Synthesis and biological evaluation of coumarin replacements of novobiocin as Hsp90 inhibitors

Ligand-based pharmacophore modelling, structure optimisation, and biological evaluation for the identification of 2-heteroarylthio-N-arylacetamides as novel HSP90 C-terminal inhibitors

Targeting Heat shock protein 90 (HSP90) C-terminus is an important strategy to develop HSP90 inhibitors without inducing heat shock response. The development of C-terminal inhibitors, however, is hampered by a lack of understanding regarding the interaction between the HSP90 C-terminus and the present inhibitors. We collected seven classical and structurally diverse HSP90 C-terminal inhibitors and constructed a ligand-based pharmacophore model. The subsequent virtual screening and structural optimisation led to the identification of 2-heteroarylthio-N-arylacetamides as novel HSP90 C-terminal inhibitors. 9 and 27 exhibited strong antitumour activity in vitro by inhibiting proliferation and inducing apoptosis in multiple cancer cell lines. These compounds disrupted the interaction between HSP90 C-terminus and peptidylprolyl isomerase D, exerting a stronger inhibitory effect than novobiocin. 27 significantly induced the degradation of HSP90 clients without triggering heat shock response. In an in vivo study using 4T1 mice breast cancer models, 9 showed a potent antitumour effect without obvious toxicity.

Small molecule inhibitors targeting heat shock protein 90: An updated review

As a molecular chaperone, heat shock protein 90 (HSP90) plays important roles in the folding, stabilization, activation, and degradation of over 500 client proteins, and is extensively involved in cell signaling, proliferation, and survival. Thus, it has emerged as an important target in a variety of diseases, including cancer, neurodegenerative diseases, and viral infections. Therefore, targeted inhibition of HSP90 provides a valuable and promising therapeutic strategy for the treatment of HSP90-related diseases. This review aims to systematically summarize the progress of research on HSP90 inhibitors in the last five years, focusing on their structural features, design strategies, and biological activities. It will refer to the natural products and their derivatives (including novobiocin derivatives, deguelin derivatives, quinone derivatives, and terpenoid derivatives), and to synthetic small molecules (including resorcinol derivatives, pyrazoles derivatives, triazole derivatives, pyrimidine derivatives, benzamide derivatives, benzothiazole derivatives, and benzofuran derivatives). In addition, the major HSP90 small-molecule inhibitors that have moved into clinical trials to date are also presented here.

Pallado-Catalyzed Cascade Synthesis of 2-Alkoxyquinolines from 1,3-Butadiynamides

A novel synthesis strategy to access 2-alkoxyquinoline derivatives via a palladium-driven cascade reaction is disclosed. Unlike classic methods based on the alkylation of 2-quinolones with alkyl halides, the present method benefits from the de novo assembly of the quinoline core starting from 1,3-butadiynamides. Palladium-catalyzed reaction cascades with N-(2-iodophenyl)-N-tosyl-1,3-butadiynamides and primary alcohols as external nucleophiles proceed under mild reaction conditions and selectively deliver a variety of differently functionalized 4-alkenyl 2-alkoxyquinolines in a single batch transformation.

A silicon-containing aryl/penta-1,4-dien-3-one/amine hybrid exhibits antiproliferative effects on breast cancer cells by targeting the HSP90 C-terminus without inducing heat-shock response

A pharmacophore-hybridized strategy based on previously reported HSP90 C-terminal inhibitors was utilized to prepare 32 aryl/penta-1,4-dien-3-one/amine hybrids. Among them, a silicon-containing compound 1z exhibited remarkable broad-spectrum antiproliferative effects on various human breast cancer cell lines. Through fluorescence polarization and AlphaScreen-based assays, we demonstrated that 1z specifically inhibited the HSP90 C-terminus without affecting HSP90 N-terminus. Furthermore, 1z effectively inhibited the HSP90 C-terminus without inducing heat-shock response (HSR), leading to the degradation of its client proteins HER2, pAKT, AKT, and CDK4, causing G1 arrest of MCF-7 and SKBr3 cells, and ultimately contributing to apoptosis of these cells through caspase-3, caspase-8, and caspase-9 activation. Additionally, the penta-1,4-dien-3-one linker in the hybrid, a large bulky lipophilic substitution in the aryl fragment at the 3'-site, and the presence of N-methylpiperazine as the amine fragment were identified as crucial factors that significantly contributed to the observed antiproliferative activity through structure-activity relationship (SAR) analysis. Lastly, we found that 1z exhibited superior thermostability compared to vibsanin B derivatives and good in vitro metabolic stability in simulated intestinal fluid, representing one of the few reported silicon-containing HSP90 C-terminal inhibitors.

