Transformation of the Non-Selective Aminocyclohexanol-Based Hsp90 Inhibitor into a Grp94-Seletive Scaffold

Pyrrole-containing hybrids as potential anticancer agents: An insight into current developments and structure-activity relationships

Cancer poses a significant threat to human health. Therefore, it is urgent to develop potent anti-cancer drugs with excellent inhibitory activity and no toxic side effects. Pyrrole and its derivatives are privileged heterocyclic compounds with significant diverse pharmacological effects. These compounds can target various aspects of cancer cells and have been applied in clinical settings or are undergoing clinical trials. As a result, pyrrole has emerged as a promising drug scaffold and has been further probed to get novel entities for the treatment of cancer. This article reviews recent research progress on anti-cancer drugs containing pyrrole. It focuses on the mechanism of action, biological activity, and structure-activity relationships of pyrrole derivatives, aiming to assist in designing and synthesizing innovative pyrrole-based anti-cancer compounds.

FRET Assays for the Identification of C. albicans HSP90-Sba1 and Human HSP90α-p23 Binding Inhibitors

Heat shock protein 90 (HSP90) is a critical target for anticancer and anti-fungal-infection therapies due to its central role as a molecular chaperone involved in protein folding and activation. In this study, we developed in vitro Förster Resonance Energy Transfer (FRET) assays to characterize the binding of C. albicans HSP90 to its co-chaperone Sba1, as well as that of the homologous human HSP90α to p23. The assay for human HSP90α binding to p23 enables selectivity assessment for compounds aimed to inhibit the binding of C. albicans HSP90 to Sba1 without affecting the physiological activity of human HSP90α. The combination of the two assays is important for antifungal drug development, while the assay for human HSP90α can potentially be used on its own for anticancer drug discovery. Since ATP binding of HSP90 is a prerequisite for HSP90-Sba1/p23 binding, ATP-competitive inhibitors can be identified with the assays. The specificity of binding of fusion protein constructs-HSP90-mNeonGreen (donor) and Sba1-mScarlet-I (acceptor)-to each other in our assay was confirmed via competitive inhibition by both non-labeled Sba1 and known ATP-competitive inhibitors. We utilized the developed assays to characterize the stability of both HSP90-Sba1 and HSP90α-p23 affinity complexes quantitatively. Kd values were determined and assessed for their precision and accuracy using the 95.5% confidence level. For HSP90-Sba1, the precision confidence interval (PCI) was found to be 70-120 (100 ± 20) nM while the accuracy confidence interval (ACI) was 100-130 nM. For HSP90α-p23, PCI was 180-260 (220 ± 40) nM and ACI was 200-270 nM. The developed assays were used to screen a nucleoside-mimetics library of 320 compounds for inhibitory activity against both C. albicans HSP90-Sba1 and human HSP90α-p23 binding. No novel active compounds were identified. Overall, the developed assays exhibited low data variability and robust signal separation, achieving Z factors > 0.5.

Enniatin A Analogues as Novel Hsp90 Inhibitors that Modulate Triple-Negative Breast Cancer

The 90 kilo-Dalton heat shock protein (Hsp90) is a molecular chaperone that facilitates the maturation of nascent polypeptides into their biologically active conformation. Because many of the >400 known client protein substrates are implicated in the development/progression of cancer, it is hypothesized that Hsp90 inhibition will simultaneously shut down numerous oncogenic pathways. Unfortunately, most of the small molecule Hsp90 inhibitors that have undergone clinical evaluation thus far have failed due to various toxicities. Therefore, the disruption of Hsp90 protein-protein interactions with cochaperones and/or client substrates has been proposed as an alternative way to achieve Hsp90 inhibition without such adverse events. The hexadepsipeptide Enniatin A (EnnA) has recently been reported to be one such inhibitor that also manifests immunogenic activity. Herein, we report preliminary structure-activity relationship (SAR) studies to determine the structural features that confer this unprecedented activity for an Hsp90 inhibitor. Our studies find that EnnA's branching moieties are necessary for its activity, but some structural modifications are tolerated.

