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ficient affinity to immunoprecipitate Hsp90 and that binding is prevented with excess ATP. When it truly is unclear regardless of whether the ATP is competing directly at the C terminal site or is acting allosterically by binding towards the N terminus and therefore preventing accessibility Everolimus at the C terminal pocket, this data demonstrates that KU174 is binding directly to Hsp90. Surface Plasma Resonance In an effort to further characterize KU174 as a direct Hsp90 inhibitor, the binding of KU174 to Hsp90 was analyzed by surface plasmon resonance spectroscopy. The kinetics of binding and dissociation were reliably fitted to a pseudo 1st order model to get a 1:1 interaction with the ka and kd calculated to be 1.04 × 103 and 0.098, respectively. The Kd estimated from the fitting from the binding curve was in close agreement with the Kd estimated from the ratio from the dissociation and association constants.
In comparison, the ka and kd for the binding of novobiocin to Hsp90 were 211 and 0.23 , with a Kd calculated from the binding curve of 0.86 mM 0.02 s.e. Therefore, the SPR analysis from the interaction of KU174 with Hsp90 indicated the compound bound directly towards the purified recombinant protein with an affinity roughly Everolimus 12 fold higher than NB. Cancer cell based Hsp90 dependent luciferase refolding assay Direct inhibition from the Hsp90 protein folding machinery was assessed making use of a cancer cell based luciferase refolding assay developed in our laboratory. Previously, the Hsp90 luciferase based refolding assay has been validated making use of rabbit reticulocyte lysates.
On the other hand, there remains concern regardless of whether the presentation of Hsp90 complexes within these lysates are physiologically relevant in cancer. Various lines of evidence suggest that Hsp90 is present in cancer cells as part of a large macromolecular complex and therefore drugs that target Hsp90 activity really should be Bosutinib engineered towards binding Hsp90 within its physiologically relevant cancer cellular environment. Based on the aforementioned limitations making use of rabbit reticulocyte lysates, a cell based luciferase assay was optimized making use of both N terminal and C terminal Hsp90 inhibitors in prostate cancer cell lines. The extent of luciferase refolding in PC3 MM2 in the presence of Nterminal or C terminal Hsp90 inhibitors was evaluated at 60 and 90 minutes. Both classes of Hsp90 inhibitors demonstrated comparable EC50 concentrations at 60 and 90 minutes with 17 AAG becoming far more potent.
Considering that a 60 minute refolding experiment resulted inside a considerable improve in luciferase activity and good signal to noise, all subsequent experiments were performed at this time point. In an effort to demonstrate assay efficiency and accuracy, the parent compound NB and an earlier, much less potent analogue, F 4 was compared to KU174 and 17AAG. As expected, NB and F 4 resulted in proper shifted dose response curves relative to KU174 with NB showing minimal activity. Subsequently, a second N terminal inhibitor, radicicol, and an inactive novobiocin analog determined in our laboratory to not bind Hsp90, KU298, were analyzed in this assay as additional positive and damaging controls, respectively.
In this experiment, radicicol demonstrated an EC50 value comparable to 17 AAG, when as expected KU298 was inactive, further supporting the specificity of this assay for Hsp90 inhibition. Lastly, to compare this assay across prostate cancer cell lines, the capacity of Hsp90 inhibitors to inhibit luciferase refolding was examined in an LNCaP LN3 luciferase expressing cell line. In agreement with our earlier results, these compounds inhibited Hsp90 dependent luciferase refolding with increased potency when comparing EC50 values between cell lines, a trend that has also been observed in other functional assays. Overall, these data demonstrate a novel method to determine on target Hsp90 inhibition making use of a functional assay in an intact cancer cell milieu.
In vivo preclinical proof of idea studies Initially, pilot pharmacokinetic studies of KU174 were conducted in the mouse and revealed substantial metabolism and clearance preventing the use of this species for efficacy studies as powerful concentrations of drug could not be achieved at the site of action. Thus, KU174 was initially tested in the rat PC3 MM2 xenograft tumor model inside a single dose pilot PK study to ensure that powerful concentrations could be reached in the tumor prior to conducting a multi dose efficacy study. A KU174 tumor to plasma ratio of 4:1 was achieved six hours immediately after a single i.p. administration of 75 mg/kg suggesting selective retention. The concentration of KU174 in the tumor correlated to 17 M, assuming a gram of tissue is equal to 1 milliliter, at this time point, which was believed to be adequate enough to observe a pharmacodynamic response depending on our in vitro data. Following this single dose study, a multi dose efficacy study was conducted making use of a rat PC3 MM2 xenograft tumor model to ensure that tumor volume could be monitored over time. In this study, KU174 was admi