The FOX study is a 2-part, multicenter, Phase 2 study of safety, pharmacokinetics, and biomarkers in children and adolescents with Duchenne muscular dystrophy previously treated with gene therapy including a randomized, double-blind, placebo-controlled Part A, followed by an open-label part B.

FOX is a 2-part, multi-center, Phase 2 study to evaluate the effect of sevasemten (EDG-5506) on safety, pharmacokinetics and biomarkers of muscle damage in approximately 48 children and adolescents with Duchenne muscular dystrophy treated with oral, once-daily sevasemten. This study will have up to a 4-week Screening period, a 12-week randomized double-blind, placebo-controlled treatment period (Part A), followed by up to a 144-week open-label extension period (Part B).

Approximately forty-eight (48) participants aged 6 to 17, inclusive, will be randomized to sevasemten or placebo in a 2:1 ratio. Three dose cohorts (Cohort 1, Cohort 2 and Cohort 3) of approximately 12 participants each will be enrolled. Approximately 12 additional participants may be added to 1 of these cohorts.

After review of emerging data, the protocol was amended so all dose cohorts receive the same dose in Part B.

Key Inclusion Criteria:

• Aged 6 to 17 with a documented mutation on the DMD gene and phenotype consistent with DMD.
• Prior receipt of an AAV-based gene therapy (≥ 2 years after documented receipt of gene therapy administration or ≥ 3 years after randomization in a randomized study).
• Able to complete stand from supine in ≤ 8 seconds at the Screening visit and able to perform the 4-stair climb in < 10 seconds at the Screening visit.
• Body weight ≥ 15 kg at the Screening visit.
• Treatment with a stable dose of corticosteroids for a minimum of 6 months prior to the Baseline visit.

Key Exclusion Criteria:

• Medical history or clinically significant physical exam/laboratory result that, in the opinion of the investigator, would render the participant unsuitable for the study. This includes venous access that would be too difficult to facilitate repeated blood sampling.
• Screening visit cardiac echocardiography showing left ventricular ejection fraction (LVEF) < 40%.
• Receipt of an investigational drug (other than the AAV-based gene therapy per Inclusion criteria) within 30 days or 5 half-lives (whichever is longer) of the Screening visit in the present study.
• Receipt of an exon-skipping therapy within 6 months prior to the Screening visit.