Histamine H3 Receptor Antagonists: Preclinical Promise for Treating Obesity and Cognitive Disorders

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

Obesity in Late-Life as a Protective Factor Against Dementia and Dementia-Related Mortality

OBJECTIVE

We estimated the conversion from cognitively normal to mild cognitive impairment (MCI) to probable dementia and death for underweight, normal, overweight, and obese older adults, where the timing of examinations is associated with the severity of dementia.

METHODS

We analyzed six waves of the National Health and Aging Trends Study (NHATS). Body mass (BMI) was computed from height and weight. Multi-state survival models (MSMs) examined misclassification probability, time-to-event ratios, and cognitive decline.

RESULTS

Participants (n = 6078) were 77 years old, 62% had overweight and/or obese BMI. After adjusting for the effects of cardiometabolic factors, age, sex, and race, obesity was protective against developing dementia (aHR=.44; 95%CI [.29-.67]) and dementia-related mortality (aHR=.63; 95%CI [.42-.95]).

DISCUSSION

We found a negative relationship between obesity and dementia and dementia-related mortality, a finding that has been underreported in the literature. The continuing obesity epidemic might complicate the diagnosis and treatment of dementia.

Dual Targeting Ligands-Histamine H3 Receptor Ligands with Monoamine Oxidase B Inhibitory Activity-In Vitro and In Vivo Evaluation

The clinical symptoms of Parkinson’s disease (PD) appear when dopamine (DA) concentrations in the striatum drops to around 20%. Simultaneous inhibitory effects on histamine H3 receptor (H3R) and MAO B can increase DA levels in the brain. A series of compounds was designed and tested in vitro for human H3R (hH3R) affinity and inhibitory activity to human MAO B (hMAO B). Results showed different activity of the compounds towards the two biological targets. Most compounds had poor affinity for hH3R (Ki > 500 nM), but very good inhibitory potency for hMAO B (IC50 < 50 nM). After further in vitro testing (modality of MAO B inhibition, permeability in PAMPA assay, cytotoxicity on human astrocyte cell lines), the most promising dual-acting ligand, 1-(3-(4-(tert-butyl)phenoxy)propyl)-2-methylpyrrolidine (13: hH3R: Ki = 25 nM; hMAO B IC50 = 4 nM) was selected for in vivo evaluation. Studies in rats of compound 13, in a dose of 3 mg/kg of body mass, confirmed its antagonistic effects for H3R (decline in food and a water consumption), decline in MAO B activity (>90%) in rat cerebral cortex (CTX), and an increase in DA content in CTX and striatum. Moreover, compound 13 caused a slight increase in noradrenaline, but a reduction in serotonin concentration in CTX. Thus, compound 13 is a promising dual-active ligand for the potential treatment of PD although further studies are needed to confirm this.

Pharmacokinetic Modeling of [11C]GSK-189254, PET Tracer Targeting H3 Receptors, in Rat Brain

The histamine H₃ receptor has been considered as a target for the treatment of various central nervous system diseases. Positron emission tomography (PET) studies with the radiolabeled potent and selective histamine H₃ receptor antagonist [¹¹C]GSK-189254 in rodents could be used to examine the mechanisms of action of novel therapeutic drugs or to assess changes of regional H₃ receptor density in animal models of neurodegenerative disease. [¹¹C]GSK-189254 was intravenously administered to healthy Wistar rats (n = 10), and a 60 min dynamic PET scan was carried out. Arterial blood samples were obtained during the scan to generate a metabolite-corrected plasma input function. PET data were analyzed using a one-tissue compartment model (1T2k), irreversible (2T3k) or reversible two-tissue compartment models (2T4k), graphical analysis (Logan and Patlak), reference tissue models (SRTM and SRTM2), and standard uptake values (SUVs). The Akaike information criterion and the standard error of the estimated parameters were used to select the most optimal quantification method. This study demonstrated that the 2T4k model with a fixed blood volume fraction and Logan graphical analysis can best describe the kinetics of [¹¹C]GSK-189254 in the rat brain. SUV_40–60 and the reference tissue-based measurements DVR(2T4k), BP_ND(SRTM), and SUV ratio could also be used as a simplified method to estimate H₃ receptor availability in case blood sampling is not feasible.

Role of Mast Cells in the Pathogenesis of Pruritus in Mastocytosis

Pruritus can be defined as an unpleasant sensation that evokes a desire to scratch and significantly impairs patients' quality of life. Pruritus is widely observed in many dermatoses, including mastocytosis, a rare disease characterized by abnormal accumulation of mast cells, which can involve skin, bone marrow, and other organs. Increasing evidence highlights the role of mast cells in neurogenic inflammation and itching. Mast cells release various pruritogenic mediators, initiating subsequent mutual communication with specific nociceptors on sensory nerve fibres. Among important mediators released by mast cells that induce pruritus, one can distinguish histamine, serotonin, proteases, as well as various cytokines. During neuronal-induced inflammation, mast cells may respond to numerous mediators, including neuropeptides, such as substance P, neurokinin A, calcitonin gene-related peptide, endothelin 1, and nerve growth factor. Currently, treatment of pruritus in mastocytosis is focused on alleviating the effects of mediators secreted by mast cells. However, a deeper understanding of the intricacies of the neurobiology of this disease could help to provide better treatment options for patients.

