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

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

On June 8, 2026, the U.S. Food and Drug Administration (FDA) approved an expanded indication for Pfizer's Hympavzi (marstacimab-hncq), a once-weekly subcutaneous therapy for hemophilia A or B. The label expansion represents a major clinical milestone, addressing high unmet needs for pediatric patients and those who have developed neutralizing antibodies (inhibitors) against traditional factor replacement therapies.

Broadened Patient Access

Hympavzi's updated label significantly expands the patient population eligible for this weekly, non-factor therapy:

"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. 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."

This makes Hympavzi the first subcutaneous non-factor therapy available in the U.S. for children aged 6 to 11 years living with hemophilia B. Because it is administered via a prefilled auto-injector pen and requires no routine treatment-related laboratory monitoring, it offers a dramatic quality-of-life improvement over frequent, technically difficult intravenous factor infusions.¹

Overcoming the Challenge of Inhibitors

A major complication in hemophilia care is the development of inhibitors (neutralizing antibodies) to factor replacement therapy, which occurs in roughly 20% of patients with hemophilia A and 3% of those with hemophilia B. Inhibitors render standard factor treatments ineffective and severely elevate the risk of uncontrolled bleeding.

Hympavzi bypasses this issue entirely through its unique mechanism of action:

"Unlike traditional factor replacement therapies, Hympavzi does not replace missing clotting factors. Instead, the monoclonal antibody targets tissue factor pathway inhibitor (TFPI), a naturally occurring anticoagulant that regulates blood clot formation. By inhibiting TFPI, the therapy is designed to help restore hemostatic balance and reduce bleeding risk..."

Robust Clinical Support

The FDA's decision was supported by compelling results from the Phase 3 BASIS and BASIS KIDS trials:

• Inhibitor Cohort (BASIS): Hympavzi demonstrated a 93% reduction in the mean treated annualized bleeding rate (ABR) compared with on-demand intravenous bypassing agents. Patients on Hympavzi experienced an ABR of 1.4, compared to 19.8 for those on on-demand treatments ($p<0.0001$).
• Pediatric Cohort (BASIS KIDS): In children aged 6 to 17 with hemophilia A or B without inhibitors, Hympavzi reduced the mean treated ABR to 1.8 (vs. a historical model-based rate of 3.6 for routine factor prophylaxis). For children with inhibitors, the observed mean treated ABR was 1.4 (vs. a historical model-based rate of 18.9 for on-demand therapy).

Originally approved in late 2024 for adults and adolescents aged 12 and older without inhibitors, Hympavzi is now established as a highly versatile, non-factor prophylactic option across the age and inhibitor spectrum for both hemophilia A and B.