Pfizer disclosed that a participant in a clinical trial of its hemophilia drug Hympavzi died after a cerebellar infarction followed by cerebral hemorrhage. The company notified investigators and stakeholders and said it informed regulators; internal and external safety reviews have been initiated. Pfizer emphasized collaboration with oversight bodies while the cause and any relationship to the investigational therapy are evaluated. The report underscores the heightened vigilance around bleeding and thrombotic risks in hemophilia trials and could prompt protocol amendments, intensified monitoring, or enrollment pauses depending on review outcomes. Sponsors and trial sites will monitor regulatory guidance and investigator communications for next steps.