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Four patients with prolactin-secreting pituitary adenomas were examined with positron emission tomography using L-[11C]methionine to monitor the effect of dopamine agonist treatment on the amino acid metabolism in the tumors. Within the first few hours after intramuscular injection of bromocriptine retard (50 mg) the amino acid metabolism decreased by 40. Two of the patients were reexamined 7 and 9 days later and showed a 70. reduction in the metabolism of the adenomas. This metabolic effect was later accompanied by significant tumor shrinkage in all adenomas. It is suggested that bromocriptine has a general and rapid effect on the protein synthesis of the prolactin-secreting pituitary adenoma cells.

Four patients with hormonally inactive pituitary adenomas were examined with positron emission tomography (PET) after injection, during different examinations, of L-[methyl-11C]methionine and D-[methyl-11C]methio-nine, respectively. After the rapid distribution phase, the enantiomer L-[11C]methionine, which is metabolically active, showed a considerable continuous irreversible trapping attributed to amino acid metabolism. The stereo-isomer D-[11C]methionine, which does not participate in protein synthesis, showed a rapid distribution within the whole adenoma tissue, with a distribution space on the order of 100. A minimal irreversible trapping was observed which could be explained by technical factors. It is concluded that PET using the two enantiomers allows a separation of passive distribution and metabolism, and that L-[11C]methionine can be used for in vivo quantitative studies of amino acid metabolism of pituitary adenomas.