The study was terminated following review of safety data.

This study will be comprised of 2 parts: 1) Part A (Multiple Ascending Dose [MAD]) will be conducted to evaluate the safety and tolerability of vesleteplirsen at MAD levels to determine the maximum tolerated dose (MTD), and 2) Part B will be conducted to further evaluate the vesleteplirsen doses selected in Part A. Participants enrolling in Part B will be those who completed Part A or Study 5051-102 (NCT03675126) and meet applicable eligibility criteria for Part B, as well as additional participants who meet applicable eligibility criteria for enrollment at the beginning of Part B.

Inclusion Criteria for participants previously treated with Vesleteplirsen:

- Has received prior Vesleteplirsen treatment in Part A of this study or in Study 5051-102.

Exclusion Criteria for participants previously treated with Vesleteplirsen and new participants enrolling into Part B:

- Presence of other clinically significant illness, including cardiac, pulmonary, hepatic, renal, hematologic, immunologic, or behavioral disease, or infection or malignancy or any other condition that, in the Investigator's opinion, could interfere with participation in the trial.

Inclusion Criteria for treatment-naïve participants enrolling into Part B:

• Has a genetic diagnosis of Duchenne muscular dystrophy (DMD) and an out-of-frame deletion mutation of the DMD gene amenable to exon 51-skipping treatment.
• Has been on a stable dose of oral corticosteroids for at least 12 weeks prior to study drug administration and the dose is expected to remain constant throughout the study (except for modifications to accommodate changes in weight), or has not received corticosteroids for at least 12 weeks prior to study drug administration.
• Has stable pulmonary function (forced vital capacity [FVC] ≥40% of predicted and no requirement for nocturnal ventilation).

Exclusion Criteria for treatment-naive participants enrolling into Part B:

• History of hypomagnesemia within 12 weeks prior to Screening.
• Initiation or change of dosing (except for modifications to accommodate changes in weight or changes in standard of care) within 12 weeks prior to Screening for any of the following: angiotensin-converting enzyme inhibitors, angiotensin receptor-blocking agents, β-blockers, or potassium.
• Initiation or change of dosing within 12 weeks prior to Screening for over-the-counter preparations, such as herbal/nonherbal supplements, vitamins, minerals, and homeopathic preparations.
• Has a left ventricular ejection fraction (LVEF) <40.0% based on an echocardiogram (ECHO) performed within 12 weeks prior to Screening or at the Screening Visit.
• Treatment with any exon 51-skipping therapy within 4 weeks prior to Screening, or with any experimental gene therapy for the treatment of DMD at any time.

Other inclusion/exclusion criteria apply.