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

A patent review of histone deacetylase 8 (HDAC8) inhibitors (2013-present)

INTRODUCTION

The processes and course of several fatal illnesses, such as cancer, inflammatory diseases, and neurological disorders are closely correlated with HDAC8. Therefore, novel HDAC8 inhibitors represent effective therapeutic possibilities that may help treat these conditions. To yet, there are not any such particular HDAC8 inhibitors available for sale. This review was conducted to examine recent HDAC8 inhibitors that have been patented over the last 10 years.

AREAS COVERED

This review focuses on HDAC8 inhibitor-related patents and their therapeutic applications that have been published within the last 10 years and are accessible through the Patentscope and Google Patents databases.

EXPERT OPINION

A handful of HDAC8 inhibitor-related patents have been submitted over the previous 10 years, more selective, and specific HDAC8 inhibitors that are intended to treat a variety of medical diseases. This could lead to the development of novel treatment approaches that target HDAC8. Employing theoretical frameworks and experimental procedures can reveal the creation of new HDAC8 inhibitors with enhanced pharmacokinetic characteristics. A thorough understanding of the role that HDAC8 inhibitors play in cancer, including the mechanisms behind HDAC8 in other disorders is necessary.

Targeting histone deacetylases in head and neck squamous cell carcinoma: molecular mechanisms and therapeutic targets

The onerous health and economic burden associated with head and neck squamous cell carcinoma (HNSCC) is a global predicament. Despite the advent of novel surgical techniques and therapeutic protocols, there is an incessant need for efficacious diagnostic and therapeutic targets to monitor the invasion, metastasis and recurrence of HNSCC due to its substantial morbidity and mortality. The differential expression patterns of histone deacetylases (HDACs), a group of enzymes responsible for modifying histones and regulating gene expression, have been demonstrated in neoplastic tissues. However, there is limited knowledge regarding the role of HDACs in HNSCC. Consequently, this review aims to summarize the existing research findings and explore the potential association between HDACs and HNSCC, offering fresh perspectives on therapeutic approaches targeting HDACs that could potentially enhance the efficacy of HNSCC treatment. Additionally, the Cancer Genome Atlas (TCGA) dataset, CPTAC, HPA, OmicShare, GeneMANIA and STRING databases are utilized to provide supplementary evidence on the differential expression of HDACs, their prognostic significance and predicting functions in HNSCC patients.

Design, Synthesis, and Biological Evaluation of Novel Quinazolin-4(3H)-One-Based Histone Deacetylase 6 (HDAC6) Inhibitors for Anticancer Activity

A series of novel quinazoline-4-(3H)-one derivatives were designed and synthesized as histone deacetylase 6 (HDAC6) inhibitors based on novel quinazoline-4-(3H)-one as the cap group and benzhydroxamic acid as the linker and metal-binding group. A total of 19 novel quinazoline-4-(3H)-one analogues (5a-5s) were obtained. The structures of the target compounds were characterized using ¹H-NMR, ¹³C-NMR, LC-MS, and elemental analyses. Characterized compounds were screened for inhibition against HDAC8 class I, HDAC4 class IIa, and HDAC6 class IIb. Among the compounds tested, 5b proved to be the most potent and selective inhibitor of HDAC6 with an IC₅₀ value 150 nM. Some of these compounds showed potent antiproliferative activity in several tumor cell lines (HCT116, MCF7, and B16). Amongst all the compounds tested for their anticancer effect against cancer cell lines, 5c emerged to be most active against the MCF-7 line with an IC₅₀ of 13.7 μM; it exhibited cell-cycle arrest in the G2 phase, as well as promoted apoptosis. Additionally, we noted a significant reduction in the colony-forming capability of cancer cells in the presence of 5c. At the intracellular level, selective inhibition of HDAC6 was enumerated by monitoring the acetylation of α-tubulin with a limited effect on acetyl-H3. Importantly, the obtained results suggested a potent effect of 5c at sub-micromolar concentrations as compared to the other molecules as HDAC6 inhibitors in vitro.

