Hsp90 inhibitors and drugs from fragment and virtual screening

We have previously reported the structure-based optimisation of a number of series of potent compounds progressed as clinical candidates for oncology through inhibition of the ATPase activity of the molecular chaperone, Hsp90. The starting point for these candidates was compounds discovered using a combination of structure-based hit identification methods. This chapter summarises the overall story of how these methods were applied. Virtual screening of commercially available compounds identified a number of classes of compounds. At the same time, an initial fragment screen identified 17 fragments of various classes that bound to the N-terminal domain of Hsp90 with weak (0.5-10 mM) affinity. A subsequent screen identified a total of 60 compounds. This collection of fragments and virtual screening hits were progressed in a number of ways. Although two fragments could be observed binding together in the active site, the synthetic effort required to link these fragments was judged too high. For the resorcinol class of fragments, limited library synthesis generated compounds in the 1-10 μM range. In addition, the resorcinol substructure was used to select commercially available compounds that were filtered using focussed docking in the Hsp90 active site to select further sets of compounds for assay. This identified structural motifs that were exploited during lead optimisation to generate AUY922, currently in Phase II clinical trials. In a separate campaign, features identified in the structures of fragments, evolved fragments and virtual screening hits bound to Hsp90 were combined to generate an oral series of compounds, progressed to preclinical candidates. The crystal structures were determined of many of the fragments bound to Hsp90 and provide examples of both maintenance and change of protein conformation on fragment binding. Finally, we analyse the extent to which our initial set of fragments recapitulates the key structural features of the Hsp90 inhibitors published to date.