PET imaging demonstrates histone deacetylase target engagement and clarifies brain penetrance of known and novel small molecule inhibitors in rat

Investigating neuroepigenetic alterations in chronic low back pain with positron emission tomography

ABSTRACT

Epigenetics has gained considerable interest as potential mediators of molecular alterations that could underlie the prolonged sensitization of nociceptors, neurons, and glia in response to various environmental stimuli. Histone acetylation and deacetylation, key processes in modulating chromatin, influence gene expression; elevated histone acetylation enhances transcriptional activity, whereas decreased acetylation leads to DNA condensation and gene repression. Altered levels of histone deacetylase (HDAC) have been detected in various animal pain models, and HDAC inhibitors have demonstrated analgesic effects in these models, indicating HDACs' involvement in chronic pain pathways. However, animal studies have predominantly examined epigenetic modulation within the spinal cord after pain induction, which may not fully reflect the complexity of chronic pain in humans. Moreover, methodological limitations have previously impeded an in-depth study of epigenetic changes in the human brain. In this study, we employed [11C]Martinostat, an HDAC-selective radiotracer, positron emission tomography to assess HDAC availability in the brains of 23 patients with chronic low back pain (cLBP) and 11 age-matched and sex-matched controls. Our data revealed a significant reduction of [11C]Martinostat binding in several brain regions associated with pain processing in patients with cLBP relative to controls, highlighting the promising potential of targeting HDAC modulation as a therapeutic strategy for cLBP.

Introducing HDAC-Targeting Radiopharmaceuticals for Glioblastoma Imaging and Therapy

Despite recent advances in multimodality therapy for glioblastoma (GB) incorporating surgery, radiotherapy, chemotherapy and targeted therapy, the overall prognosis remains poor. One of the interesting targets for GB therapy is the histone deacetylase family (HDAC). Due to their pleiotropic effects on, e.g., DNA repair, cell proliferation, differentiation, apoptosis and cell cycle, HDAC inhibitors have gained a lot of attention in the last decade as anti-cancer agents. Despite their known underlying mechanism, their therapeutic activity is not well-defined. In this review, an extensive overview is given of the current status of HDAC inhibitors for GB therapy, followed by an overview of current HDAC-targeting radiopharmaceuticals. Imaging HDAC expression or activity could provide key insights regarding the role of HDAC enzymes in gliomagenesis, thus identifying patients likely to benefit from HDACi-targeted therapy.

Fragment-Based Discovery of a Novel, Brain Penetrant, Orally Active HDAC2 Inhibitor

Fragment-based ligand discovery was successfully applied to histone deacetylase HDAC2. In addition to the anticipated hydroxamic acid- and benzamide-based fragment screening hits, a low affinity (∼1 mM) α-amino-amide zinc binding fragment was identified, as well as fragments binding to other regions of the catalytic site. This alternative zinc-binding fragment was further optimized, guided by the structural information from protein-ligand complex X-ray structures, into a sub-μM, brain penetrant, HDAC2 inhibitor (17) capable of modulating histone acetylation levels in vivo.

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.

Development and Biological Evaluation of the First Highly Potent and Specific Benzamide-Based Radiotracer [18F]BA3 for Imaging of Histone Deacetylases 1 and 2 in Brain

The degree of acetylation of lysine residues on histones influences the accessibility of DNA and, furthermore, the gene expression. Histone deacetylases (HDACs) are overexpressed in various tumour diseases, resulting in the interest in HDAC inhibitors for cancer therapy. The aim of this work is the development of a novel 18F-labelled HDAC1/2-specific inhibitor with a benzamide-based zinc-binding group to visualize these enzymes in brain tumours by positron emission tomography (PET). BA3, exhibiting high inhibitory potency for HDAC1 (IC50 = 4.8 nM) and HDAC2 (IC50 = 39.9 nM), and specificity towards HDAC3 and HDAC6 (specificity ratios >230 and >2080, respectively), was selected for radiofluorination. The two-step one-pot radiosynthesis of [18F]BA3 was performed in a TRACERlab FX2 N radiosynthesizer by a nucleophilic aliphatic substitution reaction. The automated radiosynthesis of [18F]BA3 resulted in a radiochemical yield of 1%, a radiochemical purity of >96% and a molar activity between 21 and 51 GBq/µmol (n = 5, EOS). For the characterization of BA3, in vitro and in vivo experiments were carried out. The results of these pharmacological and pharmacokinetic studies indicate a suitable inhibitory potency of BA3, whereas the applicability for non-invasive imaging of HDAC1/2 by PET requires further optimization of the properties of this compound.

