ESCHERICHIA-COLI GPT GENE SENSITIZES RAT GLIOMA-CELLS TO KILLING BY 6-THIOXANTHINE OR 6-THIOGUANINE

Genes that encode enzymes that convert inactive ''prodrugs'' into anti cancer metabolites may be therapeutically useful against brain tumors. Unlike other genes tested to date in brain tumor models, the Escheric hia coil gpt gene is unique in that it not only sensitizes cells to th e prodrug 6-thioxanthine (6TX) but also encodes resistance to a differ ent regimen (mycophenolic acid, xanthine, and hypoxanthine), thus prov iding a means to select for gpt-positive cells. In the present study, rat C6 glioma cells were infected with a retrovirus vector that transd uces th is gene. A clonal line (C6GPT-7) was derived that exhibited si gnificant 6TX susceptibility in vitro with an ID50 of 2.5 mu mol/L, wh ereas 50% growth inhibition of parental C6 cells was not achieved at c oncentrations tested (up to 50 mu mol/L). Th is line also exhibited si gnificant sensitivity to 6-thioguanine (6TG), with an ID50 of 0.05 mu mol/L, whereas 50% growth inhibition of parental C6 cells was achieved at 0.5 mu mol/L. In a ''bystander'' assay, C6GPT-7 tumor cells effici ently transferred 6TX sensitivity to C6 cells at ratios as low as 1:9 (C6GPT-7:C6). This in vitro bystander effect was abrogated when C6GPT- 7 and C6 cells were separated by a microporous membrane, suggesting th at it was not mediated by highly diffusible metabolites. In vivo both 6TX and 6TG significantly inhibited the growth of subcutaneously trans planted C6GPT-7 cells but not that of C6 cells in athymic mice. In an intracerebral model, both 6TX and 6TG exhibited significant antiprolif erative effects against tumors formed by C6GPT-7 cells. These findings provide a basis for exploring further gene therapy strategies based o n in vivo transfer of the E coil gpt gene to provide chemosensitivity against 6TX and 6TG.