Safety, Tolerability, Pharmacodynamic, Efficacy, and Pharmacokinetic Study of DYNE-251 in Participants With Duchenne Muscular Dystrophy Amenable to Exon 51 Skipping

Open to males ages 4-16

• Age 4 to 16 years inclusive, at the time of informed consent/assent.
• Male with a confirmed diagnosis of DMD and with a mutation in the dystrophin gene characterized by exon deletion amenable to exon 51 skipping.
• Upper extremity muscle group that is amenable to muscle biopsy.
• Brooke Upper Extremity Scale score of 1 or 2.
• Ambulatory or non-ambulatory. A non-ambulatory participant must have been non-ambulatory for <2 years before enrollment.
• Receiving a stable dosage of glucocorticoids for at least 12 weeks prior to the start of study drug administration, with the expectation of maintaining a stable dose during the Placebo-Controlled and Open-Label Periods of the study (unless dose adjustment is required by weight change).
• Left ventricular ejection fraction of ≥50% by echocardiogram or ≥55% by cardiac magnetic resonance imaging (MRI).

• Uncontrolled clinical symptoms and signs of congestive heart failure (CHF).
• Any change in prophylaxis/treatment for CHF within 3 months prior to the start of study treatment.
• History of major surgical procedure within 12 weeks prior to the start of study drug administration or an expectation of a major surgical procedure during the study.
• Requirement of daytime ventilator assistance.
• Percent predicted FVC <40 % (applies only for participants who are age ≥7 years).
• Receipt of eteplirsen, or alternative exon-skipping/dystrophin-modifying therapy, within 12 weeks of randomization.
• Receipt of non-exon skipping investigational drug within 4 months before the start of study drug administration.
• Receipt of gene therapy at any time.

Other inclusion and exclusion criteria may apply.