Paclitaxel and SN‐38 Overcome Cisplatin Resistance of Ovarian Cancer Cell Lines by Down‐regulating the Influx and Efflux System of Cisplatin

Cisplatin (DDP) is one of the key drugs used to treat patients with ovarian cancer, although resistance to DDP can occur. Paclitaxel and SN‐38 (an active metabolite of irinotecan (CPT‐11)) are two drugs that are effective in patients with DDP‐resistant ovarian cancer. To study how these drugs may overcome the intrinsic and/or acquired resistance of cancer cells to DDP, we investigated the effect of a combination of DDP with paclitaxel and a combination of DDP with SN‐38 on three ovarian cancer cell lines, RTSG (intrinsically resistant cell line), KF (DDP‐sensitive cell line), and KFra (acquired resistant cell line obtained from KF). We found that these combinations showed additive to synergistic antitumor activity. A time‐dependent platinum (Pt) accumulation was observed in the DDP‐sensitive KF cell line, while a decrease occurred in the KFra cell line. Little accumulation was observed in RTSG. Intracellular Pt accumulation was increased in all three cell lines by exposure to paclitaxel or SN‐38. Ouabain, a Na(+),K(+)‐ATPase inhibitor, decreased Pt accumulation in KF and KFra cell lines and inhibited the paclitaxel‐ and SN‐38‐induced increases in Pt accumulation in these cell lines. When we assessed the mRNA levels of the multidrug resistance‐associated protein (MRP), which may be an efflux pump for DDP, the combination of paclitaxel or SN‐38 with DDP down‐regulated these levels, which are up‐regulated by DDP alone. These results suggest that paclitaxel and SN‐38 overcome DDP resistance of ovarian cell lines by controlling intracellular accumulation of DDP via both the influx and efflux systems