Gefitinib ("
Iressa",
ZD1839) is an orally active, selective
epidermal growth factor receptor tyrosine kinase inhibitor, and the single agent is clinically effective in
non-small cell lung cancer. Although
gefitinib combined with various
cytotoxic agents has been reported to enhance cytotoxicity in vitro and in mouse models, the mechanism remains undetermined. Here, to explore the mechanism with
topoisomerase I inhibitors, we focused on the efflux pump of the
breast cancer resistance
protein (BCRP/ABCG2), and then examined whether
gefitinib restored
drug sensitivity in multidrug-resistant
cancer cells overexpressing BCRP. We used PC-6 human
small cell lung cancer cells and multidrug-resistant PC-6/SN2-5H cells selected with
SN-38 of the active metabolite of
irinotecan, and BCRP-overexpressing MCF-7/MX cells selected with
mitoxantrone and BCRP
cDNA transfectant MCF-7/clone 8 cells.
Drug sensitivity against anticancer drugs was determined by tetrazolium
dye assay, and intracellular
topotecan accumulation by FACScan. The
topotecan transport study was done using the plasma membrane vesicles of PC-6/SN2-5H cells. The resistant PC-6/SN2-5H cells overexpressed BCRP but not
epidermal growth factor receptor mRNA. Ten micromoles of
gefitinib reversed
topotecan,
SN-38, and
mitoxantrone resistance, and increased the intracellular
topotecan accumulation in the resistant cells but not in the parental cells. Furthermore,
gefitinib inhibited the
topotecan transport into the vesicles, and the K(i) value was 1.01 +/- 0.09 micromol/L in the Dixon plot analysis, indicating direct inhibition of BCRP by
gefitinib. However,
gefitinib was not transported into the vesicles with the high-performance liquid chromatography method. These results indicate that
gefitinib reverses BCRP-mediated drug resistance by direct inhibition other than competitive inhibition as a BCRP substrate. Combination of
gefitinib and
topoisomerase I inhibitors could be clinically effective in
cancers expressing BCRP.