Amino Acid Distribution and Metabolism in Pituitary Adenomas Using Positron Emission Tomography with D-[11C]Methionine and L-[11C]Methionine

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Abstract
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]methionine, 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 stereoisomer 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.