PBGENE-DMD Phase 1/2a Safety and Preliminary Efficacy Study in Duchenne Muscular Dystrophy (FUNCTION-DMD)

The purpose of this Phase 1/2a trial is to evaluate the safety, tolerability, and preliminary efficacy of PBGENE-DMD in patients with DMD harboring mutations amenable to excision of exons 45-55. Given the limitations of existing therapeutic strategies, PBGENE-DMD represents a novel, innovative approach with the potential for a one-time, durable correction of the underlying genetic defect in the largest molecular subset of patients with DMD.

Inclusion Criteria: 1. Males, 2 to 7 years of age, inclusive, at the time of informed consent/assent 2. Molecular confirmed DMD diagnosis (DMD mutation fully contained between exons 45 to 55 \[inclusive\]) 3. Clinical phenotype consistent with DMD in the opinion of the Investigator 4. Ability to complete age-appropriate motor testing assessments requirements. Participants aged 2 to \< 4 years at the time of screening must: 1. Be able to walk at least 10 meters independently (without assistive devices). 2. Be able to rise from the floor without physical assistance (use of a Gowers' maneuver is acceptable). Participants aged 4 to 7 years at the time of screening must: 3. Be able to walk at least 100 meters independently (without assistive devices). 4. Have an NSAA total score between 16 and 29, inclusive. 5. Participant has received age-appropriate routine childhood immunizations per the local country's national immunization schedule. 6. The participant's parent(s)/LAR(s) are willing and able to provide written informed consent prior to the initiation of any trial-specific procedures; where applicable, the participant must provide written or verbal assent in accordance with local regulations. 7. The participant and their parent(s)/LAR(s) are willing to participate in a LTFU study after the completion of this trial. Exclusion Criteria: 1. Prior treatment with any gene therapy, gene editing therapy, or cell-based therapy at any time. 2. Receipt of any investigational medication or experimental therapy within 6 months prior to Day 1. 3. Prior or ongoing use of any product designed to increase dystrophin expression, investigational, or otherwise, including exon-skipping therapies, within 6 months of the scheduled Day 1 dose or inability or unwillingness to refrain from initiating or resuming these therapies for at least 5 years following gene therapy administration. 4. Prior ongoing use of any product designed to increase dystrophin expression, investigational, or otherwise, including exon-skipping therapies, within 6 months of the scheduled Day 1 dose. 5. Concurrent enrollment in another clinical trial, unless it is observational (non-interventional). 6. A positive test for antibodies to AAV9 7. A participant has any condition that would contraindicate treatment with immunosuppression. 8. Participants with pathogenic mutations in exons 1-44 and/or exons 56-79. 9. Evidence of cardiomyopathy or clinically significant left ventricular dysfunction, defined as LVEF \