Synthesis of novobiocin derivatives and evaluation of their antigonococcal activity and pharmacokinetics

Gonorrhea has become a serious problem because the number of infected people is increasing and the multi-drug resistance of the causative bacteria, Neisseria gonorrhoeae, is progressing. To develop novel drugs against resistant N. gonorrhoeae, we focused on the antibiotic novobiocin (1). This natural product has a different mechanism of action from existing drugs for gonorrhea, which may make it effective against resistant strains. Actually, it was applied to resistant N. gonorrhoeae, and moderate antibacterial activity was confirmed. Based on this result, we investigated the development of an antigonococcal drug with 1 as the lead compound. The pharmacophore is thought to be the noviose sugar moiety, especially around the 3'-position, so we derivatized this part in order to improve antibacterial activity. As a result, we found that 5 with an methylpyrrole ester structure have a very potent antibacterial activity. This derivative also showed excellent antigonococcal activity against resistant strains in vitro, however it has poor water solubility and pharmacokinetics because it is the acidic lipid-soluble compound. Therefore, we considered introduction of a basic substituent into the molecule would result in an amphoteric compound with improved water solubility, and we investigated further derivatization. As a result of synthesizing various derivatives, we found 47 containing imidazole with strong antigonococcal activity and greatly improved water solubility. This derivative has also improved metabolism and blood concentration in vivo, and is expected to be orally absorbed. Based on these results, we believe that 47 is a very promising anti-gonococcal lead compound and has great potential for further development.

Triethylammonium salt of a synthesized dicoumarol: Structural insight and human anti-glioblastoma activities

Glioblastoma multiforme (GBM) is the most common and primary brain tumor with poor prognosis. They are removed by following tedious and life threatening surgeries. GBM stem cells (GSCs) are the main source of tumor recurrence after surgery. Hence, drugs are designed to overcome the recurrent glioblastoma malignant cells. Currently used chemotherapies are not cost effective as well as bear resistance. New and effective chemotherapeutic compounds are developed to overcome the intrinsic and acquired resistance. Dicoumarol derivative 3,3'-[(4-methoxyphenyl)methanediyl]bis(4-hydroxy-2Hchromen-2-one) (HL) and its triethylammonium salt triethylammonium3-[(4-methoxyphenyl)(4-hydroxy-2-oxo-2H-chromen-3-yl)methyl]-2-oxo-2H-chromen-4-olate (L) were synthesized and characterized using spectral and analytical techniques. The deprotonated compound L was further studied structurally using single crystal analysis. Cytotoxic studies against human glioblastoma cells A172 and LN229 were investigated both dose and time dependently and compared with the cytotoxicity of normal human astrocytes (NHA). The IC₅₀ value of HL against A172 was found to be lying within the range 2.68-0.95 μM whereas against LN229 the range was found to be 9.55-0.85 μM. Similarly, the compound L revealed range of 1.9-0.271 μM against A172 and 1.2-0.27 μM against LN229. Cell cycle arrest was observed in GBM cells treated with L compared to the control group, which suggested that L may trigger apoptosis in GBM cells according to cytotoxicity and flow cytometry results. The antioxidant activity of synthesized compounds was also investigated using DPPH free radicals.

Therapeutic Effects of Coumarins with Different Substitution Patterns

The use of derivatives of natural and synthetic origin has gained attention because of their therapeutic effects against human diseases. Coumarins are one of the most common organic molecules and are used in medicine for their pharmacological and biological effects, such as anti-inflammatory, anticoagulant, antihypertensive, anticonvulsant, antioxidant, antimicrobial, and neuroprotective, among others. In addition, coumarin derivates can modulate signaling pathways that impact several cell processes. The objective of this review is to provide a narrative overview of the use of coumarin-derived compounds as potential therapeutic agents, as it has been shown that substituents on the basic core of coumarin have therapeutic effects against several human diseases and types of cancer, including breast, lung, colorectal, liver, and kidney cancer. In published studies, molecular docking has represented a powerful tool to evaluate and explain how these compounds selectively bind to proteins involved in various cellular processes, leading to specific interactions with a beneficial impact on human health. We also included studies that evaluated molecular interactions to identify potential biological targets with beneficial effects against human diseases.

