The purpose of the study is to show that the intake of L-arginine and metformin improves muscle function and delays disease progression in patients with Duchenne's muscular dystrophy.

This is an investigator-initiated, open-label, single-center, proof-of-concept-study. The study medication consists of L-arginine and metformin. The duration of the study is 16 weeks and comprehends one screening and four study visits.

Inclusion Criteria:

• Molecular diagnosis of DMD
• Patients 7 - 10 years of age at time of screening
• Ambulant

Exclusion Criteria:

• Previous (3 months or less) or concomitant participation in another therapeutic trial
• Use of L-arginine, L-citrulline or metformin within the last 3 months
• Known individual hypersensitivity to L-citrulline or metformin
• Other chronic disease or clinical relevant limitation of renal, liver, heart function according to discretion of the investigator