BIVV001, a new class of factor VIII replacement for hemophilia A that is independent of von Willebrand factor in primates and mice.

Factor VIII (FVIII) replacement products enable comprehensive care in hemophilia A. Treatment goals in severe hemophilia A are expanding beyond low annualized bleed rates to include long-term outcomes associated with high sustained FVIII levels. Endogenous von Willebrand factor (VWF) stabilizes and protects FVIII from degradation and clearance, but it also subjects FVIII to a half-life ceiling of ∼15 to 19 hours. Increasing recombinant FVIII (rFVIII) half-life further is ultimately dependent upon uncoupling rFVIII from endogenous VWF. We have developed a new class of FVIII replacement, rFVIIIFc-VWF-XTEN (BIVV001), that is physically decoupled from endogenous VWF and has enhanced pharmacokinetic properties compared with all previous FVIII products. BIVV001 was bioengineered as a unique fusion protein consisting of a VWF-D'D3 domain fused to rFVIII via immunoglobulin-G1 Fc domains and 2 XTEN polypeptides (Amunix Pharmaceuticals, Inc, Mountain View, CA). Plasma FVIII half-life after BIVV001 administration in mice and monkeys was 25 to 31 hours and 33 to 34 hours, respectively, representing a three- to fourfold increase in FVIII half-life. Our results showed that multifaceted protein engineering, far beyond a few amino acid substitutions, could significantly improve rFVIII pharmacokinetic properties while maintaining hemostatic function. BIVV001 is the first rFVIII with the potential to significantly change the treatment paradigm for severe hemophilia A by providing optimal protection against all bleed types, with less frequent doses. The protein engineering methods described herein can also be applied to other complex proteins.

Intrinsic differences between FVIIIa mimetic bispecific antibodies and FVIII prevent assignment of FVIII-equivalence.

Essentials Non-factor VIII (FVIII) therapies for hemophilia A, such as bispecific antibodies (bsAbs), are in development. Bispecific antibodies are intrinsically different from FVIII and lack many of the same regulatory mechanisms. These differences complicate assignment and interpretation of FVIII-equivalent activity. Inability to assign FVIII equivalence compromises our capacity to assess hemostatic potential of bsAb therapies. BACKGROUND: Activated factor VIII (FVIIIa) mimetic bsAbs aim to enable prophylactic treatment of hemophilia A patients with and without inhibitors. With different mechanisms of action, benchmarking their activity against FVIII to determine efficacious yet safe dosage is difficult. OBJECTIVE: To compare the activities of sequence identical emicizumab (SI-Emi) and another bsAb, BS-027125, to recombinant FVIII (rFVIII) using clinical and nonclinical assays and to evaluate our ability to assign a FVIII-equivalent value to bsAbs and implications thereof. METHODS: Activities of SI-Emi, BS-027125, and rFVIII were measured by one-stage clotting assay, chromogenic factor Xa generation assay, and thrombin generation assay. We also assessed the activity of anti-FIXa and anti-FX bivalent homodimers of each bsAb and probed the effect of different reagents in thrombin generation assay (TGA). RESULTS: The FVIII-like activity of SI-Emi and BS-027125 ranged greatly across each assay, varying both by parameter measured within an assay and by reagents used. Notably, SI-Emi anti-FIXa bivalent homodimer had meaningful activity in several assays, whereas BS-027125 anti-FIXa bivalent homodimer only had activity in the chromogenic assay. Surprisingly, SI-Emi displayed activity in the absence of phospholipids, while BS-027125 had minimal phospholipid-independent activity. CONCLUSIONS: Bispecific antibodies demonstrate little consistency between assays tested here owing to intrinsic differences between FVIII and bsAbs. While some trends are shared, the bsAbs also differ in mechanism. These inconsistencies complicate assignment of FVIII-equivalent values to bsAbs. Ultimately, a deeper mechanistic understanding of bsAbs as well as bsAb-tailored assays are needed to monitor and predict their hemostatic potential and long-term efficacy and safety confidently.