Novel HDAC6 isoform selective chiral small molecule histone deacetylase inhibitors

Pharmacophore-based virtual screening of ZINC database, molecular modeling and designing new derivatives as potential HDAC6 inhibitors

To date, many HDAC6 inhibitors have been identified and developed but none is clinically approved as of now. Through this study, we aim to obtain novel HDAC6 selective inhibitors and provide new insights into the detailed structural design of potential HDAC6 inhibitors. A HypoGen-based 3D QSAR HDAC6 pharmacophore was built and used as a query model to screen approximately 8 million ZINC database compounds. First, the ZINC Database was filtered using ADMET, followed by pharmacophore-based library screening. Using fit value and estimated activity cutoffs, a final set of 54 ZINC hits was obtained that were further investigated using molecular docking with the crystal structure of human histone deacetylase 6 catalytic domain 2 in complex with Trichostatin A (PDB ID: 5EDU). Through detailed in silico screening of the ZINC database, we shortlisted three hits as the lead molecules for designing novel HDAC6 inhibitors with better efficacy. Docking with 5EDU, followed by ADMET and TOPKAT analysis of modified ZINC hits provided 9 novel potential HDAC6 inhibitors that possess better docking scores and 2D interactions as compared to the control ZINC hit molecules. Finally, a 50 ns MD analysis run followed by Protein-Ligand Interaction Energy (PLIE) analysis of the top scored hits provided a novel molecule N1 that showed promisingly similar results to that of Ricolinostat (a known HDAC6 inhibitor). The comparable result of the designed hits to established HDAC6 inhibitors suggests that these compounds might prove to be successful HDAC6 inhibitors in future. Designed novel hits that might act as good HDAC6 inhibitors derived from ZINC database using combined molecular docking and modeling approaches.

HDAC6: A unique HDAC family member as a cancer target

BACKGROUND

HDAC6, a structurally and functionally distinct member of the HDAC family, is an integral part of multiple cellular functions such as cell proliferation, apoptosis, senescence, DNA damage and genomic stability, all of which when deregulated contribute to carcinogenesis. Among several HDAC family members known so far, HDAC6 holds a unique position. It differs from the other HDAC family members not only in terms of its subcellular localization, but also in terms of its substrate repertoire and hence cellular functions. Recent findings have considerably expanded the research related to the substrate pool, biological functions and regulation of HDAC6. Studies in HDAC6 knockout mice highlighted the importance of HDAC6 as a cell survival player in stressful situations, making it an important anticancer target. There is ample evidence stressing the importance of HDAC6 as an anti-cancer synergistic partner of many chemotherapeutic drugs. HDAC6 inhibitors have been found to enhance the effectiveness of conventional chemotherapeutic drugs such as DNA damaging agents, proteasome inhibitors and microtubule inhibitors, thereby highlighting the importance of combination therapies involving HDAC6 inhibitors and other anti-cancer agents.

CONCLUSIONS

Here, we present a review on HDAC6 with emphasis on its role as a critical regulator of specific physiological cellular pathways which when deregulated contribute to tumorigenesis, thereby highlighting the importance of HDAC6 inhibitors as important anticancer agents alone and in combination with other chemotherapeutic drugs. We also discuss the synergistic anticancer effect of combination therapies of HDAC6 inhibitors with conventional chemotherapeutic drugs.

Borinostats: solid-phase synthesis of carborane-capped histone deacetylase inhibitors with a tailor-made selectivity profile

The elevated expression of histone deacetylases (HDACs) in various tumor types renders their inhibition an attractive strategy for epigenetic therapeutics. One key issue in the development of improved HDAC inhibitors (HDACis) is the selectivity for single HDAC isoforms over unspecific pan inhibition to minimize off-target toxicity. Utilizing the carborane moiety as a fine-tuning pharmacophore, we herein present a robust solid phase synthetic approach towards tailor-made HDACis meeting both ends of the selectivity spectrum, namely pan inhibition and highly selective HDAC6 inhibition.

