Multiple blood disorders are mediated by immunoglobulin G (IgG) autoantibodies that recognize self-antigens associated with platelet biology. Prominent examples are immune thrombocytopenia (ITP), characterized by anti-platelet glycoprotein antibodies that drive low platelet counts and a predisposition to bleeding (Mititelu A, et al. Int J Mol Sci. 2024, 25(4):2163), and heparin-induced thrombocytopenia (HIT), characterized by antibodies to platelet factor 4 (PF4) bound to the anticoagulant drug heparin, causing thrombocytopenia with platelet activation and hypercoagulability (McKenzie SE, Sachais BS. Curr Opin Hematol. 2014, 21(5):380–387). Treatment options for ITP include IVIG, rituximab, or plasmapheresis to reduce anti-platelet autoantibodies (Mititelu A, et al. Int J Mol Sci. 2024, 25(4):2163), while HIT treatment involves discontinuation of heparin and starting alternative anticoagulants while managing thrombosis (Alhanshani AA. Int J Gen Med. 2023, 16:3947–3953). In principle, patients could rapidly recover from both conditions if free and antigen-bound autoantibodies are abruptly degraded.

Streptococcus pyogenes secretes an IgG protease called IdeS (von Pawel‐Rammingen U, et al. The EMBO Journal. 2002, 21(7):1607–1615). Wild type (WT) IdeS (drug name imlifidase) has enabled platelet transfusion in a patient with severe alloimmune platelet refractoriness (Qamar A, et al. Blood. 2022, 140(Supplement 1):8562–8563) and prevented hypercoagulability and thrombocytopenia in mice administered anti-PF4/H or anti-platelet antibodies, respectively (Kizlik-Masson C, et al. Blood. 2019, 133(22):2427–2435; Johansson BP, Shannon O, Björck L. PLoS One. 2008, 3(2):e1692). However, WT IdeS is highly immunogenic and is not dosed a second time in humans outside of the first 24 hours. It also suffers from a short half-life and is mostly cleared in hours (Lorant T, et al. Am J Transplant. 2018, 18(11):2752–2762).

CYR212 is an albumin-fused variant of IdeS and an investigational drug candidate. Using the rabbit as a preclinical model (IdeS cleaves human and rabbit IgG but has minimal activity in other species), we have shown that CYR212 provides deep and rapid depletion of IgG with low immunogenicity and long half-life (Sauer M, et al. Annals of the Rheumatic Diseases. 2024, 83:1133; Sauer M, et al. Molecular Therapy. 2024, 32(4):837-837). Here, we describe immunogenicity of a CYR212 surrogate, in which human albumin is replaced with rabbit albumin, in rabbits intravenously administered 3 doses every other week (N=3 per group). At doses of 0.2 μg/kg CYR212(RSA), serum IgG was decreased by 70-85% without any detectable ADA. At high doses of 10 μg/kg, serum IgG was decreased by 97-98% with IgG class ADA being elicited by the third dose. No IgM class ADA were detected and anti-CYR212(RSA) ADA titers were low (mean <10,000 reciprocal dilution) and declining by Week 11. By comparison, WT IdeS at 10 μg/kg elicited IgM and IgG class ADA after the first dose, with increasing titers over 11 weeks that were 5,000-fold higher. CYR212(RSA) had exceptional catalytic activity and decreased endogenous IgG by >55% following a single very low dose of 0.03 μg/kg. Combined with high bioavailability (87%) of CYR212 via subcutaneous route, the results are supportive of low-volume subcutaneous administration with a long (Q2W+) dosing interval.

To demonstrate feasibility for the treatment of HIT, CYR212 was found to not only degrade free anti-PF4/H monoclonal antibody (IgG1 clone 5B9) but also degraded pre-formed immune complexes of PF4/H in vitro. Degradation of antigen-bound antibodies is considered a prerequisite for HIT treatment, since PF4/H/IgG complexes continue to activate platelets and elevate thrombosis risk even after heparin withdrawal.

Finally, therapeutic efficacy of CYR212 was demonstrated in a mouse model of ITP. Rabbit anti-platelet IgG antibodies were passively transferred to C57BL/6 mice to induce sub-lethal thrombocytopenia. CYR212 treated animals demonstrated superior recovery of platelet counts compared to the control group. Pharmacokinetics (PK) in C57BL/6 mice, humanized FcRn mice, and hFcRn/Albumin-KO mice supported modeling of long human PK with an elimination half-life of 9.6 days.

Overall, the data show CYR212 achieves rapid and deep depletion of IgG, with low immunogenicity for effective repeat doses. The profile is well suited for rapidly addressing antibody-mediated hematological emergencies and long-term maintenance therapy.

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2025
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