Written By: Pharmacally Medical News Desk
Pfizer has disclosed a death in an ongoing clinical trial evaluating Hympavzi (marstacimab), according to a formal safety letter circulated to the global hemophilia community and published on the website of the World Federation of Hemophilia (WFH).
The letter, dated December 22, 2025, reports that a participant enrolled in a long-term extension study of marstacimab experienced a cerebellar infarction, followed by a cerebral hemorrhage, and subsequently died. Pfizer stated that the event has been classified as a serious adverse event and is currently under investigation by the study investigator, the sponsor, and an independent Data Monitoring Committee. Regulatory authorities have also been notified.
According to the communication, the patient was receiving marstacimab as part of the trial protocol. At this stage, Pfizer has not established a causal relationship between the investigational therapy and the fatal event, and the company emphasized that the evaluation is ongoing.
As of now, Pfizer has not issued a standalone press release or public safety update beyond this community communication.
This report represents an early safety disclosure rather than a finalized regulatory or clinical conclusion. Pfizer has stated that further updates will be shared as the investigation progresses. Until additional details are released by the company or health authorities, the information should be viewed as preliminary.
About Marstacimab
Marstacimab, marketed as Hympavzi, is a monoclonal antibody targeting tissue factor pathway inhibitor (TFPI) and is designed to reduce bleeding episodes in people with hemophilia A or B.
In the United States, the Food and Drug Administration (FDA) approved Hympavzi (marstacimab-hncq) on October 11, 2024. The approval covers routine prophylaxis to prevent or reduce the frequency of bleeding episodes in adult and pediatric patients (12 years and older) with hemophilia A or hemophilia B who do not have factor VIII or factor IX inhibitors.
References
WFH and NBDF statement on severe adverse event with marstacimab rebalancing agent for hemophilia, World Federation of Haemophilia, 22 December 2025, https://wfh.org/wp-content/uploads/2025/12/WFH-NBDF-Statement-on-Marstacimab-Thrombotic-Event-12-22-2025_Final.pdf
Pfizer Letter to Hemophilia Community Partners, 22 December 2025, https://wfh.org/wp-content/uploads/2025/12/Letter_Marstacimab-Serious-Adverse-Event_12.22.2025.pdf