Pfizer Reports Patient Death In Marstacimab Hemophilia Extension Study

Pfizer Inc. (NYSE:PFE) disclosed the death of a participant in a long-term open-label extension study of marstacimab in patients with hemophilia A or B.

In October 2024, the FDA approved Pfizer’s Hympavzi (marstacimab-hncq) for routine prophylaxis to prevent or reduce the frequency of bleeding episodes in adults and pediatric patients with hemophilia A (congenital factor VIII deficiency) or hemophilia B (congenital factor IX deficiency).

European Hemophilia Consortium (EHC), a non-profit, non-governmental organization, reported the update.

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Details Of The Fatal Adverse Event

The patient died on December 14, 2025, after serious adverse events, including a cerebellar infarction followed by cerebral hemorrhage.

The individual had hemophilia A with inhibitors and had previously completed the active treatment phase of the parent trial before entering the extension study.

The fatal event occurred during the extension period while the patient was receiving marstacimab prophylaxis and recombinant factor VIIa for perioperative bleeding management following minor surgery.

Pfizer, in coordination with the study investigator and an independent Data Monitoring Committee, is conducting a detailed review to determine contributing factors, including underlying conditions, concomitant medications, and any potential link to treatment.

Regulatory authorities have been notified, and surgical management protocols across the marstacimab program are being reassessed.

Known Thrombotic Risks With Rebalancing Therapies

Thrombotic events are a known risk for rebalancing therapies such as marstacimab, which inhibits tissue factor pathway inhibitor to increase thrombin generation.

Similar events have been reported with other non-factor hemophilia therapies, particularly around surgical procedures and when combined with clotting factors or bypassing agents.

Marstacimab is approved in Europe for patients with hemophilia A or B without inhibitors. Its use in patients with inhibitors remains investigational and is still under clinical evaluation.

Phase 3 BASIS Study Results

In June, Pfizer released topline results from the Phase 3 BASIS study evaluating marstacimab for adults and adolescents living with hemophilia A or B with inhibitors.

The study met the primary endpoint and key secondary bleeding endpoints, demonstrating the superiority of once-weekly subcutaneous Hympavzi in improving key bleeding outcomes compared to on-demand treatment in a patient population where less burdensome treatment approaches are needed.

In December, Pfizer presented results from the Phase 3 BASIS study evaluating marstacimab for adults and adolescents living with hemophilia A or B with inhibitors.

The results demonstrated the superiority of Hympavzi in improving key bleeding outcomes compared to on-demand treatment with bypassing agents.

PFE Price Action: Pfizer shares were down 0.73% at $25.02 at the time of publication on Tuesday, according to Benzinga Pro data.

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