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 \>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 (2021) for those age ≥18 years and Bedside Schwartz Equation for those \<18 years * Participants with chronic respiratory insufficiency and on long term/hull-time ventilatory assistance requiring at least 6 hours per day for at least 21 consecutive days. * 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. * Participants who have been classified as severe cardiac risk by the Investigator. The above information is not intended to contain all considerations relevant to a participant's potential participation in a clinical trial.