This clinical trial is focused on studying the effects of a medication called fitusiran in individuals with severe hemophilia A or B. Hemophilia is a genetic disorder that affects the blood’s ability to clot, leading to excessive bleeding. The study aims to understand how well fitusiran can prevent bleeding episodes in male participants aged 12 years and older, whether or not they have developed inhibitory antibodies to factor VIII or IX, which are proteins involved in blood clotting.

Participants in the study will receive fitusiran, which is administered as an injection under the skin. The study will observe the frequency of bleeding episodes while participants are on fitusiran treatment. The trial will also compare the bleeding rates during the fitusiran treatment period with those during the standard care period, which may involve other treatments. The study will last for a period of up to 36 months, during which participants will be monitored for any changes in their bleeding patterns and overall health.

In addition to fitusiran, the study will involve other treatments such as coagulation factor VIII, antithrombin III, coagulation factor IX, eptacog alfa (activated), and factor VIII inhibitor bypassing fraction. These treatments are typically used to help manage bleeding in people with hemophilia. The study will help determine the effectiveness and safety of fitusiran in reducing bleeding episodes compared to these other treatments. Participants will be closely monitored for any side effects or adverse events throughout the study.