Long-Term Durability, Safety and Efficacy of Fitusiran Prophylaxis in People with Hemophilia a or B, with or without Inhibitors - Results from the Phase II Study

Introduction: Hemophilia is a bleeding disorder characterized by ineffective clot formation due to insufficient thrombin generation. The burden of disease for individuals with hemophilia is high, and less invasive treatment approaches are needed (Machin and Ragni. J Blood Med. 2018). Fitusiran is a once a month subcutaneously administered investigational RNA interference therapeutic targeting antithrombin as a means to improve thrombin generation and promote hemostasis in people with hemophilia A or B, with or without inhibitors. A completed Phase I study demonstrated that monthly subcutaneous administration of fitusiran was generally well tolerated and lowered antithrombin in a dose-dependent manner, resulting in increased thrombin generation and decreased bleeding frequency (Pasi et al. New Engl J Med. 2017). The aim of this abstract is to describe long-term durability, safety and efficacy of monthly fitusiran treatment for patients with hemophilia A or B, with or without inhibitors, in the Phase II open-label extension study. Methods: The fitusiran Phase I study (NCT02035605) followed by the Phase II open-label extension study (NCT02554773) included male patients, >18 years of age, with moderate or severe hemophilia A and B, with or without inhibitors. Patients received monthly fixed doses of fitusiran 50 mg or 80 mg subcutaneously. Exploratory post-hoc analysis of bleed events was used to calculate median annualized bleed rate in patients with hemophilia A and B, with or without inhibitors. Results: Thirty-four patients (hemophilia A, n=27 [13 with inhibitors and 14 without inhibitors]; hemophilia B, n=7 [2 with inhibitors and 5 without inhibitors) were enrolled in the Phase 2 open-label extension study, and were treated for up to 4.7 years with a median exposure of approximately 2.6 years (as of March 10, 2020). Once-monthly subcutaneous dosing of fitusiran prophylaxis lowered antithrombin (a reduction of between 85% to 72% from baseline) across patients over a sustained period of time. An exploratory analysis of bleeding events showed an overall median annualized bleed rate of 0.84 during the observation period (see figure). Breakthrough bleeds were managed successfully in accordance with the revised bleed management guidelines for reduced doses of bypassing agents and replacement factors. As of March 10, 2020, fitusiran was generally well tolerated and no anti-drug antibody formation was detected. Conclusions: Monthly fitusiran prophylaxis provided