Multidrug resistance-associated protein (MRP) and P-glycoprotein are drug efflux pumps conferring multidrug resistance to tumor cells and sharing numerous substrates. In order to determine a flow cytometric assay allowing to analyse MRP activity in cancerous cells in a sensitive and specific manner, cellular accumulation and efflux of the anionic fluorescent dye carboxy-2',7'-dichlorofluorescein (CDF) were studied by flow cytometry using mainly MRP-overexpressing lung GLC4/Sb30 cells and parental GLC4 cells. GLC4/Sb30 cells were found to display reduced accumulation and enhanced efflux of the dye when compared to their parental counterparts. Probenecid, a well known blocker of MRP, strongly enhanced CDF accumulation in GLC4/Sb30 cells through inhibiting efflux of the dye; it also increased CDF levels in GLC4 cells, although to a lesser extent, which may likely be linked to the low, but detectable, expression of MRP in these cells. Comparison of CDF retention with that of calcein demonstrated that the former dye was the most efficiently effluxed by GLC4/Sb30 cells. In contrast to MRP overexpression, that of P-glycoprotein was not found to alter cellular CDF labelling whereas it strongly impaired calcein staining. These results indicate that CDF is a substrate for MRP, but not for P-gp, which may likely be useful for sensitive and specific flow cytometric determination of MRP activity in clinical samples.

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