New York– Pfizer has announced positive results from a Phase 3 trial of HYMPAVZI™ (marstacimab), an investigational therapy for people with hemophilia A or B who have developed inhibitors to standard treatment. The once-weekly subcutaneous injection significantly reduced bleeding episodes compared to current on-demand treatments, according to the company.
In the 12-month BASIS trial, patients receiving HYMPAVZI showed a 93% reduction in the annualized bleeding rate (ABR) compared to those treated with bypassing agents, commonly used when standard factor replacement therapies stop working due to antibody resistance. Participants on HYMPAVZI had an average ABR of 1.39, compared to 19.78 in the control group (p < 0.0001).
The study focused on patients aged 12 and older with severe hemophilia who had developed inhibitors—immune responses that render conventional therapies ineffective. Roughly 20% of people with hemophilia A and 3% with hemophilia B are affected by these inhibitors.
HYMPAVZI, developed by Pfizer scientists, does not replace clotting factors directly. Instead, it targets tissue factor pathway inhibitor (TFPI), a natural mechanism that regulates blood clotting. By interfering with TFPI’s Kunitz 2 domain, the drug aims to restore balance in blood clot formation without relying on factor VIII or IX replacement.
No deaths or thromboembolic events were reported in the trial, and the treatment was generally well tolerated, consistent with previous studies.
“These results are highly encouraging for a group of patients who often have limited and burdensome treatment options,” said Dr. Davide Matino, principal investigator of the study. “Weekly subcutaneous delivery with strong efficacy could represent a meaningful improvement in care.”
Pfizer says it plans to present more detailed results at upcoming medical conferences and will begin discussions with regulatory agencies about potential approval of HYMPAVZI for this patient population.
