A 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 (ATLAS-KIDS)

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.

Study Overview

• Status: Recruiting
• Conditions: Hemophilia
• Intervention / Treatment: Drug: Fitusiran; Biological: Clotting factor concentrates (CFC) or bypassing agents (BPA); Biological: Antithrombin concentrate (ATIIIC)

Detailed Description

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.

• Study Type: Interventional
• Enrollment (Estimated): 85
• Phase: Phase 3

Contacts and Locations

• Study Contact: Name: Trial Transparency email recommended (Toll free for US & Canada); Phone Number: option 6 800-633-1610; Email: contact-us@sanofi.com

Study Locations

• Belgium
  • Brussels, Belgium, 1020
    • Recruiting
    • Investigational Site Number : 0560002
    • Contact: Bernard Wenderickx; Phone Number: +32 2 24 77 36 54; Email: Bernard.wenderickx@hubruxelles.be
    • Principal Investigator: Phu Quoc Lê
  • Brussels, Belgium, 1200
    • Recruiting
    • Investigational Site Number : 0560001
    • Principal Investigator: An Van Damme
    • Contact: Alix Blore; Phone Number: +32 2 764 23 37; Email: alix.blore@saintluc.uclouvain.be
• Brazil
  • São Paulo, Brazil, 05403-000
    • Recruiting
    • Hospital das Clinicas da Faculdade de Medicina da Universidade de Sao Paulo- Site Number : 0760004
  • São Paulo
    • Ribeirão Preto, São Paulo, Brazil, 14051140
      • Recruiting
      • Hospital das Clinicas da Faculdade de Medicina de Ribeirao Preto- Site Number : 0760003
      • Contact: Ana Beatriz Nicolau Perim; Phone Number: +55 16 3963 6480
      • Principal Investigator: Luciana Oliveira
• Canada
  • Quebec
    • Montreal, Quebec, Canada, H3T 1C5
      • Recruiting
      • Investigational Site Number : 1240003
• India
  • Bengaluru, India, 560034
    • Recruiting
    • Investigational Site Number : 3560004
  • Mumbai, India, 400022
    • Recruiting
    • Investigational Site Number : 3560002
  • Pune, India, 411004
    • Recruiting
    • Investigational Site Number : 3560001
  • Vellore, India, 632004
    • Recruiting
    • Investigational Site Number : 3560003
• Italy
  • Firenze
    • Florence, Firenze, Italy, 50134
      • Recruiting
      • Investigational Site Number : 3800002
  • Milano
    • Milan, Milano, Italy, 20122
      • Recruiting
      • Investigational Site Number : 3800001
      • Principal Investigator: Flora Peyvandi
      • Contact: Flora Peyvandi; Phone Number: +39 02 5503.5414; Email: flora.peyvandi@policlinico.mi.it
    • Rozzano, Milano, Italy, 20089
      • Recruiting
      • Investigational Site Number : 3800003
      • Contact: Maria Elisa Mancuso; Phone Number: +39 0282245981; Email: mariaelisa.mancuso@hunimed.eu
      • Principal Investigator: Maria Elisa Mancuso
• Poland
  • Lower Silesian Voivodeship
    • Wroclaw, Lower Silesian Voivodeship, Poland, 50-556
      • Recruiting
      • Investigational Site Number : 6160002
      • Contact: Marta Hetman; Phone Number: +48 530060737; Email: martha.hetman@gmail.com
      • Principal Investigator: Elzbieta Latos-Grazynska
• Romania
  • Iași, Romania, 700309
    • Recruiting
    • Investigational Site Number : 6420001
  • Timișoara, Romania, 300011
    • Recruiting
    • Investigational Site Number : 6420003
• Saudi Arabia
  • Riyadh, Saudi Arabia, 12746
    • Recruiting
    • Investigational Site Number : 6820001
• Spain
  • Madrid, Spain, 28046
    • Recruiting
    • Investigational Site Number : 7240002
  • Zaragoza, Spain, 50009
    • Recruiting
    • Investigational Site Number : 7240003
  • Barcelona [Barcelona]
    • Esplugues de Llobregat, Barcelona [Barcelona], Spain, 08950
      • Recruiting
      • Investigational Site Number : 7240001
• Taiwan
  • Taichung, Taiwan, 402
    • Recruiting
    • Investigational Site Number : 1580002
  • Taipei, Taiwan, 100
    • Recruiting
    • Investigational Site Number : 1580001
• Turkey (Türkiye)
  • Istanbul, Turkey (Türkiye), 34093
    • Recruiting
    • Investigational Site Number : 7920001
    • Contact: Funda Ciga; Phone Number: +90 533 420 60 57
    • Principal Investigator: Basak Koc Senol
  • Izmir, Turkey (Türkiye), 35100
    • Recruiting
    • Investigational Site Number : 7920002
    • Contact: Esra Nur Nalçakan Sağlam; Phone Number: +90 (538) 322 67 22
    • Principal Investigator: Kaan Kavakli
• United States
  • California
    • Los Angeles, California, United States, 90007
      • Recruiting
      • The Luskin Orthopaedic Institute for Children- Site Number : 8400013
    • Orange, California, United States, 92868
      • Recruiting
      • The Center for Inherited Blood Disorders- Site Number : 8400009
  • Nevada
    • Las Vegas, Nevada, United States, 89147
      • Recruiting
      • Cure 4 The Kids Foundation- Site Number : 8400001
  • New Jersey
    • Hackensack, New Jersey, United States, 07601
      • Recruiting
      • Hackensack Meridian Health - Hackensack University Medical Center- Site Number : 8400008

