Restoration of intracellular drug accumulation in MDR cell line K562r does not mean reversal of its drug resistance

Our purpose was to test whether drug sensitivity and drug accumulation in MDR cell erythroleukemic K562r could be restored by incubating cells with 3 anthracycline antibiotics in combination. Drug sensitivities of cells were assessed with MTT assay, in which doxorubicin, epirubicin, daunorubicin, or the 3-drug mixture was applied with concentrations ranging from 1 to 3125 ng.ml-1. The IC50 of K562r cells were 1.0, 1.0, 0.1, and 0.2 microgram.ml-1, respectively, about 22, 16, 10, and 20 times higher than those of K562 cells. After cells were exposed to doxorubicin (2-32 micrograms.ml-1) for 1 h, the drug concentrations in K562r cells were all higher than those in K562 cells. Similar results were obtained for epirubicin or daunorubicin. After 1-h incubation of cells with the 3-drug mixture (3 to 192 micrograms.ml-1), there were no considerable differences of drug accumulation between K562r and K562 cells with only 3 exceptions in 21 groups. It is concluded that restoration of intracellular drug accumulation in MDR cell line K562r was not correlated with reversal of its drug resistances.