AGENTS WHICH REVERSE MULTIDRUG-RESISTANCE ARE INHIBITORS OF [H-3] VINBLASTINE TRANSPORT BY ISOLATED VESICLES

Resistance of human cancer cells to multiple cytotoxic hydrophobic agents (multidrug resistance) is due to overexpression of the MDR1 gene whose product is the ATP-dependent multidrug transporter, P-glycoprotein. We have previously reported that plasma membrane vesicles partially purified from multidrug-resistant human KB carcinoma cells, but not from drug-sensitive cells, accumulated [H-3]vinblastine in an ATP-dependent manner (Horio, M., Gottesman, M.M. and Pastan, I. (1988) Proc. Natl. Acad. Sci. USA 85, 3580-3584). Certain calcium-channel blockers, quinidine, and phenothiazines are able to overcome multidrug resistance in cultured cells. In this work, the effect of these reversing agents on ATP-dependent vinblastine (VBL) transport by vesicles from drug-resistant KB cells has been characterized. Azidopine was the most potent inhibitor of ATP-dependent VBL uptake tested (ID50: concentration of inhibitor such that the transport of vinblastine is inhibited by 50%, < 1-mu-M). Verapamil, quinidine, and the tiapamil analogue RO-11-2933 were potent but less effective inhibitors (ID50 < 5-mu-M). Diltiazem, nifedipine and trifluoperazine were even less effective. These agents had no effect on Na+-dependent and Na+-independent L-leucine uptake by the vesicles, indicating that the inhibition of ATP dependent VBL transport by these agents is not a non-specific effect, as might result from leaks in the vesicle membrane. Verapamil, quinidine, azidopine and trifluoperazine increased the apparent K(m) value of vinblastine transport, suggesting that these agents may be competitive inhibitors of vinblastine transport.