Pretreatment with 5-fluorouracil enhances cytotoxicity and retention of DNA-bound platinum in a cisplatin resistant human ovarian cancer cell line

A combination of 5-fluorouracil (5-FU) and cisplatin (cis-Diamminedichloroplatinum(II); CDDP) was administered to CDDP-sensitive (A2780) and -resistant (2780(CP)) human ovarian cancer cell lines in vitro in order to investigate the effects of 5-FU pretreatment on CDDP cytotoxicity, removal of DNA-bound platinum (Pt-DNA) and cellular glutathione (GSH) level. The cells were incubated with various doses of 5-FU for 24 hours, and then exposed to various doses of CDDP after a drug-free interval of 24 hours. Pretreatment with 5-FU (0.5 - 2.0 mug/ml) augmented the cytotoxicity of CDDP in 2780(CP) cells, but did not affect A2780 cells. 2780(CP) cells lost 23.0 to 41.9% of their total Pt-DNA determined by flameless atomic absorption spectrophotometry at 6 to 24 hours after exposure to 10.0 mug/ml CDDP alone; nevertheless, 1.0 mug/ml 5-FU pretreatment caused a significant delay in removal of Pt-DNA (3.8, 1.5 and 7.2% at 6, 12 and 24 hours after exposure to CDDP, respectively). At 24 hours after 15.0 mug/ml CDDP exposure, 2780CP cells pretreated with 5-FU lost only 15.7% of their total Pt-DNA, although the platinum removal rate with CDDP alone was 28.4%. The GSH levels in 2780(CP) cells were similar in the presence or absence of 5-FU pretreatment. These data indicated that 5-FU pretreatment enhances the cytotoxicity of CDDP and reverses CDDP resistance in 2780(CP) cells and that its mechanism is related to the inhibitory effect of 5-FU on DNA repair, and not to cellular GSH levels.