Inhibitors of the heat shock response: biology and pharmacology

Liver-derived IGF-I regulates mean life span in mice

Background

Transgenic mice with low levels of global insulin-like growth factor-I (IGF-I) throughout their life span, including pre- and postnatal development, have increased longevity. This study investigated whether specific deficiency of liver-derived, endocrine IGF-I is of importance for life span.

Methods and Findings

Serum IGF-I was reduced by approximately 80% in mice with adult, liver-specific IGF-I inactivation (LI-IGF-I^-/- mice), and body weight decreased due to reduced body fat. The mean life span of LI-IGF-I^-/- mice (n = 84) increased 10% vs. control mice (n = 137) (Cox's test, p<0.01), mainly due to increased life span (16%) of female mice [LI-IGF-I^-/- mice (n = 31): 26.7±1.1 vs. control (n = 67): 23.0±0.7 months, p<0.001]. Male LI-IGF-I^-/- mice showed only a tendency for increased longevity (p = 0.10). Energy expenditure, measured as oxygen consumption during and after submaximal exercise, was increased in the LI-IGF-I^-/- mice. Moreover, microarray and RT-PCR analyses showed consistent regulation of three genes (heat shock protein 1A and 1B and connective tissue growth factor) in several body organs in the LI-IGF-I^-/- mice.

Conclusions

Adult inactivation of liver-derived, endocrine IGF-I resulted in moderately increased mean life span. Body weight and body fat decreased in LI-IGF-I^-/- mice, possibly due to increased energy expenditure during exercise. Genes earlier reported to modulate stress response and collagen aging showed consistent regulation, providing mechanisms that could underlie the increased mean life span in the LI-IGF-I^-/- mice.

To fold or not to fold: modulation and consequences of Hsp90 inhibition

BACKGROUND

The 90-kDa heat-shock proteins (Hsp90) have rapidly evolved into promising therapeutic targets for the treatment of several diseases, including cancer and neurodegenerative diseases. Hsp90 is a molecular chaperone that aids in the conformational maturation of nascent polypeptides, as well as the rematuration of denatured proteins.

DISCUSSION

Many of the Hsp90-dependent client proteins are associated with cellular growth and survival and, consequently, inhibition of Hsp90 represents a promising approach for the treatment of cancer. Conversely, stimulation of heat-shock protein levels has potential therapeutic applications for the treatment of neurodegenerative diseases that result from misfolded and aggregated proteins.

CONCLUSION

Hsp90 modulation exhibits the potential to treat unrelated disease states, from cancer to neurodegenerative diseases, and, thus, to fold or not to fold, becomes a question of great value.

HSP70 inhibition by the small-molecule 2-phenylethynesulfonamide impairs protein clearance pathways in tumor cells

The evolutionarily conserved stress-inducible HSP70 molecular chaperone plays a central role in maintaining protein quality control in response to various forms of stress. Constitutively elevated HSP70 expression is a characteristic of many tumor cells and contributes to their survival. We recently identified the small-molecule 2-phenylethyenesulfonamide (PES) as a novel HSP70 inhibitor. Here, we present evidence that PES-mediated inhibition of HSP70 family proteins in tumor cells results in an impairment of the two major protein degradation systems, namely, the autophagy-lysosome system and the proteasome pathway. HSP70 family proteins work closely with the HSP90 molecular chaperone to maintain the stability and activities of their many client proteins, and PES causes a disruption in the HSP70/HSP90 chaperone system. As a consequence, many cellular proteins, including known HSP70/HSP90 substrates, accumulate in detergent-insoluble cell fractions, indicative of aggregation and functional inactivation. Overall, PES simultaneously disrupts several cancer critical survival pathways, supporting the idea of targeting HSP70 as a potential approach for cancer therapeutics.

The hexameric structures of human heat shock protein 90

Background

The human 90-kDa heat shock protein (HSP90) functions as a dimeric molecular chaperone. HSP90 identified on the cell surface has been found to play a crucial role in cancer invasion and metastasis, and has become a validated anti-cancer target for drug development. It has been shown to self-assemble into oligomers upon heat shock or divalent cations treatment, but the functional role of the oligomeric states in the chaperone cycle is not fully understood.

Principal Findings

Here we report the crystal structure of a truncated HSP90 that contains the middle segment and the carboxy-terminal domain, termed MC-HSP90. The structure reveals an architecture with triangular bipyramid geometry, in which the building block of the hexameric assembly is a dimer. In solution, MC-HSP90 exists in three major oligomer states, namely dimer, tetramer and hexamer, which were elucidated by size exclusion chromatography and analytical ultracentrifugation. The newly discovered HSP90 isoform HSP90N that lacks the N-terminal ATPase domain also exhibited similar oligomerization states as did MC-HSP90.

Conclusions

While lacking the ATPase domain, both MC-HSP90 and HSP90N can self-assemble into a hexameric structure, spontaneously. The crystal structure of MC-HSP90 reveals that, in addition to the C-terminal dimerization domain, the residue W320 in the M domain plays a critical role in its oligomerization. This study not only demonstrates how the human MC-HSP90 forms a hexamer, but also justifies the similar formation of HSP90N by using 3D modeling analysis.

Nano self-assembly of recombinant human gelatin conjugated with α-tocopheryl succinate for Hsp90 inhibitor, 17-AAG, delivery

A wide variety of drug delivery systems have been developed for the delivery of anticancer agents. One of the most frequently used natural biomaterials in drug delivery systems is polysaccharides; however, they are difficult to digest and to eliminate from the body after systemic administration due to their high molecular weight natures and the absence of degrading enzymes. Therefore, the development of degradable and eliminable natural biomaterials is critical for successful in vivo applications. In the present study, we report the development of self-assembled biodegradable nanoparticles based on recombinant human gelatin (rHG) modified with alpha-tocopheryl succinate (TOS). The rHG-TOS nanoparticles efficiently encapsulated 17-AAG (17-allylamino-17-demethoxygeldanamycin), a small molecular anticancer drug targeting heat shock protein 90. The formation of 17-AAG-loaded nanoparticles was confirmed using TEM and dynamic light scattering analysis and found to be within the size of 90-220 nm. The loading efficiency, sustained release pattern, and stability of 17-AAG from the rHG-TOS nanoparticles were determined using HPLC. Furthermore, the passive targeting of rHG-TOS nanoparticles to the tumor area via enhanced permeability and retention effect was examined by noninvasive live animal imaging in a tumor mouse model. Finally, the 17-AAG-loaded nanoparticles were nonimmunogenic and more efficient than free 17-AAG in manifesting an anticancer effect in the tumor model. Overall, our data demonstrate rHG-TOS as a promising tool for the delivery of 17-AAG featuring therapeutic efficacy and biocompatibility.

