S851 A FULLY HUMAN BISPECIFIC ANTIBODY FUNCTIONALLY RESCUES FACTOR VIII DEFICIENCY EX VIVO

Abstract

Background:Haemophilia A is caused by deficiency of Factor VIII (F.VIII), an essential blood‐clotting cofactor. Prophylaxis or on‐demand treatment with recombinant FVIII forms the current standard of care to prevent bleeding in Haemophilia A patients in the developed world. However, approximately 30% of severe haemophilia A patients develop inhibitory antibodies against F.VIII. Moreover, FVIII treatment requires venous access which is burdensome. The recent successful clinical trials and subsequent regulatory approval of a F.VIII‐mimetic bispecific antibody, Hemlibra, in Haemophilia A patients with inhibitors have provided much needed hope for suffers and their family. However, whilst Hemilibra represents a major breakthrough in the field, there remain some concerns about balancing the risks of bleeding with those of thrombosis when using this agent.Aims:To develop a new fully human FVIII mimetic bispecific antibody that can support the assembly of a functional Factor IXa (F.IXa) and Factor X (F.X) complex, thereby replacing F.VIII function in vivo.Methods:Using Kymab's bispecific antibody platform, we developed KY1049, a fully human common light chain (CLC) bispecific antibody, to test as an effective functional mimetic of F.VIII. KY1049 bispecific heterodimer was purified using standard Protein A chromatography, followed by cation ion exchange chromatography (cIEX). The mass and heterodimer purity of the bispecific molecule was confirmed using Mass Spectrometry (MS). A combination of in vitro and ex vivo assays was used to characterize the cIEX purified bispecific antibody, including chromogenic FXase and Activated Partial Thromboplastin Time (aPTT). The affinities of the FIX arm and FX arm of KY1049 bispecific to FIX and FX, respectively, were determined using Surface Plasmon Resonance (SPR).Results:cIEX purified KY1049 molecule was confirmed as a single species by MS. After denaturing, the molecular weights of the two heavy chains and one light chain matches in silico predicted molecular weights. Purified KY1049 actively generated activated F.X (F.Xa) and functionally rescued the clotting defect in F.VIII‐depleted plasma in the aPTT assay. Importantly, ex vivo activities of KY1049 indicates its therapeutic benefit is likely to be similar in efficacy to Hemlibra. KY1049 binds F.IX and F.X simultaneously by SPR, but the affinities are different from those reported of Hemlibra.Summary/Conclusion:We have developed KY1049, a FVIII mimetic bispecific antibody, that demonstrates biological activities comparable to those reported for Hemlibra. Its therapeutic benefit is likely to be at least as effective as Hemlibra. Furthermore, initial analysis of the KY1049 molecule also showed some differences between the two molecules which could be harnessed to enhance its physical properties and activity.