Are inhibitors of histone deacetylase 8 (HDAC8) effective in hematological cancers especially acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL)?

Histone deacetylase 8 (HDAC8) aberrantly deacetylates histone and non-histone proteins. These include structural maintenance of chromosome 3 (SMC3) cohesin protein, retinoic acid induced 1 (RAI1), p53, etc and thus, regulating diverse processes such as leukemic stem cell (LSC) transformation and maintenance. HDAC8, one of the crucial HDACs, affects the gene silencing process in solid and hematological cancer progressions especially on acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). A specific HDAC8 inhibitor PCI-34051 showed promising results against both T-cell lymphoma and AML. Here, we summarize the role of HDAC8 in hematological malignancies, especially in AML and ALL. This article also introduces the structure/function of HDAC8 and a special attention has been paid to address the HDAC8 enzyme selectivity issue in hematological cancer especially against AML and ALL.

Exploring structural requirements of HDAC10 inhibitors through comparative machine learning approaches

Histone deacetylase (HDAC) inhibitors are in the limelight of anticancer drug development and research. HDAC10 is one of the class-IIb HDACs, responsible for cancer progression. The search for potent and effective HDAC10 selective inhibitors is going on. However, the absence of human HDAC10 crystal/NMR structure hampers the structure-based drug design of HDAC10 inhibitors. Different ligand-based modeling techniques are the only hope to speed up the inhibitor design. In this study, we applied different ligand-based modeling techniques on a diverse set of HDAC10 inhibitors (n = 484). Machine learning (ML) models were developed that could be used to screen unknown compounds as HDAC10 inhibitors from a large chemical database. Moreover, Bayesian classification and Recursive partitioning models were used to identify the structural fingerprints regulating the HDAC10 inhibitory activity. Additionally, a molecular docking study was performed to understand the binding pattern of the identified structural fingerprints towards the active site of HDAC10. Overall, the modeling insight might offer helpful information for medicinal chemists to design and develop efficient HDAC10 inhibitors.

HDAC8 as an emerging target in drug discovery with special emphasis on medicinal chemistry

HDAC8 catalyzes the deacetylation of both histones and nonhistone proteins. The abnormal expression of HDAC8 is associated with various pathological conditions causing cancer and other diseases like myopathies, Cornelia de Lange syndrome, renal fibrosis, and viral and parasitic infections. The substrates of HDAC8 are involved in diverse molecular mechanisms of cancer such as cell proliferation, invasion, metastasis and drug resistance. Based on the crystal structures and the key residues at the active site, HDAC8 inhibitors have been designed along the canonical pharmacophore. This article details the importance, recent advancements, and the structural and functional aspects of HDAC8 with special emphasis on the medicinal chemistry aspect of HDAC8 inhibitors that will help in developing novel epigenetic therapeutics.

HDAC8 Promotes Liver Metastasis of Colorectal Cancer via Inhibition of IRF1 and Upregulation of SUCNR1

Histone deacetylases (HDACs) are well-characterized for their involvement in tumor progression. Herein, the current study set out to unravel the association of HDAC8 with colorectal cancer (CRC). Bioinformatics analyses were carried out to retrieve the expression patterns of HDAC8 in CRC and the underlying mechanism. Following expression determination, the specific roles of HDAC8, IRF1, and SUCNR1 in CRC cell functions were analyzed following different interventions. Additionally, tumor formation and liver metastasis in nude mice were operated to verify the fore experiment. Bioinformatics analyses predicted the involvement of the HDAC8/IRF1/SUCNR1 axis in CRC. In vitro cell experiments showed that HDAC8 induced the CRC cell growth by reducing IRF1 expression. Meanwhile, IRF1 limited SUCNR1 expression by binding to its promoter. SUCNR1 triggered the growth and metastasis of CRC by inhibiting cell autophagy. HDAC8 blocked IRF1-mediated SUCNR1 inhibition and thereby inhibited autophagy, accelerating CRC cell growth. Lastly, HDAC8 facilitated the development of CRC and liver metastasis by regulating the IRF1/SUCNR1 axis in vivo. Taken together, our findings highlighted the critical role for the HDAC8/IRF1/SUCNR1 axis in the regulation of autophagy and the resultant liver metastasis in CRC.

