Synthesis and in vitro evaluation of 5-[18F]fluoroalkyl pyrimidine nucleosides for molecular imaging of herpes simplex virus type 1 thymidine kinase reporter gene expression

Two novel series of 5-fluoroalkyl-2'-deoxyuridines (FPrDU, FBuDU, FPeDU) and 2'-fluoro-2'-deoxy-5-fluoroalkylarabinouridines (FFPrAU, FFBuAU, FFPeAU) that have three, four, or five methylene units (propyl, butyl, or pentyl) at C-5 were prepared and tested as reporter probes for imaging herpes simplex virus type I thymidine kinase (HSV1-tk) gene expression. The Negishi coupling methodology was employed in efficiently synthesizing the radiolabeling precursors. All six 5-[F-18]fluoroalkyl pyrimidines were readily prepared from 3-N-benzoyl-3',5'-di-O-benzoyl-protected 5-O-mesylate Precursors in 17-35% radiochemical yield (decay-corrected). In vitro studies highlighted that all six [F-18]-labeled nucleosides selectively accumulated in cells expressing the HSV1-TK protein and there was negligible uptake in control cells. [F-18]FPrDU, [F-18]FBuDU, [F-18]FPeDU, and [F-18]FFBuAU had the best uptake profiles. Despite their selective accumulation in HSV1-tk-expressing cells, all 5-fluoroalkyl pyrimidine nucleosides had low-to-negligible cytotoxic activity (CC50 > 1000-1209 mu M). Ultimately, the results demonstrated that 5-[F-18]fluoropropyl, [F-18]fluorobutyl, and [F-18]fluoropentyl pyrimidine nucleosides have the potential to be in vivo HSV1-TK PET reporter probes over a dynamic range of reporter gene expression levels.