Rationale: Classic galactosemia is a rare inherited metabolic disease that presents in neonatal patients with a life-threatening multi-organ toxic syndrome. Although the current standard of care - a galactose-restricted diet - quickly relieves the severe neonatal clinical picture, it fails to prevent brain and gonadal sequelae. There is a need for new therapeutic strategies.

As arginine is an amino acid that is therapeutically widely used with no side effects described, we propose to use it in a pilot-clinical study. We aim to evaluate the effects of arginine in classic galactosemia patients, in order to determine its potential therapeutic role in this disease.

Objective: To evaluate the possible effect of arginine on the whole body galactose oxidative capacity in classic galactosemia patients.

Study design: Interventional pilot-clinical study with pre-post single arm design.

Study population: We aim to include 5 classic galactosemia adult patients homozygous for the p.Q188R mutation.

Intervention: All participants will receive arginine in the form of Asparten ® (arginine aspartate) during 1 month, by oral administration.

The main study parameter is whole body galactose galactose oxidative capacity.

Inclusion Criteria:

• Classic galactosemia patient homozygous for the p.Q188R mutation, diagnosed by GALT enzyme activity assay and GALT gene mutation analysis
• Eighteen years of age or older
• Capable of giving informed consent

Exclusion Criteria:

• Urea cycle disorders (assessed by post prandial amino acid profile in blood)
• Increased level of plasma uric acid
• Patients experiencing acute illness of classic galactosemia
• Pregnant women (or considering getting pregnant) or breastfeeding women