Demonstration of [C-11] 5-hydroxy-L-tryptophan uptake and decarboxylation in carcinoid tumors by specific positioning labeling in positron emission tomography

In three patients with carcinoid liver and/or lymph node metastases, we stu died the process of tumor tracer uptake and decarboxylation by means of pos itron emission tomography (PET) using 5-hydroxy-L-tryptophan (5-HTP) C-11-l abeled in the beta-position (HTP) and later the same day with 5-HTP C-11-la beled in the carboxyl group (HTC). With HTP, in which the C-11-label follow s the molecule through decarboxylation to form C-11-serotonin, a high tumor accumulation of the tracer was found. With HTC, in which the label is rapi dly eliminated from the tissues as (CO2)-C-11 if decarboxylation takes plac e, there was virtually no uptake by the tumors. By utilizing data from PET scanning with both tracers, we could quantify the decarboxylation rate and tissue accumulation of [C-11]-serotonin and hence the enzymatic action of a romatic amino acid decarboxylase. NUCL MED BIOL 27;1:33-41, 2000. (C) 2000 Elsevier Science Inc. All rights reserved.