The Coalition for Hemophilia B Statement on Marstacimab (Hympavzi®)

December 23, 2025 - The Coalition for Hemophilia B (CHB) was informed by Pfizer of a severe adverse event involving marstacimab (Hympavzi®).

The event involved an individual with hemophilia A who suffered a fatal thrombotic stroke following minor surgery. The individual had been receiving marstacimab prophylaxis for approximately three years and was participating in a long-term open-label extension clinical trial at the time of the event. The adverse event occurred outside of the United States.

Marstacimab is a rebalancing therapy that works by targeting tissue factor pathway inhibitor (TFPI) to increase thrombin generation and clot formation. Due to this mechanism of action, thrombotic events are a known adverse event of special interest, particularly in surgical settings or when used in combination with other hemostatic therapies.

Pfizer is actively investigating the circumstances surrounding this tragic event to better understand causality. The company has also confirmed that all patient support resources and informational pages will remain available during the investigation.

We extend our deepest condolences to the individual’s family and loved ones.

CHB appreciates Pfizer’s transparency and timely communication and remains committed to keeping the community informed. Individuals with questions or concerns about their treatment plans are encouraged to speak directly with their hemophilia treatment center or healthcare provider. Please see Pfizers statement below.