Differential in vivo effects of H3 receptor ligands in a new mouse dipsogenia model

Experimental Studies Indicate That ST-2223, the Antagonist of Histamine H3 and Dopamine D2/D3 Receptors, Restores Social Deficits and Neurotransmission Dysregulation in Mouse Model of Autism

Altered regulation of neurotransmitters may lead to many pathophysiological changes in brain disorders including autism spectrum disorder (ASD). Given the fact that there are no FDA-approved effective treatments for the social deficits in ASD, the present study determined the effects of chronic systemic treatment of the novel multiple-active H3R/D2R/D3R receptor antagonist ST-2223 on ASD-related social deficits in a male Black and Tan Brachyury (BTBR) mice. ST-2223 (2.5, 5, and 10 mg/kg, i.p.) significantly and dose-dependently mitigated social deficits and disturbed anxiety levels of BTBR mice (p < 0.05) in comparison to the effects of aripiprazole (1 mg/kg, i.p.). Moreover, levels of monoaminergic neurotransmitters quantified by LC-MS/MS in four brain regions including the prefrontal cortex, cerebellum, striatum, and hippocampus unveiled significant elevation of histamine (HA) in the cerebellum and striatum; dopamine (DA) in the prefrontal cortex and striatum; as well as acetylcholine (ACh) in the prefrontal cortex, striatum, and hippocampus following ST-2223 (5 mg/kg) administration (all p < 0.05). These in vivo findings demonstrate the mitigating effects of a multiple-active H3R/D2R/D3R antagonist on social deficits of assessed BTBR mice, signifying its pharmacological potential to rescue core ASD-related behaviors and altered monoaminergic neurotransmitters. Further studies on neurochemical alterations in ASD are crucial to elucidate the early neurodevelopmental variations behind the core symptoms and heterogeneity of ASD, leading to new approaches for the future therapeutic management of ASD.

Histamine H3 receptor antagonism modulates autism-like hyperactivity but not repetitive behaviors in BTBR T+Itpr3tf/J inbred mice

BACKGROUND

Autism spectrum disorders (ASDs) are a group of neurodevelopmental conditions defined by behavioral deficits in social communication and interactions, mental inflexibility and repetitive behaviors. Converging evidence from observational and preclinical studies suggest that excessive repetitive behaviors in people with ASD may be due to elevated histaminergic H3 receptor signaling in the striatum. We hypothesized that systemic administration of pharmacological histamine H3 receptor antagonists would attenuate the expression of repetitive behaviors in the BTBR T+Itpr3tf/J (BTBR) mouse inbred strain, an established mouse model presenting autism-like repetitive behaviors and novelty-induced hyperactivity. We further aimed to investigate whether agonism of the histamine H3 receptor would be sufficient to induce repetitive behaviors in the C57BL/6J control mouse strain.

METHODS

Different doses of H3 receptor agonists (i.e., (R)-α-methylhistamine and immethridine) and H3 receptor antagonists/inverse agonists (i.e., ciproxifan and pitolisant) were administered via intraperitoneal (i.p.) injection in male mice to characterize the acute effects of these compounds on ASD-related behavioral readouts.

RESULTS

The highly selective H3 receptor agonist immethridine significantly increased the time spent in stereotypic patterns in C57BL/6J mice, but this effect appeared to be driven by general sedative properties of the compound. High doses of pitolisant significantly decreased locomotor hyperactivity in novel environments in BTBR mice, without significant effects on repetitive behaviors.

CONCLUSIONS

Based on our findings, we conclude that acute H3 receptor manipulation mainly affected general motor activity levels in novel environments. Small changes in stereotyped behaviors were observed but appeared to be driven by altered general activity levels.

