FDA expands Pfizer’s HYMPAVZI indication for additional hemophilia A and B patient groups

Pfizer said the U.S. Food and Drug Administration approved an expanded indication for HYMPAVZI (marstacimab-hncq), broadening the drug’s use to two additional hemophilia A or hemophilia B populations with significant medical need. Hemophilia is a bleeding disorder caused by impaired blood clotting, with hemophilia A involving factor VIII deficiency and hemophilia B involving factor IX deficiency. For some patients, the immune system develops “inhibitors,” meaning antibodies that can neutralize factor replacement therapies and make bleeding control harder.

Under the expanded label, HYMPAVZI is now approved for routine prophylaxis, intended to prevent or reduce the frequency of bleeding episodes, for patients with hemophilia A or B ages 12 and older who have inhibitors. Pfizer also said the expanded use includes pediatric patients ages 6 to 11 with hemophilia A or B, whether or not they have inhibitors. Pfizer framed the pediatric expansion as especially meaningful for hemophilia B patients ages 6 to 11, calling HYMPAVZI the first subcutaneous, non-factor therapy available for that group in the U.S.

HYMPAVZI is described by Pfizer as an anti-tissue factor pathway inhibitor, or anti-TFPI, therapy. Instead of replacing missing clotting factors, marstacimab-hncq targets TFPI, a natural anticoagulant involved in blood clotting initiation. Pfizer said the regimen combines prophylactic bleed protection with once-weekly subcutaneous administration and, importantly, does not require routine treatment-related lab monitoring. Pfizer also noted that inhibitors arise in about 20% of people with hemophilia A and about 3% of people with hemophilia B.

Pfizer pointed to results from its Phase 3 BASIS trial, which supported the inhibitor portion of the approval for adults and adolescents 12 years and older. In Pfizer’s description, HYMPAVZI reduced the mean treated annualized bleeding rate (ABR) by 93% compared with on-demand (OD) intravenous treatment using bypassing agents, reporting 1.4 versus 19.8 treated bleeds per year (p less than 0.0001). Pfizer said the BASIS KIDS Phase 3 trial supported the pediatric label expansion, with analyses presented for children ages 6 to 17 that compared observed bleeding outcomes on HYMPAVZI to historical model-based comparisons.

For children without inhibitors, Pfizer reported a mean treated ABR of 1.8 (99% confidence interval 1.1 to 2.6) in those treated with HYMPAVZI, versus a historical model-based mean ABR of 3.6 (99% confidence interval 1.3 to 5.8) under routine prophylaxis. For children with inhibitors, Pfizer reported a mean treated ABR of 1.4 (99% confidence interval 0.5 to 4.5) on HYMPAVZI versus a historical model-based mean ABR of 18.9 (99% confidence interval 14.2 to 25.2) on on-demand therapy. Pfizer also reported that thromboembolic events (venous and arterial) occurred in two patients among 259 treated in an open-label extension study.

The company did not disclose pricing, reimbursement expectations, or a specific U.S. launch timeline in the announcement. It also did not provide, in the release text available, the complete set of dosing and monitoring instructions that would appear in an updated prescribing information document, beyond describing once-weekly subcutaneous administration and the trial dosing approach. For the pediatric inhibitor group, Pfizer’s comparisons used descriptive and historical model-based approaches for some analyses rather than a fully randomized direct comparator.

Still, the FDA’s label expansion places HYMPAVZI in a broader role within a hemophilia treatment landscape that increasingly includes non-factor therapies and gene-based options. Pfizer said it is continuing to pursue expanded regulatory approvals in other countries for this indication. Next for market watchers is how quickly clinicians and payers adopt HYMPAVZI for the newly covered inhibitor and younger pediatric groups, and whether additional data submissions refine the pediatric benefit profile over time.