Development of a thin layer chromatography method for plasma correction of [¹⁸F]fluorocholine metabolites in positron...

After 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.

First, 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.

A 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 = 0,03 ([¹⁸F]fluorocholine) and Rf = 0.78 (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% ± 6%, 58% ± 15%, 43% ± 10%, 39% ± 6% and 37% ± 6% at 2.8 min, 5.8 min, 8.8 min, 11.7 min and 14.7 min post-injection, respectively.

We 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.