DEXNIGULDIPINE-HCL IS A POTENT ALLOSTERIC INHIBITOR OF [H-3] VINBLASTINE BINDING TO P-GLYCOPROTEIN OF CCRF ADR 5000 CELLS

Cell membranes were prepared from the multidrug resistant, P-glycoprot ein expressing human lymphoblastoid cell line CCRF-ADR 5000. The P-gly coprotein of these membranes possessed high affinity binding sites for [H-3]vinblastine, with a K-d of 8 +/- 2 nM and B-max of 17 +/- 8 pmol /mg of protein. The binding of [H-3]vinblastine to P-glycoprotein was not ATP-dependent, and was inhibited by cytotoxic drugs with the follo wing potency order; vincristine > doxorubicin > etoposide. The 1,4-dih ydropyridine and multidrug resistance reversing agent, dexniguldipine- HCl, inhibited binding with a K-i value of 37 nM. The multidrug resist ance reversing agent cyclosporin A, and the cytotoxics doxorubicin and etoposide did not alter the kinetics of [H-3]vinblastine dissociation from P-glycoprotein; however, the 1,dr-dihydropyridine dexniguldipine -HCl and nicardipine accelerated dissociation of [H-3]vinblastine. The se data suggest that P-glycoprotein possesses at least two allosterica lly coupled drug acceptor sites; receptor site 1 which binds vinblasti ne, doxorubucin, etoposide and cyclosporin A, and receptor site 2 whic h binds dexniguldipine-HCl and other 1,4-dihydropyridines.