The histamine H3 receptor: an attractive target for the treatment of cognitive disorders

In silico and in vitro studies of two non-imidazole multiple targeting agents at histamine H3 receptors and cholinesterase enzymes

Recently, multi-target directed ligands have been of research interest for multifactorial disorders such as Alzheimer's disease (AD). Since H₃ receptors (H₃ Rs) and cholinesterases are involved in pathophysiology of AD, identification of dual-acting compounds capable of improving cholinergic neurotransmission is of importance in AD pharmacotherapy. In the present study, H₃ R antagonistic activity combined with anticholinesterase properties of two previously computationally identified lead compounds, that is, compound 3 (6-chloro-N-methyl-N-[3-(4-methylpiperazin-1-yl)propyl]-1H-indole-2-carboxamide) and compound 4 (7-chloro-N-[(1-methylpiperidin-3-yl)methyl]-1,2,3,4-tetrahydroisoquinoline-2-carboxamide), was tested. Moreover, molecular docking and binding free energy calculations were conducted for binding mode and affinity prediction of studied ligands toward cholinesterases. Biological evaluations revealed inhibitory activity of ligands in nanomolar (compound 3: H₃ R EC₅₀  = 0.73 nM; compound 4: H₃ R EC₅₀  = 31 nM) and micromolar values (compound 3: AChE IC₅₀  = 9.09 µM, BuChE IC₅₀  = 21.10 µM; compound 4: AChE IC₅₀  = 8.40 µM, BuChE IC₅₀  = 4.93 µM) for H₃ R antagonism and cholinesterase inhibition, respectively. Binding free energies yielded good consistency with cholinesterase inhibitory profiles. The results of this study can be used for lead optimization where dual inhibitory activity on H₃ R and cholinesterases is needed. Such ligands can exert their biological activity in a synergistic manner resulting in higher potency and efficacy.

Search for new multi-target compounds against Alzheimer's disease among histamine H3 receptor ligands

Multi-target-directed ligands seem to be an interesting approach to the treatment of complex disorders such as Alzheimer's disease. The aim of the present study was to find novel multifunctional compounds in a non-imidazole histamine H₃ receptor ligand library. Docking-based virtual screening was applied for selection of twenty-six hits which were subsequently evaluated in Ellman's assay for the inhibitory potency toward acetyl- (AChE) and butyrylcholinesterase (BuChE). The virtual screening with high success ratio enabled to choose multi-target-directed ligands. Based on docking results, all selected ligands were able to bind both catalytic and peripheral sites of AChE and BuChE. The most promising derivatives combined the flavone moiety via a six carbon atom linker with a heterocyclic moiety, such as azepane, piperidine or 3-methylpiperidine. They showed the highest inhibitory activities toward cholinesterases as well as well-balanced potencies against H₃R and both enzymes. Two derivatives were chosen - 5 (IC₅₀ = 0.46 μM (AChE); 0.44 μM (BuChE); K_i = 159.8 nM (H₃R)) and 17 (IC₅₀ = 0.50 μM (AChE); 0.76 μM (BuChE); K_i = 228.2 nM (H₃R)), and their inhibition mechanism was evaluated in kinetic studies. Both compounds displayed non-competitive mode of AChE and BuChE inhibition. Compounds 5 and 17 might serve as good lead structures for further optimization and development of novel multi-target anti-Alzheimer's agents.

Therapeutic Potential of Multifunctional Tacrine Analogues

Abstract: Tacrine is a potent inhibitor of cholinesterases (acetylcholinesterase and butyrylcholinesterase) that shows limiting clinical application by liver toxicity. In spite of this, analogues of tacrine are considered as a model inhibitor of cholinesterases in the therapy of Alzheimer’s disease. The interest in these compounds is mainly related to a high variety of their structure and biological properties. In the present review, we have described the role of cholinergic transmission and treatment strategies in Alzheimer’s disease as well as the synthesis and biological activity of several recently developed classes of multifunctional tacrine analogues and hybrids, which consist of a new paradigm to treat Alzheimer’s disease. We have also reported potential of these analogues in the treatment of Alzheimer’s diseases in various experimental systems.

c-Fos expression in the ascending arousal system induced by physical exercise in rats: Implication for memory performance

During exercise, multiple sensory information such as visual outflow, proprioception, and vestibular information promote an increase in arousal state, which may convey positive effects on cognitive abilities such as memory. Nevertheless, which of the components of the ascending arousal system (AAS) are engaged during physical activity and which of them are critical for cognitive enhancement, induced by exercise is still unclear. Two experiments were conducted, to answer these questions: in the first one, the neuronal activity of different components of the AAS was evaluated by c-Fos immunoreactivity (Fos-ir) in running rats exposed to a lock or unlock running wheel. We found a specific Fos-ir increase in the tuberomammillary nucleus (TMN) associated with physical exercise. In the second experiment sedentary and exercised rats were challenged to conduct an object recognition memory task, and the activity of the AAS after learning was evaluated by c-Fos immunoreactivity. The exercised group showed a higher performance in the object recognition memory task which gets correlated with an increase on Fos-ir in the TMN, but not with the other components of the AAS, suggesting that the increase on TMN activity induced by exercise may be the foremost contributor of the AAS to memory enhancement observed in exercised animals.

WDL-RF: predicting bioactivities of ligand molecules acting with G protein-coupled receptors by combining weighted deep learning and random forest

Motivation

Precise assessment of ligand bioactivities (including IC50, EC50, Ki, Kd, etc.) is essential for virtual screening and lead compound identification. However, not all ligands have experimentally determined activities. In particular, many G protein-coupled receptors (GPCRs), which are the largest integral membrane protein family and represent targets of nearly 40% drugs on the market, lack published experimental data about ligand interactions. Computational methods with the ability to accurately predict the bioactivity of ligands can help efficiently address this problem.

Results

We proposed a new method, WDL-RF, using weighted deep learning and random forest, to model the bioactivity of GPCR-associated ligand molecules. The pipeline of our algorithm consists of two consecutive stages: (i) molecular fingerprint generation through a new weighted deep learning method, and (ii) bioactivity calculations with a random forest model; where one uniqueness of the approach is that the model allows end-to-end learning of prediction pipelines with input ligands being of arbitrary size. The method was tested on a set of twenty-six non-redundant GPCRs that have a high number of active ligands, each with 200-4000 ligand associations. The results from our benchmark show that WDL-RF can generate bioactivity predictions with an average root-mean square error 1.33 and correlation coefficient (r2) 0.80 compared to the experimental measurements, which are significantly more accurate than the control predictors with different molecular fingerprints and descriptors. In particular, data-driven molecular fingerprint features, as extracted from the weighted deep learning models, can help solve deficiencies stemming from the use of traditional hand-crafted features and significantly increase the efficiency of short molecular fingerprints in virtual screening.

Availability and implementation

The WDL-RF web server, as well as source codes and datasets of WDL-RF, is freely available at https://zhanglab.ccmb.med.umich.edu/WDL-RF/ for academic purposes.

Supplementary information

Supplementary data are available at Bioinformatics online.

