Discovery of 2-((4-resorcinolyl)-5-aryl-1,2,3-triazol-1-yl)acetates as potent Hsp90 inhibitors with selectivity over TRAP1

As the most abundant heat shock protein (HSP), Hsp90 is actively involved in tumor cell growth and various responses to anti-carcinogenic stress. Hsp90 has thus emerged as a potential drug target. A structure-based drug design approach was applied to develop novel resorcinolyltriazole derivatives as Hsp90 inhibitors. Structure-activity relationships (SARs) and molecular docking were investigated to provide a rationale for binding affinity and paralog selectivity. Click chemistry between iodoethynylresorcinol and an azido derivative was used to synthesize a new family of 2-((4-resorcinolyl)-5-aryl-1,2,3-triazol-1-yl) acetates that exhibited Hsp90 binding affinities of 40–100 nM (IC₅₀). Among the synthesized molecules, the triazole alkyl acetates displayed the highest Hsp90 binding affinities. Their potency against Hsp90 was over 100-fold stronger than against TRAP1 and 1–3-fold stronger than against Grp94. In particular, compounds 18, 19, and 30 had Hsp90 inhibitory activities of ~45 nM (IC₅₀) and they displayed over 350-fold selectivity for Hsp90 over TRAP1.     

Discovery of novel anti-breast cancer agents derived from deguelin as inhibitors of heat shock protein 90 (HSP90)

A series of O-substituted analogues of the B,C-ring truncated scaffold of deguelin were designed as C-terminal inhibitors of heat shock protein 90 (HSP90) and investigated as novel antiproliferative agents against HER2-positive breast cancer. Among the synthesized compounds, compound 80 exhibited significant inhibition in both trastuzumab-sensitive and trastuzumab-resistant breast cancer cells, whereas compound 80 did not show any cytotoxicity in normal cells. Compound 80 markedly downregulated the expression of the major client proteins of HSP90 in both cell types, indicating that the cytotoxicity of 80 in breast cancer cells is attributed to the destabilization and inactivation of HSP90 client proteins and that HSP90 inhibition represents a promising strategy to overcome trastuzumab resistance. A molecular docking study of 80 with the homology model of a HSP90 homodimer showed that 80 fit nicely in the C-terminal domain with a higher electrostatic complementary score than that of ATP.     

Quinazoline Based HSP90 Inhibitors: Synthesis,Modeling Study and ADME Calculations Towards Breast Cancer Targeting

A new 2-thioquinazolinones series was designed and synthesized as HSP90 inhibitors based on the structure of hit compound VII obtained by virtual screening approach. Their in vitro anti-proliferative activity was evaluated against three human cancer cell lines rich in HSP90 namely; colorectal carcinoma (HCT-116), and cervical carcinoma (Hela), breast carcinoma (MCF-7). Compounds 5a, 5d, 5e and 9h showed a significant broad spectrum anti-proliferative activity against all tested cell lines. They were characterized by potent effect against breast cancer in particular with IC₅₀ of 11.73, 8.56, 7.35 and 9.48 μM, respectively against Doxorubicin (IC₅₀ 4.17 μM). HSP90 ATPase activity inhibition assay were conducted where compound 5d exhibited the best IC₅₀ with 1.58 μM compared to Tanespimycin (IC₅₀ = 2.17 μM). Compounds 5a and 9h showed higher IC₅₀ values of 3.21 and 3.41 μM, respectively. The effects of 5a, 5d and 9h on Her2 (a client proteins of HSP90) and HSP70 were evaluated in MCF-7 cells. All tested compounds were found to reduce Her2 protein expression levels and induce Hsp70 protein expression levels significantly, emphasizing that antibreast cancer effect is a consequence of HSP90 chaperone inhibition. Cell cycle analysis of MCF-7 cells treated with 5d showed cell cycle arrest at G2/M phase 38.89% and pro-apoptotic activity as indicated by annexin V-FITC staining by 22.42%. Molecular docking studies suggested mode of interaction to HSP90 via hydrogen bonding. ADME properties prediction of the active compounds suggested that they could be used as orally absorbed anticancer drug candidates.     

Amide-tethered quinoline-resorcinol conjugates as a new class of HSP90 inhibitors suppressing the growth of prostate cancer cells

The study is focused on the design and synthesis of amide tethered quinoline-resorcinol hybrid constructs as a new class of HSP90 inhibitor. In-vitro studies of the synthetic compounds led to the identification of compound 11, which possesses potent cell growth inhibitory effects against HCT116, Hep3B and PC-3 cell lines, exerted through HSP90 inhibition. Compound 11 triggers degradation of HSP90 client proteins along with concomitant induction of HSP70, demonstrates apoptosis inducing ability and causes G2M phase cell cycle arrest in PC-3 cells. Molecular modeling was used to dock compound 11 into the HSP90 active site and key interactions with the amino acid residues of the HSP90 chaperone protein were determined.     

