to a phenyl group attached to the parent molecular moiety through an oxygen atom the term 34 phenylthio 34 refers to a phenyl group attached to the parent molecular moiety through a sulfur atom the term 34 pyridyloxy 34 refers to a pyridyl group attached to the parent molecular moiety through an oxygen atom the term 34 metabolically cleavable group 34 denotes a group which is cleaved in vivo to yield the parent molecule of the structural formulae indicated above wherin m is hydrogen examples of metabolically cleavable groups include cor coor conrr and ch 2 or radicals where r is selected independently at each occurrence from alkyl trialkylsilyl carbocyclic aryl or carbocyclic aryl substituted with one or more of c 1 c 4 alkyl halogen hydroxy or c 1 c 4 alkoxy specific examples of representative metabolically cleavable groups include acetyl methoxycarbonyl benzoyl methoxymethyl and trimethylsilyl groups by 34 pharmaceutically acceptable salt 34 it is meant those salts which are within the scope of sound medical judgement suitable for use in contact with the tissues of humans and lower animals without undue toxicity irritation allergic response and the like and are commensurate with a reasonable benefit risk ratio pharmaceutically acceptable salts are well known in the art for example s m berge et al describe pharmaceutically acceptable salts in detail in j pharmaceutical sciences 1977 66 1 19 the salts can be prepared in situ during the final isolation and purification of the compounds of the invention or separately by reacting the free base function with a suitable organic acid representative acid addition salts include acetate adipate alginate ascorbate aspartate.