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 hyperphenylalaninemia 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 BH ₄ 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 30min, 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.