Following the design path of isoform-selective Hsp90 inhibitors: Small differences, great opportunities

The heat shock protein 90 (Hsp90) family consists of four highly conserved isoforms: the mitochondrial TRAP-1, the endoplasmic reticulum-localised Grp94, and the cytoplasmic Hsp90α and Hsp90β. Since the late 1990s, this family has been extensively studied as a potential target for the treatment of cancer, neurological disorders, and infectious diseases. The initial approach was to develop non-selective, so-called pan-Hsp90 ATP-competitive inhibitors of the N-terminal domain. Many of these agents were tested in clinical trials, mainly for the treatment of cancer, but none of them succeeded in the clinic. This was mainly due to the lack of efficacy and various toxicities associated with the induction of heat shock response (HSR). This lack of success has prompted a turn to new approaches of Hsp90 inhibition. Thus, inhibitors selective for a particular isoform of Hsp90 have been developed. These isoform-selective inhibitors do not induce HSR and have a more targeted effect because not all client proteins are equally dependent on all four paralogues of Hsp90. However, it is extremely difficult to develop such selective compounds because the family is highly conserved. Hsp90α and Hsp90β have an amazing 95% identity of the N-terminal ATP binding site, differing only in two amino acid residues. Therefore, the focus of this review is to fully elucidate the key structural features of the selective inhibitor classes in terms of binding site dissimilarities. In addition to a methodological characterisation of the structure-activity relationships, the main advantages of selective inhibition of the TRAP-1, Grp94, Hsp90α and Hsp90β isoforms are discussed.

Radiosynthesis and preclinical evaluation of [11C]SNX-ab as an Hsp90α,β isoform-selective PET probe for in vivo brain and tumour imaging

BACKGROUND

The molecular chaperone, Hsp90, is a key player in the protein quality control system that maintains homeostasis under cellular stress conditions. It is a homodimer with ATP-dependent activity, and is a prominent member of the chaperone machinery that stabilizes, matures and (re)folds an extensive list of client proteins. Hsp90 occurs as four isoforms, cytosolic Hsp90α and Hsp90β, mitochondrial TRAP1 and Grp94 present in the endoplasmic reticulum. An aberrant role of Hsp90 has been attributed to several cancers and neurodegenerative disorders. Consequently, Hsp90 has emerged as an attractive therapeutic target. However, pan-Hsp90 inhibition often leads to detrimental dose-limiting toxicities. Novel strategies for Hsp90-targeted therapy intend to avoid this by using isoform-specific Hsp90 inhibition. In this respect, the radiosynthesis of carbon-11 labeled SNX-ab was developed and [¹¹C]SNX-ab was evaluated as a Hsp90α,β isoform-selective PET probe, which could potentially allow to quantify in vivo Hsp90α,β expression.

RESULTS

[¹¹C]SNX-ab was synthesized with excellent radiochemical yields of 45% and high radiochemical purity (> 98%). In vitro autoradiography studies on tissue slices of healthy mouse brain, mouse B16.F10 melanoma and U87 glioblastoma using homologous (SNX-ab, SNX-0723) and heterologous (Onalespib and PU-H71) Hsp90 inhibitors demonstrated only limited reduction of tracer binding, indicating that the binding of [¹¹C]SNX-ab was not fully Hsp90-specific. Similarly, [¹¹C]SNX-ab binding to U87 cells was not efficiently inhibited by Hsp90 inhibitors. Ex vivo biodistribution studies in healthy mice revealed limited brain exposure of [¹¹C]SNX-ab and predominantly hepatobiliary clearance, which was confirmed by in vivo full-body dynamic µPET studies.

CONCLUSION

Our results suggest that [¹¹C]SNX-ab is not an ideal probe for in vivo visualization and quantification of Hsp90α/β expression levels in tumour and brain. Future research in the development of next-generation Hsp90 isoform-selective PET tracers is warranted to dissect the role played by each isoform towards disease pathology and support the development of subtype-specific Hsp90 therapeutics.

