A Study to Investigate the Safety, Tolerability, and Efficacy of SAR446268, an Adeno-associated Viral Vector-mediated Gene Therapy in Participants Aged 10 to 55 Years of Age With Non-congenital Myotonic Dystrophy Type 1

Inclusion Criteria: Participants are eligible to be included in the study only if all of the following criteria apply: * For Part A, participants must be 18 to 55 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 55 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). Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Participants with neutralizing antibodies against the AAV.SAN011 capsid * Participants with left ventricular ejection fraction \<50% * Participants with liver or biliary disease defined as having at least one of the following: * ALT \>3 x ULN and AST \>3 x ULN * Alkaline phosphatase \>3 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 * Participan…