A major global clinical trial has offered new hope to people living with haemophilia who struggle with frequent, painful bleeding episodes and treatment failure. Pfizer’s Phase 3 BASIS study, presented at the 67th American Society of Haematology Meeting in Orlando, reports that its investigational drug HYMPAVZI (marstacimab) cut treated bleeding episodes by an impressive 93 percent in adults and adolescents who carry inhibitors — a group that often has the fewest treatment options and the highest disease burden.

Haemophilia A and B are lifelong bleeding disorders caused by the lack of clotting proteins, known as factor VIII and factor IX. Most patients rely on “factor replacement therapy”, meaning the missing protein is infused into the blood to help clots form. However, some patients develop “inhibitors”, which are antibodies that neutralise these treatments, making them ineffective. This circumstance affects about 20 per cent of people with haemophilia A and about 3 per cent with haemophilia B. For families, this means more hospital visits, more pain, and more fear.

In the new trial, 48 participants with severe haemophilia and inhibitors received HYMPAVZI as a weekly “subcutaneous injection”, meaning it is given under the skin rather than into a vein. This method requires minimal preparation and no regular laboratory monitoring, reducing the burden of care. Before starting the drug, all participants had been on “bypassing agents”, a type of rescue medicine used when standard factor therapy stops working.

The findings are striking. Patients on HYMPAVZI reported a mean annualised bleeding rate (ABR) of 1.39 compared with 19.78 on on-demand therapy. The annualised bleeding rate is a simple measure of how often a patient bleeds in a year. The trial also showed a median ABR of zero among patients on HYMPAVZI, demonstrating the drug’s ability to prevent most bleeding events. Improvements were seen across spontaneous bleeds, joint bleeds, and target joint bleeds.

“The emergence of inhibitors poses significant treatment challenges and can increase disease burden for people living with haemophilia A or B,” said Davide Matino, M.D., M.Sc., Principal Investigator. “In patients with inhibitors, this study demonstrates HYMPAVZI’s potential as a safe and efficacious treatment option… and improvement in certain aspects of health-related quality of life.”

Participants also recorded better scores on standard quality-of-life measures, indicating reduced pain, better mobility, and easier daily functioning. Safety findings were reassuring, with most adverse events mild or moderate and no deaths reported.

Michael Vincent, M.D., Ph.D., of Pfizer, noted, “It is encouraging that these data demonstrate the potential of HYMPAVZI to combine efficacy, safety, and straightforward administration… and address a significant patient need.”

Pfizer has submitted the data to the U.S. FDA and European Medicines Agency. For people in low- and middle-income countries, where access to advanced therapies is limited, such innovations may signal a future where treatment becomes simpler, safer, and more protective — provided affordability and availability do not widen existing health inequities.