Development of a Novel, Small-Molecule Brain-Penetrant Histone Deacetylase Inhibitor That Enhances Spatial Memory Formation in Mice

Histone acetylation is a prominent epigenetic modification linked to the memory loss symptoms associated with neurodegenerative disease. The use of existing histone deacetylase inhibitor (HDACi) drugs for treatment is precluded by their weak blood-brain barrier (BBB) permeability and undesirable toxicity. Here, we address these shortcomings by developing a new class of disulfide-based compounds, inspired by the scaffold of the FDA-approved HDACi romidepsin (FK288). Our findings indicate that our novel compound MJM-1 increases the overall level of histone 3 (H3) acetylation in a prostate cancer cell line. In mice, MJM-1 injected intraperitoneally (i.p.) crossed the BBB and could be detected in the hippocampus, a brain region that mediates memory. Consistent with this finding, we found that the post-training i.p. administration of MJM-1 enhanced hippocampus-dependent spatial memory consolidation in male mice. Therefore, MJM-1 represents a potential lead for further optimization as a therapeutic strategy for ameliorating cognitive deficits in aging and neurodegenerative diseases.

Imaging HDACs In Vivo: Cross-Validation of the [11C]Martinostat Radioligand in the Pig Brain

PURPOSE

With the emerging knowledge about the impact of epigenetic alterations on behavior and brain disorders, the ability to measure epigenetic alterations in brain tissue in vivo has become critically important. We present the first in vivo/in vitro cross-validation of the novel positron emission tomography (PET) radioligand [¹¹C]Martinostat in the pig brain with regard to its ability to measure histone deacetylase 1-3 (HDAC1-3) levels in vivo.

PROCEDURES

Nine female Danish landrace pigs underwent 121-min dynamic PET scans with [¹¹C]Martinostat. We quantified [¹¹C]Martinostat uptake using both a simple ratio method and kinetic models with arterial input function. By the end of the scan, the animals were euthanized and the brains were extracted. We measured HDAC1-3 protein levels in frontal cortex, cerebellum vermis, and hippocampus and compared the protein levels and regional outcome values to the [¹¹C]Martinostat PET quantification.

RESULTS

[¹¹C]Martinostat distributed widely across brain regions, with the highest uptake in the cerebellum vermis and the lowest in the olfactory bulbs. Based on the Akaike information criterion, the quantification was most reliably performed by Ichise MA1 kinetic modeling, but since the radioligand displayed very slow kinetics, we also calculated standard uptake value (SUV) ratios which correlated well with V_T. The western blots revealed higher brain tissue protein levels of HDAC1/2 compared to HDAC3, and HDAC1 and HDAC2 levels were highly correlated in all three investigated brain regions. The in vivo SUV ratio measure correlated well with the in vitro HDAC1-3 levels, whereas no correlation was found between V_T values and HDAC levels.

CONCLUSIONS

We found good correlation between in vivo measured SUV ratios and in vitro measures of HDAC 1-3 proteins, supporting that [¹¹C]Martinostat provides a good in vivo measure of the cerebral HDAC1-3 protein levels.

Preclinical Evaluation of an 18F-Labeled SW-100 Derivative for PET Imaging of Histone Deacetylase 6 in the Brain

Histone deacetylase 6 (HDAC6), an enzyme involved in protein degradation, exhibits several unique properties, such as cytoplasmic localization and ubiquitin binding. HDAC6 has emerged as an interesting therapeutic target in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Techniques enabling noninvasive HDAC6 imaging in the brain could enhance understanding of its pathologic role, but development of brain-penetrating radioligands for HDACs imaging by positron emission tomography (PET) remains challenging. Here, we report the synthesis and evaluation of an ¹⁸F-labeled tetrahydroquinoline derivative, [¹⁸F]2, based on the HDAC6 selective inhibitor SW-100 as a brain HDAC6 imaging radioligand. [¹⁸F]2 was synthesized via copper-mediated radiofluorination from an arylboronic precursor, followed by removal of the catalyst by solid-phase extraction and then hydroxamic acid formation. [¹⁸F]2 demonstrated good penetration and moderate stability in the mouse brain. In mouse plasma, however, [¹⁸F]2 was rapidly metabolized to a corresponding carboxylic acid form. Blocking studies in mice with unlabeled compound 2 and HDAC6 selective inhibitors, including tubastatin A and ACY-775, demonstrated that the HDAC6 inhibitors displaced over 80% of [¹⁸F]2 taken up in the brain, indicating selective binding of [¹⁸F]2. These results suggest that [¹⁸F]2 is a potentially useful PET radioligand for brain HDAC6 imaging.