Copper-Catalyzed Cyclization of 2-Alkynylanilines to Give 2-Haloalkoxy-3-alkyl(aryl)quinolines

Herein we describe a method to produce 2-haloalkoxy-3-substituted quinolines via the cyclization of 2-alkynylanilines with TMSCF₃ and THF. This synthetic method uses inexpensive and easy-to-handle TMSCF₃ and employs a commercially available CuI catalyst to transform a broad range of 2-alkynylanilines into versatile 2-difluoromethoxy-3-substituted quinolines and 2-iodoalkoxy-3-substituted quinolines with excellent chemoselectivity.

Dipyridamole interacts with the N-terminal domain of HSP90 and antagonizes the function of the chaperone in multiple cancer cell lines

Molecular chaperone HSP90 has been considered as a promising target for anti-cancer drug development for years. However, due to the heat shock response induced by the ATP competitive inhibitors against HSP90, the therapeutic efficacies of the compounds are compromised, which consequently restricts the clinical use of HSP90-targeted inhibitors. Therefore, there is a need to discover novel HSP90-targeted modulators which exhibit acceptable inhibition activity against the chaperone and do not induce significant heat shock response in the meantime. Here in this study, we firstly developed a tip-based affinity selection-mass spectrometry platform with optimized experimental conditions/parameters for HSP90-targeted active compound screening, and then applied it to fish out inhibitors against HSP90 from a collection of 2,395 compounds composed of FDA-approved drugs and drug candidates. Dipyridamole, which acts as an anti-thrombotic agent by modulating multiple targets and has a long history of safe use, was identified to interact with HSP90's N-terminal domain. The following conducted biophysical and biochemical experiments demonstrated that Dipyridamole could bind to HSP90's ATP binding pocket and function as an ATP competitive inhibitor of the chaperone. Finally, cellular-based assays including CESTA, cell viability assessment and proteomic analysis etc. were performed to evaluate whether the interaction between HSP90 and Dipyridamole contributes to the anti-tumor effects of the compound. We then found that Dipyridamole inhibits the growth and proliferation of human cancer cells by downregulating cell cycle regulators and upregulating apoptotic cell signaling, which are potentially mediated by the binding of Dipyridamole to HSP90 and to PDEs (phosphodiesterases), respectively.

Molecular Target Interactions of Quinoline Derivatives as Anticancer Agents: A Review

One of the leading causes of death worldwide is cancer, which poses substantial risks to both society and an individual's life. Cancer therapy is still challenging, despite developments in the field and continued research into cancer prevention. The search for novel anticancer active agents with a broader cytotoxicity range is therefore continuously ongoing. The benzene ring gets fused to a pyridine ring at two carbon atoms close to one another to form the double ring structure of the heterocyclic aromatic nitrogen molecule known as quinoline (1-azanaphthalene). Quinoline derivatives contain a wide range of pharmacological activities, including antitubercular, antifungal, antibacterial, and antimalarial properties. Quinoline derivatives have also been shown to have anticancer properties. There are many quinoline derivatives widely available as anticancer drugs that act via a variety of mechanisms on various molecular targets, such as inhibition of topoisomerase, inhibition of tyrosine kinases, inhibition of heat shock protein 90 (Hsp90), inhibition of histone deacetylases (HDACs), inhibition of cell cycle arrest and apoptosis, and inhibition of tubulin polymerization.

