Increased cytotoxicity and bystander effect of 5-fluorouracil and 5 '-deoxy-5-fluorouridine in human colorectal cancer cells transfected with thymidine phosphorylase

5-Fluorouracil (5-FU) and 5'-deoxy-5-fluorouridine (5'-DFUR), a prodrug of 5-FU, are anticancer agents activated by thymidine phosphorylase (TP). Tran sfecting the human TP cDNA into cancer cells in order to sensitize them to these pyrimidine antimetabolites may he an important approach in human canc er gene therapy research. In this study, an expression vector containing th e human TP cDNA (pcTP5) was transfected into LS174T human colon carcinoma c ells. Eight stable transfectants were randomly selected and analysed. The c ytotoxic effects of 5-FU and 5'-DFUR were higher in TP;transfected cells as compared to wild-type cells. The maximal decreases in the IC50 were 80-fol d for 5-FU and 40-fold for 5'-DFUR. The increase in sensitivity to these py rimidines of TP-transfected cells significantly correlated with the increas e in both TP activity and TP expression. Transfected clone LS174T-c2 but no t wild-type cells exhibited formation of [H-3]FdUMP from [H-3]5-FU. in addi tion the LS174T-c2 clone enhanced the cytotoxic effect of 5'-DFUR, but also that of 5-FU, towards co-cultured parental cells. For both anti-cancer age nts, this bystander effect did not require cell-cell contact. These results show that both 5-FU or 5'-DFUR could be used together with a TP-suicide Ve ctor in cancer gene therapy.