Detection of the Mr 190,000 MilitidrUg Resistance Protein, MRP, with Monoclonal Antibodies'

proteins as P-gp (6, 7)•4 Like P-gp, transfection of drug-sensitive cells with a full-length MRP cDNA is sufficient to confer multidrug re sistance (7).5 However, despite the fact that both P-gp and MRP confer resistance to a similar spectrum of anticancer drugs, these two proteins share less than 15% amino acid identity (6). MAbs against P-gp have played a critical role in determining the relevance of this protein in clinical drug resistance (8). Some anti P-gp MAbs and their derivatives are being investigated for their therapeutic potential, particularly those that are able to reverse drug resistance (9—12)or participate in antibody-dependent cell-mediated cytolysis (13). P-gp-specific MAbs have also been useful in immu nolocalization studies of normal tissues and have provided important clues as to the physiological role of this protein (14, 15). The epitopes of several MAbs have been mapped, providing information about P-gp secondary structure and membrane topology (16, 17). To date, polyclonal antiseraraised against MRP-derivedsynthetic peptides have been used to quantify MRP protein levels in drug selected and transfected multidrug resistant cell lines and to examine MRP structure, biosynthesis, and subcellular distribution (7, 18, 19).@-@However, these studies have been hampered by the unsuitabil ity of polyclonal antisera for many experimental applications and by the limited availability of these immunoreagents. To obtain better probes for immunodetection of MRP, we have produced MAbs from mice immunized with membranes from multidrug resistant H69AR cells which express high levels of this M1 190,000 protein. These MRP-specific MAbs should greatly facilitate ongoing investigations of the biology and clinical relevance of this novel drug resistance protein.