A high-throughput screen identifies inhibitors of lung cancer stem cells

Metastasis is the main cause of cancer morbidity and mortality. Cancer stem cells (CSCs) are a rare subpopulation of cancer cells that can drive metastasis. The identification of CSC inhibitors and CSC-related genes is an alluring strategy for suppressing metastasis. Here, we established a simple and repeatable high-throughput CSC inhibitor screening platform that combined tumor sphere formation assays and cell viability assays. Human lung cancer cells were cocultured with 1280 pharmacologically active compounds (FDA-approved). Fifty-four candidate compounds obtained from our screening system completely or partially inhibited tumor sphere formation. A total of 5 of these 54 compounds (prochlorperazine dimaleate, thioridazine hydrochloride, ciproxifan hydrochloride, Ro 25-6981 hydrochloride, and AMN 082) completely inhibited the self-renewal of CSCs without cytotoxicity in vitro via their targets and suppressed lung cancer metastasis in vivo, suggesting that our screening platform is selective and reliable. DRD2, HRH3, and GRIN2B exhibited potent genes promoting CSCs in vitro experiments and clinical datasets. Further validation of the top hit (DRD2) and previously published studies demonstrate that our screening platform is a useful tool for CSC inhibitor and CSC-related gene screening.

Identification of N-phenyl-3-methoxy-4-pyridinones as orally bioavailable H3 receptor antagonists and β-amyloid aggregation inhibitors for the treatment of Alzheimer's disease

Based on our previous work, a series of N-phenyl-3-methoxy-4-pyridinone derivatives were designed as orally bioavailable dual functional agents for therapy of Alzheimer's disease, through introducing alkyloxy moiety into 4-pyridinone ring to avoid the possible phase II metabolism of 3-hydroxy-4-pyridinone in lead compound 3-hydroxy-2-methyl-1-(4-(3-(pyrrolidin-1-yl)propoxy)phenyl)-pyridin-4(1H)-one (4). In vitro studies indicated that most of these compounds exhibit excellent H₃ receptor antagonistic activities and potent self-induced Aβ_1-40/Aβ_1-42 aggregation inhibitory activities. In particular, 3-methoxy-1-(4-(3-(pyrrolidin-1-yl)propoxy)phenyl)-pyridin-4(1H)-one (7i) demonstrated IC₅₀ value of 0.52 nM in H₃R antagonism and good selectivity over other histamine receptor subtypes. The transmission electron microscopy (TEM) images showed that compound 7i can inhibit self-mediated Aβ_1-40/Aβ_1-42 aggregation efficiently. As expected, it exhibited desirable pharmacokinetic properties in plasma and good BBB permeability. Furthermore, compound 7i can efficiently block (R)-α-methylhistamine- induced dipsogenia and reverse scopolamine-induced learning deficits of rats. All above results indicated that compound 7i was a promising orally bioavailable dual functional agents with potential use in the treatment of Alzheimer's disease.

The Dual-Active Histamine H3 Receptor Antagonist and Acetylcholine Esterase Inhibitor E100 Alleviates Autistic-Like Behaviors and Oxidative Stress in Valproic Acid Induced Autism in Mice

The histamine H3 receptor (H3R) functions as auto- and hetero-receptors, regulating the release of brain histamine (HA) and acetylcholine (ACh), respectively. The enzyme acetylcholine esterase (AChE) is involved in the metabolism of brain ACh. Both brain HA and ACh are implicated in several cognitive disorders like Alzheimer’s disease, schizophrenia, anxiety, and narcolepsy, all of which are comorbid with autistic spectrum disorder (ASD). Therefore, the novel dual-active ligand E100 with high H3R antagonist affinity (hH3R: K_i = 203 nM) and balanced AChE inhibitory effect (EeAChE: IC₅₀ = 2 µM and EqBuChE: IC₅₀ = 2 µM) was investigated on autistic-like sociability, repetitive/compulsive behaviour, anxiety, and oxidative stress in male C57BL/6 mice model of ASD induced by prenatal exposure to valproic acid (VPA, 500 mg/kg, intraperitoneal (i.p.)). Subchronic systemic administration with E100 (5, 10, and 15 mg/kg, i.p.) significantly and dose-dependently attenuated sociability deficits of autistic (VPA) mice in three-chamber behaviour (TCB) test (all p < 0.05). Moreover, E100 significantly improved repetitive and compulsive behaviors by reducing the increased percentage of marbles buried in marble-burying behaviour (MBB) (all p < 0.05). Furthermore, pre-treatment with E100 (10 and 15 mg/kg, i.p.) corrected decreased anxiety levels (p < 0.05), however, failed to restore hyperactivity observed in elevated plus maze (EPM) test. In addition, E100 (10 mg/kg, i.p.) mitigated oxidative stress status by increasing the levels of decreased glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT), and decreasing the elevated levels of malondialdehyde (MDA) in the cerebellar tissues (all p < 0.05). Additionally, E100 (10 mg/kg, i.p.) significantly reduced the elevated levels of AChE activity in VPA mice (p < 0.05). These results demonstrate the promising effects of E100 on in-vivo VPA-induced ASD-like features in mice, and provide evidence that a potent dual-active H3R antagonist and AChE inhibitor (AChEI) is a potential drug candidate for future therapeutic management of autistic-like behaviours.

