Pfizer Reports Strong Phase 3 Results for Hympavzi in Hemophilia A and B With Inhibitors

Pfizer Inc has announced compelling topline results from the phase 3 BASIS study evaluating hympavzi (marstacimab) in patients 12 years and older with hemophilia A or B who have developed inhibitors to standard factor replacement therapies. The findings highlight both statistically significant and clinically meaningful reductions in bleeding episodes, reinforcing hympavzi’s potential as a transformative prophylactic therapy.

The BASIS study evaluated Hympavzi in a cohort of 48 individuals over 12 months, contrasting outcomes against a 6-month lead-in period of on-demand treatment using bypassing agents. Hympavzi demonstrated a 93% reduction in annualized bleeding rate (ABR), achieving an ABR of 1.39 compared to 19.78 for on-demand regimens (P < .0001). Superiority was consistent across all key bleeding-related secondary endpoints, including spontaneous bleeds, joint bleeds, target joint bleeds, and total bleeds.

Inhibitors are neutralizing antibodies that render factor replacement therapies ineffective, complicating management and increasing disease burden. Approximately 20% of patients with hemophilia A and 3% patients with hemophilia B patients develop these inhibitors, making effective prophylaxis a critical unmet need. Hympavzi, administered as a once-weekly subcutaneous injection, offers a low-preparation alternative to burdensome intravenous therapies.

“Patients with inhibitors tend to face frequent complications, and navigating the treatment landscape can introduce complexities and increase disease burden,” said Davide Matino, MD, MSc, principal investigator for the BASIS study. “The strong bleed reduction with Hympavzi compared to on-demand treatment…offers exciting potential for these patients who are in critical need of treatment options.”

Hympavzi was generally well-tolerated in the study, with no deaths or thromboembolic events reported. The safety findings align with prior studies, including those involving non-inhibitor cohorts. The therapy is delivered via a pre-filled pen, requiring minimal preparation and eliminating the need for venous access.

Hympavzi’s differentiated mechanism of action targets tissue factor pathway inhibitor (TFPI), rather than replacing clotting factors. This strategy helps restore hemostatic balance in patients with inhibitors by enabling clot formation through a distinct biological pathway.

Payers and managed care organizations should take note of several key factors relevant to future coverage considerations:

• Clinical superiority: The significant ABR reduction supports hympavzi’s role as a prophylactic standard in inhibitor-positive patients, a high-burden subgroup with limited options.
• Treatment adherence potential: The once-weekly subcutaneous format may improve patient compliance, historically hindered by the challenges of intravenous therapy.
• Economic impact: By reducing bleeding episodes and associated complications, hympavzi may lower the long-term health care utilization and costs tied to hospitalizations and joint damage.
• Regulatory positioning: Pfizer plans to initiate regulatory filings based on these results, which, if approved, would expand hympavzi’s existing indication in patients with hemophilia without inhibitors.

Pfizer is continuing full analysis of the phase 3 dataset and will present additional data at upcoming medical meetings.

Reference

Pfizer. Pfizer announces positive topline phase 3 results for hympavzi™ in hemophilia A or B with inhibitors. Accessed July 1, 2025. Published June 26, 2025. https://www.pfizer.com/news/press-release/press-release-detail/pfizer-announces-positive-topline-phase-3-results?cid=em_PfizerNewsroomAlert&ttype=em