A Study of ATL1102 or Placebo in Participants With Non-ambulatory Duchenne Muscular Dystrophy

Key Inclusion Criteria: Has a clinical diagnosis of DMD confirmed by validated genetic testing Is considered to be non-ambulatory, defined as unable to walk 10 meters without assistance or help at Screening. Male aged 10 to less than 18 years, at the time of Screening. Body weight of at least 25 kg at Screening. If receiving corticosteroid therapy, therapy was initiated at least six months prior to the baseline visit and a stable daily dose for at least 3 months prior to baseline Participant has a Performance of Upper Limb Module for DMD 2.0 (PUL 2.0) Entry Item A score ≥2. Able to perform spirometry and has sufficient Respiratory function defined as reproducible percent predicted FVC ≥50%. Has adequate cardiac function defined as left ventricular ejection fraction (LVEF) ≥45% by echocardiogram and if receiving cardiac medication, must be currently on a stable regimen and doses of cardiac therapy (at least 3 months prior to baseline Day 1) Participant and their parent/guardian/carer are willing and able to comply with scheduled visits, study medication administration and study procedures. Key Exclusion Criteria: Participation in another clinical trial (non-interventional) or administration of any investigational product or experimental product within 12 weeks or 5 half-lives (whichever is longer) preceding Day 1. Exposure to more than 3 investigational products within the 12 months prior to Day 1. History of clinically significant bleeding or coagulation abnormalities or clinically significant abnormal coagulation parameters. Currently receiving antiplatelet or anticoagulant therapy or has taken medication with an antiplatelet or anticoagulant effect within 4 weeks prior Day 1 Any evidence of clinically significant structural or functional heart abnormality (cardiomyopathy that is managed by ACEi or beta blockers is acceptable provided the LVEF inclusion criterion is met). Known history of or a positive test for hepatitis B surface antigen (HBsAg), hepatitis C (HCV) antibodies, human immunodeficiency virus (HIV) antibodies at Screening. Evidence of renal impairment and/or cystatin C >1.4 mg/L. Received a live vaccine (including intranasal influenza vaccine) within 4 weeks prior to Day 1 or planned live vaccination during the study period. Asthma (if requiring regular medication), bronchitis/chronic obstructive pulmonary disease (COPD), bronchiectasis, emphysema, pneumonia or the presence of any non-DMD respiratory illness that affects PEF and FVC or other respiratory measures. Requires day-time assisted mechanical or non-invasive ventilation (NIV) (night time NIV is permitted). Chronic use (daily intake >14 days), within one month of Day 1, of beta-2 agonists or any use of other bronchodilating medication (e.g., inhaled steroids, sympathomimetics, anticholinergics). Used carnitine, creatine, glutamine, oxatomide, idebenone or other forms of coenzyme Q10 or vitamin E or any other nutritional or antioxidant supplements or herbal medicines or anabolic steroids other than standard corticosteroids or puberty testosterone supplementation within 4 weeks of Day 1. Has an increased risk for opportunistic infections or systemic medical conditions resulting in significantly compromised immune system function