Substituent-Dependent Divergent Synthesis of 2-(3-Amino-2,4-dicyanophenyl)pyrroles, Pyrrolyldienols and 3-Amino-1-acylethylidene-2-cyanopyrrolizines via Reaction of Acylethynylpyrroles with Malononitrile

An efficient method for the synthesis of pharmaceutically and high-tech prospective 2-(3-amino-2,4-dicyanophenyl)pyrroles (in up to 88% yield) via the reaction of easily available substituted acylethynylpyrroles with malononitrile has been developed. The reaction proceeds in the KOH/MeCN system at 0 °C for 2 h. In the case of 2-acylethynylpyrroles without substituents in the pyrrole ring, the reaction changes direction: instead of the target 2-(3-amino-2,4-dicyanophenyl)pyrroles, the unexpected formation of pyrrolyldienols and products of their intramolecular cyclization, 3-amino-1-acylethylidene-2-cyanopyrrolizines, is observed.

The biology and inhibition of glucose-regulated protein 94/gp96

The 94 kDa molecular chaperone, glucose-regulated protein 94 (Grp94), has garnered interest during the last decade due to its direct association with endoplasmic reticulum (ER) stress and disease. Grp94 belongs to the Hsp90 family of molecular chaperones and is a master regulator of ER homeostasis due to its ability to fold and stabilize proteins/receptors, and to chaperone misfolded proteins for degradation. Multiple studies have demonstrated that Grp94 knockdown or inhibition leads to the degradation of client protein substrates, which leads to disruption of disease-dependent signaling pathways. As a result, small molecule inhibitors of Grp94 have become a promising therapeutic approach to target a variety of disease states. Specifically, Grp94 has proven to be a promising target for cancer, glaucoma, immune-mediated inflammation, and viral infection. Moreover, Grp94-peptide complexes have been utilized effectively as adjuvants for vaccines against a variety of disease states. This work highlights the significance of Grp94 biology and the development of therapeutics that target this molecular chaperone in multiple disease states.

Saucerneol attenuates nasopharyngeal carcinoma cells proliferation and metastasis through selectively targeting Grp94

BACKGROUND

Nasopharyngeal carcinoma (NPC) is highly prevalent in southern China. The remote metastasis of advanced NPC requires chemotherapeutic treatments to reduce the mortality. Our previous work revealed that saucerneol (SN) showed cytotoxicity against several nasopharyngeal carcinoma (NPC) cells. This work aims to investigate the effect of SN in NPC growth and exploring the mechanism of action.

STUDY DESIGN

Applying in vivo study, in vitro study and in silico study to indicate the mechanism of SN in inhibiting NPC growth.

METHODS

Saucerneol (SN) toxicity was measured with MTT assay. NPC proliferation was measured with EdU and colony formation assays, cell cycle was detected with flow cytometry. NPC migration and invasion were measured with scratch assay and matrigel transwell method. Further, human NPC xenograft tumor models were established in nude mice to evaluate the therapeutic efficacy of SN in vivo. Toxicological analysis was performed on H&E staining and IHC. Quantitative real-time PCR and Western blot analyses were used to evaluate the expression levels of key molecules in PI3K/AKT/mTOR, MAPK, NF-κB, and HIF-1α signal pathways. Target predicting was conducted using computational method, and target identification was carried out by ATPase assay and TSA.

RESULTS

SN, a potent NPC inhibitor that was previously isolated from Saururus chinensis in our lab, is proven to inhibit the proliferation and metastasis of HONE1 cell lines and inhibit the growth of human NPC xenografts in nude mice. Moreover, we further articulate the molecular mechanism of action for SN and, reveal that SN promotes the expression of cell cycle-dependent kinase inhibitory protein p21 Waf1/Cip1 through targeting Grp94 and then inhibiting PI3K/AKT signaling pathway as well as up-regulating p53 to disrupt the progression of HONE1 cells.

CONCLUSION

SN significantly inhibits NPC cells proliferation and metastasis in vitro and in vivo via selectively inhibit Grp94 and then blocking PI3K/AKT/mTOR/HIF-1α signaling pathway. This study firstly provides a novel selective Grp94 inhibitor as a NPC candidate.

