The present invention provides a compound of the formula (I): ##STR00001## wherein A is selected from N, CH and CR, where R is selected from halogen, optionally substituted alkyl, aralkyl and aryl, OH, NH.sub.2, NHR.sup.1, NR.sup.1R.sup.2 and SR.sup.3, where R.sup.1, R.sup.2 and R.sup.3 are each optionally substituted alkyl, aralkyl or aryl groups; B is selected from OH, NH.sub.2, NHR.sup.4, H and halogen, where R.sup.4 is an optionally substituted alkyl, aralkyl or aryl group; D is selected from OH, NH.sub.2, NHR.sup.5, H, halogen and SCH.sub.3, where R.sup.5 is an optionally substituted alkyl, aralkyl or aryl group; X and Y are independendy selected from H, OH and halogen, with the proviso that when one of X and Y is hydroxy or halogen, the other is hydrogen; Z is OH, or, when X is hydroxy, Z is selected from hydrogen, halogen, hydroxy, SQ and OQ, where Q is an optionally substituted alkyl, aralkyl or aryl group; and W is OH or H, with the proviso that when W is OH, then A is CR where R is as defined above; or a tautomer thereof; or a pharmaceutically acceptable salt thereof; or an ester thereof. The invention also provides pharmaceutical compositions comprising compounds of formula (I), methods of preparing compounds of formula (I), and methods of treatment using compounds of formula (I).