This exploratory pilot study was an open-label, crossover, single-center and non-randomized clinical trial designed to compare the effect of the standardly employed doses of allopurinol (400 mg/day) and febuxostat (80 mg/day) on the urinary DHA excretion in patients with APRT deficiency. The study was conducted between May 2013 and May 2015 as participants were enrolled at different times. The only study site was Landspitali - The National University Hospital of Iceland in Reykjavik, Iceland. The Data (Observational) Safety Monitoring Board (D/OSMB) constituted by the National Institutes of Health had oversight responsibility of the Data Safety Monitoring Plan for this clinical trial. The monitoring board reviewed accrual, patterns and frequencies of all adverse events, and protocol compliance every 6-12 months. All study subjects gave a written informed consent for their participation.