Pfizer disclosed a participant fatality in a long‑term extension trial of its tissue factor pathway inhibitor antagonist Hympavzi (marstacimab), reporting a cerebellar infarction followed by cerebral hemorrhage. The company notified the hemophilia community and regulatory stakeholders while the case is under investigation to assess causality and any trial safety implications. Pfizer’s communication signals heightened attention to thrombotic and cerebrovascular risks in trials of hemostasis‑modulating agents. The company and independent monitors will review the event, and regulators may request additional safety measures or data before further enrollment or approvals. The disclosure has prompted broad monitoring across ongoing hemophilia programs.