The Hsp90 machinery mediates the folding, maturation, activation, and assembly of various proteins involved in signal transduction,

transcriptional regulation, and cell cycle control [1]. Many of these client proteins are oncogenic. Therefore, a great advantage of the use of Hsp90 inhibitors is that multiple key oncogenic proteins can be disrupted simultaneously [2]. The geldanamycin see more derivative 17-allylamino-17-demethoxygeldanamycin (17-AAG), or tanespimycin, was the first Hsp90 inhibitor that entered clinical trials [3]. There are now about 14 inhibitors of Hsp90 function undergoing clinical trials, which belong to different structural classes [4]. All of them bind to a conserved pocket in the NH2-terminal ATP-binding domain of Hsp90, inhibiting its activity. Geldanamycin and its derivatives belong to the benzoquinone ansamycin class, which was found to inhibit expression of the oncogene c-myc [5] and to cause inactivation [6] and degradation of the tyrosine kinase src [7], human EGFR 2 (HER2)/Neu [8], raf

[9], and mutated p53 [10]. However, albeit most of phase I and phase II clinical trials with geldanamycin derivatives have already been completed or terminated due to clinical limitations, these drugs have proved the successful targeting of Hsp90, paving the way for the development of second-generation Hsp90 inhibitors [11], such as synthetic and small molecules, targeted also against the N-terminal ATP-binding site. One class of such small inhibitors is based on the pyrazole or resorcinol subunit, another class on the purine-scaffold, and lastly, novel Smad inhibitor C-terminal domain–based Hsp90 inhibitors are being developed as well [12]. NVP-AUY922 is a novel resorcinylic isoxazole–based Hsp90 inhibitor that has shown potent preclinical activity in cancer models [13] and in xenografts [14]. In addition, it has

shown tolerability in a phase I clinical trial [15]. The Hsp90-client cycle involves the association and dissociation of several cochaperones and is driven by the ATP-binding state of Hsp90 [2]. Thus, Hsp90 participates in two multichaperoning complexes with opposing activities: ATP-bound (mature) and ADP-bound (intermediate). A client protein initially associates with Silibinin Hsp70/Hsp40 and is loaded onto Hsp90 through p60Hop, forming the ADP-bound intermediate state. When ADP is transformed into ATP, the Hsp90 complex conformation is altered, releasing Hsp70/Hsp40 and p60Hop, allowing other cochaperones such as p23, p50cdc37, and immunophilins to bind Hsp90, forming the mature complex. Then, at this stage, Hsp90-bound ATP is hydrolyzed, and the energy released enables client protein folding. Hsp90 inhibitors such as 17-AAG inhibit the ATPase intrinsic activity of Hsp90, impeding the chaperone to achieve the mature state [16].