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

Eligibility Criteria

• * For Part A, participants must be 18 to 50 years of age inclusive, at the time of signing the informed consent.
• * For Part B, participants must be as follows:
• * 10 to 17 years of age inclusive, at the time of signing the informed consent or,
• * 18 to 50 years of age inclusive, at the time of signing the informed consent.
• * Participants with non-congenital onset DM1
• * Participants presenting with signs of DM1 including myotonia and muscle weakness, as diagnosed previously by a clinician based on medical history.
• * Participants with genetic diagnosis of DM1 \[cytosine-thymine-guanine (CTG) repeat length ≥50 in one allele from medical history\]
• * Participants who can walk independently for at least 10 meters at screening (orthoses and ankle braces allowed).
• * Participants who have been classified according to cardiac risk by the Investigator as:
• * Moderate risk participants with pacemaker and/or implantable cardioverter-defibrillator (ICD) for Part A
• * Low, moderate, or high cardiac risk for Part B
• * 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:
• * 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.