4,5,6,7-Tetrahydro-isoxazolo-[4,5-c]-pyridines as a new class of cytotoxic Hsp90 inhibitors

New Synthetic Isoxazole Derivatives Acting as Potent Inducers of Fetal Hemoglobin in Erythroid Precursor Cells Isolated from β-Thalassemic Patients

Induction of fetal hemoglobin (HbF) is highly beneficial for patients carrying β-thalassemia, and novel HbF inducers are highly needed. Here, we describe a new class of promising HbF inducers characterized by an isoxazole chemical skeleton and obtained through modification of two natural molecules, geldanamycin and radicicol. After preliminary biological assays based on benzidine staining and RT-qPCR conducted on human erythroleukemic K562 cells, we employed erythroid precursors cells (ErPCs) isolated from β-thalassemic patients. ErPCs weretreated with appropriate concentrations of isoxazole derivatives. The accumulation of globin mRNAs was studied by RT-qPCR, and hemoglobin production by HPLC. We demonstrated the high efficacy of isozaxoles in inducing HbF. Most of these derivatives displayed an activity similar to that observed using known HbF inducers, such as hydroxyurea (HU) or rapamycin; some of the analyzed compounds were able to induce HbF with more efficiency than HU. All the compounds were active in reducing the excess of free α-globin in treated ErPCs. All the compounds displayed a lack of genotoxicity. These novel isoxazoles deserve further pre-clinical study aimed at verifying whether they are suitable for the development of therapeutic protocols for β-thalassemia.

Novel O-acylated (E)-3-aryl-6,7-dihydrobenzisoxazol-4(5H)-one oximes targeting HSP90-HER2 axis in breast cancer cells

Novel O-acylated (E)-3-aryl-6,7-dihydrobenzisoxazol-4(5H)-one oximes were designed as potential HSP90 inhibitors. A series of the compounds was synthesized by oximation of (E)-3-aryl-6,7-dihydrobenzisoxazol-4(5H)-ones followed by O-acylation with acylamidobenzoic acids. The obtained compounds showed an antiproliferative effect on three breast cancer cell lines (MCF7, MDA-MB-231 and HCC1954). Compound 16s exhibited high antiproliferative potency against HCC1954 breast cancer cells with the IC₅₀ value of 6 µM was selected for in-depth evaluation. Compound 16s did not inhibit the growth of normal epithelial cells. We have demonstrated that the compound 16s can induce apoptosis in cancer cells via inhibition of HSP90 "client" proteins including a key oncogenic receptor, HER2/neu. Described here compounds can be considered for further basic and preclinical investigation as a part of HSP90/HER2-targeted therapies.

Posttranslational modification and beyond: interplay between histone deacetylase 6 and heat-shock protein 90

Posttranslational modification (PTM) and regulation of protein stability are crucial to various biological processes. Histone deacetylase 6 (HDAC6), a unique histone deacetylase with two functional catalytic domains (DD1 and DD2) and a ZnF-UBP domain (ubiquitin binding domain, BUZ), regulates a number of biological processes, including gene expression, cell motility, immune response, and the degradation of misfolded proteins. In addition to the deacetylation of histones, other nonhistone proteins have been identified as substrates for HDAC6. Hsp90, a molecular chaperone that is a critical modulator of cell signaling, is one of the lysine deacetylase substrates of HDAC6. Intriguingly, as one of the best-characterized regulators of Hsp90 acetylation, HDAC6 is the client protein of Hsp90. In addition to regulating Hsp90 at the post-translational modification level, HDAC6 also regulates Hsp90 at the gene transcription level. HDAC6 mainly regulates the Hsp90-HSF1 complex through the ZnF-UBP domain, thereby promoting the HSF1 entry into the nucleus and activating gene transcription. The mutual interaction between HDAC6 and Hsp90 plays an important role in the regulation of protein stability, cell migration, apoptosis and other functions. Plenty of of studies have indicated that blocking HDAC6/Hsp90 has a vital regulatory role in multifarious diseases, mainly in cancers. Therefore, developing inhibitors or drugs against HDAC6/Hsp90 becomes a promising development direction. Herein, we review the current knowledge on molecular regulatory mechanisms based on the interaction of HDAC6 and Hsp90 and inhibition of HDAC6 and/or Hsp90 in oncogenesis and progression, antiviral and immune-related diseases and other vital biological processes.

