Targeting histone deacetylases: Emerging applications beyond cancer.

Histone deacetylases (HDACs) are a special class of hydrolase enzymes, which through epigenetic control of cellular acetylation, play regulatory roles in various processes including chromatin packing, cytokine signaling, and gene expression. Widespread influence on cell function has implicated dysregulated HDAC activity in human disease. While traditionally an oncology target, in the past decade, there has been a notable rise in inhibition strategies within several therapeutic areas beyond cancer. This review highlights advances in four of these indications, neurodegenerative disease, metabolic disorders, cardiovascular disease, and viral infections, focusing on the role of deacetylases in disease, small molecule drug discovery, and clinical progress.

Discovery of YSR734: A Covalent HDAC Inhibitor with Cellular Activity in Acute Myeloid Leukemia and Duchenne Muscular Dystrophy.

Histone deacetylases (HDACs) have emerged as powerful epigenetic modifiers of histone/non-histone proteins via catalyzing the deacetylation of ε-N-acetyl lysines. The dysregulated activity of these Zn2+-dependent hydrolases has been broadly implicated in disease, notably cancer. Clinically, the recurring dose-limiting toxicities of first-generation HDACi sparked a paradigm shift toward safer isoform-specific molecules. With pervasive roles in aggressive diseases, there remains a need for novel approaches to target these enzymes. Herein, we report the discovery of YSR734, a first-in-class covalent HDACi, with a 2-aminobenzanilide Zn2+ chelate and a pentafluorobenzenesulfonamide electrophile. This class I selective proof of concept modified HDAC2Cys274 (catalytic domain), with nM potency against HDAC1-3, sub-µM activity in MV4-11 cells, and limited cytotoxicity in MRC-9 fibroblasts. In C2C12 myoblasts, YSR734 activated muscle-specific biomarkers myogenin/Cav3, causing potent differentiation into myotubes (applications in Duchenne Muscular Dystrophy). Current efforts are focused on improving in vivo ADME toward a preclinical covalent HDACi.

Discovery of HDAC6-Selective Inhibitor NN-390 with in Vitro Efficacy in Group 3 Medulloblastoma.

Histone deacetylase 6 (HDAC6) has been targeted in clinical studies for anticancer effects due to its role in oncogenic transformation and metastasis. Through a second-generation structure-activity relationship (SAR) study, the design, and biological evaluation of the selective HDAC6 inhibitor NN-390 is reported. With nanomolar HDAC6 potency, >200-550-fold selectivity for HDAC6 in analogous HDAC isoform functional assays, potent intracellular target engagement, and robust cellular efficacy in cancer cell lines, NN-390 is the first HDAC6-selective inhibitor to show therapeutic potential in metastatic Group 3 medulloblastoma (MB), an aggressive pediatric brain tumor often associated with leptomeningeal metastases and therapy resistance. MB stem cells contribute to these patients' poor clinical outcomes. NN-390 selectively targets this cell population with a 44.3-fold therapeutic margin between patient-derived Group 3 MB cells in comparison to healthy neural stem cells. NN-390 demonstrated a 45-fold increased potency over HDAC6-selective clinical candidate citarinostat. In summary, HDAC6-selective molecules demonstrated in vitro therapeutic potential against Group 3 MB.

Development of HDAC Inhibitors Exhibiting Therapeutic Potential in T-Cell Prolymphocytic Leukemia.

Epigenetic targeting has emerged as an efficacious therapy for hematological cancers. The rare and incurable T-cell prolymphocytic leukemia (T-PLL) is known for its aggressive clinical course. Current epigenetic agents such as histone deacetylase (HDAC) inhibitors are increasingly used for targeted therapy. Through a structure-activity relationship (SAR) study, we developed an HDAC6 inhibitor KT-531, which exhibited higher potency in T-PLL compared to other hematological cancers. KT-531 displayed strong HDAC6 inhibitory potency and selectivity, on-target biological activity, and a safe therapeutic window in nontransformed cell lines. In primary T-PLL patient cells, where HDAC6 was found to be overexpressed, KT-531 exhibited strong biological responses, and safety in healthy donor samples. Notably, combination studies in T-PLL patient samples demonstrated KT-531 synergizes with approved cancer drugs, bendamustine, idasanutlin, and venetoclax. Our work suggests HDAC inhibition in T-PLL could afford sufficient therapeutic windows to achieve durable remission either as stand-alone or in combination with targeted drugs.

