Safety and efficacy of a fitusiran antithrombin-based dose regimen in people with hemophilia A or B: the ATLAS-OLE study

Fitusiran, a subcutaneous investigational siRNA therapeutic, lowers antithrombin (AT) to increase thrombin generation and rebalance hemostasis in people with hemophilia. This phase 3 open-label extension study (ATLAS‑OLE, NCT03754790) evaluated safety and efficacy of antithrombin-based dose regimen (AT-DR) in males ≥ 12 years with severe hemophilia A/B, with/without inhibitors. The original 80 mg monthly (QM) dose regimen (ODR) was optimized to AT-DR targeting AT activity levels 15-35% to mitigate thrombotic risk (starting dose 50 mg once every 2 months [Q2M], individually adjusted to 20 mg Q2M or 20/50/80 mg QM as needed). Primary and secondary endpoints were safety and efficacy, respectively. Integrated safety analyses assessed safety of AT-DR and ODR across all fitusiran studies and integrated efficacy analyses compared efficacy of AT‑DR in ATLAS‑OLE with phase 3 parent study control groups. At interim data cut-off, 213 participants were enrolled on AT-DR (78% on Q2M regimens). Integrated safety analyses of participants receiving AT-DR (n = 286) demonstrated that AT-DR was well tolerated. In ATLAS-OLE, median (interquartile range) observed annualized bleeding rate (ABR) with AT-DR was 3.7 (0.0, 7.5). Integrated efficacy analyses demonstrated superiority of AT-DR over on-demand clotting factor concentrates (CFCs) (71% mean ABR reduction, P < 0.0001), and on-demand bypassing agents (BPAs) (73% mean ABR reduction, P = 0.0006); improvement over BPA prophylaxis (70% mean ABR reduction); and comparable ABR to CFC prophylaxis. Fitusiran AT‑DR was well tolerated and maintained bleed protection with as few as 6 injections per year. This trial was registered at www.clinicaltrials.gov as #NCT03754790.