. . . . "Particularly suitable in-vivo hydrolysable esters include lower alkyl groups, lower alkyl groups substituted by a hydroxyl group not on the ??-carbon atom and groups of the sub-formulae (a) or (b): ##STR4## wherein R7 is a hydrogen atom or a methyl group; R8 is a lower alkyl or phenyl group; R9 is a hydrogen atom or a methyl or methoxyl group; and R10 is a hydrogen atom or a methyl or methoxyl gro" .