Metabolic benefits of novel histamine H3 receptor ligands in the model of excessive eating: The importance of intrinsic activity and pharmacokinetic properties

AIMS

One of the therapeutic approaches in the treatment of obesity is the use of histamine H3 receptor ligands. Histamine plays a significant role in eating behavior because it causes a loss of appetite and is considered to be a satiety signal released during food intake.

MATERIAL AND METHODS

Histamine ligands were selected based on the preliminary studies which included determination of intrinsic activity and selected pharmacokinetic parameters. Female Wistar rats were fed palatable feed for 28 days and simultaneously the tested compounds were administered intraperitoneally at a dose of 10 mg/kg b.w./day. Rats' weight was evaluated daily and calories intake was evaluated once per week. At the end of experiment insulin and glucose tolerance tests was performed. Plasma levels of cholesterol, triglycerides, leptin, insulin, glucose, C-peptide and CRP were also determined. In order to rule out false-positive results the influence of tested compounds on spontaneous activity of rats was monitored.

RESULTS

Animals fed palatable feed and treated with KSK-61 or KSK-63 compounds showed the slowest weight gain which was comparable to the one observed in control animals. Both compounds with the highest pharmacological activity have also similar pharmacokinetic properties with quite long half-life and high volume of distribution indicating that they can freely cross most biological barriers. Some compounds, especially KSK-63, compensated for metabolic disorders.

CONCLUSION

The presented study proves that search among the active histamine H3 receptor ligands for the new therapeutic agents to treat obesity is justified. Compounds KSK-61 and KSK-63 can be considered as the leading structures.

Histamine 3 receptor inverse agonist Samelisant (SUVN-G3031): Pharmacological characterization of an investigational agent for the treatment of cognitive disorders

BACKGROUND

Central histamine H3 receptors are a family of presynaptic auto and heteroreceptors. Blockade of the presynaptic H3 receptors activates the downstream pathway(s) involved in the processes of learning and memory, making it a potential therapeutic option for ameliorating cognitive dysfunction. Samelisant (SUVN-G3031) is a potent and selective inverse agonist at the H3 receptors.

AIM

The aim of this research is to study the effects of Samelisant in diverse animal models of cognitive functions.

METHODS

The effects of Samelisant on cognitive functions were studied using social recognition, object recognition and Morris water maze tasks. Neurochemical and electrophysiological effects of Samelisant were monitored using microdialysis and electroencephalography techniques.

RESULTS

Samelisant showed procognitive effects in diverse animal models of cognition at doses ranging from 0.3 to 3 mg/kg, per os (p.o.) (social recognition and object recognition task). Samelisant significantly increased the brain acetylcholine levels in the cortex at doses of 10 and 20 mg/kg, p.o. In the Morris water maze task, combined administration of suboptimal doses of Samelisant and donepezil resulted in procognitive effects significantly larger than the either treatment. Similarly, Samelisant significantly potentiated the effects of donepezil on pharmacodynamic biomarkers of cognition i.e. acetylcholine levels in brain and neuronal theta oscillations.

CONCLUSION

Samelisant may have potential utility in the treatment of cognitive deficits associated with hypocholinergic state.

Region-specific regulation of central histaminergic H3 receptor expression in a mouse model of cow's milk allergy

Central histaminergic H3 receptor (H3R) has been extensively investigated as a potential therapeutic target for various neurological and neurodegenerative disorders. Despite promising results in preclinical rodent models, clinical trials have not provided conclusive evidence for the benefit of H3R antagonists to alleviate cognitive and behavioral symptoms of these disorders. Inconsistent pharmacological efficacies may arise from aberrant changes in H3R over time during disease development. Because H3R is involved in feedback inhibition of histamine synthesis and secretion, the expression of the autoreceptor may also be reciprocally regulated by altered histamine levels in a pathological condition. Thus, we investigated H3R expression in a mouse model of cow's milk allergy, a condition associated with increased histamine levels. Mice were sensitized to bovine whey proteins (WP) over 5 weeks and H3R protein and transcript levels were examined in the brain. Substantially increased H3R immunoreactivity was observed in various brain regions of WP-sensitized mice compared to sham mice. Elevated H3R expression was also found in the thalamic/hypothalamic region. The expression of histaminergic H1, but not H2, receptor subtype was also increased in this and the midbrain regions. Unlike the brain, all three histaminergic receptors were increased in the small intestine. These results indicated that the central histaminergic receptors were altered in WP-sensitized mice in a subtype- and region-specific manner, which likely contributed to behavioral changes we observed in these mice. Our study also suggests that altered levels of H3R could be considered during a pharmacological intervention of a neurological disease.

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.

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.

