Rational Design and Development of Radiation-Sensitizing Histone Deacetylase Inhibitors

A patent review of histone deacetylase 6 inhibitors in neurodegenerative diseases (2014-2019)

Introduction: Histone deacetylase 6 (HDAC6) is unique in comparison with other zinc-dependent HDAC family members. An increasing amount of evidence from clinical and preclinical research demonstrates the potential of HDAC6 inhibition as an effective therapeutic approach for the treatment of cancer, autoimmune diseases, as well as neurological disorders. The recently disclosed crystal structures of HDAC6-ligand complexes offer further means for achieving pharmacophore refinement, thus further accelerating the pace of HDAC6 inhibitor discovery in the last few years.Area covered: This review summarizes the latest clinical status of HDAC6 inhibitors, discusses pharmacological applications of selective HDAC6 inhibitors in neurodegenerative diseases, and describes the patent applications dealing with HDAC6 inhibitors from 2014-2019 that have not been reported in research articles.Expert opinion: Phenylhydroxamate has proven a very useful scaffold in the discovery of potent and selective HDAC6 inhibitors. However, weaknesses of the hydroxamate function such as metabolic instability and mutagenic potential limit its application in the neurological field, where long-term administration is required. The recent invention of oxadiazole-based ligands by pharmaceutical companies may provide a new opportunity to optimize the druglike properties of HDAC6 inhibitors for the treatment of neurodegenerative diseases.

Histone Deacetylases and Histone Deacetylase Inhibitors: Molecular Mechanisms of Action in Various Cancers

Epigenetic modifications such as histone modification play an important role in tumorigenesis. There are several evidence that histone deacetylases (HDACs) play a key role in cancer induction and progression by histone deacetylation. Besides, histone acetylation is being accessed as a therapeutic target because of its role in regulating gene expression. HDAC inhibitors (HDACIs) are a family of synthetic and natural compounds that differ in their target specificities and activities. They affect markedly cancer cells, inducing cell differentiation, cell cycle arrest and cell death, reduction of angiogenesis, and modulation of the immune system. Here, we summarize the mechanisms of HDACs and the HDACIs in several cancers. An online search of different sources such as PubMed, ISI, and Scopus was performed to find available data on mechanisms and pathways of HDACs and HDACIs in different cancers. The result indicated that HDACs induce cancer through multiple mechanisms in various tissues. This effect can be inhibited by HDACIs which affect cancer cell by different pathways such as cell differentiation, cell cycle arrest, and cell death. In conclusion, these findings indicate that the HDACs play a major role in carcinogenesis through various pathways, and HDACIs can inhibit HDAC activity by multiple mechanisms resulting in cell cycle arrest, cell growth inhibition, and apoptosis induction.

A phase 1 trial of Vorinostat in combination with concurrent chemoradiation therapy in the treatment of advanced staged head and neck squamous cell carcinoma

Purpose Vorinostat is a potent HDAC inhibitor that sensitizes head and neck squamous cell carcinoma (HNSCC) to cytotoxic therapy while sparing normal epithelium. The primary objective of this Phase I study was to determine the maximally tolerated dose (MTD) and safety of Vorinostat in combination with standard chemoradiation therapy treatment in HNSCC. Patients and Methods Eligible patients had pathologically confirmed Stage III, IVa, IVb HNSCC, that was unresectable or borderline resectable involving the larynx, hypopharynx, nasopharynx, and oropharynx. Vorinostat was administered at the assigned dosage level (100-400 mg, three times weekly) in a standard 3 + 3 dose escalation design. Vorinostat therapy began 1 week prior to initiation of standard, concurrent chemoradiation therapy and continued during the entire course of therapy. Results Twenty six patients met eligibility criteria and completed the entire protocol. The primary tumor sites included tonsil (12), base of tongue (9), posterior pharyngeal wall (1), larynx (4) and hypopharynx (3). Of the 26 patients, 17 were HPV-positive and 9 were HPV-negative. The MTD of Vorinostat was 300 mg administered every other day. Anemia (n = 23/26) and leukopenia (n = 20/26) were the most commonly identified toxicities. The most common Grade3/4 events included leukopenia (n = 11) and lymphopenia (n = 17). No patient had Grade IV mucositis, dermatitis or xerostomia. The median follow time was 33.8 months (range 1.6-82.9 months). Twenty four of 26 (96.2%) patients had a complete response to therapy. Conclusion Vorinostat in combination with concurrent chemoradiation therapy is a safe and highly effective treatment regimen in HNSCC. There was a high rate of complete response to therapy with toxicity rates comparable, if not favorable to existing therapies. Further investigation in Phase II and III trials is strongly recommended.

