Doxorubicin resistance in P388 leukemia—evidence for reduced drug influx

Multi‐drug resistance (MDR) in cancer cells is associated with reduced drug accumulation. Although intensively studied, the mechanism of this process remains ill‐defined. We have now developed a new, rapid and quantitative method of measuring uptake of doxorubicin by these cells, in which the fluorescence of accumulated drug is rapidly quenched by DNA in the cell nucleus. Pre‐treatment of cells with deoxyribonuclease eliminates DNA from non‐viable, permeable cells, and this obviates the spurious fluorescence quenching that made previous application of this technique useless. Our data strongly suggest that the drug passively diffuses into cells. The rate of this diffusion into drug‐resistant cells is considerably lower than that found in drug‐sensitive cells. The ratio of the rates of drug entry in these cell types could fully account for the differences between the cell lines in doxorubicin growth‐inhibitory activity. In these experiments no evidence for the previously proposed active efflux mechanism was found in either cell line.