One embodiment of the present invention is a method of treating a subject with a multi-drug resistant cancer. The method comprises administering to the subject an effective amount of a compound represented by Structural Formula (I): Y is a covalent bond or a substituted or unsubstituted straight chained hydrocarbyl group, or, Y, taken together with both >C=Z groups to which it is bonded, is a substituted or unsubstituted aromatic group. R1-R4 are independently -H, an aliphatic group, a substituted aliphatic group, an aryl group or a substituted aryl group, or R1 and R3 taken together with the carbon and nitrogen atoms to which they are bonded, and/or R2 and R4 taken together with the carbon and nitrogen atoms to which they are bonded, form a non-aromatic heterocyclic ring optionally fused to an aromatic ring. Preferably R1 and R2 are the same and R3 and R4 are the same. R5-R6 are independently -H, an aliphatic group, a substituted aliphatic group, an aryl group or a substituted aryl group. Z is =O or =S.