Crystal structure of the middle and C-terminal domains of Hsp90α labeled with a coumarin derivative reveals a potential allosteric binding site as a drug target

The 90 kDa heat-shock protein (Hsp90) is an abundant molecular chaperone that is essential to activate, stabilize and regulate the function of a plethora of client proteins. As drug targets for the treatment of cancer and neurodegenerative diseases, Hsp90 inhibitors that bind to the N-terminal ATP-binding site of Hsp90 have shown disappointing efficacy in clinical trials. Thus, allosteric regulation of the function of Hsp90 by compounds that interact with its middle and C-terminal (MC) domains is now being pursued as a mechanism to inhibit the ATPase activity and client protein-binding activity of Hsp90 without concomitant induction of the heat-shock response. Here, the crystal structure of the Hsp90αMC protein covalently linked to a coumarin derivative, MDCC {7-diethylamino-3-[N-(2-maleimidoethyl)carbamoyl]coumarin}, which is located in a hydrophobic pocket that is formed at the Hsp90αMC hexamer interface, is reported. MDCC binding leads to the hexamerization of Hsp90, and the stabilization and conformational changes of three loops that are critical for its function. A fluorescence competition assay demonstrated that other characterized coumarin and isoflavone-containing Hsp90 inhibitors compete with MDCC binding, suggesting that they could bind at a common site or that they might allosterically alter the structure of the MDCC binding site. This study provides insights into the mechanism by which the coumarin class of allosteric inhibitors potentially disrupt the function of Hsp90 by regulating its oligomerization and the burial of interaction sites involved in the ATP-dependent folding of Hsp90 clients. The hydrophobic binding pocket characterized here will provide new structural information for future drug design.

Dissecting Molecular Principles of the Hsp90 Chaperone Regulation by Allosteric Modulators Using a Hierarchical Simulation Approach and Network Modeling of Allosteric Interactions: Conformational Selection Dictates the Diversity of Protein Responses and Ligand-Specific Functional Mechanisms

Conformational plasticity of the Hsp90 molecular chaperones underlies the diversity of functional mechanisms that these versatile molecular machines employ to coordinate their vast protein clientele in the cellular environment. Despite a steady progress in studies of the Hsp90 machinery, a great deal remains unknown about molecular principles and ligand-specific functional mechanisms of the Hsp90 regulation by allosteric modulators that attracted significant attention because of their therapeutic potential. Due to structural complexity and dynamic nature of the Hsp90 responses to allosteric modulators, the atomistic details about the mode of action of these small molecules continue to be fairly scarce and controversial. In this work, we employ an integrative strategy that encompassed atomistic simulations of the Hsp90 proteins and hierarchical modeling of Hsp90-ligand binding with network analysis to explore functional mechanisms of the Hsp90 regulation by a panel of allosteric modulators (novobiocin, KU-135, KU-174, and KU-32) with different models of action. The results show that functional mechanisms of allosteric modulation in the Hsp90 proteins may be driven by conformational selection principles in which ligands elicit pre-existing states of the unbound chaperone to drive ligand-specific protein responses and distinct scenarios of Hsp90 regulation. We found that novobiocin can selectively sequester an ensemble of open chaperone conformations and inhibit the progression of the functional cycle through a cascade of cumulative dynamic changes. In contrast, KU-32 displayed unique preferences toward partially closed dynamic states, inducing robust allosteric signaling and stimulation of the ATPase cycle. The proposed model of the Hsp90 regulation by allosteric modulators reconciled diverse experimental data and showed that allosteric modulators may operate via targeted exploitation of dynamic landscapes eliciting vastly different protein responses and diverse mechanisms of action.

Natural heat shock protein 90 inhibitors in cancer and inflammation

Heat shock protein (HSP)90 is the most abundant HSPs, which are chaperone molecules whose major roles are cell protection and maintenance by means of aiding the folding, the stabilization and the remodeling of a wide range of proteins. A few hundreds of proteins depend on HSP90 chaperone activity, including kinases and transcriptional factors that play essential roles in cancer and inflammation, so that HSP90-targeted therapies have been considered as a potential strategy for the treatment of cancer and inflammatory-associated diseases. HSP90 inhibition by natural, semi-synthetic and synthetic compounds have yield promising results in pre-clinical studies and clinical trials for different types of cancers and inflammation. Natural products are a huge source of biologically active compounds widely used in drug development due to the great diversity of their metabolites which are capable to modulate several protein functions. HSP90 inhibitors have been isolated from bacteria, fungi and vegetal species. These natural compounds have a noteworthy ability to modulate HSP90 activity as well as serve as scaffolds for the development of novel synthetic or semi-synthetic inhibitors. Over a hundred clinical trials have evaluated the effect of HSP90 inhibitors as adjuvant treatment against different types of tumors and, currently, new studies are being developed to gain sight on novel promising and more effective approaches for cancer treatment. In this review, we present the naturally occurring HSP90 inhibitors and analogues, discussing their anti-cancer and anti-inflammatory effects.

