OVEREXPRESSION OF THE MDR1 GENE IN BLAST CELLS FROM PATIENTS WITH ACUTE MYELOCYTIC-LEUKEMIA IS ASSOCIATED WITH DECREASED ANTHRACYCLINE ACCUMULATION THAT CAN BE RESTORED BY CYCLOSPORINE-A

Typical multi‐drug resistance (MDR) in human and animal cell lines is caused by overactivity of a unidirectional drug efflux pump. This pump is composed of a 170‐kDa transmembrane glycoprotein (P‐glycoprotein) that is encoded by the so‐called mdrl gene. The functionally relevant characteristic of MDR cells is a defect in drug accumulation that can be restored by agents which inhibit the P‐glycoprotein pump. The purpose of our study was to find out whether Pglycoprotein inhibitors could increase the daunorubicin (DNR) accumulation in acute myelocytic leukemia (AML) cells, overexpressing the mdrl gene. Using dot blot analysis with an mdrl‐specific cDNA probe, we identified leukemic cell samples, obtained from chemotherapy‐resistant AML patients, that had relatively high levels of mdrl expression. These leukemic cells showed a reduced ability to accumulate DNR in vitro, as quantitated by flow cytometry. Addition of cyclosporin‐A (Cy‐A), a drug known to inhibit the P‐glycoprotein pump, to the incubation medium resulted in an increase (up to 60%) in steady‐state drug uptake by the leukemic cells. The degree of Cy‐A‐induced increase in drug accumulation in the leukemic cells correlated approximately with the level of overexpression of the mdrl gene. Our data indicate that Cy‐A is a good candidate for combination chemotherapy with cytotoxic drugs in clinical trials, aimed at the treatment of drug resistance in AML. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company