Design, synthesis, and anti-inflammatory activity characterization of novel brain-permeable HDAC6 inhibitors

Histone deacetylases: the critical enzymes for microglial activation involved in neuropathic pain

Neuropathic pain is a common health problem in clinical practice that can be caused by many different factors, including infection, ischemia, trauma, diabetes mellitus, nerve compression, autoimmune disorders, cancer, trigeminal neuralgia, and abuse of certain drugs. This type of pain can persistently affect patients for a long time, even after the rehabilitation of their damaged tissues. Researchers have identified the crucial role of microglial activation in the pathogenesis of neuropathic pain. Furthermore, emerging evidence has shown that the expression and/or activities of different histone deacetylases (HDACs) can modulate microglial function and neuropathic pain. In this review, we will summarize and discuss the functions and mechanisms of HDACs in microglial activation and neuropathic pain development. Additionally, we will also list the emerging HDAC inhibitors or activators that may contribute to therapeutic advancement in alleviating neuropathic pain.

Histone Deacetylase (HDAC) Inhibitors as a Novel Therapeutic Option Against Fibrotic and Inflammatory Diseases

Histone deacetylases (HDACs) are enzymes that play an essential role in the onset and progression of cancer. As a consequence, a variety of HDAC inhibitors (HDACis) have been developed as potent anticancer agents, several of which have been approved by the FDA for cancer treatment. However, recent accumulated research results have suggested that HDACs are also involved in several other pathophysiological conditions, such as fibrotic, inflammatory, neurodegenerative, and autoimmune diseases. Very recently, the HDAC inhibitor givinostat has been approved by the FDA for an indication beyond cancer: the treatment of Duchenne muscular dystrophy. In recent years, more and more HDACis have been developed as tools to understand the role that HDACs play in various disorders and as a novel therapeutic approach to fight various diseases other than cancer. In the present perspective article, we discuss the development and study of HDACis as anti-fibrotic and anti-inflammatory agents, covering the period from 2020-2024. We envision that the discovery of selective inhibitors targeting specific HDAC isozymes will allow the elucidation of the role of HDACs in various pathological processes and will lead to the development of promising treatments for such diseases.

Molecular Imaging Reveals Antineuroinflammatory Effects of HDAC6 Inhibition in Stroke Models

Ischemic stroke is a devastating disease that causes neuronal death, neuroinflammation, and other cerebral damage. However, effective therapeutic strategies for ischemic stroke are still lacking. Histone deacetylase 6 (HDAC6) has been implicated in the pathogenesis of ischemic stroke, and the pharmacological inhibition of HDAC6 has shown promising neuroprotective effects. In this study, we utilized positron emission tomography (PET) imaging with the HDAC6-specific radioligand [18F]PB118 to investigate the dynamic changes of HDAC6 expression in the brain after ischemic injury. The results revealed a significant decline in [18F]PB118 uptake in the ipsilateral hemisphere on the first day after ischemia, followed by a gradual increase on days 4 and 7. To evaluate the therapeutic potential of HDAC6 inhibitors, we developed a novel brain-permeable and potent HDAC6 inhibitor, PB131, and assessed its neuroprotective effects in an ischemic stroke mouse model. PET imaging studies demonstrated that PB131 treatment alleviated the decline in [18F]PB118 uptake and reduced the infarct size in middle cerebral artery occlusion mice. Furthermore, PET imaging with the TSPO-specific radioligand [18F]FEPPA revealed that PB131 significantly suppressed neuroinflammation in the ischemic brain. These findings provide insights into the dynamic changes of HDAC6 in ischemic stroke and the potential of HDAC6 inhibitors as novel therapeutic agents for this condition.

The Effect of Pexidartinib on Neuropathic Pain via Influences on Microglia and Neuroinflammation in Mice

BACKGROUND

Chronic pain is a debilitating medical condition that lacks effective treatments. Increasing evidence suggests that microglia and neuroinflammation underlie pain pathophysiology, which therefore supports a potential strategy for developing pain therapeutics. Here, our study is testing the hypothesis that the promise of pain amelioration can be achieved using the small-molecule pexidartinib (PLX-3397), a previously food and drug administration (FDA)-approved cancer medicine and a colony-stimulating factor-1 receptor (CSF-1R) inhibitor that display microglia-depleting properties.

METHOD

We used the previously reported chronic constriction injury (CCI) mouse model, in which PLX-3397 or vehicle was orally administrated to mice daily for 21 days, then applied to the CCI model, followed by PLX-3397 or vehicle administration for an additional 28 days. Additionally, we examined microglia-related neuroinflammation markers using positron emission tomography (PET) neuroimaging and immunofluorescence (IF).