Translational role of natural coumarins and their derivatives as anticancer agents

Natural coumarins and their derivatives isolated from various plants or microorganisms have inherent antioxidant, antibacterial, antifungal, antiviral and anticancer properties among many biological activities. Some of these coumarins and their derivatives lead to self-programmed cancer cell death (apoptosis) via different mechanisms, which will be discussed. The link between bacterial and viral infections to cancer compels us to highlight fascinating reports from coumarin isolation from microorganisms; comment on the recent bioavailability studies of natural or derived coumarins; and discuss our perspectives with respect to bioisosterism in coumarins, p-glycoprotein inhibition and covalent modification, and bioprobes. Overall, this review hopes to stimulate and offer in particular medicinal chemists and the reader in general an outlook on natural coumarins and their derivatives with potential for cancer therapy.

In silico identification and analysis of the binding site for aminocoumarin type inhibitors in the C-terminal domain of Hsp90

Hsp90 contains two Nucleotide Binding Sites (NBS): one each in its N-terminal domain (NTD) and C-terminal domain (CTD), respectively. Previously we used computational techniques to locate a nucleotide-binding site in the CTD. Nucleotide binding at this site stabilized the structurally labile region within this domain, thus providing a rationale for increased resistance to thermal denaturation and proteolysis. A scan for ligand-binding sites in CTD revealed four potential sites with the requisite volume to accommodate aminocoumarins and -derived inhibitors. Only one of these reproducibly formed docked complexes with inhibitors and showed excellent interactions with residues lining the site. Fortuitously, it was identical to the aforementioned nucleotide-binding site thus providing an explanation for the reported direct competition between inhibitors and nucleotides. Further studies with carefully chosen inhibitors and some inactive analogues provided an explanation for the known Structure-Activity Relationships (SAR) of aminocoumarin and -derived inhibitors. We also performed similar studies of the NTD to discern the reason(s) for its inability to bind aminocoumarins, given the family resemblance to prokaryotic Top-IV and Gyr-B. Our studies permitted the identification of the putative inhibitor binding site in the CTD, an explanation for increased resistance to thermal denaturation and proteolysis upon inhibitor binding as well as direct competition with ATP.

Discovery of new molecular entities able to strongly interfere with Hsp90 C-terminal domain

Heat shock protein 90 (Hsp90) is an ATP dependent molecular chaperone deeply involved in the complex network of cellular signaling governing some key functions, such as cell proliferation and survival, invasion and angiogenesis. Over the past years the N-terminal protein domain has been fully investigated as attractive strategy against cancer, but despite the many efforts lavished in the field, none of the N-terminal binders (termed "classical inhibitors"), currently in clinical trials, have yet successfully reached the market, because of the detrimental heat shock response (HSR) that showed to induce; thus, recently, the selective inhibition of Hsp90 C-terminal domain has powerfully emerged as a more promising alternative strategy for anti-cancer therapy, not eliciting this cell rescue cascade. However, the structural complexity of the target protein and, mostly, the lack of a co-crystal structure of C-terminal domain-ligand, essential to drive the identification of new hits, represent the largest hurdles in the development of new selective C-terminal inhibitors. Continuing our investigations on the identification of new anticancer drug candidates, by using an orthogonal screening approach, here we describe two new potent C-terminal inhibitors able to induce cancer cell death and a considerable down-regulation of Hsp90 client oncoproteins, without triggering the undesired heat shock response.

Development of noviomimetics that modulate molecular chaperones and manifest neuroprotective effects

Heat shock protein 90 (Hsp90) is a chaperone under investigation for the treatment of cancer and neurodegenerative diseases. Neuroprotective Hsp90 C-terminal inhibitors derived from novobiocin (novologues) include KU-32 and KU-596. These novologues modulate molecular chaperones and result in an induction of Heat Shock Protein 70 (Hsp70). "Noviomimetics" replace the synthetically complex noviose sugar with a simple cyclohexyl moiety to maintain biological efficacy as compared to novologues KU-596 and KU-32. In this study, we further explore the development of noviomimetics and evaluate their efficacy using a luciferase refolding assay, immunoblot analysis, a c-jun assay, and an assay measuring mitochondrial bioenergetics. These new noviomimetics were designed and synthesized and found to induce Hsp70 and improve biological activity. Noviomimetics 39e and 40a were found to induce Hsp70 and exhibit promising effects in cellular assays.