Inhibitors of the Plasmodium falciparum Hsp90 towards Selective Antimalarial Drug Design: The Past, Present and Future

Malaria is still one of the major killer parasitic diseases in tropical settings, posing a public health threat. The development of antimalarial drug resistance is reversing the gains made in attempts to control the disease. The parasite leads a complex life cycle that has adapted to outwit almost all known antimalarial drugs to date, including the first line of treatment, artesunate. There is a high unmet need to develop new strategies and identify novel therapeutics to reverse antimalarial drug resistance development. Among the strategies, here we focus and discuss the merits of the development of antimalarials targeting the Heat shock protein 90 (Hsp90) due to the central role it plays in protein quality control.

HSP90 inhibitors reduce cholesterol storage in Niemann-Pick type C1 mutant fibroblasts

Niemann-Pick type C1 (NPC1) disease is a lysosomal lipid storage disorder caused by mutations of the NPC1 gene. More than 300 disease-associated mutations are reported in patients, resulting in abnormal accumulation of unesterified cholesterol, glycosphingolipids, and other lipids in late endosomes and lysosomes (LE/Ly) of many cell types. Previously, we showed that treatment of many different NPC1 mutant fibroblasts with histone deacetylase inhibitors resulted in reduction of cholesterol storage, and we found that this was associated with enhanced exit of the NPC1 protein from the endoplasmic reticulum and delivery to LE/Ly. This suggested that histone deacetylase inhibitors may work through changes in protein chaperones to enhance the folding of NPC1 mutants, allowing them to be delivered to LE/Ly. In this study, we evaluated the effect of several HSP90 inhibitors on NPC1I1061T skin fibroblasts. We found that HSP90 inhibition resulted in clearance of cholesterol from LE/Ly, and this was associated with enhanced delivery of the mutant NPC1I1061T protein to LE/Ly. We also observed that inhibition of HSP90 increased the expression of HSP70, and overexpression of HSP70 also reduced cholesterol storage in NPC1I1061T fibroblasts. However, we did not see correction of cholesterol storage by arimoclomol, a drug that is reported to increase HSP70 expression, at doses up to 0.5 mM. The increase in other chaperones as a consequence of HSP90 improves folding of NPC1 protein and relieves cholesterol accumulation in NPC1 mutant fibroblasts.

Selective Inhibition of the Hsp90α Isoform

The 90 kDa heat shock protein (Hsp90) is a molecular chaperone that processes nascent polypeptides into their biologically active conformations. Many of these proteins contribute to the progression of cancer, and consequently, inhibition of the Hsp90 protein folding machinery represents an innovative approach toward cancer chemotherapy. However, clinical trials with Hsp90 N-terminal inhibitors have encountered deleterious side effects and toxicities, which appear to result from the pan-inhibition of all four Hsp90 isoforms. Therefore, the development of isoform-selective Hsp90 inhibitors is sought to delineate the pathological role played by each isoform. Herein, we describe a structure-based approach that was used to design the first Hsp90α-selective inhibitors, which exhibit >50-fold selectivity versus other Hsp90 isoforms.

Molecular Chaperone GRP94/GP96 in Cancers: Oncogenesis and Therapeutic Target

During tumor development and progression, intrinsic and extrinsic factors trigger endoplasmic reticulum (ER) stress and the unfolded protein response, resulting in the increased expression of molecular chaperones to cope with the stress and maintain tumor cell survival. Heat shock protein (HSP) GRP94, also known as GP96, is an ER paralog of HSP90 and has been shown to promote survival signaling during tumor-induced stress and modulate the immune response through its multiple clients, including TLRs, integrins, LRP6, GARP, IGF, and HER2. Clinically, elevated expression of GRP94 correlates with an aggressive phenotype and poor clinical outcome in a variety of cancers. Thus, GRP94 is a potential molecular marker and therapeutic target in malignancies. In this review, we will undergo deep molecular profiling of GRP94 in tumor development and summarize the individual roles of GRP94 in common cancers, including breast cancer, colon cancer, lung cancer, liver cancer, multiple myeloma, and others. Finally, we will briefly review the therapeutic potential of selectively targeting GRP94 for the treatment of cancers.

