Pharmaceutical compounds

PCT No. PCT/GB95/00300 Sec. 371 Date Nov. 4, 1996 Sec. 102(e) Date Nov. 4, 1996 PCT Filed Feb. 14, 1995 PCT Pub. No. WO95/21830 PCT Pub. Date Aug. 17, 1995Piperazine derivatives of formula (A): (A) wherein --- denotes an optional bond, provided that either --a-- and --c-- are both bonds and --b-- and --d-- are not bonds, or --b-- and --d-- are both bonds and --a-- and --c-- are not bonds; each of R14 and R15 is independently selected from hydrogen and C1-C6 alkyl which is unsubstituted or substituted by an organic functional group; and each of R1 to R10, which may be the same or different, is independently hydrogen or an organic functional group; and the pharmaceutically acceptable salts and esters thereof have utility as modulators of multiple drug resistance (MDR).