The histamine H3R antagonist DL77 attenuates autistic behaviors in a prenatal valproic acid-induced mouse model of autism

Autistic spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairment in social communication and restricted/repetitive behavior patterns or interests. Antagonists targeting histamine H3 receptor (H3R) are considered potential therapeutic agents for the therapeutic management of different brain disorders, e.g., cognitive impairments. Therefore, the effects of subchronic treatment with the potent and selective H3R antagonist DL77 (5, 10, or 15 mg/kg, i.p.) on sociability, social novelty, anxiety, and aggressive/repetitive behavior in male Tuck-Ordinary (TO) mice with ASD-like behaviors induced by prenatal exposure to valproic acid (VPA, 500 mg/kg, i.p.) were evaluated using the three-chamber test (TCT), marble burying test (MBT), nestlet shredding test (NST), and elevated plus maze (EPM) test. The results showed that VPA-exposed mice exhibited significantly lower sociability and social novelty preference compared to VPA-exposed mice that were pretreated with DL77 (10 or 15 mg/kg, i.p.). VPA-exposed mice presented a significantly higher percentage of buried marbles in MBT and shredded nestlet significantly more in NST compared to the control groups. However, VPA-exposed animals pretreated with DL77 (10 or 15 mg/kg, i.p.) buried a reduced percentage of marbles in MBT and presented a significantly lower percentage of shredding behavior in NST. On the other hand, pretreatment with DL77 (5, 10, or 15 mg/kg, i.p.) failed to restore the disturbed anxiety levels and hyperactivity observed in VPA-exposed animals in EPM, whereas the reference drug donepezil (DOZ, 1 mg/kg, i.p.) significantly palliated the anxiety and reduced the hyperactivity measures of VPA-exposed mice. Furthermore, pretreatment with DL77 (10 or 15 mg/kg, i.p.) modulated oxidative stress status by increasing GSH and decreasing MDA, and it attenuated the proinflammatory cytokines IL-1β, IL-6 and TNF-α exacerbated by lipopolysaccharide (LPS) challenge, in VPA-exposed mouse brain tissue. Taken together, these results provide evidence that modulation of brain histaminergic neurotransmission, such as by subchronic administration of the H3R antagonist DL77, may serve as an effective pharmacological therapeutic target to rescue ASD-like behaviors in VPA-exposed animals, although further investigations are necessary to corroborate and expand these initial data.

Thirst sensation and oral dryness following alcohol intake

Substantial acute and chronic intakes of alcohol or ethanol (EtOH) severely influence oral sensations, such as thirst and oral dryness (dry mouth, xerostomia). Thirst sensation and oral dryness are primarily caused by the activation of neurons in brain regions, including the circumventricular organs and hypothalamus, which are referred to as the dipsogenic center, and by a decrease in salivary secretion, respectively. The sensation of thirst experienced after heavy-alcohol drinking is widely regarded as a consequence of EtOH-induced diuresis; however, EtOH in high doses induces anti-diuresis. Recently, it has been proposed that the ethanol metabolite acetaldehyde induces thirst via two distinct processes in the central nervous system from EtOH-induced diuresis, based on the results of animal experiments. The present review describes new insights regarding the induction mechanism of thirst sensation and oral dryness after drinking alcohol.