The Development of Hsp90β-Selective Inhibitors to Overcome Detriments Associated with pan-Hsp90 Inhibition

The 90 kD heat shock proteins (Hsp90) are molecular chaperones that are responsible for the folding of select proteins, many of which are directly associated with cancer progression. Consequently, inhibition of the Hsp90 protein folding machinery results in a combinatorial attack on numerous oncogenic pathways. Seventeen small-molecule inhibitors of Hsp90 have entered clinical trials for the treatment of cancer, all of which bind the Hsp90 N-terminus and exhibit pan-inhibitory activity against all four Hsp90 isoforms, which may lead to adverse effects. The development of Hsp90 isoform-selective inhibitors represents an alternative approach toward the treatment of cancer and may limit some of these detriments. Described herein, is a structure-based approach to develop isoform-selective inhibitors of Hsp90β, which induces the degradation of select Hsp90 clients without concomitant induction of Hsp90 levels. Together, these initial studies support the development of Hsp90β-selective inhibitors as a method for overcoming the detriments associated with pan-inhibition.

Modulation of protein fate decision by small molecules: targeting molecular chaperone machinery

Modulation of protein fate decision and protein homeostasis plays a significant role in altering the protein level, which acts as an orientation to develop drugs with new mechanisms. The molecular chaperones exert significant biological functions on modulation of protein fate decision and protein homeostasis under constantly changing environmental conditions through extensive protein–protein interactions (PPIs) with their client proteins. With the help of molecular chaperone machinery, the processes of protein folding, trafficking, quality control and degradation of client proteins could be arranged properly. The core members of molecular chaperones, including heat shock proteins (HSPs) family and their co-chaperones, are emerging as potential drug targets since they are involved in numerous disease conditions. Development of small molecule modulators targeting not only chaperones themselves but also the PPIs among chaperones, co-chaperones and clients is attracting more and more attention. These modulators are widely used as chemical tools to study chaperone networks as well as potential drug candidates for a broader set of diseases. Here, we reviewed the key checkpoints of molecular chaperone machinery HSPs as well as their co-chaperones to discuss the small molecules targeting on them for modulation of protein fate decision.

Ni complexes of an alane/tris(phosphine) ligand built around a strongly Lewis acidic tris(N-pyrrolyl)aluminum

Syntheses of a new tripodal alane/tris(phosphine) ligand (AlP₃) based on 2-(diisopropylphosphino)pyrrole, and AlP₃-supported Ni complexes are reported.

The Alpha Isoform of Heat Shock Protein 90 and the Co-chaperones p23 and Cdc37 Promote Opioid Anti-nociception in the Brain

Opioid activation of the mu opioid receptor (MOR) promotes signaling cascades that evoke both analgesic responses to pain and side effects like addiction and dependence. Manipulation of these cascades, such as by biased agonism, has great promise to improve opioid therapy. However, the signaling cascades of the MOR are in general poorly understood, providing few targets for drug development. In our earlier work, we identified Heat shock protein 90 (Hsp90) as a novel and crucial regulator of opioid anti-nociception in the brain by promoting ERK MAPK activation. In this study, we sought to identify the molecular isoforms and co-chaperones by which Hsp90 carried out this role, which could provide specific targets for future clinical intervention. We used novel selective small molecule inhibitors as well as CRISPR/Cas9 gene editing constructs delivered by the intracerebroventricular (icv) route to the brains of adult CD-1 mice to target Hsp90 isoforms (Hsp90α/β, Grp94) and co-chaperones (p23, Cdc37, Aha1). We found that inhibition of the isoform Hsp90α fully blocked morphine anti-nociception in a model of post-surgical paw incision pain, while blocking ERK and JNK MAPK activation, suggesting Hsp90α as the main regulator of opioid response in the brain. We further found that inhibition of the co-chaperones p23 and Cdc37 blocked morphine anti-nociception, suggesting that these co-chaperones assist Hsp90α in promoting opioid anti-nociception. Lastly, we used cycloheximide treatment in the brain to demonstrate that rapid protein translation within 30 min of opioid treatment is required for Hsp90 regulation of opioid response. Together these studies provide insight into the molecular mechanisms by which Hsp90 promotes opioid anti-nociception. These findings thus both improve our basic science knowledge of MOR signal transduction and could provide future targets for clinical intervention to improve opioid therapy.

