-8M and 1 x 10-10M, between 5 x 10-8M and 5 x 10-11M, between 5 x 10-8M and 1 x 10-11M, between 5 x 10-8M and 5 x 10-12M, between 5 x 10-8M and 1 x 10-12M, 1 x 10-8M and 5 x 10-9M, between 1 x 10-8M and 1 x 10-9M, between 1 x 10-8M and 5 x 10-10M, between 1 x 10-8M and 1 x 10-10M, between 1 x 10-8M and 5 x 10-11M, between 1 x 10-8M and 1 x 10-11M, between 1 x 10-8M and 5 x 10-12M, between 1 x 10-8M and 1 x 10-12M, between 5 x 10-9M and 1 x 10-9M, between 5 x 10-9M and 5 x 10-10M, between 5 x 10-9M and 1 x 10-10M, between 5 x 10-9M and 5 x 10-11M, between 5 x 10-9M and 1 x 10-11M, between 5 x 10-9M and 5 x 10-12M, between 5 x 10-9M and 1 x 10-12M, between 1 x 10-9M and 5 x 10-10M, between 1 x 10-9M and 1 x 10-10M, between 1 x 10-9M and 5 x 10-11M, between 1 x 10-9M and 1 x 10-11M, between 1 x 10-9M and 5 x 10-12M, between 1 x 10-9M and 1 x 10-12M, between 5 x 10-10M and 1 x 10-10M, between 5 x 10-10M and 5 x 10-11M, between, 1 x 10-10M and 5 x 10-11M, 1 x 10-10M and 1 x 10-11M, between 1 x 10-10M and 5 x 10-12M, between 1 x 10-10M and 1 x 10-12M, between 5 x 10-11M and 1 x 10-12M, between 5 x 10-11M and 5 x 10-12M, between 5 x 10-11M and 1 x 10-12M, between 1 x 10-11M and 5 x 10-12M, or between 1 x 10-11M and 1 x 10-12M, as measured by SPR or biolayer interferometry (e.g., Biacore analysis).

[0078] Similarly, in at least one embodiment, the feline FcRn antagonist activity can be determined by measuring binding affinity to the feline FcRn protein comprising a heterodimer of a feline FCGRT chain (e.g., SEQ ID NO: 25) and a feline B2M chain (e.g., SEQ ID NO: 26). Accordingly, in at least oneembodiment, a feline FcRn antagonist comprising an Fc fragment of the present disclosure binds to feline FcRn with a dissociation constant (Kd) of less than 5 x 10-6M, less than 1 x 10-6M, less than 5 x 10-7M, less than 1 x 10-7M, less than 5 x 10-8M, less than 1 x 10-8M, less than 5 x 10-9M, less than 1 x 10-9M, less than 5 x 10-10M, less than 1 x 10-10M, less than 5 x 10-11M, less than 1 x 10-11M, less than 5 x 10-12M, or less than 1 x 10-12M, as measured by SPR or biolayer interferometry (e.g., Biacore analysis).

[0079] In some embodiments, a feline FcRn antagonist comprising an Fc fragment of the present disclosure binds to feline FcRn with a Kd of between 5 x 10-6M and 1 x 10-6M, between 5 x 10-6M and 5 x 10-7M, between 5 x 10-6M and 1 x 10-7M, between 5 x 10-6M and 5 x 10-8M, 5 x 10-6M and 1 x 10-8M, between 5 x 10-6M and 5 x 10-9M, between 5 x 10-6M and 1 x 10-9M, between 5 x 10-6M and 5 x 10-10M, between 5 x 10-6M and 1 x 10-10M, between 5 x 10-6M and 5 x 10-11M, between 5 x 10-6M and 1 x 10-11M, between 5 x 10-6M and 5 x 10-12 M, between 5 x 10-6M and 1 x 10-12M, between 1 x 10-6M and 5 x 10-7M, between 1 x 10-6M and 1 x 10-7M, between 1 x 10-6M and 5 x 10-8M, 1 x 10-6M and 1 x 10-8M, between 1 x 10-6M and 5 x 10-9M, between 1 x 10-6M and 1 x 10-9M, between 1 x 10-6M and 5 x 10-10M, between 1 x 10-6M and 1 x 10-10M, between 1 x 10-6M and 5 x 10-11M, between 1 x 10-6M and 1 x 10-11M, between 1 x 10-6M and 5 x 10-12M, between 1 x 10-6M and 1 x 10-12M, between 5 x 10-7M and 1 x 10-7M, between 5 x 10-7M and 5 x 10-8M, 5 x 10-7M and 1 x 10-8M, between 5 x 10-7M and 5 x 10-9M, between 5 x 10-7M and 1 x 10-9M, between 5 x 10-7M and 5 x 10-10M, between 5 x 10-7M and 1 x 10-10M, between 5 x 10-7M and 5 x 10-11M, between 5 x 10-7M and 1 x 10-11M, between 5 x 10-7M and 5 x 10-12M, between 5 x 10-7M and 1 x 10-12M, between 1 x 10-7M and 5 x 10-8M, 1 x 10-7M and 1 x 10-8M, between 1 x 10-7M and 5 x 10-9M, between 1 x 10-7M and 1 x 10-9M, between 1 x 10-7M and 5 x 10-10M, between 1 x 10-7M and 1 x 10-10M, between 1 x 10-7M and 5 x 10-11M, between 1 x 10-7M and 1 x 10-11M, between 1 x 10-7M and 5 x 10-12M, between 1 x 10-7M and 1 x 10-12M, between 5 x 10-8M and 1 x 10-8M, between 5 x 10-8M and 5 x 10-9M, between 5 x 10-8M and 1 x 10-9M, between 5 x 10-8M and 5 x 10-10M, between 5 x 10-8M and 1 x 10-10M, between 5 x 10-8M and 5 x 10-11M, between 5 x 10-8M and 1 x 10-11M, between 5 x 10-8M and 5 x 10-12M, between 5 x 10-8M and 1 x 10-12M, 1 x 10-8M and 5 x 10-9M, between 1 x 10-8M and 1 x 10-9M, between 1 x 10-8M and 5 x 10-10M, between 1 x 10-8M and 1 x 10-10M, between 1 x 10-8M and 5 x 10-11M, between 1 x 10-8M and 1 x 10-11M, between 1 x 10-8M and 5 x 10-12M, between 1 x 10-8M and 1 x 10-12M, between 5 x 10-9M and 1 x 10-9M, between 5 x 10-9M and 5 x 10-10M, between 5 x 10-9M and 1 x 10-10M, between 5 x 10-9M and 5 x 10-11M, between 5 x 10-9M and 1 x 10-11M, between 5 x 10-9M and 5 x 10-12M, between 5 x 10-9M and 1 x 10-12M, between 1 x 10-9M and 5 x 10-10M, between 1 x 10-9M and 1 x 10-10M, between 1 x 10-9M and 5 x 10-11M, between 1 x 10-9M and 1 x 10-11M, between 1 x 10-9M and 5 x 10-12M, between 1 x 10-9M and 1 x 10-12M, between 5 x 10-10M and 1 x 10-10M, between 5 x 10-10M and 5 x 10-11M, between, 1 x 10-10M and 5 x 10-11M, 1 x 10-10M and 1 x 10-11M, between 1 x 10-10M and 5 x 10-12M, between 1 x 10-10M and 1 x 10-12M, between 5 x 10-11M and 1 x 10-12M, between 5 x 10-11M and 5 x 10-12M, between 5 x 10-11M and 1 x 10-12M, between 1 x 10-11M and 5 x 10-12M, or between 1 x 10-11M and 1 x 10-12M, as measured by SPR or biolayer interferometry (e.g., Biacore analysis).