Non-Imidazole Histamine H₃ Ligands. Part VII. Synthesis, In Vitro and In Vivo Characterization of 5-Substituted-2-thiazol-4-n-propylpiperazines

H₃ receptors present on histaminergic and non-histaminergic neurons, act as autoreceptors or heteroreceptors controlling neurotransmitter release and synthesis. Previous, studies have found that the compound N-methyl-N-3-phenylalkyl-2-[2-(4-n-propylpiperazin-1-yl)-1,3-thiazol-5-yl]ethan-1-amine (ADS-531, 2c) exhibits high in vitro potency toward H₃ guinea pig jejunal receptors, with pA₂ = 8.27. To optimize the structure of the lead compound ADS-531, a series of 5-substituted-2-thiazol-4-n-propylpiperazines 3 were synthesized and subjected to in vitro pharmacological characterization; the alkyl chain between position 2 of the thiazole ring and the terminal secondary N-methylamino function was elongated from three to four methylene groups and the N-methylamino functionality was substituted by benzyl-, 2-phenylethyl-, and 3-phenyl-propyl- moieties. SAR studies on novel non-imidazole, 5-substituted-2-thiazol-4-n-propyl-piperazines 3 showed that the most active compound 3a (pA₂ = 8.38), additionally possessed a weak competitive H₁-antagonistic activity. Therefore, compound ADS-531, which did not exhibit any H₁-antagonistic activity, was chosen for further evaluation for its affinity to the recombinant rat and human histamine H₃ receptors (rH₃R and hH₃R, respectively). ADS-531 exhibited nanomolar affinity for both rH₃R and hH₃R receptors. It was also shown that, ADS-531 given subchronically to rats (s.c. 3 mg/kg, 5 days) penetrated the brain, where it affected dopamine, noradrenaline and serotonin concentration; however, it did not affect histamine concentration nor feeding behavior.

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.

Determination of receptor occupancy in the presence of mass dose: [11C]GSK189254 PET imaging of histamine H3 receptor occupancy by PF-03654746

Measurements of drug occupancies using positron emission tomography (PET) can be biased if the radioligand concentration exceeds "tracer" levels. Negative bias would also arise in successive PET scans if clearance of the radioligand is slow, resulting in a carryover effect. We developed a method to (1) estimate the in vivo dissociation constant K_d of a radioligand from PET studies displaying a non-tracer carryover (NTCO) effect and (2) correct the NTCO bias in occupancy studies taking into account the plasma concentration of the radioligand and its in vivo K_d. This method was applied in a study of healthy human subjects with the histamine H₃ receptor radioligand [¹¹C]GSK189254 to measure the PK-occupancy relationship of the H₃ antagonist PF-03654746. From three test/retest studies, [¹¹C]GSK189254 K_d was estimated to be 9.5 ± 5.9 pM. Oral administration of 0.1 to 4 mg of PF-03654746 resulted in occupancy estimates of 71%-97% and 30%-93% at 3 and 24 h post-drug, respectively. NTCO correction adjusted the occupancy estimates by 0%-15%. Analysis of the relationship between corrected occupancies and PF-03654746 plasma levels indicated that PF-03654746 can fully occupy H₃ binding sites ( RO_max = 100%), and its IC₅₀ was estimated to be 0.144 ± 0.010 ng/mL. The uncorrected IC₅₀ was 26% higher.

Monoaminergic and Histaminergic Strategies and Treatments in Brain Diseases

The monoaminergic systems are the target of several drugs for the treatment of mood, motor and cognitive disorders as well as neurological conditions. In most cases, advances have occurred through serendipity, except for Parkinson's disease where the pathophysiology led almost immediately to the introduction of dopamine restoring agents. Extensive neuropharmacological studies first showed that the primary target of antipsychotics, antidepressants, and anxiolytic drugs were specific components of the monoaminergic systems. Later, some dramatic side effects associated with older medicines were shown to disappear with new chemical compounds targeting the origin of the therapeutic benefit more specifically. The increased knowledge regarding the function and interaction of the monoaminergic systems in the brain resulting from in vivo neurochemical and neurophysiological studies indicated new monoaminergic targets that could achieve the efficacy of the older medicines with fewer side-effects. Yet, this accumulated knowledge regarding monoamines did not produce valuable strategies for diseases where no monoaminergic drug has been shown to be effective. Here, we emphasize the new therapeutic and monoaminergic-based strategies for the treatment of psychiatric diseases. We will consider three main groups of diseases, based on the evidence of monoamines involvement (schizophrenia, depression, obesity), the identification of monoamines in the diseases processes (Parkinson's disease, addiction) and the prospect of the involvement of monoaminergic mechanisms (epilepsy, Alzheimer's disease, stroke). In most cases, the clinically available monoaminergic drugs induce widespread modifications of amine tone or excitability through neurobiological networks and exemplify the overlap between therapeutic approaches to psychiatric and neurological conditions. More recent developments that have resulted in improved drug specificity and responses will be discussed in this review.

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.

