Study to Assess the Safety, Tolerability, and Efficacy of Viltolarsen in Ambulant and Non-Ambulant Boys With DMD (Galactic53)

Inclusion Criteria: 1. Patient (if age 18 years or older) or patient's parent(s) or legal guardian(s) has (have) provided written informed consent and Health Insurance Portability and Accountability Act authorization, where applicable, prior to any study-related procedures; patients younger than age 18 years will be asked to give written or verbal assent according to local requirements; 2. Patient has a confirmed diagnosis of DMD defined as: 1. Patient is male with clinical signs compatible with DMD; and 2. Patient has a confirmed DMD mutation(s) in the dystrophin gene that is amenable to skipping of exon 53 to restore the dystrophin messenger ribonucleic acid reading frame including determination of unambiguously defined exon boundaries (using techniques such as multiplex ligation-dependent probe amplification, comparative genomic hybridization array, or other techniques with similar capability); 3. Patient is more than 8 years of age at time of first infusion in the study; 4. Patient has a Brooke scale rating of 3 or better OR an upright FVC 30% or greater at Screening; 5. Patient, if sexually active, is willing to abstain from sexual intercourse or employ a barrier or medical method of contraception during and for 3 months following completion of IP administration; 6. Patient and patient's parent(s)/guardian(s) (if patient is \<18 years of age) and/or caregiver(s) are willing and able to comply with scheduled visits, IP administration plan, and study procedures; 7. Patient must be on a stable dose of glucocorticoid (GC) or not treated with GC for at least 3 months prior to the first dose of IP and is expected to remain on stable dose of GC treatment or off GC for the duration of the study. Other inclusion criteria may apply Exclusion Criteria: 1. Patient has had an acute illness within 4 weeks prior to the first dose of IP; 2. Patient has evidence of symptomatic cardiomyopathy (New York Heart Association Class III or higher); 3. Patient requires ventilation support while awake during the day; 4. Patient has an allergy or hypersensitivity to IP or any of its constituents; 5. Patient has severe behavioral or cognitive problems that preclude participation in the study, in the opinion of the investigator; 6. Patient has a previous or ongoing medical condition, medical history, physical findings, or laboratory abnormalities that could affect patient 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; 7. Patient has had surgery within 3 months prior to the first anticipated administration of IP or has known plans to have surgery during the Treatment Period; 8. Patient has positive test results for hepatitis B antigen, hepatitis C antibody, or human immunodeficiency virus antibody at Screening; 9. Patient has been diagnosed with asthma that requires chronic treatment with a long-acting beta agonist; 10. Patient has relevant history of or current drug or alcohol abuse or use of any tobacco/marijuana products by smoking or vaping within 3 months prior to treatment with IP; 11. Patient is currently taking any other investigational drug or has taken any other investigational drug within 3 months prior to the first dose of IP or within 5 times the half-life of a medication, whichever is longer; 12. Patient has taken any gene therapy; 13. Patient is currently taking any other exon skipping agent or has taken any other exon skipping agent within 3 months prior to the first dose of IP; 14. Patient has hydronephrosis, hydroureter, renal or urinary tract calculi, or ureteral stenosis by renal ultrasound; 15. Patient was previously enrolled in an interventional study of viltolarsen. Other exclusion criteria may apply