Study to Assess the Efficacy and Safety of Viltolarsen in Ambulant Boys With DMD (RACER53)

Inclusion Criteria: * Male ≥ 4 years and \< 8 years of age * Confirmed DMD mutation(s) in the dystrophin gene that is amenable to skipping of exon 53 to restore the dystrophin mRNA reading frame * Able to walk independently without assistive devices * TTSTAND \< 10 seconds * Stable dose of glucocorticoid (GC) for at least 3 months prior to study entry and is expected to remain on stable dose of GC treatment for the duration of the study * Other inclusion criteria may apply Exclusion Criteria: * Current or history of chronic systemic fungal or viral infections * Acute illness within 4 weeks prior to the first dose of study drug * Evidence of symptomatic cardiomyopathy (Note: Asymptomatic cardiac abnormality on investigation would not be exclusionary) * Allergy or hypersensitivity to the study drug or to any of its constituents * Severe behavioral or cognitive problems that preclude participation in the study, in the opinion of the investigator * Previous or ongoing medical condition, medical history, physical findings or laboratory abnormalities that could affect safety, make it unlikely that treatment and follow-up will be correctly completed or impair the assessment of study results, in the opinion of the investigator; * Surgery within the 3 months prior to the first dose of study drug or surgery is planned for anytime during the duration of the study * Participant has positive test results for hepatitis B antigen, hepatitis C antibody or human immunodeficiency virus (HIV) * Currently taking any other investigational drug or has taken any other investigational drug within 3 months prior to the first dose of study drug or within 5 times the half-life of a medication, whichever is longer * Previously enrolled in an interventional study of viltolarsen * Currently taking any other exon skipping agent or has taken any other exon skipping agent within 3 months prior to the first dose of study drug * Having taken any gene therapy * Other exclusion criteria may apply