Implantable HDAC-inhibiting chemotherapeutics derived from hydrophobic amino acids for localized anticancer therapy

Epigenetic targeting of different cancers by inhibiting particular histone deacetylase (HDAC) isozymes is a promising treatment approach against cancer. Development of locally-implantable molecular inhibitors of HDAC (henceforth called HDACi) promises high tumour site concentration and reduced systemic degradation of the HDACi. Herein, we report the design of such implantable HDACi based on amphiphilic derivatives of hydrophobic amino acids endowed with a hydroxamic acid (hxa)-based zinc-binding residue. The amino acids present in HDACi influenced the HDAC isozyme that could be inhibited most effectively; the l-phenylalanine derivative 4e inhibited the HDAC6 isozyme most potently (IC50 ∼ 88 nM), while the l-isoleucine derivative 4h was most effective against the isozyme HDAC2 (IC50 ∼ 94 nM). We also noticed that the l-Phe derivative 4e was up to 5× more potent towards inhibiting HDAC6 than its optical antipode 4f derived from d-Phe. This was rationalized in terms of the varying extent of penetration of the enantiomeric inhibitors inside the catalytic tunnel of the enzyme. Since the isozymes HDAC6 and HDAC2 are overexpressed in different cancer cells, 4e and 4h elicited selective anticancer activity in different cancer cell lines. Additive therapeutic action of the combination therapy of 4e and 4h was observed on lung cancer cells that overexpress both these isozymes. Further, 4e formed implantable self-assembled hydrogels that achieved sustained and selective killing of cancer cells in the vicinity of implantation.

Synthesis and Evaluation of 18F-INER-1577-3 as a Central Nervous System (CNS) Histone Deacetylase Imaging Agent

BACKGROUND

Epigenetic dysfunction is implicated in many neurologic, psychiatric and oncologic diseases. Consequently, histone deacetylases (HDACs) inhibitors have been developed as therapeutic and imaging agents for these diseases. However, only a few radiotracers have been developed as HDACs imaging agents for the central nervous system (CNS). We report herein the synthesis and evaluation of [¹⁸F]INER-1577-3 ([¹⁸F]5) as an HDACs imaging agent for CNS.

METHODS

[¹⁸F]INER-1577-3 ([¹⁸F]5) was synthesized by two methods: one-step (A) and two-step (B) methods. Briefly, radiofluorination of the corresponding precursors (11, 12) with K[¹⁸F]/K2.2.2 followed by purifications with HPLC gave ([¹⁸F]5). The quality of [¹⁸F]INER- 1577-3 synthesized by these methods was verified by HPLC and TLC as compared to an authentic sample. The inhibitions of [¹⁸F]INER-1577-3 and related HDACs inhibitors on tumor cells growth were carried out with breast cancer cell line 4T1 and MCF-7. The whole-body and brain uptake of [¹⁸F]INER-1577-3 in rats and AD mice were determined using a micro-PET scanner and the data was analyzed using PMOD.

RESULTS

The radiochemical yield of [¹⁸F]INER-1577-3 synthesized by these two methods was 1.4 % (Method A) and 8.8% (Method B) (EOB), respectively. The synthesis time was 115 min and 100 min, respectively, from EOB. The inhibition studies showed that INER-1577-3 has a significant inhibitory effect in HDAC6 and HDAC8 but not HDAC2. PET studies in rats and AD mice showed a maximum at about 15 min postinjection for the whole brain of a rat (0.47 ± 0.03 %ID/g), SAMP8 mice (5.63 ± 1.09 %ID/g) and SAMR1 mice (7.23 ± 1.21 %ID/g).

CONCLUSION

This study showed that INER-1577-3 can inhibit tumor cell growth and is one of a few HDACs inhibitors that can penetrate the blood-brain barrier (BBB) and monitor HDAC activities in AD mice. Thus, [¹⁸F]INER-1577-3 may be a potent HDACs imaging agent, especially for CNS.

