Decreased intracellular mitoxantrone in resistant MCF-7 breast cancer cells is attributed to an energy dependent efflux

Abstract

Intracellular drug accumulation was studied in two drug resistant variants of the human breast cancer MCF-7 (MCF/7) cell line selected with mitoxantrone (MCF7/Mitox) and doxorubicin (MCF7/D40). Earlier studies show that both cell lines have similar cell cycle characteristics, and both are multidrug resistant. Previously, P-glycoprotein was detected in MCF7/D40, but not in MCF7/Mitox. Both cell lines, however, display decreased drug accumulation. The P-glycoprotein chemo-modulator verapamil increased mitoxantrone accumulation 1.6 fold in MCF7/D40 cells, thus achieving identical intracellular drug levels to the MCF7/S cell line. Verapamil had little effect on drug accumulation in MCF7/Mitox cells. Rapid influx of mitoxantrone from 5 seconds to 60 seconds was not significantly different between MCF7/Mitox and MCF7/S. Influx in the MCF7/D40 cell line was greater than in the MCF7/Mitox or MCF7/S cell lines. Decreased drug accumulation was found to be at least partly due to enhanced drug efflux. Depletion of 73.9% to 88.9% of cellular ATP by sodium azide (NaN3) decreased the efflux of mitoxantrone in each cell line, thus demonstrating an energy dependence of drug efflux.