The role of heat shock protein 90 in regulating pain, opioid signaling, and opioid antinociception

Heat shock protein 90 (Hsp90) is one of the central signal transduction regulators of the cell. Via client interactions with hundreds of proteins, including receptors, receptor regulatory kinases, and downstream signaling regulators, Hsp90 has a crucial and wide-ranging impact on signaling in response to numerous drugs with impacts on resultant physiology and behavior. Despite this importance, however, Hsp90 has barely been studied in the context of pain and the opioid receptor system, leaving open the possibility that Hsp90 could be manipulated to improve pain therapeutic outcomes, a current area of massive medical need. In this review, we will highlight the known roles of Hsp90 in directly regulating the initiation and maintenance of the pain state. We will also explore how Hsp90 regulates signaling and antinociceptive responses to opioid analgesic drugs, with a special emphasis on ERK MAPK signaling. Understanding this new and growing area will improve our understanding of how Hsp90 regulates signaling and physiology, and also may provide new ways to treat pain, and perhaps reduce the severe impact of the ongoing opioid addiction and overdose crisis.

The Role of Heat Shock Proteins in Regulating Receptor Signal Transduction

Heat shock proteins (Hsp) are a class of stress-inducible proteins that mainly act as molecular protein chaperones. This chaperone activity is diverse, including assisting in nascent protein folding and regulating client protein location and translocation within the cell. The main proteins within the Hsp family, particularly Hsp70 and Hsp90, also have a highly diverse and numerous set of protein clients, which when combined with the high expression levels of Hsp proteins (2%-6% of total protein content) establishes these molecules as "central regulators" of cell protein physiology. Among the client proteins, Hsps regulate numerous signal-transduction and receptor-regulatory kinases, and indeed directly regulate some receptors themselves. This also makes the Hsps, particularly Hsp90, central regulators of signal-transduction machinery, with important impacts on endogenous and drug ligand responses. Among these roles, Hsp90 in particular acts to maintain mature signaling kinases in a metastable conformation permissive for signaling activation. In this review, we will focus on the roles of the Hsps, with a special focus on Hsp90, in regulating receptor signaling and subsequent physiologic responses. We will also explore potential means to manipulate Hsp function to improve receptor-targeted therapies. Overall, Hsps are important regulators of receptor signaling that are receiving increasing interest and exploration, particularly as Hsp90 inhibitors progress toward clinical approval for the treatment of cancer. Understanding the complex interplay of Hsp regulation of receptor signaling may provide important avenues to improve patient treatment.

Physiological function of myocilin and its role in the pathogenesis of glaucoma in the trabecular meshwork (Review)

Myocilin is highly expressed in the trabecular meshwork (TM), which plays an important role in the regulation of intraocular pressure (IOP). Myocilin abnormalities may cause dysfunction of the TM, potentially leading to increased IOP. High IOP is a well‑known primary risk factor for glaucoma. Myocilin mutations are common among glaucoma patients, and they are implicated in juvenile‑onset open‑angle glaucoma (JOAG) and adult‑onset primary open‑angle glaucoma (POAG). Aggregation of aberrant mutant myocilins is closely associated with glaucoma pathogenesis. The aim of the present review was to discuss the recent findings regarding the major physiological functions of myocilin, such as intra‑ and extracellular proteolytic processes. We also aimed to discuss the risk factors associated with myocilin and the development of glaucoma, such as misfolded/mutant myocilin, imbalance of myocilin and extracellular proteins, and instability of mutant myocilin associated with temperature. Finally, we further outlined certain issues that are yet to be resolved, which may represent the basis for future studies on the role of myocilin in glaucoma.

