Metoprolol Safety and Efficacy in the Prevention of Cardiomyopathy in Patients with Duchenne Muscular Dystrophy (DMD)

Lab values

• Current or previous permanent use of any beta-blocker medication 2. Treatment with another investigational drug or other intervention within 3 months prior to screening 3. Clinically significant bradycardia at rest or by Holter ECG, based on age and sex adjusted normal values, atrioventricular block higher than first degree at rest, or second degree Wenckebach at night, pauses longer than 2.5 seconds 4. Presence of pacemaker or ICD 5. Clinical signs or symptoms of heart failure 6. Left ventricular Ejection Fraction (LVEF) <57% (assessed by Teichholtz echocardiography) 7. Inability to obtain adequate quality echocardiography images (necessary to monitor for primary endpoint and safety) 8. Known allergic reactions to components of the IMPs

• Subject's parent(s) or legal guardian(s) has (have) provided written informed consent, where applicable, prior to any study-related procedures; participants will be asked to give written or verbal assent according to requirements (>16 years old) 2. Stated willingness to comply with all study procedures and availability for the duration of the study 3. Male, ≥ 8 years and <17 years of age at time of enrolment in the study 4. Ability to take oral medication and be willing to adhere to the study intervention regimen 5. Subject has confirmed diagnosis of DMD, as defined as clinical picture consistent with typical DMD and: • Dystrophin immunofluorescence and/or immunoblot showing complete dystrophin deficiency, or • Identifiable mutation within the DMD gene (deletion/duplication of one or more exons), where reading frame can be predicted as 'out-offrame' or • Complete dystrophin gene sequencing showing an alteration (point mutation, duplication, other) that is expected to preclude production of the dystrophin protein (i.e., nonsense mutation, deletion/duplication leading to a downstream stop codon) 6. Taking ACEi treatment at minimum required doses as outlined in the protocol for at least 30 days