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.

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.

Cherry-picked ligands at histamine receptor subtypes

Histamine, a biogenic amine, is considered as a principle mediator of multiple physiological effects through binding to its H1, H2, H3, and H4 receptors (H1-H4Rs). Currently, the HRs have gained attention as important targets for the treatment of several diseases and disorders ranging from allergy to Alzheimer's disease and immune deficiency. Accordingly, medicinal chemistry studies exploring histamine-like molecules and their physicochemical properties by binding and interacting with the four HRs has led to the development of a diversity of agonists and antagonists that display selectivity for each HR subtype. An overview on H1-R4Rs and developed ligands representing some key steps in development is provided here combined with a short description of structure-activity relationships for each class. Main chemical diversities, pharmacophores, and pharmacological profiles of most innovative H1-H4R agonists and antagonists are highlighted. Therefore, this overview should support the rational choice for the optimal ligand selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies. This article is part of the Special Issue entitled 'Histamine Receptors'.

The novel non-imidazole histamine H3 receptor antagonist DL77 reduces voluntary alcohol intake and ethanol-induced conditioned place preference in mice

It has become clear that histamine H3 receptors (H3R) have been implicated in modulating ethanol intake and preference in laboratory animals. The novel non-imidazole H3R antagonist DL77 with excellent selectivity profile shows high in-vivo potency as well as in-vitro antagonist affinity with ED50 of 2.1 ± 0.2 mg/kg and pKi=8.08, respectively. In the present study, and applying an unlimited access two-bottle choice procedure, the anti-alcohol effects of the H3R antagonist, DL77 (0, 3, 10 and 30 mg/kg; i.p.), were investigated in adult mice. In this C57BL/6 line, effects of DL77 on voluntary alcohol intake and preference, as well as on total fluid intake were evaluated. Results have shown that DL77, dose-dependently, reduced both ethanol intake and preference. These effects were very selective as both saccharin and quinine, used to control for taste sensitivity, and intakes were not affected following DL77 pre-application. More importantly, systemic administration of DL77 (10 mg/kg) during acquisition inhibited ethanol-induced conditioned-place preference (EtOH-CPP) as measured using an unbiased protocol. The anti-alcohol activity observed for DL77 was abrogated when mice were pretreated with the selective H3R agonist R-(α)-methyl-histamine (RAMH) (10 mg/kg), or with the CNS penetrant H1R antagonist pyrilamine (PYR) (10mg/kg). These results suggest that DL77 has a predominant role in two in vivo effects of ethanol. Therefore, signaling via H3R is essential for ethanol-related consumption and conditioned reward and may represent a novel therapeutic pharmacological target to tackle ethanol abuse and alcoholism.

Proxyfan acts as a neutral antagonist of histamine H3 receptors in the feeding-related hypothalamic ventromedial nucleus

BACKGROUND AND PURPOSE

Centrally acting histamine H(3) receptor ligands are proposed as potential treatments for obesity, although the value of inverse agonists at these receptors is still debated. Functional inhibition of H(3) autoreceptors activates neurones in a hypothalamic 'satiety' centre. The H(3) receptor antagonist, proxyfan was used as a tool to assess the action of histaminergic compounds in this model.

EXPERIMENTAL APPROACH

We compared the actions of histamine on feeding with those of an H(3) receptor agonist (imetit) and inverse agonist (thioperamide) in rats and mice. Sites of action were identified by immunohistochemistry and the hypothalamic ventromedial nucleus (VMN) was investigated using electrophysiological techniques.

KEY RESULTS

Central histamine or thioperamide decreased fast-induced feeding, whereas imetit increased feeding. Systemic thioperamide entered the brain to activate hypothalamic feeding centres and to reduce feeding without causing any adverse behaviours. Thioperamide activated neurones in the VMN through an action on histamine autoreceptors, whilst imetit had the opposite effect. Proxyfan administered alone did not affect either feeding or electrical activity. However, it blocked the actions of both thioperamide and imetit, acting as a neutral antagonist in this system.

