A Study to Test a Medicine (Fitusiran) for Preventing Bleeds in People With Severe Hemophilia Who Previously Received Preventive Treatment With Emicizumab

Who May Qualify: - Male participants must be ≥12 years of age inclusive, at the time of signing the willing to sign a consent form - Diagnosis of severe congenital hemophilia A (FVIII \< 1%) as evidenced by a central laboratory measurement at screening or documented medical record evidence. - Inhibitor titer of ≥0.6 BU/mL at Screening, or - Inhibitor titer of \<0.6 BU/mL at Screening with medical record evidence of 2 consecutive titers ≥0.6 BU/mL, or - Inhibitor titer of \<0.6 BU/mL at Screening with medical record evidence of anamnestic response. - Participants who are currently on the full labeled dose of emicizumab prophylaxis, irrespective of inhibitor/non-inhibitor status. - Signed willing to sign a consent form/assent which includes compliance with the requirements and restrictions listed in the willing to sign a consent form form (ICF) and in this protocol. Who Should NOT Join This Trial: Participants are excluded from the study if any of the following criteria apply: - Known coexisting bleeding disorders - History of antiphospholipid antibody syndrome. - History of arterial or venous thromboembolism, atrial fibrillation, significant valvular disease, myocardial infarction, angina, transient ischemic attack, or stroke. Participants who have experienced thrombosis associated with indwelling venous access may be enrolled. - Presence of clinically significant liver disease - Current or prior participation in a fitusiran trial - Current or prior participation in a gene therapy trial - AT activity \<60% at Screening, as determined by central laboratory measurement - Coexisting thrombophilic disorder - Hepatitis C virus antibody positive, except participants who have negative Hepatitis C viral load and no evidence of cirrhosis - Presence of acute hepatitis, ie, hepatitis A, hepatitis E. - Presence of acute or chronic hepatitis B infection - Known to be HIV positive with CD4 count \<200 cells/μL. - Reduced renal function ...See full criteria on ClinicalTrials.gov Always talk to your doctor about whether this trial is right for you.