Modulation of resistance to idarubicin by the cyclosporin PSC 833 (valspodar) in multidrug-resistant cells.

Idarubicin (IDA) is an anthracycline anticancer drug utilized in the treatment of acute leukemias. There are conflicting data published with regard to the cross-resistance of IDA in multidrug-resistant (MDR) cells expressing P-glycoprotein (P-gp). We evaluated the cytotoxicity and cellular accumulation of IDA in a panel of anthracycline-selected MDR cell lines. Leukemia K562/R7 cells and sarcoma MES-SA/Dx5 cells expressing high levels of the MDR1 (ABCB1) gene were resistant to IDA (42-fold and 150-fold, respectively). In both of these cell lines, resistance to IDA was equivalent to that for doxorubicin, the drug used to select for the MDR variants. The P-gp inhibitor PSC 833 (valspodar) at 2 microM completely restored sensitivity to IDA. IDA accumulation was decreased 12-fold in MES-SA/Dx5 cells vs parental cell line, and drug uptake was restored to control levels by PSC 833. Reduced intracellular IDA was correlated with P-gp content by flow cytometry. Experiments in NIH3T3 murine cells transfected with the human MDR1 gene substantiated the findings of cross-resistance to IDA and reversal of resistance by PSC 833. Our data indicate that IDA is a high-affinity substrate for P-gp.