Pro-apoptotic activity of novel synthetic isoxazole derivatives exhibiting inhibitory activity against tumor cell growth in vitro

In order to develop potential anticancer agents stimulating apoptosis, novel 3,4-isoxazolediamide and 4,5,6,7-tetrahydro-isoxazolo-[4,5-c]-pyridine derivatives have been synthetized. The original structures of geldanamycin and radicicol, which are known natural heat shock protein (HSP) inhibitors, were deeply modified because both of them exhibit several drawbacks, such as poor solubility, hepatotoxicity, intrinsic chemical instability or deprivation of the in vivo activity. This novel class of synthetic compounds containing the isoxazole nucleus exhibited potent and selective inhibition of HSP90 in previous studies. Biological assays (focusing on in vitro antiproliferative effects and pro-apoptotic activity) in human erythroleukemic K562 cells (as a model system referring to tumor cells grown in suspension), glioblastoma U251-MG and glioblastoma temozolomide (TMZ)-resistant T98G cell lines (two model systems referring to tumor cells grown attached to the flask), were performed. Almost all isoxazole derivatives demonstrated significant antiproliferative and pro-apoptotic activities, showing induction of both early and late apoptosis of K562 cells. Different effects were observed on the glioma U251-MG and T98G cells, depending on the structure of the analogues. Antiproliferative and pro-apoptotic activities in K562 cells were associated with the activation of the erythroid differentiation program. The present study demonstrated that 3,4-isoxazolediamide and 4,5,6,7-tetrahydro-isoxazolo-[4,5-c]-pyridine derivatives should be considered for in vivo studies focusing on the development of anticancer drugs acting, at least partially, via activation of apoptosis.

The synthetic and therapeutic expedition of isoxazole and its analogs

Isoxazole, constituting an important family of five-membered heterocycles with one oxygen atom and one nitrogen atom at adjacent positions is of immense importance because of its wide spectrum of biological activities and therapeutic potential. It is, therefore, of prime importance that the development of new synthetic strategies and designing of new isoxazole derivatives should be based on the most recent knowledge emerging from the latest research. This review is an endeavor to highlight the progress in the chemistry and biological activity of isoxazole derivatives which could provide a low-height flying bird's eye view of isoxazole derivatives to the medicinal chemists for the development of clinically viable drugs using this information.

TRAP1: a viable therapeutic target for future cancer treatments?

INTRODUCTION

HSP90 molecular chaperones (i.e., HSP90α, HSP90β, GRP94 and TRAP1) are potential therapeutic targets to design novel anticancer agents. However, despite numerous designed HSP90 inhibitors, most of them have failed due to unfavorable toxicity profiles and lack of specificity toward different HSP90 paralogs. Indeed, a major limitation in this field is the high structural homology between different HSP90 chaperones, which significantly limits our capacity to design paralog-specific inhibitors. Area covered: This review examines the relevance of TRAP1 in tumor development and progression, with an emphasis on its oncogenic/oncosuppressive role in specific human malignancies and its multifaceted and context-dependent functions in cancer cells. Herein, we discuss the rationale for considering TRAP1 as a potential molecular target and the strategies used to date, to achieve its compartmentalized inhibition directly in mitochondria. Expert opinion: TRAP1 targeting may represent a promising strategy for cancer therapy, based on the increasing and compelling evidence supporting TRAP1 involvement in human carcinogenesis. However, considering the complexity of TRAP1 biology, future strategies of drug discovery need to improve selectivity and specificity toward TRAP1 respect to other HSP90 paralogs. The characterization of specific human malignancies suitable for TRAP1 targeting is also mandatory.