Recent development of multifunctional agents as potential drug candidates for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a complex and progressive neurodegenerative disorder. The available therapy is limited to the symptomatic treatment and its efficacy remains unsatisfactory. In view of the prevalence and expected increase in the incidence of AD, the development of an effective therapy is crucial for public health. Due to the multifactorial aetiology of this disease, the multi-target-directed ligand (MTDL) approach is a promising method in search for new drugs for AD. This review updates information on the development of multifunctional potential anti-AD agents published within the last three years. The majority of the recently reported structures are acetylcholinesterase inhibitors, often endowed with some additional properties. These properties enrich the pharmacological profile of the compounds giving hope for not only symptomatic but also causal treatment of the disease. Among these advantageous properties, the most often reported are an amyloid-β antiaggregation activity, inhibition of β-secretase and monoamine oxidase, an antioxidant and metal chelating activity, NOreleasing ability and interaction with cannabinoid, NMDA or histamine H3 receptors. The majority of novel molecules possess heterodimeric structures, able to interact with multiple targets by combining different pharmacophores, original or derived from natural products or existing therapeutics (tacrine, donepezil, galantamine, memantine). Among the described compounds, several seem to be promising drug candidates, while others may serve as a valuable inspiration in the search for new effective therapies for AD.

Procognitive properties of drugs with single and multitargeting H3 receptor antagonist activities

The histamine H3 receptor (H3 R) is an important modulator of numerous central control mechanisms. Novel lead optimizations for H3 R antagonists/inverse agonists involved studies of structure-activity relationships, cross-affinities, and pharmacokinetic properties of promising ligands. Blockade of inhibitory histamine H3 autoreceptors reinforces histaminergic transmission, while antagonism of H3 heteroreceptors accelerates the corticolimbic liberation of acetylcholine, norepinephrine, glutamate, dopamine, serotonin and gamma-aminobutyric acid (GABA). The H3 R positioned at numerous neurotransmission crossroads indicates therapeutic applications of small-molecule H3 R modulators in a number of psychiatric and neurodegenerative diseases with various clinical candidates available. Dual target drugs displaying H3 R antagonism/inverse agonism with inhibition of acetylcholine esterase (AChE), histamine N-methyltransferase (HMT), or serotonin transporter (SERT) are novel class of procognitive agents. Main chemical diversities, pharmacophores, and pharmacological profiles of procognitive agents acting as H3 R antagonists/inverse agonists and dual H3 R antagonists/inverse agonists with inhibiting activity on AChE, HMT, or SERT are highlighted here.

Synthesis, biological evaluation, and computational studies of Tri- and tetracyclic nitrogen-bridgehead compounds as potent dual-acting AChE inhibitors and hH3 receptor antagonists

Combination of AChE inhibiting and histamine H3 receptor antagonizing properties in a single molecule might show synergistic effects to improve cognitive deficits in Alzheimer's disease, since both pharmacological actions are able to enhance cholinergic neurotransmission in the cortex. However, whereas AChE inhibitors prevent hydrolysis of acetylcholine also peripherally, histamine H3 antagonists will raise acetylcholine levels mostly in the brain due to predominant occurrence of the receptor in the central nervous system. In this work, we designed and synthesized two novel classes of tri- and tetracyclic nitrogen-bridgehead compounds acting as dual AChE inhibitors and histamine H3 antagonists by combining the nitrogen-bridgehead moiety of novel AChE inhibitors with a second N-basic fragment based on the piperidinylpropoxy pharmacophore with different spacer lengths. Intensive structure-activity relationships (SARs) with regard to both biological targets led to compound 41 which showed balanced affinities as hAChE inhibitor with IC50 = 33.9 nM, and hH3R antagonism with Ki = 76.2 nM with greater than 200-fold selectivity over the other histamine receptor subtypes. Molecular docking studies were performed to explain the potent AChE inhibition of the target compounds and molecular dynamics studies to explain high affinity at the hH3R.

Discovery of (1R,6S)-5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one (R,S-4a): histamine H(3) receptor inverse agonist demonstrating potent cognitive enhancing and wake promoting activity

A series of fused cyclopropyl-4,5-dihydropyridazin-3-one (3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one) phenoxypiperidine analogs was designed and synthesized, leading to the identification of (1R,6S)-5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one (R,S-4a) as a second-generation pyridazin-3-one H3R antagonist. Compound R,S-4a was a potent H3R functional antagonist in vivo in the rat dipsogenia model, demonstrated potent wake activity in the rat EEG/EMG model, and enhanced short-term memory in the rat social recognition memory model at doses as low as 0.03-0.3 mg/kg po.

