Compounds of formula ##STR1## wherein R and R.sup.3, which may be the same or different, each is hydrogen, an alkyl group containing 1 to 5 carbon atoms, or an optionally substituted aryl or aralkyl group, substituents being selected from the group consisting of halogen, alkoxy and alkyl groups containing 1 to 5 carbon atoms, said alkyl groups in turn being optionally halo-substituted; R.sup.1 and R.sup.2, which may be the same or different, each is hydrogen, an alkyl group containing 1 to 5 carbon atoms, or an optionally substituted aryl or aralkyl group, substituents being as indicated above or R.sup.1 and R.sup.2 taken together are an alkylene group containing 2 or 3 carbon atoms; m and p, which may be the same or different each is a number from 0 to 3; n is the number 2 or 3; and X is a group of the formula R.sub.x.sup.4 --CO--, R.sub.x.sup.4 --COO--, R.sub.x.sup.4 --CONH--, R.sub.x.sup.4 --NHCO--, or R.sub.x.sup.4 --O-- and Y is a group of formula R.sub.y.sup.4 --CO--, R.sub.y.sup.4 --COO--, R.sub.y.sup.4 --CONH--, R.sub.y.sup.4 --NHCO--, or R.sub.y.sup.4 --O-- which R.sub.y.sup.4 and R.sub.x.sup.4, which may be the same of different, each is an optionally substituted aryl, aralkyl or aryloxyalkyl group, substitutents being as indicated above, and the alkyl moiety in the aralkyl and aryloxyalkyl groups containing 1 to 5 carbon atoms and can be optionally substituted by a cyloalkyl group containing 3 to 8 carbon atoms; and physiologically acceptable acid addition salts thereof, have been found to be active in restoring drug-sensitivity to cancer cells that have become multidrug-resistant. They can, therefore, be used as adjuvants in oncology, and they can be administered simultaneously, separately or at intervals in combination with one or more conventional anti-cancer agents for treating malignant tumors and corresponding metastases.Compounds and salts which have not been previously described in British Patent Specification No. 883013 as well as their preparation and pharmaceutical compositions containing them are also disclosed.