Purification of the precursor for the automated radiosynthesis of [18F]FCWAY by counter-current chromatography

Radiolabeled FCWAY (N-{2-[4-(2-methoxyphenyl)piperazino]}-N-(2-pyridinyl) trans-4-fluorocyclohexanecarboxamide) was prepared for human positron emission tomography (PET) studies by a simple one-step radiosynthesis. The LC–MS analysis of the products indicated that it contained impurities which may interfere with FCWAY uptake of 5-HT1A receptors and that these impurities were derived from an impurity originally present in the precursor preparation. Since preparative HPLC failed to resolve one of the impurities from the precursor, preparative-scale high-speed counter-current chromatography (HSCCC) was used for purification of this FCWAY precursor. A suitable two-phase solvent system composed of cyclohexane–ethyl acetate–methanol–water at a volume ratio of 1:1:1:1 or 4:5:4:5 was selected based on the partition coefficients of the precursor and impurity as determined by a LC–MS method. Using the second solvent ratio of 4:5:4:5 with the organic phase as a mobile phase, a 2.57 g amount of precursor preparation was successfully purified yielding 2.2 g of the pure precursor by a single run.