Impaired infectivity of ritonavir-resistant HIV is rescued by heat shock protein 90AB1

Certain ritonavir resistance mutations impair HIV infectivity through incomplete Gag processing by the mutant viral protease. Analysis of the mutant virus phenotype indicates that accumulation of capsid-spacer peptide 1 precursor protein in virus particles impairs HIV infectivity and that the protease mutant virus is arrested during the early postentry stage of HIV infection before proviral DNA synthesis. However, activation of the target cell can rescue this defect, implying that specific host factors expressed in activated cells can compensate for the defect in ritonavir-resistant HIV. This ability to rescue impaired HIV replication presented a unique opportunity to identify host factors involved in postentry HIV replication, and we designed a functional genetic screen so that expression of a given host factor extracted from activated T cells would lead directly to its discovery by rescuing mutant virus replication in nonactivated T cells. We identified the cellular heat shock protein 90 kDa α (cytosolic), class B member 1 (HSP90AB1) as a host factor that can rescue impaired replication of ritonavir-resistant HIV. Moreover, we show that pharmacologic inhibition of HSP90AB1 with 17-(allylamino)-17-demethoxygeldanamycin (tanespimycin) has potent in vitro anti-HIV activity and that ritonavir-resistant HIV is hypersensitive to the drug. These results suggest a possible role for HSP90AB1 in postentry HIV replication and may provide an attractive target for therapeutic intervention.

Protein folding in the cytoplasm and the heat shock response

Proteins generally must fold into precise three-dimensional conformations to fulfill their biological functions. In the cell, this fundamental process is aided by molecular chaperones, which act in preventing protein misfolding and aggregation. How this machinery assists newly synthesized polypeptide chains in navigating the complex folding energy landscape is now being understood in considerable detail. The mechanisms that ensure the maintenance of a functional proteome under normal and stress conditions are also of great medical relevance, as the aggregation of proteins that escape the cellular quality control underlies a range of debilitating diseases, including many age-of-onset neurodegenerative disorders.

Hsp90/Cdc37 chaperone/co-chaperone complex, a novel junction anticancer target elucidated by the mode of action of herbal drug Withaferin A

BACKGROUND

HSPs (Heat shock proteins) are highly conserved ubiquitous proteins among species which are involved in maintaining appropriate folding and conformation of other proteins and are thus referred to as molecular chaperones. Hsp90 (Heat-shock protein 90 kDa) is one of a group of molecular chaperones responsible for managing protein folding and quality control in cell environment. However it is also involved in the maturation and stabilization of a wide range of oncogenic client proteins which are crucial for oncogenesis and malignant progression. Hsp90 requires a series of co-chaperones to assemble into a super-chaperone complex for its function. These co-chaperones bind and leave the complex at various stages to regulate the chaperoning process. Arresting the chaperone cycle at these stages by targeting different co-chaperone/Hsp90 interactions seems to be quite a viable alternative and is likely to achieve similar consequences as that of Hsp90 direct inhibition with added favors of high specificity and reduced side effect profile. The study conducted here is an attempt to explore the potential of Withania somnifera's major constituent WA (Withaferin A) in attenuating the Hsp90/Cdc37 chaperone/co-chaperone interactions for enhanced tumor arresting activity and to elucidate the underlying mode of action using computational approaches.

RESULTS

Formation of active Hsp90/Cdc37 complex is one of the essential steps for facilitation of chaperone client interaction, non-assembly of which can lead to prevention of the chaperone-client association resulting in apoptosis of tumor cells. From our flexible docking analysis of WA into active Hsp90/Cdc37 complex in which key interfacing residues of the complex were kept flexible, disruption of the active association complex can be discerned. While docking of WA into segregated Hsp90 leaves the interface residues untouched. Thus the molecular docking analysis of WA into Hsp90 and active Hsp90/Cdc37 complex conducted in this study provides significant evidence in support of the proposed mechanism of chaperone assembly suppression by inhibition or disruption of active Hsp90/Cdc37 complex formation being accounted by non-assembly of the catalytically active Hsp90/Cdc37 complex. Results from the molecular dynamics simulations in water show that the trajectories of the protein complexed with ligand WA are stable over a considerably long time period of 4 ns, with the energies of the complex being lowered in comparison to the un-docked association complex, suggesting the thermodynamic stability of WA complexed Hsp90/Cdc37.

CONCLUSIONS

The molecular chaperone Hsp90 has been a promising target for cancer therapy. Cancer is a disease marked by genetic instability. Thus specific inhibition of individual proteins or signalling pathways holds a great potential for subversion of this genetic plasticity of cancers. This study is a step forward in this direction. Our computational analysis provided a rationalization to the ability of naturally occurring WA to alter the chaperone signalling pathway. The large value of binding energy involved in binding of WA to the active Hsp90/Cdc37 complex consolidates the thermodynamic stability of the binding. Our docking results obtained substantiate the hypothesis that WA has the potential to inhibit the association of chaperone (Hsp90) to its co-chaperone (Cdc37) by disrupting the stability of attachment of Hsp90 to Cdc37. Conclusively our results strongly suggest that withaferin A is a potent anticancer agent as ascertained by its potent Hsp90-client modulating capability.

A phase I study of the heat shock protein 90 inhibitor alvespimycin (17-DMAG) given intravenously to patients with advanced solid tumors

PURPOSE

A phase I study to define toxicity and recommend a phase II dose of the HSP90 inhibitor alvespimycin (17-DMAG; 17-dimethylaminoethylamino-17-demethoxygeldanamycin). Secondary endpoints included evaluation of pharmacokinetic profile, tumor response, and definition of a biologically effective dose (BED).

PATIENTS AND METHODS

Patients with advanced solid cancers were treated with weekly, intravenous (i.v.) 17-DMAG. An accelerated titration dose escalation design was used. The maximum tolerated dose (MTD) was the highest dose at which ≤ 1/6 patients experienced dose limiting toxicity (DLT). Dose de-escalation from the MTD was planned with mandatory, sequential tumor biopsies to determine a BED. Pharmacokinetic and pharmacodynamic assays were validated prior to patient accrual.