[0080] In some embodiments of the canine or feline FcRn antagonists of the present disclosure, the FcRn antagonists comprising an Fc fragment comprise a label or are conjugated to a second moiety. The terms “label” and “detectable label” mean a moiety attached to an Fc fragment or its analyte to render a reaction (for example, binding) between the members of the specific binding pair, detectable. The labeled member of the specific binding pair is referred to as “detectably labeled.” Thus, the term “labeled binding protein” refers to a protein with a label incorporated that provides for the identification of the binding protein. In some embodiments, the label is a detectable marker that can produce a signal that is detectable by visual or instrumental means, for example, incorporation of a radiolabeled amino acid or attachment to a polypeptide of biotinyl moieties that can be detected by marked avidin (for example, streptavidin containing a fluorescent marker or enzymatic activity that can be detected by optical or colorimetric methods). Examples of labels for polypeptides include, but are not limited to, the following: radioisotopes or radionuclides (for example,3H,14C,35S,90Y,99Tc,111In,125I,131I,177Lu,166Ho, or153Sm); chromogens, fluorescent labels (for example, FITC, rhodamine, lanthanide phosphors), enzymatic labels (for example, horseradish peroxidase, luciferase, alkaline phosphatase); chemiluminescent markers; biotinyl groups; predetermined polypeptide epitopes recognized by a secondary reporter (for example, leucine zipper pair sequences, binding sites for secondary antibodies, metal binding domains, epitope tags); and magnetic agents, such as gadolinium chelates. Representative examples of labels commonly employed for immunoassays include moieties that produce light, for example, acridinium compounds, and moieties that produce fluorescence, for example, fluorescein. In this regard, the moiety itself may not be detectably labeled but may become detectable upon reaction with yet another moiety.

[0081] Pharmaceutical Compositions

[0082] The terms “pharmaceutical formulation” and “pharmaceutical composition” refer to a preparation which is in such form as to permit the biological activity of the active ingredient(s) to be effective, and which contains no additional components that are unacceptably toxic to a subject to which the formulation would be administered.

[0083] A pharmaceutically acceptable carrier is non-toxic to recipients at the dosages and concentrations employed and is compatible with other ingredients of the formulation. The pharmaceutically acceptable carrier is appropriate for the formulation employed. Examples of pharmaceutically acceptable carriers include alumina; aluminum stearate; lecithin; serum proteins, such as human serum albumin, canine or other animal albumin; buffers such as phosphate, citrate, tromethamine or HEPES buffers; glycine; sorbic acid; potassium sorbate; partial glyceride mixtures of saturated vegetable fatty acids; water; salts or electrolytes, such as protamine sulfate, disodium hydrogen phosphate, potassium hydrogen phosphate, sodium chloride, zinc salts, colloidal silica, or magnesium trisilicate; polyvinyl pyrrolidone, cellulose- based substances; polyethylene glycol; sucrose; mannitol; or amino acids including, but not limited to, arginine.

[0084] The pharmaceutical composition can be stored in lyophilized form. Thus, in some embodiments, the preparation process includes a lyophilization step. The lyophilized composition may then be reformulated, typically as an aqueous composition suitable for parenteral administration, prior toadministration to the dog. In other embodiments, particularly where the antibody is highly stable to thermal and oxidative denaturation, the pharmaceutical composition can be stored as a liquid, i.e., as an aqueous composition, which may be administered directly, or with appropriate dilution, to the dog. A lyophilized composition can be reconstituted with sterile Water for Injection (WFI). Bacteriostatic reagents, such benzyl alcohol, may be included. Thus, the invention provides pharmaceutical compositions in solid or liquid form.

[0085] The pH of the pharmaceutical compositions may be in the range of from about pH 5 to about pH 8, when administered. The compositions of the invention are sterile if they are to be used for therapeutic purposes. Sterility can be achieved by any of several means known in the art, including by filtration through sterile filtration membranes (e.g., 0.2-micron membranes). Sterility may be maintained with or without anti-bacterial agents.