Synthesis, anticancer activity, SAR and binding mode of interaction studies of substituted pentanoic acids: part II

Aim: Our previous results suggest that phenyl/naphthylacetyl pentanoic acid derivatives may exhibit dual MMP-2 and HDAC8 inhibitory activities and show effective cytotoxic properties. Methodology: Here, 13 new compounds (C1-C13) were synthesized and characterized. Along with these new compounds, 16 previously reported phenyl/napthylacetyl pentanoic acid derivatives (C14-C29) were biologically evaluated. Results: Compounds C6 and C27 showed good cytotoxicity against leukemia cell line Jurkat E6.1. The mechanisms of cytotoxicity of these compounds were confirmed by DNA deformation assay and reactive oxygen species assay. MMP-2 and HDAC8 expression assays suggested the dual inhibiting property of these two compounds. These findings were supported by results of molecular docking studies. In silico pharmacokinetic properties showed compounds C6 and C27 have high gastrointestinal absorption. Conclusion: This study highlights the action of phenyl/naphthylacetyl pentanoic acid derivatives as anticancer agents.

Dissecting Histone Deacetylase 3 in Multiple Disease Conditions: Selective Inhibition as a Promising Therapeutic Strategy

The acetylation of histone and non-histone proteins has been implicated in several disease states. Modulation of such epigenetic modifications has therefore made histone deacetylases (HDACs) important drug targets. HDAC3, among various class I HDACs, has been signified as a potentially validated target in multiple diseases, namely, cancer, neurodegenerative diseases, diabetes, obesity, cardiovascular disorders, autoimmune diseases, inflammatory diseases, parasitic infections, and HIV. However, only a handful of HDAC3-selective inhibitors have been reported in spite of continuous efforts in design and development of HDAC3-selective inhibitors. In this Perspective, the roles of HDAC3 in various diseases as well as numerous potent and HDAC3-selective inhibitors have been discussed in detail. It will surely open up a new vista in the discovery of newer, more effective, and more selective HDAC3 inhibitors.

Discovery of novel N-substituted thiazolidinediones (TZDs) as HDAC8 inhibitors: in-silico studies, synthesis, and biological evaluation

Epigenetics plays a fundamental role in cancer progression, and developing agents that regulate epigenetics is crucial for cancer management. Among Class I and Class II HDACs, HDAC8 is one of the essential epigenetic players in cancer progression. Therefore, we designed, synthesized, purified, and structurally characterized novel compounds containing N-substituted TZD (P1-P25). Cell viability assay of all compounds on leukemic cell lines (CEM, K562, and KCL22) showed the cytotoxic potential of P8, P9, P10, P12, P19, and P25. In-vitro screening of different HDACs isoforms revealed that P19 was the most potent and selective inhibitor for HDAC8 (IC50 - 9.3 μM). Thermal shift analysis (TSA) confirmed the binding of P19 to HDAC8. In-vitro screening of all compounds on the transport activity of GLUT1, GLUT4, and GLUT5 indicated that P19 inhibited GLUT1 (IC50 - 28.2 μM). P10 and P19 induced apoptotic cell death in CEM cells (55.19% and 60.97% respectively) and P19 was less cytotoxic on normal WBCs (CC50 - 104.2 μM) and human fibroblasts (HS27) (CC50 - 105.0 μM). Thus, among this novel series of TZD derivatives, compound P19 was most promising HDAC8 inhibitor and cytotoxic on leukemic cells. Thus, P19 could serve as a lead for further development of optimized molecules with enhanced selectivity and potency.

