Gefitinib inhibits the ATP-binding site of the tyrosine kinase associated with the epidermal growth factor receptor. It is conceivable that gefitinib may inhibit functions of ATP-binding cassette (ABC) transporters by binding at their ATP-binding sites. The aim of this study is to systematically explore the combined effect of gefitinib and chemotherapeutic agents in gefitinib-insensitive multidrug resistant (MDR) cells that overexpress ABC transporters. MCF7 breast carcinoma cells and CL1 lung adenocarcinoma cells were both insensitive to gefitinib. MDR cancer cells were developed by stepwise escalating concentrations of each chemotherapeutic agent in culture media. Cells that overexpress P-glycoprotein (MCF7/Adr and CL1/Pac), breast cancer-resistant protein (MCF7/TPT and CL1/Tpt), and MDR-associated protein 1 (MCF7/Vp) were used in this study. All resistant mutants were insensitive to gefitinib. Gefitinib (0.3-3 mu mol/L) added to culture media had no effect on IC50 values of paclitaxel, topotecan, doxorubicin, or etoposide in wild-type MCF7 or CL1 cells. In contrast, these concentrations of gefitinib caused a dose-dependent reversal of resistance to paclitaxel in CL1/Pac cells, to doxorubicin in MCF7/ADR cells, and to topotecan in CL1/Tpt and MCF7/TPT cells. Gefitinib had no influence on sensitivity to etoposide in MDR-associated protein1 overexpressing MCF7/VP cells. Topotecan efflux was inhibited and accumulation was partially restored in CL1/Tpt and MCF7/TPT cells when cells were incubated simultaneously with gefitinib. Our results suggest that the interaction of gefitinib and chemotherapeutic agents does occur in cells expressing one of these two proteins.
