A p-[18F]fluoroethoxyphenyl bicyclic nucleoside analogue as a potential positron emission tomography imaging agent for varicella-zoster virus thymidine kinase gene expression

Abstract

We recently reported a new positron emission tomography (PET) reporter gene, namely, varicella-zoster virus thymidine kinase (VZV-tk) in combination, with carbon-11 or fluorine-18 labeled m-alkoxyphenyl bicyclic nucleoside analogues (BCNAs) as PET reporter probes. We now report the synthesis and evaluation of p-alkoxyphenyl-BCNA tracers ([11C]-4 and [18F]-5), which are found to be superior to the m-alkoxyphenyl-BCNA tracers. In particular, the fluorine-18 labeled tracer ([18F]-5, IC50 of 5 is 4.2 microM) shows a higher accumulation in VZV-tk expressing cells than the previously reported m-methoxyphenyl BCNA. [11C]-4 and [18F]-5 were synthesized by heating the phenol precursor 3 with 11CH 3I and 18FCH 2CH 2Br, respectively, as alkylating agents. In vitro evaluation of [11C]-4 and [18F]-5 in 293T cells showed about 14- and 54-fold higher uptake, respectively, into VZV-tk gene-transduced cells compared to control cells. LC-MS analysis confirmed the formation of monophosphate derivative of 5 upon catalysis by VZV TK. In vivo studies of this new reporter gene/probe system are in progress.