Serum levels of 5-hydroxyindole derivates after administration of L-5-hydroxytryptophan ethyl ester

Serum levels of 5-hydroxytryptophan ethyl ester, 5-hydroxytryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid (5-HTPE, 5-HTP, 5-HT and 5-HIAA, respectively), were measured after intravenous administration of L-5-HTPE in humans, premedicated with a peripheral decarboxylase inhibitor. Semi-automated methods for the estimation of the 5-hydroxyindole derivatives are described. Only serum levels of 5-HTP and 5-HIAA were found to be increased, indicating that the ethyl ester is rapidly hydrolyzed and that the decarboxylation of 5-HTP is in part inhibited. The levels of 5-HTP increased during the infusion, but dropped rapidly when the intravenous administration was terminated. Serum levels of 5-HIAA remained constant for at least 6 hours, although during the same period the levels of 5-HTP do change markedly. The persistently increased serum levels of 5-HIAA suggests, that this metabolite is formed from the 5-HT stored in peripheral or central tissue. Serum levels of 5-HIAA may therefore be indicative of changes of 5-HT metabolism during drug treatment. Levels of 5-HTP may be used for the estimation of the availability of the ethylester of 5-HTP.