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

Design, synthesis, and biological evaluation of indazole-based histone deacetylase 6 inhibitors

Histone deacetylases (HDACs) have been explored as anticancer targets for over two decades, with six HDAC inhibitors approved for clinical use. However, these pan-HDAC inhibitors exhibit off-target effects, necessitating the development of isoform-selective inhibitors. Among HDACs, HDAC6 has garnered attention due to its dual catalytic domains, cytoplasmic localization, and zinc-finger ubiquitin-binding domain (Zf-UBD). Its role in gene expression, proliferation, protein homeostasis, and cell cycle regulation make it an attractive anticancer target. Here, we report on the design and synthesis of indazole-based HDAC6 inhibitors and evaluate the impact of zinc-binding group (ZBG) modifications on pharmacokinetics. Compound 5j emerged as a selective and potent HDAC6 inhibitor (IC50 = 1.8 ± 0.3 nM), exhibiting strong antiproliferative activity against HCT116 cells (GI50 = 3.1 ± 0.6 μM). It preferentially induced α-tubulin acetylation over histone H3 at concentrations as low as 0.5 μM which is a hallmark of HDAC6 selective inhibition. However, its hydroxamic acid-based ZBG resulted in a very low oral bioavailability (1.2 %). To address this limitation, compound 12 was synthesized with an ethyl hydrazide ZBG, significantly improving oral bioavailability (53 %). These findings highlight compound 12 as a promising lead for further pharmacophore optimization, paving the way for clinically viable HDAC6 selective inhibitors with enhanced drug-like properties.

HDAC6 facilitates LUAD progression by inducing EMT and enhancing macrophage polarization towards the M2 phenotype

Histone deacetylase 6 (HDAC6) plays a critical role in lung adenocarcinoma (LUAD) prognosis and the tumor immune microenvironment (TIME). This study, utilizing public datasets and experimental validation, revealed that HDAC6 is upregulated in LUAD, correlating with poor survival outcomes and an immunosuppressive TIME characterized by increased Tregs, CAFs, M2 macrophages, and MDSCs. HDAC6-high patients showed reduced immunotherapy response. HDAC6 knockout inhibited tumor growth, suppressed PI3K/AKT/mTOR signaling and EMT, and enhanced apoptosis and M1 macrophage recruitment. HDAC6 inhibition synergized with anti-PD-1 therapy, suggesting a potential combinatorial strategy for LUAD treatment. HDAC6 serves as a key prognostic marker and therapeutic target in LUAD.

Histone Deacetylase 6 Brain PET in Amyotrophic Lateral Sclerosis-Frontotemporal Spectrum Disorder

OBJECTIVE

[18F]EKZ-001 is a positron emission tomography (PET) tracer targeting histone deacetylase 6 (HDAC6), an enzyme responsible for intracellular transport and clearance of misfolded proteins. HDAC6 modulation is a promising treatment strategy in neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Apart from motor symptoms, people with ALS (pwALS) can show a variable degree of cognitive impairment as part of the ALS-frontotemporal spectrum disorder (ALS-FTSD). This work assessed [18F]EKZ-001 binding in pwALS with variable involvement of FTSD.

METHODS

Twenty-four pwALS (13M/11F, 61 ± 10 years) and 12 healthy controls (HC) (6M/6F, 58 ± 3 years) were included. Thirteen pwALS were cognitively normal (ALS-CN), and eleven pwALS presented with FTSD (ALS-FTSD) ranging from mild cognitive or behavioral impairment to FTD, according to their performance on the Edinburgh cognitive and behavioral ALS screen (ECAS). All subjects underwent dynamic PET-MR imaging with arterial sampling, and regional distribution volumes (VT) were calculated using a Logan graphical analysis.

RESULTS

[18F]EKZ-001 VT was significantly lower in pwALS compared to HC. For ALS-CN, the largest reduction was found in the brainstem. For ALS-FTSD, reductions were more widespread in both gray and white matter. No differences in VT were found between pwALS with and without a C9orf72 mutation. [18F]EKZ-001 VT was not correlated with ECAS scores, age, or disease duration.

INTERPRETATION

[18F]EKZ-001 binding is lower throughout the brain in pwALS compared to HC. This may be related to a compensatory mechanism to repair intracellular transport defects in ALS or to reduced HDAC6 enzyme availability for [18F]EKZ-001 binding due to sequestration of HDAC6 within protein aggregates.

Tubulin/HDAC dual-target inhibitors: Insights from design strategies, SARs, and therapeutic potential

Microtubules, one of the cytoskeletons in eukaryotic cells, maintain the proper operation of several cellular functions. Additionally, they are regulated by the acetylation of HDAC6 and SIRT2 which affects microtubule dynamics. Given the fact that tubulin and HDAC inhibitors play a synergistic effect in the treatment of many cancers, the development of tubulin/HDAC dual-target inhibitors is conducive to addressing multiple limitations including drug resistance, dose toxicity, and unpredictable pharmacokinetic properties. At present, tubulin/HDAC dual-target inhibitors have been obtained in three main ways: uncleavable linked pharmacophores, cleavable linked pharmacophores, and modification of single-target drugs. Their therapeutic efficacy has been verified in vivo and in vitro assays. In this article, we reviewed the research progress of tubulin/HDAC dual inhibitors from design strategies, SARs, and biological activities, which may provide help for the discovery of novel tubulin/HDAC dual inhibitors.

HDAC6 mediates tumorigenesis during mitosis and the development of targeted deactivating agents

Epigenetics, particularly deacetylation, plays a critical role in tumorigenesis as many carcinogens are under tight control by post-translational modification. HDAC6, an important and special histone deacetylase (HDAC) family member, has been indicated to increase carcinogenesis through various functions. Recent studies demonstrated the effects of HDAC6 inhibitors in mitotic arrest, however, detailed mechanisms still remain unknown. Herein, we review and summarize HDAC6-associated proteins that have been implicated in important roles in mitosis. We also discuss the development of medicinal agents targeting HDAC6.