Melatonin- and Ferulic Acid-Based HDAC6 Selective Inhibitors Exhibit Pronounced Immunomodulatory Effects In Vitro and Neuroprotective Effects in a Pharmacological Alzheimer's Disease Mouse Model

The structures of melatonin and ferulic acid were merged into tertiary amide-based histone deacetylase 6 (HDAC6) inhibitors to develop multi-target-directed inhibitors for neurodegenerative diseases to incorporate antioxidant effects without losing affinity and selectivity at HDAC6. Structure-activity relationships led to compound 10b as a hybrid molecule showing pronounced and selective inhibition of HDAC6 (IC50 = 30.7 nM, > 25-fold selectivity over other subtypes). This compound shows comparable DPPH radical scavenging ability to ferulic acid, comparable ORAC value to melatonin and comparable Cu²⁺ chelating ability to EDTA. It also lacks neurotoxicity on HT-22 cells, exhibits a pronounced immunomodulatory effect, and is active in vivo showing significantly higher efficacy in an AD mouse model to prevent both Aβ25-35-induced spatial working and long-term memory dysfunction at lower dose (0.3 mg/kg) compared to positive control HDAC6 inhibitor ACY1215 and an equimolar mixture of the three entities ACY1215, melatonin and ferulic acid, suggesting potentially disease-modifying properties.

Recent advances in small molecular modulators targeting histone deacetylase 6

Histone deacetylase 6 (HDAC6) is a unique isozyme in the HDAC family with various distinguished characters. HDAC6 is predominantly localized in the cytoplasm and has several specific nonhistone substrates, such as α-tubulin, cortactin, Hsp90, tau and peroxiredoxins. Accumulating evidence reveals that targeting HDAC6 may serve as a promising therapeutic strategy for the treatment of cancers, neurological disorders and immune diseases, making the development of HDAC6 inhibitors particularly attractive. Recently, multitarget drug design and proteolysis targeting chimera technology have also been applied in the discovery of novel small molecular modulators targeting HDAC6. In this review, we briefly describe the structural features and biological functions of HDAC6 and discuss the recent advances in HDAC6 modulators, including selective inhibitors, chimeric inhibitors and proteolysis targeting chimeras for multiple therapeutic purposes.

Discovery of specific HDAC6 inhibitor with anti-metastatic effects in pancreatic cancer cells through virtual screening and biological evaluation

Histone deacetylase 6 (HDAC6) has been demonstrated to play a major role in cell motility and aggresome formation, and HDAC6 inhibition is therefore considered as a promising epigenetic strategy for cancer treatment. At present, only a minority of compounds have been reported as HDAC6 inhibitors, so specific HDAC6 inhibitors with safety profile need to be discovered urgently. In this paper, HDAC6 inhibitors with diverse structures were used to generate the pharmacophore model by ligand-based method, which contained two hydrogen bond acceptors and two hydrophobic groups. A combined virtual screening based on pharmacophore model and molecular docking was adopted to screen potential HDAC6 inhibitors. Subsequently, the HDAC6 inhibitory activity of the hit compounds were evaluated using an in vitro enzyme binding inhibition assay. The experimental results illustrated that cefoperazone sodium had the strongest inhibitory effect on HDAC6 among the six screened compounds, and its IC₅₀ value was 8.59 ± 1.06 μM. Cefoperazone sodium significantly catalyzed the hyperacetylation of α-tubulin but not histone H3, proving that cefoperazone sodium was a selective inhibitor of HDAC6. Since the expression of HDAC6 plays an important role in cancer metastasis, the effects of cefoperazone sodium on migration and invasion of human pancreatic cancer PANC-1 cells were further investigated by wound healing and transwell chamber assays. It was found that cefoperazone sodium could evidently inhibit the migration and invasion of PANC-1 cells. Furthermore, the binding pattern of inhibitor at the active site of the crystal structure was revealed by molecular docking, providing a reference value for the structural design and optimization of HDAC6 inhibitors. This study provides a systematic virtual screening approach for discovering HDAC6 active inhibitors, and by which the specific effect of cefoperazone sodium against HDAC6 was found, suggesting its potential application on cancer therapy.

Rh(iii)-catalyzed spiroannulation of 3-arylquinoxalin-2(1H)-ones with alkynes: practical access to spiroquinoxalinones

The use of imines as a H acceptor for Rh(iii)-catalyzed spirocyclization of 3-arylquinoxalinones and alkynes via a C–H functionalization/[3 + 2] annulation sequence has been achieved.

