Discovery of a fluorescent probe with HDAC6 selective inhibition

Probing metalloenzyme dynamics in living systems: Contemporary advances in fluorescence imaging tools and applications

Metalloenzymes are essential to cellular function, and their overexpression or enhanced activation are potential therapeutic targets. However, the study of metalloenzymes in vitro presents various challenges, leading many to develop tools to study them in their native cellular environment. Small-molecule fluorescence probes are commonly used to monitor metalloenzyme function, activity, and distribution in situ. These include probes that are activity-based (fluorescence is mediated by enzyme activity) or binding-based (fluorescence is mediated by interactions with the enzyme upon binding its metal cofactor). We discuss recent innovations that overcome key design challenges, such as the rapid diffusion of activity-based probes, the difficulty of probing redox-active enzymes, the selectivity of binding-based probes, and the poor penetration depth of fluorescence, and describe novel applications of these tools.

Histone deacetylase 6's function in viral infection, innate immunity, and disease: latest advances

In the family of histone-deacetylases, histone deacetylase 6 (HDAC6) stands out. The cytoplasmic class IIb histone deacetylase (HDAC) family is essential for many cellular functions. It plays a crucial and debatable regulatory role in innate antiviral immunity. This review summarises the current state of our understanding of HDAC6's structure and function in light of the three mechanisms by which it controls DNA and RNA virus infection: cytoskeleton regulation, host innate immune response, and autophagy degradation of host or viral proteins. In addition, we summed up how HDAC6 inhibitors are used to treat a wide range of diseases, and how its upstream signaling plays a role in the antiviral mechanism. Together, the findings of this review highlight HDAC6's importance as a new therapeutic target in antiviral immunity, innate immune response, and some diseases, all of which offer promising new avenues for the development of drugs targeting the immune response.

Discovery of the First Irreversible HDAC6 Isoform Selective Inhibitor with Potent Anti-Multiple Myeloma Activity

In our previous research, a series of phenylsulfonylfuroxan-based hydroxamates were developed, among which compound 1 exhibited remarkable in vitro and in vivo antitumor potency due to its histone deacetylase (HDAC) inhibitory and nitric oxide (NO)-donating activities. Herein, the in-depth study of compound 1 revealed that this HDAC inhibitor-NO donor hybrid could enduringly increase the intracellular levels of acetyl histones and acetyl α-tubulin, which could be ascribed to its irreversible inhibition toward class I HDACs and HDAC6. Structural modification of compound 1 led to a novel phenylsulfonylfuroxan-based hydroxamate 4, which exhibited considerable HDAC6 inhibitory activity and selectivity. Furthermore, compound 4 could inhibit intracellular HDAC6 both selectively and irreversibly. To the best of our knowledge, this is the first research reporting the irreversible inhibition of HDAC6. It was also demonstrated that compared with ACY-241 (a reversible HDAC6 inhibitor in clinical trials), the irreversible HDAC6 selective inhibitor 4 exhibited not only superior anti-multiple myeloma activity but also improved therapeutic index.

Affinity probes based on small-molecule inhibitors for tumor imaging

Methods for molecular imaging of target areas, including optical imaging, radionuclide imaging, magnetic resonance imaging and other imaging technologies, are helpful for the early diagnosis and precise treatment of cancers. In addition to cancer management, small-molecule inhibitors are also used for developing cancer target probes since they act as the tight-binding ligands of overexpressed proteins in cancer cells. This review aims to summarize the structural designs of affinity probes based on small-molecule inhibitors from the aspects of the inhibitor, linker, dye and radionuclide, and discusses the influence of the modification of these structures on affinity and pharmacokinetics. We also present examples of inhibitor affinity probes in clinical applications, and these summaries will provide insights for future research and clinical translations.

