FDA Approves Pfizer’s Hympavzi for Hemophilia A, B

The US Food and Drug Administration (FDA) has approved Hympavzi (marstacimab, Pfizer) as routine prophylaxis to prevent or reduce the frequency of bleeding episodes in patients 12 years or older who have hemophilia A without factor VIII inhibitors or hemophilia B without factor IX inhibitors. 

The once-weekly subcutaneous injection targets an anticoagulation protein called tissue factor pathway inhibitor (TFPI). Reducing TFPI’s amount and activity in the blood subsequently increases the amount of thrombin, a pro-clotting enzyme, in circulation.

“Today’s approval of Hympavzi provides patients with hemophilia a new treatment option that is the first of its kind to work by targeting a protein in the blood clotting process,” Ann Farrell, MD, director of FDA’s Division of Non-Malignant Hematology, said in an agency press release. 

Hympavzi is the first non-factor, once-weekly treatment for hemophilia B in the United States. The subcutaneous injection emicizumab (Hemlibra, Genentech), which works by a different mechanism, is already on the market for hemophilia A. 

The current approval was based on the open-label BASIS trial in 116 men and boys with either severe hemophilia A or B without factor inhibitors. 

During the trial’s first 6 months, patients received standard treatment with clotting factor replacement either on-demand (33 patients) or prophylactically (83 patients). Patients were then switched to Hympavzi prophylaxis for a year. 

Among patients receiving on-demand standard treatment during the first 6 months, the annualized bleeding rate was 38 episodes. That rate fell to 3.2 episodes during treatment with Hympavzi. 

Among patients receiving prophylactic standard treatment during the first 6 months, the estimated annualized bleeding rate was 7.85 episodes, which then fell to 5.08 during the year of Hympavzi prophylaxis, FDA said.

Injection-site reactions, headaches, and itching were the most common side effects with marstacimab, occurring in 3% or more of patients. Labeling warns of the potential for circulating blood clots, hypersensitivity, and embryofetal toxicity. Marstacimab is supplied in prefilled syringes. 

Marstacimab is Pfizer’s second hemophilia approval in 2024. FDA approved the company’s hemophilia B gene therapy fidanacogene elaparvovec (Beqvez) in April. 

Pfizer noted in a press release that results for another arm of the BASIS trial in patients with clotting factor inhibitors are expected in 2025. 

M. Alexander Otto is a physician assistant with a master’s degree in medical science and a journalism degree from Newhouse. He is an award-winning medical journalist who worked for several major news outlets before joining Medscape. Alex is also an MIT Knight Science Journalism fellow. Email: aotto@mdedge.com