Discovery of a Potent Grp94 Selective Inhibitor with Anti-Inflammatory Efficacy in a Mouse Model of Ulcerative Colitis

As the endoplasmic reticulum paralogue of Hsp90, Grp94 chaperones a small set of client proteins associated with some diseases, including cancer, primary open-angle glaucoma, and inflammatory disorders. Grp94-selective inhibition has been a potential therapeutic strategy for these diseases. In this study, inspired by the conclusion that ligand-induced "Phe199 shift" effect is the structural basis of Grp94-selective inhibition, a series of novel Grp94 selective inhibitors incorporating "benzamide" moiety were developed, among which compound 54 manifested the most potent Grp94 inhibitory activity with an IC₅₀ value of 2 nM and over 1000-fold selectivity to Grp94 against Hsp90α. In a DSS-induced mouse model of ulcerative colitis (UC), compound 54 exhibited significant anti-inflammatory efficacy. This work provides a potent Grp94 selective inhibitor as probe compound for the biological study of Grp94 and represents the first study that confirms the potential therapeutic efficacy of Grp94-selective inhibitors against UC.

An update on the discovery and development of selective heat shock protein inhibitors as anti-cancer therapy

INTRODUCTION

Over the years, not a single HSP inhibitor has progressed into the post-market phase of drug development despite the success recorded in various pre-clinical and clinical studies. The inability of existing drugs to specifically target oncogenic HSPs has majorly accounted for these setbacks. Recent combinatorial strategies that incorporated computer-aided drug design (CADD) techniques are geared towards the development of highly specific HSP inhibitors with increased activities and minimal toxicities. Areas covered: In this review, strategic therapeutic approaches that have recently aided the development of selective HSP inhibitors were highlighted. Also, the significant contributions of CADD techniques over the years were discussed in detail. This article further describes promising computational paradigms and their applications towards the discovery of highly specific inhibitors of oncogenic HSPs. Expert opinion: The recent shift towards highly selective and specific HSP inhibition has shown great promise as evidenced by the development of paralog/isoform-selective HSP drugs. It could be further augmented with computer-aided drug design strategies, which incorporate reliable methods that would greatly enhance the design and optimization of novel inhibitors with improved activities and minimal toxicities.

Structure Based Design of a Grp94-Selective Inhibitor: Exploiting a Key Residue in Grp94 To Optimize Paralog-Selective Binding

Grp94 and Hsp90, the ER and cytoplasmic hsp90 paralogs, share a conserved ATP-binding pocket that has been targeted for therapeutics. Paralog-selective inhibitors may lead to drugs with fewer side effects. Here, we analyzed 1 (BnIm), a benzyl imidazole resorcinylic inhibitor, for its mode of binding. The structures of 1 bound to Hsp90 and Grp94 reveal large conformational changes in Grp94 but not Hsp90 that expose site 2, a binding pocket adjacent to the central ATP cavity that is ordinarily blocked. The Grp94:1 structure reveals a flipped pose of the resorcinylic scaffold that inserts into the exposed site 2. We exploited this flipped binding pose to develop a Grp94-selective derivative of 1. Our structural analysis shows that the ability of the ligand to insert its benzyl imidazole substituent into site 1, a different side pocket off the ATP binding cavity, is the key to exposing site 2 in Grp94.

Isoform-selective Hsp90 inhibition rescues model of hereditary open-angle glaucoma

The heat shock protein 90 (Hsp90) family of molecular chaperones regulates protein homeostasis, folding, and degradation. The ER-resident Hsp90 isoform, glucose-regulated protein 94 (Grp94), promotes the aggregation of mutant forms of myocilin, a protein associated with primary open-angle glaucoma. While inhibition of Grp94 promotes the degradation of mutant myocilin in vitro, to date no Grp94-selective inhibitors have been investigated in vivo. Here, a Grp94-selective inhibitor facilitated mutant myocilin degradation and rescued phenotypes in a transgenic mouse model of hereditary primary open-angle glaucoma. Ocular toxicities previously associated with pan-Hsp90 inhibitors were not evident with our Grp94-selective inhibitor, 4-Br-BnIm. Our study suggests that selective inhibition of a distinct Hsp90 family member holds translational promise for ocular and other diseases associated with cell stress and protein misfolding.

