A compound of formula (I) and pharmaceutically acceptable salts, solvates and hydrolysable esters thereof wherein: X.sup.1 is O or S; X.sup.2 is O or S; n is 1 3, One of Y and Z is N, and the other is O; R.sup.1 and R.sup.2 are independently halogen, H, CH.sub.3, OCH.sub.3 or C.sub.1-3 alkyl; Each R.sup.3 independently is halogen, CF.sub.3, C.sub.1-6 straight or branched alkyl; R.sup.4 and R.sup.5 are independently H or C.sub.1-3 alkyl. y is 0, 1, 2, 3, 4 or 5.