Binding of inhibitors to active-site mutants of CD1, the enigmatic catalytic domain of histone deacetylase 6

Small molecules targeting HDAC6 for cancer treatment: Current progress and novel strategies

Histone deacetylase 6 (HDAC6) plays a crucial role in the initiation and progression of various cancers, as its overexpression is linked to tumor growth, invasion, migration, survival, apoptosis, and angiogenesis. Therefore, HDAC6 has emerged as an attractive target for anticancer drug discovery in the past decade. However, the development of conventional HDAC6 inhibitors has been hampered by their limited clinical efficacy, acquired resistance, and inability to inhibit non-enzymatic functions of HDAC6. To overcome these challenges, new strategies, such as dual-acting inhibitors, targeted protein degradation (TPD) technologies (including PROTACs, HyT), are essential to enhance the anticancer activity of HDAC6 inhibitors. In this review, we focus on the recent advances in the design and development of HDAC6 modulators, including isoform-selective HDAC6 inhibitors, HDAC6-based dual-target inhibitors, and targeted protein degraders (PROTACs, HyT), from the perspectives of rational design, pharmacodynamics, pharmacokinetics, and clinical status. Finally, we discuss the challenges and future directions for HDAC6-based drug discovery for cancer therapy.

HDAC6-Activatable Multifunctional Near-Infrared Probe for Glioma Cell Detection and Elimination

Glioblastoma multiforme (GBM) is a highly aggressive primary brain tumor associated with limited treatment options and high drug resistance, presenting significant challenges in the pursuit of effective treatment strategies. Epigenetic modifications have emerged as promising diagnostic biomarkers and therapeutic targets for GBM. For instance, histone deacetylase 6 (HDAC6) has been identified as a potential pharmacological target for GBM. Furthermore, the overexpression of monoamine oxidase A (MAO A) in glioma has been linked to tumor progression, making it an attractive target for therapy. In this study, we successfully engineered HDAC-MB, an activatable multifunctional small-molecule probe with the goal of efficiently detecting and killing glioma cells. HDAC-MB can be selectively activated by HDAC6, leading to the "turn on" of near-infrared fluorescence and effective inhibition of MAO A, along with potent photodynamic therapy (PDT) effects. Consequently, HDAC-MB not only enables the imaging of HDAC6 in live glioma cells but also exhibits the synergistic effect of MAO A inhibition and PDT, effectively inhibiting glioma invasion and inducing cellular apoptosis. The distinctive combination of features displayed by HDAC-MB positions it as a versatile and highly effective tool for the accurate diagnosis and treatment of glioma cells. This opens up opportunities to enhance therapy outcomes and explore future applications in glioma theranostics.

Design, synthesis, and biological evaluation of HDAC6 inhibitors targeting L1 loop and serine 531 residue

Histone deacetylases (HDACs) are a group of enzymes that remove acetyl groups from histones, leading to the silencing of genes. Targeting specific isoforms of HDACs has emerged as a promising approach for cancer therapy, as it can overcome drawbacks associated with pan-HDAC inhibitors. HDAC6 is a unique HDAC isoform that deacetylates non-histone proteins and is primarily located in the cytoplasm. It also has two catalytic domains and a zinc-finger ubiquitin binding domain (Zf-UBD) unlike other HDACs. HDAC6 plays a critical role in various cellular processes, including cell motility, protein degradation, cell proliferation, and transcription. Hence, the deregulation of HDAC6 is associated with various malignancies. In this study, we report the design and synthesis of a series of HDAC6 inhibitors. We evaluated the synthesized compounds by HDAC enzyme assay and identified that compound 8g exhibited an IC50 value of 21 nM and 40-fold selective activity towards HDAC6. We also assessed the effect of compound 8g on various cell lines and determined its ability to increase protein acetylation levels by Western blotting. Furthermore, the increased acetylation of α-tubulin resulted in microtubule polymerization and changes in cell morphology. Our molecular docking study supported these findings by demonstrating that compound 8g binds well to the catalytic pocket via L1 loop of HDAC6 enzyme. Altogether, compound 8g represents a preferential HDAC6 inhibitor that could serve as a lead for the development of more potent and specific inhibitors.

Novel pyrrolo[2,1-c][1,4]benzodiazepine-3,11-dione (PBD) derivatives as selective HDAC6 inhibitors to suppress tumor metastasis and invasion in vitro and in vivo

Selective inhibition of histone deacetylase 6 (HDAC6) has been emerged as a promising approach to cancer treatment. As a pivotal strategy for drug discovery,molecular hybridization was introduced in this study and a series of pyrrolo[2,1-c][1,4] benzodiazepine-3,11-diones (PBDs) based hydroxamic acids was rationally designed and synthesizedas novel selective HDAC6 inhibitors. Preliminary in vitro enzyme inhibition assay and structure-activity relationship (SAR) discussion confirmed our design strategy and met the expectation. Several of the compounds showed high potent against HDAC6 enzyme in vitro, and compound A7 with a long aliphatic linker was revealed to have the similar activity as the positive control tubastatin A. Further in vitro characterization of A7 demonstrates the metastasis inhibitory potency in MDA-MB-231 cell line and western blotting showed that A7 could induce the upregulation of Ac-α-tubulin, but not induce the excessive acetylation of histone H3, which indicated that the compound had HDAC6 targeting effect in MDA-MB-231 cells. In vivo study revealed that compound A7 has satisfactory inhibitory effects onliver and lung metastasis of breast cancer in mice. Molecular docking released that A7 could fit well with the receptor and interact with some key residues, which lays a foundation for further structural modifications to elucidate the interaction mode between compounds and target protein. This pharmacological investigation workflow provided a reasonable and reference methodto examine the pharmacological effects of inhibiting HDAC6 with a single molecule, either in vitro or in vivo. All of these results suggested that A7 is a promising lead compound that could lead to the further development of novel selective HDAC6 inhibitors for the treatment of tumor metastasis.

Mercaptoacetamide: A promising zinc-binding group for the discovery of selective histone deacetylase 6 inhibitors

Histone deacetylase 6 (HDAC6) is a zinc-dependent HDAC that mainly modulates the acetylation status of non-histone substrates, such as α-tubulin and heat shock protein 90 (HSP90). The activity of HDAC6 plays a critical role in cell proliferation, protein trafficking and degradation, cell shape, migration, as well as regulation of immunomodulatory factors. For this reason, HDAC6 influences the progress of cancers, neurodegenerative disorders, and autoimmune responses. In the last few years, the discovery of selective HDAC6 inhibitors (HDAC6is) has become an attractive research area as five HDAC6is are being investigated in phase I/II clinical trials. However, the hydroxamic acid functional group still represents the predominant zinc-binding group (ZBG), that often suffers from poor pharmacokinetics and mutagenic potential, thus impairing the application of hydroxamate-based HDAC6is for long-term therapies. On the other hand, mercaptoacetamide (MCA)-based HDAC6is comprise a class of compounds that, in some cases, display nanomolar HDAC6 potency and a thousand-fold selectivity over class I HDAC isozymes. Moreover, MCA-based HDAC6is lack the mutagenicity associated with the hydroxamate function and display pharmacological effects, demonstrating the potential of this particular ZBG to improve upon the drug-like properties of HDAC6is. Herein, we summarize for the first time the structure-activity relationships (SARs) of MCA-based HDAC6is, discuss their HDAC6 selectivity at the molecular level using inhibitor-HDAC co-crystal structures, and further provide our perspective regarding their drug metabolism, pharmacokinetics, and pharmacological properties.