Discriminations of active from inactive HDAC8 inhibitors Part II: Bayesian classification study to find molecular fingerprints

In continuation of our earlier work (Doi: 10.1080/07391102.2019.1661876), a statistically validated and robust Bayesian model was developed on a large diverse set of HDAC8 inhibitors. The training set comprised of 676 small molecules and 293 compounds were considered as test set molecules. The findings of this analysis will help to explore some major directions regarding the HDAC8 inhibitor designing approach. Acrylamide (G1-G3, G9), N-substituted 2-phenylimidazole (G4-G8, G9, G12-G13, G16-G19), benzimidazole (G10-G11), piperidine substituted pyrrole (G13-G14) groups, alkyl/aryl amide (G15) and aryloxy carboxamide (G20) fingerprints were found to play a crucial role in HDAC8 inhibitory activity whereas -CH-N=CH- (B1, B4-B6, B14) motif, benzamide (B2-B3, B9-B13, B16-B17) groups and heptazepine (B7-B8, B15, B18-B20) group were found to influence negatively the HDAC8 inhibitory activity. The importance of such fingerprints was further validated by the HDAC8 enzyme and related inhibitor interactions at the receptor level. These results are in close agreement with those of our previous work that validate each other. Moreover, this comparative learning may enrich future endeavours regarding the designing strategy of HDAC8 inhibitors.

HDAC8 inhibitor attenuates airway responses to antigen stimulus through synchronously suppressing galectin-3 expression and reducing macrophage-2 polarization

BACKGROUND

This study was to investigate of the mechanism by which histone deacetylase (HDAC) 8 inhibitor ameliorated airway hyperresponsiveness (AHR) and allergic airway inflammation.

METHODS

Mice were sensitized and then treated with budesonide (BUD) or PCI-34051 (PCI) prior to exposing to normal saline (NS) or ovalbumin (OVA). The raw264.7 cells were treated with interleukin (IL)-4 and PCI or shRNA alone. Repetitive measurements of enhanced pause (Penh) were executed by increasing concentrations of acetyl-β-methacholine chloride (0 - 50 mg/ml). Cells in bronchoalveolar lavage fluid (BALF) and pathological changes of lungs were examined, respectively. The expression levels of HDAC8, Galecitn (Gal)-3, CD68, CD86, CD163, Arg1 and NOS2 in lungs were measured. Co-regulation of HDAC8 and Gal-3 proteins was observed by immunofluorescence staining and co-immunoprecipitation assay (Co-IP).

RESULTS

Significant increases in Penh and IL-4 level were detected with a large inflammatory infiltrate, comprised predominantly of macrophages and eosinophils, into the BALF in OVA-exposed lungs. HDAC8, Gal-3, CD68, CD86, CD163, Arg1 and NOS2 proteins were over-expressed with the significant changes in the Arg1 and NOS2 mRNA levels in the lungs and the IL-4-treated cells. PCI intervention obviously reduced the counts of CD163+ cells. Furthermore, Gal-3 knockdown suppressed Arg1 expression in the cells. Immunofluorescence staining displayed simultaneous changes in HDAC8 and Gal-3 expression in the investigated samples. Treatment with PCI resulted in synchronous reduction of HDAC8 and Gal-3 expression in the Co-IP complexes.

CONCLUSIONS

The HDAC8 inhibitor ameliorates AHR and airway inflammation in animal model of allergic asthma through reducing HDAC8-Gal-3 interaction and M2 macrophage polarization.

Synthesis, anticancer activity, structure–activity relationship and binding mode of interaction studies of substituted pentanoic acids

Aim: Simultaneous inhibition of MMP-2 and HDAC8 may be an effective strategy to target cancer. Methodology: In continuation of our earlier efforts, a series of substituted pentanoic acids (1–18) were synthesized and checked for their biological activity along with some earlier reported compounds (19 –35). Results: Compounds 18 and 31 were found to induce apoptosis effectively in a dose-dependent fashion in Jurkat-E6.1 cell line. They reduced the expression of both MMP-2 and HDAC8 effectively. 31 also produced prominent intensity of fluorescence to bring nick in Jurkat-E6.1 cells. 31 also showed cellular arrest in sub-G0 phase. Conclusion: Such compounds may be useful to battle against cancer.

