An Open-label, Parallel, Phase 3, Two-arm Study to Investigate the Efficacy and Safety of Fitusiran Prophylaxis in Male Participants Aged 1 to Less Than 12 Years With Hemophilia A or B With or Without Inhibitory Antibodies to Factors VIII or IX
This is a parallel, Phase 3, two-arm, open-label study to evaluate the efficacy and safety of treatment with fitusiran prophylaxis administered to male pediatric participants (aged 1 to \<12 years) who have severe hemophilia A or B, with or without inhibitory antibodies to FVIII or FIX. Number of participants: Approximately 85 participants will be enrolled into the study: * Approximately 60 fitusiran-naïve participants with severe hemophilia A or B, with or without inhibitors (fitusiran-naïve arm), and * Approximately 25 participants with severe hemophilia A or B with inhibitors rolling over from the EFC15467\* dose confirmation study (roll-over arm). * Fitusiran has been investigated in the pediatric population in study EFC15467, which enrolled male participants aged 1 to \<12 years with hemophilia A or B with inhibitors to examine the safety and tolerability of fitusiran in the pediatric population. Participants will be enrolled into 1 of 2 arms: * Fitusiran-naïve: these participants have not previously received fitusiran, and they will undergo screening and study eligibility assessments. Once enrolled, they will go through a 24-week standard of care (SOC) period before starting fitusiran prophylaxis. * Roll-over participants from the EFC15467 study: only participants who are still on active treatment in study EFC15467 and consenting to study EFC17905 will be eligible to roll over. They will not need to undergo screening or further eligibility assessments. They will directly enroll into the fitusiran treatment period and continue treatment on their current fitusiran dose. The duration of fitusiran treatment will be up to 160 weeks for the fitusiran-naïve arm and up to 60 weeks for the roll-over arm.
∙ Participants not previously exposed to fitusiran are eligible to be included in the study only if all of the following criteria apply:
• Participant must be 1 to \<12 years of age at the time of enrollment.
• Participants must have severe hemophilia A or B (FVIII \<1% or FIX ≤2%) as evidenced by a central laboratory measurement at screening or documented medical record evidence.
• Participants must meet inhibitor or non-inhibitor status as defined below:
∙ Inhibitor:
∙ Requiring use of BPA for prophylaxis or BPA as on-demand therapy for any bleeding episodes for at least the last 3 months prior to screening, and 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 (eg, anaphylaxis) or nephrotic syndrome
∙ Non-inhibitor:
∙ Requiring use of clotting factor concentrates (CFCs) for prophylaxis or CFCs as on-demand therapy for any bleeding episodes for at least the last 3 months prior to screening, and meet each of the following criterion:
• Nijmegen-modified Bethesda assay inhibitor titer of \<0.6 BU/mL at screening, AND
• No use of BPA to treat bleeding episodes for at least the last 3 months prior to screening
‣ Participants must have adequate peripheral venous access, as determined by the Investigator, to allow the blood draws required by the study protocol.
⁃ Male: There are no contraceptive requirements for this study except where required by local regulations.
⁃ Capable of giving signed informed consent/assent. A signed written informed consent must be obtained from parent(s)/legal guardian (hereafter referred to as the parent), as well as a written or oral assent obtained from participant, per local and national requirements.