Recent advances in HDAC-targeted imaging probes for cancer detection

Histone Deacetylases (HDACs) are abnormally high expressed in various cancers and play a crucial role in regulating gene expression. While HDAC-targeted inhibitors have been rapidly developed and approved in the last twenty years, noninvasive monitoring and visualizing the expression levels of HDACs in tumor tissues might help to early diagnosis in cancer and predict the response to HDAC-targeted cancer therapy. In this review, we summarize the recent advancements in the development of HDAC-targeted probes and their applications in cancer imaging and image-guided surgery. We also discuss the design strategies, advantages and disadvantages of these probes. We hope that this review will provide guidance for the design of HDAC-targeted imaging probes and clinical applications in future.

First Fluorescent Acetylspermidine Deacetylation Assay for HDAC10 Identifies Selective Inhibitors with Cellular Target Engagement

Histone deacetylases (HDACs) are important epigenetic regulators involved in many diseases, especially cancer. Five HDAC inhibitors have been approved for anticancer therapy and many are in clinical trials. Among the 11 zinc-dependent HDACs, HDAC10 has received relatively little attention by drug discovery campaigns, despite its involvement, e. g., in the pathogenesis of neuroblastoma. This is due in part to a lack of robust enzymatic conversion assays. In contrast to the protein lysine deacetylase and deacylase activity of most other HDAC subtypes, it has recently been shown that HDAC10 has strong preferences for deacetylation of oligoamine substrates like acetyl-putrescine or -spermidine. Hence, it is also termed a polyamine deacetylase (PDAC). Here, we present the first fluorescent enzymatic conversion assay for HDAC10 using an aminocoumarin-labelled acetyl-spermidine derivative to measure its PDAC activity, which is suitable for high-throughput screening. Using this assay, we identified potent inhibitors of HDAC10-mediated spermidine deacetylation in vitro. Based on the oligoamine preference of HDAC10, we also designed inhibitors with a basic moiety in appropriate distance to the zinc binding hydroxamate that showed potent inhibition of HDAC10 with high selectivity, and we solved a HDAC10-inhibitor structure using X-ray crystallography. We could demonstrate selective cellular target engagement for HDAC10 but a lysosomal phenotype in neuroblastoma cells that was previously associated with HDAC10 inhibition was not observed. Thus, we have developed new chemical probes for HDAC10 that allow further clarification of the biological role of this enzyme.

Histone Deacetylases as Epigenetic Targets for Treating Parkinson's Disease

Parkinson's disease (PD) is a chronic progressive neurodegenerative disease that is increasingly becoming a global threat to the health and life of the elderly worldwide. Although there are some drugs clinically available for treating PD, these treatments can only alleviate the symptoms of PD patients but cannot completely cure the disease. Therefore, exploring other potential mechanisms to develop more effective treatments that can modify the course of PD is still highly desirable. Over the last two decades, histone deacetylases, as an important group of epigenetic targets, have attracted much attention in drug discovery. This review focused on the current knowledge about histone deacetylases involved in PD pathophysiology and their inhibitors used in PD studies. Further perspectives related to small molecules that can inhibit or degrade histone deacetylases to treat PD were also discussed.

HDAC Inhibitors: Innovative Strategies for Their Design and Applications

Histone deacetylases (HDACs) are a large family of epigenetic metalloenzymes that are involved in gene transcription and regulation, cell proliferation, differentiation, migration, and death, as well as angiogenesis. Particularly, disorders of the HDACs expression are linked to the development of many types of cancer and neurodegenerative diseases, making them interesting molecular targets for the design of new efficient drugs and imaging agents that facilitate an early diagnosis of these diseases. Thus, their selective inhibition or degradation are the basis for new therapies. This is supported by the fact that many HDAC inhibitors (HDACis) are currently under clinical research for cancer therapy, and the Food and Drug Administration (FDA) has already approved some of them. In this review, we will focus on the recent advances and latest discoveries of innovative strategies in the development and applications of compounds that demonstrate inhibitory or degradation activity against HDACs, such as PROteolysis-TArgeting Chimeras (PROTACs), tumor-targeted HDACis (e.g., folate conjugates and nanoparticles), and imaging probes (positron emission tomography (PET) and fluorescent ligands).

