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Probing daunorubicin accumulation defects in non‐P‐glycoprotein expressing multidrug‐resistant cell lines using digitonin

Multi drug resistance (MDR) in tumor cells is frequently associated with reduced cellular cytostatic drug accumulation, caused by the drug efflux protein, P‐glycoprotein (Pgp). The action of Pgp in tumor cells can be detected by measuring the increase of daunorubicin accumulation upon blocking Pgp with drugs such as verapamil. A number of MDR cell lines have been described, characterized by decreased drug accumulation without Pgp being present. For such non‐Pgp MDR cells no gene probes or functional assays are available to study this phenotype in clinical tumor specimens. We have worked out a method which enables the detection of drug‐transport‐related decreases in cellular daunorubicin accumulations without the need for the use of specific Pgp blockers. The cells used were SW‐1573‐, GLC4‐ and HT1080‐sensitive cell lines, which accumulated (corrected for DNA content) 272%, 1,288% and 203% more daunorubicin than the non‐Pgp MDR sub lines SW‐1573/2RI20, GLC4/ADR and HTI080/DR4. When the plasma membranes of these MDR lines were permeabilized with 20 μM digitonin an increase to 282%, 1,260% and 239% of 14C‐daunorubicin control accumulation was measured (at pH = 7.35). The intracellular pH measured with BCECF was the same in parent and corresponding MDR cells, excluding the role of pH differences in the measured effects. This method provides a tool allowing the detection of cellular mechanisms (including Pgp) which are related to active outward transport of daunorubicin.