Comparison of [18F]FHBG and [14C]FIAU for imaging of HSV1-tk reporter gene expression: adenoviral infection vs stable transfection.

Earlier studies involving comparison of different reporter probes have shown conflicting results between pyrimidine nucleosides [e.g., 2'-fluoro-2'-deoxy-1-beta- d-arabinofuranosyl-5-iodouracil (FIAU)] and acycloguanosine derivatives [e.g., penciclovir (PCV), 9-(4-fluoro-3-hydroxymethylbutyl)guanine (FHBG)]. We hypothesized that this reported discrepancy may be related to how the reporter gene is delivered to the cells-stably transfected vs adenoviral infection. We directly compared the uptake characteristics of [(18)F]FHBG, [(3)H]PCV, and [(14)C]FIAU in cell culture and in vivo using an adenoviral mediated gene transfer model and stably transfected cells. We further compared the uptake of three reporter probes using both HSV1-tk and a mutant HSV1-sr39tk expressing cells to assess the optimal reporter probe/reporter gene combination. [(14)C]FIAU accumulation was greater than that of [(3)H]PCV and [(18)F]FHBG in control cells and in HSV1-tk stably transfected cells ( P<0.001). After infection of C6 cells with AdCMV- HSV1-tk (1.5x10(8) pfu), [(18)F]FHBG and [(3)H]PCV accumulation was significantly greater than that of [(14)C]FIAU ( P<0.01). [(18)F]FHBG and [(3)H]PCV accumulated to a significantly greater extent than [(14)C]FIAU in C6-stb-sr39tk+ and AdCMV- HSV1-sr39tk infected C6 cells ( P<0.001). Results from the nude mice supported the results in cell culture. [(14)C]FIAU led to significantly higher %ID/g in C6-stb-tk+ xenografts than [(18)F]FHBG ( P<0.05); however, compared with [(14)C]FIAU, [(18)F]FHBG led to as high %ID/g in HSV1-tk expressing hepatocytes and to significantly greater %ID/g in C6-stb-sr39tk+ xenografts and HSV1-sr39tk expressing hepatocytes. Dynamic sequential images showed that [(18)F]FHBG was well retained in HSV1-sr39tk expressing cells (C6-stb-sr39tk+) for at least 4 h after injection, while it was rapidly cleared from HSV1-tk expressing cells (MH3924A-stb-tk+). [(14)C]FIAU accumulated in HSV1-tk stably expressing cells to a greater extent than either [(3)H]PCV or [(18)F]FHBG. However, the accumulation of [(3)H]PCV and [(18)F]FHBG in adenoviral infected C6 cells or hepatocytes was equivalent to or greater than that of [(14)C]FIAU. These results may be due to intracellular biochemical changes (e.g., thymidine) when cells are infected with adenovirus. For adenoviral studies, the [(18)F]FHBG/ HSV1-sr39tk combination was shown to be more sensitive than the [(14)C]FIAU/ HSV1-tk combination HSV1-tk.