Fluorescent molecular probe-based activity and inhibition monitoring of histone deacetylases

Extensive studies in recent decades have revealed that gene expression regulation is not limited to genetic mutations but also to processes that do not alter the genetic sequence. Post-translational histone modification is one of these processes in addition to DNA or RNA modifications. Histone modifications are essential in controlling histone functions and play a vital role in cellular gene expression. The reversible histone acetylation, regulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), is an example of such modifications. HDACs are involved in the deacetylation of histones and lead to the termination of gene expression. Although this cellular process is essential, upregulation of HDACs is found in numerous cancers. Therefore, research related to the activity and inhibition monitoring of HDACs is necessary to gain profound knowledge of these enzymes and evaluate the success of the therapeutic approach. In this perspective, methodology derived from fluorescent molecular probes is one of the preferable methods. Herein, we describe fluorescent probes developed to target HDACs by considering their activity and inhibition characteristics.

An HDAC8-selective fluorescent probe for imaging in living tumor cell lines and tissue slices

Histone deacetylase 8 (HDAC8) has been used as a therapeutic target for many cancers as it is highly expressed in neuroblastoma cells and breast cancer cells. HDAC8-selective fluorescent probes need to be urgently developed. Herein, two novel fluorescent probes, namely NP-C6-PCI and AM-C6-PCI, based on the conjugation of 1,8-naphthalimide with a highly selective inhibitor of HDAC8 (PCI-34051) were reported. Compared with PCI-34051 (KD = 6.25 × 10-5 M), NP-C6-PCI (KD = 8.05 × 10-6 M) and AM-C6-PCI (KD = 7.42 × 10-6 M) showed great selectivity toward HDAC8. Two fluorescent probes exhibited high fluorescence intensity under λex = 450 nm and a large Stokes shift (100 nm). NP-C6-PCI was selected for cell and tissue imaging due to the similarity in the bioactivity of NP-C6-PCI with PCI-34051. The ability of NP-C6-PCI to target imaging HDAC8 in SH-SY5Y and MDA-MB-231 tumor cells was demonstrated. Furthermore, NP-C6-PCI was applied to imaging SH-SY5Y tumor tissue slices to indicate the relative expression level of HDAC8. Therefore, this HDAC8-selective fluorescent probe can be expected for applications in HDAC8-targeted drug screening as well as in pathologic diagnoses.

Development of selective HDAC6 inhibitors with in vitro and in vivo anti-multiple myeloma activity

Histone deacetylase 6 (HDAC6) is a promising therapeutic target for the treatment of cancers, neurodegenerative diseases and autoimmune disorders. Herein a novel series of pyrrolo[2,3-d]pyrimidine-based HDAC inhibitors were designed, synthesized and biologically evaluated, among which compounds 7a, 12a1, and 16a1 exhibited potent inhibitory activities and selectivities against HDAC6. Notably, compared with the well-known HDAC6 inhibitor Tubastatin A, our pyrrolo[2,3-d]pyrimidine-based HDAC6 inhibitors showed superior in vitro antiproliferative activity against human multiple myeloma cell lines RPMI 8226, U266 and MM.1S, while maintaining the low cytotoxicity against human breast cancer cell line MDA-MB-231 and two normal cell lines. The HDAC6 selective inhibition of one representative compound 12a1 in RPMI 8226 cells was confirmed by western blot analysis. Although pyrrolo[2,3-d]pyrimidine is a privileged structure in many kinase inhibitors, compound 12a1 showed negligible inhibition against several kinases including JAK family members and Akt1, indicating its acceptable off-target profile. Besides, compound 12a1 exhibited desirable metabolic stability in mouse liver microsome. The in vivo anti-multiple myeloma potency of 12a1, alone and in combination with bortezomib, was demonstrated in a RPMI 8226 xenograft model.