Synthesis and Biological Evaluation of Stilbene Analogues as Hsp90 C-Terminal Inhibitors

The design, synthesis, and biological evaluation of stilbene-based novobiocin analogues is reported. Replacement of the biaryl amide side chain with a triazole side chain produced compounds that exhibited good antiproliferative activities. Heat shock protein 90 (Hsp90) inhibition was observed when N-methylpiperidine was replaced with acyclic tertiary amines on the stilbene analogues that also contain a triazole-derived side chain. These studies revealed that ≈24 Å is the optimal length for compounds that exhibit good antiproliferative activity as a result of Hsp90 inhibition.

Resorcinol-Based Grp94-Selective Inhibitors

Glucose regulated protein 94 (Grp94) is the endoplasmic reticulum resident of the 90 kDa heat shock protein (Hsp90) family and represents a promising therapeutic target for the treatment of several diseases. Grp94 is the most unique member of the 90 kDa heat shock protein family due to a five amino acid insertion into its primary sequence, which creates hydrophobic subpockets exclusive to Grp94 that can be utilized for selective inhibition. The first resorcinol-based Grp94-selective inhibitor to take advantage of the hydrophobic S2 subpocket has been developed and shown to manifest low nanomolar affinity and ∼10-fold selectivity for Grp94. Furthermore, these Grp94-selective inhibitors manifest low micromolar GI₅₀ values against multiple myeloma cells, supporting Grp94 as an emerging target for the treatment of this disease.

Second Generation Grp94-Selective Inhibitors Provide Opportunities for the Inhibition of Metastatic Cancer

Glucose regulated protein 94 (Grp94) is the endoplasmic reticulum (ER) resident isoform of the 90 kDa heat shock protein (Hsp90) family and its inhibition represents a promising therapeutic target for the treatment of many diseases. Modification of the first generation cis-amide bioisostere imidazole to alter the angle between the resorcinol ring and the benzyl side chain via cis-amide replacements produced compounds with improved Grp94 affinity and selectivity. Structure-activity relationship studies led to the discovery of compound 30, which exhibits 540 nm affinity and 73-fold selectivity towards Grp94. Grp94 is responsible for the maturation and trafficking of proteins associated with cell signaling and motility, including select integrins. The Grp94-selective inhibitor 30 was shown to exhibit potent anti-migratory effects against multiple aggressive and metastatic cancers.

Osmotic Stress Induced Cell Death in Wheat Is Alleviated by Tauroursodeoxycholic Acid and Involves Endoplasmic Reticulum Stress-Related Gene Expression

Although, tauroursodeoxycholic acid (TUDCA) has been widely studied in mammalian cells because of its role in inhibiting apoptosis, its effects on plants remain almost unknown, especially in the case of crops such as wheat. In this study, we conducted a series of experiments to explore the effects and mechanisms of action of TUDCA on wheat growth and cell death induced by osmotic stress. Our results show that TUDCA: (1) ameliorates the impact of osmotic stress on wheat height, fresh weight, and water content; (2) alleviates the decrease in chlorophyll content as well as membrane damage caused by osmotic stress; (3) decreases the accumulation of reactive oxygen species (ROS) by increasing the activity of antioxidant enzymes under osmotic stress; and (4) to some extent alleviates osmotic stress-induced cell death probably by regulating endoplasmic reticulum (ER) stress-related gene expression, for example expression of the basic leucine zipper genes bZIP60B and bZIP60D, the binding proteins BiP1 and BiP2, the protein disulfide isomerase PDIL8-1, and the glucose-regulated protein GRP94. We also propose a model that illustrates how TUDCA alleviates osmotic stress-related wheat cell death, which provides an important theoretical basis for improving plant stress adaptation and elucidates the mechanisms of ER stress-related plant osmotic stress resistance.