On December 23 local time, Pfizer Inc. announced that a patient participating in a long-term study of its hemophilia drug had died following severe side effects. Pfizer’s stock closed down more than 1% on the same day.
According to the European Haemophilia Consortium, a patient support organization, the patient died on December 14 from a brain hemorrhage following a stroke.
The long-term study involved Pfizer's long-acting hemophilia therapy, Hympavzi. The drug, administered via weekly injections, was approved in the United States last year for the treatment of patients aged 12 and older with hemophilia A or B. It works by targeting specific clotting proteins to prevent or reduce bleeding episodes.
Regarding the fatality, Pfizer responded in a statement: "Pfizer, along with trial investigators and an independent external Data Monitoring Committee, is actively gathering information to better understand the complex and multifactorial circumstances surrounding this event."
The company further stated that, based on its current understanding and the aggregate clinical data collected to date, it does not expect this incident to impact the overall safety profile for patients receiving the treatment.
Hemophilia is caused by defects in the genes responsible for regulating clotting factors, leading to spontaneous or severe bleeding after injury or surgery. In addition to long-acting factor therapies, gene therapies have emerged in recent years as another treatment option.
Previously, Pfizer developed Beqvez, a one-time gene therapy for adults with moderate-to-severe hemophilia B, which received U.S. marketing approval. However, due to high costs and weak demand for such treatments, Pfizer announced earlier this year that it would cease the global development and commercialization of its hemophilia gene therapy programs.

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