The study will evaluate the safety and tolerability of GEN6050X gene therapy in Duchenne muscular dystrophy (DMD) patients amenable to exon 50 skipping.

GEN6050X is an intravenously administered human DMD exon 50 skipping base editing drug containing dual single-stranded adeno-associated virus serotype 9 (ss.AAV9) vectors.

The study is a first-in-human, single-arm, open-label, single-center clinical trial to evaluate safety and tolerability of a single intravenous infusion of GEN6050X in ambulatory boys with DMD. Other objectives include pharmacokinetics, pharmacodynamics, and the preliminary clinical efficacy of GEN6050X over 52 weeks. A total of three ambulatory pediatric participants (aged 4 to 9 years old) are expected to enroll, each receiving a dose of 5×10^13 vg/kg. These participants will be dosed in a staggered fashion.

Safety assessments will include monitoring of adverse events (AEs), laboratory tests, electrocardiograms (ECGs), vital signs, and physical examinations throughout the study duration. In addition, a comprehensive short-term prophylactic immunosuppression regimen(including rituximab and sirolimus) will be administered prior to treatment in order to mitigate potential immune response.

Inclusion Criteria:

1. Subject age: 4-10 years old (including 10 years old)
2. Gender: Male
3. Patients with DMD gene exon deletion types confirmed by molecular diagnosis: 8-49, 20-49, 22-49, 51, 51-53, 51-55, 51-57, 51-59, 51-60, 51-67, 51-69, 51-75 or 51-78 and other mutations amenable to exon 50 skipping.
4. The participant is able to walk independently and completes the 10-meter walk test without assistance.
5. Participant is able to complete time to stand from supine independently in less than 30s.
6. The participant is able to cooperate with motor assessment testing.
7. Receipt of glucocorticoids for 6 months and a stable daily dose for at least 12 weeks prior to study entry
8. Ability to tolerate muscle biopsies under anesthesia with no contraindications to these procedures.

Exclusion Criteria:

1. Participants are in the active period of viral infection, including infections such as TORCH virus, Epstein-Barr(EB) virus, and severe acute respiratory syndrome coronavirus 2 (SARS-COV-2).

2. Received a live attenuated vaccine within 3 months prior to receiving GEN6050X, or was exposed to an influenza (or other inactivated) vaccine within 30 days prior to receiving GEN6050X, or received systemic antiviral, anti-infective, and/or interferon therapy.

3. Serological tests found HIV, Hepatitis B Virus(HBV), hepatitis C virus(HCV), and syphilis infection.

4. Severe infection (e.g., pneumonia, pyelonephritis, or meningitis) within 4 weeks prior to receiving gene therapy.

5. With clear symptoms of cardiomyopathy, echocardiography shows that the left ventricular ejection fraction is less than 40%.

6. Need for continuous or intermittent assisted support from a ventilator.

7. Diagnosed with autoimmune disease or receiving related treatment for autoimmune disease.

8. The following indicators are abnormal in laboratory biochemical testing:

   γ-glutamyl transpeptidase (GGT) above the 2-fold upper limit and total bilirubin above 1.5 times the upper limit, cystatin C (cystatin C) > 1.27 mg/L, hemoglobin (Hgb) < 100 or >200 g/L; Leukocytes (WBC) > 18.5×10^9/L or platelet ≤ 125×10^9/L.

9. The titer of AAV9 neutralizing antibody determined by cell suppression assay > 1:50.

10. Patients have received any gene therapy (e.g., adeno associated virus(AAV) gene therapy), cell therapy (e.g., stem cell transplantation), in vivo editing, or ex vivo editing therapy (e.g., CRISPR-Cas9, TALEN) in the past.

11. Participant has any contraindication to immunosuppressive therapy.

12. Has a medical condition or extenuating circumstance that, in the opinion of the principal investigator, is unsuitable for participation in the clinical trial.

13. The family does not wish to disclose the patient's study participation to the attending physician and other medical providers.