SHERIDAN, WYOMING – December 10, 2025 – For many people living with hemophilia, treatment is already demanding—regular infusions, constant planning, and the worry of a bleed that can disrupt everything. But for those who develop “inhibitors,” standard therapies may stop working, turning daily life into a much more dangerous balancing act. New Phase 3 data from Pfizer now suggest that the once-weekly treatment HYMPAVZI® (marstacimab) could offer these patients a very different outlook.
When hemophilia treatments suddenly stop working
Hemophilia A and B are rare genetic bleeding disorders where the blood can’t clot properly. For decades, the usual approach has been factor replacement therapy, giving patients the clotting factor their bodies are missing.
For some, though, the body develops “inhibitors”—antibodies that neutralize these replacement factors and make them ineffective. That can mean more bleeds, more joint damage, more hospital visits and a much heavier emotional and physical burden on patients and families.
“The emergence of inhibitors poses significant treatment challenges and can increase disease burden for people living with hemophilia A or B,”4 said Davide Matino, M.D., M.Sc., BASIS Principal Investigator, Associate Professor of Medicine, 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.”
What the HYMPAVZI study found
The Phase 3 BASIS study followed adults and adolescents with severe hemophilia A or hemophilia B who had developed inhibitors. First, participants were observed for six months on their usual on-demand intravenous treatment with bypassing agents. Then, during a 12-month active treatment phase, 48 participants received HYMPAVZI: a 300 mg loading dose followed by a 150 mg once-weekly subcutaneous injection.
Compared with their earlier on-demand therapy, patients on HYMPAVZI saw:
- A 93% reduction in mean treated annualized bleeding rate (ABR)
- A median ABR of 0 on HYMPAVZI versus 16.42 on prior treatment
- Superiority across all bleeding-related endpoints, including spontaneous bleeds, joint bleeds, target joint bleeds, and total treated and untreated bleeds
Importantly for everyday life, HYMPAVZI is given as a once-weekly injection under the skin, with minimal preparation and no routine treatment-related lab monitoring. For people used to frequent IV infusions and vein access challenges, that kind of simplification can feel like getting a piece of normal life back.
Beyond numbers: what fewer bleeds can mean day to day
The BASIS trial also looked at health-related quality of life—and here, too, HYMPAVZI showed meaningful improvements. Using validated tools like the Haem-A-QoL and EQ-5D-5L, patients reported:
- Less physical burden from pain, swelling and movement difficulties
- Better overall scores across multiple life domains and daily activities
- Improvements in mobility, self-care, usual activities and discomfort levels
While every person’s experience is unique, fewer bleeds and easier dosing can translate into practical things: more confidence at work or school, less fear around travel, and more freedom to plan family life without constantly checking treatment schedules and infusion supplies.
HYMPAVZI was generally well tolerated in the study, with no deaths or thromboembolic (blood clot) events reported in the safety population. Most side effects were mild or moderate, with the most common including COVID-19, upper respiratory infections, increases in a blood clotting marker (fibrin D-dimer) and headache. One treatment-related serious rash led to study discontinuation and resolved.
“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, M.D., Ph.D., Chief Inflammation & Immunology Officer, 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.”
Mini FAQ: what this means for patients and families
Is HYMPAVZI already approved for people with inhibitors?
Pfizer has submitted these Phase 3 data to regulators including the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) for review. Approval decisions for patients with inhibitors are still pending and may vary by country.
Who is HYMPAVZI currently approved for?
HYMPAVZI is currently approved in more than 40 countries for eligible patients 12 years and older with hemophilia A without factor VIII inhibitors, or hemophilia B without factor IX inhibitors. Always check your local prescribing information and talk with your hemophilia care team.
How does HYMPAVZI work?
Unlike factor replacement, HYMPAVZI targets tissue factor pathway inhibitor (TFPI), one of the body’s natural “brakes” on clotting. By blocking a specific part of TFPI, it aims to rebalance bleeding and clot formation, helping protect against bleeds with once-weekly injections.
Is HYMPAVZI right for me or my child?
Only your hematologist or specialist team can answer that. They’ll consider your bleeding history, inhibitor status, other health conditions and personal preferences around treatment schedules and administration.
For anyone living with hemophilia and inhibitors, these new data offer a cautiously optimistic signal: a potential future where bleed protection, safety and simpler dosing can coexist—and where life plans are shaped more by personal goals than by bleeding episodes.
Learn more about HYMPAVZI and Pfizer’s work in hemophilia at https://www.Pfizer.com.