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

This is a single center, randomized, double-blind, placebo controlled study. The study medication consists of L-citrulline and metformin, respectively placebo given orally; 2.5 g L-citrulline or placebo will be given 3 times daily, metformin containing 250 mg or placebo will be administered 3 times daily. The duration of the study is 26 weeks and comprehends one screening and three study visits.

Amendment 1: Amended eligibility criteria

Inclusion Criteria:

• Molecular diagnosis of DMD
• Patients 6.5 - 10 years of age at time of screening
• Ambulant
• Ability to walk 150 m in the 6 min walking distance (6MWT)
• D1 subdomain of the MFM scale >40%
• stable treatment with steroids for >6 months or steroid naïve patients

Exclusion Criteria:

• Previous (3 months or less) or concomitant participation in another therapeutic trial
• Use of L-citrulline, L-arginine or metformin within the last 3 months
• Known individual hypersensitivity to L-citrulline or metformin
• known or suspected malignancy
• Other chronic disease or clinical relevant limitation of renal, liver, heart function according to discretion of investigator
• start of cortisone treatment or change in dosage <6 months prior to screening