CONCLUSIONS AND IMPLICATIONS

The H(3) receptor inverse agonist, thioperamide, potently reduced appetite without adverse behavioural effects. This action was blocked by proxyfan, acting as a neutral antagonist in this model and, therefore, this compound is useful in determining the selectivity of H(3) receptor-directed drugs. A major action of thioperamide is through presynaptic autoreceptors, inducing stimulation by endogenous histamine of postsynaptic H(1 ) receptors on anorectic hypothalamic neurones.

CEP-26401 (irdabisant), a potent and selective histamine H₃ receptor antagonist/inverse agonist with cognition-enhancing and wake-promoting activities

CEP-26401 [irdabisant; 6-{4-[3-((R)-2-methyl-pyrrolidin-1-yl)-propoxy]-phenyl}-2H-pyridazin-3-one HCl] is a novel, potent histamine H₃ receptor (H₃R) antagonist/inverse agonist with drug-like properties. High affinity of CEP-26401 for H₃R was demonstrated in radioligand binding displacement assays in rat brain membranes (K(i) = 2.7 ± 0.3 nM) and recombinant rat and human H₃R-expressing systems (K(i) = 7.2 ± 0.4 and 2.0 ± 1.0 nM, respectively). CEP-26401 displayed potent antagonist and inverse agonist activities in [³⁵S]guanosine 5'-O-(γ-thio)triphosphate binding assays. After oral dosing of CEP-26401, occupancy of H₃R was estimated by the inhibition of ex vivo binding in rat cortical slices (OCC₅₀ = 0.1 ± 0.003 mg/kg), and antagonism of the H₃R agonist R-α-methylhistamine- induced drinking response in the rat dipsogenia model was demonstrated in a similar dose range (ED₅₀ = 0.06 mg/kg). CEP-26401 improved performance in the rat social recognition model of short-term memory at doses of 0.01 to 0.1 mg/kg p.o. and was wake-promoting at 3 to 30 mg/kg p.o. In DBA/2NCrl mice, CEP-26401 at 10 and 30 mg/kg i.p. increased prepulse inhibition (PPI), whereas the antipsychotic risperidone was effective at 0.3 and 1 mg/kg i.p. Coadministration of CEP-26401 and risperidone at subefficacious doses (3 and 0.1 mg/kg i.p., respectively) increased PPI. These results demonstrate potent behavioral effects of CEP-26401 in rodent models and suggest that this novel H₃R antagonist may have therapeutic utility in the treatment of cognitive and attentional disorders. CEP-26401 may also have therapeutic utility in treating schizophrenia or as adjunctive therapy to approved antipsychotics.

Discovery of two clinical histamine H(3) receptor antagonists: trans-N-ethyl-3-fluoro-3-[3-fluoro-4-(pyrrolidinylmethyl)phenyl]cyclobutanecarboxamide (PF-03654746) and trans-3-fluoro-3-[3-fluoro-4-(pyrrolidin-1-ylmethyl)phenyl]-N-(2-methylpropyl)cyclobutanecarboxamide (PF-03654764)

The discovery of two histamine H(3) antagonist clinical candidates is disclosed. The pathway to identification of the two clinical candidates, 6 (PF-03654746) and 7 (PF-03654764) required five hypothesis driven design cycles. The key to success in identifying these clinical candidates was the development of a compound design strategy that leveraged medicinal chemistry knowledge and traditional assays in conjunction with computational and in vitro safety tools. Overall, clinical compounds 6 and 7 exceeded conservative safety margins and possessed optimal pharmacological and pharmacokinetic profiles, thus achieving our initial goal of identifying compounds with fully aligned oral drug attributes, "best-in-class" molecules.

