FDA Expands Pfizer’s Hympavzi Approval to Pediatric Hemophilia Patients and Those with Inhibitors

The FDA has approved an expanded indication for Pfizer's marstacimab-hncq (brand name Hympavzi), extending use of the once-weekly subcutaneous therapy to additional patient populations living with hemophilia A or B.¹ The approval now includes patients aged 12 years and older with inhibitors and pediatric patients ages 6 to 11 years with or without inhibitors, according to a June 8 company announcement.¹

With the expanded label, Hympavzi is indicated in the US for routine prophylaxis to prevent or reduce bleeding episodes in adults and children aged 6 years and older with hemophilia A or B, regardless of inhibitor status.¹

The decision addresses areas of unmet need within the hemophilia community, particularly among patients who develop inhibitors to factor replacement therapies and younger children who may face challenges associated with frequent intravenous treatments.

According to Pfizer, the approval makes Hympavzi the first subcutaneous non-factor therapy available for children ages 6 to 11 years with hemophilia B.¹ The therapy is administered once weekly using a prefilled auto-injector pen and does not require routine treatment-related laboratory monitoring.¹

A significant challenge for some patients is the development of inhibitors, antibodies that neutralize factor replacement therapies. Approximately 20% of patients with hemophilia A and 3% of patients with hemophilia B develop inhibitors, limiting available treatment options and increasing the risk of uncontrolled bleeding.¹

“For children who have to deal with bleeding episodes from an early age and for people living with hemophilia who develop inhibitors, treatment options have been limited and are often burdensome,” said Guy Young, MD, director of the Hemostasis and Thrombosis Center at Children's Hospital Los Angeles, in the company announcement.^1

What clinical data supported the FDA decision?

The approval was supported by data from the Phase 3 BASIS and BASIS KIDS studies.¹ In the inhibitor cohort of the BASIS trial, Hympavzi demonstrated a 93% reduction in mean treated annualized bleeding rate compared with on-demand intravenous bypassing agents.^1,2 Patients receiving Hympavzi experienced a mean treated annualized bleeding rate of 1.4 versus 19.8 for those receiving on-demand treatment (p<0.0001).^1,2

Additional support came from interim data from the Phase 3 BASIS KIDS study evaluating children ages 6 to 17 years with hemophilia A or B.^1,3 Among children without inhibitors, treatment with Hympavzi resulted in a mean treated annualized bleeding rate of 1.8 compared with a historical model-based rate of 3.6 for routine prophylaxis.^1,3

Among children with inhibitors, the observed mean treated annualized bleeding rate was 1.4 compared with a historical model-based rate of 18.9 for on-demand therapy.^1,3 In children ages 6 to 11 years, model-based mean treated annualized bleeding rates ranged from 1.3 to 1.4, depending on inhibitor status and prior treatment history.^1,3

How does Hympavzi differ from traditional hemophilia treatments?

Unlike traditional factor replacement therapies, Hympavzi does not replace missing clotting factors.¹ Instead, the monoclonal antibody targets tissue factor pathway inhibitor (TFPI), a naturally occurring anticoagulant that regulates blood clot formation.¹

By inhibiting TFPI, the therapy is designed to help restore hemostatic balance and reduce bleeding risk while offering a less burdensome administration schedule than intravenous factor replacement.¹

Hympavzi became the first anti-TFPI therapy approved in the US and EU in 2024 for certain patients with hemophilia A or B without inhibitors.¹ It was also the first hemophilia therapy approved in those regions to be administered via a prefilled auto-injector pen.¹

References

1. U.S. FDA approves Pfizer's HYMPAVZI for the treatment of two additional hemophilia A or B patient populations with significant medical need. (2026 Jun 8). Business Wire. Accessed June 8, 2026. https://www.pfizer.com/news/press-release/press-release-detail/us-fda-approves-pfizers-hympavzi-treatment-two-additional
2. A study to evaluate marstacimab in participants with hemophilia with inhibitors (BASIS). ClinicalTrials.gov. Accessed June 8, 2026. https://clinicaltrials.gov/study/NCT03938792
3. A study to investigate marstacimab in children with hemophilia (BASIS KIDS). ClinicalTrials.gov. Accessed June 8, 2026. https://clinicaltrials.gov/study/NCT05611801