The invention provides a compound for use as an inhibitor of Hsp90, the compound having the formula (I): or salts, tautomers, solvates and N-oxides thereof; wherein R1 is hydroxy or hydrogen; R2 is hydroxy; methoxy or hydrogen; provided that at least one of R1 and R2 is hydroxy; R3 is selected from hydrogen; halogen; cyano; C 1-5 hydrocarbyl and C 1-5 hydrocarbyloxy; wherein the C 1-5 hydrocarbyl and C 1-5 hydrocarbyloxy moieties are each optionally substituted by one or more substituents selected from hydroxy, halogen, C 1-2 alkoxy, amino, mono- and di-C 1-2 alkylamino, and aryl and heteroaryl groups of 5 to 12 ring members; R4 is selected from hydrogen; a group -(O)n-R7 where n is 0 or 1 and R7 is an acyclic C 1-5 hydrocarbyl group or a monocyclic carbocyclic or heterocyclic group having 3 to 7 ring members; halogen; cyano; hydroxy; amino; and mono- or di-C 1-5 hydrocarbyl-amino, wherein the acyclic Ci-5 hydrocarbyl group and the mono and di-C 1-5 hydrocarbylamino moieties in each instance are optionally substituted by one or more substituents selected from hydroxy, halogen, C 1-2 alkoxy, amino, mono- and di-C 1-2 alkylamino, and aryl and heteroaryl groups of 5 to 12 ring members; or R3 and R4 together form a monocyclic carbocyclic or heterocyclic ring of 5 to 7 ring members; and R5 and R6 are as defined in the claims.