Exploring the Potential of Spherical Harmonics and PCVM for Compounds Activity Prediction

Biologically active chemical compounds may provide remedies for several diseases. Meanwhile, Machine Learning techniques applied to Drug Discovery, which are cheaper and faster than wet-lab experiments, have the capability to more effectively identify molecules with the expected pharmacological activity. Therefore, it is urgent and essential to develop more representative descriptors and reliable classification methods to accurately predict molecular activity. In this paper, we investigate the potential of a novel representation based on Spherical Harmonics fed into Probabilistic Classification Vector Machines classifier, namely SHPCVM, to compound the activity prediction task. We make use of representation learning to acquire the features which describe the molecules as precise as possible. To verify the performance of SHPCVM ten-fold cross-validation tests are performed on twenty-one G protein-coupled receptors (GPCRs). Experimental outcomes (accuracy of 0.86) assessed by the classification accuracy, precision, recall, Matthews’ Correlation Coefficient and Cohen’s kappa reveal that using our Spherical Harmonics-based representation which is relatively short and Probabilistic Classification Vector Machines can achieve very satisfactory performance results for GPCRs.

Obese- and allergic-related asthma phenotypes among children across the United States

OBJECTIVES

Pediatric asthma is heterogeneous with phenotypes that reflect differing underlying inflammation and pathophysiology. Little is known about the national prevalence of certain obesity- and allergy-related asthma phenotypes or associated characteristics. We therefore assessed the national prevalence, risk factors, and caregiver-reported severity of four asthma phenotypes: not-allergic-not-obese, allergic-not-obese, obese-not-allergic, and allergic-and-obese.

METHODS

We analyzed data from the 2007-2008 National Survey of Children's Health (NSCH) of 10-17 year-olds with caregiver-reported asthma. We described sociodemographic and health risk factors of each phenotype and then applied logistic and ordinal regression models to identify associated risk factors and level of severity of the phenotypes.

RESULTS

Among 4427 children with asthma in this NSCH cohort, the association between race and phenotype was statistically significant (p < 0.0001); white children with asthma were most likely to have allergic-not-obese asthma while black and Hispanic children with asthma were most likely to have the obese-nonallergic phenotype (p < 0.001). Attention-deficit disorder/attention-deficit hyperactivity disorder was more likely to be present in allergic-not-obese children (odds ratio (OR) 1.50, confidence interval (CI) 1.14-1.98, p = 0.004). The phenotype with the highest risk for more severe compared to mild asthma was the obese-and-allergic asthma phenotype (OR 3.34, CI 2.23-5.01, p < 0.001).

CONCLUSIONS

Allergic-not-obese asthma comprised half of our studied asthma phenotypes, while obesity-related asthma (with or without allergic components) comprised one-fifth of asthma phenotypes in this cohort representative of the US population. Children with both obese and allergic asthma are most likely to have severe asthma. Future management of childhood asthma might consider more tailoring of treatment and management plans based upon different childhood asthma phenotypes.

Novel Tetrahydroquinazolinamines as Selective Histamine 3 Receptor Antagonists for the Treatment of Obesity

The histamine 3 receptor (H3R) is a presynaptic receptor, which modulates several neurotransmitters including histamine and various essential physiological processes, such as feeding, arousal, cognition, and pain. The H3R is considered as a drug target for the treatment of several central nervous system disorders. We have synthesized and identified a novel series of 4-aryl-6-methyl-5,6,7,8-tetrahydroquinazolinamines that act as selective H3R antagonists. Among all the synthesized compounds, in vitro and docking studies suggested that the 4-methoxy-phenyl-substituted tetrahydroquinazolinamine compound 4c has potent and selective H3R antagonist activity (IC₅₀ < 0.04 μM). Compound 4c did not exhibit any activity on the hERG ion channel and pan-assay interference compounds liability. Pharmacokinetic studies showed that 4c crosses the blood brain barrier, and in vivo studies demonstrated that 4c induces anorexia and weight loss in obese, but not in lean mice. These data reveal the therapeutic potential of 4c as an anti-obesity candidate drug via antagonizing the H3R.

7-Pyrrolidinethoxy-4'-Methoxyisoflavone Prevents Amyloid β-Induced Injury by Regulating Histamine H3 Receptor-Mediated cAMP/CREB and AKT/GSK3β Pathways

In studies on the treatment of Alzheimer’s disease (AD), in which cognition is enhanced even modestly or selectively, it has been considered that the histamine H3 receptor (H3R) may be a potential target. In this study, we aimed at evaluating the ability of 7-pyrrolidinethoxy-4′-methoxyisoflavone (indicated as LC1405), a novel potential H3R antagonist identified from our H3R antagonist screening system, to ameliorate amyloid β (Aβ)-induced cognitive deficits, and to explore the underlying mechanisms that are related to H3R-modulated signaling. Our results demonstrated that LC1405 effectively reduced the progression of Aβ-associated disorders, such as improved learning and memory capabilities, preserved tissues from suffering neurodegeneration and ultrastructural abnormalities, and ameliorated cholinergic dysfunction in an APP/PS1 double transgenic mouse model of AD. In an in vitro model, LC1405 protected neuronal cells against copper-induced Aβ toxicity, as demonstrated by the improvement in cell viability and decrease in neuronal apoptotic ratio. In addition, treatment with LC1405 resulted in the up-regulation of acetylcholine (ACh) or histamine release and provided neuroprotection through cellular signaling cascades involving H3R-mediated cAMP/CREB and AKT/GSK3β pathways. Furthermore, the beneficial effects of LC1405 on Aβ-mediated toxicity and H3R-mediated cAMP/CREB and AKT/GSK3β axes were reversed after pharmacological activation of H3R. In conclusion, our results demonstrated that LC1405 blocked Aβ-induced toxicity through H3R-modulated signaling transduction both in vitro and in vivo. The results also suggested that LC1405 might have translational potential as a complementary therapy to control disease progression in AD patients who developed cognitive deficits with H3R-related ACh neurotransmission abnormality.

Histamine H3 receptor antagonists/inverse agonists: Where do they go?

Since the discovery of the histamine H₃ receptor in 1983, tremendous advances in the pharmacological aspects of H₃ receptor antagonists/inverse agonists have been accomplished in preclinical studies. At present, there are several drug candidates that reached clinical trial studies for various indications. However, entrance of these candidates to the pharmaceutical market is not free from challenges, and a variety of difficulties is engaged with their developmental process. In this review, the potential role of H₃ receptors in the pathophysiology of various central nervous system, metabolic and allergic diseases is discussed. Thereafter, the current status for H₃ receptor antagonists/inverse agonists in ongoing clinical trial studies is reviewed and obstacles in developing these agents are emphasized.

Central nervous system effects of the histamine-3 receptor antagonist CEP-26401, in comparison with modafinil and donepezil, after a single dose in a cross-over study in healthy volunteers

AIMS

In previous studies, the histamine-3 receptor antagonist CEP-26401 had a subtle effect on spatial working memory, with the best effect seen at the lowest dose tested (20 μg), and a dose-dependent disruption of sleep. In the current study, 3 low-dose levels of CEP-26401 were compared with modafinil and donepezil.

METHODS

In this double-blind, placebo- and positive-controlled, randomized, partial 6-way cross-over study, 40 healthy subjects received single doses of placebo, CEP-26401 (5, 25 or 125 μg) or modafinil 200 mg or donepezil 10 mg. Pharmacokinetic and pharmacodynamic measurements were performed predose and at designated time points postdose.

RESULTS

The main endpoint between-errors of the spatial working memory-10-boxes task only improved for the 125 μg dose of CEP-26401 with a difference of 2.92 (confidence interval [CI] -1.21 to 7.05), 3.24 (CI -1.57 to 8.04) and 7.45 (CI 2.72 to 12.19) for respectively the 5, 25 and 125 μg dose of CEP-26401, -1.65 (CI -0.572 to 1.96) for modafinil and - 3.55 (CI -7.13 to 0.03) for donepezil. CEP-26401 induced an improvement of adaptive tracking, saccadic peak velocity and reaction time during N-back, but a dose-related inhibition of sleep and slight worsening of several cognitive parameters at the highest dose. CEP-26401 significantly changed several subjective visual analogue scales, which was strongest at 25 μg, causing the same energizing and happy feeling as modafinil, but with a more relaxed undertone.

