A multidrug-resistant (MDR) variant, K562/Dox, was selected from repea
ted exposure of human erythroleukemia cell line K562 to doxorubicin (D
ox). K562/Dox displayed typical MDR features with respect to its cross
-resistance to a variety of functionally and structurally unrelated co
mpounds: vincristine (Vin), Dox, mitomycin C, reduced steady-state int
racellular anthracycline accumulation, and elevated P-glycoprotein exp
ression/mdrl mRNA transcription/mdrl gene amplification. Nevertheless,
by incubation of cells with Dox/epirubicin (Epi)/daunorubicin (Dau) (
5-80 mu g/ml), the initial drug uptake was similar (p > 0.05) in K562/
Dox and K562 cells, suggesting P-glycoprotein-mediated drug efflux wou
ld not occur unless a relatively high cellular drug concentration was
reached. After 8 h incubation of cells with 50 ng/ml Dox (5 times high
er than its IC50 to K562 cells), there were only slight differences (p
> 0.05) in intracellular drug levels between K562/Dox and K562 cells,
clearly indicating that K562/Dox, circumventing drug toxicity in this
case, was irrelevant to reduced drug accumulation caused by P-glycopr
otein. Similar results were obtained when Epi or Dau was applied. Desp
ite complete restoration of anthracycline accumulation in K562/Dox cel
ls in the presence of 6 mu mol/l verapamil, the reversal of their drug
resistance was incomplete. These results suggest that P-glycoprotein-
mediated drug efflux possibly did not play a primary role in the drug
resistance of K562/Dox cells.