Phase 1/2a for Safety, PK and PD of SQY51 in Paediatric and Adult Patients Duchenne Muscular Dystrophy

INCLUSION CRITERIA FOR PHASE 1: * Boys of ≥6 years of age and ≥ 16 kg body weight. * Ambulatory or non-ambulatory status, * Patients and, if minor, their legal guardians, who are willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. * Diagnosed with Duchenne Muscular Dystrophy (DMD), genotypically confirmed with DMD mutations amenable to exon-51 skipping. * Stable hepatic and renal function. * Left ventricular ejection fraction (LVEF) at screening ≥40%. * If clinically indicated, approved concomitant treatment within standards of care guidelines for DMD, such as antihypertensive, vasodilators, lipid lowering, thyroid replacement, vitamins, mineral substitution, gastric protectors, and nutritional supplements. * Non-invasive mechanical ventilation is permissive if \< 16 h/day. * Being affiliated with a French social security. * Informed consent form signed by the patient or, if minor, by the legal guardian(s). INCLUSION CRITERIA FOR PHASE 2a: Patients must have completed Phase 1 of the study. EXCLUSION CRITERIA FOR PHASE 1 AND 2a: * Patient with any serious medical/surgical or psychiatric condition/illness/history that in the opinion of the investigator would jeopardize patient's safety or would interfere with the study assessments/results, including insufficient vaccination against infectious diseases as recommended by national guidelines, medical history of infection with Hepatitis B,C and HIV. * Patient with any known allergies to products likely to be used in the study (e.g., antiseptics, anesthetics), known hypersensitivity to any of the ingredients, or excipients of the study drug). * Patient who participated in other investigational study within the last three months, including those with investigational drugs that aim at restoring dystrophin expression such as other antisense oligomers. * Patient that received gene therapy. * Patient with intellectual disability or behavioral problem such that they cannot comply with the study procedure. * Patient with advanced cardiomyopathy and LVEF \< 40%. Patients with dysrhythmias and being treated for dysrhythmias. Patients with non-treated tachycardia. * Patient for which orthopedic surgery is planned during the time of the study. * Tracheostomized patients and dependent on invasive mechanical ventilation. Non-invasive mechanical ventilation ≥ 16 h/day. Predicted vital forced capacity \< 20%. Medical history with more than two respiratory decompensations requiring hospitalization during the previous year. No respiratory decompensation in the four months preceding enrolment. * Patients on medications that can restore dystrophin expression, tamoxifen and other drugs without indication for DMD or paediatric population. * Abnormal laboratory values in the clinically significant range.