Marstacimab for the Treatment of Hemophilia A or B

Johnny Mahlangu Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of Witwatersrand, Johannesburg, South AfricaCorrespondence: Johnny Mahlangu, Department of Molecular Medicine and Haematology, Faculty of Health Sciences, University of Witwatersrand, 7 York Road, Parktown, Johannesburg, South Africa, Tel +27114898413, Email [email protected]: Hemophilia A and B, caused by deficiencies of coagulation factors VIII or IX, result in impaired thrombin generation with consequent spontaneous or trauma-related bleeding, particularly hemarthroses. Although prophylactic factor replacement therapy remains the global standard of care for hemophilia, it has significant limitations, including intravenous administration, breakthrough bleeding, inhibitor development, and deteriorating arthropathy despite prophylaxis. Marstacimab, an IgG1 monoclonal antibody targeting tissue factor pathway inhibitor (TFPI), represents a novel non-factor approach. Marstacimab restores thrombin generation via the extrinsic pathway, bypassing intrinsic pathway deficiencies and offering prophylactic benefit independent of inhibitor status. Clinical evaluation across Phase 1b/2 and Phase 3 (BASIS) trials and ongoing extension studies has demonstrated robust efficacy. In Phase 3, marstacimab reduced annualized bleed rates by 91.6% compared with episodic treatment and was non-inferior to factor prophylaxis. Bleed control was sustained though zero-bleed outcomes were not uniformly achieved. Pharmacokinetic data support once-weekly fixed dosing independent of body weight, simplifying administration and potentially improving adherence. Across all studies, marstacimab demonstrated a favorable safety profile. Injection site reactions were the most common adverse events, while anti-drug antibodies, including neutralizing types, were transient and without clinical impact. Marstacimab, the first FDA-approved anti-TFPI antibody for prophylaxis in hemophilia A and B without inhibitors, addresses key unmet needs, particularly for hemophilia B patients lacking subcutaneous (SC) prophylaxis options. Its novel mechanism, ease of administration, and sustained efficacy position it as a significant therapeutic advance. Marstacimab‘s exact role in the hemophilia treatment armamentarium is yet to be established with the availability of coming real-world experience data. The long-term studies remain essential to fully demonstrate its role, especially in populations with inhibitors and in the context of evolving non-factor therapies. This review summarises currently available clinical data and contextualises these in light of other treatments in hemophilia.Keywords: hemophilia A, hemophilia B, marstacimab, tissue factor pathway inhibitor, TFPI, non-factor therapy, subcutaneous administration, monoclonal antibodies