RESULTS

Twenty-five patients received 17-DMAG (range 2.5-106 mg/m(2)). At 106 mg/m(2) of 17-DMAG 2/4 patients experienced DLT, including one treatment-related death. No DLT occurred at 80 mg/m(2). Common adverse events were gastrointestinal, liver function changes, and ocular. Area under the curve and mean peak concentration increased proportionally with 17-DMAG doses 80 mg/m(2) or less. In peripheral blood mononuclear cells significant (P < 0.05) HSP72 induction was detected (≥ 20 mg/m(2)) and sustained for 96 hours (≥ 40 mg/m(2)). Plasma HSP72 levels were greatest in the two patients who experienced DLT. At 80 mg/m(2) client protein (CDK4, LCK) depletion was detected and tumor samples from 3 of 5 patients confirmed HSP90 inhibition. Clinical activity included complete response (castration refractory prostate cancer, CRPC 124 weeks), partial response (melanoma, 159 weeks), and stable disease (chondrosarcoma, CRPC, and renal cancer for 28, 59, and 76 weeks, respectively).

CONCLUSIONS

The recommended phase II dose of 17-DMAG is 80 mg/m(2) weekly i.v.

Characterization of Hsp70 gene in Chironomus riparius: expression in response to endocrine disrupting pollutants as a marker of ecotoxicological stress

We characterized the Hsp70 cDNA in Chironomus riparius and evaluated its expression profile under different environmental stressors. It is highly conserved, at both DNA and protein levels, displaying many of the hallmarks of Hsps and sharing 80-96% of overall amino acid identities with homologous sequences from other diptera. The changes are mainly concentrated in the C-terminal domain of the protein. Phylogenetic analysis was consistent with the known classification of insects. The Hsp70 gene was located by in situ hybridization in region III-3A at the third polytene chromosome, a locus activated upon heat shock as shown by RNA pol II binding. As C. riparius is widely used in aquatic ecotoxicology testing, we studied Hsp70 gene induction in fourth instar aquatic larvae submitted to heat shock and selected environmental pollutants classified as potential endocrine disruptors. RT-PCR analysis showed that Hsp70 mRNA levels increased significantly (p<0.05) after short-term acute exposures to a temperature shift (HS), cadmium chloride (Cd), butyl benzyl phthalate (BBP), diethylhexyl phthalate (DEHP), bisphenol A (BPA), 4-nonylphenol (NP) and ethinylestradiol (EE). However, neither pentachlorophenol (PCP) nor tributyltin (TBTO) treatments were able to activate the Hsp70 gene. The cognate form, Hsc70, was also analysed and, unlike Hsp70, was not altered by any of the different treatments assayed. Moreover, at the times tested, there was no significant mortality of the larvae. The rapid upregulation of the Hsp70 gene suggests that it is sensitive and selective for different environmental pollutants, and could be used as an early molecular endpoint in ecotoxicological studies.

Application of chemoproteomics to drug discovery: identification of a clinical candidate targeting hsp90

A chemoproteomics-based drug discovery strategy is presented that utilizes a highly parallel screening platform, encompassing more than 1000 targets, with a focused chemical library prior to target selection. This chemoproteomics-based process enables a data-driven selection of both the biological target and chemical hit after the screen is complete. The methodology has been exemplified for the purine binding proteome (proteins utilizing ATP, NAD, FAD). Screening of an 8000 member library yielded over 1500 unique protein-ligand interactions, which included novel hits for the oncology target Hsp90. The approach, which also provides broad target selectivity information, was used to drive the identification of a potent and orally active Hsp90 inhibitor, SNX-5422, which is currently in phase 1 clinical studies.

Mixed pH-sensitive polymeric micelles for combination drug delivery

PURPOSE

To prepare mixed polymeric micelles that can carry two different drugs, doxorubicin (DOX) and 17-hydroxyethylamino-17-demethoxygeldanamycin (GDM-OH), for combination cancer chemotherapy.

METHODS

The pH-sensitive micelles were prepared from poly(ethylene glycol)-poly(aspartate hydrazide) block copolymers to which either DOX or GDM-OH is conjugated through acid-labile hydrazone bond (individual micelles). Mixed micelles were formed not only by simply mixing two different individual micelles in aqueous solutions (aqueous mixed micelles) but also by evaporating organic solvents from the organic/aqueous mixed solvents in which two block copolymers possessing different drugs were dissolved homogeneously (organic mixed micelles). Particle size measurements, pH-dependent drug release tests, cytotoxicity assays and western blot analysis were subsequently conducted.

RESULTS

Individual and aqueous/organic mixed micelles showed clinically relevant particle size (<100 nm) and pH-dependent drug release patterns. Mixed polymer micelles suppress cancer cell growth effectively in a drug concentration, mixing method and schedule-dependent way.

CONCLUSION

Combination chemotherapy using polymeric micelles seems to minimize a schedule-dependent change in combination drug efficacy in comparison to drug combination using DMSO formulations.

Inhibiting heat-shock protein 90 reverses sensory hypoalgesia in diabetic mice

Increasing the expression of Hsp70 (heat-shock protein 70) can inhibit sensory neuron degeneration after axotomy. Since the onset of DPN (diabetic peripheral neuropathy) is associated with the gradual decline of sensory neuron function, we evaluated whether increasing Hsp70 was sufficient to improve several indices of neuronal function. Hsp90 is the master regulator of the heat-shock response and its inhibition can up-regulate Hsp70. KU-32 (N-{7-[(2R,3R,4S,5R)-3,4-dihydroxy-5-methoxy-6,6-dimethyl-tetrahydro-2H-pyran-2-yloxy]-8-methyl-2-oxo-2H-chromen-3-yl}acetamide) was developed as a novel, novobiocin-based, C-terminal inhibitor of Hsp90 whose ability to increase Hsp70 expression is linked to the presence of an acetamide substitution of the prenylated benzamide moiety of novobiocin. KU-32 protected against glucose-induced death of embryonic DRG (dorsal root ganglia) neurons cultured for 3 days in vitro. Similarly, KU-32 significantly decreased neuregulin 1-induced degeneration of myelinated Schwann cell DRG neuron co-cultures prepared from WT (wild-type) mice. This protection was lost if the co-cultures were prepared from Hsp70.1 and Hsp70.3 KO (knockout) mice. KU-32 is readily bioavailable and was administered once a week for 6 weeks at a dose of 20 mg/kg to WT and Hsp70 KO mice that had been rendered diabetic with streptozotocin for 12 weeks. After 12 weeks of diabetes, both WT and Hsp70 KO mice developed deficits in NCV (nerve conduction velocity) and a sensory hypoalgesia. Although KU-32 did not improve glucose levels, HbA1c (glycated haemoglobin) or insulin levels, it reversed the NCV and sensory deficits in WT but not Hsp70 KO mice. These studies provide the first evidence that targeting molecular chaperones reverses the sensory hypoalgesia associated with DPN.

