Synthesis of¹⁸F-labeled acyclic purine and pyrimidine nucleosides intended for monitoring gene expression

Non-invasive imaging of genes which are introduced into cells is a useful method for gene therapy monitoring. The labeling of acyclic purine and pyrimidine nucleoside derivatives with fluorine-18 required for the HSV-1 tk imaging approach with positron emission tomography (PET) is described. The methoxytritylated and tosylated precursors were radiolabeled using a K[F-18]F/kryptofix 2.2.2 (TM) Complex, followed by removal of the protecting groups under acidic conditions and HPLC purification. The radiochemical yields of the F-18-tracers amount to 5%-15% (decay corrected) after a synthesis time of 85-95 min; the radiochemical purity was > 98% with an average specific activity of 19 GBq/mu mol at the end of synthesis.