Emerging Hsp90 inhibitors: from discovery to clinic

Hsp90 is a chaperone with important roles in maintaining transformation and in elevating the survival and growth potential of cancer cells. Activation of signaling pathways mediated by Hsp90 protein clients is necessary for cell proliferation, regulation of cell cycle progression and apoptosis. Additionally, gain-of-function mutations responsible for transformation often require Hsp90 for the maintenance of their folded, functionally active conformations. These characteristics promise Hsp90 as an important target in cancer therapy and prompt for the identification, development and clinical translation of small molecule inhibitors of the chaperone. This review intends to update the reader on the status of several existing and emerging classes of direct inhibitors of Hsp90 ATPase activity.     

Purine-scaffold Hsp90 inhibitors

Heat shock protein 90 (Hsp90) has emerged as an important target, the inhibition of which is aimed at a wide range of cell transformations that lead to malignancy. A clinical evaluation of 17-AAG, the first Hsp90 inhibitor to enter the clinic, revealed evidence of activity for the compound at a manageable level of toxicity. However, clinical results have also demonstrated the limitations of this drug. The identification of novel Hsp90 inhibitor classes with improved structural characteristics and better pharmacological profiles has therefore become a major focus of interest in the field of cancer therapeutics. This feature describes the purine-scaffold class of Hsp90 inhibitors, focusing on research efforts from the discovery stage to the clinical translation of such compounds.     

Medicinal chemistry of Hsp90 inhibitors

Heat shock protein (Hsp90) inhibitors are an increasingly interesting and important class of compounds where the first in class, natural product derived inhibitors such as 17-allylaminogeldanamycin (17-AAG), are entering late stage clinical development. Recently the emergence of synthetic, small molecule inhibitors has been described and both NVP-AUY922 and BIIB021 have entered clinical development. The medicinal chemistry of these and other published small molecule Hsp90 inhibitors is described in this review.     

Hsp90抑制剂的研究进展

热休克蛋白90(Hsp90)是生物进化过程中高度保守的一类蛋白,作为分子伴侣对细胞中众多信号蛋白的构象成熟和功能稳定进行调控.近年来的研究证明,Hsp90与肿瘤发生、发展、生物学行为及其预后有密切的关系,已成为新兴的抗肿瘤药物作用靶点,众多Hsp90抑制剂应运而生,同时基于结构的小分子抑制剂的设计也引起日益广泛的关注,本文就Hsp90的结构特点及其相关抑制剂进行综述,以期对Hsp90抑制剂的发现和优化提供依据.     

Natural product origins of Hsp90 inhibitors

The currently used Hsp90 inhibitors, geldanamycin, herbimycin A and radicicol, were isolated many years ago from Streptomyces and fungi originally for their antiprotozoal activity, herbicidal activity and antifungal activity, respectively. In the mid 1980s, it was found that the benzoquinone ansamycin antibiotics (herbimycin A, geldanamycin, and macbecin) reversed v-Src transformed cells to normal phenotypes, and Bcr-abl was subsequently suggested to be the molecular target for the treatment of chronic myelogenous leukemia through a study using herbimycin A for its selective antioncogenic activity. In 1994, these ansamycins were found to bind to Hsp90 and to cause the degradation of client proteins including Src kinases; further efforts to develop anticancer drugs were made using geldanamycin analogs, and 17AAG was chosen as the best candidate for clinical trials. The number of novel natural products isolated from microbial origins is continuing to increase and is doubling every 10 years. Thus, screening of bioactive substances from natural origins, using assays including defined targets, and developing leads toward drugs via optimized derivatization is a conventional but still promising strategy for drug discovery and development.     

Development and application of Hsp90 inhibitors

Heat shock protein 90 has emerged as an important target in several diseases. The present review will discuss our understanding of the role played by Hsp90 in regulating and maintaining the transformed phenotype in cancers and neurodegenerative diseases, as well as recent findings on its roles in fungal and viral infections. It will also update the reader on the preclinical development and clinical translation of Hsp90 inhibitors.     