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child
• Accepts Healthy Volunteers: No

Description

Inclusion Criteria:

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.

Exclusion Criteria:

Participants not previously exposed to fitusiran are excluded from the study if any of the following criteria apply:

• Known co-existing bleeding disorders other than hemophilia A or B.
• Presence of clinically significant liver disease.
• History of antiphospholipid antibody syndrome.
• History of arterial or venous thromboembolism, unrelated to an indwelling venous access
• Any condition (eg, medical concern), which in the opinion of the Investigator, would make the participant unsuitable for dosing or which could interfere with the study compliance, the participant's safety and/or the participant's participation in the completion of the treatment period of the study.
• History of multiple drug allergies or history of allergic reaction to an oligonucleotide or GalNAc.
• Subjects with a central or peripheral indwelling catheter, with a history of venous access complications (such as infections, thrombosis) leading to hospitalization and/or systemic anticoagulation therapy in the last 12 months.
• At screening, anticipated need of surgery during the study or planned surgery scheduled to occur during the study.
• Completion of a surgical procedure within 14 days prior to screening, or currently receiving additional BPA infusion for postoperative hemostasis.
• History of intolerance to SC injection(s).
• Current participation in ITI therapy.
• The use of emicizumab (Hemlibra®) or any non-factor bleed management treatment within 6 months prior to screening
• Prior gene therapy
• Current or future participation in another clinical study, scheduled to occur during this study, involving an investigational product other than fitusiran or an investigational device.
• AT activity <60% at screening, as determined by central laboratory analysis.
• Co-existing thrombophilic disorder.
• Presence of an active Hepatitis C virus infection
• Presence of acute hepatitis A or Hepatitis E virus infection.
• Presence of acute or chronic hepatitis B virus infection.
• Platelet count ≤100 000/μL.
• Presence of acute infection at screening.
• Human immunodeficiency virus (HIV) positive with a CD4 count of <400 cells/μL.
• Estimated glomerular filtration rate ≤45 mL/min/1.73 m2 (using the Schwartz formula).

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

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Treatment
• Allocation: N/A
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Experimental: Fitusiran-naïve arm; Participants will go through a 24-week standard of care (SOC) period before receiving a selected dose of fitusiran at regular interval. If a fitusiran dose adjustment is needed during the study, participants will follow a specific dosing regimen as per study protocol.	Drug: Fitusiran; Pharmaceutical form: solution for injection in PBS Route of administration: subcutaneous; Other Names: SAR439774; Qfitlia™; Biological: Clotting factor concentrates (CFC) or bypassing agents (BPA); Pharmaceutical form: solution for injection Route of administration: intravenous injection; Biological: Antithrombin concentrate (ATIIIC); Pharmaceutical form: solution for injection Route of administration: intravenous injection
Experimental: EFC15647 roll-over arm; Participants will continue receiving their current fitusiran dose from EFC15467. If a fitusiran dose adjustment is needed during the study, participants will follow a specific dosing regimen as per study protocol.	Drug: Fitusiran; Pharmaceutical form: solution for injection in PBS Route of administration: subcutaneous; Other Names: SAR439774; Qfitlia™; Biological: Clotting factor concentrates (CFC) or bypassing agents (BPA); Pharmaceutical form: solution for injection Route of administration: intravenous injection; Biological: Antithrombin concentrate (ATIIIC); Pharmaceutical form: solution for injection Route of administration: intravenous injection