A Phase II trial of 17-allylamino, 17-demethoxygeldanamycin (17-AAG, tanespimycin) in patients with metastatic melanoma

PURPOSE

A Phase II study to screen for anti-melanoma activity of the heat shock protein 90 (HSP90) inhibitor, 17-AAG (17-allylamino-17-demethoxygeldanamycin) was performed. The primary endpoint was the rate of disease stabilisation in patients with progressive, metastatic melanoma treated with 17-AAG. Secondary endpoints were to determine: the toxicity of 17-AAG, the duration of response(s), median survival and further study the pharmacokinetics and pharmacodynamics of 17-AAG.

PATIENTS AND METHODS

Patients with metastatic melanoma (progressive disease documented ≤6 months of entering study) were treated with weekly, intravenous 17-AAG. A Simon one sample two stage minimax design was used. A stable disease rate of ≥25% at 6 months was considered compatible with 17-AAG having activity.

RESULTS

Fourteen patients (8 male: 6 female) were entered, eleven received 17-AAG (performance status 0 or 1). Median age was 60 (range 29-81) years. The majority (93%) received prior chemotherapy and had stage M1c disease (71%). Toxicity was rarely ≥ Grade 2 in severity and commonly included fatigue, headache and gastrointestinal disturbances. One of eleven patients treated with 17-AAG had stable disease for 6 months and median survival for all patients was 173 days. The study was closed prematurely prior to completion of the first stage of recruitment and limited planned pharmacokinetic and pharmacodynamic analyses.

CONCLUSION

Some evidence of 17-AAG activity was observed although early study termination meant study endpoints were not reached. Stable disease rates can be incorporated into trials screening for anti-melanoma activity and further study of HSP90 inhibitors in melanoma should be considered.

Binding of a small molecule at a protein-protein interface regulates the chaperone activity of hsp70-hsp40

Heat shock protein 70 (Hsp70) is a highly conserved molecular chaperone that plays multiple roles in protein homeostasis. In these various tasks, the activity of Hsp70 is shaped by interactions with co-chaperones, such as Hsp40. The Hsp40 family of co-chaperones binds to Hsp70 through a conserved J-domain, and these factors stimulate ATPase and protein-folding activity. Using chemical screens, we identified a compound, 115-7c, which acts as an artificial co-chaperone for Hsp70. Specifically, the activities of 115-7c mirrored those of a Hsp40; the compound stimulated the ATPase and protein-folding activities of a prokaryotic Hsp70 (DnaK) and partially compensated for a Hsp40 loss-of-function mutation in yeast. Consistent with these observations, NMR and mutagenesis studies indicate that the binding site for 115-7c is adjacent to a region on DnaK that is required for J-domain-mediated stimulation. Interestingly, we found that 115-7c and the Hsp40 do not compete for binding but act in concert. Using this information, we introduced additional steric bulk to 115-7c and converted it into an inhibitor. Thus, these chemical probes either promote or inhibit chaperone functions by regulating Hsp70-Hsp40 complex assembly at a native protein-protein interface. This unexpected mechanism may provide new avenues for exploring how chaperones and co-chaperones cooperate to shape protein homeostasis.

Stochastic modelling of the eukaryotic heat shock response

The heat shock response (HSR) is a highly evolutionarily conserved defence mechanism allowing the cell to promptly react to elevated temperature conditions and other forms of stress. It has been subject to intense research for at least two main reasons. First, it is considered a promising candidate for deciphering the engineering principles underlying regulatory networks. Second, heat shock proteins (main actors of the HSR) play crucial role in many fundamental cellular processes. Therefore, profound understanding of the heat shock response would have far-reaching ramifications for the cell biology. Recently, a new deterministic model of the eukaryotic heat shock response has been proposed in the literature. It is very attractive since it consists of only the minimum number of components required by any functional regulatory network, while yet being capable of biological validation. However, it admits small molecule populations of some of the considered metabolites. In this paper a stochastic model corresponding to the deterministic one is constructed and the outcomes of these two models are confronted. The aim with this comparison is to show that, in the case of the heat shock response, the approximation of a discrete system with a continuous model is a reasonable approach. This is not always the truth, especially when the numbers of molecules of the considered species are small. By making the effort of performing and analysing 1000 stochastic simulations, we investigate the range of behaviour the stochastic model is likely to exhibit. We demonstrate that the obtained results agree well with the dynamics displayed by the continuous model, which strengthens the trust in the deterministic description. A proof of the existence and uniqueness of the stationary distribution of the Markov chain underlying the stochastic model is given. Moreover, the obtained view of the stochastic dynamics and the performed comparison to the outcome of the continuous formulation provide more insight into the dynamics of the heat shock response mechanism.

HSP90 manages the ends

The telomere environment requires an efficient means to assemble and disassemble a multitude of structures to operate correctly and to help achieve cellular homeostasis. Telomeres are challenged by a common binding specificity displayed by many of the protein components for telomeric DNA, which could result in competitive DNA interactions, and by a cell cycle-restricted timing of events, which enforces a narrow working period in which to perform numerous tasks. In this review, we discuss how the HSP90 molecular chaperone network avoids these obstacles and facilitates an effective operation of the telomere system.

The enhancement of antiproliferative and proapoptotic activity of HDAC inhibitors by curcumin is mediated by Hsp90 inhibition

Curcumin, a natural polyphenol, has been described to exhibit effects on signaling pathways, leading to induction of apoptosis. In this study, we observed that curcumin inhibited Hsp90 activity causing depletion of client proteins implicated in survival pathways. Based on this observation, this study was designed to investigate the cellular effects of curcumin combination with the pan-HDAC inhibitors, vorinostat and panobinostat, which induce hyperacetylation of Hsp90, resulting in inhibition of its chaperone function. The results showed that, at subtoxic concentrations, curcumin markedly sensitized tumor cells to vorinostat- and panobinostat-induced growth inhibition and apoptosis. The sensitization was associated with persistent depletion of Hsp90 client proteins (EGFR, Raf-1, Akt, and survivin). In conclusion, our findings document a novel mechanism of action of curcumin and support the therapeutic potential of curcumin/HDAC inhibitors combination, because the synergistic interaction was observed at pharmacologically achievable concentrations, which were ineffective when each drug was used alone.

