Pfizer's Stock Plunges 1.31% as Hympavzi Tied to Fatal Adverse Event Trading Volume Ranks 48th

Market Snapshot

, 2025, amid heightened scrutiny over safety concerns tied to its hemophilia drug Hympavzi. , ranking 48th in market activity for the day. The decline reflects investor anxiety following reports of a patient death linked to the drug, which has already faced regulatory and clinical challenges in its development and approval process.

Key Drivers Behind the Decline

The primary catalyst for the stock’s decline was the announcement that a patient participating in a long-term clinical trial of Hympavzi experienced a fatal adverse event. The individual, who had , suffered a cerebellar infarction followed by cerebral hemorrhage while receiving the drug as part of a post-surgical hemostatic management protocol. , prompting Pfizer to initiate a comprehensive review of the case in collaboration with the Data Monitoring Committee and regulatory authorities.

The incident has raised concerns about the safety profile of Hympavzi, a once-weekly subcutaneous therapy approved in 2024 for hemophilia A or B patients without inhibitors. While the drug targets a novel mechanism—blocking the to enhance thrombin generation—it is not yet approved for use in patients with inhibitors, a subset of hemophilia patients who develop antibodies against traditional clotting factor treatments. The patient in question was part of an open-label extension study evaluating Hympavzi’s long-term safety in this unapproved population, compounding risks associated with off-label use.

Pfizer emphasized in statements that the event is under active investigation and that current clinical data do not indicate a broader safety risk for approved patients. However, the company has notified regulatory bodies and is revising surgical management protocols for its marstacimab development program. The fatal outcome aligns with known thrombotic risks of rebalancing therapies like Hympavzi, which have previously been associated with adverse events in perioperative settings. Similar incidents have been reported with other non-factor hemophilia treatments, underscoring the complexity of managing clotting disorders in high-risk scenarios.

The news comes amid a broader period of setbacks for Pfizer’s hematology portfolio. Earlier in 2025, the company discontinued its gene therapy due to low demand and halted the sickle cell disease drug Oxbryta after safety concerns emerged in post-market data. These prior challenges have heightened sensitivity to adverse events in the sector, with investors scrutinizing the company’s ability to navigate clinical and commercial risks. The Hympavzi incident now adds to a narrative of unmet expectations in blood disorder treatments, potentially affecting market confidence in the drug’s long-term viability.

Pfizer’s response has focused on transparency and collaboration, with the company working to gather data on the patient’s underlying conditions, concurrent medications, and potential treatment-related factors. While the firm maintains that the event does not reflect systemic safety issues, the incident has already drawn attention from hemophilia advocacy groups, which have highlighted the need for caution in off-label applications of the drug. The outcome of the ongoing review will likely influence regulatory scrutiny and investor sentiment, with implications for Hympavzi’s market trajectory and Pfizer’s broader pipeline in rare diseases.