CN133, a Novel Brain-Penetrating Histone Deacetylase Inhibitor, Hampers Tumor Growth in Patient-Derived Pediatric Posterior Fossa Ependymoma Models

Pediatric ependymoma (EPN) is a highly aggressive tumor of the central nervous system that remains incurable in 40% of cases. In children, the majority of cases develop in the posterior fossa and can be classified into two distinct molecular entities: EPN posterior fossa A (PF-EPN-A) and EPN posterior fossa B (PF-EPN-B). Patients with PF-EPN-A have poor outcome and are in demand of new therapies. In general, PF-EPN-A tumors show a balanced chromosome copy number profile and have no recurrent somatic nucleotide variants. However, these tumors present abundant epigenetic deregulations, thereby suggesting that epigenetic therapies could provide new opportunities for PF-EPN-A patients. In vitro epigenetic drug screening of 11 compounds showed that histone deacetylase inhibitors (HDACi) had the highest anti-proliferative activity in two PF-EPN-A patient-derived cell lines. Further screening of 5 new brain-penetrating HDACi showed that CN133 induced apoptosis in vitro, reduced tumor growth in vivo and significantly extended the survival of mice with orthotopically-implanted EPN tumors by modulation of the unfolded protein response, PI3K/Akt/mTOR signaling, and apoptotic pathways among others. In summary, our results provide solid preclinical evidence for the use of CN133 as a new therapeutic agent against PF-EPN-A tumors.

In vivo human brain expression of histone deacetylases in bipolar disorder

The etiology of bipolar disorder (BD) is unknown and the neurobiological underpinnings are not fully understood. Both genetic and environmental factors contribute to the risk of BD, which may be linked through epigenetic mechanisms, including those regulated by histone deacetylase (HDAC) enzymes. This study measures in vivo HDAC expression in individuals with BD for the first time using the HDAC-specific radiotracer [¹¹C]Martinostat. Eleven participants with BD and 11 age- and sex-matched control participants (CON) completed a simultaneous magnetic resonance - positron emission tomography (MR-PET) scan with [¹¹C]Martinostat. Lower [¹¹C]Martinostat uptake was found in the right amygdala of BD compared to CON. We assessed uptake in the dorsolateral prefrontal cortex (DLPFC) to compare previous findings of lower uptake in the DLPFC in schizophrenia and found no group differences in BD. Exploratory whole-brain voxelwise analysis showed lower [¹¹C]Martinostat uptake in the bilateral thalamus, orbitofrontal cortex, right hippocampus, and right amygdala in BD compared to CON. Furthermore, regional [¹¹C]Martinostat uptake was associated with emotion regulation in BD in fronto-limbic areas, which aligns with findings from previous structural, functional, and molecular neuroimaging studies in BD. Regional [¹¹C]Martinostat uptake was associated with attention in BD in fronto-parietal and temporal regions. These findings indicate a potential role of HDACs in BD pathophysiology. In particular, HDAC expression levels may modulate attention and emotion regulation, which represent two core clinical features of BD.

Clinical validation of the novel HDAC6 radiotracer [18F]EKZ-001 in the human brain

Purpose

Histone deacetylase 6 (HDAC6) is a cytoplasmic enzyme that modulates intracellular transport and protein quality control. Inhibition of HDAC6 deacetylase activity has shown beneficial effects in disease models, including Alzheimer’s disease and amyotrophic lateral sclerosis. This first-in-human positron emission tomography (PET) study evaluated the brain binding of [¹⁸F]EKZ-001 ([¹⁸F]Bavarostat), a radiotracer selective for HDAC6, in healthy adult subjects.

Methods

Biodistribution and radiation dosimetry studies were performed in four healthy subjects (2M/2F, 23.5 ± 2.4 years) using sequential whole-body PET/CT. The most appropriate kinetic model to quantify brain uptake was determined in 12 healthy subjects (6M/6F, 57.6 ± 3.7 years) from 120-min dynamic PET/MR scans using a radiometabolite-corrected arterial plasma input function. Four subjects underwent retest scans (2M/2F, 57.3 ± 5.6 years) with a 1-day interscan interval to determine test-retest variability (TRV). Regional volume of distribution (V_T) was calculated using one-tissue and two-tissue compartment models (1-2TCM) and Logan graphical analysis (LGA), with time-stability assessed. V_T differences between males and females were evaluated using volume of interest and whole-brain voxel-wise approaches.

Results

The effective dose was 39.1 ± 7.0 μSv/MBq. Based on the Akaike information criterion, 2TCM was the preferred model compared to 1TCM. Regional LGA V_T were in agreement with 2TCM V_T, however demonstrated a lower absolute TRV of 7.7 ± 4.9%. Regional V_T values were relatively homogeneous with highest values in the hippocampus and entorhinal cortex. Reduction of acquisition time was achieved with a 0 to 60-min scan followed by a 90 to 120-min scan. Males demonstrated significantly higher V_T than females in the majority of cortical and subcortical brain regions. No relevant radiotracer related adverse events were reported.

Conclusion

[¹⁸F]EKZ-001 is safe and appropriate for quantifying HDAC6 expression in the human brain with Logan graphical analysis as the preferred quantitative approach. Males showed higher HDAC6 expression across the brain compared to females.

