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

Tetrahydrobiopterin (BH4) 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 BH4 supply in the PAH-expressing tissue. Previous experiments with liver PAH-deficient transgenic mice that expressed PAH in skeletal muscle demonstrated transient correction of hyperphenylataninernia only with hourly parenteral BH4 administration. In this report, the fate of intravenously administered BH4 is examined. The conclusions are that (1) BH4 administered intravenously is rapidly taken up by liver and kidney, and (2) uptake of BH4 into muscle is relatively low. The levels of BH4 achieved in skeletal muscle following IV injection are only 10% of the amount expected were BH4 freely and equally distributed across all tissues. The half-life of BH4 in muscle is approximately 30 min, necessitating repeated injections to maintain muscle BH4 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 BH4 supply in PAH-expressing muscle. (C) 2003 Elsevier Inc. All rights reserved.