Alternate strategies of Hsp90 modulation for the treatment of cancer and other diseases

The 90 kDa heat shock protein (Hsp90) has become a validated target for the development of anti-cancer agents. Several Hsp90 inhibitors are currently under clinical trial investigation for the treatment of cancer. All of these agents inhibit Hsp90's protein folding activity by binding to the N-terminal ATP binding site of the Hsp90 molecular chaperone. Administration of these investigational drugs elicits induction of the heat shock response, or the overexpression of several Hsps, which exhibit antiapoptotic and pro-survival effects that may complicate the application of these inhibitors. To circumvent this issue, alternate mechanisms for Hsp90 inhibition that do not elicit the heat shock response have been identified and pursued. After providing background on the structure, function, and mechanism of the Hsp90 protein folding machinery, this review describes several mechanisms of Hsp90 modulation via small molecules that do not induce the heat shock response.

Detection of the ATPase activity of the molecular chaperones Hsp90 and Hsp72 using the TranscreenerTM ADP assay kit

The molecular chaperone heat shock protein 90 (Hsp90) is required for the correct folding and stability of a number of client proteins that are important for the growth and maintenance of cancer cells. Heat shock protein 72 (Hsp72), a co-chaperone of Hsp90, is also emerging as an attractive cancer drug target. Both proteins bind and hydrolyze adenosine triphosphate (ATP), and ATPase activity is essential for their function. Inhibition of Hsp90 ATPase activity leads to the degradation of client proteins, resulting in cell growth inhibition and apoptosis. Several small-molecule inhibitors of the ATPase activity of Hsp90 have been described and are currently being evaluated clinically for the treatment of cancer. A number of methods for the measurement of ATPase activity have been previously used, but not all of these are ideally suited to screening cascades in drug discovery projects. The authors have evaluated the use of commercial reagents (Transcreener ADP) for the measurement of ATPase activity of both yeast and human Hsp90 (ATP K(m) approximately 500 microM) and human Hsp72 (ATP K(m) ~1 microM). The low ATPase activity of human Hsp90 and its stimulation by the co-chaperone Aha1 was measured with ease using reduced incubation times, generating robust data (Z' = 0.75). The potency of several small-molecule inhibitors of both Hsp90 and Hsp72 was determined using the Transcreener reagents and compared well to that determined using other assay formats.

Phase I study of the heat shock protein 90 inhibitor alvespimycin (KOS-1022, 17-DMAG) administered intravenously twice weekly to patients with acute myeloid leukemia

Heat shock protein 90 (Hsp90) is a molecular chaperone with many oncogenic client proteins. The small-molecule Hsp90 inhibitor alvespimycin, a geldanamycin derivative, is being developed for various malignancies. This phase 1 study examined the maximum-tolerated dose (MTD), safety and pharmacokinetic/pharmacodynamic profiles of alvespimycin in patients with advanced acute myeloid leukemia (AML). Patients with advanced AML received escalating doses of intravenous alvespimycin (8-32 mg/m(2)), twice weekly, for 2 of 3 weeks. Dose-limiting toxicities (DLTs) were assessed during cycle 1. A total of 24 enrolled patients were evaluable for toxicity. Alvespimycin was well tolerated; the MTD was 24 mg/m(2) twice weekly. Common toxicities included neutropenic fever, fatigue, nausea and diarrhea. Cardiac DLTs occurred at 32 mg/m(2) (elevated troponin and myocardial infarction). Pharmacokinetics revealed linear increases in C(max) and area under the curve (AUC) from 8 to 32 mg/m(2) and minor accumulation upon repeated doses. Pharmacodynamic analyses on day 15 revealed increased apoptosis and Hsp70 levels when compared with baseline within marrow blasts. Antileukemia activity occurred in 3 of 17 evaluable patients (complete remission with incomplete blood count recovery). The twice-weekly administered alvespimycin was well tolerated in patients with advanced AML, showing linear pharmacokinetics, target inhibition and signs of clinical activity. We determined a recommended phase 2 dose of 24 mg/m(2).

Withaferin A targets heat shock protein 90 in pancreatic cancer cells

The purpose of this study is to investigate the efficacy and the mechanism of Hsp90 inhibition of Withaferin A (WA), a steroidal lactone occurring in Withania somnifera, in pancreatic cancer in vitro and in vivo. Withaferin A exhibited potent antiproliferative activity against pancreatic cancer cells in vitro (with IC(50)s of 1.24, 2.93 and 2.78 microM) in pancreatic cancer cell lines Panc-1, MiaPaCa2 and BxPc3, respectively. Annexin V staining showed that WA induced significant apoptosis in Panc-1 cells in a dose-dependent manner. Western blotting demonstrated that WA inhibited Hsp90 chaperone activity to induce degradation of Hsp90 client proteins (Akt, Cdk4 and glucocorticoid receptor), which was reversed by the proteasomal inhibitor, MG132. WA-biotin pull down assay of Hsp90 using Panc-1 cancer cell lysates and purified Hsp90 showed that WA-biotin binds to C-terminus of Hsp90 which was competitively blocked by unlabeled WA. Co-immunoprecipitation exhibited that WA (10 microM) disrupted Hsp90-Cdc37 complexes from 1 to 24h post-treatment, while it neither blocked ATP binding to Hsp90, nor changed Hsp90-P23 association. WA (3, 6mg/kg) inhibited tumor growth in pancreatic Panc-1 xenografts by 30% and 58%, respectively. These data demonstrate that Withaferin A binds Hsp90, inhibits Hsp90 chaperone activity through an ATP-independent mechanism, results in Hsp90 client protein degradation, and exhibits in vivo anticancer activity against pancreatic cancer.

