Novel Histone Deacetylase Class IIa Selective Substrate Radiotracers for PET Imaging of Epigenetic Regulation in the Brain

Computationally Guided Design, Synthesis, and Evaluation of Novel Non-Hydroxamic Histone Deacetylase Inhibitors, Based on N-Trifluoroacetamide as a Zinc-Binding Group, Against Breast Cancer

Background: Histone deacetylases (HDACs) are enzymes that deacetylate histone proteins, impacting the transcriptional repression and activation of cancer-associated genes such as P53 and Ras. The overexpression of HDACs in breast cancer (BC) underscores their significance as therapeutic targets for modulating gene expression through epigenetic regulation. Methods: In this study, a novel series of SAHA (suberoylanilide hydroxamic acid) analogs were designed using an in silico ligand-based strategy. These analogs were then synthesized and evaluated for their HDAC-inhibitory capacity as well as their antiproliferative capacity on breast cancer cells. These compounds retained an aliphatic LINKER, mimicking the natural substrate acetyl-lysine, while differing from the hydroxamic fragment present in SAHA. Results: The synthesized compounds exhibited HDAC inhibitory activity, suggesting potential for binding to these pharmacological targets. Compounds 5b, 6a, and 6b were identified as promising candidates in the evaluation on breast cancer cell lines MCF-7 and MDA-MB-231 at 72 h. Specifically, compound 6b, which contains an N-trifluoroacetyl group as a zinc-binding group (ZBG), demonstrated an IC50 of 76.7 µM in the MDA-MB-231 cell line and 45.7 µM in the MCF-7 cell line. In the non-tumorigenic cell line, the compound exhibited an IC50 of 154.6 µM. Conversely, SAHA exhibited an almost negligible safety margin with regard to its cytotoxic activity when compared to breast cancer cells and healthy cells (MCF-10A). This observation underscores the elevated toxicity exhibited by hydroxamic acid-derived molecules. Conclusions: The bioisosteric modification of ZBG by N-trifluoroacetyl in 6a and 6b demonstrated favorable cytotoxic activity, exhibiting a higher safety margin. This study underscores the challenge of identifying novel ZBGs to replace hydroxamic acid in the development of HDAC inhibitors, with the objective of enhancing their physicochemical and toxicological profile for utilization in BC treatment.

Epigenetic Control of an Auxiliary Subunit of Voltage-Gated Sodium Channels Regulates the Strength of Drug-Cue Associations and Relapse-Like Cocaine Seeking

BACKGROUND

Repeated use of addictive drugs produces long-lasting and prepotent drug-cue associations that increase vulnerability for relapse in individuals with a substance use disorder. Epigenetic factors, such as HDAC5 (histone deacetylase 5), play a key role in regulating the formation of drug-cue associations, but the underlying mechanisms remain unclear.

METHODS

We used a combination of molecular biology, cultured cells, tandem mass spectrometry, deacetylase activity measurements, co-immunoprecipitation, and molecular dynamics simulations to assess HDAC5 structure-activity relationships. In male and female Long Evans rats, we used viral-mediated expression of HDAC5 mutants in the nucleus accumbens (NAc) to test effects on cocaine intravenous self-administration and cue-reinstated cocaine seeking. We also used in silico analysis of single-nucleus RNA sequencing data, quantitative reverse transcriptase-polymerase chain reaction, viral-mediated expression of Scn4b short hairpin RNA, patch-clamp electrophysiology, and rat cocaine or sucrose SA to assess Scn4b's effects on NAc intrinsic excitability and cued reward seeking.

RESULTS

We discovered that 2 conserved cysteines located near HDAC5's catalytic domain were required for its intrinsic deacetylase activity and that HDAC5's deacetylase activity was required in NAc medium spiny neurons (MSNs) to limit relapse-like cue-reinstated cocaine seeking. Moreover, we found that HDAC5 limited cocaine-seeking, but not sucrose-seeking, behavior by reducing NAc MSN intrinsic excitability through the deacetylase-dependent repression of Scn4b, which codes for an auxiliary subunit of voltage-gated sodium channels.