[0086] Uses of Fc Fragments as FcRn Antagonists and Associated Pharmaceutical Compositions

[0087] In some embodiments, it is contemplated that the canine or feline Fc fragments or pharmaceutical compositions comprising the canine or feline Fc fragments of the present disclosure can be used for treating an autoantibody associated conditions, disorder, or disease, in a canine or feline subject, such as a canine or feline companion animal. In such uses, a therapeutically effective amount can be used, which is any amount in which any toxic or detrimental effects of the substance / molecule, agonist or antagonist are outweighed by the therapeutically beneficial effects. A therapeutically effective amount may be delivered in one or more administrations. A therapeutically effective amount refers to an amount effective, at dosages and for periods of time necessary, to achieve the desired therapeutic or prophylactic result.

[0088] In some embodiments, the canine or feline Fc fragments of the present disclosure or a pharmaceutical composition comprising a canine or feline Fc fragments of the present disclosure is administered parenterally, by subcutaneous administration, intravenous infusion, or intramuscular injection. In some embodiments, a canine or feline Fc fragment or pharmaceutical composition comprising a canine or feline Fc fragment of the present disclosure is administered as a bolus injection or by continuous infusion over a period of time. In some embodiments, the canine or feline Fc fragments or pharmaceutical composition comprising a canine or feline Fc fragments is administered by an intramuscular, an intraperitoneal, an intracerebrospinal, a subcutaneous, an intra-arterial, an intrasynovial, an intrathecal, or an inhalation route.

[0089] In at least one embodiment of their use, an FcRn antagonist comprising a canine or feline Fc fragment of the present disclosure alone, or combined with other autoimmune treatment agents, as described herein, may be administered in an amount in the range of 0.01 mg / kg body weight to 100 mg / kg body weight per dose. It is also contemplated that a FcRn antagonist comprising a canine or feline Fc fragment of the present disclosure, alone or combined with other autoimmune treatment agents can be administered to a companion animal at one time or over a series of treatments. For example, a FcRn antagonist comprising a canine or feline Fc fragment of the present disclosure or a pharmaceuticalcomposition comprising a canine or feline Fc fragment of the present disclosure may be administered at least once, more than once, at least twice, at least three times, at least four times, or at least five times.

[0090] In some embodiments, an FcRn antagonist comprising a canine or feline Fc fragment of the present disclosure can be administered as a therapeutically effective dose once per day, per week for at least two or three consecutive weeks, and in some embodiments, this cycle of treatment is repeated two or more times, optionally interspersed with one or more weeks of no treatment. In other embodiments, the therapeutically effective dose is administered once per day for two to five consecutive days, and in some embodiments, this cycle of treatment is repeated two or more times, optionally interspersed with one or more days or weeks of no treatment.

[0091] Administration can be “in combination with” one or more further therapeutic agents includes simultaneous (concurrent) and consecutive or sequential administration in any order. The term “concurrently” is used herein to refer to administration of two or more therapeutic agents, where at least part of the administration overlaps in time or where the administration of one therapeutic agent falls within a short period of time relative to administration of the other therapeutic agent. For example, the two or more therapeutic agents are administered with a time separation of no more than about a specified number of minutes. The term “sequentially” is used herein to refer to administration of two or more therapeutic agents where the administration of one or more agent(s) continues after discontinuing the administration of one or more other agent(s), or wherein administration of one or more agent(s) begins before the administration of one or more other agent(s). For example, administration of the two or more therapeutic agents are administered with a time separation of more than about a specified number of minutes. As used herein, “in conjunction with” refers to administration of one treatment modality in addition to another treatment modality. As such, “in conjunction with” refers to administration of one treatment modality before, during or after administration of the other treatment modality to the animal.

[0092] Various methods known in the art for detecting specific antibody-antigen binding can be used to assay the FcRn antagonists of the present disclosure, including in their preparation and use. Exemplary immunoassays which can be conducted include fluorescence polarization immunoassay (FPIA), fluorescence immunoassay (FIA), enzyme immunoassay (EIA), nephelometric inhibition immunoassay (NIA), enzyme linked immunosorbent assay (ELISA), and radioimmunoassay (RIA). An indicator moiety, or label group, can be attached to the subject antibodies and is selected so as to meet the needs of various uses of the method which are often dictated by the availability of assay equipment and compatible immunoassay procedures. Appropriate labels include, without limitation, radionuclides (for example125I,131I,35S,3H, or32P), enzymes (for example, alkaline phosphatase, horseradish peroxidase, luciferase, or p-galactosidase), fluorescent moieties or proteins (for example, fluorescein, rhodamine, phycoerythrin, GFP, or BFP), or luminescent moieties (for example, Qdot™ nanoparticles supplied by the Quantum Dot Corporation, Palo Alto, Calif.). General techniques to be used in performing the various immunoassays noted above are known to those of ordinary skill in the art.EXAMPLES

[0093] Various features and embodiments of the disclosure are illustrated in the following representative examples, which are intended to be illustrative, and not limiting. Those skilled in the art will readily appreciate that the specific examples are only illustrative of the invention as described more fully in the claims which follow thereafter. Every embodiment and feature described in the application should be understood to be interchangeable and combinable with every embodiment contained within.

[0094] Example 1: Design and recombinant expression of canine and feline FcRn antagonists comprising a canine or feline Fc fragment

[0095] This example illustrates the design and recombinant expression of Fc fragments useful as FcRn antagonists that are derived from the canine Fc region sequence or the feline Fc region sequence.

[0096] The canine Fc fragment polypeptide of SEQ ID NO: 1 was designed based on a portion of the canine Fc region which includes the hinge sequence, was expressed in CHO cells as part of a dimeric protein including two Fc fragment chains disulfide linked at the hinge. The dimeric protein was determined to be >95 % monodispersed by gel filtration chromatography. The expressed protein was purified using a Protein A column.

[0097] The feline Fc fragment polypeptide of SEQ ID NO: 4 was designed based on a portion of the feline Fc region which includes the hinge sequence and was expressed in CHO cells as part of a dimeric protein including two Fc fragment chains disulfide linked at the hinge. The dimeric protein was determined to be >95 % monodispersed by gel filtration chromatography. The expressed protein was purified using a Protein A column.

