FDA Approves Alhemo (Fitusiran) for Hemophilia A and B: First Monthly Subcutaneous Treatment

What Is Fitusiran and How Does It Work for Hemophilia?

Fitusiran, developed by Sanofi under the brand name Alhemo, has received FDA approval for routine prophylaxis to prevent or reduce the frequency of bleeding episodes in adults and adolescents with hemophilia A or B, with or without factor inhibitors. Unlike traditional factor replacement therapies that supplement the missing clotting factor, fitusiran uses RNA interference (RNAi) technology to silence the gene encoding antithrombin (SERPINC1) in hepatocytes, reducing antithrombin levels and thereby lowering the threshold for thrombin generation.

This mechanism is particularly significant for the estimated 25–30% of severe hemophilia A patients who develop neutralizing antibodies (inhibitors) against factor VIII, rendering standard factor replacement ineffective. In the Phase 3 ATLAS-INH trial, fitusiran demonstrated a substantial reduction in annualized bleeding rate (ABR) compared to on-demand bypassing agents in inhibitor patients. Similarly, in the ATLAS-A/B trial enrolling non-inhibitor patients, fitusiran significantly reduced ABR compared with on-demand factor replacement. The once-monthly subcutaneous injection offers a marked convenience advantage over the frequent intravenous infusions required by factor concentrates.

How Does Alhemo Compare to Hemlibra and Other Hemophilia Treatments?

Roche's emicizumab (Hemlibra), first approved by the FDA in 2017, established the paradigm for non-factor prophylaxis in hemophilia A with inhibitors and has become a dominant prophylactic therapy globally. Fitusiran and emicizumab have not been compared in a head-to-head trial, but both have demonstrated low annualized bleeding rates in their respective clinical programs. Importantly, fitusiran covers both hemophilia A and B, whereas emicizumab is only indicated for hemophilia A, giving fitusiran a broader addressable population including hemophilia B patients worldwide.

Safety monitoring has been a central focus following a clinical hold in 2017 after a fatal cerebral sinus thrombosis during early trials. The revised dosing protocol, designed to target moderate antithrombin reduction rather than near-complete suppression, has shown an improved safety profile across the ATLAS program. Thromboembolic events remained a monitored risk but occurred at low rates in later trials. Sanofi has priced Alhemo in a range comparable to other non-factor prophylactic therapies, and the company has indicated willingness to enter outcomes-based contracts with major insurers.