The present invention provides compounds having the formula: ##STR00001## wherein A is CH or N; B is chosen from OH, NH.sub.2, NHR, H or halogen; D is chosen from OH, NH.sub.2, NHR, H, halogen or SCH.sub.3; R is an optionally substituted alkyl, aralkyl or aryl group; and X and Y are independently selected from H, OH or halogen except that when one of X and Y is hydroxy or halogen, the other is hydrogen; and Z is OH or, when X is hydroxy, Z is selected from hydrogen, halogen, hydroxy, SQ or OQ, Q is an optionally substituted alkyl, aralkyl or aryl group; or a tautomer thereof; or a pharmaceutically acceptable salt thereof; or an ester thereof; or a prodrug thereof; and compounds having the formula: ##STR00002## wherein A, X, Y, Z and R are defined for compounds of formula (I) where first shown above; E is chosen from CO.sub.2H or a corresponding salt form, CO.sub.2R, CN, CONH.sub.2, CONHR or CONR.sub.2; and G is chosen from NH.sub.2, NHCOR, NHCONHR or NHCSNHR; or a tautomer thereof, or a pharmaceutically acceptable salt thereof, or an ester thereof, or a prodrug thereof.The present invention also provides the use of the above compounds as pharmaceuticals, pharmaceutical compositions containing the compounds and processes for preparing the compounds.