Increased levels of pro-survival chaperones such as Hsp27 [44] and Hsp70 due to elevation in the heat shock response [45] have been proposed as possible NQO1-unrelated causes of resistance to benzoquinone ansamycins [46]. In our system however, Hsp70 protein levels were not significantly induced after 17-AAG treatment in resistant cells. Inaccessibility Panobinostat price of Hsp90 inhibitors to the Hsp90 isoforms located in mitochondria

[47], which contribute to apoptosis inhibition, may be another plausible cause of resistance. Furthermore, mutations or alterations in posttranslational modifications in the Hsp90 itself may contribute to Hsp90 inhibitor resistance [46].

Our cellular models, however, were sensitive to NVP-AUY922, which is based on resorcinol and not structurally related to benzoquinones [14]. This inhibitor is not dependent on the presence of NQO1 and we have demonstrated that NVP-AUY922 sensitivity Small molecule library does not correlate with NQO1 activity (Figure 8C). In a clinical setting, it is more useful to use NVP-AUY922 that offers several advantages over benzoquinones: no liver toxicity and no NQO1 or other reductase requirement for its function. Furthermore, we have shown in this report for the first time that this novel Hsp90 inhibitor is very potent in combination with other drugs such as gemcitabine, oxaliplatin, AZD6244, or NVP-BEZ235 in cell lines that are not very responsive to these drugs ( Figure 11). Moreover, it has

been shown that NVP-AUY922 is able to sensitize prostate cancer cell to radiation [48]. Therefore, NVP-AUY922 has a great potential to be used not only as a single agent but also in combination with chemotherapy or radiation therapy, even when these agents are not very effective when used alone. NVP-AUY922 is a more potent inhibitor than 17-AAG Succinyl-CoA in pancreatic and colorectal cellular models, as demonstrated by inhibition of cell proliferation and colony formation, cell death induction, HER receptor depletion, and inhibition of ERK and Akt signaling pathways. Some of these models show resistance to 17-AAG, especially pancreatic carcinoma cell lines. The ABC transporters examined are not involved in resistance to the Hsp90 inhibitors 17-AAG and NVP-AUY922. The use of NQO1 as a biomarker of response to Hsp90 inhibitors is limited only to 17-AAG and not to NVP-AUY922 and is dependent on the cellular context. Moreover, we show that rather than a marker of response to 17-AAG, NQO1 is a marker of sensitivity, as cells devoid of this enzyme can still respond to 17-AAG. Therefore, the utilization of non-benzoquinone compounds such as NVP-AUY922 is more appropriate.