Fitusiran: The first approved siRNA therapy for hemophilia via reducing plasma antithrombin levels

Abstract

Hemophilia is a coagulation disorder caused by deficiencies in clotting factors and is primarily classified into hemophilia A (factor VIII deficiency) and hemophilia B (factor IX deficiency). Current treatment relies predominantly on replacement therapy, where patients receive clotting factor concentrates either episodically or prophylactically to achieve hemostasis or mitigate bleeding risks. Despite its efficacy, this approach presents limitations, including suboptimal treatment accessibility due to high costs, reduced patient compliance from frequent intravenous administration, and therapeutic failure in patients developing plasma inhibitors. These challenges underscore the need for novel therapeutics addressing unmet clinical demands. On March 28, 2025, the US Food and Drug Administration (FDA) approved fitusiran, a small interfering RNA (siRNA) therapeutic for hemophilia. This first-in-class agent demonstrates pan-hemophilia efficacy by targeting antithrombin to enhance thrombin activity, irrespective of factor VIII/IX deficiency status or plasma inhibitor presence. By pioneering a mechanism of antithrombin suppression, enabling sustained therapeutic action, and facilitating precision monitoring protocols, fitusiran has the potential to redefine hemophilia treatment paradigms.