Sanofi hit pause on the development program for fitusiran, its Alnylam-partnered RNA-silencing hemophilia treatment, thanks to new side effects seen in clinical trials, a trio of hemophilia organizations announced on Friday.
The company voluntarily placed the global dosing hold after “the identification of new adverse events,” according to a joint statement from the World Federation of Hemophilia, European Hemophilia Consortium and the National Hemophilia Foundation on Friday.
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Sanofi picked up the global rights to fitusiran in 2018 when the French pharma and Alnylam retooled a development deal that had previously divided fitusiran and the now-approved amyloidosis drug Onpattro (patirsiran) along geographic lines. It is developing fitusiran for hemophilia A and B.
This isn’t the first time the program has been paused for safety. Fitusiran ran into an FDA clinical hold in 2017 after a patient with hemophilia A died from a thrombotic event—or blood clot—in a phase 2 study. Alnylam suspended dosing in the trial and worked on a strategy to improve safety monitoring and mitigate the risk of fatal blood clots affecting more patients.
Sanofi announced long-term data from that phase 2 study in June of this year, showing the drug staved off bleeding episodes for up to 57 months, or nearly five years.
People with hemophilia A make a faulty version of a blood clotting factor called factor VIII, or too little of it, while people with hemophilia B make a defective version of factor IX. These patients receive frequent infusions of these clotting factors, often multiple times a week, to control bleeding.
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Given once a month as an injection just under the skin, fitusiran targets antithrombin, a protein that prevents blood clotting. The hope is that lowering antithrombin levels will boost levels of thrombin, an enzyme that causes blood to clot.