DISCUSSION

Of the doses tested, the 25 μg dose of CEP-26401 had the most optimal balance between favourable subjective effects and sleep inhibition. Whether CEP-26401 can have beneficial effects in clinical practice remains to be studied.

Imaging histamine H3 receptors with [18 F]FMH3: Test-retest and occupancy studies in the non-human primate

A positron emission tomography (PET) radioligand, [¹⁸ F]FMH3, has been developed to interrogate histamine receptor subtype 3 (H3R), where dysfunction at this site is linked with obesity, sleep abnormality, and cognitive disorders. [¹⁸ F]FMH3 was evaluated for imaging central H3R sites in non-human primates through test-retest (TRT) and dose-receptor occupancy studies with two selective H3R antagonists in order to support clinical investigations. Two adult female baboons underwent [¹⁸ F]FMH3 PET brain scans in the HR+, at repeated baseline (n = 7) and following administration of escalating doses of ABT-239 (0.003-0.1m/kg, n = 4) and ciproxifan (0.5-2.1 mg/kg, n = 7). Volume of distribution (V_T ) in brain regions was estimated using the 2-tissue compartment model. TRT variability of V_T across repeated baseline scans was reported as % coefficient of variation (COV). ABT-239 and ciproxifan occupancy at H3R was estimated using the occupancy plot, and the relationship of occupancy with dose and plasma levels was determined. In baboons, distribution of [¹⁸ F]FMH3 was high in the striatum, intermediate in cortical regions, and low in the brain stem. COV of baseline V_T was 7.0 ± 3.5%, averaged across regions and animals. Dose-dependent effects of ABT-239 and ciproxifan measured the brain. ED50 and EC50, respectively, were 0.011 mg/kg and 0.942 ng/ml for ABT-239 and 0.73 mg/kg and 208.3 ng/ml for ciproxifan. [¹⁸ F]FMH3 demonstrated high TRT reliability and can be used to measure occupancy of H3R-targeted drugs. Validation in non-human primates support [¹⁸ F]FMH3 PET studies toward clinical investigations of H3R.

Chronic histamine 3 receptor antagonism alleviates depression like conditions in mice via modulation of brain-derived neurotrophic factor and hypothalamus-pituitary adrenal axis

The last two decades of research has established histamine (HA) as a neurotransmitter. Since H3R antagonists are known to modulate several neurotransmitters besides HA, H3R antagonists have shown potential for the treatment of different central nervous system disorders, including depression. However, molecular mechanisms underlying the beneficial effects of H3R antagonism in depression are not clear, yet. In the present study, we investigated the antidepressant potential of ciproxifan, a selective H3R antagonist, in chronic unpredictable stress (CUS) model of depression in C57BL/6 J mice. We observed that chronic treatment of CUS mice with ciproxifan (3 mg/kg i.p.; for three weeks) alleviates depression-like symptoms such as helplessness measured by forced swim and tail suspension test (FST and TST), anhedonia measured by sucrose preference test (SPT) and social deficit measured in social behavior test. Chronic ciproxifan treatment restored CUS induced BDNF expression in the prefrontal cortex (PFC) and hippocampus. We also observed that ciproxifan modulates CUS induced NUCB2/nesfatin-1 and CRH expression in the hypothalamus and plasma corticosterone. We also determined the direct effect of HA on BDNF expression in neurons by western blotting and immunocytochemistry, and found that HA significantly induced BDNF expression, which was blocked by the H4R selective antagonist, but not by other HA receptor selective antagonists. Furthermore, ciproxifan significantly modulated NMDA glutamate receptor subunits NR2B and NR2A. Thus, these results suggest that increased HA signaling in the brain produces antidepressant-like effects in mice and modulates BDNF expression and HPA-axis.

Discovery and Development of N-[4-(1-Cyclobutylpiperidin-4-yloxy)phenyl]-2-(morpholin-4-yl)acetamide Dihydrochloride (SUVN-G3031): A Novel, Potent, Selective, and Orally Active Histamine H3 Receptor Inverse Agonist with Robust Wake-Promoting Activity

A series of chemical optimizations guided by in vitro affinity at a histamine H₃ receptor (H₃R), physicochemical properties, and pharmacokinetics in rats resulted in identification of N-[4-(1-cyclobutyl-piperidin-4-yloxy)phenyl]-2-(morpholin-4-yl)acetamide dihydrochloride (17v, SUVN-G3031) as a clinical candidate. Compound 17v is a potent (hH₃R K_i = 8.73 nM) inverse agonist at H₃R with selectivity over other 70 targets, Compound 17v has adequate oral exposures and favorable elimination half-lives both in rats and dogs. It demonstrated high receptor occupancy and marked wake-promoting effects with decreased rapid-eye-movement sleep in orexin-B saporin lesioned rats supporting its potential therapeutic utility in treating human sleep disorders. It had no effect on the locomotor activity at doses several fold higher than its efficacious dose. It is devoid of hERG and phospholipidosis issues. Phase-1 evaluation for safety, tolerability, and pharmacokinetics, and long-term safety studies in animals have been successfully completed without any concern for further development.

Design, synthesis, and in vitro and in vivo characterization of 1-{4-[4-(substituted)piperazin-1-yl]butyl}guanidines and their piperidine analogues as histamine H3 receptor antagonists

Previously, we have shown that 1-substituted-[4-(7-phenoxyheptylpiperazin-1-yl)butyl]guanidine with electron withdrawing substituents at position 4 in the benzyl moiety exhibits high in vitro affinities toward the guinea pig jejunal histamine H₃ receptor with pA ₂ ranging from 8.49 to 8.43. Here, we present data on the impact of replacement of the piperazine scaffold by the piperidine ring (compounds 2a and 2b), moving benzyl- and 4-trifluoromethylbenzyl substituents from position 1 to 3 of the guanidine moiety (compounds 2c and 2d), which decreases the guanidine basicity (compound 2e), and the influence of individual synthons (compounds 2f-h), present in the lead compounds 1b and 1c, on the antagonistic activity against the histamine H₃ receptor. Additionally, the most active compounds 1a, 1c, and 1d were evaluated for their affinity to the rat histamine H₃ receptor and the human histamine H₃ and H₄ receptors. It was also shown that compounds 1a, 1c and 1d, given parenterally for five days, reduced the food intake of rats and did not influence the brain histamine or noradrenaline concentrations; however, significantly reduced serotonin and dopamine concentrations were found in rats administered with compounds 1a and 1c, respectively.

Comprehensive review of mechanisms of pathogenesis involved in Alzheimer's disease and potential therapeutic strategies

AD is a progressive neurodegenerative disorder and a leading cause of dementia in an aging population worldwide. The enormous challenge which AD possesses to global healthcare makes it as urgent as ever for the researchers to develop innovative treatment strategies to fight this disease. An in-depth analysis of the extensive available data associated with the AD is needed for a more comprehensive understanding of underlying molecular mechanisms and pathophysiological pathways associated with the onset and progression of the AD. The currently understood pathological and biochemical manifestations include cholinergic, Aβ, tau, excitotoxicity, oxidative stress, ApoE, CREB signaling pathways, insulin resistance, etc. However, these hypotheses have been criticized with several conflicting reports for their involvement in the disease progression. Several issues need to be addressed such as benefits to cost ratio with cholinesterase therapy, the dilemma of AChE selectivity over BChE, BBB permeability of peptidic BACE-1 inhibitors, hurdles related to the implementation of vaccination and immunization therapy, and clinical failure of candidates related to newly available targets. The present review provides an insight to the different molecular mechanisms involved in the development and progression of the AD and potential therapeutic strategies, enlightening perceptions into structural information of conventional and novel targets along with the successful applications of computational approaches for the design of target-specific inhibitors.

