Key Inclusion Criteria:  Genetic diagnosis of DM1 (CTG repeat length ≥ 100) Clinician assessed signs of DM1 Ability to walk independently (orthoses and ankle braces allowed) for at least 10 meters at screening  Key Exclusion Criteria:  Diabetes that is not adequately controlled BMI > 35 kg/m2 Uncontrolled hypertension Congenital DM1 History of tibialis anterior (TA) biopsy within 3 months of Day 1 or planning to undergo TA biopsies during study period Recently treated with an investigational drug Treatment with anti-myotonic medication within 14 days of Day 1  Note: Other protocol defined Inclusion/Exclusion criteria may apply.