FDA Grants Priority Review For Pfizer’s Marstacimab for Hemophilia A or B

FDA granted priority review for the supplemental Biologics License Application (sBLA) for marstacimab, marketed as HYMPAVZI.¹ This medicine is intended for the treatment of patients with hemophilia A or B, and the application seeks to expand the current indication to include patients aged 6 and older with inhibitors, as well as pediatric patients aged 6 to 11 without inhibitors. This review comes as the landscape of hemophilia treatment is shifting toward non-factor prophylactic options that simplify administration and address biological resistance.

Marstacimab is notable as the first anti-tissue factor pathway inhibitor (TFPI) approved in the United States and the European Union for hemophilia A or B. Unlike traditional factor replacement therapies that replace missing factor VIII or factor IX, marstacimab targets the Kunitz 2 domain of TFPI.¹ This mechanism is intended to re-establish the balance between bleeding and clot formation. The product's use of a pre-filled, once-weekly subcutaneous auto-injector pen is a critical design element, as it eliminates the need for the routine treatment-related lab monitoring and complex preparation often associated with intravenous infusions.

How Do Recent Clinical Trial Outcomes Address the Challenges of Pediatric Patients?

The clinical evidence supporting these expanded indications stems from the BASIS and BASIS KIDS Phase 3 trials. These studies address populations with a high degree of unmet medical need, particularly those who have developed inhibitors, referring to antibodies that neutralize factor replacement therapies and render them ineffective. Michael Vincent, MD, PhD, chief inflammation & immunology officer at Pfizer, emphasized the importance of these developments in a press release:¹ “There is a significant medical need for younger patients with hemophilia and for those who have developed inhibitors, which neutralize factor replacement therapies and render them ineffective.” He further noted, “Based on the findings in the BASIS clinical trial program and if approved, we believe HYMPAVZI has the potential to become a transformative option for these patients that have limited or burdensome treatment options today. We look forward to progressing discussions with regulators to make this medicine available for patients.”

Repeated bleeding episodes in children can lead to permanent joint damage due to the vulnerability of growing cartilage and bone. Guy Young, MD, director of the Hemostasis and Thrombosis Center at Children's Hospital, Los Angeles, explained the necessity of preventative care for this demographic,¹ “for children living with hemophilia A or B between ages 6 and 11, treatment approaches that prevent bleeds are particularly important to protect growing joints.” He added that “HYMPAVZI would address a critical unmet medical need for these patients and those with inhibitors if approved, particularly patients ages 6 to 11 with hemophilia B who do not have non-factor treatment options available today.”¹

The BASIS trial evaluated the efficacy and safety of marstacimab by measuring the treated annualized bleeding rate over a 12-month active treatment period. In the BASIS KIDS study, 68 patients aged 6-11 were treated with a regimen consisting of a 150 mg loading dose followed by 75 mg once weekly.¹ These trials are essential to understanding the safety profile of anti-TFPI therapies, which may include risks such as thromboembolic events or allergic reactions. Beyond the United States, the use of marstacimab for patients 12 years and older with inhibitors is also under review by the EMA. This global regulatory activity underscores the industry's drive to provide stable, subcutaneous alternatives for more than 800,000 people living with hemophilia worldwide.

How Are Other Pfizer Biologics Progressing?

Beyond advancements in hematology, the pharmaceutical industry is witnessing a parallel evolution in long-acting metabolic therapies. Topline results from the Phase 2b VESPER-3 trial for the GLP-1 receptor agonist PF-08653944 demonstrated that switching from weekly titration to monthly maintenance dosing achieved significant weight loss, reaching 12.3% at 28 weeks.²

Why Does the Transition to Monthly Maintenance Dosing Matter for Metabolic Therapy?

A four-fold reduction in dosing frequency addresses significant barriers to patient adherence and simplifies supply logistics. Jim List, MD, PhD, stated in a press release,² “These topline results from the Phase 2b VESPER-3 study reinforce the potential of PF’3944 as a monthly treatment with competitive efficacy.” Dr. List noted that PF-08653944 serves as a key anchor in addressing critical care gaps.

How Does the Expansion of Long-Acting Injectable Pipelines Influence Development Strategies?

Advancing 10 Phase 3 trials in 2026 highlights a strategic push to address the global obesity epidemic and its associated comorbidities.² This shift toward monthly subcutaneous options within the GLP-1 receptor agonist class reflects a developmental priority to provide less burdensome treatment alternatives for chronic metabolic conditions.

References

1. Pfizer. FDA Grants Priority Review for HYMPAVZI® (marstacimab) sBLA for the Treatment of Two Hemophilia A or B Patient Populations with Significant Medical Need. Press Release. Feb 6, 2026.
2. Pfizer. Pfizer’s Ultra-Long-Acting Injectable GLP-1 RA Shows Robust and Continued Weight Loss with Monthly Dosing in Phase 2b Trial. Press Release. Press Release. Feb 3, 2026.