Radiosynthesis,biological evaluation and preliminary microPET study of 18F-labeled 5-resorcinolic triazolone derivative based on ganetespib targeting HSP90

Heat-shock protein 90 (HSP90) is a molecular chaperone that activates oncogenic transformation in several solid tumors, including lung and breast cancers. Ganetespib, a most promising candidate among several HSP90 inhibitors under clinical trials, has entered Phase III clinical trials for cancer therapy. Despite numerous evidences validating HSP90 as a target of anticancer, there are few studies on PET agents targeting oncogenic HSP90. In this study, we synthesized and biologically evaluated a novel ¹⁸F-labeled 5-resorcinolic triazolone derivative (1, [¹⁸F]PTP-Ganetespib) based on ganetespib. [¹⁸F]PTP-Ganetespib was labeled by click chemistry of Ganetespib-PEG-Alkyne (10) and [¹⁸F]PEG-N₃ (11) with 37.3?±?5.11% of radiochemical yield and 99.7?±?0.09% of radiochemical purity. [¹⁸F]PTP-Ganetespib showed proper LogP (0.96?±?0.06) and good stability in human serum over 97% for 2?h. [¹⁸F]PTP-Ganetespib showed high uptakes in breast cancer cells containing triple negative breast cancer (TNBC) MDA-MB-231 and Her2-negative MCF-7 cells, which are target breast cancer cell lines of HSP90 inhibitor, ganetespib, as an anticancer. Blocking of HSP90 by the pretreatment of ganetespib exhibited significantly decreased accumulation of [¹⁸F]PTP-Ganetespib in MDA-MB-231 and MCF-7 cells, indicating the specific binding of [¹⁸F]PTP-Ganetespib to MDA-MB-231 and MCF-7 cells with high HSP90 expression. In the biodistribution and microPET imaging studies, the initial uptake into tumor was weaker than in other thoracic and abdominal organs, but [¹⁸F]PTP-Ganetespib was retained relatively longer in the tumor than other organs. The uptake of [¹⁸F]PTP-Ganetespib in tumors was not sufficient for further development as a tumor-specific PET imaging agent by itself, but this preliminary PET imaging study of [¹⁸F]PTP-Ganetespib can be basis for developing new PET imaging agents based on HSP90 inhibitor, ganetespib.     

Native Size-Exclusion Chromatography–Based Mass Spectrometry Reveals New Components of the Early Heat Shock Protein 90 Inhibition Response Among Limited Global Changes

The molecular chaperone heat shock protein 90 (HSP90) works in concert with co-chaperones to stabilize its client proteins, which include multiple drivers of oncogenesis and malignant progression. Pharmacologic inhibitors of HSP90 have been observed to exert a wide range of effects on the proteome, including depletion of client proteins, induction of heat shock proteins, dissociation of co-chaperones from HSP90, disruption of client protein signaling networks, and recruitment of the protein ubiquitylation and degradation machinery—suggesting widespread remodeling of cellular protein complexes. However, proteomics studies to date have focused on inhibitor-induced changes in total protein levels, often overlooking protein complex alterations. Here, we use size-exclusion chromatography in combination with mass spectrometry (SEC-MS) to characterize the early changes in native protein complexes following treatment with the HSP90 inhibitor tanespimycin (17-AAG) for 8 h in the HT29 colon adenocarcinoma cell line. After confirming the signature cellular response to HSP90 inhibition (e.g., induction of heat shock proteins, decreased total levels of client proteins), we were surprised to find only modest perturbations to the global distribution of protein elution profiles in inhibitor-treated HT29 cells at this relatively early time-point. Similarly, co-chaperones that co-eluted with HSP90 displayed no clear difference between control and treated conditions. However, two distinct analysis strategies identified multiple inhibitor-induced changes, including known and unknown components of the HSP90-dependent proteome. We validate two of these—the actin-binding protein Anillin and the mitochondrial isocitrate dehydrogenase 3 complex—as novel HSP90 inhibitor-modulated proteins. We present this dataset as a resource for the HSP90, proteostasis, and cancer communities (https://www.bioinformatics.babraham.ac.uk/shiny/HSP90/SEC-MS/), laying the groundwork for future mechanistic and therapeutic studies related to HSP90 pharmacology. Data are available via ProteomeXchange with identifier PXD033459.     