Molecular Pathology and Pharmacological Treatment of Autism Spectrum Disorder-Like Phenotypes Using Rodent Models

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder with a high prevalence rate. The core symptoms of ASD patients are impaired social communication and repetitive behavior. Genetic and environmental factors contribute to pathophysiology of ASD. Regarding environmental risk factors, it is known that valproic acid (VPA) exposure during pregnancy increases the chance of ASD among offspring. Over a decade of animal model studies have shown that maternal treatment with VPA in rodents recapitulates ASD-like pathophysiology at a molecular, cellular and behavioral level. Here, we review the prevailing theories of ASD pathogenesis, including excitatory/inhibitory imbalance, neurotransmitter dysfunction, dysfunction of mTOR and endocannabinoid signaling pathways, neuroinflammation and epigenetic alterations that have been associated with ASD. We also describe the evidence linking neuropathological changes to ASD-like behavioral abnormalities in maternal VPA-treated rodents. In addition to obtaining an understanding of the neuropathological mechanisms, the VPA-induced ASD-like animal models also serve as a good platform for testing pharmacological reagents that might be use treating ASD. We therefore have summarized the various pharmacological studies that have targeted the classical neurotransmitter systems, the endocannabinoids, the Wnt signal pathway and neuroinflammation. These approaches have been shown to often be able to ameliorate the ASD-like phenotypes induced by maternal VPA treatments.

Repurposing potential of 1st generation H1-specific antihistamines as anti-filovirus therapeutics

Ebola and Marburg are filoviruses and biosafety level 4 pathogens responsible for causing severe hemorrhagic fevers in humans with mortality rates up to 90%. The most recent outbreak in West Africa resulted in approximately 11,310 deaths in 28,616 reported cases. Currently there are no FDA-approved vaccines or therapeutics to treat infections of these deadly viruses. Recently we screened an FDA-approved drug library and identified numerous G protein-coupled receptor (GPCR) antagonists including antihistamines possessing anti-filovirus properties. Antihistamines are attractive targets for drug repurposing because of their low cost and ease of access due to wide use. In this report we identify common over the counter antihistamines, such as diphenhydramine (Benadryl) and chlorcyclizine (Ahist) as potential candidates for repurposing as anti-filovirus agents. Furthermore, we demonstrate that this potential is wide-spread through the 1st generation of H₁-specific antihistamines but is not present in newer drugs or drugs targeting H₂, H₃ and H₄ receptors. We showed that the filovirus entry inhibition is not dependent on the classical antagonism of cell surface histamine or muscarinic acetylcholine receptors but occurs in the endosome, like the cathepsin inhibitor CA-074. Finally, using extensive docking studies we showed the potential for these drugs to bind directly to the EBOV-GP at the same site as toremifene. These findings suggest that the 1st generation antihistamines are excellent candidates for repurposing as anti-filovirus therapeutics and can be further optimized for removal of unwanted histamine or muscarinic receptor interactions without loss of anti-filovirus efficacy.

Design, synthesis, and biological evaluation of novel oxadiazole- and thiazole-based histamine H3R ligands

Histamine H₃ receptor (H₃R) is largely expressed in the CNS and modulation of the H₃R function can affect histamine synthesis and liberation, and modulate the release of many other neurotransmitters. Targeting H₃R with antagonists/inverse agonists may have therapeutic applications in neurodegenerative disorders, gastrointestinal and inflammatory diseases. This prompted us to design and synthesize azole-based H₃R ligands, i.e. having oxadiazole- or thiazole-based core structures. While ligands of oxadiazole scaffold were almost inactive, thiazole-based ligands were very potent and several exhibited binding affinities in a nanomolar concentration range. Ligands combining 4-cyanophenyl moiety as arbitrary region, para-xylene or piperidine carbamoyl linkers, and/or pyrrolidine or piperidine basic heads were found to be the most active within this series of thiazole-based H₃R ligands. The most active ligands were in silico screened for ADMET properties and drug-likeness. They fulfilled Lipinski's and Veber's rules and exhibited potential activities for oral administration, blood-brain barrier penetration, low hepatotoxicity, combined with an overall good toxicity profile.

Histamine H3 receptor density is negatively correlated with neural activity related to working memory in humans

Background

The histamine H₃ receptor is regarded as a drug target for cognitive impairments in psychiatric disorders. H₃ receptors are expressed in neocortical areas, including the prefrontal cortex, the key region of cognitive functions such as working memory. However, the role of prefrontal H₃ receptors in working memory has not yet been clarified. Therefore, using functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) techniques, we aimed to investigate the association between the neural activity of working memory and the density of H₃ receptors in the prefrontal cortex.

Findings

Ten healthy volunteers underwent both fMRI and PET scans. The N-back task was used to assess the neural activities related to working memory. H₃ receptor density was measured with the selective PET radioligand [¹¹C] TASP457. The neural activity of the right dorsolateral prefrontal cortex during the performance of the N-back task was negatively correlated with the density of H₃ receptors in this region.

Conclusions

Higher neural activity of working memory was associated with lower H₃ receptor density in the right dorsolateral prefrontal cortex. This finding elucidates the role of H₃ receptors in working memory and indicates the potential of H₃ receptors as a therapeutic target for the cognitive impairments associated with neuropsychiatric disorders.

Current Enlightenment About Etiology and Pharmacological Treatment of Autism Spectrum Disorder

Autistic Spectrum Disorder (ASD) is a complex neurodevelopmental brain disorder characterized by two core behavioral symptoms, namely impairments in social communication and restricted/repetitive behavior. The molecular mechanisms underlying ASD are not well understood. Recent genetic as well as non-genetic animal models contributed significantly in understanding the pathophysiology of ASD, as they establish autism-like behavior in mice and rats. Among the genetic causes, several chromosomal mutations including duplications or deletions could be possible causative factors of ASD. In addition, the biochemical basis suggests that several brain neurotransmitters, e.g., dopamine (DA), serotonin (5-HT), gamma-amino butyric acid (GABA), acetylcholine (ACh), glutamate (Glu) and histamine (HA) participate in the onset and progression of ASD. Despite of convincible understanding, risperidone and aripiprazole are the only two drugs available clinically for improving behavioral symptoms of ASD following approval by Food and Drug Administration (FDA). Till date, up to our knowledge there is no other drug approved for clinical usage specifically for ASD symptoms. However, many novel drug candidates and classes of compounds are underway for ASD at different phases of preclinical and clinical drug development. In this review, the diversity of numerous aetiological factors and the alterations in variety of neurotransmitter generation, release and function linked to ASD are discussed with focus on drugs currently used to manage neuropsychiatric symptoms related to ASD. The review also highlights the clinical development of drugs with emphasis on their pharmacological targets aiming at improving core symptoms in ASD.

Prenatal Alcohol Exposure Increases Histamine H3 Receptor-Mediated Inhibition of Glutamatergic Neurotransmission in Rat Dentate Gyrus

BACKGROUND

We have reported that prenatal alcohol exposure (PAE)-induced deficits in dentate gyrus, long-term potentiation (LTP), and memory are ameliorated by the histamine H3 receptor inverse agonist ABT-239. Curiously, ABT-239 did not enhance LTP or memory in control offspring. Here, we initiated an investigation of how PAE alters histaminergic neurotransmission in the dentate gyrus and other brain regions employing combined radiohistochemical and electrophysiological approaches in vitro to examine histamine H3 receptor number and function.

