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 1 thymidine kinase (HSV1-tk) gene expression. The Negishi coupling methodology was employed in efficiently synthesizing the radiolabeling precursors. All six 5-[18F]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 [18F]-labeled nucleosides selectively accumulated in cells expressing the HSV1-TK protein and there was negligible uptake in control cells. [18F]FPrDU, [18F]FBuDU, [18F]FPeDU, and [18F]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 μM). Ultimately, the results demonstrated that 5-[18F]fluoropropyl, [18F]fluorobutyl, and [18F]fluoropentyl pyrimidine nucleosides have the potential to be in vivo HSV1-TK PET reporter probes over a dynamic range of reporter gene expression levels.