HDAC6‑selective inhibitor synergistically enhances the anticancer activity of immunomodulatory drugs in multiple myeloma

Nonselective histone deacetylase (HDAC) inhibitors have therapeutic effects, but exhibit dose‑limiting toxicities in patients with multiple myeloma (MM). The present study investigated the interaction between the HDAC6 inhibitor, A452, and immunomodulatory drugs (IMiDs) on dexamethasone (Dex)‑sensitive and ‑resistant MM cells compared with the current clinically tested HDAC6 inhibitor, ACY‑1215. It was shown that the combination of the HDAC6‑selective inhibitor, A452, with either of the IMiDs tested (lenalidomide or pomalidomide) led to the synergistic inhibition of cell growth, a decrease in the viability of MM cells and in an increase in the levels of apoptosis. Furthermore, enhanced cell death was associated with the inactivation of AKT and extracellular signal‑regulated kinase (ERK)1/2. Of note, A452 in combination with IMiDs induced synergistic MM cytotoxicity without altering the expression of cereblon and thereby, the synergistic downregulation of IKAROS family zinc finger (IKZF)1/3, c‑Myc and interferon regulatory factor 4 (IRF4). Furthermore, combined treatment with A452 and IMiDs induced the synergistic upregulation of PD‑L1. More importantly, this combination treatment was effective in the Dex‑resistant MM cells. Overall, the findings of this study indicate that A452 is more effective as an anticancer agent than ACY‑1215. Taken together, these findings suggest that a combination of the HDAC6‑selective inhibitor, A452, and IMiDs may prove to be beneficial in the treatment of patients with MM.

Highly efficient Ir-catalyzed asymmetric hydrogenation of benzoxazinones and derivatives with a Brønsted acid cocatalyst

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, affording various chiral dihydrobenzoxazinones and derivatives with excellent results.

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, which may display a new activation mode with a single anion-binding interaction among the substrate, cocatalyst Brønsted acid and ligand. This synthetic approach afforded a series of chiral dihydrobenzoxazinones and derivatives with excellent results (>99% conversion, 88–96% yields, 91–>99% ee, up to 40 500 TON). A key to success is the utilization of a strong Brønsted acid as the cocatalyst, such as hydrochloric acid, to form a possible single anion-binding interaction with the substrate and catalyst, which greatly contributed to the improvement of reactivity and enantioselectivity. Importantly, a creative and efficient synthetic route was developed to construct the important intermediate for the potential IgE/IgG receptor modulator through our catalytic methodology system.

Synthesis and Investigation of Therapeutic Potential of Isoform-Specific HDAC8 Inhibitors for the Treatment of Cutaneous T Cell Lymphoma

BACKGROUND

The available treatment option for any type of cancer including CTCL is chemotherapy and radiation therapy which indiscriminately persuade on the normal cells. One way out for selective destruction of CTCL cells without damaging normal cells is the use of histone deacetylase inhibitors (HDACi). Despite promising results in the treatment of CTCL, these HDACi have shown a broadband inhibition profile, moderately selective for one HDAC class but not for a particular isotype. The prevalence of drug-induced side effects leaves open a narrow window of speculation that the decreased therapeutic efficacy and observed side effects may be most likely due to non specific HDAC isoform inhibition. The aim of this paper is to synthesis and evaluates HDAC8 isoform specific inhibitors.

METHODS

Based on the preliminary report on the design and in silico studies of 52 hydroxamic acid derivatives bearing multi-substituent heteroaromatic rings with chiral amine linker, five compounds were shortlisted and synthesized by microwave assisted approach and high yielding synthetic protocol. A series of in vitro assays in addition to HDAC8 inhibitory activity was used to evaluate the synthesised compounds.

RESULTS

Inhibitors 1e, 2e, 3e, 4e and 5e exerted the anti-proliferative activities against CTCL cell lines at 20- 100 µM concentrations. Both the pyrimidine- and pyridine-based probes exhibited μM inhibitory activity against HDAC8. The pyrimidine-based probe 1e displayed remarkable HDAC8 selectivity superior to that of the standard drug, SAHA with an IC50 at 0.1µM.

CONCLUSION

Our study demonstrated that simple modifications at different portions of pharmacophore in the hydroxamic acid analogues are effective for improving both HDAC8 inhibitory activity and isoform selectivity. Potent and highly isoform-selective HDAC8 inhibitors were identified. These findings would be expedient for further development of HDAC8-selective inhibitors.