Non-imidazole histamine H3 ligands: part V. synthesis and preliminary pharmacological investigation of 1-[2-thiazol-4-yl- and 1-[2-thiazol-5-yl-(2-aminoethyl)]-4-n-propylpiperazine derivatives

Series of 1-[2-thiazol-4-yl-(2-aminoethyl)]- and 1-[2-thiazol-5-yl-(2-aminoethyl)]-4-n-propylpiperazine derivatives have been prepared and in vitro tested as H3-receptor antagonists (the electrically evoked contraction of the guinea-pig jejunum). It appeared that by comparison of homologous pairs, the 1-[2-thiazol-5-yl-(2-aminoethyl)]-4-n-propylpiperazines (3a,b and 4a-d) have much higher potency than their analogous 1-[2-thiazol-4-yl-(2-aminoethyl)]-4-n-propylpiperazines (2a-k). Based on the obtained results, we observed the 5-position of 2-methyl-2-R-aminoethyl substituents in the thiazole ring is favourable for histamine H3 receptor antagonist activity, whereas its presence in position 4 leads, almost in each case, to strong decrease of activity.

Assessment of the abuse liability of ABT-288, a novel histamine H₃ receptor antagonist

RATIONALE

Histamine H3 receptor antagonists, such as ABT-288, have been shown to possess cognitive-enhancing and wakefulness-promoting effects. On the surface, this might suggest that H3 antagonists possess psychomotor stimulant-like effects and, as such, may have the potential for abuse.

OBJECTIVES

The aim of the present study was to further characterize whether ABT-288 possesses stimulant-like properties and whether its pharmacology gives rise to abuse liability.

METHODS

The locomotor-stimulant effects of ABT-288 were measured in mice and rats, and potential development of sensitization was addressed. Drug discrimination was used to assess amphetamine-like stimulus properties, and drug self-administration was used to evaluate reinforcing effects of ABT-288. The potential development of physical dependence was also studied.

RESULTS

ABT-288 lacked locomotor-stimulant effects in both rats and mice. Repeated administration of ABT-288 did not result in cross-sensitization to the stimulant effects of d-amphetamine in mice, suggesting that there is little overlap in circuitries upon which the two drugs interact for motor activity. ABT-288 did not produce amphetamine-like discriminative stimulus effects in drug discrimination studies nor was it self-administered by rats trained to self-administer cocaine. There were no signs of physical dependence upon termination of repeated administration of ABT-288 for 30 days.

CONCLUSIONS

The sum of these preclinical data, the first of their kind applied to H3 antagonists, indicates that ABT-288 is unlikely to possess a high potential for abuse in the human population and suggests that H3 antagonists, as a class, are similar in this regard.

Preclinical evaluation of non-imidazole histamine H3 receptor antagonists in comparison to atypical antipsychotics for the treatment of cognitive deficits associated with schizophrenia

Cognitive deficits associated with schizophrenia (CDS) are implicated as a core symptom cluster of the disease and are associated with poor daily life functioning. Unfortunately, current antipsychotic agents provide little alleviation of CDS, representing a critical unmet therapeutic need. Here we investigated the effects of ABT-239 and A-431404, non-imidazole histamine H(3) receptor (H(3)R) antagonists, in animal models with relevance to CDS. As N-methyl-d-aspartate receptor hypofunction is considered an important factor in the pathogenesis of schizophrenia, acute administration of ketamine or MK-801 was used to induce cognitive impairments. The assays employed in the current studies were spontaneous alternation in cross-maze, used as an indication of working memory, and inhibitory avoidance (IA), used to assess long-term memory retention. Risperidone and olanzapine were also tested to directly compare the effects of H(3)R antagonists to two widely used antipsychotics. ABT-239 and A-431404, but not risperidone and olanzapine, attenuated ketamine-induced deficits on spontaneous alternation in cross-maze, while none of these compounds affected alternation performance on their own. ABT-239 and A-431404 also attenuated MK-801-induced impairments in IA; no effects were observed when given alone. Risperidone and olanzapine, however, failed to attenuate MK-801-induced deficits in IA and produced dose-dependent impairments when given alone. ABT-239 was also investigated in methylazoxymethanol acetate (MAM) treated rats, a neurodevelopmental model for schizophrenia. Chronic, but not acute, treatment with ABT-239 significantly improved spontaneous alternation impairments in MAM rats tested in cross-maze. In summary, these results suggest H(3)R antagonists may have the potential to ameliorate CDS.

Randomized crossover study of the histamine H3 inverse agonist MK-0249 for the treatment of cognitive impairment in patients with schizophrenia

BACKGROUND

Current antipsychotic treatments have little impact on the cognitive deficits associated with schizophrenia. It has been proposed that agents which promote histamine release may enhance cognition. We evaluated whether the H3 inverse agonist MK-0249 might improve cognitive deficits in patients with schizophrenia.

METHODS

Outpatients (N=55) with schizophrenia between ages 21 and 55 who were clinically stable, experienced no more than mild to moderate overall symptoms (PANSS score total 36-75), and were taking a stable dose of antipsychotic medication were randomized to MK-0249 10mg and placebo in a multi-center, randomized, double-blind, 2-period (4-weeks per period), cross-over study. The primary efficacy endpoint was the mean change from baseline at 4-weeks of treatment in the total cognitive score on the Brief Assessment of Cognition in Schizophrenia (BACS) Battery. Other assessments of cognition were also performed.

