Fitusiran Prophylaxis in Male Pediatric Subjects Aged 1 to Less Than 12 Years With Hemophilia A or B

Inclusion Criteria: * Male, aged 1 to \<12 years at the time of enrollment. * Severe hemophilia A or B (Factor VIII (FVIII) \<1% or Factor IX (FIX) ≤2%) * Participants must have inhibitory antibodies to FVIII or FIX and must meet one of the following Nijmegen-modified Bethesda assay results criteria: * 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 1 inhibitor titer ≥0.6 BU/mL and a history of anamnestic response or severe allergic reaction (anaphylaxis or nephrotic syndrome) * Adequate peripheral venous access, as determined by the Investigator, to allow the blood draws required by the study protocol * Weight requirements at the time of enrollment: 8 to \<45 kg * Willing and able to comply with the study requirements and to provide signed written informed consent obtained from parent(s)/legal guardian (hereinafter the "parent") and written or oral assent obtained from participant, per local and national requirements Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Known co-existing bleeding disorders other than hemophilia A or B * Antithrombin (AT) activity \<60% at Screening * Co-existing thrombophilic disorder * Clinically significant liver disease * Active Hepatitis C virus infection * Acute or chronic Hepatitis B virus…