The HDAC6-selective inhibitor is effective against non-Hodgkin lymphoma and synergizes with ibrutinib in follicular lymphoma

Follicular lymphoma (FL) is the most common indolent B-cell non-Hodgkin lymphoma (NHL) with genetic alterations of BCL-2, KMT2B, and KMT6. FL is refractory to conventional chemotherapy and is still incurable in most patients. Thus, new drugs and/or novel combination treatment strategies are needed to further improve FL patient outcome. We investigated the efficacy of the histone deacetylase 6 (HDAC6) inhibitor A452 combined with a Bruton's tyrosine kinase (BTK) inhibitor ibrutinib on NHL and the underlying mechanisms compared with the current clinically tested HDAC6 inhibitor ACY-1215. We first showed that FL is the most sensitive to HDAC6 inhibitor. We showed that combining A452 with ibrutinib led to the synergistic inhibition of cell growth and decreased viability of FL cells, as well as increased levels of apoptosis. Similar synergistic interactions occur in chronic lymphocytic leukemia (CLL) and germinal center diffuse large B-cell lymphoma cells (DLBCL). Enhanced cell death is associated with AKT and ERK1/2 inactivation and increased DNA damage (induction of γH2A.X and reduction of pChk1/2). In addition, A452 downregulates c-Myc, an effect significantly enhanced by ibruninib. Although ACY-1215 is less potent than A452, it displays synergism with ibrutinib. Overall, our results suggest that A452 is more effective as an anticancer agent than ACY-1215 in FL. These findings suggest that a combination of HDAC6-selective inhibitor and ibrutinib is a potent therapeutic strategy for NHL including FL.

Combined comparative molecular field analysis, comparative molecular similarity indices analysis, molecular docking and molecular dynamics studies of histone deacetylase 6 inhibitors

Human histone deacetylase isoform 6 (HDAC6) has been shown to have an immense role in cell motility and aggresome formation and is being an attractive selective target for the treatment of multiple tumour types and neurodegenerative conditions. The discovery of selective HDAC6 inhibitors with new chemical functionalities is therefore of utmost interest to researchers. In order to examine the structural requirements for HDAC6-specific inhibitors and to derive predictive model which can be used for designing new selective HDAC6 inhibitors, a three-dimensional quantitative structure-activity relationship study was carried out on a diverse set of ligands using common feature-based pharmacophore alignment followed by employing comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) techniques. The models displayed high correlation of 0.978 and 0.991 for best CoMFA and CoMSIA models, respectively, and a good statistical significance. The model could be used for predicting activities of the test set compounds as well as for deriving useful information regarding steric, electrostatic, hydrophobic properties of the molecules used in this study. Further, the training and test set molecules were docked into the HDAC6 binding site and molecular dynamics was carried out to suggest structural requirements for design of new inhibitors.

Efficient access to chiral dihydrobenzoxazinones via Rh-catalyzed hydrogenation

Rh/(S)-DTBM-SegPhos-catalyzed asymmetric hydrogenation of prochiral (Z)-2-(2-oxo-2H-benzo[b][1,4]oxazin-3(4H)-ylidene)acetate esters was successfully developed. A series of chiral dihydrobenzoxazinones were prepared through this efficient methodology with good to excellent results (up to >99% conversion, 93% yield and >99% ee), which are important motifs in the biologically active molecules.

Rh/(S)-DTBM-SegPhos-catalyzed asymmetric hydrogenation of prochiral (Z)-2-(2-oxo-2H-benzo[b][1,4]oxazin-3(4H)-ylidene)acetate esters was successfully developed to prepare various chiral dihydrobenzoxazinones with good to excellent results.

Histone deacetylase 8 (HDAC8) and its inhibitors with selectivity to other isoforms: An overview

The histone deacetylases (HDACs) enzymes provided crucial role in transcriptional regulation of cells through deacetylation of nuclear histone proteins. Discoveries related to the HDAC8 enzyme activity signified the importance of HDAC8 isoform in cell proliferation, tumorigenesis, cancer, neuronal disorders, parasitic/viral infections and other epigenetic regulations. The pan-HDAC inhibitors can confront these conditions but have chances to affect epigenetic functions of other HDAC isoforms. Designing of selective HDAC8 inhibitors is a key feature to combat the pathophysiological and diseased conditions involving the HDAC8 activity. This review is concerned about the structural and positional aspects of HDAC8 in the HDAC family. It also covers the contributions of HDAC8 in the pathophysiological conditions, a preliminary discussion about the recent scenario of HDAC8 inhibitors. This review might help to deliver the structural, functional and computational information in order to identify and design potent and selective HDAC8 inhibitors for target specific treatment of diseases involving HDAC8 enzymatic activity.