CONCLUSIONS

Our findings suggest that HDAC5's control of NAc Scn4b expression governs the formation of cocaine-cue, but not sucrose-cue, associations through modulation of NAc MSN intrinsic excitability and drug-induced NAc plasticity mechanisms.

Histone Deacetylases (HDACs) Roles in Inflammation-mediated Diseases; Current Knowledge

The histone acetyl transferases (HATs) and histone deacetylases (HDACs), which are mostly recognized for their involvement in regulating chromatin remodeling via histone acetylation/deacetylation, have been shown to also change several non-histone proteins to regulate other cellular processes. Acetylation affects the activity or function of cytokine receptors, nuclear hormone receptors, intracellular signaling molecules, and transcription factors in connection to inflammation. Some small-molecule HDAC inhibitors are utilized as anticancer medications in clinical settings due to their capability to regulate cellular growth arrest, differentiation, and death. Here, we summarize our present knowledge of the innate and adaptive immunological pathways that classical HDAC enzymes control. The aim is to justify the targeted (or non-targeted) use of inhibitors against certain HDAC enzymes in inflammatory diseases such as arthritis, inflammatory bowel diseases (IBD), airways inflammation and neurological diseases.

High-Contrast PET Imaging with [18F]NT160, a Class-IIa Histone Deacetylase Probe for In Vivo Imaging of Epigenetic Machinery in the Central Nervous System

We utilized positron emission tomography (PET) imaging in vivo to map the spatiotemporal biodistribution/expression of class-IIa histone deacetylases (class-IIa HDACs) in the central nervous system (CNS). Herein we report an improved radiosynthesis of [18F]NT160 using 4-hydroxy-TEMPO which led to a significant improvement in radiochemical yield and molar activity. PET imaging with [18F]NT160, a highly potent class-IIa HDAC inhibitor, led to high-quality and high-contrast images of the brain. [18F]NT160 displayed excellent pharmacokinetic and imaging characteristics: brain uptake is high in gray matter regions, tissue kinetics are appropriate for a 18F-tracer, and specific binding for class-IIa HDACs is demonstrated by self-blockade. Higher uptake with [18F]NT160 was observed in the hippocampus, thalamus, and cortex while the uptake in the cerebellum was relatively low. Overall, our current studies with [18F]NT160 will likely facilitate the development and clinical translation of PET tracers for imaging of class-IIa HDACs biodistribution/expression in cancer and the CNS.

The Impact of Fluorination on the Design of Histone Deacetylase Inhibitors

In recent years, histone deacetylases (HDACs) have emerged as promising targets in the treatment of cancer. The approach is to inhibit HDACs with drugs known as HDAC inhibitors (HDACis). Such HDACis are broadly classified according to their chemical structure, e.g., hydroxamic acids, benzamides, thiols, short-chain fatty acids, and cyclic peptides. Fluorination plays an important role in the medicinal-chemical design of new active representatives. As a result of the introduction of fluorine into the chemical structure, parameters such as potency or selectivity towards isoforms of HDACs can be increased. However, the impact of fluorination cannot always be clearly deduced. Nevertheless, a change in lipophilicity and, hence, solubility, as well as permeability, can influence the potency. The selectivity towards certain HDACs isoforms can be explained by special interactions of fluorinated compounds with the structure of the slightly different enzymes. Another aspect is that for a more detailed investigation of newly synthesized fluorine-containing active compounds, fluorination is often used for the purpose of labeling. Aside from the isotope ¹⁹F, which can be detected by nuclear magnetic resonance spectroscopy, the positron emission tomography of ¹⁸F plays a major role. However, to our best knowledge, a survey of the general effects of fluorination on HDACis development is lacking in the literature to date. Therefore, the aim of this review is to highlight the introduction of fluorine in the course of chemical synthesis and the impact on biological activity, using selected examples of recently developed fluorinated HDACis.

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.