Hsp90 inhibitors and drug resistance in cancer: the potential benefits of combination therapies of Hsp90 inhibitors and other anti-cancer drugs

Hsp90 is a chaperone protein that interacts with client proteins that are known to be in the cell cycle, signaling and chromatin-remodeling pathways. Hsp90 inhibitors act additively or synergistically with many other drugs in the treatment of both solid tumors and leukemias in murine tumor models and humans. Hsp90 inhibitors potentiate the actions of anti-cancer drugs that target Hsp90 client proteins, including trastuzumab (Herceptin?) which targets Her2/Erb2B, as Hsp90 inhibition elicits the drug effects in cancer cell lines that are otherwise resistant to the drug. A phase II study of the Hsp90 inhibitor 17-AAG and trastuzumab showed that this combination therapy has anticancer activity in patients with HER2-positive metastatic breast cancer progressing on trastuzumab. In this review, we discuss the results of Hsp90 inhibitors in combination with trastuzumab and other cancer drugs. We also discuss recent results from yeast focused on the genetics of drug resistance when Hsp90 is inhibited and the implications that this might have in understanding the effects of genetic variation in treating cancer in humans.     

Recent advances in Hsp90 inhibitors as antitumor agents

One promising therapeutic strategy for treating cancer is to specifically target signal transduction pathways that have a key role in oncogenic transformation and malignant progression. Hsp90 is an emerging therapeutic target of interest for the treatment of cancer. It is responsible for modulating cellular response to stress by maintaining the function of numerous signalling proteins - known as 'client proteins' - that are associated with cancer cell survival and proliferation. Many cancers result from specific mutations in, or aberrant expression of, these client proteins. Small molecule Hsp90 inhibitors bind to the ATP binding pocket, inhibit chaperone function and could potentially result in cytostasis or cell death. Consequently, many client proteins are targeted for degradation via the ubiquitin-proteasome pathway including receptor and non receptor kinases (Erb-B2, epidermal growth factor receptor, and Src family kinases), serine/threonine kinases (c-Raf-1 and Cdk4), steroid hormone receptors (androgen and estrogen), and apoptosis regulators such as mutant p53. Inhibition of Hsp90 function has also proven effective in killing cancer cells that have developed resistance to targeted therapies such as kinase inhibitors. This review is intended to update recent developments in new Hsp90 inhibitors as antitumors agents, the design, biological evaluation and their clinical trials studies.     

Discovery and development of pyrazole-scaffold Hsp90 inhibitors

This review explains why the chaperone Hsp90 is an exciting protein target for the discovery of new drugs to treat cancer in the clinic, and summarises the properties of natural product derived inhibitors before relating the discovery and current state of development of synthetic pyrazole compounds. Blockade of Hsp90 results in reduced cellular levels of several proteins implicated in cancer including CDK4, ERBB2 and C-RAF, and causes simultaneous inhibition of cancer cell proliferation in culture and of tumor xenograft growth in vivo. Hsp90 has an ATPase domain that is necessary for its Hsp chaperone function, and X-ray crystallography has shown that natural product inhibitors (geldanamycin, radicicol) of Hsp90 function bind to this domain. High throughput assays focusing on the ATPase activity of Hsp90 were developed and used to discover novel chemical starting points for cancer drug discovery. The discovery, synthesis and SAR of 3,4-diaryl pyrazoles is described. X-Ray crystallography of protein-inhibitor complexes revealed important interactions involving the resorcinol substituent at C-3, and these X-ray structures strongly influenced subsequent medicinal chemistry research that has resulted in highly potent inhibitors with sub-micromolar activity in cells. SAR and X-ray data are summarised for analogues in which the 4-phenyl substituent is replaced by amides or piperazine derivatives. Prospects for the pyrazoles as they progress towards clinical development are discussed in relation to current Phase I trials with derivatives of geldanamycin.     

