As all cell lines respond to NVP-AUY922, the increase in Hsp70 is very significant and occurs rapidly. In the HCUVA-CC-34 primary culture however, EGFR depletion, ERK1/2 phosphorylation, and Hsp70

up-regulation are not very dramatic, which explain the moderate effects of this drug in anchorage-dependent and anchorage-independent growth assays. Experiments are Doramapimod manufacturer underway to try to identify a possible mechanism of resistance of HCUVA-CC-34 and other colorectal cellular models to NVP-AUY922. Since all our cellular models, apart from the exception just mentioned, were sensitive to NVP-AUY922, we sought to find markers of sensitivity/resistance to 17-AAG. In fact, phospho-kinase arrays were performed in 17-AAG–sensitive as well as in 17-AAG–resistant cell lines with the intention to find putative markers. However, we could not clearly associate differences found between cell lines to resistance to this drug. As it has been suggested that ABC transporters may play a role in resistance to Hsp90 inhibitors, we analyzed Mdr-1, MRP1, and BRCP1 protein levels

in these cell lines and found that none of the 17-AAG–resistant pancreatic and colorectal carcinoma cell lines expressed these transporters, find more with the exception of Caco-2 cells that express very low levels of BRCP1. However, many of the 17-AAG–sensitive cell lines express some of these ABC transporters (Figure 7). Therefore, we can rule out the role of these ABC transporters

in 17-AAG resistance. In addition to Pgp (Mdr-1), it has been suggested in several reports that NQO1/DT-diaphorase is necessary for benzoquinone ansamycin function. This enzyme is able to metabolize quinones to the corresponding hydroquinones, which are more stable and bind Hsp90 with greater affinity. We have found that the 17-AAG–resistant pancreatic carcinoma PANC-1 and CFPAC-1 cells lack NQO1 protein and activity (Figure 8), confirming the results previously reported by Siegel et al. [39]. The 17-AAG–resistant Caco-2 cells also lack NQO1 protein and enzymatic activity. However, LoVo cells, which are also devoid of NQO1 enzyme (Figure 8), are still responsive to 17-AAG, as demonstrated especially in soft Sclareol agar assays and cell cycle analyses (Figure 2 and Figure 3). We speculate that other reductases, albeit with less potency, may be able to reduce 17-AAG to 17-AAGH2 in these cells. Another possibility is that although less potent, the nonreduced benzoquinones may also have an activity and be able to exert the same effects as their reduced counterparts at higher concentrations. When we blocked NQO1 activity in 17-AAG–sensitive cell lines with ES936, these cells were still growth inhibited by 17-AAG (Figure 9).