Effect of Carrier on Organ Distribution of 75Se-Selenomethionine

Abstract 75 Se-selenomethionine was the first amino acid used in clinical scintillation scanning. It can be produced with a high specific activity by biosynthesis (specific activity about 300 mCi/mg) or chemical synthesis (specific activity about 6 mCi/mg). Synthetic methods have been developed recently for the incorporation of the short-lived radionuclides carbon-11 and fluorine-18 into organic molecules. The final products of these syntheses have, however, relatively low specific activities (about 1.0 μCi/mg). This made it important to study the effect of carrier on the organ distribution of labeled organic compounds. For that purpose, mice were injected with 75 Se-selenomethionine, with varying amounts of d,l-methioine, d,l-selenomethionine, and l-methionine added. With increasing amounts of selenomethionine carrier there was a decrease of uptake to a maximum of 50% in all organs, while with methionine as carrier the liver uptake increased. As a result, selenomethionine carrier, unlike methionine, does not cause a decrease in the ratios of pancreas/liver concentration. The fact that methionine and selenomethionine follow separate pathways is significant for external visualization. It indicates that radioactive organic compounds of low specific activity may be useful for clinical scintigraphy, as long as one can ensure that the nonradioactive carrier is identical to the labeled compound.