DIOC(2)(3) IS NOT A SUBSTRATE FOR MULTIDRUG-RESISTANCE PROTEIN (MRP)-MEDIATED DRUG EFFLUX

Multidrug resistance (MDR) is often related to expression of P-glycopr otein (Pgp) or Multidrug Resistance Protein (MRP). Pgp-mediated MDR ca n be evaluated by determining cellular retention of fluorescent substr ates by now cytometry. This study determined if agents used to evaluat e Pgp function also can be used to evaluate MRP function. Cellular ret ention of doxorubicin (Dox), Rhodamine-123 (Rh-123), and 3,3'-diethylo xacarbocyanine iodide (DiOC(2)(3)) were studied in MRP-expressing cell lines (HL60/Adr and HT1080/DR4), whereas a Pgp expressing cell fine ( A2780/Dx5) served as a positive control. Overexpression of Pgp correla ted inversely with retention of Dox, Rh-123, and DiOC(2)(3); however, under identical experimental conditions (1 h reincubation in drug-free medium), no retention difference of the three agents was detected bet ween parental and MRP-expressing resistant cells. Upon extending the r eincubation time to 4 h, an efflux of Rh-123 and Dox in the resistant Lines became apparent and even more pronounced after 24 h; however, st ill no efflux was detectable for DiOC(2)(3). Incubation of the cells w ith a modulator of MDR, PAK-104P, negated the observed drug efflux in Pgp and MRP expressing cells, which correlated with increased sensitiv ity of the MDR Lines to doxorubicin. Thus both Dox and Rh-123 can be u sed to evaluate MRP-function, but DiOC(2)(3) can not. (C) 1996 Wiley-L iss, Inc.