Alnylam, in conjunction with its development partner Sanofi, is developing fitusiran for the treatment of hemophilia A and B. Fitusiran is one of the most advanced drugs that acts via RNA interference (RNAi), specifically targeting antithrombin (AT) mRNA. It has been shown that AT deficiency can improve thrombin generation and promote hemostasis in hemophilia. Acting at the final common pathway in the clotting cascade, fitusiran’s mechanism is therefore amenable to all hemophilia patients.
In September 2017, Alnylam and Sanofi disclosed a fatal thromboembolic event in a hemophilia A patient in an open-label extension of a Phase II trial. At this point, development was suspended pending further investigation. After the clinical hold was lifted in December 2017, dosing restarted in both the newly initiated Phase III ATLAS program, and the Phase II open-label extension. Alnylam and the US Food and Drug Administration reached an agreement on new clinical risk mitigation measures, including reducing the dose of replacement factor or bypassing agent used to treat any breakthrough bleeds.
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