Evaluation of acyllysine isostere interactions with the aromatic pocket of the AF9 YEATS domain

Amide-π interactions, in which an amide interacts with an aromatic group, are ubiquitous in biology, yet remain understudied relative to other noncovalent interactions. Recently, we demonstrated that an electrostatically tunable amide-π interaction is key to recognition of histone acyllysine by the AF9 YEATS domain, a reader protein which has emerged as a therapeutic target due to its dysregulation in cancer. Amide isosteres are commonly employed in drug discovery, often to prevent degradation by proteases, and have proven valuable in achieving selectivity when targeting epigenetic proteins. However, like amide-π interactions, interactions of amide isosteres with aromatic rings have not been thoroughly studied despite widespread use. Herein, we evaluate the recognition of a series of amide isosteres by the AF9 YEATS domain using genetic code expansion to evaluate the amide isostere-π interaction. We show that compared to the amide-π interaction with the native ligand, each isostere exhibits similar electrostatic tunability with an aromatic residue in the binding pocket, demonstrating that the isosteres maintain similar interactions with the aromatic residue. We identify a urea-containing ligand that binds with enhanced affinity for the AF9 YEATS domain, offering a promising starting point for inhibitor development. Furthermore, we demonstrate that carbamate and urea isosteres of crotonyllysine are resistant to enzymatic removal by SIRT1, a protein that cleaves acyl post-translational modifications, further indicating the potential of amide isosteres in YEATS domain inhibitor development. These results also provide experimental precedent for interactions of these common drug discovery moieties with aromatic rings that can inform computational methods.

Low- and high-cocaine intake affects the spatial and temporal dynamics of class IIa HDAC expression-activity in the nucleus accumbens and hippocampus of male rats as measured by [18F]TFAHA PET/CT neuroimaging

Repeated cocaine alters neuronal function in the nucleus accumbens (NAc), a brain region involved in cocaine taking, and in hippocampus (HC), known for contextual and associative learning. [¹⁸F]TFAHA is a histone deacetylase (HDAC) class IIa-specific radiotracer for positron emission tomography (PET)-imaging developed by our group to study epigenetic mechanisms. Here, [¹⁸F]TFAHA was used to conduct PET-imaging coupled with computed tomography (CT) of rat brains at baseline and after repeated cocaine intravenous self-administration (cocaine-IVSA) in low-intake versus high-intake cocaine groups. A 3 h-access FR1-schedule of cocaine-IVSA (0.5 mg/kg/infusion) for 12 continuous days was used with male Sprague Dawley rats following jugular vein catheterization. PET/CT neuroimaging with [¹⁸F]TFAHA was acquired in a dynamic mode over 40 min post-radiotracer administration at baseline and on day 12 of cocaine-IVSA using a longitudinal, repeated design. This study shows that high-cocaine intake significantly decreases class IIa HDAC expression-activity in NAc, while low-cocaine intake significantly decreases expression-activity in HC in male rats. These findings suggest the individual rats with low-cocaine intake had epigenetic changes in HC, where drug-associative changes occur. Alternatively, individuals with high-cocaine intake had robust epigenetic changes in NAc, where rewared-related behaviors originate. These findings are the first longitudinal data obtained in vivo to implicate class IIa HDACs in the persistent behavioral effects of cocaine. Furthermore, our results are consistent with published research implicating class IIa HDACs in cocaine-induced brain changes and studies suggesting a relationship between an individual's drug-taking behavior and regional pattern of epigenetic changes in the brain.

Trifluoroacetyl Lysine as a Bromodomain Binding Mimic of Lysine Acetylation

Genetic code expansion has proven invaluable to the elucidation of functions of defined protein modifications through the site-specific incorporation of noncanonical amino acids. The use of nonhydrolyzable derivatives of post-translational modifications can greatly increase site stoichiometry and half-life. Investigating acetyllysine reader domain (bromodomain) interactions with acetylated nonhistone proteins is challenging due to the limited tools available and dynamic nature of this post-translational modification. Here, we demonstrate that bromodomains bind acetyllysine peptides and those substituted with an acetyllysine derivative, trifluoroacetyllysine, with similar affinity and selectivity. Importantly, both trifluoroacetyllysine and acetyllysine can be site-specifically incorporated into proteins expressed in bacterial and mammalian cells, and the strong electron-withdrawing trifluoro substituent makes the latter resistant to deacetylation by sirtuins (SIRTs). The controlled expression of SIRT-resistant, site-specifically acetylated transcription factors expands the set of available tools for determining the function of acetylation, and it serves as a template for investigating bromodomain interactions with acetylated transcription factors.

