KR30031 is a novel P-glycoprotein inhibitor with potential anticancer activity. KR30031 is a verapamil analog with fewer cardiovascular effects. The ability of KR-30031 to reduce this efflux transport is equal to that of verapamil, a well-known P-glycoprotein inhibitor. The bioavailability of paclitaxel could be enhanced by coadministration of a P-glycoprotein inhibitor, KR-30031.