[0098] Example 2: Design and recombinant expression of canine and feline Fc fragment with Fc region YTE or Y mutations

[0099] In human IgGs, the “YTE” mutations in the CH2 domain 252Y, 254T, and 256E (EU numbering), can greatly enhance its binding to human FcRn. This example illustrates the design and recombinant expression of the canine Fc fragment of SEQ ID NO: 1 and feline Fc fragment of SEQ ID NO: 4 described in Example 1 with either just the “Y” Fc region amino acid substitution of L252Y (corresponding L37Y relative to SEQ ID NO: 1), or the set of all three “YTE” Fc region amino acid substitutions of L252Y, A254T, and T256E (corresponding to L37Y, A39T, and T41E relative to SEQ ID NO: 1).

[0100] The canine Fc fragment polypeptide of SEQ ID NO: 1 was designed for recombinant expression having the 252Y mutation (EU numbering) as SEQ ID NO: 2. The canine Fc fragment polypeptide of SEQ ID NO: 1 with the “YTE” mutations 252Y, 254T, and 256E (EU numbering) was designed for recombinant expression as SEQ ID NO: 3. These engineered canine Fc fragment “Y” and “YTE”polypeptides of SEQ ID NO: 2 and 3, respectively, were expressed in CHO cells as part of a dimeric protein and purified using a Protein A column.

[0101] Similarly, the feline Fc fragment polypeptide of SEQ ID NO: 4 was designed for recombinant expression having the S252Y mutation (EU numbering; or S39Y relative to SEQ ID NO: 4) as SEQ ID NO: 5. The feline Fc fragment polypeptide of SEQ ID NO: 4 with the “YTE” mutations S252Y, S254T, and T256E (EU numbering; or S39Y, S41T, and T43E relative to SEQ ID NO: 4) was designed for recombinant expression as SEQ ID NO: 6. These engineered feline Fc fragment “Y” and “YTE” polypeptides of SEQ ID NO: 5 and 6, respectively, were expressed in CHO cells as part of a dimeric protein and purified using a Protein A column.

[0102] Example 3: FcRn binding activity of canine and feline Fc fragments

[0103] This example illustrates a Biacore analysis study of the ability of the canine and feline Fc fragments described in Examples 1 and 2 to specifically bind to canine or feline FcRn protein.

[0104] Materials and methods

[0105] The Biacore binding analysis conditions and parameters used in the study are summarized below in Tables A-D.

[0106] Table A: Biacore instrumentation Name Manufacturer Cat No. Biacore 8K Cytiva 29327020Name Manufacturer Cat No. Mi l t ll P b tt l i Bi 1 1

[0008] a e C: eagens use Name Manufacturer Cat No.

[0109] Table D: FcRn antigens Name Conc. MW Isotype / fusion L kD t Full name Cat No. Lot No. 1- ) - )

[0110] Preparation of running buffer: running buffer is diluted from 10× HBS-EP+ buffer.

[0111] The immobilization on a sensor of the canine and feline FcRn proteins was performed at 25 °C while HBS-EP+ was used as the running buffer. The sensor chip surface of flow cells 1 and 2 were activated by freshly mixed 50 mmol / L N-Hydroxysuccinimide (NHS) and 200 mmol / L 1-ethyl-3-(3- dimethylaminopropyl) carbodiimide hydrochloride (EDC) for 100 s (10 µL / min).

[0112] Afterwards, diluted in 10 mmol / L NaAC was injected into the flow cell 2 to achieve conjugation of appropriate Response Unit respectively, while flow cell 1 was set as blank.

[0113] After the amine coupling reaction, the remaining active coupling sites on chip surface were blocked with 100 s injection of 1 mol / L ethanolamine hydrochloride.

[0114] The assay was performed at 25 °C and the running buffer was HBS-EP+.

[0115] The Fc fragment samples were diluted into multiple concentrations and injected over the sensor chip surface as association phase, followed by injecting running buffer as dissociation phase. The association phase of the Fc fragments to the immobilized FcRn proteins was performed at pH 5.5 and the dissociation phase of antibody to antigen is performed at pH 7.4. The SPR running configuration parameters are summarized in Table E.

[0116] Table E: Running configuration of SPR measurement Immobilization Ligand antigen

[0117] All the data was processed using the Biacore 8K Evaluation Software version 4.0. Flow cell 1 and blank injections of running buffer in each cycle were used as double reference for Resonance Units subtraction.

[0118] Results

[0119] The binding measurement results are summarized in Table F.

[0120] Table F: Affinity data of antigen to antibodies FcRn Protein Fc Fragment i l Chi² (RU²) ka (1 / Ms) kd (1 / s) KD (M) Rmax (RU)Canine FcRn & SEQ ID NO: 3 B2M (YTE) 1.58E-01 NA NA 2.06E-08 15.5 Feline FcRn & SEQ ID NO: 4canine and feline Fc fragments exhibit enhanced binding to canine and feline FcRn, respectively, relative to the “wild-type” Fc fragments (SEQ ID NOs: 1 and 4) without these mutations. Surprisingly, unlike in human IgG, where the combination of the YTE mutations results in significantly stronger binding to FcRn compared to the Y mutation alone, for both canine and feline Fc fragment polypeptides disclosed herein, the addition of the TE mutations to the single Y mutation had little effect on binding to canine FcRn, and a slightly negative effect on binding to feline FcRn. Therefore, the single Y mutation was selected for further development of the Fc fragments as described in the following examples.

[0122] Example 4: Canine Fc fragments with intra-chain and inter-chain disulfide bonds that enhance stability

[0123] The canine “Y” Fc fragment of SEQ ID NO: 2 was further engineered with an inter-chain disulfide as shown by the polypeptide of SEQ ID NO: 7. This Fc fragment polypeptide of SEQ ID NO: 7 was highly expressed in mammalian cells and was >95% monodispersed.