Spatial and temporal dynamics of HDACs class IIa following mild traumatic brain injury in adult rats

The fundamental role of epigenetic regulatory mechanisms involved in neuroplasticity and adaptive responses to traumatic brain injury (TBI) is gaining increased recognition. TBI-induced neurodegeneration is associated with several changes in the expression-activity of various epigenetic regulatory enzymes, including histone deacetylases (HDACs). In this study, PET/CT with 6-([18F]trifluoroacetamido)-1- hexanoicanilide ([18F]TFAHA) to image spatial and temporal dynamics of HDACs class IIa expression-activity in brains of adult rats subjected to a weight drop model of diffuse, non-penetrating, mild traumatic brain injury (mTBI). The mTBI model was validated by histopathological and immunohistochemical analyses of brain tissue sections for localization and magnitude of expression of heat-shock protein-70 kDa (HSP70), amyloid precursor protein (APP), cannabinoid receptor-2 (CB2), ionized calcium-binding adapter protein-1 (IBA1), histone deacetylase-4 and -5 (HDAC4 and HDAC5). In comparison to baseline, the expression-activities of HDAC4 and HDAC5 were downregulated in the hippocampus, nucleus accumbens, peri-3rd ventricular part of the thalamus, and substantia nigra at 1-3 days post mTBI, and remained low at 7-8 days post mTBI. Reduced levels of HDAC4 and HDAC5 expression observed in neurons of these brain regions post mTBI were associated with the reduced nuclear and neuropil levels of HDAC4 and HDAC5 with the shift to perinuclear localization of these enzymes. These results support the rationale for the development of therapeutic strategies to upregulate expression-activity of HDACs class IIa post-TBI. PET/CT (MRI) with [18F]TFAHA can facilitate the development and clinical translation of unique therapeutic approaches to upregulate the expression and activity of HDACs class IIa enzymes in the brain after TBI.

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).

Design, synthesis, biochemical evaluation, radiolabeling and in vivo imaging with high affinity class-IIa histone deacetylase inhibitor for molecular imaging and targeted therapy

Herein, we describe the design, synthesis and deciphering of the key characteristics of the structure activity relationship (SAR) of trifluoromethyloxadiazole (TFMO) bearing class-IIa HDAC inhibitors. Our medicinal chemistry campaign of 23 compounds identified compound 1 as a highly potent inhibitor with sub nM affinity to class-IIa HDAC4 isoform. Therefore, We radiolabeled compound 1 (named thereafter as NT160) with [¹⁸F]fluoride thus producing the identical [¹⁸F]-NT160 as a diagnostic tool for positron emission tomography (PET). [¹⁸F]-NT160 was produced in high radiochemical purity (>95%), moderate radiochemical yield (2−5%) and moderate molar activity in the range of 0.30−0.85 GBq/umol (8.0−23.0 mCi/umol). We also established that [¹⁸F]-NT160 can cross the blood brain barrier and bind to class-IIa HDACs in vivo. The combination of [¹⁸F]-NT160 and 1 represent a novel theranostic pair using the same molecule to enable diagnostic PET imaging with [¹⁸F]-NT160 followed by targeted therapy with NT160.

