Tetrahydropterin therapy for hyperphenylalaninemia caused by defective synthesis of tetrahydrobiopterin

A patient with hyperphenylalaninemia caused by a defect in the synthesis of tetrahydrobiopterin was treated with 6‐methyltetrahydropterin. This synthetic analog of the naturally occurring hydroxylation cofactor tetrahydrobiopterin, when given orally at a daily dose of 20 mg per kilogram of body weight increased depressed plasma and cerebrospinal fluid levels of norepinephrine. At a daily dose of 8 mg/kg, this pterin increased depressed cerebrospinal fluid levels of the biogenic amine metabolites dihydroxyphenylacetic acid, homovanillic acid, and 5‐hydroxyindoleacetic acid. At these doses of 6‐methyltetrahydropterin, there was an improvement of the patient's neurological symptoms, including a pronounced decrease in eye rolling and drooling and a marked increase in muscle strength, coordination, and physical activity.