Efficient modular phosphorus-containing ligands for stereoselective catalysis

Over several years, our research team has contributed to ligand design for metal-catalyzed stereoselective transformations. This has been achieved with the synthesis and application to organic transformations of interest of an array of structurally diverse P-containing ligands. These range from highly modular enantiopure phosphine–phosphite ligands to supramolecularly regulated enantioselective phosphorus-based catalysts. Our research in supramolecular interactions has also led to the discovery of an unprecedented halogen-bonded rhodium-catalyst.

A452, an HDAC6-selective inhibitor, synergistically enhances the anticancer activity of chemotherapeutic agents in colorectal cancer cells

Although histone deacetylase inhibitors (HDACi) alone could be clinically useful, these are most recently used in combination with other anticancer agents in clinical trials for cancer treatment. Recently, we reported the anticancer activity of an HDAC6-selective inhibitor A452 toward various cancer cell types. This study aims to present a potent synergistic antiproliferative effect of A452/anticancer agent treatment in colorectal cancer cells (CRC) cells, independently of the p53 status. A452 in combination with irinotecan, or SAHA is more potent than either drug alone in the apoptotic pathway as evidenced by activated caspase-3 and PARP, increased Bak and pp38, decreased Bcl-xL, pERK, and pAKT, and induced apoptotic cells. Furthermore, A452 enhances DNA damage induced by anticancer agents as indicated by the increased accumulation of γH2AX and the activation of the checkpoint kinase Chk2. The silencing of HDAC6 enhances the cell growth inhibition and cell death caused by anticancer agents. In addition, A452 induces the synergistic suppression of cell migration and invasion. This study suggests a mechanism by which HDAC6-selective inhibition can enhance the efficacy of specific anticancer agents in CRC cells.

The HDAC6 inhibitor ACY‑1215 enhances the anticancer activity of oxaliplatin in colorectal cancer cells

ACY‑1215, also known as ricolinostat, is a leading histone deacetylase 6 inhibitor, which is currently being tested in clinical trials for hematological malignancies. Previous studies have reported that ACY‑1215 is not potent enough as a monotherapy for the treatment of colorectal cancer (CRC), which generally requires combination therapy for successful treatment. Therefore, the present study aimed to determine whether the synergistic interaction detected between ACY‑1215 and anticancer agents in hematological cancers could occur in solid tumors. The results of the present study indicated that ACY‑1215 exerted a potent synergistic anti-proliferative effect when used in combination with anticancer agents in CRC cells. The combination of ACY‑1215 and oxaliplatin was more potent than either drug alone, as indicated by an increase in apoptotic cells and their effects on the apoptotic pathway; ACY‑1215 and oxaliplatin cotreatment activated caspase‑3 and poly (ADP ribose) polymerase, increased B‑cell lymphoma (Bcl)‑2 homologous antagonist/killer expression, and decreased Bcl‑extra large protein, phosphorylated-extracellular signal-regulated kinase and phosphorylated-protein kinase B expression. In addition, combined treatment of ACY‑1215 and anticancer agents induced synergistic upregulation of programmed death‑ligand 1. These findings suggested that a therapeutic strategy that combines ACY‑1215 and oxaliplatin warrants attention for the treatment of solid tumors, including CRC.

Design, synthesis, biological evaluation and in vivo testing of dual phosphodiesterase 5 (PDE5) and histone deacetylase 6 (HDAC6)-selective inhibitors for the treatment of Alzheimer's disease

We have identified chemical probes that act as dual phosphodiesterase 5 (PDE5) and histone deacetylase 6 (HDAC6)-selective inhibitors (>1 log unit difference versus class I HDACs) to decipher the contribution of HDAC isoforms to the positive impact of dual-acting PDE5 and HDAC inhibitors on mouse models of Alzheimer's disease (AD) and fine-tune this systems therapeutics approach. Structure- and knowledge-based approaches led to the design of first-in-class molecules with the desired target compound profile: dual PDE5 and HDAC6-selective inhibitors. Compound 44b, which fulfilled the biochemical, functional and ADME-Tox profiling requirements and exhibited adequate pharmacokinetic properties, was selected as pharmacological tool compound and tested in a mouse model of AD (Tg2576) in vivo.