Evaluation of Class IIa Histone Deacetylases Expression and In Vivo Epigenetic Imaging in a Transgenic Mouse Model of Alzheimer's Disease

Epigenetic regulation by histone deacetylase (HDAC) is associated with synaptic plasticity and memory formation, and its aberrant expression has been linked to cognitive disorders, including Alzheimer's disease (AD). This study aimed to investigate the role of class IIa HDAC expression in AD and monitor it in vivo using a novel radiotracer, 6-(tri-fluoroacetamido)-1-hexanoicanilide ([¹⁸F]TFAHA). A human neural cell culture model with familial AD (FAD) mutations was established and used for in vitro assays. Positron emission tomography (PET) imaging with [¹⁸F]TFAHA was performed in a 3xTg AD mouse model for in vivo evaluation. The results showed a significant increase in HDAC4 expression in response to amyloid-β (Aβ) deposition in the cell model. Moreover, treatment with an HDAC4 selective inhibitor significantly upregulated the expression of neuronal memory-/synaptic plasticity-related genes. In [¹⁸F]TFAHA-PET imaging, whole brain or regional uptake was significantly higher in 3xTg AD mice compared with WT mice at 8 and 11 months of age. Our study demonstrated a correlation between class IIa HDACs and Aβs, the therapeutic benefit of a selective inhibitor, and the potential of using [¹⁸F]TFAHA as an epigenetic radiotracer for AD, which might facilitate the development of AD-related neuroimaging approaches and therapies.

Noninvasive quantification of SIRT1 expression-activity and pharmacologic inhibition in a rat model of intracerebral glioma using 2-[18F]BzAHA PET/CT/MRI

Background

Several studies demonstrated that glioblastoma multiforme progression and recurrence is linked to epigenetic regulatory mechanisms. Sirtuin 1 (SIRT1) plays an important role in glioma progression, invasion, and treatment response and is a potential therapeutic target. The aim of this study is to test the feasibility of 2-[¹⁸F]BzAHA for quantitative imaging of SIRT1 expression–activity and monitoring pharmacologic inhibition in a rat model of intracerebral glioma.

Methods

Sprague Dawley rats bearing 9L (N = 12) intracerebral gliomas were injected with 2-[¹⁸F]BzAHA (300–500 µCi/animal i.v.) and dynamic positron-emission tomography (PET) imaging was performed for 60 min. Then, SIRT1 expression in 9L tumors (N = 6) was studied by immunofluorescence microscopy (IF). Two days later, rats with 9L gliomas were treated either with SIRT1 specific inhibitor EX-527 (5 mg/kg, i.p.; N = 3) or with histone deacetylases class IIa specific inhibitor MC1568 (30 mg/kg, i.p.; N = 3) and 30 min later were injected i.v. with 2-[¹⁸F]BzAHA. PET-computerized tomography-magnetic resonance (PET/CT/MR) images acquired after EX-527 and MC1568 treatments were co-registered with baseline images.

Results

Standard uptake values (SUVs) of 2-[¹⁸F]BzAHA in 9L tumors measured at 20 min post-radiotracer administration were 1.11 ± 0.058 and had a tumor-to-brainstem SUV ratio of 2.73 ± 0.141. IF of 9L gliomas revealed heterogeneous upregulation of SIRT1, especially in hypoxic and peri-necrotic regions. Significant reduction in 2-[¹⁸F]BzAHA SUV and distribution volume in 9L tumors was observed after administration of EX-527, but not MC1568.

Conclusions

PET/CT/MRI with 2-[¹⁸F]BzAHA can facilitate studies to elucidate the roles of SIRT1 in gliomagenesis and progression, as well as to optimize therapeutic doses of novel SIRT1 inhibitors.

Molecular imaging HDACs class IIa expression-activity and pharmacologic inhibition in intracerebral glioma models in rats using PET/CT/(MRI) with [18F]TFAHA