Synthesis and evaluation of Hsp90 inhibitors that contain the 1,4-naphthoquinone scaffold

High-throughput screening of a library of diverse molecules has identified the 1,4-naphthoquinone scaffold as a new class of Hsp90 inhibitors. The synthesis and evaluation of a rationally-designed series of analogues containing the naphthoquinone core scaffold has provided key structure–activity relationships for these compounds. The most active inhibitors exhibited potent in vitro activity with low micromolar IC₅₀ values in anti-proliferation and Her2 degradation assays. In addition, 3g, 12, and 13a induced the degradation of oncogenic Hsp90 client proteins, a hallmark of Hsp90 inhibition. The identification of these naphthoquinones as Hsp90 inhibitors provides a new scaffold upon which improved Hsp90 inhibitors can be developed.     

Suberoylanilide hydroxamic acid induces ROS-mediated cleavage of HSP90 in leukemia cells

Heat shock protein 90 (HSP90) is a molecular chaperone that supports stability of client proteins. We found that HSP90 was cleaved to 55 kDa protein after treatment with histone deacetylase (HDAC) inhibitors including suberoylanilide hydroxamic acid (SAHA) in several leukemia cell lines. We further analyzed molecular changes induced by SAHA in K562 cells. The SAHA-induced cleavage of HSP90 was blocked by a pan-caspase inhibitor, z-VAD-fmk, implying that the process is dependent on caspase activity. However, the experiments using antagonistic and agonistic Fas antibodies revealed that the cleavage of HSP90 was not dependent on Fas signaling. SAHA induced generation of reactive oxygen species (ROS), and the cleavage of HSP90 was blocked by a ROS scavenger N-acetylcystein (NAC). We also confirmed that hydrogen peroxide (H₂O₂) induced cleavage of HSP90 in a similar manner. Caspase 2, 3, 4, 6, 8, and 10 were activated by treatment with SAHA, and the activities were reduced by the pretreatment of NAC. Treatment of the cells with caspase 10 inhihitor, but not other inhibitors of caspases activated by SAHA, prevented cleavage of HSP90 by SAHA. SAHA-induced ROS generation and HSP90 cleavage were dependent on newly synthesized unknown proteins. Taken together, our results suggest that the cleavage of HSP90 by SAHA is mediated by ROS generation and caspase 10 activation. HSP90 cleavage may provide an additional mechanism involved in anti-cancer effects of HDAC inhibitors.

Electronic supplementary material

The online version of this article (doi:10.1007/s12192-014-0533-4) contains supplementary material, which is available to authorized users.     

Recruitment of a Cytoplasmic Chaperone Relay by the A2A Adenosine Receptor

The adenosine A_2A receptor is a prototypical rhodopsin-like G protein-coupled receptor but has several unique structural features, in particular a long C terminus (of >120 residues) devoid of a palmitoylation site. It is known to interact with several accessory proteins other than those canonically involved in signaling. However, it is evident that many more proteins must interact with the A_2A receptor, if the trafficking trajectory of the receptor is taken into account from its site of synthesis in the endoplasmic reticulum (ER) to its disposal by the lysosome. Affinity-tagged versions of the A_2A receptor were expressed in HEK293 cells to identify interacting partners residing in the ER by a proteomics approach based on tandem affinity purification. The receptor-protein complexes were purified in quantities sufficient for analysis by mass spectrometry. We identified molecular chaperones (heat-shock proteins HSP90α and HSP70-1A) that interact with and retain partially folded A_2A receptor prior to ER exit. Complex formation between the A_2A receptor and HSP90α (but not HSP90β) and HSP70-1A was confirmed by co-affinity precipitation. HSP90 inhibitors also enhanced surface expression of the receptor in PC12 cells, which endogenously express the A_2A receptor. Finally, proteins of the HSP relay machinery (e.g. HOP/HSC70-HSP90 organizing protein and P23/HSP90 co-chaperone) were recovered in complexes with the A_2A receptor. These observations are consistent with the proposed chaperone/coat protein complex II exchange model. This posits that cytosolic HSP proteins are sequentially recruited to folding intermediates of the A_2A receptor. Release of HSP90 is required prior to recruitment of coat protein complex II components. This prevents premature ER export of partially folded receptors.     

