RESUMO
[(18)F]FHOMP (6-((1-[(18)F]-fluoro-3-hydroxypropan-2-yloxy)methyl)-5-methylpyrimidine-2,4(1H,3H)-dione), a C-6 substituted pyrimidine derivative, has been synthesized and evaluated as a potential PET agent for imaging herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene expression. [(18)F]FHOMP was prepared by the reaction of the tosylated precursor with tetrabutylammonium [(18)F]-fluoride followed by acidic cleavage of the protecting groups. In vitro cell accumulation of [(18)F]FHOMP and [(18)F]FHBG (reference) was studied with HSV1-tk transfected HEK293 (HEK293TK+) cells. Small animal PET and biodistribution studies were performed with HEK293TK+ xenograft-bearing nude mice. The role of equilibrative nucleoside transporter 1 (ENT1) in the transport and uptake of [(18)F] FHOMP was also examined in nude mice after treatment with ENT1 inhibitor nitrobenzylmercaptopurine ribonucleoside phosphate (NBMPR-P). [(18)F]FHOMP was obtained in a radiochemical yield of ~25% (decay corrected) and the radiochemical purity was greater than 95%. The uptake of [(18)F]FHOMP in HSV1-TK containing HEK293TK+ cells was 52 times (at 30 min) and 244 times (at 180 min) higher than in control HEK293 cells. The uptake ratios between HEK293TK+ and HEK293 control cells for [(18)F]FHBG were significantly lower i.e. 5 (at 30 min) and 81 (240 min). In vivo, [(18)F]FHOMP accumulated to a similar extend in HEK293TK+ xenografts as [(18)F]FHBG but with a higher general background. Blocking of ENT1 reduced [(18)F]FHOMP uptake into brain from a standardized uptake value (SUV) of 0.10±0.01 to 0.06±0.02, but did not reduce the general background signal in PET. Although [(18)F]FHOMP does not outperform [(18)F]FHBG in its in vivo performance, this novel C-6 pyrimidine derivative may be a useful probe for monitoring HSV1-tk gene expression in vivo.
RESUMO
INTRODUCTION: We report on the synthesis, radiolabeling, in vitro and in vivo characterization of N-Me-[(18)F]FHBT (6-(3-[(18)F]fluoro-2-(hydroxymethyl)propyl)-1,5-dimethylpyrimidin-2,4(1H,3H)-dione), a C-6-substituted N-1-methylated pyrimidine derivative as a reporter probe for imaging herpes simplex virus type 1 thymidine kinase (HSV1-TK) expression. METHODS: N-Me-[(18)F]FHBT was synthesized via a standard nucleophilic substitution reaction followed by acidic cleavage of the methoxytrityl protecting group. Cell uptake was studied in vitro with control HEK293 (human embryonic kidney cells) and HEK293 cells stably transfected with nonmutant HSV1-tk (HEK293TK+ cells). Positron emission tomography (PET) imaging and biodistribution studies of N-Me-[(18)F]FHBT or [(18)F]FHBG were performed in nude mice bearing xenografts of HEK293 control and TK+ cells. RESULTS: N-Me-[(18)F]FHBT was obtained in a two-step reaction in an overall maximal radiochemical yield (decay-corrected) of 5% and a radiochemical purity >96%. The tracer uptake in HSV1-TK containing HEK293TK+ cells was 14.5 times (at 30 min) and 55.4 times (at 240 min) higher than in control HEK293 cells. In mice, N-Me-[(18)F]FHBT and [(18)F]FHBG accumulated significantly and exhibited similar radioactivity levels in the HEK293TK+ xenografts; however, standardized uptake values ratios between HEK293TK+ and HEK293 control xenografts were higher for [(18)F]FHBG than for N-Me-[(18)F]FHBT. Both tracers showed high gall bladder and abdominal activity. CONCLUSION: The biological evaluations demonstrated the feasibility of using N-methylated C-6-substituted pyrimidine derivative N-Me-[(18)F]FHBT as a PET radiotracer for monitoring HSV1-TK expression in vivo.