HDAC class IIa enzymes (HDAC4, 5, 7, 9) are important for glioma progression, invasion, responses to TMZ and radiotherapy, and prognosis. In this study, we demonstrated the efficacy of PET/CT/(MRI) with [¹⁸F]TFAHA for non-invasive and quantitative imaging of HDAC class IIa expression-activity in intracerebral 9L and U87-MG gliomas in rats. Increased accumulation of [¹⁸F]TFAHA in 9L and U87-MG tumors was observed at 20 min post radiotracer administration with SUV of 1.45 ± 0.05 and 1.08 ± 0.05, respectively, and tumor-to-cortex SUV ratios of 1.74 ± 0.07 and 1.44 ± 0.03, respectively. [¹⁸F]TFAHA accumulation was also observed in normal brain structures known to overexpress HDACs class IIa: hippocampus, n.accumbens, PAG, and cerebellum. These results were confirmed by immunohistochemical staining of brain tissue sections revealing the upregulation of HDACs 4, 5, and 9, and HIF-1α, hypoacetylation of H2AK5ac, H2BK5ac, H3K9ac, H4K8ac, and downregulation of KLF4. Significant reduction in [¹⁸F]TFAHA accumulation in 9L tumors was observed after administration of HDACs class IIa specific inhibitor MC1568, but not the SIRT1 specific inhibitor EX-527. Thus, PET/CT/(MRI) with [¹⁸F]TFAHA can facilitate studies to elucidate the roles of HDAC class IIa enzymes in gliomagenesis and progression and to optimize therapeutic doses of novel HDACs class IIa inhibitors in gliomas.

A Novel Substrate Radiotracer for Molecular Imaging of SIRT2 Expression and Activity with Positron Emission Tomography

PURPOSE

The purpose of this study was to develop a SIRT2-specific substrate-type radiotracer for non-invasive PET imaging of epigenetic regulatory processes mediated by SIRT2 in normal and disease tissues.

PROCEDURES

A library of compounds containing tert-butyloxycarbonyl-lysine-aminomethylcoumarin backbone was derivatized with fluoroalkyl chains 3-16 carbons in length. SIRT2 most efficiently cleaved the myristoyl, followed by 12-fluorododecanoic and 10-fluorodecanoic groups (K_cat/K_m 716.5 ± 72.8, 615.4 ± 50.5, 269.5 ± 52.1/s mol, respectively). Radiosynthesis of 12- [¹⁸F]fluorododecanoic aminohexanoicanilide (12-[¹⁸F]DDAHA) was achieved by nucleophilic radiofluorination of 12-iododecanoic-AHA precursor.

RESULTS

A significantly higher accumulation of 12-[¹⁸F]DDAHA was observed in MCF-7 and MDA-MB-435 cells in vitro as compared to U87, MiaPaCa, and MCF10A, which was consistent with levels of SIRT2 expression. Initial in vivo studies using 12-[¹⁸F]DDAHA conducted in a 9L glioma-bearing rats were discouraging, due to rapid defluorination of this radiotracer upon intravenous administration, as evidenced by significant accumulation of F-18 radioactivity in the skull and other bones, which confounded the interpretation of images of radiotracer accumulation within the tumor and other regions of the brain.

CONCLUSIONS

The next generation of SIRT2-specific radiotracers resistant to systemic defluorination should be developed using alternative sites of radiofluorination on the aliphatic chain of DDAHA. A SIRT2-selective radiotracer may provide information about SIRT2 expression and activity in tumors and normal organs and tissues, which may help to better understand the roles of SIRT2 in different diseases.

Unpacking the Pathogen Box-An Open Source Tool for Fighting Neglected Tropical Disease

The Pathogen Box is a 400-strong collection of drug-like compounds, selected for their potential against several of the world's most important neglected tropical diseases, including trypanosomiasis, leishmaniasis, cryptosporidiosis, toxoplasmosis, filariasis, schistosomiasis, dengue virus and trichuriasis, in addition to malaria and tuberculosis. This library represents an ensemble of numerous successful drug discovery programmes from around the globe, aimed at providing a powerful resource to stimulate open source drug discovery for diseases threatening the most vulnerable communities in the world. This review seeks to provide an in-depth analysis of the literature pertaining to the compounds in the Pathogen Box, including structure-activity relationship highlights, mechanisms of action, related compounds with reported activity against different diseases, and, where appropriate, discussion on the known and putative targets of compounds, thereby providing context and increasing the accessibility of the Pathogen Box to the drug discovery community.