Discovery and development of purine-scaffold Hsp90 inhibitors

The heat-shock protein 90 (Hsp90), an important target in cancer and other diseases, has become recently the focus of several drug discovery and development efforts. The initially identified natural-product inhibitors of Hsp90, such as geldanamycin, played a major role in elucidating its biological function and in determining its clinical relevance. Upcoming synthetic inhibitors, such as the purine-scaffold class, furthered our understanding on Hsp90 in cancer and neurodegenerative diseases and delivered what are promised to be clinical candidates with favorable pharmacologic profiles. This review intends to inform the reader on efforts ranging from the discovery of purine-scaffold Hsp90 inhibitors to their clinical translation as well as on their use as chemical tools to dissect the roles of Hsp90 in pathogenic systems.     

Development of purine-scaffold small molecule inhibitors of Hsp90

The Hsp90 chaperones play a key role in regulating the physiology of cells exposed to environmental stress and in maintaining the malignant phenotype in tumor cells. Agents that interfere with the function of the chaperone may thus be beneficial in the treatment of cancers. The ansamycins (geldanamycin and herbimycin) and the unrelated natural product radicicol were found to bind to the N-terminal pocket of Hsp90 and inhibit its function. However, translation of these compounds to the clinic was impeded by stability and hepatoxicity issues. 17AAG, a derivative of geldanamycin, was found to be less hepatotoxic and is currently undergoing Phase I clinical trial. Unfortunately, 17AAG is insoluble, difficult to formulate and it is not yet clear if therapeutically effective doses can be administered without escalating non-Hsp90 associated toxicities. Additionally, for reasons not yet completely understood, a subset of tumor cells are insensitive to the action of the drug. The development of novel agents that lack the drawbacks of the natural products is thus necessary. Here we present an overview of such efforts with focus on a new class of purine-scaffold Hsp90 inhibitors developed by rational design.     

Targeting Hsp90: small-molecule inhibitors and their clinical development

The Hsp90 multichaperone complex has important roles in the development and progression of malignant transformation. Several small-molecule inhibitors of Hsp90 of diverse chemotypes have shown potent antitumor activity in a wide-range of malignancies, and are currently in clinical or late-stage preclinical investigation. This review intends to update the reader on advances made over the past two years in the clinical development of Hsp90 inhibitors in advanced cancers. It will refer to the two 17-AAG formulations, tanespimycin and IPI-504, and to synthetic small molecules, among which are the purine-scaffold Hsp90 inhibitor CNF2024/BIIB021, the isoxazole derivative VER-52296/NVP-AUY922, and the carbazol-4-one benzamide derivative SNX-5422, and will present our current knowledge on their clinical performance.     

Overview: translating Hsp90 biology into Hsp90 drugs

The Hsp90 molecular chaperone has emerged as one of the most exciting targets for cancer drug development. Hsp90 is overexpressed in many malignancies, very likely as a result of the stress that is induced both by the hostile cancer microenvironment and also by the mutation and abberant expression of oncoproteins. A particularly attractive feature of Hsp90 as a cancer drug target is that it is required for the conformational stability and function of a wide range of oncogenic 'client' proteins, including c-Raf-1, Cdk4, ErbB2, mutant p53, c-Met, Polo-1 and telomerase hTERT. Inhibition of Hsp90 should therefore block multiple mission critical oncogenic pathways in the cancer cell, leading to inhibition of all the hallmark traits of malignancy. This combinatorial blockade of oncogenic targets should give rise to board spectrum antitumour activity across multiple cancer types. The 'druggability' of Hsp90 was confirmed by the discovery that the natural products geldanamycin and radicicol, which have anticancer activity, exert their biological effects by inhibiting the essential ATPase activity associated with the N-terminal domain of the protein. The first-in-class Hsp90 inhibitor has entered clinical trial and provided proof of concept that Hsp90 can be inhibited and clinical benefit seen at non-toxic doses. Further development is underway and a related analogue 17DMAG also shows promise in preclinical models. In addition, novel Hsp90 inhibitors have been identified using methods such as high throughput screening and x-ray crystallography. The opportunities and challenges involved in translating the fast moving biology of Hsp90 into patient benefit is discussed.     

