The invention provides a compound for use as an inhibitor of Hsp90, the compound having the formula (I): or salts, tautomers, solvates or N-oxides thereof; wherein: A is N or a group CR3; R1 is a monocyclic or bicyclic carbocyclic or heterocyclic ring of 5 to 10 ring members of which up to two ring members may be heteroatoms selected from N, O and S and the remainder are carbon atoms, the carbocyclic or heterocyclic ring being optionally substituted by one or more substituent groups independently selected from R10; and R2, R3 and R10 are as defined in the claims.