Antidiabetic properties of the histamine H3 receptor protean agonist proxyfan

Proxyfan is a histamine H3 receptor protean agonist that can produce a spectrum of pharmacological effects including agonist, inverse agonist, and antagonist. We have discovered that proxyfan (10 mg/kg orally) significantly improved glucose excursion after an ip glucose tolerance test in either lean or high-fat/cholesterol diet-induced obese mice. It also reduced plasma glucose levels comparable to that of metformin (300 mg/kg orally) in a nongenetic type 2 diabetes mouse model. The dose-dependent decrease in glucose excursion correlated with inhibition of ex vivo H3 receptor binding in the cerebral cortex. In addition, glucose levels were significantly reduced compared with vehicle-treated mice after intracerebroventricular administration of proxyfan, suggesting the involvement of central H3 receptors. Proxyfan-induced reduction of glucose excursion was not observed in the H3 receptor knockout mice, suggesting that proxyfan mediates this effect through H3 receptors. Proxyfan reduced glucose excursion by significantly increasing plasma insulin levels in a glucose-independent manner. However, no difference in insulin sensitivity was observed in proxyfan-treated mice. The H1 receptor antagonist chlorpheniramine and the H2 receptor antagonist zolantidine had modest effects on glucose excursion, and neither inhibited the glucose excursion reduced by proxyfan. The H3 receptor antagonist/inverse agonist, thioperamide, had weaker effects on glucose excursion compared with proxyfan, whereas the H3 receptor agonist imetit did not affect glucose excursion. In conclusion, these findings demonstrate, for the first time, that manipulation of central histamine H3 receptor by proxyfan can significantly improve glucose excursion by increasing plasma insulin levels via a glucose-independent mechanism.

CXCR2 inverse agonism detected by arrestin redistribution

To study CXCR2 modulated arrestin redistribution, the authors employed arrestin as a fusion protein containing either the Aequorea victoria-derived enhanced green fluorescent protein (EGFP) or a recently developed mutant of eqFP611, a red fluorescent protein derived from Entacmaea quadricolor. This mutant, referred to as RFP611, had earlier been found to assume a dimeric quarternary structure. It was therefore employed in this work as a "tandem" (td) construct for pseudo-monomeric fusion protein labeling. Both arrestin fusion proteins, containing either td-RFP611 (Arr-td-RFP611) or enhanced green fluorescent protein (EGFP; Arr-EGFP), were found to colocalize with internalized fluorescently labeled Gro-alpha a few minutes after Gro-alpha addition. Intriguingly, however, Arr-td-RFP611 and Arr-EGFP displayed distinct cellular distribution patterns in the absence of any CXCR2-activating ligand. Under these conditions, Arr-td-RFP611 showed a largely homogeneous cytosolic distribution, whereas Arr-EGFP segregated, to a large degree, into granular spots. These observations indicate a higher sensitivity of Arr EGFP to the constitutive activity of CXCR2 and, accordingly, an increased arrestin redistribution to coated pits and endocytic vesicles. In support of this interpretation, the authors found the known CXCR2 antagonist Sch527123 to act as an inverse agonist with respect to Arr-EGFP redistribution. The inverse agonistic properties of Sch527123 were confirmed in vitro in a guanine nucleotide binding assay, revealing an IC(50) value similar to that observed for Arr-EGFP redistribution. Thus, the redistribution assay, when based on Arr-EGFP, enables the profiling of antagonistic test compounds with respect to inverse agonism. When based on Arr-td-RFP611, the assay may be employed to study CXCR2 agonism or neutral antagonism.

Use of the H3 receptor antagonist radioligand [3H]-A-349821 to reveal in vivo receptor occupancy of cognition enhancing H3 receptor antagonists

BACKGROUND AND PURPOSE

The histamine H3 receptor antagonist radioligand [3H]-A-349821 was characterized as a radiotracer for assessing in vivo receptor occupancy by H3 receptor antagonists that affect behaviour. This model was established as an alternative to ex vivo binding methods, for relating antagonist H3 receptor occupancy to blood levels and efficacy in preclinical models.

EXPERIMENTAL APPROACH

In vivo cerebral cortical H3 receptor occupancy by [3H]-A-349821 was determined in rats from differences in [3H]-A-349821 levels in the isolated cortex and cerebellum, a brain region with low levels of H3 receptors. Comparisons were made to relate antagonist H3 receptor occupancy to blood levels and efficacy in a preclinical model of cognition, the five-trial inhibitory avoidance response in rat pups.