Structure–activity relationships of hydroxamate-based histone deacetylase-8 inhibitors: reality behind anticancer drug discovery

The pan-histone deacetylase (HDAC) inhibitors comprise a fish-like structural orientation where hydrophobic aryl- and zinc-binding groups act as head and tail, respectively of a fish. The linker moiety correlates the body of the fish linking head and tail groups. Despite these pan-HDAC inhibitors, selective HDAC-8 inhibitors are still in demand as a safe remedy. HDAC-8 is involved in invasion and metastasis in cancer. This review deals with the rationale behind HDAC-8 inhibitory activity and selectivity along with detailed structure–activity relationships of diverse hydroxamate-based HDAC-8 inhibitors. HDAC-8 inhibitory potency may be increased by modifying the fish-like pharmacophoric features of such type of pan-HDAC inhibitors. This review may provide a preliminary basis to design and optimize new lead molecules with higher HDAC-8 inhibitory activity. This work may surely enlighten in providing useful information in the field of target-specific anticancer therapy.

Anti-cancer effects of naturally derived compounds targeting histone deacetylase 6-related pathways

Alterations of the epigenetic machinery, affecting multiple biological functions, represent a major hallmark enabling the development of tumors. Among epigenetic regulatory proteins, histone deacetylase (HDAC)6 has emerged as an interesting potential therapeutic target towards a variety of diseases including cancer. Accordingly, this isoenzyme regulates many vital cellular regulatory processes and pathways essential to physiological homeostasis, as well as tumor multistep transformation involving initiation, promotion, progression and metastasis. In this review, we will consequently discuss the critical implications of HDAC6 in distinct mechanisms relevant to physiological and cancerous conditions, as well as the anticancer properties of synthetic, natural and natural-derived compounds through the modulation of HDAC6-related pathways.

Recent advances in the discovery of potent and selective HDAC6 inhibitors

Histone deacetylase HDAC6, a member of the class IIb HDAC family, is unique among HDAC enzymes in having two active catalytic domains, and has unique physiological function. In addition to the modification of histone, HDAC6 targets specific substrates including α-tubulin and HSP90, and are involved in protein trafficking and degradation, cell shape and migration. Selective HDAC6 inhibitors are an emerging class of pharmaceuticals due to the involvement of HDAC6 in different pathways related to neurodegenerative diseases, cancer, and immunology. Therefore, extensive investigations have been made in the discovery of selective HDAC6 inhibitors. Based on their different zinc binding groups (ZBGs), in this review, HDAC6 inhibitors are grouped as hydroxamic acids, a sulfur containing ZBG based derivatives and other ZBG-derived compounds, and their enzymatic inhibitory activity, selectivity and other biological activities are introduced and summarized.

The therapeutic hope for HDAC6 inhibitors in malignancy and chronic disease

Recent years have witnessed an emergence of a new class of therapeutic agents, termed histone deacetylase 6 (HDAC6) inhibitors. HDAC6 is one isoform of a family of HDAC enzymes that catalyse the removal of functional acetyl groups from proteins. It stands out from its cousins in almost exclusively deacetylating cytoplasmic proteins, in exerting deacetylation-independent effects and in the success that has been achieved in developing relatively isoform-specific inhibitors of its enzymatic action that have reached clinical trial. HDAC6 plays a pivotal role in the removal of misfolded proteins and it is this role that has been most successfully targeted to date. HDAC6 inhibitors are being investigated for use in combination with proteasome inhibitors for the treatment of lymphoid malignancies, whereby HDAC6-dependent protein disposal currently limits the cytotoxic effectiveness of the latter. Similarly, numerous recent studies have linked altered HDAC6 activity to the pathogenesis of neurodegenerative diseases that are characterized by misfolded protein accumulation. It seems likely though that the function of HDAC6 is not limited to malignancy and neurodegeneration, the deacetylase being implicated in a number of other cellular processes and diseases including in cardiovascular disease, inflammation, renal fibrosis and cystogenesis. Here, we review the unique features of HDAC6 that make it so appealing as a drug target and its currently understood role in health and disease. Whether HDAC6 inhibition will ultimately find a clinical niche in the treatment of malignancy or prevalent complex chronic diseases remains to be determined.

