Plasma Accumulation and Metabolism of Orally Administered Single Dose L‐5‐Hydroxytryptophan in Man

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Abstract
Single oral doses of L‐5‐hydroxytryptophan (5‐HTP) were administered in combination with L‐aromatic amino acid decarboxylase inhibitors. The time courses of plasma concentrations of 5‐HTP, 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) and the concentrations of 5‐HT in blood platelets were measured. Carbidopa enhanced the rise in plasma concentrations of 5‐HTP 5–15 fold and counteracted the increase in plasma 5‐HIAA levels induced by 5‐HTP alone. A single dose of the decarboxylase inhibitor was equipotent to 14 days' pretreatment. Plasma or platelet concentrations of 5‐HT failed to reflect the metabolism of 5‐HTP. The ratio of 5‐HTP to carbidopa influenced the systemic bioavailability of single dose administered 5‐HTP indicating dose dependent absorption kinetics. Co‐administration of L‐dopa with 5‐HTP and decarboxylase inhibitors had no effect on gastrointestinal absorption of 5‐HTP in six parkinsonian patients.

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