Global pharma giant Pfizer is touting a major breakthrough for patients with rare blood disease hemophilia who face some of the toughest treatment challenges.

The company announced the data of its Phase 3 trial showing its investigational therapy HYMPAVZI (marstacimab) slashed bleeding rates in patients with hemophilia A or B who develop inhibitors, by more than 90% compared with standard on-demand treatment.

Results from the pivotal BASIS study have been unveiled at the 67th American Society of Hematology (ASH) Annual Meeting in Orlando and simultaneously published in Blood.

Inhibitors—antibodies that neutralize factor replacement therapies—can render standard treatments ineffective and leave patients with limited options. Globally, more than 800,000 people live with hemophilia, and about 20% of those with hemophilia A and 3% with hemophilia B develop inhibitors that block FVIII or FIX therapies.

“The emergence of inhibitors poses significant treatment challenges and can increase disease burden for people living with hemophilia A or B,” said Davide Matino, BASIS Principal Investigator and Associate Professor of Medicine at McMaster University.

“In patients with inhibitors, this study demonstrates HYMPAVZI’s potential as a safe and efficacious treatment option that not only significantly reduced bleeding episodes via a once-weekly subcutaneous administration, but also demonstrated improvement in certain aspects of health-related quality of life.”

HYMPAVZI’s safety profile was generally favorable. No deaths or thromboembolic events occurred among 51 participants in the safety group. Most adverse events were mild or moderate; only one serious treatment-related event—rash—led to discontinuation.

“It is encouraging that these data demonstrate the potential of HYMPAVZI to combine efficacy, safety, and straightforward administration for adults and adolescents living with hemophilia A or B with inhibitors and address a significant patient need,” said Michael Vincent, Chief Inflammation & Immunology Officer at Pfizer.

“We look forward to potentially making this treatment available for these patients as Pfizer continues its ongoing effort spanning more than 40 years to improve hemophilia care.”

Pfizer has submitted the new data to both the US FDA and the European Medicines Agency. HYMPAVZI is already cleared in more than 40 countries for patients with hemophilia A or B without inhibitors.

HYMPAVZI works differently from traditional factor-replacement therapies: instead of replacing missing clotting factors, it blocks tissue factor pathway inhibitor (TFPI), a natural brake on the body’s clotting response. By targeting TFPI’s Kunitz 2 domain, it aims to restore clotting balance and prevent bleeds using a simple once-weekly injection, as per Pfizer.