A small molecule inhibitor of inducible heat shock protein 70

The multifunctional, stress-inducible molecular chaperone HSP70 has important roles in aiding protein folding and maintaining protein homeostasis. HSP70 expression is elevated in many cancers, contributing to tumor cell survival and resistance to therapy. We have determined that a small molecule called 2-phenylethynesulfonamide (PES) interacts selectively with HSP70 and leads to a disruption of the association between HSP70 and several of its cochaperones and substrate proteins. Treatment of cultured tumor cells with PES promotes cell death that is associated with protein aggregation, impaired autophagy, and inhibition of lysosomal function. Moreover, this small molecule is able to suppress tumor development and enhance survival in a mouse model of Myc-induced lymphomagenesis. The data demonstrate that PES disrupts actions of HSP70 in multiple cell signaling pathways, offering an opportunity to better understand the diverse functions of this molecular chaperone and also to aid in the development of new cancer therapies.

(-)-Epigallocatechin-3-gallate inhibits Hsp90 function by impairing Hsp90 association with cochaperones in pancreatic cancer cell line Mia Paca-2

(-)-Epigallocatechin-3-gallate [(-)-EGCG], the most abundant polyphenolic catechin in green tea, showed chemoprevention and anticancer activities. (-)-EGCG was reported to bind to the C-terminal domain of heat shock protein 90 (Hsp90). The purpose of this study is to investigate (-)-EGCG as a novel Hsp90 inhibitor to impair Hsp90 superchaperone complex for simultaneous downregulation of oncogenic proteins in pancreatic cancer cells. MTS assay showed that (-)-EGCG exhibited antiproliferative activity against pancreatic cancer cell line Mia Paca-2 in vitro with IC50 below 50 muM. (-)-EGCG increased caspase-3 activity up to 3-fold in a time- and concentration-dependent manner. Western blotting analysis demonstrated that (-)-EGCG induced downregulation of oncogenic Hsp90 client proteins by approximately 70-95%, including Akt, Cdk4, Raf-1, Her-2, and pERK. Co-immunoprecipitation showed that (-)-EGCG decreased the association of cochaperones p23 and Hsc70 with Hsp90 by more than 50%, while it had little effect on the ATP binding to Hsp90. Proteolytic fingerprinting assay confirmed direct binding between (-)-EGCG and the Hsp90 C-terminal domain. These data suggest that the binding of (-)-EGCG to Hsp90 impairs the association of Hsp90 with its cochaperones, thereby inducing degradation of Hsp90 client proteins, resulting antiproliferating effects in pancreatic cancer cells.

KU135, a novel novobiocin-derived C-terminal inhibitor of the 90-kDa heat shock protein, exerts potent antiproliferative effects in human leukemic cells

The 90-kDa heat shock protein (Hsp90) assists in the proper folding of numerous mutated or overexpressed signal transduction proteins that are involved in cancer. Consequently, there is considerable interest in developing chemotherapeutic drugs that specifically disrupt the function of Hsp90. Here, we investigated the extent to which a novel novobiocin-derived C-terminal Hsp90 inhibitor, designated KU135, induced antiproliferative effects in Jurkat T-lymphocytes. The results indicated that KU135 bound directly to Hsp90, caused the degradation of known Hsp90 client proteins, and induced more potent antiproliferative effects than the established N-terminal Hsp90 inhibitor 17-allylamino-demethoxygeldanamycin (17-AAG). Closer examination of the cellular response to KU135 and 17-AAG revealed that only 17-AAG induced a strong up-regulation of Hsp70 and Hsp90. In addition, KU135 caused wild-type cells to undergo G(2)/M arrest, whereas cells treated with 17-AAG accumulated in G(1). Furthermore, KU135 but not 17-AAG was found to be a potent inducer of mitochondria-mediated apoptosis as evidenced, in part, by the fact that cell death was inhibited to a similar extent by Bcl-2/Bcl-x(L) overexpression or the depletion of apoptotic protease-activating factor-1 (Apaf-1). Together, these data suggest that KU135 inhibits cell proliferation by regulating signaling pathways that are mechanistically different from those targeted by 17-AAG and as such represents a novel opportunity for Hsp90 inhibition.

Emerging drugs in the treatment of pancreatic cancer

BACKGROUND

Pancreatic cancer is the fourth leading cause of cancer-related death in the US. However, there is a growing belief that novel biological agents could improve survival of patients with this cancer. Gemcitabine-based chemotherapy remains the cornerstone treatment for advanced pancreatic cancers. So far, the current targeted agents that have been used in combination with gemcitabine have failed to improve clinical outcomes. This failure may stem from the heterogeneous molecular pathogenesis of pancreatic cancers, which involves several oncogenic pathways and defined genetic mutations.

OBJECTIVE

The aims of this review are: i) to define the existing treatments available at present for patients with pancreatic cancers in the neo-adjuvant, adjuvant, locally advanced and metastatic settings; ii) to highlight the molecular heterogeneity of the cancers and the rationale for targeting specific oncogenic pathways; iii) to give an overview of targeted agents that may potentially have an impact in the treatment of pancreatic cancers.

CONCLUSIONS

Molecular pathogenesis of pancreatic cancer involves several pathways and defined genetic mutations. Targeting these complex molecular pathways with a combination of novel biological and chemotherapeutic agents could potentially improve patient outcome.

Biological and chemical approaches to diseases of proteostasis deficiency

Many diseases appear to be caused by the misregulation of protein maintenance. Such diseases of protein homeostasis, or "proteostasis," include loss-of-function diseases (cystic fibrosis) and gain-of-toxic-function diseases (Alzheimer's, Parkinson's, and Huntington's disease). Proteostasis is maintained by the proteostasis network, which comprises pathways that control protein synthesis, folding, trafficking, aggregation, disaggregation, and degradation. The decreased ability of the proteostasis network to cope with inherited misfolding-prone proteins, aging, and/or metabolic/environmental stress appears to trigger or exacerbate proteostasis diseases. Herein, we review recent evidence supporting the principle that proteostasis is influenced both by an adjustable proteostasis network capacity and protein folding energetics, which together determine the balance between folding efficiency, misfolding, protein degradation, and aggregation. We review how small molecules can enhance proteostasis by binding to and stabilizing specific proteins (pharmacologic chaperones) or by increasing the proteostasis network capacity (proteostasis regulators). We propose that such therapeutic strategies, including combination therapies, represent a new approach for treating a range of diverse human maladies.