3D-QSAR, molecular docking, and molecular dynamic simulations for prediction of new Hsp90 inhibitors based on isoxazole scaffold

Heat shock protein 90(Hsp90), as a molecular chaperone, play a crucial role in folding and proper function of many proteins. Hsp90 inhibitors containing isoxazole scaffold are currently being used in the treatment of cancer as tumor suppressers. Here in the present studies, new compounds based on isoxazole scaffold were predicted using a combination of molecular modeling techniques including three-dimensional quantitative structure-activity relationship (3D-QSAR), molecular docking and molecular dynamic (MD) simulations. Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were also done. The steric and electrostatic contour map of CoMFA and CoMSIA were created. Hydrophobic, hydrogen bond donor and acceptor of CoMSIA model also were generated, and new compounds were predicted by CoMFA and CoMSIA contour maps. To investigate the binding modes of the predicted compounds in the active site of Hsp90, a molecular docking simulation was carried out. MD simulations were also conducted to evaluate the obtained results on the best predicted compound and the best reported Hsp90 inhibitors in the 3D-QSAR model. Findings indicate that the predicted ligands were stable in the active site of Hsp90.

Targeting Class I Histone Deacetylases in a "Complex" Environment

Histone deacetylase (HDAC) inhibitors are proven anticancer therapeutics and have potential in the treatment of many other diseases including HIV infection, Alzheimer's disease, and Friedreich's ataxia. A problem with the currently available HDAC inhibitors is that they have limited specificity and target multiple deacetylases. Designing isoform-selective inhibitors has proven challenging due to similarities in the structure and chemistry of HDAC active sites. However, the fact that HDACs 1, 2, and 3 are recruited to several large multi-subunit complexes, each with particular biological functions, raises the possibility of specifically inhibiting individual complexes. This may be assisted by recent structural and functional information about the assembly of these complexes. Here, we review the available structural information and discuss potential targeting strategies.

Synthesis of 5,6-dihydro-4H-benzo[d]isoxazol-7-one and 5,6-dihydro-4H-isoxazolo[5,4-c]pyridin-7-one derivatives as potential Hsp90 inhibitors

A novel class of 5,6-dihydro-4H-benzo[d]isoxazol-7-ones and 5,6-dihydro-4H-isoxazolo[5,4-c]pyridin-7-ones was designed, synthesized, and assayed to investigate the affinity toward Hsp90 protein. The synthetic route was based on a 1,3-dipolar cycloaddition of nitriloxides, generated in situ from suitable benzaldoximes, with 2-bromocyclohex-2-enones or 3-bromo-5,6-dihydro-1H-pyridin-2-ones. Whereas all the compounds bearing a benzamide group on the bicyclic scaffold were devoid of activity, the derivatives carrying a resorcinol-like fragment showed a remarkable inhibitory effect on Hsp90. Docking calculations were performed to investigate the orientation of the new compounds within the binding site of the enzyme.

Synthesis and biological evaluation of novel Δ2-isoxazoline fused cyclopentane derivatives as potential antimicrobial and anticancer agents

Novel Δ²-isoxazoline fused cyclopentane derivatives have been synthesized and evaluated for their antimicrobial and anticancer activities.

Exploring in vitro and in vivo Hsp90 inhibitors activity against human protozoan parasites

A set of compounds, previously selected as potent Hsp90α inhibitors, has been studied on a panel of human parasites. 5-Aryl-3,4-isoxazolediamide derivatives (1) were active against two protozoa, Trypanosoma brucei rhodesiense and Plasmodium falciparum, with a good tolerability toward cytotoxicity on non-malignant L6 rat myoblast cell line, unlike the 1,5-diaryl,4-carboxamides-1,2,3-triazole derivatives (2) which, while showing a single-digit nM range activity against the same protozoa, were also highly cytotoxic on L6 cells. In a subsequent in vivo study, two isoxazolediamide derivatives, 1a and 1b, were very efficacious on the sleeping sickness-causing agent with a clear parasitaemia during treatment. These data, however, showed that not all protozoa are sensitive to Hsp90 inhibitors, as well as not all Hsp90 inhibitors are equally active on parasites.

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.