METHODS

Long-Evans rat dams voluntarily consumed either a 0% or 5% ethanol solution 4 hours each day throughout gestation. This pattern of drinking, which produces a mean peak maternal serum ethanol concentration of 60.8 ± 5.8 mg/dl, did not affect maternal weight gain, litter size, or offspring birthweight.

RESULTS

Radiohistochemical studies in adult offspring revealed that specific [3 H]-A349821 binding to histamine H3 receptors was not different in PAE rats compared to controls. However, H3 receptor-mediated Gi /Go protein-effector coupling, as measured by methimepip-stimulated [35 S]-GTPγS binding, was significantly increased in cerebral cortex, cerebellum, and dentate gyrus of PAE rats compared to control. A LIGAND analysis of detailed methimepip concentration-response curves in dentate gyrus indicated that PAE significantly elevates receptor-effector coupling by a lower affinity H3 receptor population without significantly altering the affinities of H3 receptor subpopulations. In agreement with the [35 S]-GTPγS studies, a similar range of methimepip concentrations also inhibited electrically evoked field excitatory postsynaptic potential responses and increased paired-pulse ratio, a measure of decreased glutamate release, to a significantly greater extent in dentate gyrus slices from PAE rats than in controls.

CONCLUSIONS

These results suggest that a PAE-induced elevation in H3 receptor-mediated inhibition of glutamate release from perforant path terminals as 1 mechanism contributing the LTP deficits previously observed in the dentate gyrus of PAE rats, as well as providing a mechanistic basis for the efficacy of H3 receptor inverse agonists for ameliorating these deficits.

A mathematical model for histamine synthesis, release, and control in varicosities

Background

Histamine (HA), a small molecule that is synthesized from the amino acid histidine, plays an important role in the immune system where it is associated with allergies, inflammation, and T-cell regulation. In the brain, histamine is stored in mast cells and other non-neuronal cells and also acts as a neurotransmitter. The histamine neuron cell bodies are in the tuberomammillary (TM) nucleus of the hypothalamus and these neurons send projections throughout the central nervous system (CNS), in particular to the cerebral cortex, amygdala, basal ganglia, hippocampus, thalamus, retina, and spinal cord. HA neurons make few synapses, but release HA from the cell bodies and from varicosities when the neurons fire. Thus the HA neural system seems to modulate and control the HA concentration in projection regions. It is known that high HA levels in the extracellular space inhibit serotonin release, so HA may play a role in the etiology of depression.

Results

We compare model predictions to classical physiological experiments on HA half-life, the concentration of brain HA after histidine loading, and brain HA after histidine is dramatically increased or decreased in the diet. The model predictions are also consistent with in vivo experiments in which extracellular HA is measured, using Fast Scan Cyclic Voltammetry, in the premammillary nucleus (PM) after a 2 s antidromic stimulation of the TM, both without and in the presence of the H₃ autoreceptor antagonist thioperamide. We show that the model predicts well the temporal behavior of HA in the extracellular space over 30 s in both experiments.

Conclusions

Our ability to measure in vivo histamine dynamics in the extracellular space after stimulation presents a real opportunity to understand brain function and control. The observed extracellular dynamics depends on synthesis, storage, neuronal firing, release, reuptake, glial cells, and control by autoreceptors, as well as the behavioral state of the animal (for example, depression) or the presence of neuroinflammation. In this complicated situation, the mathematical model will be useful for interpreting data and conducting in silico experiments to understand causal mechanisms. And, better understanding can suggest new therapeutic drug targets.

Histamine N-methyltransferase regulates aggression and the sleep-wake cycle

Histamine is a neurotransmitter that regulates diverse physiological functions including the sleep-wake cycle. Recent studies have reported that histaminergic dysfunction in the brain is associated with neuropsychiatric disorders. Histamine N-methyltransferase (HNMT) is an enzyme expressed in the central nervous system that specifically metabolises histamine; yet, the exact physiological roles of HNMT are unknown. Accordingly, we phenotyped Hnmt knockout mice (KO) to determine the relevance of HNMT to various brain functions. First, we showed that HNMT deficiency enhanced brain histamine concentrations, confirming a role for HNMT in histamine inactivation. Next, we performed comprehensive behavioural testing and determined that KO mice exhibited high aggressive behaviours in the resident-intruder and aggressive biting behaviour tests. High aggression in KO mice was suppressed by treatment with zolantidine, a histamine H2 receptor (H2R) antagonist, indicating that abnormal H2R activation promoted aggression in KO mice. A sleep analysis revealed that KO mice exhibited prolonged bouts of awakening during the light (inactive) period and compensatory sleep during the dark (active) period. Abnormal sleep behaviour was suppressed by treatment with pyrilamine, a H1R antagonist, prior to light period, suggesting that excessive H1R activation led to the dysregulation of sleep-wake cycles in KO mice. These observations inform the physiological roles of HNMT.

The Histamine H3 Receptor Antagonist E159 Reverses Memory Deficits Induced by Dizocilpine in Passive Avoidance and Novel Object Recognition Paradigm in Rats

The involvement of histamine H3 receptors (H3Rs) in memory is well known, and the potential of H3R antagonists in therapeutic management of neuropsychiatric diseases, e.g., Alzheimer disease (AD) is well established. Therefore, the effects of histamine H3 receptor (H3R) antagonist E159 (2.5–10 mg/kg, i.p.) in adult male rats on dizocilpine (DIZ)-induced memory deficits were studied in passive avoidance paradigm (PAP) and in novel object recognition (NOR) using pitolisant (PIT) and donepezil (DOZ) as standard drugs. Upon acute systemic pretreatment of E159 at three different doses, namely 2.5, 5, and 10 mg/kg, i.p., 2.5 and 5 but not 10 mg/kg of E159 counteracted the DIZ (0.1 mg)-induced memory deficits, and this E159 (2.5 mg)-elicited memory-improving effects in DIZ-induced amnesic model were moderately abrogated after acute systemic administration of scopolamine (SCO), H2R antagonist zolantidine (ZOL), but not with H1R antagonist pyrilamine to the animals. Moreover, the observed memory-enhancing effects of E159 (2.5 mg/kg, i.p.) were strongly abrogated when animals were administered with a combination of SCO and ZOL. Furthermore, the E159 (2.5 mg)-provided significant memory-improving effect of in DIZ-induced short-term memory (STM) impairment in NOR was comparable to the DOZ-provided memory-enhancing effect, and was abolished when animals were injected with the CNS-penetrant histamine H3R agonist R-(α)-methylhistamine (RAMH). However, E159 at a dose of 2.5 mg/kg failed to exhibit procognitive effect on DIZ-induced long-term memory (LTM) in NOR. Furthermore, the results observed revealed that E159 (2.5 mg/kg) did not alter anxiety levels and locomotor activity of animals naive to elevated-plus maze (EPM), demonstrating that improved performances with E159 (2.5 mg/kg) in PAP or NOR are unrelated to changes in emotional responding or in spontaneous locomotor activity. These results provide evidence for the potential of drugs targeting H3Rs for the treatment of neuropsychiatric disorders, e.g., AD.

