FDA grants priority review to marstacimab (HYMPAVZI) for younger children and patients with hemophilia inhibitors

The FDA has accepted and granted Priority Review to a supplemental Biologics License Application (sBLA) for HYMPAVZI (marstacimab) from Pfizer Inc., seeking to expand its indication to include patients aged 6 years and older with hemophilia A or B with inhibitors, as well as pediatric patients aged 6 to 11 years with hemophilia A or B without inhibitors.¹

Marstacimab is currently approved in the United States for patients aged 12 years and older with hemophilia A without factor VIII (FVIII) inhibitors or hemophilia B without factor IX (FIX) inhibitors.² The FDA has set a Prescription Drug User Fee Act (PDUFA) action date in the second quarter of 2026.

If approved, marstacimab would extend a once-weekly subcutaneous prophylactic option to younger children and to patients with inhibitors, a population with limited treatment options. According to Pfizer, the therapy requires minimal preparation and does not require routine treatment-related laboratory monitoring.

“There is a significant medical need for younger patients with hemophilia and for those who have developed inhibitors, which neutralize factor replacement therapies and render them ineffective,” said Michael Vincent, MD, PhD, chief inflammation & immunology officer, Pfizer. “Based on the findings in the BASIS clinical trial program and if approved, we believe HYMPAVZI has the potential to become a transformative option for these patients that have limited or burdensome treatment options today. We look forward to progressing discussions with regulators to make this medicine available for patients.”

Hemophilia is typically diagnosed in early childhood and affects more than 800,000 people worldwide. The disorder is characterized by deficiency of clotting factor VIII in hemophilia A or factor IX in hemophilia B, resulting in impaired blood clotting. Recurrent bleeding episodes, including hemarthroses, may lead to joint scarring and long-term joint damage. Children may be particularly vulnerable because growing cartilage and bone can be affected by repeated bleeding.

Inhibitors develop in approximately 20% of individuals with hemophilia A and 3% of those with hemophilia B. In these patients, antibodies neutralize infused clotting factors, limiting the effectiveness of factor replacement therapy. Individuals who are refractory to immune tolerance induction therapy may have few therapeutic options.

“For children living with hemophilia A or B between ages 6 and 11, treatment approaches that prevent bleeds are particularly important to protect growing joints,” said Guy Young, MD, director, Hemostasis and Thrombosis Center at Children's Hospital, Los Angeles. “HYMPAVZI would address a critical unmet medical need for these patients and those with inhibitors if approved, particularly patients ages 6 to 11 with hemophilia B who do not have non-factor treatment options available today.”

The FDA grants Priority Review to therapies that may offer significant advances in treatment or provide options where no adequate therapy exists. The designation shortens the standard sBLA review period by four months. In addition, the FDA granted Breakthrough Therapy Designation to marstacimab for routine prophylaxis in pediatric patients aged 6 to younger than 12 years with hemophilia B, with and without inhibitors.

The sBLA submission for adults and adolescents with inhibitors is supported by data from the phase 3 BASIS trial (NCT03938792), a global, open-label, multicenter study evaluating efficacy and safety in participants aged 12 to younger than 75 years with severe hemophilia A or moderately severe to severe hemophilia B, with or without inhibitors. The inhibitor cohort included 48 participants treated with marstacimab during a 12-month active treatment period (ATP), compared with prior on-demand intravenous bypassing agents administered during a 6-month observational period. The primary endpoint assessed the treated annualized bleeding rate (ABR) during the ATP versus prior on-demand therapy.

The pediatric submission is supported by data from the phase 3 BASIS KIDS trial (NCT05611801), which evaluated marstacimab in children aged 1 to younger than 18 years with severe hemophilia A or moderately severe to severe hemophilia B, with or without inhibitors. Among 68 children aged 6 to 11 years, treated ABR during a 12-month ATP was compared with ABR during a 12-month period of prior routine prophylaxis with factor replacement therapy or bypassing agents.

Marstacimab is designed to target tissue factor pathway inhibitor (TFPI), a natural regulator of blood clot initiation, rather than replacing missing clotting factors. The therapy is administered subcutaneously once weekly using a pre-filled auto-injector pen.¹

Marstacimab has received regulatory approvals in more than 40 countries for eligible patients with hemophilia A without FVIII inhibitors or hemophilia B without FIX inhibitors. The expanded indication remains under review in the United States, and the use of marstacimab in patients aged 12 years and older with hemophilia A or B with inhibitors is also under review by the European Medicines Agency.¹

References

1. Pfizer. FDA Grants Priority Review for HYMPAVZI^® (marstacimab) sBLA for the Treatment of Two Hemophilia A or B Patient Populations with Significant Medical Need. Business Wire. February 6, 2026. Accessed February 6, 2026. https://www.businesswire.com/news/home/20260205107500/en/FDA-Grants-Priority-Review-for-HYMPAVZI-marstacimab-sBLA-for-the-Treatment-of-Two-Hemophilia-A-or-B-Patient-Populations-with-Significant-Medical-Need
2. FDA. FDA Approves New Treatment for Hemophilia A or B. FDA. October 11, 2024. Accessed February 6, 2026. https://www.fda.gov/news-events/press-announcements/fda-approves-new-treatment-hemophilia-or-b#:~:text=Product%20is%20First%20Non%2DFactor,IX%20inhibitors%20(neutralizing%20antibodies).