Structural exploration of tetrahydroisoquinoline derivatives as HDAC8 inhibitors through multi-QSAR modeling study

Histone deacetylase 8 (HDAC8) is one of the crucial HDACs responsible for influencing the epigenetic functions of the body. Overexpression of HDAC8 is found to be involved in numerous disease conditions such as tumorigenesis, cell proliferation, cancer, viral infections, neuronal disorders and other epigenetic diseases. Therefore, inhibition of HDAC8 is a primary method to combat these diseases. In this article, a multi-QSAR modeling study on tetrahydroisoquinoline derivatives was conducted to identify important contributions of the structural features of these compounds toward HDAC8 inhibition. All these QSAR modeling techniques were individually validated and justified the observations of each other. The results implied that the tetrahydroisoquinoline moiety may be effective as a cap group than as a linker moiety for HDAC8 inhibition. Different substitutions at the tetrahydroisoquinoline scaffold were also found to be crucial in modulating HDAC8 inhibition.Communicated by Ramaswamy H. Sarma.

Selective and nonselective HDAC8 inhibitors: a therapeutic patent review

Histone deacetylase 8 (HDAC8) is one of the attractive therapeutic anticancer targets. HDAC8 has been overexpressed in a variety of human cancers. Therefore, HDAC8 inhibitors offer beneficial effects in the treatment of solid and hematological tumors. Different HDAC inhibitors entered into different phases of clinical studies. However, selectivity towards specific HDAC8 enzyme is still demanding. In this patent review, a number of patented selective and nonselective HDAC8 inhibitors along with their implication as anticancer agents have been discussed in details. Molecules should possess modified fish-like structural arrangement to impart potency and selectivity towards HDAC8. This comprehensive patent analysis will surely provide newer aspects of designing selective HDAC8 inhibitors targeted to anticancer therapy in future.

Histone deacetylase 8 triggers the migration of triple negative breast cancer cells via regulation of YAP signals

Triple-negative breast cancer (TNBC) shows highly aggressive clinical behaviors and poor prognosis compared to other breast cancer subtypes. Histone deacetylases (HDACs) can regulate the progression of various cancers, but the role of HDAC8 in TNBC remains unexplored. Here, we found that HDAC8 enhanced the in vitro migration abilities of breast cancer cells. Targeted inhibition of HDAC8 via si-HDAC8 and its selective inhibitor PCI34051 could suppress the migration of cells. In TNBC cells, HDAC8 stabilized the expression and increased the nuclear localization of YAP, a major downstream effector of Hippo pathway. While silencing YAP could attenuate HDAC8 triggered migration of TNBC cells. Mechanistically, HDAC8 suppressed the phosphorylation of YAP^Ser127, which was related to its cytoplasmic sequestration degradation. Our data revealed that HDAC8 could trigger the migration of TNBC cells via regulation of Hippo-YAP signals, suggesting that HDAC8 might be a potential target for TNBC therapy.

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.

Diverse classes of HDAC8 inhibitors: in search of molecular fingerprints that regulate activity

AIM

HDAC8 is one of the crucial enzymes involved in malignancy. Structural explorations of HDAC8 inhibitory activity and selectivity are required.

MATERIALS & METHODS

A mathematical framework was constructed to explore important molecular fragments responsible for HDAC8 inhibition. Bayesian classification models were developed on a large set of structurally diverse HDAC8 inhibitors.

RESULTS

This study helps to understand the structural importance of HDAC8 inhibitors. The hydrophobic aryl cap function is important for HDAC8 inhibition whereas benzamide moiety shows a negative impact on HDAC8 inhibition.

CONCLUSION

This work validates our previously proposed structural features for better HDAC8 inhibition. The comparative learning between the statistical and intelligent methods will surely enrich future drug design aspects of HDAC8 inhibitors.