• Number of participants reported with adverse events [ Time Frame: 160 weeks ]
  Number of participants reported with treatment-emergent adverse events (TEAEs)
• Fitusiran plasma concentrations [ Time Frame: Day 1 and Day 85 ]
  Plasma samples will be collected for measurement of plasma concentrations of fitusiran at specific time points postdose on Day 1 and predose on Day 85

• Number of participants reported with adverse events [ Time Frame: 160 weeks ]
  Number of participants reported with treatment-emergent adverse events (TEAEs)
• Pharmacokinetics (PK): Maximum plasma concentration (Cmax) [ Time Frame: Day 1, Day 29, Day 57 ]
  Plasma samples will be collected for measurement of plasma concentrations of fitusiran such as Cmax.
• Pharmacokinetics (PK): Time to reach maximum plasma concentration (Tmax) [ Time Frame: Day 1, Day 29, Day 57 ]
  To evaluate time to reach Cmax
• Pharmacokinetics (PK): Ctrough [ Time Frame: Day 1, Day 29, Day 57 ]
  To evaluate concentration observed just before investigational medicinal product (IMP) administration during repeated dosing (Ctrough)

Primary Objective:

- To confirm appropriate dose levels of fitusiran when administered to male pediatric participants (ages 1 to <12 years of age) with severe hemophilia A or B

Secondary Objective:

• To characterize the safety and tolerability
• To determine fitusiran plasma concentrations at selected time points

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:

• 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 infection
• Acute Hepatitis A or hepatitis E infection
• HIV positive with a CD4 count of <400 cells/μL
• History of arterial or venous thromboembolism, unrelated to an indwelling venous access
• Inadequate renal function
• History of multiple drug allergies or history of allergic reaction to an oligonucleotide or N-Acetylgalactosamine (GalNAc)
• Participants with central or peripheral indwelling catheters, with history of venous access complications leading to hospitalization and/or systemic anticoagulation therapy
• History of intolerance to subcutaneous (SC) injection(s)
• The use of emicizumab (Hemlibra®) within 6 months prior to screening
• Any other conditions or comorbidities that would make the patient unsuitable for enrollment or could interfere with participation in or completion of the study, per Investigator judgment

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.