[0124] An intra-chain disulfide was introduced to canine Fc variant (SEQ ID NO: 8). The protein is highly expressed in mammalian cell and >95% was monodispersed. The Tm, measured by differential scanning fluorescence (DSF), was increased from 70 C to 81 C.

[0125] Both inter-chain and intra-chain disulfide were introduced to canine Fc variant (SEQ ID NO: 9). The protein is highly expressed in mammalian cell and >95% was monodispersed.

[0126] Introduction of either inter-chain or intra-chain or both disulfides (SEQ ID NO: 7.8, 9) did alter the binding affinity to canine FcRn compared to parental canine Fc variant (SEQ ID NO: 2) based on Biacore analysis.

[0127] Example 5: Feline Fc fragments with intra-chain and inter-chain disulfide bonds that enhance stability

[0128] An inter-chain disulfide was introduced to feline Fc variant (SEQ ID NO: 10). The protein is highly expressed in mammalian cell and >95% was monodispersed.

[0129] An intra-chain disulfide was introduced to feline Fc variant (SEQ ID NO: 11). The protein is highly expressed in mammalian cell and >95% was monodispersed. The Tm, measured by differential scanning fluorescence (DSF), was increased from 70 C to 85 C.

[0130] Both inter-chain and intra-chain disulfide were introduced to feline Fc variant (SEQ ID NO: 12). The protein is highly expressed in mammalian cell and >95% was monodispersed.

[0131] Introduction of either inter-chain or intra-chain or both disulfides (SEQ ID NO: 10, 11, 12) did alter the binding affinity to feline FcRn compared to parental feline Fc variant (SEQ ID NO: 5) based on Biacore analysis.

[0132] Example 6: Optimization of affinity to canine FcRn for improved therapeutic index

[0133] To create a high potency canine FcRn antagonist (blocker), it may be helpful for the antagonist to have increased affinity to canine FcRn at both low pH (~pH6.0) and at neutral pH. But too high an affinity of the FcRn antagonist for canine FcRn will shorten the half-life of the antagonist. Therefore, it is critical to establish an optimal affinity of the antagonist (Fc fragment) for canine FcRn that balances potency in vitro with pharmacokinetic half-life in vivo. It is a surprising result of the FcRn antagonists of the present disclosure that an optimal affinity is achieved in the Kd range from 0.1 μM to 100 pM at neutral pH, and in the range of 0.1 nM to 1 pM at low pH. Mutations in the Fc fragment may be combined from the various positions as disclosed herein, not limited as listed, to “tune” the FcRn affinity into the optimal range of Kd. A variety of other mutations are provided herein (see e.g., Fc fragments of Table 1) and known in the art that may be combined from various positions of the Fc fragment.

[0134] One exemplary Fc fragment prepared herein that provides an optimal Kd for a canine FcRn antagonist is the polypeptide of SEQ ID NO: 13. This FcRn antagonist Fc fragment combines the Y mutation at position 252 (Eu numbering) and a Y mutation at position 434 (Eu numbering). Position 434 may also be mutated F, H, R, W, or S. An in vivo study of the Fc fragment of SEQ ID NO: 13 is described in Example 8 below.

[0135] Example 7: Optimization of affinity to feline FcRn for improved therapeutic index

[0136] To create high potency feline FcRn antagonist (blocker), it is helpful to have increased affinity to feline FcRn at both low pH (~pH 6.0) and neutral pH. But too high an affinity of a blocker can shorten the in vivo half-life of the Fc fragment as an FcRn antagonist. An optimal feline FcRn antagonist for in vivo use can be prepared by engineering FcRn binding affinity over pH range. The optimal affinity (Kd) should be “tuned” to a range of 0.1 μM to 100 pM at neutral pH and 0.1 nM to 1 pM at low pH by engineering the Fc fragment with mutations. A variety of mutations are provided herein (see e.g., Fc fragments of Table 1) and known in the art that may be combined from various positions of the Fc fragment.

[0137] One exemplary Fc fragment prepared herein that provides an optimal Kd for a feline FcRn antagonist is the polypeptide of SEQ ID NO: 18. This FcRn antagonist Fc fragment combines the Y mutation at position 252 (Eu numbering) and a Y mutation at position 434 (Eu numbering). Position 434 may also be mutated F, H, R, or W. An in vivo study of the Fc fragment of SEQ ID NO: 18 is described in Example 9 below.

[0138] Example 8: Pharmacodynamic study of a canine FcRn antagonist Fc fragment

[0139] This example illustrates a pharmacodynamic study over a 14 day period of a canine FcRn antagonist comprising the Fc fragment polypeptide of SEQ ID NO: 13 administered to dogs and the resultant effect of this treatment on IgG levels in the dogs.

[0140] Materials and methods

[0141] Two dogs were dosed intravenously (IV) at 10 mg / kg with the Fc fragment polypeptide of SEQ ID NO: 13 on Day 0. Serum was collected for pharmacodynamic analysis of IgG concentrations prior to dosing and then following dosing at: 0.5 h, 8 h, 24 h, 36 h, 48 h post-dose, and then daily until Day 14. Prior to dosing, samples were taken from the dogs and baseline IgG concentrations of 2503 and 1714 mg / dL were determined. Following dosing, IgG levels were measured and were found to drop, until Day 7. As shown in FIG.1, at Day 7 minimum IgG concentrations of 1732 and 1062 mg / dL, respectively, were achieved.

[0142] Results

[0143] As shown by the results plotted in FIG.2, the percent change of IgG concentrations on Day 7 dogs were -30 % and -38 %, respectively. This study confirms that a single IV dose of the canine FcRn antagonist Fc fragment polypeptide of SEQ ID NO: 13 at 10 mg / kg (IV) is able to decrease IgG concentrations by 1 / 3 (approx. -33%). There were no adverse events reported during this study, thus 10 mg / kg is considered safe and well tolerated.