Design, synthesis and biological evaluation of a series of CNS penetrant HDAC inhibitors structurally derived from amyloid-β probes

To develop novel CNS penetrant HDAC inhibitors, a new series of HDAC inhibitors having benzoheterocycle were designed, synthesized, and biologically evaluated. Among the synthesized compounds, benzothiazole derivative 9b exhibited a remarkable anti-proliferative activity (GI₅₀ = 2.01 μM) against SH-SY5Y cancer cell line in a dose and time-dependent manner, better than the reference drug SAHA (GI₅₀ = 2.90 μM). Moreover, compound 9b effectively promoted the accumulation of acetylated Histone H3 and α-tubulin through inhibition of HDAC1 and HDAC6 enzymes, respectively. HDAC enzyme assay also confirmed that compound 9b efficiently inhibited HDAC1 and HDAC6 isoforms with IC₅₀ values of 84.9 nM and 95.9 nM. Furthermore, compound 9b inhibited colony formation capacity of SH-SY5Y cells, which is considered a hallmark of cell carcinogenesis and metastatic potential. The theoretical prediction, in vitro PAMPA-BBB assay, and in vivo brain pharmacokinetic studies confirmed that compound 9b had much higher BBB permeability than SAHA. In silico docking study demonstrated that compound 9b fitted in the substrate binding pocket of HDAC1 and HDAC6. Taken together, compound 9b provided a novel scaffold for developing CNS penetrant HDAC inhibitors and therapeutic potential for CNS-related diseases.

Neuroepigenetic signatures of age and sex in the living human brain

Age- and sex-related alterations in gene transcription have been demonstrated, however the underlying mechanisms are unresolved. Neuroepigenetic pathways regulate gene transcription in the brain. Here, we measure in vivo expression of the epigenetic enzymes, histone deacetylases (HDACs), across healthy human aging and between sexes using [¹¹C]Martinostat positron emission tomography (PET) neuroimaging (n = 41). Relative HDAC expression increases with age in cerebral white matter, and correlates with age-associated disruptions in white matter microstructure. A post mortem study confirmed that HDAC1 and HDAC2 paralogs are elevated in white matter tissue from elderly donors. There are also sex-specific in vivo HDAC expression differences in brain regions associated with emotion and memory, including the amygdala and hippocampus. Hippocampus and white matter HDAC expression negatively correlates with emotion regulation skills (n = 23). Age and sex are associated with HDAC expression in vivo, which could drive age- and sex-related transcriptional changes and impact human behavior.

Gene transcription is known to vary with age and sex, although the underlying mechanisms are unresolved. Here, the authors show that epigenetic enzymes known as HDACs, which regulate gene transcription, are increasingly expressed with age in the living human brain, with sex differences also observed.

Frontal Cortex Epigenetic Dysregulation During the Progression of Alzheimer's Disease

Although the frontal cortex plays an important role in cognitive function and undergoes neuronal dysfunction in Alzheimer's disease (AD), the factors driving these cellular alterations remain unknown. Recent studies suggest that alterations in epigenetic regulation play a pivotal role in this process in AD. We evaluated frontal cortex histone deacetylase (HDAC) and sirtuin (SIRT) levels in tissue obtained from subjects with a premortem diagnosis of no-cognitive impairment (NCI), mild cognitive impairment (MCI), mild to moderate AD (mAD), and severe AD (sAD) using quantitative western blotting. Immunoblots revealed significant increases in HDAC1 and HDAC3 in MCI and mAD, followed by a decrease in sAD compared to NCI. HDAC2 levels remained stable across clinical groups. HDAC4 was significantly increased in MCI and mAD, but not in sAD compared to NCI. HDAC6 significantly increased during disease progression, while SIRT1 decreased in MCI, mAD, and sAD compared to NCI. HDAC1 levels negatively correlated with perceptual speed, while SIRT1 positively correlated with perceptual speed, episodic memory, global cognitive score, and Mini-Mental State Examination. HDAC1 positively, while SIRT1 negatively correlated with cortical neurofibrillary tangle counts. These findings suggest that dysregulation of epigenetic proteins contribute to neuronal dysfunction and cognitive decline in the early stage of AD.

PET neuroimaging reveals histone deacetylase dysregulation in schizophrenia

BACKGROUND

Patients with schizophrenia (SCZ) experience chronic cognitive deficits. Histone deacetylases (HDACs) are enzymes that regulate cognitive circuitry; however, the role of HDACs in cognitive disorders, including SCZ, remains unknown in humans. We previously determined that HDAC2 mRNA levels were lower in dorsolateral prefrontal cortex (DLPFC) tissue from donors with SCZ compared with controls. Here we investigated the relationship between in vivo HDAC expression and cognitive impairment in patients with SCZ and matched healthy controls using [11C]Martinostat positron emission tomography (PET).