Development of Phenyl Cyclohexylcarboxamides as a Novel Class of Hsp90 C-terminal Inhibitors

Inhibition of the heat shock protein 90 (Hsp90) C-terminus represents a promising therapeutic strategy for the treatment of cancer. Novobiocin, a coumarin antibiotic, was the first Hsp90 C-terminal inhibitor identified, however, it manifested poor anti-proliferative activity (SKBr3, IC50 ≈700 μm). Subsequent structure-activity relationship (SAR) studies on novobiocin led to development of several analogues that exhibited improved anti-proliferative activity against several cancer cell lines. Recent studies demonstrate that the biphenyl core could be used in lieu of the coumarin ring system, which resulted in more efficacious analogues. In continuation of previous efforts, the work described herein has identified the phenyl cyclohexyl core as a novel scaffold for Hsp90 C-terminal inhibition. Structure-activity relationship (SAR) studies on this scaffold led to the development of compounds that manifest mid-nanomolar activity against SKBr3 and MCF-7 breast cancer cell lines through Hsp90 inhibition.

Design, synthesis and biological evaluation of alkylamino biphenylamides as Hsp90 C-terminal inhibitors

Hsp90 is a promising therapeutic target for the development of anti-cancer agents due to its integral role in the stability and function of proteins associated with all ten hallmarks of cancer. Novobiocin, a coumarin antibiotic, was the first natural product identified that targeted the Hsp90 C-terminal domain and manifested anti-proliferative activity (SKBr3 IC50∼700μM). Subsequent structural investigations on novobiocin led to analogues with significantly improved anti-proliferative activity against multiple cancer cell lines. In an effort to develop more efficacious and diverse analogues, it was recently found that the coumarin ring of novobiocin could be replaced with the biphenyl core without compromising activity. Based on these prior studies, a series of alkylamino biphenylamides was designed, synthesized and evaluated for anti-proliferative activity against two breast cancer cell lines. SAR studies demonstrated that the incorporation of an alkylamino side chain onto the biphenyl core improved anti-proliferative activity and resulted in compounds that exhibit sub-micromolar to mid-nanomolar activity through Hsp90 inhibition. Importantly, these studies indicate the presence of a hydrophilic region about the central core that can be exploited for the design of new inhibitors.

Diverging Novobiocin Anti-Cancer Activity from Neuroprotective Activity through Modification of the Amide Tail

Novobiocin is a natural product that binds the Hsp90 C-terminus and manifests Hsp90 inhibitory activity. Structural investigations on novobiocin led to the development of both anti-cancer and neuroprotective agents. The varied pharmacological activity manifested by these novobiocin analogs prompted the investigation of structure-function studies to identify these contradictory effects, which revealed that modifications to the amide side chain produce either anti-cancer or neuroprotective activity. Compounds that exhibit neuroprotective activity contain a short alkyl or cycloalkyl amide side chain. In contrast, anti-cancer agents contain five or more carbons, disrupt interactions between Hsp90α and Aha1, and induce the degradation of Hsp90-dependent client proteins.

New dihydropyrimidin-2(1H)-one based Hsp90 C-terminal inhibitors

The inhibition of the C-terminal domain of heat shock protein 90 (Hsp90) is emerging as a novel strategy for cancer therapy, therefore the identification of a new class of C-terminal inhibitors is strongly required.

Identification of the key structural elements of a dihydropyrimidinone core driving toward more potent Hsp90 C-terminal inhibitors

Dramatic improvement in the biological activity of DHPM derivatives as a new class of Hsp90 C-terminal inhibitors for cancer therapy.

