Multidrug resistance is a major obstacle to cancer treatment. Using an expression cDNA library transfer approach to elucidating the molecular basis of non-P-glycoprotein-mediated multidrug resistance, we previously established that expression of multidrug resistance protein (MRP), an ATP-binding cassette superfamily transporter, confers multidrug resistance (G. D. Kruh et al., Cancer Res., 54: 1649–1652, 1994). In the present study, we generated NIH/3T3 MRP transfectants without using chemotherapeutic drugs to facilitate the pharmacological analysis of the MRP phenotype. MRP transfectants displayed increased resistance to several lipophilic drugs, including doxorubicin, daunorubicin, etoposide, actinomycin D, vincristine, and vinblastine. However, increased resistance was not observed for Taxol, a drug for which transfection of MDR1 confers high levels of resistance. Verapamil increased the sensitivity of MRP transfectants relative to control transfectants, but reversal was incomplete for doxorubicin and etoposide, the drugs for which MRP conferred the highest resistance levels. For the latter two drugs, MRP transfectants, which were ∼8- and ∼10-fold more sensitive than control cells in the absence of verapamil, exhibited 3.8- and 3.3-fold relative sensitization with 10 µMverapamil, respectively, but remained ∼2 and ∼3-fold more resistant than control cells. Analysis of drug kinetics using radiolabeled daunorubicin revealed decreased accumulation and increased efflux in MRP transfectants. Confocal microscopic analysis of intracellular daunorubicin in MRP transfectants was consistent with reduced intracellular drug concentrations, and also revealed an altered pattern of intracellular drug distribution characterized by the initial accumulation of drug in a perinuclear location, followed by the development of a punctate pattern of drug scattered throughout the cytoplasm

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