METHODS

In a case-control study, relative [11C]Martinostat uptake was compared between 14 patients with SCZ or schizoaffective disorder (SCZ/SAD) and 17 controls using hypothesis-driven region-of-interest analysis and unbiased whole brain voxel-wise approaches. Clinical measures, including the MATRICS consensus cognitive battery, were administered.

RESULTS

Relative HDAC expression was lower in the DLPFC of patients with SCZ/SAD compared with controls, and HDAC expression positively correlated with cognitive performance scores across groups. Patients with SCZ/SAD also showed lower relative HDAC expression in the dorsomedial prefrontal cortex and orbitofrontal gyrus, and higher relative HDAC expression in the cerebral white matter, pons, and cerebellum compared with controls.

CONCLUSIONS

These findings provide in vivo evidence of HDAC dysregulation in patients with SCZ and suggest that altered HDAC expression may impact cognitive function in humans.

FUNDING

National Institute of Mental Health (NIMH), Brain and Behavior Foundation, Massachusetts General Hospital (MGH), Athinoula A. Martinos Center for Biomedical Imaging, National Institute of Biomedical Imaging and Bioengineering (NIBIB), NIH Shared Instrumentation Grant Program.

HDAC2 dysregulation in the nucleus basalis of Meynert during the progression of Alzheimer's disease

AIMS

Alzheimer's disease (AD) is characterized by degeneration of cholinergic basal forebrain (CBF) neurons in the nucleus basalis of Meynert (nbM), which provides the major cholinergic input to the cortical mantle and is related to cognitive decline in patients with AD. Cortical histone deacetylase (HDAC) dysregulation has been associated with neuronal degeneration during AD progression. However, whether HDAC alterations play a role in CBF degeneration during AD onset is unknown. We investigated global HDAC protein levels and nuclear HDAC2 immunoreactivity in tissue containing the nbM, changes and their association with neurofibrillary tangles (NFTs) during the progression of AD.

METHODS

We used semi-quantitative western blotting and immunohistochemistry to evaluate HDAC and sirtuin (SIRT) levels in individuals that died with a premortem clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI), mild/moderate AD (mAD) or severe AD (sAD). Quantitative immunohistochemistry was used to identify HDAC2 protein levels in individual cholinergic nbM nuclei and their colocalization with the early phosphorylated tau marker AT8, the late-stage apoptotic tau marker TauC3 and Thioflavin-S, a marker of β-pleated sheet structures in NFTs.

RESULTS

In AD patients, HDAC2 protein levels were dysregulated in the basal forebrain region containing cholinergic neurons of the nbM. HDAC2 nuclear immunoreactivity was reduced in individual cholinergic nbM neurons across disease stages. HDAC2 nuclear reactivity correlated with multiple cognitive domains and with NFT formation.

CONCLUSIONS

These findings suggest that HDAC2 dysregulation contributes to cholinergic nbM neuronal dysfunction, NFT pathology, and cognitive decline during clinical progression of AD.

Design, Synthesis, and Blood-Brain Barrier Transport Study of Pyrilamine Derivatives as Histone Deacetylase Inhibitors

We designed and synthesized a pyrilamine derivative 1 as a selective class I HDAC inhibitor that targets pyrilamine-sensitive proton-coupled organic cation antiporter (PYSOCA) at the blood-brain barrier (BBB). Introduction of pyrilamine moiety to benzamide type HDAC inhibitors kept selective class I HDAC inhibitory activity and increased BBB permeability. Our BBB transport study showed that compound 1 is a substrate of PYSOCA. Thus, our findings suggest that the hybrid method of HDAC inhibitor and substrate of PYSOCA such as pyrilamine is useful for development of HDAC inhibitors with increased BBB permeability.

Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

Hydrolytic enzymes are a large class of biological catalysts that play a vital role in a plethora of critical biochemical processes required to maintain human health. However, the expression and/or activity of these important enzymes can change in many different diseases and therefore represent exciting targets for the development of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radiotracers. This review focuses on recently reported radiolabeled substrates, reversible inhibitors, and irreversible inhibitors investigated as PET and SPECT tracers for imaging hydrolytic enzymes. By learning from the most successful examples of tracer development for hydrolytic enzymes, it appears that an early focus on careful enzyme kinetics and cell-based studies are key factors for identifying potentially useful new molecular imaging agents.