[0144] Example 9: Pharmacodynamic study of a feline FcRn antagonist Fc fragment

[0145] This example illustrates a pharmacodynamic study over a 14 day period of a feline FcRn antagonist comprising the Fc fragment polypeptide of SEQ ID NO: 18 administered to cats and the resultant effect of this treatment on IgG levels in the cats.

[0146] Materials and methods

[0147] Two cats were dosed intravenously (IV) at 10 mg / kg with Fc fragment polypeptide of SEQ ID NO: 18 on Day 0. Serum was collected for pharmacodynamic analysis of IgG concentrations prior to dosing, and then post-dosing at 0.5 h, 8 h, 24 h, 36 h, 48 h, and daily until Day 14. Prior to dosing, samples were taken from the cats for determination of baseline IgG concentrations. Following dosing, serum IgG levels are determined and the results plotted until Day 14 for each cat.

[0148] Results

[0149] No serious adverse events were observed in the treated cats over the course of the study. Thus, a 10 mg / kg I.V. dose of the Fc fragment polypeptide of SEQ ID NO: 18 is considered safe and well- tolerated by cats. Percentage changes of serum IgG concentrations determined during this study will confirm whether the single IV dose of 10 mg / kg of the Fc fragment polypeptide of SEQ ID NO: 18 is able to decrease IgG concentrations in treated cats as was shown for dogs using the Fc fragment polypeptide of SEQ ID NO: 13 as described in Example 8.

[0150] While the foregoing disclosure of the present invention has been described in some detail by way of example and illustration for purposes of clarity and understanding, this disclosure including the examples, descriptions, and embodiments described herein are for illustrative purposes, are intended to be exemplary, and should not be construed as limiting the present disclosure. It will be clear to one skilled in the art that various modifications or changes to the examples, descriptions, and embodiments described herein can be made and are to be included within the spirit and purview of this disclosure and the appended claims. Further, one of skill in the art will recognize a number of equivalent methods and procedure to those described herein. All such equivalents are to be understood to be within the scope of the present disclosure and are covered by the appended claims.

[0151] Additional embodiments of the invention are set forth in the following claims.