Intracellular fluorescence competition assay for inhibitor engagement of histone deacetylase

An intracellular fluorescence competition assay was developed to assess the capability of inhibitor candidates to engage histone deacetylase (HDAC) inside living cells and thus diminish cell uptake and staining by the HDAC-targeted fluorescent probe APS. Fluorescence cell microscopy and flow cytometry showed that pre-incubation of living cells with candidate inhibitors led to diminished cell uptake of the fluorescent probe. The assay was effective because the fluorescent probe (APS) possessed the required performance properties, including bright fluorescence, ready membrane diffusion, selective intracellular HDAC affinity, and negligible acute cytotoxicity. The concept of an intracellular fluorescence competition assay is generalizable and has broad applicability since it obviates the requirement to use the isolated biomacromolecule target for screening of molecular candidates with target affinity.

Overcome the tumor immunotherapy resistance by combination of the HDAC6 inhibitors with antitumor immunomodulatory agents

Tumor immunotherapy is currently subject of intense scientific and clinical developments. In previous decade, therapists used natural immune system from the human body to treat several diseases. Although tumor immune disease is a big challenge, combinatorial therapeutic strategy has been succeeded to show the clinical significance. In this context, we discuss the HDAC6 and tumor immune diseases relationship. Also, we summarized the current state of knowledge that based on the combination treatments of the HDAC6 inhibitors (HDAC6is) with antitumor immunomodulatory agents. We observed that, the combination therapies slow down the tumor immune diseases by blocking the aggresome and proteasome pathway. The combination therapy was able to reduce M2 macrophage and increasing PD-L1 blockade sensitivity. Most importantly, multiple combinations of HDAC6is with other agents may consider as potential strategies to treat tumor immune diseases, by reducing the side effects and improve efficacy for the future clinical development.

Mixed alkoxy/hydroxy 1,8-naphthalimides: expanded fluorescence colour palette and in vitro bioactivity

An efficient and functional group tolerant route to access hydroxy 1,8-naphthalimides has been used to synthesise a range of mono- and disubstituted hydroxy-1,8-naphthalimides with fluorescence emissions covering the visible spectrum. The dialkoxy substituted compounds prepared possess high quantum yields (up to 0.95) and long fluorescent lifetimes (up to 14 ns). The method has been used to generate scriptaid analogues that successfully inhibit HDAC6 in vitro with tubulin acetylation assays confirming that these compounds are more effective than tubastatin.

Next-generation of selective histone deacetylase inhibitors

Histone deacetylases (HDACs) are clinically validated epigenetic drug targets for cancer treatment. HDACs inhibitors (HDACis) have been successfully applied against a series of cancers. First-generation inhibitors are mainly pan-HDACis that target multiple isoforms which might lead to serious side effects. At present, the next-generation HDACis are mainly focused on being class- or isoform-selective which can provide improved risk–benefit profiles compared to non-selective inhibitors. Because of the rapid development in next-generation HDACis, it is necessary to have an updated and state-of-the-art overview. Here, we summarize the strategies and achievements of the selective HDACis.

Histone deacetylases (HDACs) are clinically validated epigenetic drug targets for cancer treatment.

Fluorescent "keep-on" type pharmacophore obtained from dynamic combinatorial library of Schiff bases

We established a novel principle for fluorescence detection of a target protein. A low-molecular-weight fluorescent pharmacophore, as a targeted probe, was selected from a dynamic combinatorial library of Schiff bases. The pharmacophore retains its fluorescence when bound to the hydrophobic site of the target, whereas it loses it because of hydrolysis when unbound. Graphical abstract We describe a novel concept for detection of a target protein (i.e., HSA) by using a keep-on-type fluorescent pharmacophore which is discovered from a dynamic combinatorial library of Schiff bases. When the target is absent, the keep-on-pharmacophore is degraded by hydrolysis, with the result that we can see no fluorescence.

A highly HDAC6-selective inhibitor acts as a fluorescent probe

HDAC6 receives great attention because of its therapeutic potential for the treatment of various diseases.