RESULTS

A total of 46 patients completed the study. MK-0249 10mg did not demonstrate a statistically significant difference compared to placebo in the mean change from baseline in the total cognitive score on the BACS battery after 4-weeks of treatment (-0.1, 95% CI: -2.3, 2.1) or with regard to secondary measures of attention/processing speed, episodic memory, or working memory after 4-weeks of treatment. The incidence of adverse events was greater during the MK-0249 treatment period (25/52 patients, 48.1%) compared to the placebo treatment period (15/51 patients, 29.4%).

CONCLUSION

MK-0249 10mg once daily was not superior to placebo in the treatment of cognitive impairment in patients with schizophrenia after 4-weeks. (Clinicaltrials.gov: NCT00506077).

Synthesis, pharmacological evaluation and QSAR modeling of mono-substituted 4-phenylpiperidines and 4-phenylpiperazines

A series of mono-substituted 4-phenylpiperidines and -piperazines have been synthesized and their effects on the dopaminergic system tested in vivo. The structure activity relationship (SAR) revealed that the position and physicochemical character of the aromatic substituent proved to be critical for the levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the brain of freely moving rats. In order to investigate how the structural properties of these compounds affect the response, a set of tabulated and calculated physicochemical descriptors were modeled against the in vivo effects using partial least square (PLS) regression. Furthermore, the binding affinities to the dopamine D2 (DA D2) receptor and monoamine oxidase A (MAO A) enzyme were determined for a chosen subset and QSAR models using the same descriptors as in the in vivo model were produced to investigate the mechanisms leading to the observed DOPAC response. These models, in combination with a strong correlation between the levels of striatal DOPAC and the affinities to DA D2 and MAO A, provides a comprehensive understanding of the biological response for compounds in this class.

The histamine autoreceptor is a short isoform of the H₃ receptor

BACKGROUND AND PURPOSE The histamine H(3) receptor was identified as the autoreceptor of brain histaminergic neurons. After its cloning, functional H(3) receptor isoforms generated by a deletion in the third intracellular loop were found in the brain. Here, we determined if this autoreceptor was the long or the short isoform. EXPERIMENTAL APPROACH We hypothesized that the deletion would affect H(3) receptor stereoselectivity. The effects of the enantiomers of two chiral ligands, N(α)-methyl-α-chloromethylhistamine (N(α) Me-αClMeHA) and sopromidine, were investigated on cAMP formation at the H(3(445)) and H(3(413)) receptor isoforms, common to all species. They were further compared with their effects at autoreceptors. They were also compared on [(35)S]GTPγ[S] binding to membranes of rat cerebral cortex, striatum and hypothalamus, the richest area in autoreceptors. KEY RESULTS The stereoselectivity of N(α) Me-αClMeHA enantiomers as agonists was similar at the H(3(413)) receptor isoform and autoreceptors, but lower at the long isoform. While (S) sopromidine did not discriminate between the isoforms, (R) sopromidine was an antagonist at the H(3(413)) receptor isoform and autoreceptors, but a full agonist at the long isoform. In rat brain, stereoselectivity of N(α) Me-αClMeHA was higher in the hypothalamus than in cerebral cortex or striatum, whereas the opposite pattern was found for sopromidine. CONCLUSIONS AND IMPLICATIONS The pharmacological profiles of H(3) receptor isoforms differed markedly, showing that the function of autoreceptors was fulfilled by a short isoform, such as the H(3(413)) receptor. Development of drugs selectively targeting autoreceptors might enhance their therapeutic efficacy and/or decrease incidence of side effects.

Identification and characterization of ZEL-H16 as a novel agonist of the histamine H3 receptor

The histamine H3 receptor (H3R) has been recognized as a promising target for the treatment of various central and peripheral nervous system diseases. In this study, a non-imidazole compound, ZEL-H16, was identified as a novel histamine H3 receptor agonist. ZEL-H16 was found to bind to human H3R with a Ki value of approximately 2.07 nM and 4.36 nM to rat H3R. Further characterization indicated that ZEL-H16 behaved as a partial agonist on the inhibition of forskolin-stimulated cAMP accumulation (the efficacy was 60% of that of histamine) and activation of ERK1/2 signaling (the efficacy was 50% of that of histamine) at H3 receptors, but acted as a full agonist just like histamin in the guinea-pig ileum contraction assay. These effects were blocked by pertussis toxin and H3 receptor specific antagonist thioperamide. ZEL-H16 showed no agonist or antagonist activities at the cloned human histamine H1, H2, and H4 receptors and other biogenic amine GPCRs in the CRE-driven reporter assay. Furthermore, our present data demonstrated that treatment of ZEL-H16 resulted in intensive H3 receptor internalization and delayed recycling to the cell surface as compared to that of control with treatment of histamine. Thus, ZEL-H16 is a novel and potent nonimidazole agonist of H3R, which might serve as a pharmacological tool for future investigations or as possible therapeutic agent of H3R.

