This is a 48-week multicentre, parallel, randomised, double-blind, placebo controlled phase 3 safety and efficacy trial. There are two treatment arms: Tamoxifen (verum) and placebo (control), with treatment allocation of 1:1.~The investigators plan to screen at least 79 and to enroll at least 71 ambulant DMD patients aged between 6.5 and 12 years (group A) and 6 - 20 non-ambulant DMD patients aged between 10 and 16 years (group B). In order to reach statistical power, 60 ambulant patients (group A) need to complete the trial. Treatment with 20 mg Tamoxifen once daily will be given for the total trial duration of 48 weeks.~Only patients with glucocorticoids (standard treatment of care) will be included in group A (ambulant patients) and only non-glucocorticoid users in group B. At baseline as well as at the end of the study clinical, laboratory, and MRI measurements will be performed. These include the Motor Function Measure (MFM) scale, timed function tests, the 6 minute walking distance, quantitative muscle testing (QMT) and quantitative thigh muscle MRI, questionnaires. A physical examination, an ECG, vital signs as well as safety laboratory blood analyses will be performed at every visit. Furthermore, an x-ray of the hand and a dual energy x-ray absorptiometry (DEXA)-scan will be performed at baseline and at the end of the study.~An open label extension (OLE) trial for participants of the TAMDMD main study will be performed. All TAMDMD patients on TAM or placebo are offered to enter this OLE. All OLE patients will receive 20 mg of TAM daily during 48 weeks. The same study specific assessments as in the double-blind randomized phase will be performed during the OLE phase
A randomised, double blind, placebo controlled, 48-week clinical trial with a core population (group A) of 79 ambulant 6.5 to 12 years old Duchenne's muscular dystrophy (DMD) patients that are under stable standard treatment of care with glucocorticoids. Furthermore, the investigators plan to include 6-20 non-ambulant patients who do not receive glucocorticoids (as parallel group B), 10 to 16 years old, to obtain efficacy and safety data in a broader DMD population. All patients will receive 20 mg of tamoxifen (TAM) or placebo once daily during 48 weeks.~An open label extension (OLE) trial for participants of the TAMDMD main study will be performed. All TAMDMD patients on TAM or placebo are offered to enter this OLE.