[0152] The disclosures of all publications, patent applications, patents, or other documents mentioned herein are expressly incorporated by reference in their entirety for all purposes to the same extent as if each such individual publication, patent, patent application or other document were individually specifically indicated to be incorporated by reference herein in its entirety for all purposes and were set forth in its entirety herein. In case of conflict, the present specification, including specified terms, will control. References 1. Alfaidi, Nouf, Salama Karmastaji, Alexandria Matic, and Vera Bril.2024. “FcRn Inhibitor Therapies in Neurologic Diseases.” CNS Drugs 1–17. 2. Almela, Ramón M. n.d. “Review of Pemphigus Foliaceus in Dogs and Cats.” 3. Anderson, J. G., A. Kol, P. Bizikova, B. P. Stapelton, K. Ford, A. Villarreal, R. J. Jimenez, D. Vasilatis, and B. G. Murphy.2020. “Immunopathogenesis of Canine Chronic Ulcerative Stomatitis.” PloS One 15(1):e0227386. 4. Anderson, J. G., S. Peralta, A. Kol, P. H. Kass, and B. Murphy.2017. “Clinical and Histopathologic Characterization of Canine Chronic Ulcerative Stomatitis.” Veterinary Pathology 54(3):511–19. 5. Balch, Andrea, and Andrew Mackin.2007. “Canine Immune-Mediated Hemolytic Anemia: Pathophysiology, Clinical Signs, and Diagnosis.” Compend 29(4):217–25. 6. Canapp Jr, Sherman O., Leah A. Cohn, David J. Maggs, Margaret A. Miller, Marie E. Kerl, and Dennis P. O’Brien.2001. “Xerostomia, Xerophthalmia, and Plasmacytic Infiltrates of the Salivary Glands (Sjögren’s-like Syndrome) in a Cat.” Journal of the American Veterinary Medical Association 218(1):59–65. 7. Dodi, Pier Luigi.2015. “Immune-Mediated Keratoconjunctivitis Sicca in Dogs: Current Perspectives on Management.” Veterinary Medicine: Research and Reports 341–47. 8. Goggs, Robert, SG Dennis, Andrea Di Bella, Karen R. Humm, G. McLauchlan, Carmel Mooney, Alison Ridyard, Simon Tappin, David Walker, and S. 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Hermida-Prieto, Manuel, Javier García-Castro, and Luis Mariñas-Pardo.2021. “Systemic Treatment of Immune-Mediated Keratoconjunctivitis Sicca with Allogeneic Stem Cells Improves the Schirmer Tear Test Score in a Canine Spontaneous Model of Disease.” Journal of Clinical Medicine 10(24):5981. Iwasaki, T., T. Olivry, J. C. Lapiere, L. S. Chan, C. Peavey, Y. Y. Liu, J. C. R. Jones, P. J. Ihrke, and D. T. Woodley.1995. “Canine Bullous Pemphigoid (BP): Identification of the 180-Kd Canine BP Antigen by Circulating Autoantibodies.” Veterinary Pathology 32(4):387–93. Iwata, Yohei, Kazuhiro Komura, Masanari Kodera, Toshikazu Usuda, Yoko Yokoyama, Toshihide Hara, Eiji Muroi, Fumihide Ogawa, Motoi Takenaka, and Shinichi Sato.2008. “Correlation of IgE Autoantibody to BP180 with a Severe Form of Bullous Pemphigoid.” Archives of Dermatology 144(1):41–48. Jackson, Hilary A., Thierry Olivry, Frederic Berget, Stanley M. Dunston, Catherine Bonnefont, and Luc Chabanne.2004. “Immunopathology of Vesicular Cutaneous Lupus Erythematosus in the Rough Collie and Shetland Sheepdog: A Canine Homologue of Subacute Cutaneous Lupus Erythematosus in Humans.” Veterinary Dermatology 15(4):230–39. Jaffry, Mustafa, Daniel L. Menkes, Anam Shaikh, Kranthi Mandava, Om Kothari, Kazim Jaffry, and Nizar Souayah.2023. “Neonatal Fc Receptor Inhibitor Therapeutics in Neuromuscular Disease.” Journal of Clinical Neuromuscular Disease 24(4):188–98. Lee, Da Bin, Frank JM Verstraete, and Boaz Arzi.2020. “An Update on Feline Chronic Gingivostomatitis.” Veterinary Clinics: Small Animal Practice 50(5):973–82. Levy, Britt J., Lisa B. Mamo, and Petra Bizikova.2020. “Detection of Circulating Anti‐ keratinocyte Autoantibodies in Feline Pemphigus Foliaceus.” Veterinary Dermatology 31(5):378-e100. Lommer, Milinda J.2013. “Oral Inflammation in Small Animals.” Veterinary Clinics: Small Animal Practice 43(3):555–71. Mariette, Xavier, and Lindsey A. Criswell.2018. “Primary Sjögren’s Syndrome.” New England Journal of Medicine 378(10):931–39.Nelke, Christopher, Marianna Spatola, Christina B. Schroeter, Heinz Wiendl, and Jan D. Lünemann.2022. “Neonatal Fc Receptor–Targeted Therapies in Neurology.” Neurotherapeutics 19(3):729–40. Nishifuji, Koji, Kumiko Yoshida-Yamakita, and Toshiroh Iwasaki.2005. “A Canine Pemphigus Foliaceus Case Showing Parallel Relationship of Disease Activity and Titer of Serum Anti- Keratinocyte Cell Surface Antibodies.” Journal of Veterinary Medical Science 67(9):943–45. Oksenberg, Jorge, David Brassat, Mattias Olsson, Sven Hagnerud, David UR Hedelius, and Per- Arne Oldenborg.2006. “Hematologic Diseases: Autoimmune Hemolytic Anemia and Immune Thrombocytopenic Purpura.” Immunogenetics of Autoimmune Disease 135–43. Olivry, T., L. S. Chan, L. Xu, P. Chace, S. M. Dunston, M. Fahey, and M. P. Marinkovich.1999. “Novel Feline Autoimmune Blistering Disease Resembling Bullous Pemphigoid in Humans: IgG Autoantibodies Target the NC16A Ectodomain of Type XVII Collagen (BP180 / BPAG2).” Veterinary Pathology 36(4):328–35. Olivry, Thierry.2006. “A Review of Autoimmune Skin Diseases in Domestic Animals: I– Superficial Pemphigus.” Veterinary Dermatology 17(5):291–305. Olivry, Thierry, and Hilary A. Jackson.2001. “Diagnosing New Autoimmune Blistering Skin Diseases of Dogs and Cats.” Clinical Techniques in Small Animal Practice 16(4):225–29. Olivry, Thierry, Sohaila Joubeh, Stanley M. Dunston, Takafumi Nishiyama, and Reza F. Ghohestani.2003. “Desmoglein‐3 Is a Target Autoantigen in Spontaneous Canine Pemphigus Vulgaris.” Experimental Dermatology 12(2):198–203. Piek, Christine J.2011. “Canine Idiopathic Immune-Mediated Haemolytic Anaemia: A Review with Recommendations for Future Research.” Veterinary Quarterly 31(3):129–41. Pyzik, Michal, Kine M. K. Sand, Jonathan J. Hubbard, Jan Terje Andersen, Inger Sandlie, and Richard S. Blumberg.2019. “The Neonatal Fc Receptor (FcRn): A Misnomer?” Frontiers in Immunology 10. doi: 10.3389 / fimmu.2019.01540. Rischmueller, Maureen, Joanna Tieu, and Susan Lester.2016. “Primary Sjögren’s Syndrome.” Best Practice & Research Clinical Rheumatology 30(1):189–220. Rosenkrantz, Wayne S.2004. “Pemphigus: Current Therapy.” Veterinary Dermatology 15(2):90–98. Scott, Danny W., William H. Miller, and Hollis N. Erb.2013. “Feline Dermatology at Cornell University: 1407 Cases (1988–2003).” Journal of Feline Medicine and Surgery 15(4):307–16. Swann, James W., Oliver A. Garden, Claire L. Fellman, Barbara Glanemann, Robert Goggs, Dana N. LeVine, Andrew J. Mackin, and Nathaniel T. Whitley.2019. “ACVIM Consensus Statement on the Treatment of Immune‐mediated Hemolytic Anemia in Dogs.” Journal of Veterinary Internal Medicine 33(3):1141–72.

Claims

CLAIMS What is claimed is:

1. A canine FcRn antagonist comprising a canine Fc fragment having an amino acid substitution L252Y and an amino acid substitution selected from N434Y, N434F, N434H, N434R, and N434W, wherein the amino acid substitution is relative to SEQ ID NO: 1, and the amino acid position is based on Eu numbering; optionally, wherein the Fc fragment comprises an amino acid sequence having 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 greater identity to a sequence selected from SEQ ID NO: 13, 14, 15, 16, and 17.

2. The canine FcRn antagonist of claim 1, wherein the canine Fc fragment further comprises an amino acid substitution relative to SEQ ID NO: 1 selected from A254T and T256E, wherein the amino acid position is based on Eu numbering.

3. The canine FcRn antagonist of any one of claims 1-2, wherein the canine Fc fragment further comprises an amino acid substitution relative to SEQ ID NO: 1 of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule.

4. The canine FcRn antagonist of claim 3, wherein the amino acid substitution relative to SEQ ID NO: 1 is T396C, wherein the amino acid position is based on Eu numbering.

5. The canine FcRn antagonist of any one of claims 1-2, wherein the canine Fc fragment further comprises at least two amino acid substitutions relative to SEQ ID NO: 1 of a C at positions capable of forming an intradomain disulfide bridge.