Pharmacological properties and procognitive effects of ABT-288, a potent and selective histamine H3 receptor antagonist

Blockade of the histamine H(3) receptor (H(3)R) enhances central neurotransmitter release, making it an attractive target for the treatment of cognitive disorders. Here, we present in vitro and in vivo pharmacological profiles for the H(3)R antagonist 2-[4'-((3aR,6aR)-5-methyl-hexahydro-pyrrolo[3,4-b]pyrrol-1-yl)-biphenyl-4-yl]-2H-pyridazin-3-one (ABT-288). ABT-288 is a competitive antagonist with high affinity and selectivity for human and rat H(3)Rs (K(i) = 1.9 and 8.2 nM, respectively) that enhances the release of acetylcholine and dopamine in rat prefrontal cortex. In rat behavioral tests, ABT-288 improved acquisition of a five-trial inhibitory avoidance test in rat pups (0.001-0.03 mg/kg), social recognition memory in adult rats (0.03-0.1 mg/kg), and spatial learning and reference memory in a rat water maze test (0.1-1.0 mg/kg). ABT-288 attenuated methamphetamine-induced hyperactivity in mice. In vivo rat brain H(3)R occupancy of ABT-288 was assessed in relation to rodent doses and exposure levels in behavioral tests. ABT-288 demonstrated a number of other favorable attributes, including good pharmacokinetics and oral bioavailability of 37 to 66%, with a wide central nervous system and cardiovascular safety margin. Thus, ABT-288 is a selective H(3)R antagonist with broad procognitive efficacy in rodents and excellent drug-like properties that support its advancement to the clinical area.

Randomized clinical study of a histamine H3 receptor antagonist for the treatment of adults with attention-deficit hyperactivity disorder

BACKGROUND

Psychostimulants, including methylphenidate and amphetamine preparations, are commonly prescribed for the treatment of attention-deficit hyperactivity disorder (ADHD) in children and adults. Histamine H3 receptors reside on non-histamine neurons and regulate other neurotransmitters (e.g. acetylcholine, noradrenaline [norepinephrine]) suggesting that H3 antagonists have the potential to improve attention and impulsivity. Research indicates that H3 receptor antagonists due to their novel mechanism of action may have a unique treatment effect offering an important alternative for the treatment of ADHD. Bavisant (JNJ-31001074) is a highly selective, orally active antagonist of the human H3 receptor with a novel mechanism of action, involving wakefulness and cognition, with potential as a treatment for ADHD.

OBJECTIVE

The objective of this study was to evaluate the efficacy, safety and tolerability of three dosages of bavisant compared with placebo in adults with ADHD.

STUDY DESIGN

This randomized, double-blind, placebo- and active-controlled, parallel-group, multicentre study evaluated three dosages of bavisant (1 mg/day, 3 mg/day or 10 mg/day) and two active controls in adults with ADHD. The study consisted of a screening phase of up to 14 days, a 42-day double-blind treatment phase and a 7-day post-treatment follow-up phase. Efficacy and safety assessments were performed.

SETTING

The study was conducted at 37 study centres in the US from April 2009 through January 2010.

PARTICIPANTS

Men and women aged 18-55 years with an established diagnosis of ADHD as confirmed by clinician and self-report diagnostic measures were enrolled.

INTERVENTION

Participants were randomly assigned equally to one of six treatment groups: placebo, bavisant 1 mg/day, 3 mg/day or 10 mg/day, atomoxetine hydrochloride 80 mg/day or osmotic-release oral system (OROS) methylphenidate hydrochloride 54 mg/day.

MAIN OUTCOME MEASURE

The primary efficacy endpoint was the change in the Attention Deficit Hyperactivity Disorder Rating Scale, Version IV (ADHD-RS-IV) total score from baseline (day 1) to the end of the treatment phase (day 42), and included all randomized participants who received one or more doses of study drug and had baseline and one or more post-baseline assessments (intent-to-treat [ITT] population). Safety assessments included treatment-emergent adverse events (TEAEs), laboratory tests and ECG readings.

RESULTS

430 participants were randomized, 424 received one or more doses of study medication and 335 (78%) of those randomized completed the study. Study participants had a mean age of 33.9 years and were predominantly White men. Mean treatment duration ranged from 31.4 to 38.8 days across groups. Mean change from baseline in the total ADHD-RS-IV score at day 42 (primary efficacy endpoint) was -8.8 in the placebo group versus -9.3, -11.2 and -12.2 in the bavisant 1 mg/day, 3 mg/day and 10 mg/day groups, respectively; the change in the 10 mg/day group was not statistically superior to placebo (p=0.161), and hence statistical comparisons of the 1 mg/day and 3 mg/day groups with placebo based on a step-down closed testing procedure were not performed. Mean change from baseline in the total ADHD-RS-IV score at day 42 was superior to placebo in the atomoxetine (-15.3) and OROS methylphenidate (-15.7) groups (p<0.005). Secondary efficacy assessments demonstrated a similar pattern with a non-significant trend towards improvement in the bavisant groups. The two lower dosages showed a good tolerability profile, but the higher dosage of bavisant was less well tolerated, as evidenced by the incidence of total TEAEs (61.8%, 82.4%, 89.0%), and discontinuations due to TEAEs (4.4%, 7.4%, 19.2%) in the bavisant 1 mg/day, 3 mg/day and 10 mg/day groups, respectively, compared with 58.9% and 2.7%, respectively on placebo. In the atomoxetine and OROS methylphenidate groups, the incidence of total TEAEs was 83.8% and 82.4% and discontinuations due to TEAEs was 10.8% and 8.8%, respectively.