KEY RESULTS

In adult rats, [3H]-A-349821, 1.5 microg x kg(-1), penetrated into the brain and cleared more rapidly from cerebellum than cortex; optimally, [3H]-A-349821 levels were twofold higher in the latter. With increasing [3H]-A-349821 doses, cortical H3 receptor occupancy was saturable with a binding capacity consistent with in vitro binding in cortex membranes. In studies using tracer [3H]-A-349821 doses, ABT-239 and other H3 receptor antagonists inhibited H3 receptor occupancy by [3H]-A-349821 in a dose-dependent manner. Blood levels of the antagonists corresponding to H3 receptor occupancy were consistent with blood levels associated with efficacy in the five-trial inhibitory avoidance response.

CONCLUSIONS AND IMPLICATIONS

When employed as an occupancy radiotracer, [3H]-A-349821 provided valid measurements of in vivo H3 receptor occupancy, which may be helpful in guiding and interpreting clinical studies of H3 receptor antagonists.

Pharmacological characterization of JNJ-28583867, a histamine H3 receptor antagonist and serotonin reuptake inhibitor

Wake-promoting agents such as modafinil are used in the clinic as adjuncts to antidepressant therapy in order to alleviate lethargy. The wake-promoting action of histamine H(3) receptor antagonists has been evidenced in numerous animal studies. They may therefore be a viable strategy for use as an antidepressant therapy in conjunction with selective serotonin reuptake inhibitors. JNJ-28583867 (2-Methyl-4-(4-methylsulfanyl-phenyl)-7-(3-morpholin-4-yl-propoxy)-1,2,3,4-tetrahydro-isoquinoline) is a selective and potent histamine H(3) receptor antagonist (K(i)=10.6 nM) and inhibitor of the serotonin transporter (SERT) (K(i)=3.7 nM), with 30-fold selectivity for SERT over the dopamine and norepinephrine transporters. After subcutaneous administration, JNJ-28583867 occupied both the histamine H(3) receptor and the SERT in rat brain at low doses (<1 mg/kg). JNJ-28583867 blocked imetit-induced drinking (3-10 mg/kg i.p.), confirming in vivo functional activity at the histamine H(3) receptor and also significantly increased cortical extracellular levels of serotonin at doses of 0.3 mg/kg (s.c.) and higher. Smaller increases in cortical extracellular levels of norepinephrine and dopamine were also observed. JNJ-28583867 (3-30 mg/kg p.o.) showed antidepressant-like activity in the mouse tail suspension test. JNJ-28583867 (1-3 mg/kg s.c.) caused a dose-dependent increase in the time spent awake mirrored by a decrease in NREM. Concomitantly, JNJ-28583867 produced a potent suppression of REM sleep from the dose of 1 mg/kg onwards. JNJ-28583867 has good oral bioavailability in the rat (32%), a half-life of 6.9 h and a C(max) of 260 ng/ml after 10 mg/kg p.o. In summary, JNJ-28583867 is a combined histamine H(3) receptor antagonist-SERT inhibitor with in vivo efficacy in biochemical and behavioral models of depression and wakefulness.

Constitutive activity of the histamine H3 receptor

Constitutive activity has been mainly recorded for numerous overexpressed and/or mutated receptors. The histamine H(3) receptor (H(3)R) is a target of choice to study the physiological relevance of this process. In rodent brain, postsynaptic H(3)Rs show high constitutive activity, and presynaptic H(3) autoreceptors that show constitutive activity have a predominant role in inhibiting the activity of histamine neurons. H(3)R inverse agonists abrogate this constitutive brake and enhance histamine release in vivo. Some of these inverse agonists have entered clinical trials for the treatment of cognitive and food intake disorders. Studies performed in vitro and in vivo with proxyfan show that this H(3)R ligand is a 'protean agonist' - that is, a ligand with a spectrum of activity ranging from full agonism to full inverse agonism depending on the level of H(3)R constitutive activity. Consistent with its physiological and therapeutic relevance, the constitutive activity of H(3)R thus has a major function in the brain and regulates the activity of H(3)R-targeted drugs.

