Cell surface localization of FVIII mimetic therapy

About This Grant

PROJECT SUMMARY/ABSTRACT Hemophilia A (HA) is an inherited bleeding disorder caused by a deficiency of coagulation factor VIII (FVIII), leading to severe hemorrhage if untreated. Treatment with prophylactic FVIII concentrates is considered the standard of care in severe HA (<1% FVIII activity), but 30% of patients with HA (pwHA) develop neutralizing alloantibodies to FVIII called inhibitors, negating the efficacy of these concentrates. Bypassing agents (BPA), such as activated prothrombin complex concentrate (APCC) and recombinant activated factor VII (rFVIIa), are the therapeutic mainstays for acute bleeding in these patients but are less effective than FVIII and carry an inherent risk of thrombosis. Emicizumab, a bispecific antibody that binds to activated factor IX (FIXa) and factor X (FX), mimics activated FVIII (FVIIIa) by holding FIXa and FX in proximity, thereby enabling FX activation even in the presence of inhibitors. While emicizumab has revolutionized care of patients with hemophilia A with inhibitors (HAWI), individuals on the drug still require additional hemostatic therapy for breakthrough bleeds, trauma, or surgical procedures. However, treatment is limited to rFVIIa because of life-threatening thrombosis and/or thrombotic microangiopathy (TMA) observed with standard-dose APCC co-administration. My preliminary data have shown that emicizumab readily promotes FX activation on endothelial surfaces, and shows dose- dependence to FIXa, whereas FVIIIa has known saturable kinetics due to its high-affinity FIXa binding. It is unknown to what degree emicizumab acts on platelet surfaces as compared to FVIIIa, and how this affects potential combination hemostatic therapy, or how this knowledge could improve implementation and monitoring of therapy. While FVIIIa has several mechanisms that localize it to platelet surfaces, emicizumab does not have a known mechanism to accomplish this. Thus, I hypothesize that FVIIIa and emicizumab differ in the cellular localization of their activity and that their distinct cell-surface preferences have important consequences: 1) it may allow the use of commercially available FIX products (which contain trace FIXa), as a BPA alternative in people with HAWI on emicizumab, and 2) a novel whole blood thrombin generation assay may better reflect how effective the therapy may be in an individual. This proposal is designed to test this hypothesis by investigating the cellular localization of FVIII mimetics using novel in vitro assays and a canine model. In Aim 1, I will determine the physiologic/therapeutic surfaces of emicizumab as compared to FVIIIa, and how combined surfaces may modulate FX activation. In Aim 2, I will evaluate the ability of commercially available FIX products to enhance FXa generation in the presence of emicizumab. And in Aim 3, I will determine whether bleed frequency correlates with whole blood thrombin generation, a newly developed assay that works in human and canine whole blood. This will establish a potential method to predict and monitor efficacy of non-factor therapies.