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Annualized treated bleeding rate (ABR) in the fitusiran primary efficacy period and in the SOC period	A bleeding episode is defined as any occurrence of hemorrhage that requires administration of CFCs or BPAs	Day 85 to Day 421 (fitusiran primary efficacy period); Day -168 to Day -1 (SOC period)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Annualized spontaneous bleeding rate (AsBR) in the fitusiran primary efficacy period and in the SOC period	A spontaneous bleeding episode is a bleeding event that occurs for no apparent or known reason, particularly into the joints, muscles, and soft tissues	Day 85 to Day 421 (fitusiran primary efficacy period); Day -168 to Day -1 (SOC period)
Annualized joint bleeding rate (AjBR) in the fitusiran primary efficacy period and in the SOC period	A joint bleeding episode is characterized by an unusual sensation in the joint ("aura") in combination with 1) increasing swelling or warmth over the skin over the joint, 2) increasing pain, or 3) progressive loss of range of motion or difficulty in using the limb as compared with baseline	Day 85 to Day 421 (fitusiran primary efficacy period); Day -168 to Day -1 (SOC period)
ABR in the fitusiran treatment period (160 weeks) for fitusiran-naïve participants	A bleeding episode is defined as any occurrence of hemorrhage that requires administration of CFCs or BPAs	Day 1 to Day 1121
ABR in the fitusiran treatment period (60 weeks) for rolled-over participants	A bleeding episode is defined as any occurrence of hemorrhage that requires administration of CFCs or BPAs	Day 1 to Day 421
Change in physical activity	Change in physical activity as measured via the PROMIS questionnaire	From Day 1 to Day 421; From Week -24 to Day -1
Change in pain intensity	Change in pain intensity as measured via the PROMIS questionnaire	From Day 1 to Day 421; From Week -24 to Day -1
Change in HRQoL	Change in HRQoL as measured by the EuroQoL 5-dimension questionnaire	From Day 1 to Day 421; From Week -24 to Day -1
Incidence, severity, seriousness, and relatedness of adverse events (AEs)	All AEs are collected from the signing of the informed consent form (ICF) until last AT follow up visit	Date of signed ICF (Day -228 to Day -169) until last AT follow-up visit (Day 1121 + approximately 24 weeks)
Change in total score and domain scores	Change in total score and domain scores is assessed by the Hemophilia Joint Health Score	From Day 1 to Day 421; From Week -24 to Day -1
Target joints resolution	Target joints resolution assessed per ISTH criteria	At Day 421

Collaborators and Investigators

• Sponsor: Sanofi

Publications and helpful links

Helpful Links

• EFC17905 Plain Language Results Summary
• EFC17905 (ATLAS-KIDS) website for potential participants

Study record dates

Study Major Dates

• Study Start (Actual): December 18, 2025
• Primary Completion (Estimated): August 4, 2029
• Study Completion (Estimated): December 30, 2031

Study Registration Dates

• First Submitted: December 9, 2025
• First Submitted That Met QC Criteria: December 9, 2025
• First Posted (Actual): December 16, 2025

Study Record Updates

• Last Update Posted (Actual): May 26, 2026
• Last Update Submitted That Met QC Criteria: May 22, 2026
• Last Verified: May 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Genetic Diseases, Inborn; Hematologic Diseases; Blood Coagulation Disorders; Hemorrhagic Disorders; Blood Coagulation Disorders, Inherited; Coagulation Protein Disorders; Congenital, Hereditary, and Neonatal Diseases and Abnormalities; Hemic and Lymphatic Diseases; Hemophilia A; fitusiran
• Other Study ID Numbers: EFC17905 (Sanofi Identifier); U1111-1280-7028 (Registry Identifier: ICTRP); 2025-521858-42 (Registry Identifier: CTIS)

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: YES
• IPD Plan Description: Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: Yes
• Studies a U.S. FDA-regulated device product: No