Compounds of formula (1) are inhibitors of HSP90 activity in vitro or in vivo, and of use in the treatment of inter alia, cancer: wherein R2 is a group of formula -(Ar1)m-(Alk1)P-(Z)r-(Alk2)S-Q wherein Ar1 is an optionally substituted aryl or heteroaryl radical, Alk' and Alk 2 are optionally substituted divalent C1-C3 alkylene or C2-C3 alkenylene radicals, m, p, r and s are independently 0 or 1, Z is -0-, -S-, -(C=O)-, -(C=S)-, -S02-, -C(=O)O-, -C(=O) NR A- , -C(=S)NR A-, -S02NR A-, -NR AC(=O)_, -NR AS02- or-NR A-wherein R A is hydrogen or C1-C6 alkyl, and Q is hydrogen or an optionally substituted carbocyclic or heterocyclic radical; R3 is hydrogen, an optional substituent, or an optionally substituted (C1-C6)alkyl, aryl or heteroaryl radical; and R4 is a carboxylic ester, carboxamide or sulfonamide group.