Blood metabolism of [methyl- 11C]choline; implications for in vivo imaging with positron emission tomography

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Abstract
[methyl-11C]Choline (11C-choline) is a radioligand potentially useful for oncological positron emission tomography (PET). As a first step towards the development of a kinetic model for quantification of 11C-choline uptake, blood metabolism of 11C-choline during PET imaging was studied in humans. High-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC) were used for the analysis of 11C-choline and its radioactive metabolites. Prior to human PET imaging we studied ex vivo the biodistribution and metabolism of intravenously administered 11C-choline in rats. Our results revealed that the radioactivity accumulated particularly in kidney, lung, adrenal gland and liver. Chromatographic analysis showed that the level of unmetabolized 11C-choline in rat plasma decreased from 42%±20% (mean±SD) at 5 min to 21%±10% at 15 min after injection. In accordance with these findings, in humans the unmetabolized 11C-choline represents 62%±19% of the total radioactivity in arterial plasma at 5 min after injection and 27%±12% at 15 min. In human venous plasma the corresponding values were 85%±12% and 48%±12% at 5 and 10 min, respectively. The major metabolite observed in both human and rat plasma was identified as 11C-betaine. In human arterial plasma this maximally represented 82%±9% of the total radioactivity at 25 min after radiotracer injection. By 20 min after injection, the 11C-choline and 11C-betaine in human arterial plasma reached a plateau, and their fractional activities remained nearly constant thereafter. Although most of the circulating 11C-choline in blood is transported to tissues, it does not disappear totally from blood within the first 40 min after tracer injection.

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