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Outcomes of Marstacimab in North American Participants With Hemophilia A and B Without Inhibitors: Subanalysis of the Phase 3 BASIS Trial Anjali A1, Pengling 2, Jennifer 3, Travis 3, Andrew 4. 1Stead Family Department of Pediatrics, Carver School of Medicine, University of Iowa, Iowa City, IA, USA.2Pfizer Inc, Cambridge, MA, USA.3Pfizer Inc, New York, NY, USA.4Pfizer Inc, Collegeville, PA, USA

Abstract

Introduction BASIS (NCT03938792) is an open-label, phase 3, multi-center study evaluating efficacy of anti-tissue factor pathway inhibitor antibody marstacimab in individuals with severe hemophilia A (HA; factor VIII <1%) or moderately severe to severe hemophilia B (HB; factor IX ≤2%) with or without inhibitors. Marstacimab significantly reduced the annualized bleeding rate (ABR) of treated bleeds (ABR_treated) compared with prior on-demand (OD) or routine prophylaxis (RP) factor replacement therapy. This subanalysis describes baseline characteristics and outcomes in North American (Canada, United States, and Mexico) participants with HA or HB without inhibitors. Objective To evaluate efficacy and safety of marstacimab in North American participants with HA or HB without inhibitors enrolled in BASIS. Methods The BASIS trial included a 6-month observational phase (OP), during which participants continued their OD or RP factor replacement therapy regimen, followed by a 12-month active treatment phase (ATP). During the ATP, participants received subcutaneous marstacimab (300-mg loading dose, followed by 150-mg once weekly [QW]). After 6 months, participants meeting prespecified dose escalation criteria (weight ≥50 kg, ≥2 spontaneous bleeds requiring treatment in 6 months) could increase their dose to 300 mg QW. Efficacy outcomes included ABR_treated and other bleeding events. Safety outcomes included incidence of treatment-emergent adverse events (TEAEs), serious adverse events (SAEs), and immunogenicity (assessed by positive antidrug antibody [ADAb+] and neutralizing antibody [NAb+] titers). Results Fourteen (OD n=3; RP n=11) North American participants with hemophilia without inhibitors were enrolled. Median age was 29 years; 85.7% had HA and 64.3% had ≥1 target joint at baseline. All were White (100.0%). Among OD participants, mean ABR_treated (1.41 vs 26.41) and median ABR_treated (1.19 vs 33.93) decreased during ATP vs OP. Respectively, similar reductions were observed with mean joint bleeds (1.41 vs 24.44), spontaneous bleeds (1.41 vs 23.75), target joint bleeds (1.41 vs 17.15), and total ABR (2.14 vs 37.88). No TEAEs or SAEs occurred. Among RP participants, mean ABR_treated (2.58 vs 2.15) and median ABR_treated (1.01 vs 0.00) were comparable during the ATP vs OP. Respectively, similar results were observed in mean joint bleeds (1.84 vs. 1.27), spontaneous bleeds (0.92 vs. 2.15), target joint bleeds (0.46 vs 0.35), and total ABR (2.76 vs 2.34). TEAEs occurred in 8/11 (72.7%) RP participants; 2/11 (18.2%) experienced TEAEs related to treatment. No SAEs occurred. Overall, 3/14 (21.4%) participants were ADAb+ and none were NAb+. One (7.1%) participant dose-escalated during the ATP. Conclusions In the North American subgroup of BASIS participants with HA or HB without inhibitors, marstacimab was generally well tolerated. OD participants demonstrated substantial numerical reductions in ABR, whereas participants previously on RP maintained stable bleed control. These North American–specific findings are consistent with outcomes observed in the overall BASIS population.

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