Synthesis and evaluation of 2′-deoxy-2′-¹⁸F-fluoro- 5-fluoro-1-β-D-arabinofuranosyluracil as a potential PET imaging agent for suicide gene expression

2′-Deoxy-2′-¹⁸F-fluoro-5-fluoro-1-β-D- arabinofuranosyluracil (¹⁸F-FFAU) has been synthesized and evaluated in HT-29 cells as a potential PET agent for herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression. Methods: 2-Deoxy-2- ¹⁸F-fluoro-1,3,5-tri-O-benzoyl-α-D-arabinofuranose was prepared by the reaction of the respective 2-triflate with tetrabutylammonium ¹⁸F-fluoride. The fluorosugar was converted to its 1-bromo derivative and coupled with protected 5-fluorouracil. The crude product was hydrolyzed in base and purified by high-performance liquid chromatography to obtain the ¹⁸F-FFAU. In vitro studies were performed in HT-29 cells by incubation at various time points. In vivo studies including biodistribution and microPET were performed in tumor-bearing nude mice. Results: The radiochemical yield was 20%-30% decay corrected with an average of 25% in 4 runs. Radiochemical purity was >99% and average specific activity was 85 GBq/μmol (2,300 mCi/μmol) (end of synthesis). In vitro accumulation of ³H-FFAU in HSV1-tk-expressing cells was ∼180-fold (P < 0.001) higher than that in the wild-type cells between 30 and 120 min. In vivo uptake of ³H-FFAU in HSV1-tk-positive tumors at 2 h was ∼8-fold (P < 0.001) higher than that in the control tumors. Tumor uptake (percentage injected dose per gram of tissue) and the uptake ratio (tk-positive to wild type) of ³H-FFAU in tk-positive cells was higher compared with those of our earlier studies using 2′-¹⁴C-deoxy-2′-fluoro-5-methyl-1- β-D-arabinofuranosyluracil (¹⁴C-FMAU) and 9-(4- ¹⁸F-fluoro-3-hydroxymethylbutyl) guanine (¹⁸F-FHBG) in the same cell lines. micro-PET on tumor-bearing nude mice also demonstrated a very high uptake of ¹⁸F-FFAU in tk-positive tumors compared with that of the control tumor without significant accumulation in other organs. Conclusion: These results demonstrate that ¹⁸F-FFAU has superior biodistribution characteristics and significantly higher in vivo uptake in HSV1-tk-expressing tumor compared with previously studied agents.