Detection of MRP functional activity: calcein AM but not BCECF AM as a Multidrug Resistance-related Protein (MRP1) substrate.

Resistance to anticancer drugs has been attributed to an array of cellular changes. The multidrug resistance-related protein (MRP1) is an efflux pump whose overexpression confers resistance to several classes of drugs, such as the anthracyclines, epipodophyllotoxins, and vinca alkaloids. These drugs are mainstays in cancer therapy. MRP1 overexpression is hypothesized to be a causative agent of clinical treatment failure. Consistently accurate methods for detecting this protein are necessary to further understand its biology and delineate its possible clinical relevance. Flow cytometric analysis of multidrug resistance (MDR) is a valuable method to evaluate both antigen expression and function. Using flow cytometry, we assayed MRP1 functional activity in pediatric leukemic blasts and an array of MDR+ and WT cell lines. We conclude that calcein AM, when used in a retention assay with MRP1-specific modulators, is able to reliably detect MRP functional activity. 2'-7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF AM) transport is not indicative of MRP1 overexpression. .