Synthesis and Hypoxia Selective Radiosensitization Potential of β-2-FAZA and β-3-FAZL: Fluorinated Azomycin β-Nucleosides

18F-Labelled fluoroazomycin arabinoside ([18F]FAZA) is a 2-nitroimidazole (azomycin) based PET tracer used extensively in cancer clinics to diagnose tumour hypoxia. The hypoxia-specific uptake and rapid blood clearance kinetics of FAZA contribute to good tumor-to-background ratios (T/B ratios) and high image contrast. However, FAZA, an α- configuration nucleoside, is not transported by cellular nucleoside transporters. It enters cells only via diffusion, therefore not achieving the high uptake and T/B ratios characteristic of actively transported radiopharmaceuticals. The present work describes the synthesis, physicochemical properties and preliminary assessment of the radiosensitization properties of two novel azomycin nucleosides, 1-β-D-(2-deoxy-2-fluoroarabinofuranosyl)-2-nitroimidazole (β-2-FAZA) and 1-β-D-(3- deoxy-3-fluorolyxofuranosyl)-2-nitroimidazole (β-3-FAZL) (fluorination yields 60% and 55%, respectively). The tosylated precursors required to synthesize the corresponding F-18 labeled radiopharmaceuticals are also reported. The partition coefficients (P) for β-2-FAZA (1.0) and β-3-FAZL (0.95) were marginally lower than reported for FAZA (1.1). The radiosensitization properties of both these compounds are similar to that of FAZA, with sensitizer enhancement ratios (SER) of ∼1.8 for HCT-116 cells.