Development of a thin layer chromatography method for plasma correction of [18F]-fluoromethylcholine metabolites in positron emission tomography quantification studies in humans.

Abstract

INTRODUCTION\nAfter its intravenous injection, [18F]fluorocholine is oxidized by choline-oxidase into its main plasma metabolite, [18F]fluorobetaine. If PET kinetic modeling quantification of [18F]fluorocholine uptake is intended, the plasma input time-activity-curve of the parent tracer must be obtained, i.e., the fraction of the total plasma radioactivity corresponding to the nonmetabolized [18F]fluorocholine at each time has to be known. Hence our aim was to develop an easy-routine Thin-Layer-Chromatography (TLC) method to separate and quantify the relative fractions of [18F]fluorocholine and [18F]fluorobetaine as a function of time during PET imaging in humans.\n\n\nMETHODS\nFirst, we tested several combinations of solvents systems and layers to select the one showing the best resolution on non-radioactive standards. Thereafter, [18F]fluorobetaine was obtained through chemical oxidation of an [18F]fluorocholine sample at diferent incubation times and we applied the selected TLC-system to aliquots of this oxidation solution, both in a saline and in human deproteinized plasma matrices. The plates were detected by a radio-TLC-scanner. This TLC-system was finally applied to arterial plasma samples from 9 patients with high-grade-glioma undergoing brain PET imaging and a parent fraction curve was obtained in each of them.\n\n\nRESULTS\nA TLC-system based on Silica-Gel-60//MeOH-NH3 was selected from the choline/betaine non-radioactive standards assay. Radiochromatograms of [18F]fluorocholine oxidation solution yielded two separated and well-defined peaks, Rf\u202f=\u202f0,03 ([18F]fluorocholine) and Rf\u202f=\u202f0.78 (18F]fluorobetaine) consistent with those observed on non-radioactive standards. During the oxidation, the [18F]fluorocholine radioactivity peak decreased progressively at several incubation times, while the other peak ([18F]fluorobetaine) increased accordingly. The mean values of the parent fraction of [18F]fluorocholine of the 9 patients studied (mean+/-SD) were 94%\u202f±\u202f6%, 58%\u202f±\u202f15%, 43%\u202f±\u202f10%, 39%\u202f±\u202f6% and 37%\u202f±\u202f6% at 2.8\u202fmin, 5.8\u202fmin, 8.8\u202fmin, 11.7\u202fmin and 14.7\u202fmin post-injection, respectively.\n\n\nCONCLUSIONS\nWe have developed a TLC-system, easy to perform in a standard radiopharmacy unit, that enables the metabolite correction of arterial input function of [18F]fluorocholine in patients undergoing PET oncologic quantitative imaging.