Identification of new Hsp90 inhibitors by structure-based virtual screening

Structure-based virtual screening identified pyrimidine-2,4,6-trione and 4H-1,2,4-triazole-3-thiol as novel scaffolds of Hsp90 ATPase inhibitors. Their binding modes in the ATP-binding pocket of Hsp90 were analyzed using AutoDoc program combined with molecular dynamics (MD) simulations.

Expression of heat shock proteins after ultrasound exposure in HL-60 cells

One of the important cellular defense mechanisms against stress is the induction of heat shock proteins (HSPs). We have recently demonstrated that a low frequency electromagnetic field is unable to induce the heat shock response (HSR). In the present study, we expanded our investigations to the induction of HSPs, particularly Hsp72, by ultrasound (US). Human promyelocytic leukemia HL-60 cells were exposed in suspension to US at 1, 3 and 10 MHz, as well as combinations of two of these frequencies. The ability of US to induce Hsp72 was tested for different frequencies, intensities and exposure times. In addition, the water bath temperature was varied from 30 to 36 degrees C. The Hsp72 protein expression was determined 4 and 24 h after treatment. We found that the amount of Hsp72 increased with increasing US frequency, reaching its highest level of about 1800%, induced by 10 MHz. After increasing the temperature of the water bath, the amount of Hsp72 in the treated cells was also increased, whereas no induction was observed at 30 degrees C. For all treatment conditions, ultrasound of 1 MHz was unable to significantly induce Hsp72. At 10 MHz, the exposure time was varied from 0 to 20 min. We found that the induction of Hsp72 took place after 5 min of exposure. For a fixed level of absorbed US energy, the continuous regime, as well as a pulsation of 1:2 (5 ms on and 5 ms off) induced the same Hsp72 level. Pulsation of 1:5 (2 ms on and 8 ms off) and 1:10 (1 ms on and 9 ms off) did not show any effect. A single sonication of 20 min, as well as a fractionated sonication of two 10 min exposures induced the same level of Hsp72, whereas four exposures of 5 min reduced the Hsp72 level. At the optimum exposure conditions (10 MHz, 10 min), the concentration of other HSPs was also determined. Hsp27 showed no effect but Hsp32, Hsp40 and Hsp72 were induced. Taken together, these results suggest a synergistic interaction between heat and US.

Co-ordinated stage-dependent enhancement of Plasmodium falciparum antioxidant enzymes and heat shock protein expression in parasites growing in oxidatively stressed or G6PD-deficient red blood cells

Background

Plasmodium falciparum-parasitized red blood cells (RBCs) are equipped with protective antioxidant enzymes and heat shock proteins (HSPs). The latter are only considered to protect against thermal stress. Important issues are poorly explored: first, it is insufficiently known how both systems are expressed in relation to the parasite developmental stage; secondly, it is unknown whether P. falciparum HSPs are redox-responsive, in view of redox sensitivity of HSP in eukaryotic cells; thirdly, it is poorly known how the antioxidant defense machinery would respond to increased oxidative stress or inhibited antioxidant defense. Those issues are interesting as several antimalarials increase the oxidative stress or block antioxidant defense in the parasitized RBC. In addition, numerous inhibitors of HSPs are currently developed for cancer therapy and might be tested as anti-malarials. Thus, the joint disruption of the parasite antioxidant enzymes/HSP system would interfere with parasite growth and open new perspectives for anti-malaria therapy.

Methods

Stage-dependent mRNA expression of ten representative P. falciparum antioxidant enzymes and hsp60/70–2/70–3/75/90 was studied by quantitative real-time RT-PCR in parasites growing in normal RBCs, in RBCs oxidatively-stressed by moderate H2O2 generation and in G6PD-deficient RBCs. Protein expression of antioxidant enzymes was assayed by Western blotting. The pentosephosphate-pathway flux was measured in isolated parasites after Sendai-virus lysis of RBC membrane.

Results

In parasites growing in normal RBCs, mRNA expression of antioxidant enzymes and HSPs displayed co-ordinated stage-dependent modulation, being low at ring, highest at early trophozoite and again very low at schizont stage. Additional exogenous oxidative stress or growth in antioxidant blunted G6PD-deficient RBCs indicated remarkable flexibility of both systems, manifested by enhanced, co-ordinated mRNA expression of antioxidant enzymes and HSPs. Protein expression of antioxidant enzymes was also increased in oxidatively-stressed trophozoites.

Conclusion

Results indicated that mRNA expression of parasite antioxidant enzymes and HSPs was co-ordinated and stage-dependent. Secondly, both systems were redox-responsive and showed remarkably increased and co-ordinated expression in oxidatively-stressed parasites and in parasites growing in antioxidant blunted G6PD-deficient RBCs. Lastly, as important anti-malarials either increase oxidant stress or impair antioxidant defense, results may encourage the inclusion of anti-HSP molecules in anti-malarial combined drugs.

New developments in Hsp90 inhibitors as anti-cancer therapeutics: mechanisms, clinical perspective and more potential

The molecular chaperone Hsp90 (heat shock protein 90) is a promising target in cancer therapy. Preclinical and clinical evaluations of a variety of Hsp90 inhibitors have shown anti-tumor effect as a single agent and in combination with chemotherapy. Current Hsp90 inhibitors are categorized into several classes based on distinct modes of inhibition, including (i) blockade of ATP binding, (ii) disruption of co-chaperone/Hsp90 interactions, (iii) antagonism of client/Hsp90 associations and (iv) interference with post-translational modifications of Hsp90. The different functions of Hsp90 isoforms and the isoform selectivity of drugs need further investigation. The correlation of cell surface Hsp90 with cancer metastasis and the emerging involvement of Hsp90 inhibition in cancer stem cells have become exciting areas that could be exploited. Therefore, the aim of this review is (1) to summarize the up-to-date knowledge of mechanistic studies and clinical prospect of currently available Hsp90 inhibitors, (2) to enhance our perspectives for designing and discovering novel Hsp90 inhibitors, and (3) to provide an insight into less-understood potential of Hsp90 inhibition in cancer therapy.

