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 [F-18]FHBG, [H-3]PCV, and [C-14]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. [C-14]FIAU accumulation was greater than that of [H-3]PCV and [F-18]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), [F-18]FHBG and [H-3]PCV accumulation was significantly greater than that of [C-14]FIAU (P<0.01). [F-18]FHBG and [H-3]PCV accumulated to a significantly greater extent than [C-14]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. [C-14]FIAU led to significantly higher %ID/g in C6-stb-tk+ xenografts than [F-18]FHBG (P<0.05); however, compared with [C-14]FIAU, [F-18]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 [F-18]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+). [C-14]FIAU accumulated in HSV1-tk stably expressing cells to a greater extent than either [H-3]PCV or [F-18]FHBG. However, the accumulation of [H-3]PCV and [F-18]FHBG in adenoviral infected C6 cells or hepatocytes was equivalent to or greater than that of [C-14]FIAU. These results may be due to intracellular biochemical changes (e.g., thymidine) when cells are infected with adenovirus. For adenoviral studies, the [F-18]FHBG/HSV1-sr39tk combination was shown to be more sensitive than the [C-14]FIAU/HSV1-tk combination HSV1-tk.