11C-Labeling of Aryl Ketones as Candidate Histamine Subtype-3 Receptor PET Radioligands through Pd(0)-Mediated 11C-Carbonylative Coupling

Pd(0)-mediated coupling between iodoarenes, [¹¹C]carbon monoxide and aryltributylstannanes has been used to prepare simple model [¹¹C]aryl ketones. Here, we aimed to label four 2-aminoethylbenzofuran chemotype based molecules ([¹¹C]1–4) in the carbonyl position, as prospective positron emission tomography (PET) radioligands for the histamine subtype 3 receptor (H3R) by adapting this methodology with use of aryltrimethylstannanes. Radiosynthesis was successfully performed on a platform equipped with a mini-autoclave and a liquid handling robotic arm, within a lead-shielded hot-cell. Candidate radioligands were readily formulated in saline containing ethanol (10%, v/v) and ascorbic acid (0.5 mg/10 mL). Yields for preclinical use were in the range of 5–9%, decay-corrected from cyclotron-produced [¹¹C]CO₂ and molar activities were >115 GBq/µmol at end of synthesis. Radiochemical purities exceeded >97%.

Design, synthesis, biological evaluation, and molecular docking of novel flavones as H3 R inhibitors

A series of novel flavone derivatives were designed, synthesized, and evaluated for their H₃ R inhibitory activity. The results showed that four compounds exhibited significant anti-H₃ R activity. Molecular docking experiments indicated that a salt bridge, hydrogen-bonding, and hydrophobic interactions all contributed to interactions between inhibitors and H₃ R.

Mechanistic characterization of S 38093, a novel inverse agonist at histamine H3 receptors

Histaminergic H3 inverse agonists, by stimulating central histamine release, represent attractive drug candidates to treat cognitive disorders. The present studies aimed to describe the mechanistic profile of S 38093 a novel H3 receptors inverse agonist. S 38093 displays a moderate affinity for rat, mouse and human H3 receptors (Ki=8.8, 1.44 and 1.2µM, respectively) with no affinity for other histaminergic receptors. In cellular models, the compound was able to antagonize mice H3 receptors (K_B=0.65µM) and to suppress cAMP decrease induced by an H3 agonist via human H3 receptors (K_B=0.11µM). The antagonism properties of the compound were confirmed by electrophysiological studies on rat hippocampal slices (from 0.1μM). In cells expressing a high H3 density, S 38093 behaved as a moderate inverse agonist at rat and human H3 receptors (EC₅₀=9 and 1.7µM, respectively). S 38093 was rapidly absorbed in mouse and rat (T_max=0.25-0.5h), slowly in monkey (2h), with a bioavailability ranging from 20% to 60% and t_1/2 ranging from 1.5 to 7.4h. The compound was widely distributed with a moderate volume of distribution and low protein binding. The brain distribution of S 38093 was rapid and high. In mice, S 38093 significantly increased ex vivo N-tele-Methylhistamine cerebral levels from 3mg/kg p.o. and antagonized R-α-Methylhistamine-induced dipsogenia from 10mg/kg i.p. Taken together, these data suggest that S 38093, a novel H3 inverse agonist, is a good candidate for further in vivo evaluations, in particular in animal models of cognition.

S 38093, a histamine H3 antagonist/inverse agonist, promotes hippocampal neurogenesis and improves context discrimination task in aged mice

Strategies designed to increase adult hippocampal neurogenesis (AHN) may have therapeutic potential for reversing memory impairments. H₃ receptor antagonists/inverse agonists also may be useful for treating cognitive deficits. However, it remains unclear whether these ligands have effects on AHN. The present study aimed to investigate the effects of a 28-day treatment with S 38093, a novel brain-penetrant antagonist/inverse agonist of H₃ receptors, on AHN (proliferation, maturation and survival) in 3-month-old and in aged 16-month-old mice. In addition, the effects of S 38093 treatment on 7-month-old APPSWE Tg2576 transgenic mice, a model of Alzheimer's disease, were also assessed. In all tested models, chronic treatment with S 38093 stimulated all steps of AHN. In aged animals, S 38093 induced a reversal of age-dependent effects on hippocampal brain-derived neurotrophic factor (BDNF) BDNF-IX, BDNF-IV and BDNF-I transcripts and increased vascular endothelial growth factor (VEGF) expression. Finally, the effects of chronic administration of S 38093 were assessed on a neurogenesis-dependent "context discrimination (CS) test" in aged mice. While ageing altered mouse CS, chronic S 38093 treatment significantly improved CS. Taken together, these results provide evidence that chronic S 38093 treatment increases adult hippocampal neurogenesis and may provide an innovative strategy to improve age-associated cognitive deficits.

The Obesity-Impulsivity Axis: Potential Metabolic Interventions in Chronic Psychiatric Patients

Pathological impulsivity is encountered in a broad range of psychiatric conditions and is thought to be a risk factor for aggression directed against oneself or others. Recently, a strong association was found between impulsivity and obesity which may explain the high prevalence of metabolic disorders in individuals with mental illness even in the absence of exposure to psychotropic drugs. As the overlapping neurobiology of impulsivity and obesity is being unraveled, the question asked louder and louder is whether they should be treated concomitantly. The treatment of obesity and metabolic dysregulations in chronic psychiatric patients is currently underutilized and often initiated late, making correction more difficult to achieve. Addressing obesity and metabolic dysfunction in a preventive manner may not only lower morbidity and mortality but also the excessive impulsivity, decreasing the risk for aggression. In this review, we take a look beyond psychopharmacological interventions and discuss dietary and physical therapy approaches.

Histamine H3 Receptor Antagonist Prevents Memory Deficits and Synaptic Plasticity Disruption Following Isoflurane Exposure

BACKGROUND

Exposure to pharmacological concentration of inhaled anesthetics such as isoflurane can cause short- or long-term cognitive impairments in preclinical studies. The selective antagonists of the histamine H3 receptors are considered as a promising group of novel therapeutic agents for the treatment of cognitive disorders. In this study, we investigated whether ciproxifan, a nonimidazole antagonist of H3 histamine receptors, could overcome the functional and electrophysiological sequela associated with isoflurane anesthesia.

METHODS

Adult male Sprague Dawley rats were exposed to 1.4% isoflurane or vehicle gas for 2 h. The memory tests (novel object recognition and passive avoidance) as well as in vivo hippocampal excitatory synaptic potentials were recorded 24 h postanesthesia. Locomotor activity, anxiety, and nociception 24 h after isoflurane were also examined. The drugs (ciproxifan 3 mg/kg or saline) were intraperitoneally injected 30 min prior to the behavioral tests or long-term potentiation induction.

RESULTS

Animals that were previously (24 h) exposed to 1.4% isoflurane for 2 h displayed no preference for novel objects and had impaired retention of a passive avoidance response at 1 h after sample phase. Treating isoflurane-exposed rats with ciproxifan significantly improved the memory performance, as evidenced by an increased discrimination ratio in objects recognition and prolonged retention time in passive avoidance test. Accordingly, hippocampus long-term potentiation was reduced in animals that received isoflurane, while administration of ciproxifan completely abolished the effect of isoflurane exposure on synaptic plasticity. Neither isoflurane nor ciproxifan altered motor performance, anxiety, and nociceptive responses.

CONCLUSION

These results suggest that H3R in the CNS, probably in the hippocampus, may serve as therapeutic target for improvement of anesthesia-associated cognitive deficits.

