22 December 2025
Pfizer has announced the death of a participant enrolled in the long-term open-label extension study (B7841007) of marstacimab in individuals with haemophilia A or B, with or without inhibitors. The participant died on December 14, 2025, after experiencing serious adverse events, including a cerebellar infarction followed by cerebral haemorrhage.
The individual, who had haemophilia A with inhibitors, previously participated in the active treatment phase of the parent trial (B7841005) in 2022 and transitioned into the extension study in 2023. The fatal event occurred during the extension period while the participant was receiving marstacimab prophylaxis followed by recombinant factor VIIa administered for perioperative haemostatic management after minor surgery.
Pfizer, working with the study investigator and an independent external Data Monitoring Committee, is conducting a comprehensive review to assess the circumstances of the event. This includes evaluation of underlying medical conditions, concurrent medications, and the potential relationship to study treatment. Relevant regulatory authorities and investigators have been notified, and surgical management protocols within the marstacimab development program are under review.
Thrombotic events are considered an adverse event for rebalancing therapies such as marstacimab, which acts by inhibiting tissue factor pathway inhibitor (TFPI) to enhance thrombin generation. Similar events have been reported with other non-factor haemophilia treatments, particularly in perioperative contexts and when used in combination with clotting factor concentrates or bypassing agents.
Marstacimab has received regulatory approval from the European Medicines Agency for the treatment of haemophilia A or B in patients without inhibitors. Its use in patients with inhibitors has not been approved and continues to be evaluated in clinical studies.
The EHC will continue to follow developments and review any additional information shared by regulatory authorities and Pfizer as the investigation progresses.
