The Coalition for Hemophilia B Update on Marstacimab (Hympavzi®)

January 21, 2026

In December, we shared the difficult news of the passing of a participant with hemophilia A with inhibitors who was enrolled in a long term extension clinical trial evaluating marstacimab. We know this news was deeply concerning, and we want to provide a brief update based on information currently available.

According to information shared with us by the study sponsor, Pfizer, the circumstances surrounding this event are complex and involved multiple contributing factors. These include co existing medical conditions, the use of concomitant therapies, and a recent medical procedure. An independent external Data Monitoring Committee, along with the study investigator and sponsor, is actively reviewing the case to better understand what occurred.

At this time, no identifying or sensitive patient information can be shared. This review process is ongoing, and additional scientific details are expected to be evaluated and presented at an upcoming medical congress in February.

We remain committed to transparency and to sharing information with the community as it becomes available through appropriate scientific and regulatory channels. We also recognize that news like this can raise questions and concerns, and we encourage individuals and families to discuss any questions they may have about clinical trials or treatment options with their healthcare providers.

Most importantly, we continue to hold the participant and their family in our thoughts. Every individual in our community matters, and any loss is felt deeply.

We will provide further updates if and when additional information is appropriate to share.