FDA nods label expansion for Pfizer’s Hympavzi to pediatric use

Edited by Robert McTiernan··4 min read

The US FDA approved an expanded indication for Hympavzi (marstacimab-hncq), Pfizer’s (NYSE: PFE) anti-tissue factor pathway inhibitor antibody, extending its use to patients with hemophilia A or B aged 12 years and older who have developed inhibitors, and to pediatric patients aged 6 to 11 years with or without inhibitors. The label expansion makes Hympavzi the first subcutaneous non-factor therapy available for children aged 6 to 11 with hemophilia B — a population that had no approved option in this drug class. The product had received FDA Priority Review and Breakthrough Therapy Designation for the younger pediatric population.

Hympavzi is now indicated for routine prophylaxis to prevent or reduce bleeding episodes across adults and pediatric patients aged 6 and older with hemophilia A or B, regardless of inhibitor status. It is administered as a once-weekly subcutaneous injection using a pre-filled auto-injector pen, without routine treatment-related laboratory monitoring. The drug targets the Kunitz 2 domain of tissue factor pathway inhibitor (TFPI), a natural anticoagulant that suppresses the initiation of clotting. By inhibiting TFPI, marstacimab aims to restore hemostatic balance in patients whose clotting cascade is impaired by factor VIII or factor IX deficiency, independent of whether inhibitory antibodies have developed against those factors.

Clinical support for the expanded indication came from two Phase III studies. The pivotal BASIS trial (NCT03938792) enrolled adolescents and adults aged 12 and older with hemophilia A or B and inhibitors, demonstrating that Hympavzi reduced mean treated annualized bleeding rate (ABR) by 93% compared with on-demand intravenous bypassing agent therapy (1.4 vs. 19.8; p < 0.001). Interim data from the BASIS KIDS trial (NCT05611801) in children aged 6 to 17 years showed a mean treated ABR of 1.8 in patients without inhibitors — compared with a historical model-based mean of 3.6 on prior prophylaxis — and 1.4 in those with inhibitors, against a historical comparator ABR of 18.9 on on-demand therapy. The most commonly reported adverse reactions included injection site reactions, headache, pyrexia, and arthralgia. Thromboembolic events were observed in two of 259 patients in an open-label extension study, and are noted as a warning in the US label.

The most clinically significant aspect of this label expansion is the inhibitor population. Inhibitors — neutralizing antibodies against factor VIII or IX — develop in approximately 20% of people with hemophilia A and 3% of those with hemophilia B, and they render standard factor replacement therapies largely ineffective. For these patients, treatment has historically relied on bypassing agents administered intravenously, often on-demand rather than prophylactically, and with a substantially higher bleeding burden. A once-weekly subcutaneous prophylactic option that is agnostic to inhibitor status represents a meaningful shift in how this population can be managed.

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Hympavzi enters an increasingly competitive non-factor prophylaxis landscape. Genentech’s emicizumab (Hemlibra) remains the established standard for hemophilia A with or without inhibitors, but is not indicated for hemophilia B — a distinction that gives marstacimab a structural advantage in that subpopulation. Sanofi’s fitusiran (Qfitlia), an RNAi-based antithrombin-lowering agent approved in early 2025 for hemophilia A and B with or without inhibitors in patients aged 12 and older, is the most direct competitor by indication scope. Both fitusiran and marstacimab target the anticoagulant arm of the coagulation system rather than replacing deficient clotting factors, but through distinct mechanisms. Looking further ahead, Novo Nordisk’s denecimig has reported Phase III data showing a 99% reduction in bleeding in hemophilia A, adding further competitive pressure to the non-factor subcutaneous class.

The European Commission had previously granted marketing authorization for Hympavzi in patients aged 12 and older with inhibitors, as reported earlier, providing regulatory alignment across major markets for the inhibitor indication. Pfizer said it is continuing to pursue expanded approvals in additional countries.

For Pfizer, the expanded label consolidates Hympavzi’s position as a broad-spectrum hemophilia prophylactic — one that now spans both disease subtypes, both inhibitor categories, and a pediatric age range extending to 6 years. The pediatric hemophilia B population aged 6 to 11 represents a particularly underserved group, where the absence of prior subcutaneous non-factor options had left clinicians reliant on intravenous regimens that carry adherence and access challenges. Whether the clinical differentiation from fitusiran — particularly the extension to younger children — translates into prescribing preference will depend on real-world tolerability data and payer positioning, neither of which is yet established for the expanded populations.

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