The fate of intravenously administered tetrahydrobiopterin and its implications for heterologous gene therapy of phenylketonuria.

Tetrahydrobiopterin (BH(4)) is a required cofactor for the enzymatic activity of phenylalanine hydroxylase (PAH) and is synthesized de novo from GTP in several tissues. Heterologous expression of PAH in tissues other than liver is a potential novel therapy for human phenylketonuria that is completely dependent upon BH(4) supply in the PAH-expressing tissue. Previous experiments with liver PAH-deficient transgenic mice that expressed PAH in skeletal muscle demonstrated transient correction of hyperphenylalaninemia only with hourly parenteral BH(4) administration. In this report, the fate of intravenously administered BH(4) is examined. The conclusions are that (1) BH(4) administered intravenously is rapidly taken up by liver and kidney, and (2) uptake of BH(4) into muscle is relatively low. The levels of BH(4) achieved in skeletal muscle following IV injection are only 10% of the amount expected were BH(4) freely and equally distributed across all tissues. The half-life of BH(4) in muscle is approximately 30 min, necessitating repeated injections to maintain muscle BH(4) content sufficient to support phenylalanine hydroxylation. The efficacy of heterologous muscle-directed gene therapy for the treatment of PKU will likely be limited by the BH(4) supply in PAH-expressing muscle.