Organelle-specific Hsp90 inhibitors

Heat shock protein 90 (Hsp90) is an ATP-dependent molecular chaperone that is involved in the folding, activation, and stabilization of numerous oncogenic proteins. It has become an attractive therapeutic target, especially for eradicating malignant cancers and overcoming chemotherapy resistance. The Hsp90 family in mammalian cells is composed of four major homologs: Hsp90α, Hsp90β, 94-kDa glucose-regulated protein (Grp94), and TNF receptor-associated protein 1 (Trap1). Hsp90α and Hsp90β are mainly localized in the cytoplasm, while Grp94 and Trap1 reside in the endoplasmic reticulum and the mitochondria, respectively. Additionally, some Hsp90 s are secreted from the cytoplasm, commonly called extracellular Hsp90. Interestingly, each Hsp90 isoform is localized in a particular organelle, possesses a unique biological function, and participates in various physiological and pathological processes. To inhibit the organelle-specific Hsp90 chaperone function, there have been significant efforts to accumulate Hsp90 inhibitors in particular cellular compartments. This review introduces current studies regarding the delivery of Hsp90 inhibitors to subcellular organelles, particularly to the extracellular matrix and the mitochondria, and discusses their biological insights and therapeutic implications.

Coumarins as anticancer agents: a review on synthetic strategies, mechanism of action and SAR studies

Coumarins are fused benzene and pyrone ring systems which prompt biological investigation to assess their potential therapeutic significance. It possesses immeasurable anticancer potential with minimum side effects depending on the substitutions on the basic nucleus. Coumarins have a tremendous ability to regulate diverse range of cellular pathways that can be explored for selective anticancer activity. This is the first standalone review that emphasis on the assorted retrosynthetic approaches, important targets for molecularly targeted cancer therapy and structure activity relationship studies that highlight the chemical groups responsible for evoking the anticancer potential of coumarin derivatives reported from 2011 to 2014.

Alternative approaches to Hsp90 modulation for the treatment of cancer

Hsp90 is responsible for the conformational maturation of newly synthesized polypeptides (client proteins) and the re-maturation of denatured proteins via the Hsp90 chaperone cycle. Inhibition of the Hsp90 N-terminus has emerged as a clinically relevant strategy for anticancer chemotherapeutics due to the involvement of clients in a variety of oncogenic pathways. Several immunophilins, co-chaperones and partner proteins are also necessary for Hsp90 chaperoning activity. Alternative strategies to inhibit Hsp90 function include disruption of the C-terminal dimerization domain and the Hsp90 heteroprotein complex. C-terminal inhibitors and Hsp90 co-chaperone disruptors prevent cancer cell proliferation similar to N-terminal inhibitors and destabilize client proteins without induction of heat shock proteins. Herein, current Hsp90 inhibitors, the chaperone cycle, and regulation of this cycle will be discussed.

KU675, a Concomitant Heat-Shock Protein Inhibitor of Hsp90 and Hsc70 that Manifests Isoform Selectivity for Hsp90α in Prostate Cancer 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. Inhibiting Hsp90 consequently is an attractive strategy for cancer therapy as the concomitant degradation of multiple oncoproteins may lead to effective antineoplastic agents. Here we report a novel C-terminal Hsp90 inhibitor, designated KU675, that exhibits potent antiproliferative and cytotoxic activity along with client protein degradation without induction of the heat-shock response in both androgen-dependent and -independent prostate cancer cell lines. In addition, KU675 demonstrates direct inhibition of Hsp90 complexes as measured by the inhibition of luciferase refolding in prostate cancer cells. In direct binding studies, the internal fluorescence signal of KU675 was used to determine the binding affinity of KU675 to recombinant Hsp90α, Hsp90β, and Hsc70 proteins. The binding affinity (Kd) for Hsp90α was determined to be 191 μM, whereas the Kd for Hsp90β was 726 μM, demonstrating a preference for Hsp90α. Western blot experiments with four different prostate cancer cell lines treated with KU675 supported this selectivity by inducing the degradation of Hsp90α -: dependent client proteins. KU675 also displayed binding to Hsc70 with a Kd value at 76.3 μM, which was supported in cellular by lower levels of Hsc70-specific client proteins on Western blot analyses. Overall, these findings suggest that KU675 is an Hsp90 C-terminal inhibitor, as well as a dual inhibitor of Hsc70, and may have potential use for the treatment of cancer.

