Overview

This is a multicenter, open-label, non-randomized study to investigate the safety, tolerability, and efficacy of a single intravenous (IV) infusion of SGT-003 in participants with Duchenne muscular dystrophy. There will be 2 cohorts in this study. Cohort 1 will include participants 4 to <7 years of age. Cohort 2 will include participants 7 to <12 years of age. All participants will receive SGT-003 and will be enrolled in the study for 5 total years for long-term follow up.

Eligibility

Inclusion Criteria:

• Cohort 1: 4 to <7 years of age
• Cohort 2: 7 to <12 years of age
• Participants who are ambulatory. Ambulatory as defined as "being able to walk without the use of an assistive device."
• Established clinical diagnosis of DMD and documented dystrophin gene mutation predictive of DMD phenotype confirmed by Sponsor genetic testing.
• Negative for AAV antibodies.
• On a stable dose of at least 0.5 mg/kg/day of oral daily prednisone or 0.75 mg/kg/day deflazacort for ≥12 weeks prior to entering the study.
• Meet 10-meter walk/run time criteria
• Meet time to rise from supine criteria
• Participant has body weight: ≤50 kg

Exclusion Criteria:

• Treatment with dystrophin modifying drugs within 3 months prior to screening.
• Current or prior treatment with an approved or investigational gene transfer drug.
• Exposure to certain approved or investigational drugs within 3 months prior to screening or 5 half-lives since last administration, whichever is longer.
• Established clinical diagnosis of DMD that is associated with any deletion mutation in exons 1 to 11 or 42 to 45, inclusive, in the DMD gene as documented by a genetic report and confirmed by Sponsor genetic testing.

Other inclusion or exclusion criteria apply.