The Search for Potent, Small-Molecule HDACIs in Cancer Treatment: A Decade After Vorinostat

Histone deacetylases (HDACs) play a crucial role in the remodeling of chromatin, and are involved in the epigenetic regulation of gene expression. In the last decade, inhibition of HDACs came out as a target for specific epigenetic changes associated with cancer and other diseases. Until now, more than 20 HDAC inhibitors (HDACIs) have entered clinical studies, and some of them (e.g., vorinostat, romidepsin) have been approved for the treatment of cutaneous T-cell lymphoma. This review provides an overview of current knowledge, progress, and molecular mechanisms of HDACIs, covering a period from 2011 until 2015.

Sn(ii)-Mediated facile approach for the synthesis of 2-aryl-2H-indazole-3-phosphonates and their anticancer activities

The efficient synthesis of 2-aryl-2H-indazole-3-phosphonates has been achieved successfully via a SnCl₂·2H₂O mediated one-pot method.

Ultrasound mediated, green innovation for the synthesis of polysubstituted 1,4-dihydropyridines

An atom efficient protocol via a one-pot four-component cyclocondensation reaction catalyzed by copper(i) iodide in aqueous medium under ultrasonic irradiation has been developed for the synthesis of novel polysubstituted 1,4-dihydropyridines.

Rhodium-catalyzed asymmetric arylation of N- and O-containing cyclic aldimines: facile and efficient access to highly optically active 3,4-dihydrobenzo[1,4]oxazin-2-ones and dihydroquinoxalinones

A highly enantioselective Rh-catalyzed arylation of benzoxazinones and quinoxalinones with arylboroxines was realized for the first time by employing a simple sulfur-olefin as the ligand.

Histone deacetylase 6 in health and disease

Histone deacetylase (HDAC)6 is a member of the class IIb HDAC family. This enzyme is zinc-dependent and mainly localized in the cytoplasm. HDAC6 is a unique isoenzyme with two functional catalytic domains and specific physiological roles. Indeed, HDAC6 deacetylates various substrates including α-tubulin and HSP90α, and is involved in protein trafficking and degradation, cell shape and migration. Consequently, deregulation of HDAC6 activity was associated to a variety of diseases including cancer, neurodegenerative diseases and pathological autoimmune response. Therefore, HDAC6 represents an interesting potential therapeutic target. In this review, we discuss structural features of this histone deacetylase, regulation of its expression and activity, biological functions, implication in human disease initiation and progression. Finally will describe novel and selective HDAC6 inhibitors.

Microwave-Assisted Multicomponent Synthesis of Dihydroquinoxalinones on Soluble Polymer Support

A one-pot and two-step reaction of four components, including aldehydes, Fmoc-protected α-amino acid, isocyanide, and soluble polymer-supported 4-fluoro-3-amino benzoate ester, was developed for an efficient synthesis of dihydroquinoxalinones under microwave irradiation. Fmoc deprotection followed by intramolecular cyclization of the diamide gave a facile access to novel bicyclic quinoxalin-2-ones. On the basis of this approach, a new route to synthesize this privileged scaffold from the diamide intermediate was designed and accomplished with high yields. Use of multicomponent reaction (MCR) has been shown to display a good functional group tolerance, while the product isolation and purification is straightforward by precipitation and washings. Current library discusses the synthesis and diversification of 21 compounds produced using this strategy.

Synthesis of 1- and 4-substituted piperazin-2-ones via Jocic-type reactions with N-substituted diamines

Enantiomerically-enriched trichloromethyl-containing alcohols are transformed regioselectively into enantiomerically-enriched 1-substituted piperazinones by modified Jocic reactions.

Synthesis of benzothiophene-based hydroxamic acids as potent and selective HDAC6 inhibitors

A small library of 3-[(4-hydroxycarbamoylphenyl)aminomethyl]benzothiophenes was prepared, leading to the identification of three representatives as potent and selective HDAC6 inhibitors.

