Marstacimab Prophylaxis in Hemophilia A/B Without Inhibitors: Results from the Phase 3 BASIS Trial Open Access

• Marstacimab, a monoclonal antibody, targets the tissue factor pathway inhibitor to rebalance hemostasis.

• Marstacimab reduced bleeding events and was generally well tolerated with no unanticipated side effects.

Marstacimab is a monoclonal antibody that targets the tissue factor pathway inhibitor to rebalance hemostasis. Previous phase 1 and 2 trials established marstacimab safety and efficacy in adults with severe hemophilia A or B. BASIS is an open-label, phase 3 trial of marstacimab in males aged 12-74 years with severe hemophilia A (factor VIII <1%) or moderately severe to severe hemophilia B (factor IX ≤2%). Participants without inhibitors received on-demand (OD) or routine prophylaxis (RP) factor replacement during a 6-month observational phase (OP) before receiving once-weekly subcutaneous 150 mg marstacimab prophylaxis during a 12-month active treatment phase (ATP). Primary endpoints were annualized bleeding rate (ABR) for treated bleeds vs prior OD or RP during the OP and safety. Of 128 participants enrolled in the OP, 116 received marstacimab in the ATP. In the OD group (n=33), mean ABR (95% CI) decreased from 39.86 (33.05-48.07) in the OP to 3.20 (2.10-4.88) in the ATP, demonstrating superiority of marstacimab (estimated ABR ratio, 0.080 [0.057-0.113]; P < .0001). In the RP group (n=83), mean ABR decreased from 7.90 (5.14-10.66) in the OP to 5.09 (3.40-6.78) in the ATP, demonstrating noninferiority and superiority of marstacimab (estimated ABR difference, -2.81 [-5.42 to -0.20]; P = .0349). There were no deaths or thromboembolic events. Weekly subcutaneous marstacimab reduced ABR compared with OD or RP therapy in the OP in individuals with severe hemophilia A or moderately severe to severe hemophilia B without inhibitors. Marstacimab was safe and well tolerated with no unanticipated side effects. This trial was registered at www.clinicaltrials.gov as # NCT03938792.