Open-label Study of WVE-N531 in Patients With Duchenne Muscular Dystrophy (FORWARD-53)

Inclusion Criteria: Part A and Part B: 1. Part A patients may be screened for Part B upon completion of a washout period of ≥18 weeks from last dose in Part A. New patients may also be screened for Part B 2. Diagnosis of DMD based on clinical phenotype. 3. Documented mutation in the DMD gene associated with DMD that is amenable to exon 53 intervention 4. Score of ≥1 on item 1 or 2 of the shoulder component of the Performance of the Upper Limb (PUL) (Part B ). 5. Ambulatory or non-ambulatory male 6. Stable pulmonary and cardiac function, as measured by the following: (Part B): 1\. Reproducible percent predicted forced vital capacity (FVC) ≥50%; 2. Left ventricular ejection fraction (LVEF) \>55% in patients \<10 years of age and \>45% in patients ≥10 years of age, as measured (and documented) by echocardiogram (ECHO) and/or cardiac magnetic resonance imaging (MRI), within 6 months prior to enrollment into the study. 7.Adequate muscle at Screening to perform open muscle biopsies, preferably deltoid. 8\. Currently on a stable corticosteroid therapy regimen, defined as initiation of systemic corticosteroid therapy that occurred ≥6 months prior to Screening and no changes in dose ≤3 months prior to Screening visit (Part B ). Part C 1. New patients to be screened for Part C. 2. Diagnosis of DMD based on clinical phenotype. 3. Documented mutation in the DMD gene associated with DMD that is amenable to exon 53 intervention 4. Score of ≥1 on item 1 or 2 of the shoulder component of the Performance of the Upper Limb (PUL) . 5. Ambulatory male 6. Stable pulmonary and cardiac function, as measured by the following: 1\. Reproducible percent predicted forced vital capacity (FVC) ≥50%; 2. Left ventricular ejection fraction (LVEF) \>55% in patients as measured (and documented) by echocardiogram (ECHO) and/or cardiac magnetic resonance imaging (MRI), within 6 months prior to enrollment into the study. 7\. Adequate muscle at Screening to perform open muscle biopsies, preferably deltoid. 8\. Currently on a stable corticosteroid therapy regimen, defined as initiation of systemic corticosteroid therapy that occurred ≥6 months prior to Screening and no changes in dose ≤3 months prior to Screening visit . Exclusion Criteria: 1. Clinically significant medical finding on the physical examination other than DMD that, in the judgment of the Investigator, will make the patient unsuitable for participation in, and/or completion of the study procedures. 2. Part B and Part C: Major surgery within 3 months prior to Day 1 or planned major surgery for any time during the study. 3. Part B: Diagnosis of active alcohol, cannabinoid, or other substance use disorder (except nicotine) within 6 months prior to the Screening visit 4. Part C: Any recreational substance use (including prescribed cannabinoids), with the exception of nicotine, irrespective of legality, within 2 months prior to Screening and/or unwilling to refrain from such use for the duration of the study.