A Study to Investigate the Safety, Tolerability, and Efficacy of SAR446268, an Adeno-associated Viral Vector-mediated Gene Therapy in Participants Aged 10 to 50 Years of Age With Non-congenital Myotonic Dystrophy Type 1

Exclusion Criteria

• •Participants with neutralizing antibodies against the AAV.SAN011 capsid
• •Participants with left ventricular ejection fraction (LVEF) \<50%
• •Participants with liver or biliary disease defined as having at least one of the following:
• •* Alanine aminotransferase (ALT) \>2 x ULN and aspartate aminotransferase (AST) \>2 x ULN
• •* Alkaline phosphatase \>2 x ULN
• •* Total bilirubin \>1.5 x ULN (unless has a genetically confirmed diagnosis of Gilbert's syndrome)
• •* Direct bilirubin ≥1.5 x ULN
• •* Participants with International normalized ratio \>1.5
• •Participants with renal disease defined as:
• •• Serum creatinine \>1.5 x ULN and/or estimated glomerular filtration rate \<60 mL/min/1.73 m2 as determined by Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) (2021) for those age ≥18 years and Bedside Schwartz Equation for those \<18 years
• •Participants with chronic respiratory insufficiency and on non-invasive ventilatory support, nighttime ventilatory support or full-time ventilation.
• •Participants with contraindication to corticosteroid or with conditions that could worsen in the presence of corticosteroids, as determined by the Investigator.
• •Participants with active hepatitis B or C infection; HBsAg (+), or HCV RNA (+), or current antiviral therapy for either.
• •Participants with HBcAb (+) who are not amenable for prophylactic anti-HBV therapy or pre-emptive therapy guided by serial HBV DNA monitoring during the corticosteroids therapy.
• •Participants at high risk for tuberculosis reactivation during the corticosteroids therapy as determined by the Investigator.
• •Participants with a known HIV infection
• •Participants with serious intercurrent illness that, in the opinion of the Investigator, would preclude participation in the study or potentially decrease survival.
• •Participants with recent history of or current drug or alcohol abuse in the past 12 months prior to screening.
• •Participants with history of tibialis anterior biopsy within 12 weeks from Day 1 or planning to undergo tibialis anterior biopsies during the duration of this clinical trial.
• •Participants with significant developmental delay, intellectual disability, or behavioral neuropsychiatric manifestations as determined by the Investigator.
• •Participants with previous systemic corticosteroids treatment at doses of \>5 mg/day within 15 days of Day 1
• •Participants with previous treatment with anti-myotonic medication within 15 days of Day 1
• •Participants not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions, or participants potentially at risk of noncompliance to study procedures.
• •The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.