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The Medicines and Healthcare products Regulatory Agency has approved marstacimab (Hympavzi, Pfizer) to prevent and reduce bleeding in patients with severe haemophilia A or B.

The treatment is indicated for patients aged 12 years or older who weigh at least 35 kg and have not developed inhibitors to factor VIII or IX.

Haemophilia A and B are rare genetic bleeding disorders caused by deficiency of coagulation factors: Factor VIII for haemophilia A and factor IX for haemophilia B.

Between 2022 and 2023, the UK recorded 9316 cases of haemophilia A and 2069 cases of haemophilia B, according to national registry data. Severe cases in women are rare.

Mechanism of Action

Marstacimab is a human monoclonal antibody that targets tissue factor pathway inhibitor. This inhibition enables an alternative blood clotting pathway that does not involve factor VIII or factor IX.

The drug is administered via subcutaneous injection using a pre-filled syringe or pen. The recommended dose is 300 mg initially, followed by 150 mg once weekly.

A higher weekly dose of 300 mg may be considered for patients weighing > 50 kg.

Phase 3 Trial Results

MRHA approval was based on results from the phase 3 BASIS trial. The study included 116 male patients with severe haemophilia A or moderately severe to severe haemophilia B, all without inhibitors to factor VIII or IX.

Participants received either marstacimab weekly for 12 months or standard treatment — either routine prophylaxis or an on-demand intravenous regimen with factor VIII or factor IX — for 6 months.

Primary endpoints were annualised bleeding rate (ABR) and safety outcomes. Marstacimab reduced ABR by 35% compared with routine prophylaxis and by 92% compared with on-demand treatment.

For secondary endpoints, the drug showed superiority in bleeding-related outcomes compared with on-demand treatment and noninferiority to routine prophylaxis. Quality-of-life improvements were noninferior to prophylaxis but not significant compared with on-demand therapy.

An extension study followed 87 participants for an additional 16 months. It confirmed consistent reductions in ABR compared with both standard treatment approaches.

Safety Profile

Marstacimab was generally well-tolerated. Its safety profile was consistent with earlier phase 1 and 2 data. No major differences were observed between adult and adolescent participants.

Common adverse events included COVID-19, haemorrhage, hepatic disorder, hypersensitivity, hypertension, and injection site reactions.

Marstacimab was also recommended for approval by the European Medicines Agency in September 2024 and authorised by the US Food and Drug Administration in October.

Prescribing guidance for marstacimab is available in the summary of product characteristics published on the MRHA’s website.

Annie Lennon is a medical journalist. Her writing appears on Medscape Medical News, WebMD, and Medical News Today, among other outlets.