Interrogating Substrate Selectivity and Composition of Endogenous Histone Deacetylase Complexes with Chemical Probes

Histone deacetylases (HDACs) regulate the function and activity of numerous cellular proteins by removing acetylation marks from regulatory lysine residues. We have developed peptide-based HDAC probes that contain hydroxamate amino acids of various lengths to replace modified lysine residues in the context of known acetylation sites. The interaction profiles of all human HDACs were studied with three sets of probes, which derived from different acetylation sites, and sequence context was found to have a strong impact on substrate recognition and composition of HDAC complexes. By investigating K382 acetylation of the tumor suppressor p53 as an example, we further demonstrate that the interaction profiles reflect the catalytic activities of respective HDACs. These results underline the utility of the newly established probes for deciphering not only activity, but also substrate selectivity and composition of endogenous HDAC complexes, which can hardly be achieved otherwise.

The enhanced apoptosis and antiproliferative response to combined treatment with valproate and nicotinamide in MCF-7 breast cancer cells

Acetylation of histone is a major player in epigenetic modifications, resulting in open chromatin structures and, hence, permissive conditions for transcription-factor recruitment to the promoters, followed by initiation of transcription. Histone deacetylase inhibitors arrest cancer cell growth and cause apoptosis with low toxicity thereby constituting a promising treatment for cancer. In this study, we examined the antiproliferative effects of valproate with a combination of nicotinamide in the MCF-7 cell line. MCF-7 was treated with various concentrations of valproate. The MTT assay showed that the viability of MCF-7 cells was inhibited and the cell activity was decreased. Viability percent of valproate and nicotinamide combined treatment cells (28 ± 2) was 1.78 times increased compared with the valproate-alone (0.5 mM) treated cells (50 ± 2). Colony formation in soft agar indicated that valproate at 0.3 mM, when used alone, weakly suppressed proliferation of cells (82 ± 3) and the combination treatment of valproate + nicotinamide strongly suppressed cell proliferation (51 ± 3). The flow cytometric and microscopic analyses of HDACI combined with treated cells indicated strong apoptosis induction and nuclear morphological alterations greater than those of valproate alone. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) analysis confirmed the efficiency of the HDAC inhibitor combination, revealing the effectively upregulated p16 and p21. Furthermore, to investigate the role of acetyl-histone H3 levels, western blot analyses have been performed and high levels of acetylated histone H3 were detected in valproate- and nicotinamide-treated cells. These results suggest that the combination treatment of valproate with nicotinamide exerts significant antitumor activity and could be a promising therapeutic candidate to treat human breast cancer.

Matrix metalloproteinases: potential therapy to prevent the development of second malignancies after breast radiotherapy

Radiotherapy is widely used in the treatment of patients with breast cancer, but ionizing radiation-induced carcinogenesis has been described in several studies. Matrix metalloproteinases (MMPs) are a wide family of proteases secreted by tumour and microenvironmental cells that are directly linked with invasion and metastasis through complete extracellular matrix (ECM) breakage. In the past decade, MMPs have been associated with other carcinogenesis steps, including tumour growth and angiogenesis promotion. Moreover, in vitro studies have demonstrated an enhanced migration, invasiveness, and angiogenic ability of cancer cells after radiation exposure through an increase in MMP activity. These findings are consistent with clinical observations of breast cancer metastases raised in bone, lung and brain tissues after radiotherapy. The aim of this review was to analyse the current state of research on MMPs and report new insights into the potential of MMP-targeted therapy in combination with radiotherapy to decrease the risk of radiation-induced second malignancies and to improve the overall survival of breast cancer patients.