Unique Molecular Interaction with the Histone Deacetylase 6 Catalytic Tunnel: Crystallographic and Biological Characterization of a Model Chemotype.

Histone deacetylase 6 (HDAC6) is involved in multiple regulatory processes, ranging from cellular stress to intracellular transport. Inhibition of aberrant HDAC6 activity in several cancers and neurological diseases has been shown to be efficacious in both preclinical and clinical studies. While selective HDAC6 targeting has been pursued as an alternative to pan-HDAC drugs, identifying truly selective molecular templates has not been trivial. Herein, we report a structure-activity relationship study yielding TO-317, which potently binds HDAC6 catalytic domain 2 (Ki = 0.7 nM) and inhibits the enzyme function (IC50 = 2 nM). TO-317 exhibits 158-fold selectivity for HDAC6 over other HDAC isozymes by binding the catalytic Zn2+ and, uniquely, making a never seen before direct hydrogen bond with the Zn2+ coordinating residue, His614. This novel structural motif targeting the second-sphere His614 interaction, observed in a 1.84 Å resolution crystal structure with drHDAC6 from zebrafish, can provide new pharmacophores for identifying enthalpically driven, high-affinity, HDAC6-selective inhibitors.

Characterization of Conformationally Constrained Benzanilide Scaffolds for Potent and Selective HDAC8 Targeting.

Histone deacetylases (HDACs) are an attractive therapeutic target for a variety of human diseases. Currently, all four FDA-approved HDAC-targeting drugs are nonselective, pan-HDAC inhibitors, exhibiting adverse side effects at therapeutic doses. Although selective HDAC inhibition has been proposed to mitigate toxicity, the targeted catalytic domains are highly conserved. Herein, we describe a series of rationally designed, conformationally constrained, benzanilide foldamers which selectively bind the catalytic tunnel of HDAC8. The series includes benzanilides, MMH371, MMH409, and MMH410, which exhibit potent in vitro HDAC8 activity (IC50 = 66, 23, and 66 nM, respectively) and up to 410-fold selectivity for HDAC8 over the next targeted HDAC. Experimental and computational analyses of the benzanilide structure docked with human HDAC8 enzyme showed the adoption of a low-energy L-shaped conformer that favors HDAC8 selectivity. The conformationally constrained HDAC8 inhibitors present an alternative biological probe for further determining the clinical utility and safety of pharmacological knockdown of HDAC8 in diseased cells.

PTG-0861: A novel HDAC6-selective inhibitor as a therapeutic strategy in acute myeloid leukaemia.

Dysregulated Histone Deacetylase (HDAC) activity across multiple human pathologies have highlighted this family of epigenetic enzymes as critical druggable targets, amenable to small molecule intervention. While efficacious, current approaches using non-selective HDAC inhibitors (HDACi) have been shown to cause a range of undesirable clinical toxicities. To circumvent this, recent efforts have focused on the design of highly selective HDACi as a novel therapeutic strategy. Beyond roles in regulating transcription, the unique HDAC6 (with two catalytic domains) regulates the deacetylation of α-tubulin; promoting growth factor-controlled cell motility, cell division, and metastatic hallmarks. Recent studies have linked aberrant HDAC6 function in various hematological cancers including acute myeloid leukaemia and multiple myeloma. Herein, we report the discovery, in vitro characterization, and biological evaluation of PTG-0861 (JG-265), a novel HDAC6-selective inhibitor with strong isozyme-selectivity (∼36× ) and low nanomolar potency (IC50 = 5.92 nM) against HDAC6. This selectivity profile was rationalized via in silico docking studies and also observed in cellulo through cellular target engagement. Moreover, PTG-0861 achieved relevant potency against several blood cancer cell lines (e.g. MV4-11, MM1S), whilst showing limited cytotoxicity against non-malignant cells (e.g. NHF, HUVEC) and CD-1 mice. In examining compound stability and cellular permeability, PTG-0861 revealed a promising in vitro pharmacokinetic (PK) profile. Altogether, in this study we identified a novel and potent HDAC6-selective inhibitor (∼4× more selective than current clinical standards - citarinostat, ricolinostat), which achieves cellular target engagement, efficacy in hematological cancer cells with a promising safety profile and in vitro PK.

