Pharmaceutical compounds

A piperazinedione derivative of formula (I), wherein one of R and R is H and the other is a group -COR wherein R is selected from: (i) hydroxy; (ii): (A) wherein n is 0 or 1 and m is 0, 1, 2, 3 or 4, at least one of n and m being other than 0, and either (a) R is H or C1-C6 alkyl and R is C1-C6 alkyl optionally substituted by one or two phenyl groups, or (b) R and R together with the nitrogen atom to which they are attached, form a heterocyclic group selected from (B1) and (B2), wherein R and R , which are the same or different, are each hydrogen or C1-C6 alkoxy, or R and R together form a methylenedioxy group; (iii): (C) wherein R is C1-C6 alkyl optionally substituted by hydroxy, C2-C6 alkenyl or a phenyl ring optionally substituted by a C1-C6 alkoxy; and (iv): (D) wherein p is 1, 2 or 3 and R is C2-C6 alkenyl or a phenyl ring optionally substituted by C1-C6 alkoxy; and pharmaceutically acceptable salts thereof have activity as modulators of multi-drug resistance.