Advances in the Development of PET Ligands Targeting Histone Deacetylases for the Assessment of Neurodegenerative Diseases

Epigenetic alterations of gene expression have emerged as a key factor in several neurodegenerative diseases. In particular, inhibitors targeting histone deacetylases (HDACs), which are enzymes responsible for deacetylation of histones and other proteins, show therapeutic effects in animal neurodegenerative disease models. However, the details of the interaction between changes in HDAC levels in the brain and disease progression remain unknown. In this review, we focus on recent advances in development of radioligands for HDAC imaging in the brain with positron emission tomography (PET). We summarize the results of radiosynthesis and biological evaluation of the HDAC ligands to identify their successful results and challenges. Since 2006, several small molecules that are radiolabeled with a radioisotope such as carbon-11 or fluorine-18 have been developed and evaluated using various assays including in vitro HDAC binding assays and PET imaging in rodents and non-human primates. Although most compounds do not readily cross the blood-brain barrier, adamantane-conjugated radioligands tend to show good brain uptake. Until now, only one HDAC radioligand has been tested clinically in a brain PET study. Further PET imaging studies to clarify age-related and disease-related changes in HDACs in disease models and humans will increase our understanding of the roles of HDACs in neurodegenerative diseases.

Fluid and imaging biomarkers for Alzheimer's disease: Where we stand and where to head to

There is increasing evidence that a number of potentially informative biomarkers for Alzheimer disease (AD) can improve the accuracy of diagnosing this form of dementia, especially when used as a panel of diagnostic assays and interpreted in the context of neuroimaging and clinical data. Moreover, by combining the power of CSF biomarkers with neuroimaging techniques to visualize Aβ deposits (or neurodegenerative lesions), it might be possible to better identify individuals at greatest risk for developing MCI and converting to AD. The objective of this article was to review recent progress in selected imaging and chemical biomarkers for prediction, early diagnosis and progression of AD. We present our view point of a scenario that places CSF and imaging markers on the verge of general utility based on accuracy levels that already match (or even surpass) current clinical precision.

Expression of HDAC2 but Not HDAC1 Transcript Is Reduced in Dorsolateral Prefrontal Cortex of Patients with Schizophrenia

Postmortem brain studies support dysregulated expression of the histone deacetylase enzymes, HDAC1 and HDAC2, as a central feature in diseases including schizophrenia, bipolar disorder, and depression. Our objective was to investigate HDAC expression in a large postmortem sample set representing healthy and disease brains. We used >700 well-characterized samples from patients diagnosed with schizophrenia (n = 175), major depressive disorder (n = 135), and bipolar disorder (n = 61) to measure HDAC1 and HDAC2 transcript levels by quantitative real-time PCR in dorsolateral prefrontal cortex (DLPFC) and caudate compared to control samples. HDAC expression was calculated relative to the geometric mean of β-2-microglobulin, β-glucuronidase, and β-actin. In adult-age DLPFC, HDAC2 was decreased by 34% in schizophrenia samples compared to controls (p < 10^–4). HDAC2 was significantly upregulated in major depressive disorder samples by 17% versus controls (p = 0.002). Neither smoking history nor therapeutic drugs impacted HDAC2 levels and no HDAC1 patient-control differences were observed. In caudate, HDAC levels were unchanged between patient and control groups. In control DLPFC, age fetal week 14 to 97 years (n = 326), both HDAC1 and HDAC2 levels sharply declined around birth and stabilized thereafter. Using by far the largest postmortem sample set on this topic, our major finding (decreased HDAC2 transcript) showed notable specificity in disease (schizophrenia but not major depressive disorder), HDAC subtype (HDAC2 but not HDAC1) and brain region (DLPFC but not caudate). These differences shape understanding of regional components of neural circuitry in the diseased brain and set a benchmark to quantify HDAC density and distribution using in vivo neuroimaging tools.

Epigenetic Mechanisms in Psychiatric Diseases and Epigenetic Therapy

Preclinical Research Epigenetic mechanisms refer covalent modification of DNA and histone proteins that control transcriptional regulation of gene expression. Epigenetic regulation is involved in the development of the nervous system and plays an important role in the pathophysiology of psychiatric disorders, including depression, bipolar disorder, and schizophrenia. Epigenetic drugs, including histone deacetylation and DNA methylation inhibitors have received increased attention for the management of psychiatric diseases. The purpose of this review is to discuss the potential of epigenetic drugs to treat these disorders and to clarify the mechanisms by which they regulate the dysfunctional genes in the brain. Drug Dev Res 77 : 407-413, 2016. © 2016 Wiley Periodicals, Inc.