RESULTS

We showed that PLX-3397 significantly ameliorated pain-related behavioral changes throughout the entire experimental period after CCI (vehicle versus PLX-3397 at day 14, effect size: 2.57, P = .002). Microglia changes were first analyzed by live-animal PET neuroimaging, revealing PLX-3397-associated reduction of microglia by probing receptor-interacting serine/threonine-protein kinase 1 (RIPK1), a protein primarily expressed in microglia, which were further corroborated by postmortem immunohistochemistry (IHC) analysis using antibodies for microglia, including ionized Ca2+ binding adaptor molecule 1 (Iba-1) (somatosensory cortex, hindlimb area; vehicle versus PLX-3397, effect size 3.6, P = .011) and RIPK1 (somatosensory cortex, hindlimb area; vehicle versus PLX-3397, effect size 2.9, P = .023. The expression of both markers decreased in the PLX-3397 group. Furthermore, we found that PLX-3397 led to significant reductions in various proteins, including inducible nitric oxide synthase (iNOS) (somatosensory cortex, hindlimb area; vehicle versus PLX-3397, effect size: 2.3, P = .048), involved in neuroinflammation through IHC.

CONCLUSIONS

Collectively, our study showed PLX-3397-related efficacy in ameliorating pain linked to the reduction of microglia and neuroinflammation in mice. Furthermore, our research provided new proof-of-concept data supporting the promise of testing PLX-3397 as an analgesic.

Histone deacetylases and their inhibitors in inflammatory diseases

Despite considerable research efforts, inflammatory diseases remain a heavy burden on human health, causing significant economic losses annually. Histone deacetylases (HDACs) play a significant role in regulating inflammation (via histone and non-histone protein deacetylation) and chromatin structure and gene expression regulation. Herein, we present a detailed description of the different HDACs and their functions and analyze the role of HDACs in inflammatory diseases, including pro-inflammatory cytokine production reduction, immune cell function modulation, and anti-inflammatory cell activity enhancement. Although HDAC inhibitors have shown broad inflammatory disease treatment potentials, their clinical applicability remains limited because of their non-specific effects, adverse effects, and drug resistance. With further research and insight, these inhibitors are expected to become important tools for the treatment of a wide range of inflammatory diseases. This review aims to explore the mechanisms and application prospects of HDACs and their inhibitors in multiple inflammatory diseases.

Insight in Quinazoline-based HDAC Inhibitors as Anti-cancer Agents

Cancer remains a primary cause of death globally, and effective treatments are still limited. While chemotherapy has notably enhanced survival rates, it brings about numerous side effects. Consequently, the ongoing challenge persists in developing potent anti-cancer agents with minimal toxicity. The versatile nature of the quinazoline moiety has positioned it as a pivotal component in the development of various antitumor agents, showcasing its promising role in innovative cancer therapeutics. This concise review aims to reveal the potential of quinazolines in creating anticancer medications that target histone deacetylases (HDACs).

Structure-Based Discovery of A Small Molecule Inhibitor of Histone Deacetylase 6 (HDAC6) that Significantly Reduces Alzheimer's Disease Neuropathology

Histone deacetylase 6 (HDAC6) is one of the key histone deacetylases (HDACs) that regulates various cellular functions including clearance of misfolded protein and immunological responses. Considerable evidence suggests that HDAC6 is closely related to amyloid and tau pathology, the two primary hallmarks of Alzheimer's disease (AD). It is still unclear whether HDAC6 expression changes with amyloid deposition in AD during disease progression or HDAC6 may be regulating amyloid phagocytosis or neuroinflammation or other neuropathological changes in AD. In this work, the pathological accumulation of HDAC6 in AD brains over age as well as the relationship of its regulatory activity - with amyloid pathogenesis and pathophysiological alterations is aimed to be enlightened using the newly developed HDAC6 inhibitor (HDAC6i) PB118 in microglia BV2 cell and 3D-AD human neural culture model. Results suggest that the structure-based rational design led to biologically compelling HDAC6i PB118 with multiple mechanisms that clear Aβ deposits by upregulating phagocytosis, improve tubulin/microtubule network by enhancing acetyl α-tubulin levels, regulate different cytokines and chemokines responsible for inflammation, and significantly reduce phospho-tau (p-tau) levels associated with AD. These findings indicate that HDAC6 plays key roles in the pathophysiology of AD and potentially serves as a suitable pharmacological target through chemical biology-based drug discovery in AD.