6. The canine FcRn antagonist of claim 5, wherein the at least two amino acid substitutions relative to SEQ ID NO: 1 are A343C and A433C, wherein the amino acid position is based on Eu numbering.

7. The canine FcRn antagonist of any one of claims 1-2, wherein the canine Fc fragment further comprises at least two amino acid substitutions relative to SEQ ID NO: 1 of a C at positions capable of forming an intradomain disulfide bridge, and an amino acid substitution relative to SEQ ID NO: 1 of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule.

8. The canine FcRn antagonist of any one of claims 1-7, wherein the canine Fc fragment further comprises at least a portion of a canine heavy chain constant region selected from IgG-A, IgG-B, IgG-C, and IgG-D.

9. The canine FcRn antagonist of any one of claims 1-8, wherein the canine Fc fragment has an amino acid sequence of 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 greater identity to a sequence selected from SEQ ID NO: 1, 2, 3, 7, 8, 9, 13, 14, 15, 16, and 17.

10. A feline FcRn antagonist comprising a feline Fc fragment having an amino acid substitution S252Y, wherein the amino acid substitution is relative to SEQ ID NO: 4, and the amino acid position is based on Eu numbering; optionally, wherein the feline Fc fragment comprises an amino acid sequence having 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 greater identity to a sequence of SEQ ID NO:

5.

11. The feline FcRn antagonist of claim 10, wherein the feline Fc fragment further comprises an amino acid substitution at position S434 selected from S434Y, S434F, S434H, S434R, wherein the amino acid substitution is relative to SEQ ID NO: 4, and the amino acid position is based on Eu numbering; optionally, wherein the feline Fc fragment comprises an amino acid sequence having 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 greater identity to a sequence selected from SEQ ID NO: 18, 19, 20, 21, and 22.

12. The feline FcRn antagonist of any one of claims 10-11, wherein the feline Fc fragment further comprises an amino acid substitution relative to SEQ ID NO: 4 selected from A254T and T256E, wherein the amino acid position is based on Eu numbering.

13. The feline FcRn antagonist of any one of claims 10-12, wherein the feline Fc fragment further comprises an amino acid substitution relative to SEQ ID NO: 4 of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule.

14. The feline FcRn antagonist of claim 13, wherein the amino acid substitution relative to SEQ ID NO: 4 is T396C, wherein the amino acid position is based on Eu numbering.

15. The feline FcRn antagonist of any one of claims 10-12, wherein the feline Fc fragment further comprises at least two amino acid substitutions of a C at positions capable of forming an intradomain disulfide bridge.

16. The feline FcRn antagonist of claim 15, wherein the two amino acid substitutions relative to SEQ ID NO: 4 of a C are P343C and A433C, wherein the amino acid position is based on Eu numbering.

17. The feline FcRn antagonist of any one of claims 10-12, wherein the feline Fc fragment further comprises at least two amino acid substitutions relative to SEQ ID NO: 4 of a C at positions capable of forming an intradomain disulfide bridge, and an amino acid substitution relative to SEQ ID NO: 4 of a C at a position capable of forming an interdomain disulfide bridge with a second Fc fragment molecule.

18. The feline FcRn antagonist of any one of claims 10-17, wherein the feline Fc fragment further comprises at least a portion of a feline heavy chain constant region selected from IgG-A(IgG1), and IgG- B(IgG2).

19. The feline FcRn antagonist of any one of claims 10-18, wherein the feline Fc fragment has an amino acid sequence of 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 greater identity to a sequence selected from SEQ ID NO: 4, 5, 6, 10, 11, 12, 18, 19, 20, 21, and 22.

20. An isolated nucleic acid or vector encoding the FcRn antagonist of any one of claims 1-19.

21. An isolated host cell comprising the nucleic acid or vector of claim 20.

22. A method of producing an FcRn antagonist comprising culturing the host cell of claim 21 and isolating the FcRn antagonist.

23. A pharmaceutical composition comprising the FcRn antagonist of any one of claims 1-19 and a pharmaceutically acceptable carrier.

24. A method of treating a canine or a feline having an autoantibody associated disease or condition, the method comprising administering to the canine or feline a therapeutically effective amount of the FcRn antagonist of any one of claims 1-19 or the pharmaceutical composition of claim 23.

25. The method of claim 24, wherein the autoantibody associated disease or condition is an autoimmune disease.

26. The method of claim 24, wherein the autoantibody associated disease or condition is pemphigus.

27. The method of claim 24, wherein the autoantibody associated disease or condition is bullous pemphigoid.

28. The method of claim 24, wherein the autoantibody associated disease or condition is lupus erythematosus.

29. The method of claim 24, wherein the autoantibody associated disease or condition is stomatitis.

30. The method of claim 24, wherein the autoantibody associated disease or condition is myasthenia gravis (gMG).

31. The method of any one of claims 24-30, wherein the FcRn antagonist or the pharmaceutical composition is administered parenterally.

32. The method of any one of claims 24-30, wherein the FcRn antagonist or the pharmaceutical composition is administered by an intramuscular route, an intraperitoneal route, an intracerebrospinal route, a subcutaneous route, an intra-arterial route, an intrasynovial route, an intrathecal route, or an inhalation route.

33. The method of any one of claims 24-32, wherein the FcRn antagonist is administered in an amount in the range of 0.01 mg / kg body weight to 100 mg / kg body weight per dose.

34. The method of any one of claims 24-33, wherein the FcRn antagonist is administered intravenously in an amount of at least 10 mg / kg body weight per dose; optionally, wherein the amount administered is from about 10 mg / kg to about 75 mg / kg body weight per dose.

35. The method of any one of claims 24-34, wherein at 7 days after administration of a dose of the FcRn antagonist, serum IgG concentration in the treated canine or feline is reduced by at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, or more.

36. The method of any one of claims 24-35, wherein at 14 days after administration of a dose of the FcRn antagonist, serum IgG concentration in the treated canine or feline is reduced by at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, or more.

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