Piperazine derivatives of formula (A): ##STR1## wherein .sub.-------- denotes an optional bond, provided that either .sub.----.sup.a.sub.---- and .sub.----.sup.c.sub.---- are both bonds and .sub.----.sup.b.sub.---- and .sub.----.sup.d.sub.---- are not bonds, or .sub.----.sup.b.sub.---- and .sub.----.sup.d.sub.---- are both bonds and .sub.----.sup.a.sub.---- and .sub.----.sup.c.sub.---- are not bonds; each of R.sub.14 and R.sub.15 is independently selected from hydrogen and C.sub.1 -C.sub.6 alkyl which is unsubstituted or substituted by an organic functional group; and each of R.sub.1 to R.sub.10, 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).