Recent advances in acetylcholinesterase Inhibitors and Reactivators: an update on the patent literature (2012-2015)

INTRODUCTION

Acetylcholinesterase (AChE) is the major enzyme that hydrolyzes acetylcholine, a key neurotransmitter for synaptic transmission, into acetic acid and choline. Mild inhibition of AChE has been shown to have therapeutic relevance in Alzheimer's disease (AD), myasthenia gravis, and glaucoma among others. In contrast, strong inhibition of AChE can lead to cholinergic poisoning. To combat this, AChE reactivators have to be developed to remove the offending AChE inhibitor, restoring acetylcholine levels to normal. Areas covered: This article covers recent advances in the development of acetylcholinesterase modulators, including both inhibitors of acetylcholinesterase for the efforts in development of new chemical entities for treatment of AD, as well as re-activators for resurrection of organophosphate bound acetylcholinesterase. Expert opinion: Over the past three years, research efforts have continued to identify novel small molecules as AChE inhibitors for both CNS and peripheral diseases. The more recent patent activity has focused on three AChE ligand design areas: derivatives of known AChE ligands, natural product based scaffolds and multifunctional ligands, all of which have produced some unique chemical matter with AChE inhibition activities in the mid picomolar to low micromolar ranges. New AChE inhibitors with polypharmacology or dual inhibitory activity have also emerged as highlighted by new AChE inhibitors with dual activity at L-type calcium channels, GSK-3, BACE1 and H3, although most only show low micromolar activity, thus further research is warranted. New small molecule reactivators of organophosphate-inhibited AChE have also been disclosed, which focused on the design of neutral ligands with improved pharmaceutical properties and blood-brain barrier (BBB) penetration. Gratifyingly, some research in this area is moving away from the traditional quaternary pyridinium oximes AChE reactivators, while still employing the necessary reactivation group (oximes). However, selectivity over inhibition of native AChE enzyme, effectiveness of reactivation, broad-spectrum reactivation against multiple organophosphates and reactivation of aged-enzyme continue to be hurdles for this area of research.

Isoform-Specific Biased Agonism of Histamine H3 Receptor Agonists

The human histamine H₃ receptor (hH₃R) is subject to extensive gene splicing that gives rise to a large number of functional and nonfunctional isoforms. Despite the general acceptance that G protein-coupled receptors can adopt different ligand-induced conformations that give rise to biased signaling, this has not been studied for the H₃R; further, it is unknown whether splice variants of the same receptor engender the same or differential biased signaling. Herein, we profiled the pharmacology of histamine receptor agonists at the two most abundant hH₃R splice variants (hH₃R₄₄₅ and hH₃R₃₆₅) across seven signaling endpoints. Both isoforms engender biased signaling, notably for 4-[3-(benzyloxy)propyl]-1H-imidazole (proxyfan) [e.g., strong bias toward phosphorylation of glycogen synthase kinase 3β (GSK3β) via the full-length receptor] and its congener 3-(1H-imidazol-4-yl)propyl-(4-iodophenyl)-methyl ether (iodoproxyfan), which are strongly consistent with the former's designation as a "protean" agonist. The 80 amino acid IL3 deleted isoform hH₃R₃₆₅ is more permissive in its signaling than hH₃R₄₄₅: 2-(1H-imidazol-5-yl)ethyl imidothiocarbamate (imetit), proxyfan, and iodoproxyfan were all markedly biased away from calcium signaling, and principal component analysis of the full data set revealed divergent profiles for all five agonists. However, most interesting was the identification of differential biased signaling between the two isoforms. Strikingly, hH₃R₃₆₅ was completely unable to stimulate GSK3β phosphorylation, an endpoint robustly activated by the full-length receptor. To the best of our knowledge, this is the first quantitative example of differential biased signaling via isoforms of the same G protein-coupled receptor that are simultaneously expressed in vivo and gives rise to the possibility of selective pharmacological targeting of individual receptor splice variants.

Histamine H3 receptors and its antagonism as a novel mechanism for antipsychotic effect: a current preclinical & clinical perspective

Histamine H3 receptors are present as autoreceptors on histaminergic neurons and as heteroreceptors on nonhistaminergic neurones. They control the release and synthesis of histamine and several other key neurotransmitters in the brain. H3 antagonism may be a novel approach to develop a new class of antipsychotic medications given the gathering evidence reporting therapeutic efficacy in several central nervous system disorders. Several medications such as cariprazine, lurasidone, LY214002, bexarotene, rasagiline, raloxifene, BL-1020 and ITI-070 are being developed to treat the negative symptoms and cognitive impairments of schizophrenia. These medications works through diverse mechanisms which include agonism at metabotropic glutamate receptor (mGluR2/3), partial agonism at dopamine D2, D3 and serotonin 5-HT1A receptors, antagonism at D2, 5-HT2A, 5-HT2B and 5-HT7 receptors, combined dopamine antagonism with GABA agonist activity, inhibition of monoamine oxidase-B, modulation of oestrogen receptor, and activation of nuclear retinoid X receptor. However, still specific safe therapy for psychosis remains at large. Schizophrenia is a severe neuropsychiatric disorder result both from hyper- and hypo-dopaminergic transmission causing positive and negative symptoms, respectively. Pharmacological stimulation of dopamine release in the prefrontal cortex has been a viable approach in treating negative symptoms and cognitive deficits of schizophrenia symptoms that are currently not well treated and continue to represent significant unmet medical challenges. Administration of H3 antagonists/inverse agonists increase extracellular dopamine concentrations in rat prefrontal cortex, but not in the striatum suggesting that antagonism via H3 receptor may be a potential target for treating negative symptoms and cognitive deficits associated with schizophrenia. Further, insights are emerging into the potential role of histamine H3 receptors as a target of antiobesity therapeutics which is one of the limiting adverse effects of second generation schizophrenia medications. The recent failures of two promising H3 compounds in clinical trial dampened the interest in seeking antipsychotic like activities of H3 receptor antagonists. However, due to the inconclusive nature of many of these studies, the development of H3 compounds via H3 antagonism/inverse agonism approach still hold lot of promises and may be developed as a novel class of drugs for schizophrenia and its related complications e.g. weight gain.

Ligand autoradiographical quantification of histamine H3 receptor in human dementia with Lewy bodies

Dementia with Lewy bodies (DLB) is a serious age-dependent human neurodegenerative disease, with multiple debilitating symptoms, including dementia, psychosis and significant motor deficits, but with little or no effective treatments. This comparative ligand autoradiographical study has quantified histamine H₃ receptors (H₃R) in a series of major cortical and basal ganglia structures in human DLB and Alzheimer’s (AD) post-mortem cases using the highly selective radioligand, [³H] GSK189254.

In the main, the levels of H₃ receptor were largely preserved in DLB cases when compared with aged-matched controls. However, we provide new evidence showing variable levels in the globus pallidus, and, moreover, raised levels of Pallidum H₃ correlated with positive psychotic symptoms, in particular delusions and visual hallucinations, but not symptoms associated with depression. Furthermore, no correlation was detected for H₃ receptor levels to MMSE or IUPRS symptom severity.

This study suggests that H₃R antagonists have scope for treating the psychotic symptomologies in DLB and other human brain disorders.