Inhibitors of the heat shock protein 90 activity: a novel class of tumor radiosensitizers

The 90 kDa heat shock protein (HSP90) is one of major chaperones of eukaryotes which catalyzes maturation and activation of its client proteins. Among the identified client proteins there are oncogene products, hormone or growth factor receptors and key components of signaling pathways responsible for the malignant growth of tumors or their resistance to chemotherapy and radiotherapy. In the case of inhibition of the HSP90 chaperone function, such proteins are inactivated and degraded soon that leads to simultaneous blocking several pathways essential for proliferation and survival of malignant cells; therefore, pharmacological inhibitors of the HSP90 chaperone activity could be used in anticancer therapy. At present, several HSP90 inhibitors are in preclinical testing or I-III Phase clinical trials as mono-agents or in combinations with other anticancer drugs or radiation. In the present review, all the data are summarized which characterize HSP90 inhibitors as effective radiosensitizers of tumor cells. Molecular mechanisms and selectivity of the radiosensitizing action of HSP90 inhibitors are here discussed as well as a possibility of their application to improve the outcome of radiotherapy.     

Synthesis and biological evaluation of novobiocin analogues as potential heat shock protein 90 inhibitors

Recent studies have shown that novobiocin (NB), a member of the coumermycin (CA) family of antibiotics with demonstrated DNA gyrase inhibitory activity, inhibits Heat shock protein 90 (HSP90) by binding weakly to a putative ATP-binding site within its C-terminus. To develop more potent HSP90 inhibitors that target this site and to define structure–activity relationships (SARs) for this class of compounds, we have synthesized twenty seven 3-amido-7-noviosylcoumarin analogues starting from NB and CA. These were evaluated for evidence of HSP90 inhibition using several biological assays including inhibition of cell proliferation and cell cycle arrest, induction of the heat shock response, inhibition of luciferase-refolding in vitro, and depletion of the HSP90 client protein c-erbB-2/HER-2/neu (HER2). This SAR study revealed that a substantial increase in biological activity can be achieved by introduction of an indole-2-carboxamide group in place of 4-hydroxy-isopentylbenzamido group at C-3 of NB in addition to removal/derivatization of the 4-hydroxyl group from the coumarin ring. Methylation of the 4-hydroxyl group in the coumarin moiety moderately increased biological activity as shown by compounds 11 and 13. Our most potent new analogue 19 demonstrated biological activities consistent with known HSP90-binding agents, but with greater potency than NB.     

Systematic identification of the HSP90 candidate regulated proteome

HSP90 is a central player in the folding and maturation of many proteins. More than two hundred HSP90 clients have been identified by classical biochemical techniques including important signaling proteins with high relevance to human cancer pathways. HSP90 inhibition has thus become an attractive therapeutic concept and multiple molecules are currently in clinical trials. It is therefore of fundamental biological and medical importance to identify, ideally, all HSP90 clients and HSP90 regulated proteins. To this end, we have taken a global and a chemical proteomic approach in geldanamycin treated cancer cell lines using stable isotope labeling with amino acids in cell culture and quantitative mass spectrometry. We identified >6200 proteins in four different human cell lines and ~1600 proteins showed significant regulation upon drug treatment. Gene ontology and pathway/network analysis revealed common and cell-type specific regulatory effects with strong connections to unfolded protein binding and protein kinase activity. Of the 288 identified protein kinases, 98 were geldanamycin treatment including >50 kinases not formerly known to be regulated by HSP90. Protein turn-over measurements using pulsed stable isotope labeling with amino acids in cell culture showed that protein down-regulation by HSP90 inhibition correlates with protein half-life in many cases. Protein kinases show significantly shorter half lives than other proteins highlighting both challenges and opportunities for HSP90 inhibition in cancer therapy. The proteomic responses of the HSP90 drugs geldanamycin and PU-H71 were highly similar suggesting that both drugs work by similar molecular mechanisms. Using HSP90 immunoprecipitation, we validated several kinases (AXL, DDR1, TRIO) and other signaling proteins (BIRC6, ISG15, FLII), as novel clients of HSP90. Taken together, our study broadly defines the cellular proteome response to HSP90 inhibition and provides a rich resource for further investigation relevant for the treatment of cancer.     

