Research Summary

Marstacimab Supports Breakthrough Bleed Control in Hemophilia A or B

Edited by:
Ashton L. Stahl

Key Highlights

  • Of 453 acute breakthrough bleeds, 83.2% were treated with a single factor replacement therapy (FRT) infusion.
  • Marstacimab continued as scheduled during 13 of 15 procedures, and FRT was used for 13 procedures.
  • No thromboembolic events or postoperative bleeding occurred during the 12-month active treatment phase.

Most acute breakthrough bleeds among participants receiving marstacimab prophylaxis for hemophilia A or B without inhibitors were controlled with a single factor replacement therapy (FRT) infusion, according to a phase 3 BASIS study analysis published in the Journal of Thrombosis and Haemostasis. Minor and dental procedures were also generally managed without interrupting marstacimab, and no thromboembolic or postoperative bleeding events occurred during the active treatment phase.

This subanalysis evaluated the noninhibitor cohort of BASIS, an open-label, one-way crossover, multicenter phase 3 trial. After a 6-month observational phase, participants received a 300-mg subcutaneous marstacimab loading dose followed by 150 mg once weekly during a 12-month active treatment phase. Participants meeting protocol-specified weight and spontaneous bleeding criteria after 6 months could escalate to 300 mg once weekly. Investigators descriptively analyzed the incidence of bleeding, FRT use, surgical and medical procedures, and perioperative outcomes.

Study Findings

Among 116 participants who received at least 1 marstacimab dose, 75 (64.7%) experienced 453 acute breakthrough bleeds treated with FRT. Most acute bleeds were spontaneous, accounting for 334 of 453 events, and 365 occurred in joints. Overall, 377 of 453 acute bleeds (83.2%) were managed with a single FRT infusion. Plasma-derived or recombinant standard half-life products were used for 401 of 453 bleeds (88.5%).

Thirteen participants underwent 15 procedures, including 8 dental procedures and 7 other procedures. Marstacimab continued as scheduled during 13 procedures (86.7%), and 13 procedures were managed with FRT. Three participants received marstacimab on the day of the procedure. No postoperative bleeding, thromboembolic events, or deaths occurred. One treatment-related serious adverse event of grade 1 peripheral swelling was reported.

Clinical Implications

According to the study authors, the findings support the feasibility of managing breakthrough bleeds with standard FRT and conducting minor or dental procedures with individualized FRT supplementation during marstacimab prophylaxis. They emphasized that additional studies are needed to establish management strategies for major surgery, optimal perioperative FRT dosing, the long-term safety of combining marstacimab with FRT, and patient-reported outcomes.

The authors noted that participants scheduled for major surgery were excluded, thereby restricting the applicability of the findings to higher-risk procedures. They also identified inconsistent documentation of antifibrinolytic use and geographic differences in FRT availability and dosing as limitations. Pfizer sponsored the study.

Expert Commentary

“Marstacimab prophylaxis effectively supported management of breakthrough bleeds and minor surgical procedures in participants with hemophilia A or B without inhibitors,” the researchers concluded.

Reference

Mahlangu J, Matino D, Benítez Hidalgo O, et al. Management of breakthrough bleeds and surgical procedures in participants with hemophilia A or B without inhibitors receiving marstacimab prophylaxis in the phase 3 BASIS study. J Thromb Haemost. 2026;24:2779-2788. doi:10.1016/j.jtha.2026.04.026