FDA Approves HYMPAVZI for Children and Hemophilia Patients With Inhibitors

Pfizer has secured a significant U.S. Food and Drug Administration (FDA) approval expansion for HYMPAVZI (marstacimab-hncq), extending use to patients aged 12 years and older with hemophilia A or B who develop inhibitors, and to children aged 6 to 11 years with or without inhibitors. The therapy is now approved for routine prophylaxis to prevent or reduce bleeding episodes in adults and children aged six years and older, regardless of inhibitor status.

First-in-Class for Hemophilia B

The decision establishes HYMPAVZI as the first subcutaneous non-factor therapy available for children aged 6 to 11 years with hemophilia B a population historically underserved by existing treatments. Unlike factor replacement therapies, HYMPAVZI targets tissue factor pathway inhibitor (TFPI), blocking the Kunitz 2 domain to restore hemostatic balance without replacing missing clotting factors. The once-weekly subcutaneous injection does not require routine laboratory monitoring, simplifying disease management.

Addressing High-Unmet-Need Populations

Hemophilia affects more than 800,000 people worldwide, with approximately 20% of patients with hemophilia A and 3% of those with hemophilia B developing inhibitors that neutralize standard factor replacement therapies. These patients face heightened risk of uncontrolled bleeding and limited treatment options.

Phase 3 Evidence

The FDA’s decision was supported by pivotal Phase 3 data. In the BASIS trial (NCT03938792), HYMPAVZI reduced mean treated annualized bleeding rate (ABR) by 93% compared with on-demand bypassing-agent therapy (1.4 vs. 19.8; p<0.0001).

Interim results from BASIS KIDS (NCT05611801) demonstrated low bleeding rates across pediatric populations, with mean treated ABRs of 1.8 in children without inhibitors and 1.4 in those with inhibitors. Younger children aged 6 to 11 years achieved median ABRs of 1.0, highlighting consistent bleed protection.

Safety Profile

The most common adverse events included injection-site reactions, headache, pyrexia, arthralgia, diarrhea, pruritus, and rash. Two thromboembolic events were reported among 259 patients in the open-label extension study. The product label carries warnings for thromboembolic events, hypersensitivity reactions, embryofetal toxicity, and laboratory coagulation abnormalities.

Competitive Positioning

HYMPAVZI’s expansion positions Pfizer uniquely in hemophilia B, where Roche’s Hemlibra (emicizumab) is not indicated. The approval strengthens Pfizer’s competitive standing in non-factor therapies, particularly in pediatric populations with limited alternatives.

Clinical Perspectives in Practice

Guy Young, MD, Director of the Hemostasis and Thrombosis Center at Children’s Hospital Los Angeles, emphasized that patients with inhibitors and young children have historically faced burdensome treatment regimens. He noted the potential of once-weekly administration to simplify care while maintaining bleed protection. Pfizer’s Chief U.S. Commercial Officer, Aamir Malik, highlighted the milestone as addressing a critical unmet need, especially for children aged 6 to 11 years with hemophilia B.

Expanding Clinical Access Worldwide

The application received FDA Priority Review and previously secured Breakthrough Therapy Designation for younger pediatric patients with hemophilia B. HYMPAVZI has approvals in more than 40 countries for hemophilia A or B without inhibitors and recently gained European authorization for inhibitor-associated hemophilia in patients aged 12 years and older. Pfizer is pursuing further regulatory expansions worldwide.

Reference

U.S. FDA Approves Pfizer’s HYMPAVZI for the Treatment of Two Additional Hemophilia A or B Patient Populations with Significant Medical Need | Pfizer