Histone deacetylase inhibition decreases proliferation and potentiates the effect of ionizing radiation in atypical teratoid/rhabdoid tumor cells

Atypical teratoid/rhabdoid tumor (ATRT) is a highly malignant central nervous system neoplasm that primarily occurs in children less than 3 years of age. Because of poor outcomes with intense and toxic multimodality treatment, new therapies are urgently needed. Histone deacetylase inhibitors (HDIs) have been evaluated as novel agents for multiple malignancies and have been shown to function as radiosensitizers. They act as epigenetic modifiers and lead to re-expression of inappropriately repressed genes, proteins, and cellular functions. Because of the underlying chromatin remodeling gene mutation in ATRT, HDIs are ideal candidates for therapeutic evaluation. To evaluate the role of HDIs against ATRT in vitro, we assessed the effect of drug treatment on proliferation, apoptosis, and gene expression. In addition, we examined HDI pretreatment as a radiosensitization strategy for ATRT. 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium with phenazine methosulfate electron coupling reagent (MTS) and clonogenic assays demonstrated that HDI treatment significantly reduces the proliferative capacity of BT-12 and BT-16 ATRT cells. In addition, the HDI SNDX-275 was able to induce apoptosis in both cell lines and induced p21(Waf1/Cip1) protein expression as measured by Western blot. Evaluation of differential gene expression by microarray and pathway analysis after HDI treatment demonstrated alterations of several key ATRT cellular functions. Finally, we showed that HDI pretreatment effectively potentiates the effect of ionizing radiation on ATRT cells as measured by clonogenic assay. Our findings suggest that the addition of HDIs to ATRT therapy may prove to be beneficial, especially when administered in combination with current treatment modalities, such as radiation.

Histone deacetylase cytoplasmic trapping by a novel fluorescent HDAC inhibitor

Inhibitors of histone deacetylases (HDAC) are an important emerging class of drugs for the treatment of cancers. HDAC inhibitors are currently under evaluation in clinical trials as single agents and as sensitizers in combinations with chemotherapies and radiation therapy. Although these drugs have important effects on cancer cell growth and functions, the mechanisms underlying HDAC inhibitor activities remain to be fully defined. By using rational drug design, compound 2, a fluorescent class II HDAC targeting inhibitor, was synthesized and observed to accumulate in the cytoplasmic compartments of treated cells, but not in the nuclei. Furthermore, immunostaining of inhibitor exposed cells for HDAC4 showed accumulation of this enzyme in the cytoplasmic compartment with concomitant increased acetylation of tubulin and nuclear histones. These observations support a mechanism by which nuclear histone acetylation is increased as a result of HDAC4 trapping and sequestration in the cytoplasm after binding to compound 2. The HDAC inhibitor offers potential as a novel theranostic agent, combining diagnostic and therapeutic properties in the same molecule.

Development of vorinostat: current applications and future perspectives for cancer therapy

Vorinostat is a potent histone deacetylase inhibitor that blocks the catalytic site of these enzymes. A large number of cellular proteins are modified post-translationally by acetylation, leading to altered structure and/or function. Many of these proteins, such as core nucleosomal histones and transcription factors, function in key cellular processes and signal transduction pathways that regulate cell growth, migration, and differentiation. At concentrations that are non-toxic to normal cells, vorinostat dramatically alters cellular acetylation patterns and causes growth arrest and death and in a wide range of transformed cells, both in vitro and in animal tumor models. Vorinostat has shown promising clinical activity against hematologic and solid tumors at doses that have been well tolerated by patients. Recent non-clinical experiments that explored the effects of vorinostat in combination with other chemotherapeutic agents have begun to illuminate potential mechanisms of action for this histone deacetylase inhibitor and are providing guidance for new avenues of clinical investigation.

The role of histone deacetylases in prostate cancer

Epigenetic modifications play a key role in the patho-physiology of prostate cancer. Histone deacetylases (HDACs) play major roles in prostate cancer progression. HDACs are part of a transcriptional co-repressor complex that influences various tumor suppressor genes. Because of the significant roles played by HDACs in various human cancers, HDAC inhibitors are emerging as a new class of chemotherapeutic agents. HDAC inhibitors have been shown to induce cell growth arrest, differentiation and/or apoptosis in prostate cancer. The combined use of HDAC inhibitors with other chemotherapeutic agents or radiotherapy in cancer treatment has shown promising results. Various HDAC inhibitors are in different stages of clinical trials. In this review, we discuss the molecular mechanism(s) through which HDACs influence prostate cancer progression and the potential roles of HDAC inhibitors in prostate cancer prevention and therapy.