The Efficacy and Safety of Metoprolol as add-on Treatment to Standard of Care in Preventing Cardiomyopathy in Patients With DMD

Patients will be evaluated for inclusion during a Screening period of up to one year (possible to extend for another year if warranted). Written informed consent from the patient and/or parent/legal guardian and assent, if applicable, to participate in the study must be obtained prior to beginning any study-related procedures. Once eligibility is confirmed, patients will undergo Screening Assessments. Two groups will receive either metoprolol succinate (Treatment 1 - Intervention) at a dose depending on the weight category as described below or matching placebo (Treatment 2 - Control) to maintain the study blinding, one per day for up to 60 months. All patients (both arms) will receive ACEi (e.g. perindopril, enarenal, lisinopril) or, if indicated, ARB (e.g. losartan) as a standard of care at an appropriate therapeutic dose (see below) through all the study duration. A total of approximately 150 subjects will be randomized into the study. Subjects will be assessed for safety and tolerability, clinical efficacy, at scheduled visits throughout the study. Adverse events, including SAEs, and concomitant medications will be recorded throughout the study in eCRF record. Access credentials for ePROP will be dispensed to the participant or his legal guardian when appropriate at the Enrolment Visit to record vital signs (HR and NIBP) and peak-flow-meter readings, AEs, changes to concomitant medications taken during the study, and any missed or incomplete doses of study medication. Site study staff will contact the parent(s)/ legal guardian(s) by telephone at quarterly to ensure that the tablet dispensing is proceeding according to protocol and to address any questions the parent(s)/guardian(s) may have. All subjects will periodically return to the clinical site for assessments according to the Visit Plan. The study is comprised of a Pre-screening, Screening-Enrolment-Randomization Double-Blind Treatment Period further subdivided into Phases. Subjects will be enrolled into this study at the time written informed consent is given and randomized to treatment only after completion of all Pre-treatment Screening and Enrolment assessments. Study drug dosing will occur at home on all days starting from Enrolment Visit until the Final Visit. In the event that relevant clinical or laboratory parameters remain abnormal at the time of discharge from the study, the subject will be followed medically, as clinically indicated. Any subject who prematurely discontinues the study should return to the study centre for scheduled assessments at the time of early withdrawal, whenever possible assuming the subject has not withdrawn consent.