A compound of formula (I) or a pharmaceutically acceptable salt, solvate, or hydrolysable ester thereof, Wherein: R.sup.1 and R.sup.2 are independently hydrogen or C.sub.1-3 alkyl; X represents a bond, CH.sub.2 or O; R.sup.3 and R.sup.4 are independently hydrogen, C.sub.1-6 alkyl, OCH.sub.3, CF.sub.3, allyl or halogen; X.sup.1 is CH.sub.2, SO.sub.2, or CO; R.sup.5 is --C.sub.1-6 alkyl (optionally substituted by C.sub.1-6alkoxy or C.sub.1-6alkylthio), --C.sub.2-6 alkenyl, --C.sub.0-6 alkyl phenyl (wherein the phenyl is optionally substituted by one or more CF.sub.3, halogen, C.sub.1-3 alkyl, C.sub.1-3 alkoxy), --COC.sub.1-6 alkyl, SO.sub.2C.sub.1-6 alkyl; R.sup.6 is phenyl or a 6 membered heteroaryl group containing 1, 2 or 3 N atoms wherein the phenyl or heteroaryl group is optionally substituted with 1, 2 or 3 moieties selected from the group consisting of C.sub.1-6 alkyl, halogen, --OC.sub.1-6 alkyl, --SO.sub.2C.sub.1-3 alkyl, phenyl (optionally substituted by one or more groups selected from halogen, CF.sub.3, C.sub.1-3 alkyl, OC.sub.1-3 alkyl, acetyl, CN).