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

Abstract

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) andanother 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 assaystested 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. K E Y W O R D S bispecific antibodies, blood coagulation tests, factor VIII, hemophilia A, hemostasisThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Management of bleeding and invasive procedures in haemophilia A patients with inhibitor treated with emicizumab (Hemlibra®): Proposals from the French network on inherited bleeding disorders (MHEMO), the French Reference Centre on Haemophilia, in collaboration with the French Working Group on Perioperative Haemostasis (GIHP)

“…However, there is still not enough data to expand this attitude. Moreover, the apparent procoagulant activity of emicizumab according to thrombin generation assay may vary substantially according to trigger reagent or phospholipid composition …”

Emicizumab prophylaxis in patients with haemophilia A with and without inhibitors

“…[4][5][6] Thus, safer, less immunogenic therapies are needed to treat haemophilia. 13,14 Whether bleed frequency and bleed severity, long-term joint health and quality of life are comparable to that achieved with factor VIII, therefore, remains unknown. 7-10 While emicizumab mimics factor VIII by bridging factor IXa and factor X, 11,12 it differs from factor VIII biochemically, with no phospholipid binding, only single-site interaction with factor IXa, and limited self-regulation.…”

“…[7][8][9][10] While emicizumab mimics factor VIII by bridging factor IXa and factor X, 11,12 it differs from factor VIII biochemically, with no phospholipid binding, only single-site interaction with factor IXa, and limited self-regulation. 13,14 Whether bleed frequency and bleed severity, long-term joint health and quality of life are comparable to that achieved with factor VIII, therefore, remains unknown. Recent studies indicate emicizumab is safe and effective for prophylaxis in infants with haemophilia A 15,16 and in immune tolerance induction (ITI) in inhibitor patients, 16,17 and, while costly, is cost-effective in those with inhibitors.…”

Bispecific Antibodies and Advances in Non–Gene Therapy Options in Hemophilia

“…In this assay, maximum coagulation velocity correlated well with the concentration of emicizumab. Similarly, rotational thromboelastometry (ROTEM) and thrombin generation assays could provide valuable data for monitoring emicizumab activity 31‐33 . Nonactivated ROTEM without the addition of APTT or PTT reagents could be especially informative and may be the simplest method for monitoring emicizumab therapy 31 …”