Using different tracers of tumor metabolism, the application of PET for monitoring gene therapy with the suicide gene herpes simplex virus thymidine kinase (HSVtk) is investigated in this in vitro study.
Methods: Morris hepatoma cells were transfected with a retroviral vector bearing the HSVtk gene, and different clones were established by selection with the neomycin analog G418. Thereafter, uptake measurements using fluorodeoxyglucose (FDG), 3-O-methylglucose, aminoisobutyric acid and methionine were performed in a thymidine kinase (TK)-expressing cell line and in control cells bearing the empty vector in the presence of different concentrations of ganciclovir (GCV). These experiments were done up to 48 hr after the onset of therapy. The values were expressed as Bq/well or as Bq/10(5) cells.
Results: During GCV treatment therapy, a decrease of the uptake/well was measured for all tracers in the TK-expressing cell line. After normalization to the viable cell number, the uptake for FDG and 3-O-methylglucose increases up to 195% after 24 hr incubation with GCV. A high-pressure liquid chromatography analysis revealed a decline of the FDG-6-phosphate fraction after 48 hr incubation with GCV. Consequently, a normalization of FDG uptake was observed after this incubation period, whereas the 3-O-methylglucose uptake was still increased. Experiments performed with different amounts of TK-expressing cells and control cells showed that these effects are dependent on the percentage of TK-expressing cells. The aminoisobutyric acid uptake decreases to 47%, while the methionine uptake decreases in the acid-insoluble fraction (to 17%) and increases in the acid-soluble fraction (to 150%).
Conclusion: These data indicate that combinations of the PET tracers used in these experiments may be applied for monitoring gene therapy with HSVtk. The increase in FDG and 3-O-methylglucose uptake in vitro is interpreted as stress reaction of the tumor cells. However, an uncoupling of transport and phosphorylation was observed after 48 hr incubation. The amino acid uptake experiments point to an inhibition of protein synthesis as well as of the neutral amino acid transport.