Small Molecule Inhibitors to Disrupt Protein-protein Interactions of Heat Shock Protein 90 Chaperone Machinery

Heat shock protein 90 (Hsp90) is an adenosine triphosphate dependent molecular chaperone in eukaryotic cells that regulates the activation and maintenance of numerous regulatory and signaling proteins including epidermal growth factor receptor, human epidermal growth factor receptor 2, mesenchymal-epithelial transition factor, cyclin-dependent kinase-4, protein kinase B, hypoxia-inducible factor 1α, and matrix metalloproteinase-2. Since many of Hsp90 clients are oncogenic proteins, Hsp90 has become an attractive therapeutic target for treatment of cancer. To discover small molecule inhibitors targeting Hsp90 chaperone machinery, several strategies have been employed, which results in three classes of inhibitors such as N-terminal inhibitors, C-terminal inhibitors, and inhibitors disrupting protein-protein interactions of Hsp90 chaperone machinery. Developing small molecule inhibitors that modulate protein-protein interactions of Hsp90 is a challenging task, although it offers many alternative opportunities for therapeutic intervention. The lack of well-defined binding pocket and starting points for drug design challenges medicinal chemists to discover small molecule inhibitors disrupting protein-protein interactions of Hsp90. The present review will focus on the current studies on small molecule inhibitors disrupting protein-protein interactions of Hsp90 chaperone machinery, provide biological background on the structure, function and mechanism of Hsp90's protein-protein interactions, and discuss the challenges and promise of its small molecule modulations.

Design, synthesis, and biological evaluation of ring-constrained novobiocin analogues as hsp90 C-terminal inhibitors

Hsp90 C-terminal inhibitors represent a novel and alternative chemotherapeutic approach for the treatment of cancer. Novobiocin was the first natural product identified as an Hsp90 C-terminal inhibitor; however, it manifests poor antiproliferative activity. In contrast to N-terminal inhibitors, novobiocin does not induce the pro-survival heat shock response. Structural investigations on novobiocin have elucidated some structure-activity relationships and several promising compounds. On the basis of structure-activity relationships and computational studies, a library of ring-constrained novobiocin analogues was designed, synthesized, and evaluated in antiproliferative assays. Results obtained from these studies provide insights into the Hsp90 C-terminal binding site, and new analogues that were developed manifest low micromolar to mid-nanomolar antiproliferative activity resulting from Hsp90 inhibition.

Design, synthesis and biological evaluation of biphenylamide derivatives as Hsp90 C-terminal inhibitors

Modulation of Hsp90 C-terminal function represents a promising therapeutic approach for the treatment of cancer and neurodegenerative diseases. Current drug discovery efforts toward Hsp90 C-terminal inhibition focus on novobiocin, an antibiotic that was transformed into an Hsp90 inhibitor. Based on structural information obtained during the development of novobiocin derivatives and molecular docking studies, scaffolds containing a biphenyl moiety in lieu of the coumarin ring present in novobiocin were identified as new Hsp90 C-terminal inhibitors. Structure-activity relationship studies produced new derivatives that inhibit the proliferation of breast cancer cell lines at nanomolar concentrations, which corresponded directly with Hsp90 inhibition.

Progress in the discovery and development of heat shock protein 90 (Hsp90) inhibitors

The discovery and clinical development of heat shock protein 90 (Hsp90) inhibitors continue to progress. A number of Hsp90 inhibitors are in clinical trials, and preclinical discoveries of new chemotypes that bind to distinct regions in the protein as well as isoform selective compounds are active areas of research. This review will highlight progress in the field since 2010.

Synthesis, DNA binding, docking and photocleavage studies of quinolinyl chalcones

A series of simple quinoline–chalcone conjugates have been synthesized and evaluated for their nucleolytic activity. The compounds 3c and 3d exhibited promising DNA binding and DNA photocleavage studies.

Triazole Containing Novobiocin and Biphenyl Amides as Hsp90 C-Terminal Inhibitors

Hsp90 C-terminal inhibitors are advantageous for the development of new cancer chemotherapeutics due to their ability to segregate client protein degradation from induction of the prosurvival heat shock response, which is a major detriment associated with Hsp90 N-terminal inhibitors under clinical investigation. Based upon prior SAR trends, a 1,2,3-triazole side chain was placed in lieu of the aryl side chain and attached to both the coumarin and biphenyl scaffold. Antiproliferative studies against SKBr3 and MCF-7 breast cancer cell lines demonstrated these triazole-containing compounds to exhibit improved activity. These compounds were shown to manifest Hsp90 inhibitory activity through Western blot analysis and represent a new scaffold upon which more potent inhibitors can be pursued.