A Phase III Double-blind Study With Idebenone in Patients With Duchenne Muscular Dystrophy (DMD) Taking Glucocorticoid Steroids

Inclusion Criteria: 1. Male patients with a 35% ≤ FVC ≤ 80% of predicted value at Screening and at Baseline and who, in the opinion of the investigator are in the respiratory function decline phase. 2. Minimum 10 years old at Screening. 3. Signed and dated Informed Consent Form. 4. Documented diagnosis of DMD (severe dystrophinopathy) and clinical features consistent of typical DMD at diagnosis (i.e. documented delayed motor skills and muscle weakness by age 5 years). DMD should be confirmed by mutation analysis in the dystrophin gene or by substantially reduced levels of dystrophin protein (i.e. absent or \<5% of normal) on Western blot or immunostaining. 5. Chronic use of systemic glucocorticoid steroids for DMD related conditions continuously for at least 12 months prior to Baseline without any dose adjustments on a mg/kg basis in the last 6 months (only dose adjustments determined by weight changes are allowed). 6. Ability to provide reliable FVC values at Screening and Baseline, and reproducible within 15% (relative change) at Baseline compared to Screening. 7. Patients assessed by the Investigator as willing and able to comply with the requirements of the study, possess the required cognitive abilities and are able to swallow study medication. 8. Patients who prior to Screening have been immunized with 23-valent pneumococcal polysaccharide vaccine or any other pneumococcal polysaccharide vaccine as per national recommendations, as well as annually immunized with inactivated influenza vaccine. Exclusion Criteria: 1. Symptomatic heart failure (defined as patients with structural heart disease, dyspnea, fatigue and impaired tolerance to exercise; Stage C by the ACCF/AHA guideline or NYHA Classes III-IV) and/or symptomatic ventricular arrhythmias. 2. Ongoing participation in any other therapeutic trial and/or intake of any investigational drug within 90 days prior to Baseline (only exception allowed is use of Deflazacort in the US as part of the Expanded Access Program, or any approved corticosteroid product in trial for regimen optimization, for which the patient met the inclusion criterion 5). 3. Ongoing exon-skipping therapy or read-through gene therapy for DMD; previous exon-skipping or read-through gene therapy is allowed if the stop date was more than 6 months prior to Screening. 4. Planned or expected spinal fusion surgery during the study period (as judged by the Investigator; i.e. due to rapidly progressing scoliosis), previous spinal fusion surgery is allowed if it took place more than 6 months prior to Screening. 5. Asthma, bronchitis/COPD, bronchiectasis, emphysema, pneumonia or presence of any other non-DMD respiratory illness that affects respiratory function. 6. Chronic use of beta2-agonists or any use of other bronchodilating/bronchoconstricting medication (inhaled steroids, sympathomimetics, anticholinergics, antihistamines); chronic use is defined as a daily intake for more than 14 days. 7. Any bronchopulmonary illness that required treatment with antibiotics within 3 months prior to Screening. 8. Moderate or severe hepatic impairment (use as guidance Child-Pugh class B \[7 to 9 points\] or Child-Pugh class C \[10 to 15 points\] - see Appendix B) or severe renal impairment (eGFR \<30 mL/min/1.73 m2). 9. Prior or ongoing medical condition or laboratory abnormality that in the Investigator's opinion may put the patient at significant risk may confound the study results or may interfere significantly with the patient's participation in the study. 10. Relevant history of or current drug or alcohol abuse, or use of any tobacco or marijuana products/smoking. 11. Known individual hypersensitivity to idebenone or to any of the ingredients or excipients of the study medication. 12. Daytime ventilator assistance (defined as use of any assisted ventilation while awake). Note: Patients who suffer from a severe, unstable condition including (but not limited to) cancer, auto-immune diseases, hematological diseases, metabolic disorders or immunodeficiencies, and who are at risk of an aggravation unrelated to the study condition, can only be included in the study if accepted in writing by the Sponsor's Senior Clinical Research Physician.