Ligand Based Pharmacophore Modeling and Virtual Screening Studies to Design Novel HDAC2 Inhibitors

Histone deacetylases 2 (HDAC2), Class I histone deacetylase (HDAC) family, emerged as an important therapeutic target for the treatment of various cancers. A total of 48 inhibitors of two different chemotypes were used to generate pharmacophore model using 3D QSAR pharmacophore generation (HypoGen algorithm) module in Discovery Studio. The best HypoGen model consists of four pharmacophore features namely, one hydrogen bond acceptor (HBA), and one hydrogen donor (HBD), one hydrophobic (HYP) and one aromatic centres, (RA). This model was validated against 20 test set compounds and this model was utilized as a 3D query for virtual screening to validate against NCI and Maybridge database and the hits further screened by Lipinski's rule of 5, and a total of 382 hit compounds from NCI and 243 hit compounds from Maybridge were found and were subjected to molecular docking in the active site of HDAC2 (PDB: 3MAX). Finally eight hit compounds, NSC108392, NSC127064, NSC110782, and NSC748337 from NCI database and MFCD01935795, MFCD00830779, MFCD00661790, and MFCD00124221 from Maybridge database, were considered as novel potential HDAC2 inhibitors.

Small molecule inhibitors of zinc-dependent histone deacetylases

Lysine acetylation is an ancient, evolutionarily conserved, reversible post-translational modification. A multitude of diverse cellular functions are regulated by this dynamic modification, including energy and metabolism, protein folding, transcription, and translation. Gene expression can be manipulated through changes in histone acetylation status, and this process is controlled by the function of 2 opposing enzymes: histone acetyl transferases and histone deacetylases (HDACs). The zinc-dependent HDACs are a family of hydrolases that remove acetyl groups from lysines, and their function can be modulated by the action of small molecule ligands. Inhibition through competitive binding of the catalytic domain of these enzymes has been achieved by a diverse array of small molecule chemotypes. Structural biology has aided the development of potent, and in some cases highly isoform-selective, inhibitors that have demonstrated utility in a number of neurological disease models. Continued development and characterization of highly optimized small molecule inhibitors of HDAC enzymes will help refine our understanding of their function and, optimistically, lead to novel therapeutic treatment alternatives for a host of neurological disorders.

Development of a histone deacetylase 6 inhibitor and its biological effects

Development of isoform-selective histone deacetylase (HDAC) inhibitors is important in elucidating the function of individual HDAC enzymes and their potential as therapeutic agents. Among the eleven zinc-dependent HDACs in humans, HDAC6 is structurally and functionally unique. Here, we show that a hydroxamic acid-based small-molecule N-hydroxy-4-(2-[(2-hydroxyethyl)(phenyl)amino]-2-oxoethyl)benzamide (HPOB) selectively inhibits HDAC6 catalytic activity in vivo and in vitro. HPOB causes growth inhibition of normal and transformed cells but does not induce cell death. HPOB enhances the effectiveness of DNA-damaging anticancer drugs in transformed cells but not normal cells. HPOB does not block the ubiquitin-binding activity of HDAC6. The HDAC6-selective inhibitor HPOB has therapeutic potential in combination therapy to enhance the potency of anticancer drugs.

Development and therapeutic implications of selective histone deacetylase 6 inhibitors

This Perspective provides an in depth look at the numerous disease states in which histone deacetylase 6 (HDAC6) has been implicated. The physiological pathways, protein-protein interactions, and non-histone substrates relating to different pathological conditions are discussed with regard to HDAC6. Furthermore, the compounds and methods used to modulate HDAC6 activity are profiled. The latter half of this Perspective analyzes reported HDAC6 selective inhibitors in terms of structure, potency, and selectivity over the other HDAC isoforms with the intent of providing a comprehensive overview of the molecular tools available. Potential obstacles and future directions of HDAC6 research are also presented.

HDAC6 and ovarian cancer

The special class IIb histone deacetylase, HDAC6, plays a prominent role in many cellular processes related to cancer, including oncogenesis, the cell stress response, motility, and myriad signaling pathways. Many of the lessons learned from other cancers can be applied to ovarian cancer as well. HDAC6 interacts with diverse proteins such as HSP90, cortactin, tubulin, dynein, p300, Bax, and GRK2 in both the nucleus and cytoplasm to carry out these cancerous functions. Not all pro-cancer interactions of HDAC6 involve deacetylation. The idea of using HDAC6 as a target for cancer treatment continues to expand in recent years, and more potent and specific HDAC6 inhibitors are required to effectively down-regulate the tumor-prone cell signaling pathways responsible for ovarian cancer.