Drug and multidrug resistant modulators of Formula (C): ##STR1## where Z is selected from the group consisting of --S--, --S(O).sub.w --, and --CH.sub.2 --, where w is 1 or 2; and pharmaceutically acceptable salts or solvates thereof are described and claimed. Use of the new compounds in the preparation of pharmaceutical compositions is described and claimed. In addition, methods for treating drug and multidrug resistance in various diseases using a compound of this invention are described and claimed. Also, methods of enhancing oral bioavailability of a drug and methods of enhancing bioavailability of a drug to the brain using a compound of this invention are described and claimed.