Molecular Imaging of Sirtuin1 Expression-Activity in Rat Brain Using Positron-Emission Tomography-Magnetic-Resonance Imaging with [18F]-2-Fluorobenzoylaminohexanoicanilide

Sirtuin 1 (SIRT1) is a class III histone deacetylase that plays significant roles in the regulation of lifespan, metabolism, memory, and circadian rhythms and in the mechanisms of many diseases. However, methods of monitoring the pharmacodynamics of SIRT1-targeted drugs are limited to blood sampling because of the invasive nature of biopsies. For the noninvasive monitoring of the spatial and temporal dynamics of SIRT1 expression-activity in vivo by PET-CT-MRI, we developed a novel substrate-type radiotracer, [¹⁸F]-2-fluorobenzoylaminohexanoicanilide (2-[¹⁸F]BzAHA). PET-CT-MRI studies in rats demonstrated increased accumulation of 2-[¹⁸F]BzAHA-derived radioactivity in the hypothalamus, hippocampus, nucleus accumbens, and locus coeruleus, consistent with autoradiographic and immunofluorescent (IMF) analyses of brain-tissue sections. Pretreatment with the SIRT1 specific inhibitor, EX-527 (5 mg/kg, ip), resulted in about a 20% reduction of 2-[¹⁸F]BzAHA-derived-radioactivity accumulation in these structures. In vivo imaging of SIRT1 expression-activity should facilitate studies that improve the understanding of SIRT1-mediated regulation in the brain and aid in the development and clinical translation of SIRT1-targeted therapies.

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.

Neuroepigenetics of Post-Traumatic Stress Disorder

While diagnosis of PTSD is based on behavioral symptom clusters that are most directly associated with brain function, epigenetic studies of PTSD in humans to date have been limited to peripheral tissues. Animal models of PTSD have been key for understanding the epigenetic alterations in the brain most directly relevant to endophenotypes of PTSD, in particular those pertaining to fear memory and stress response. This chapter provides an overview of neuroepigenetic studies based on animal models of PTSD, with an emphasis on the effect of stress on fear memory. Where relevant, we also describe human-based studies with relevance to neuroepigenetic insights gleaned from animal work and suggest promising directions for future studies of PTSD neuroepigenetics in living humans that combine peripheral epigenetic measures with measures of central nervous system activity, structure and function.

Evaluation of Fluorine-18-Labeled α1(I)-N-Telopeptide Analogs as Substrate-Based Radiotracers for PET Imaging of Melanoma-Associated Lysyl Oxidase

Accumulating evidence suggests an unequivocal role of lysyl oxidases as key players of tumor progression and metastasis, which renders this enzyme family highly attractive for targeted non-invasive functional imaging of tumors. Considering their function in matrix remodeling, malignant melanoma appears as particularly interesting neoplasia in this respect. For the development of radiotracers that enable PET imaging of the melanoma-associated lysyl oxidase activity, substrates derived from the type I collagen α1 N-telopeptide were labeled with fluorine-18 using N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) as prosthetic reagent. With regards to potential crosslinking to tumor-associated collagen in vivo, their interaction with triple-helical type I collagen was studied by SPR. A mouse model of human melanoma was established on the basis of the A375 cell line, for which the expression of the oncologically relevant lysyl oxidase isoforms LOX and LOXL2 was demonstrated in Western blot and immunohistochemical experiments. The radiopharmacological profiles of the peptidic radiotracers were evaluated in normal rats and A375 melanoma-bearing mice by ex vivo metabolite analysis, whole-body biodistribution studies and dynamic PET imaging. Out of three ¹⁸F-labeled telopeptide analogs, the one with the most favorable substrate properties has shown favorable tumor uptake and tumor-to-muscle ratio. Lysyl oxidase-mediated tumor uptake was proven by pharmacological inhibition using β-aminopropionitrile and by employing negative-control analogs of impeded or abolished targeting capability. The latter were obtained by substituting the lysine residue by ornithine and norleucine, respectively. Comparing the tumor uptake of the lysine-containing peptide with that of the non-functional analogs indicate the feasibility of lysyl oxidase imaging in melanoma using substrate-based radiotracers.

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.