Development of a Fluorinated Class-I HDAC Radiotracer Reveals Key Chemical Determinants of Brain Penetrance

Despite major efforts, our knowledge about many brain diseases remains remarkably limited. Epigenetic dysregulation has been one of the few leads toward identifying the causes and potential treatments of psychiatric disease over the past decade. A new positron emission tomography radiotracer, [(11)C]Martinostat, has enabled the study of histone deacetylase in living human subjects. A unique property of [(11)C]Martinostat is its profound brain penetrance, a feature that is challenging to engineer intentionally. In order to understand determining factors for the high brain-uptake of Martinostat, a series of compounds was evaluated in rodents and nonhuman primates. The study revealed the major structural contributors to brain uptake, as well as a more clinically relevant fluorinated HDAC radiotracer with comparable behavior to Martinostat, yet longer half-life.

Kinetic Analysis and Quantification of [¹¹C]Martinostat for in Vivo HDAC Imaging of the Brain

Epigenetic mechanisms mediated by histone deacetylases (HDACs) have been implicated in a wide-range of CNS disorders and may offer new therapeutic opportunities. In vivo evaluation of HDAC density and drug occupancy has become possible with [(11)C]Martinostat, which exhibits selectivity for a subset of class I/IIb HDAC enzymes. In this study, we characterize the kinetic properties of [(11)C]Martinostat in the nonhuman primate (NHP) brain in preparation for human neuroimaging studies. The goal of this work was to determine whether classic compartmental analysis techniques were appropriate and to further determine if arterial plasma is required for future NHP studies. Using an arterial plasma input function, several analysis approaches were evaluated for robust outcome measurements. [(11)C]Martinostat showed high baseline distribution volume (VT) ranging from 29.9 to 54.4 mL/cm(3) in the brain and large changes in occupancy (up to 99%) with a blocking dose approaching full enzyme saturation. An averaged nondisplaceable tissue uptake (VND) of 8.6 ± 3.7 mL/cm(3) suggests high specific binding of [(11)C]Martinostat. From a two-tissue compartment model, [(11)C]Martinostat exhibits a high K1 (averaged K1 of 0.65 mL/cm(3)/min) and a small k4 (average of 0.0085 min(-1)). Our study supports that [(11)C]Martinostat can be used to detect changes in HDAC density and occupancy in vivo and that simplified analysis not using arterial blood could be appropriate.

A philosophy for CNS radiotracer design

Decades after its discovery, positron emission tomography (PET) remains the premier tool for imaging neurochemistry in living humans. Technological improvements in radiolabeling methods, camera design, and image analysis have kept PET in the forefront. In addition, the use of PET imaging has expanded because researchers have developed new radiotracers that visualize receptors, transporters, enzymes, and other molecular targets within the human brain.

However, of the thousands of proteins in the central nervous system (CNS), researchers have successfully imaged fewer than 40 human proteins. To address the critical need for new radiotracers, this Account expounds on the decisions, strategies, and pitfalls of CNS radiotracer development based on our current experience in this area.

We discuss the five key components of radiotracer development for human imaging: choosing a biomedical question, selection of a biological target, design of the radiotracer chemical structure, evaluation of candidate radiotracers, and analysis of preclinical imaging. It is particularly important to analyze the market of scientists or companies who might use a new radiotracer and carefully select a relevant biomedical question(s) for that audience. In the selection of a specific biological target, we emphasize how target localization and identity can constrain this process and discuss the optimal target density and affinity ratios needed for binding-based radiotracers. In addition, we discuss various PET test–retest variability requirements for monitoring changes in density, occupancy, or functionality for new radiotracers.

In the synthesis of new radiotracer structures, high-throughput, modular syntheses have proved valuable, and these processes provide compounds with sites for late-stage radioisotope installation. As a result, researchers can manage the time constraints associated with the limited half-lives of isotopes. In order to evaluate brain uptake, a number of methods are available to predict bioavailability, blood–brain barrier (BBB) permeability, and the associated issues of nonspecific binding and metabolic stability. To evaluate the synthesized chemical library, researchers need to consider high-throughput affinity assays, the analysis of specific binding, and the importance of fast binding kinetics. Finally, we describe how we initially assess preclinical radiotracer imaging, using brain uptake, specific binding, and preliminary kinetic analysis to identify promising radiotracers that may be useful for human brain imaging. Although we discuss these five design components separately and linearly in this Account, in practice we develop new PET-based radiotracers using these design components nonlinearly and iteratively to develop new compounds in the most efficient way possible.