CONCLUSION

Bavisant, a highly selective, wakefulness-promoting H3 antagonist, did not display significant clinical effectiveness in the treatment of adults with ADHD.

CLINICAL TRIAL REGISTRATION NUMBER

NCT00880217.

Discovery of a potent thiadiazole class of histamine h3 receptor antagonist for the treatment of diabetes

A series of novel 2-piperidinopiperidine thiadiazoles were synthesized and evaluated as new leads of histamine H3 receptor antagonists. The 4-(5-([1,4'-bipiperidin]-1'-yl)-1,3,4-thiadiazol-2-yl)-2-(pyridin-2-yl)morpholine (5u) displayed excellent potency and ex vivo receptor occupancy. Compound 5u was also evaluated in vivo for antidiabetic efficacy in STZ diet-induced obesity type 2 diabetic mice for 2 or 12 days. Non-fasting glucose levels were significantly reduced as compared with vehicle-treated mice. In addition, 5u dose dependently blocked the increase of HbA1c after 12 days of treatment.

Novel sulfonylurea derivatives as H3 receptor antagonists. Preliminary SAR studies

The combination of antagonism at histamine H(3) receptor and the stimulation of insulin secretion have been proposed as an approach to new dual therapeutic agents for the treatment of type 2 diabetes mellitus associated with obesity. We have designed and synthesized a new series of non-imidazole derivatives, based on a basic amine ring connected through an alkyl spacer of variable length to a phenoxysulfonylurea moiety. These compounds were initially evaluated for histamine H(3) receptor binding affinities, suggesting that a propoxy chain linker between the amine and the core ring could be essential for optimal binding affinity. Compound 56, 1-(naphthalen-1-yl)-3-[(p-(3-pyrrolidin-1-ylpropoxy)benzene)]sulfonylurea exhibited the best H(3) antagonism affinity. However, since all these derivatives failed to block K(ATP) channels, the link of these two related moieties should not be considered a good pharmacophore for obtaining new dual H(3) antagonists with insulinotropic activity, suggesting the necessity to propose a new chemical hybrid prototype.

Involvement of histamine receptors in the atypical antipsychotic profile of clozapine: a reassessment in vitro and in vivo

RATIONALE

The basis of the unique clinical profile of the antipsychotic clozapine is not yet elucidated. Brain histamine receptors may play a role in schizophrenia and its treatment, but their involvement in the profile of clozapine remained unknown.

OBJECTIVES

We explored the properties of clozapine and its two metabolites, N-desmethylclozapine (NDMC) and clozapine N-oxide, at the four human histaminergic receptors. We compared their active concentrations with their blood concentrations in patients treated by clozapine. We investigated the changes in receptor densities induced in rat brain by repeated administration of a therapeutic dose of clozapine.

RESULTS

Clozapine and NDMC behaved as very potent, and partial, H(1)-receptor inverse agonists, weak, and full, H(2)-receptor inverse agonists, moderate, and protean, H(3)-receptor agonists, and moderate, and partial, H(4)-receptor agonists. Taking into account their micromolar mean blood concentrations found in 75 treated patients, and assuming that they are enriched in human brain as they are in rat brain, a full occupation of H(1)-, H(3)-, and H(4)-receptors, and a partial occupation of H(2) receptors, is expected. In agreement, repeated administration of clozapine at a therapeutic dose (20 mg/kg/day for 20 days) induced an up-regulation of H(1)- and H(2)-receptors in rat brain.

CONCLUSIONS

Clozapine and its active metabolite NDMC interact with the four human histamine receptors at clinically relevant concentrations. This interaction may substantiate, at least in part, the atypical antipsychotic profile of clozapine, as well as its central and peripheral side effects such as sedation and weight gain.

4-phenoxypiperidine pyridazin-3-one histamine H(3) receptor inverse agonists demonstrating potent and robust wake promoting activity

Structure-activity relationships for a series of phenoxypiperidine pyridazin-3-one H(3)R antagonists/inverse agonists are disclosed. The search for compounds with improved hERG and DAT selectivity without the formation of in vivo active metabolites identified 6-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-4,4-dimethyl-4,5-dihydro-2H-pyridazin-3-one 17b. Compound 17b met discovery flow criteria, demonstrated potent H(3)R functional antagonism in vivo in the rat dipsogenia model and potent wake activity in the rat EEG/EMG model at doses as low as 0.1 mg/kg ip.