Investigation of B,C-ring truncated deguelin derivatives as heat shock protein 90 (HSP90) inhibitors for use as anti-breast cancer agents

On the basis of deguelin, a series of the B,C-ring truncated surrogates with N-substituted amide linkers were investigated as HSP90 inhibitors. The structure activity relationship of the template was studied by incorporating various substitutions on the nitrogen of the amide linker and examining their HIF-1α inhibition. Among them, compound 57 showed potent HIF-1α inhibition and cytotoxicity in triple-negative breast cancer lines in a dose-dependent manner. Compound 57 downregulated expression and phosphorylation of major client proteins of HSP90 including AKT, ERK and STAT3, indicating that its antitumor activity was derived from the inhibition of HSP90 function. The molecular modeling of 57 demonstrated that 57 bound well to the C-terminal ATP-binding pocket in the open conformation of the hHSP90 homodimer with hydrogen bonding and pi-cation interactions. Overall, compound 57 is a potential antitumor agent for triple-negative breast cancer as a HSP90 C-terminal inhibitor.     

Combination of a Selective HSP90α/β Inhibitor and a RAS-RAF-MEK-ERK Signaling Pathway Inhibitor Triggers Synergistic Cytotoxicity in Multiple Myeloma Cells

Heat shock protein (HSP)90 inhibitors have shown significant anti-tumor activities in preclinical settings in both solid and hematological tumors. We previously reported that the novel, orally available HSP90α/β inhibitor TAS-116 shows significant anti-MM activities. In this study, we further examined the combination effect of TAS-116 with a RAS-RAF-MEK-ERK signaling pathway inhibitor in RAS- or BRAF-mutated MM cell lines. TAS-116 monotherapy significantly inhibited growth of RAS-mutated MM cell lines and was associated with decreased expression of downstream target proteins of the RAS-RAF-MEK-ERK signaling pathway. Moreover, TAS-116 showed synergistic growth inhibitory effects with the farnesyltransferase inhibitor tipifarnib, the BRAF inhibitor dabrafenib, and the MEK inhibitor selumetinib. Importantly, treatment with these inhibitors paradoxically enhanced p-C-Raf, p-MEK, and p-ERK activity, which was abrogated by TAS-116. TAS-116 also enhanced dabrafenib-induced MM cytotoxicity associated with mitochondrial damage-induced apoptosis, even in the BRAF-mutated U266 MM cell line. This enhanced apoptosis in RAS-mutated MM triggered by combination treatment was observed even in the presence of bone marrow stromal cells. Taken together, our results provide the rationale for novel combination treatment with HSP90α/β inhibitor and RAS-RAF-MEK-ERK signaling pathway inhibitors to improve outcomes in patients with in RAS- or BRAF-mutated MM.     

High-throughput screening assay for inhibitors of heat-shock protein 90 ATPase activity

The molecular chaperone heat-shock protein 90 (HSP90) plays a key role in the cell by stabilizing a number of client proteins, many of which are oncogenic. The intrinsic ATPase activity of HSP90 is essential to this activity. HSP90 is a new cancer drug target as inhibition results in simultaneous disruption of several key signaling pathways, leading to a combinatorial approach to the treatment of malignancy. Inhibitors of HSP90 ATPase activity including the benzoquinone ansamycins, geldanamycin and 17-allylamino-17-demethoxygeldanamycin, and radicicol have been described. A high-throughput screen has been developed to identify small-molecule inhibitors that could be developed as therapeutic agents with improved pharmacological properties. A colorimetric assay for inorganic phosphate, based on the formation of a phosphomolybdate complex and subsequent reaction with malachite green, was used to measure the ATPase activity of yeast HSP90. The Km for ATP determined in the assay was 510+/-70 microM. The known HSP90 inhibitors geldanamycin and radicicol gave IC(50) values of 4.8 and 0.9 microM respectively, which compare with values found using the conventional coupled-enzyme assay. The assay was robust and reproducible (2-8% CV) and used to screen a compound collection of approximately 56,000 compounds in 384-well format with Z' factors between 0.6 and 0.8.     

Synthesis and evaluation of 7-azaindole derivatives bearing benzocycloalkanone motifs as protein kinase inhibitors

Protein kinases are important drug targets, especially in the area of oncology. This paper reports the synthesis and biological evaluation of new 7-azaindole derivatives bearing benzocycloalkanone motifs as potential protein kinase inhibitors. Four compounds 8g, 8h, 8i, and 8l were discovered to inhibit cyclin-dependent kinase 9 (CDK9/CyclinT) and/or Haspin kinase in the micromolar to nanomolar range. 8l was identified as the most potent Haspin inhibitor (IC₅₀ = 14 nM), while 8g and 8h acted as dual inhibitors of CDK9/CyclinT and Haspin. These novel compounds constitute a promising starting point for the discovery of dual protein kinase inhibitors that have potential to be developed as anticancer agents, since both CDK9/CyclinT and Haspin are considered to be drug targets in oncology.