Phase 1/2 Study in Boys With Duchenne Muscular Dystrophy

Inclusion Criteria: * Written informed consent from parent or legal guardian prior to participation and, for patients who are 7 years of age, written assent from patient * Diagnosis of DMD based on a clinical phenotype with increased serum CK and the presence of a mutation in the dystrophin gene known to be associated with a DMD phenotype * Ability to walk independently (assistive devices are permitted) * Adequate immunization for influenza and varicella Exclusion Criteria: * Use of corticosteroids within prior 6 months of treatment initiation or planning to initiate steroid therapy within the next 6 months * Other prior or ongoing significant medical conditions * Exposure to another investigational drug (such as eteplirsen or idebenone) within 28 days prior to start of study treatment or ongoing participation in any other therapeutic clinical trial * Note: There are separate criteria for patients who participated in Part A versus newly enrolling patients. New patients must meet all of the Part A entry criteria to participate in Part B. Patients who participated in Part A must meet the following criteria to participate in Part B: * Completed Part A * Continue to meet all of the Part A entry criteria, including an absence of safety concerns (however, patients may be ≥8 years of age) There are no entry criteria for Part C; all patients who complete Part B will automatically continue in Part C