The "Cyclopropyl Fragment" is a Versatile Player that Frequently Appears in Preclinical/Clinical Drug Molecules

Recently, there has been an increasing use of the cyclopropyl ring in drug development to transition drug candidates from the preclinical to clinical stage. Important features of the cyclopropane ring are, the (1) coplanarity of the three carbon atoms, (2) relatively shorter (1.51 Å) C-C bonds, (3) enhanced π-character of C-C bonds, and (4) C-H bonds are shorter and stronger than those in alkanes. The present review will focus on the contributions that a cyclopropyl ring makes to the properties of drugs containing it. Consequently, the cyclopropyl ring addresses multiple roadblocks that can occur during drug discovery such as (a) enhancing potency, (b) reducing off-target effects,

A voltammetric and mathematical analysis of histaminergic modulation of serotonin in the mouse hypothalamus

Histamine and serotonin are neuromodulators which facilitate numerous, diverse neurological functions. Being co-localized in many brain regions, these two neurotransmitters are thought to modulate one another's chemistry and are often implicated in the etiology of disease. Thus, it is desirable to interpret the in vivo chemistry underlying neurotransmission of these two molecules to better define their roles in health and disease. In this work, we describe a voltammetric approach to monitoring serotonin and histamine simultaneously in real time. Via electrical stimulation of the axonal bundles in the medial forebrain bundle, histamine release was evoked in the mouse premammillary nucleus. We found that histamine release was accompanied by a rapid, potent inhibition of serotonin in a concentration-dependent manner. We developed mathematical models to capture the experimental time courses of histamine and serotonin, which necessitated incorporation of an inhibitory receptor on serotonin neurons. We employed pharmacological experiments to verify that this serotonin inhibition was mediated by H3 receptors. Our novel approach provides fundamental mechanistic insights that can be used to examine the full extent of interconnectivity between histamine and serotonin in the brain. Histamine and serotonin are co-implicated in many of the brain's functions. In this paper, we develop a novel voltammetric method for simultaneous real-time monitoring of histamine and serotonin in the mouse premammillary nucleus. Electrical stimulation of the medial forebrain bundle evokes histamine and inhibits serotonin release. We show voltammetrically, mathematically, and pharmacologically that this serotonin inhibition is H3 receptor mediated.

Histamine H3 receptor as a potential target for cognitive symptoms in neuropsychiatric diseases

The potential contributions of the brain histaminergic system in neurodegenerative diseases, and the possiblity of histamine-targeting treatments is attracting considerable interests. The histamine H3 receptor (H3R) is expressed mainly in the central nervous system, and is, consequently, an attractive pharmacological target. Although recently described clinical trials have been disappointing in attention deficit hyperactivity disorder (ADHD) and schizophrenia (SCH), numerous H3R antagonists, including pitolisant, demonstrate potential in the treatment of narcolepsy, excessive daytime sleepiness associated with cognitive impairment, epilepsy, and Alzheimer's disease (AD). This review focuses on the recent preclinical as well as clinical results that support the relevance of H3R antagonists for the treatment of cognitive symptoms in neuropsychiatric diseases, namely AD, epilepsy and SCH. The review summarizes the role of histaminergic neurotransmission with focus on these brain disorders, as well as the effects of numerous H3R antagonists on animal models and humans.

Drug Design for CNS Diseases: Polypharmacological Profiling of Compounds Using Cheminformatic, 3D-QSAR and Virtual Screening Methodologies

HIGHLIGHTS

Many CNS targets are being explored for multi-target drug design

New databases and cheminformatic methods enable prediction of primary pharmaceutical target and off-targets of compounds

QSAR, virtual screening and docking methods increase the potential of rational drug design

The diverse cerebral mechanisms implicated in Central Nervous System (CNS) diseases together with the heterogeneous and overlapping nature of phenotypes indicated that multitarget strategies may be appropriate for the improved treatment of complex brain diseases. Understanding how the neurotransmitter systems interact is also important in optimizing therapeutic strategies. Pharmacological intervention on one target will often influence another one, such as the well-established serotonin-dopamine interaction or the dopamine-glutamate interaction. It is now accepted that drug action can involve plural targets and that polypharmacological interaction with multiple targets, to address disease in more subtle and effective ways, is a key concept for development of novel drug candidates against complex CNS diseases. A multi-target therapeutic strategy for Alzheimer‘s disease resulted in the development of very effective Multi-Target Designed Ligands (MTDL) that act on both the cholinergic and monoaminergic systems, and also retard the progression of neurodegeneration by inhibiting amyloid aggregation. Many compounds already in databases have been investigated as ligands for multiple targets in drug-discovery programs. A probabilistic method, the Parzen-Rosenblatt Window approach, was used to build a “predictor” model using data collected from the ChEMBL database. The model can be used to predict both the primary pharmaceutical target and off-targets of a compound based on its structure. Several multi-target ligands were selected for further study, as compounds with possible additional beneficial pharmacological activities. Based on all these findings, it is concluded that multipotent ligands targeting AChE/MAO-A/MAO-B and also D₁-R/D₂-R/5-HT_2A-R/H₃-R are promising novel drug candidates with improved efficacy and beneficial neuroleptic and procognitive activities in treatment of Alzheimer's and related neurodegenerative diseases. Structural information for drug targets permits docking and virtual screening and exploration of the molecular determinants of binding, hence facilitating the design of multi-targeted drugs. The crystal structures and models of enzymes of the monoaminergic and cholinergic systems have been used to investigate the structural origins of target selectivity and to identify molecular determinants, in order to design MTDLs.

Multiple Targeting Approaches on Histamine H3 Receptor Antagonists

With the very recent market approval of pitolisant (Wakix®), the interest in clinical applications of novel multifunctional histamine H₃ receptor antagonists has clearly increased. Since histamine H₃ receptor antagonists in clinical development have been tested for a variety of different indications, the combination of pharmacological properties in one molecule for improved pharmacological effects and reduced unwanted side-effects is rationally based on the increasing knowledge on the complex neurotransmitter regulations. The polypharmacological approaches on histamine H₃ receptor antagonists on different G-protein coupled receptors, transporters, enzymes as well as on NO-signaling mechanism are described, supported with some lead structures.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

[(11)C]TASP457, a novel PET ligand for histamine H3 receptors in human brain

PURPOSE

The histamine H3 receptors are presynaptic neuroreceptors that inhibit the release of histamine and other neurotransmitters. The receptors are considered a drug target for sleep disorders and neuropsychiatric disorders with cognitive decline. We developed a novel PET ligand for the H3 receptors, [(11)C]TASP0410457 ([(11)C]TASP457), with high affinity, selectivity and favorable kinetic properties in the monkey, and evaluated its kinetics and radiation safety profile for quantifying the H3 receptors in human brain.

METHODS

Ten healthy men were scanned for 120 min with a PET scanner for brain quantification and three healthy men were scanned for radiation dosimetry after injection of 386 ± 6.2 MBq and 190 ± 7.5 MBq of [(11)C]TASP457, respectively. For brain quantification, arterial blood sampling and metabolite analysis were performed using high-performance liquid chromatography. Distribution volumes (V T) in brain regions were determined by compartment and graphical analyses using the Logan plot and Ichise multilinear analysis (MA1). For dosimetry, radiation absorbed doses were estimated using the Medical Internal Radiation Dose scheme.

RESULTS

[(11)C]TASP457 PET showed high uptake (standardized uptake values in the range of about 3 - 6) in the brain and fast washout in cortical regions and slow washout in the pallidum. The two-tissue compartment model and graphical analyses estimated V T with excellent identification using 60-min scan data (about 16 mL/cm(3) in the pallidum, 9 - 14 in the basal ganglia, 6 - 9 in cortical regions, and 5 in the pons), which represents the known distribution of histamine H3 receptors. For parametric imaging, MA1 is recommended because of minimal underestimation with small intersubject variability. The organs with the highest radiation doses were the pancreas, kidneys, and liver. The effective dose delivered by [(11)C]TASP457 was 6.9 μSv/MBq.

CONCLUSION

[(11)C]TASP457 is a useful novel PET ligand for the investigation of the density of histamine H3 receptors in human brain.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.