Heat shock protein 90 as a drug target: some like it hot

Heat shock protein 90 (HSP90) is a ubiquitously expressed chaperone that is involved in the posttranslational folding and stability of proteins. Inhibition at the NH(2)-terminal ATP-binding site leads to the degradation of client proteins by the ubiquitin proteasome pathway. Inhibition of HSP90 leads to the degradation of known oncogenes, such as ERB-B2, BRAF, and BCR-ABL, leading to the combinatorial blockade of multiple signal transduction pathways, such as the RAS-RAF-mitogen-activated protein/extracellular signal-regulated kinase kinase-extracellular signal-regulated kinase and phosphatidylinositol 3-kinase pathways. Multiple structurally diverse HSP90 inhibitors are undergoing early clinical evaluation. The clinical focus of these drugs should be solid tumors, such as breast, prostate, and lung cancers, along with malignant melanoma, in addition to hematologic malignancies, such as chronic myeloid leukemia and multiple myeloma. HSP90 inhibitors can be used as single agents or in combination with other targeted treatments or conventional forms of treatment such as chemotherapy and radiotherapy. Clinical trials evaluating efficacy of these agents should include innovative designs to capture cytostasis evidenced by clinical nonprogression and enrichment of patient populations by molecular characterization. The results of clinical trials evaluating the efficacy of drugs targeting this exciting target are awaited.

Targeting HSP90 for cancer therapy

Heat-shock proteins (HSPs) are molecular chaperones that regulate protein folding to ensure correct conformation and translocation and to avoid protein aggregation. Heat-shock proteins are increased in many solid tumours and haematological malignancies. Many oncogenic proteins responsible for the transformation of cells to cancerous forms are client proteins of HSP90. Targeting HSP90 with chemical inhibitors would degrade these oncogenic proteins, and thus serve as useful anticancer agents. This review provides an overview of the HSP chaperone machinery and the structure and function of HSP90. We also highlight the key oncogenic proteins that are regulated by HSP90 and describe how inhibition of HSP90 could alter the activity of multiple signalling proteins, receptors and transcriptional factors implicated in carcinogenesis.

Inhibition of heat shock induction of heat shock protein 70 and enhancement of heat shock protein 27 phosphorylation by quercetin derivatives

Inhibitors of heat-induced heat shock protein 70 (HSP70) expression have the potential to enhance the therapeutic effectiveness of heat-induced radiosensitization of tumors. Among known small molecule inhibitors, quercetin has the advantage of being easily modified for structure-activity studies. Herein, we report the ability of five monomethyl and five carbomethoxymethyl derivatives of quercetin to inhibit heat-induced HSP70 expression and enhance HSP27 phosphorylation in human cells. While quercetin and several derivatives inhibit HSP70 induction and enhance HSP27 phosphorylation at Ser78, other analogues selectively inhibit HSP70 induction without enhancing HSP27 phosphorylation that would otherwise aid in cell survival. We also show that good inhibitors of HSP70 induction are also good inhibitors of both CK2 and CamKII, kinases that are known to activate HSP70 expression by phosphorylation of heat shock transcription factor 1. Derivatives that show poor inhibition of either or both kinases are not good inhibitors of HSP70 induction, suggesting that quercetin's effectiveness is due to its ability to inhibit both kinases.

Transcription inhibition of heat shock proteins: a strategy for combination of 17-allylamino-17-demethoxygeldanamycin and actinomycin d

The heat shock protein (HSP) 90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) is currently in clinical trials because of its unique mechanism of action and antitumor activity. However, 17-AAG triggers the transcription and elevation of antiapoptotic HSP90, HSP70, and HSP27, which lead to chemoresistance in tumor cells. We hypothesized that inhibiting HSP90, HSP70, and HSP27 transcription may enhance 17-AAG-induced cell death in multiple myeloma cell lines. Actinomycin D (Act D), a clinically used agent and transcription inhibitor, was combined with 17-AAG. The concentrations for 17-AAG and Act D were selected based on the target actions and plasma levels during therapy. Inducible and constitutive HSP27, HSP70, and HSP90 mRNA and protein levels were measured by real-time reverse transcription-PCR and immunoblot assays. Compared with no treatment, Act D alone decreased HSP mRNA levels in MM.1S and RPMI-8226 cell lines. Combining Act D with 17-AAG did not attenuate 17-AAG-mediated increases in transcript levels of inducible HSP70; however, constitutive HSP mRNA levels were decreased. In contrast to its effect on mRNA levels, Act D was able to abrogate 17-AAG-mediated increases in all HSP protein levels. The cytotoxicity of combined Act D and 17-AAG was assessed. Treatment with Act D alone caused <40% cell death, whereas the combination of 17-AAG and Act D resulted in an increase of cell death in both multiple myeloma cell lines. In conclusion, these results indicate that 17-AAG-mediated induction of HSP70 and HSP27 expression can be attenuated by Act D and therefore can potentially improve the clinical treatment of multiple myeloma.

Emerging treatments for multiple myeloma: beyond immunomodulatory drugs and bortezomib

The successful clinical development of thalidomide, bortezomib, and lenalidomide not only transformed the therapeutic management of multiple myeloma (MM) but also catalyzed a renewed interest in the development of additional classes of novel agents for this disease. This review focuses on a series of new therapeutics that have shown promising preclinical results, as well as encouraging safety profiles and early evidence of anti-MM activity in clinical studies, either alone or in combination with other, conventional or novel, anti-MM treatments. These agents include second-generation proteasome inhibitors and immunomodulatory agents, as well as members of other therapeutic classes, such as histone deacetylase inhibitors (HDAC), heat shock protein 90 (Hsp90) inhibitors, and the alkylphospholipid Akt inhibitor perifosine.

An AlphaScreen-based high-throughput screen to identify inhibitors of Hsp90-cochaperone interaction

Hsp90 has emerged as an important anticancer drug target because of its essential role in promoting the folding and maturation of many oncogenic proteins. The authors describe the development of the first high-throughput screen, based on AlphaScreen technology, to identify a novel type of Hsp90 inhibitors that interrupt its interaction with the cochaperone HOP. The assay used the 20-mer C-terminal peptide of Hsp90 and the TPR2A domain of HOP. Assay specificity was demonstrated by measuring different interactions using synthetic peptides, with measured IC50s in good agreement with reported values. The assay was stable over 12 h and tolerated DMSO up to 5%. The authors first validated the assay by screening against 20,000 compounds in a 384-well format. After further optimization into a 1536-well format, it was screened against an NIH Chemical Genomics Center library of 76,134 compounds, with a signal-to-background ratio of 78 and Z' factor of 0.77. The present assay can be used for discovery of novel small-molecule Hsp90 inhibitors that can be used as chemical probes to investigate the role of cochaperones in Hsp90 function. Such molecules have the potential to be developed into novel anticancer drugs, for use alone or in combination with other Hsp90 inhibitors.