The multidrug resistance protein (ABCC1 or MRP1) causes resistance to multiple drugs through reduced drug accumulation. We have previously demonstrated direct interaction between MRP1 and unmodified drugs using photoreactive drug analogues. In this study, we describe the use of [I-125]iodoaryl azido-rhodamine123 (IAARh123)-a photoactive drug analogue of rhodamine 123, to study the effects of mild detergents and denaturing agents on MRP1-drug binding in membrane vesicles prepared from HeLa cells transfected with the MRP1 cDNA. Our results show that the zwitterionic detergent CHAPS and a nonionic detergent Brij35 inhibited the photolabeling of MRP1 with IAARh123. Sodium deoxycholate (SDC) and octyl-beta-glucoside (OG), structurally similar to CHAPS and Brij35 and disrupting the lipid bilayer, showed a modest increase of MRP1 photolabeling with IAARh123. Proteolytic digestion of lAARh123 photolabeled MRP1 labeled-in the presence or absence of various detergents (CHAPS, SDC, or OG) revealed identical photolabeled peptides. Consistent with the drug-binding results, non-toxic concentrations of CHAPS and Brij35 reversed vincristine and etoposide (VP16) toxicity in MRP1 expressing cells. Taken together, the results of this study show that MRP1-drug interaction can occur outside the lipid bilayer environment. However, this interaction is inhibited with certain mild detergents. (C) 2003 Elsevier Science (USA). All rights reserved.