FAQ: What the Expanded Marstacimab Approval Means for Hemophilia A, B

The U.S. Food and Drug Administration (FDA) has expanded the approval of marstacimab-hncq (Hympavzi), widening the drug's use in hemophilia A and hemophilia B.¹ "A treatment that can reduce bleeding with straightforward, once-weekly administration has the potential to fundamentally change how patients and caregivers approach this disease," said Guy Young, MD, director of the Hemostasis and Thrombosis Center at Children's Hospital Los Angeles, in a statement.¹ Below, HCPLive answers what clinicians need to know about the expanded indication.

What did the FDA just approve for marstacimab?

On June 8, 2026, the FDA expanded the approval of marstacimab-hncq (Hympavzi) to include children 6 to 11 years old and patients 12 years and older with factor inhibitors, broadening its use in hemophilia A and hemophilia B.¹

How does marstacimab work?

Marstacimab targets tissue factor pathway inhibitor (TFPI), a natural anticoagulant, rather than replacing the missing clotting factor. By inhibiting TFPI's Kunitz 2 domain, the once-weekly subcutaneous therapy is designed to rebalance clotting activity in patients previously managed only with factor replacement or bypass therapies.¹

Why do factor inhibitors complicate treatment?

Factor inhibitors are antibodies that neutralize infused clotting factor, making standard factor replacement ineffective and leaving bypass agents, which carry their own thrombotic risk and dosing burden, as the main prophylactic option. Patients with inhibitors have historically had fewer treatment choices and higher rates of breakthrough bleeding than those without them.²

What data support the expanded approval?

In the phase 3 BASIS trial, patients 12 years and older with inhibitors treated with marstacimab saw a 93% reduction in mean annualized bleeding rate compared with on-demand bypass therapy (1.4 vs 19.8; P < .0001).² In interim data from the BASIS KIDS trial, children 6 to 17 without inhibitors had a mean treated annualized bleeding rate of 1.8, versus a historical rate of 3.6, while children with inhibitors had a mean treated rate of 1.4, versus a historical rate of 18.9. Among children 6 to 11 specifically, model-based mean annualized bleeding rates ranged from 1.3 to 1.4.¹

What are the most common adverse reactions?

Injection site reactions, headache, fever, and joint pain were the most common adverse reactions reported across the trials. Serious risks include thromboembolic events and hypersensitivity reactions, both of which require ongoing monitoring during treatment.¹,²

Why does this expansion matter clinically?

This expansion makes marstacimab the first subcutaneous non-factor therapy available to young children with hemophilia B, a population that has relied almost exclusively on intravenous factor infusions or bypass agents for prophylaxis. For clinicians managing pediatric patients or those with inhibitors, it adds a once-weekly option to a treatment landscape that has historically required more frequent infusions.¹,²

References:

1. Pfizer. US FDA approves Pfizer's Hympavzi for the treatment of two additional hemophilia A or B patient populations with significant medical need. Published June 8, 2026. Accessed September 21, 2026. https://www.pfizer.com/news/press-release/press-release-detail/us-fda-approves-pfizers-hympavzi-treatment-two-additional

2. Matino D, Acharya SS, Taylor CT, Sun P, Agathon D, Raje S, Gould T, Palladino A, Mahlangu J. Efficacy and safety of marstacimab prophylaxis in hemophilia A/B with inhibitors: results from the phase 3 BASIS trial. Blood. 2026;147(9):920-931. doi:10.1182/blood.2025031065