Tricyclic series of heat shock protein 90 (Hsp90) inhibitors part I: discovery of tricyclic imidazo[4,5-c]pyridines as potent inhibitors of the Hsp90 molecular chaperone

A novel class of heat shock protein 90 (Hsp90) inhibitors was developed after a low throughput screen (LTS) of a focused library containing approximately 21K compounds selected by virtual screening. The initial [1-{3-H-imidazo[4-5-c]pyridin-2-yl}-3,4-dihydro-2H-pyrido[2,1-a]isoindole-6-one] (1) compound showed moderate activity (IC(50) = 7.6 μM on Hsp82, the yeast homologue of Hsp90). A high-resolution X-ray structure shows that compound 1 binds into an "induced" hydrophobic pocket, 10-15 ? away from the ATP/resorcinol binding site. Iterative cycles of structure-based drug design (SBDD) and chemical synthesis led to the design and preparation of analogues with improved affinity. These optimized molecules make productive interactions within the ATP binding site as reported by other Hsp90 inhibitors. This resulted in compound 8, which is a highly potent inhibitor in biochemical and cellular assays (K(d) = 0.35 nM on Hsp90; IC(50) = 30 nM on SKBr3 mammary carcinoma cells) and in an in vivo leukemia model.     

Using natural product inhibitors to validate Hsp90 as a molecular target in cancer

Heat shock protein 90 (Hsp90) is a molecular chaperone whose association is required for stability and function of multiple mutated, chimeric, and over-expressed signaling proteins that promote cancer cell growth and/or survival. Hsp90 client proteins include telomerase, mutated p53, Bcr-Abl, Raf-1, Akt, HER2/Neu (ErbB2), mutated B-Raf, mutated EGF receptor, and HIF-1alpha. Hsp90 inhibitors, by interacting specifically with a single molecular target, cause inactivation, destabilization and eventual degradation of Hsp90 client proteins, and they have shown promising anti-tumor activity in various preclinical tumor models. One Hsp90 inhibitor, 17-AAG, is currently in Phase II clinical trial and other inhibitors will shortly be entering the clinic. Hsp90 inhibitors are unique in that, although they are directed towards a specific molecular target, they simultaneously inhibit multiple signaling pathways on which cancer cells depend for growth and survival. Identification of benzoquinone ansamycins as the first Hsp90 inhibitors allowed investigators to determine the biologic effects, at first in vitro and then in vivo, of pharmacologic inhibition of Hsp90. These studies rapidly enhanced our understanding of Hsp90 function and led to the identification of radicicol as a structurally distinct Hsp90 inhibitor. Additional target-based screening uncovered novobiocin as a third structurally distinct small molecule with Hsp90 inhibitory properties. Use of novobiocin, in turn, led to identification of a previously uncharacterized C-terminal ATP binding site in the chaperone. Small molecule inhibitors of Hsp90 have been very useful in understanding Hsp90 biology and in validating this protein as a molecular target for anti-cancer drug development.     

Hsp90抑制剂的研究

Hsp90是一类普遍存在于各种细胞的分子伴侣,参与调节肿瘤细胞的发生、增殖、转移和血管生成等过程。通过对Hsp90的抑制可实现对多种肿瘤信号通路进行调控,从而有效控制肿瘤。Hsp90抑制剂是现在靶向治疗肿瘤的研究热点,其单独使用或与其他药物联合应用都有较好的抗肿瘤活性。本文主要对微生物来源的Hsp90抑制剂及其衍生物的研究进展进行综述。