An orally bioavailable, ATP-competitive, small-molecule inhibitor of heat shock protein 90 (Hsp90) with potential antineoplastic activity. Hsp90 inhibitor XL888 specifically binds to Hsp90, inhibiting its chaperone function and promoting the proteasomal degradation of oncogenic signalling proteins involved in tumour cell proliferation and survival; inhibition of tumour cell proliferation may result. Hsp90, a chaperone complex protein upregulated in a variety of tumour cell types, regulates the folding and degradation of many oncogenic signalling proteins, including Her-2 and Met.