Advances in covalent kinase inhibitors.

Over the past decade, covalent kinase inhibitors (CKI) have seen a resurgence in drug discovery. Covalency affords a unique set of advantages as well as challenges relative to their non-covalent counterpart. After reversible protein target recognition and binding, covalent inhibitors irreversibly modify a proximal nucleophilic residue on the protein via reaction with an electrophile. To date, the acrylamide group remains the predominantly employed electrophile in CKI development, with its incorporation in the majority of clinical candidates and FDA approved covalent therapies. Nonetheless, in recent years considerable efforts have ensued to characterize alternative electrophiles that exhibit irreversible or reversibly covalent binding mechanisms towards cysteine thiols and other amino acids. This review article provides a comprehensive overview of CKIs reported in the literature over a decade period, 2007-2018. Emphasis is placed on the rationale behind warhead choice, optimization approach, and inhibitor design. Current FDA approved CKIs are also highlighted, in addition to a detailed analysis of the common trends and themes observed within the listed data set.

Class I/IIb-Selective HDAC Inhibitor Exhibits Oral Bioavailability and Therapeutic Efficacy in Acute Myeloid Leukemia.

The HDAC inhibitor 4-tert-butyl-N-(4-(hydroxycarbamoyl)phenyl)benzamide (AES-350, 51) was identified as a promising preclinical candidate for the treatment of acute myeloid leukemia (AML), an aggressive malignancy with a meagre 24% 5-year survival rate. Through screening of low-molecular-weight analogues derived from the previously discovered novel HDAC inhibitor, AES-135, compound 51 demonstrated greater HDAC isoform selectivity, higher cytotoxicity in MV4-11 cells, an improved therapeutic window, and more efficient absorption through cellular and lipid membranes. Compound 51 also demonstrated improved oral bioavailability compared to SAHA in mouse models. A broad spectrum of experiments, including FACS, ELISA, and Western blotting, were performed to support our hypothesis that 51 dose-dependently triggers apoptosis in AML cells through HDAC inhibition.

Identification and Characterization of AES-135, a Hydroxamic Acid-Based HDAC Inhibitor That Prolongs Survival in an Orthotopic Mouse Model of Pancreatic Cancer.

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive, incurable cancer with a 20% 1 year survival rate. While standard-of-care therapy can prolong life in a small fraction of cases, PDAC is inherently resistant to current treatments, and novel therapies are urgently required. Histone deacetylase (HDAC) inhibitors are effective in killing pancreatic cancer cells in in vitro PDAC studies, and although there are a few clinical studies investigating combination therapy including HDAC inhibitors, no HDAC drug or combination therapy with an HDAC drug has been approved for the treatment of PDAC. We developed an inhibitor of HDACs, AES-135, that exhibits nanomolar inhibitory activity against HDAC3, HDAC6, and HDAC11 in biochemical assays. In a three-dimensional coculture model, AES-135 kills low-passage patient-derived tumor spheroids selectively over surrounding cancer-associated fibroblasts and has excellent pharmacokinetic properties in vivo. In an orthotopic murine model of pancreatic cancer, AES-135 prolongs survival significantly, therefore representing a candidate for further preclinical testing.