SCHEDULE-DEPENDENT REVERSION OF ACQUIRED CISPLATIN RESISTANCE BY 5-FLUOROURACIL IN A NEWLY ESTABLISHED CISPLATIN-RESISTANT HST-1 HUMAN SQUAMOUS CARCINOMA CELL-LINE

In vitro continuous stepwise exposure of HST-1 human squamous carcinom a cell line to cisplatin (CDDP) for I2 months resulted in a 3.5-fold s tably resistant subline designated HST-1/CP0.2. Compared with parental cells, this cell line showed a 1.8-fold increase in cellular glutathi one (GSH) and a 50% reduction in initial numbers of DNA interstrand cr oss-links (ICLs), despite similar levels of intracellular platinum acc umulation. Evaluation of the kinetics of DNA ICL removal at nearly equ ivalent levels of DNA ICL formation indicated that HST-1/CP0.2 cells a ppeared to remove DNA ICLs more rapidly than do HST-1 parental cells. Thus, both elevated cellular GSH and increased DNA repair capacity wou ld be the major factors contributing to CDDP resistance. Pretreatment of HST-1/CP0.2 cells with 5-FU, with drug-free intervals of 24 to 48 h r before exposure to CDDP, completely reversed CDDP resistance, or eve n increased the sensitivity to a level greater than that of parental c ells, whereas the opposite sequence had no effect on resistance. In pa rallel with augmentation of the cytotoxicity, the levels of cellular G SH were significantly reduced over 48 hr by 5-FU pretreatment. However , depletion of cellular GSH using buthionine sulfoximine resulted in p artial reversal of CDDP resistance, indicating that reduction of cellu lar GSH alone is not sufficient for complete reversal of CDDP resistan ce. Our data, together with evidence that 5-FU modulates the repair of platinum-DNA cross-links, suggest that schedule-dependent, complete r eversal of CDDP resistance by 5-FU might be attributed to its inhibito ry effects on both GSH levels and the repair of platinum-DNA adducts. Thus, optimization for the drug administration schedule is important w hen aiming at therapeutic synergy and circumvention of acquired CDDP r esistance. (C) 1996 Wiley-Liss, Inc.