Structurally novel histamine H3 receptor antagonists GSK207040 and GSK334429 improve scopolamine-induced memory impairment and capsaicin-induced secondary allodynia in rats

GSK207040 (5-[(3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy]-N-methyl-2-pyrazinecarboxamide) and GSK334429 (1-(1-methylethyl)-4-({1-[6-(trifluoromethyl)-3-pyridinyl]-4-piperidinyl}carbonyl)hexahydro-1H-1,4-diazepine) are novel and selective non-imidazole histamine H(3) receptor antagonists from distinct chemical series with high affinity for human (pK(i)=9.67+/-0.06 and 9.49+/-0.09, respectively) and rat (pK(i)=9.08+/-0.16 and 9.12+/-0.14, respectively) H(3) receptors expressed in cerebral cortex. At the human recombinant H(3) receptor, GSK207040 and GSK334429 were potent functional antagonists (pA(2)=9.26+/-0.04 and 8.84+/-0.04, respectively versus H(3) agonist-induced changes in cAMP) and exhibited inverse agonist properties (pIC(50)=9.20+/-0.36 and 8.59+/-0.04 versus basal GTPgammaS binding). Following oral administration, GSK207040 and GSK334429 potently inhibited cortical ex vivo [(3)H]-R-alpha-methylhistamine binding (ED(50)=0.03 and 0.35 mg/kg, respectively). Functional antagonism of central H(3) receptors was demonstrated by blockade of R-alpha-methylhistamine-induced dipsogenia in rats (ID(50)=0.02 and 0.11 mg/kg p.o. for GSK207040 and GSK334429, respectively). In more pathophysiologically relevant pharmacodynamic models, GSK207040 (0.1, 0.3, 1 and 3mg/kg p.o.) and GSK334429 (0.3, 1 and 3mg/kg p.o.) significantly reversed amnesia induced by the cholinergic antagonist scopolamine in a passive avoidance paradigm. In addition, GSK207040 (0.1, 0.3 and 1mg/kg p.o.) and GSK334429 (3 and 10mg/kg p.o.) significantly reversed capsaicin-induced reductions in paw withdrawal threshold, suggesting for the first time that blockade of H(3) receptors may be able to reduce tactile allodynia. Novel H(3) receptor antagonists such as GSK207040 and GSK334429 may therefore have therapeutic potential not only in dementia but also in neuropathic pain.

Inverse agonism or neutral antagonism at G-protein coupled receptors: A medicinal chemistry challenge worth pursuing?

The identification of constitutive, or intrinsic, activity of G-protein coupled receptors has had major impact on receptor theory, the identification of agents that inhibit this ligand-independent receptor activity has led, in turn, to the concept of inverse agonism. It has subsequently emerged that the majority, around 85%, of all known G-protein coupled receptor antagonists are, in fact, inverse agonists. Agents that affect only ligand-dependent receptor activation, i.e. have no effect on constitutive receptor signalling, are termed neutral antagonists and turn out to be relatively rare in pharmacology. Is this relevant for medicinal chemistry? That question is difficult to answer with certainty because there has been little or no effort to understand the structure activity relationships of neutral antagonist vs. inverse agonist molecules. In this review, we suggest that these pharmacological differences may well be translated to differential effects in the whole animal and in medicine. We argue that having either option to inhibit a particular receptor may reveal differences in efficacy and tolerability thus increasing the potential value of a G-protein coupled receptor inhibitor programme. However, since inverse agonists appear to constitute a default inhibitor mode, a systematic survey of the structure activity relationships around what makes a neutral antagonist will be an essential first step towards this goal.

Evidence for tolerance following repeated dosing in rats with ciproxifan, but not with A-304121

Blockade of presynaptic histamine H(3) receptors with potent and selective ligands improves cognitive function in rodents and there is significant interest in developing such drugs for long-term symptomatic treatment of CNS disorders such as attention deficit hyperactivity disorder (ADHD). Unfortunately, little is known about the effects of repeated exposure to H(3) receptor antagonists/inverse agonists. We therefore investigated the effects of acute and repeated daily administration of two potent, brain penetrating H(3) receptor antagonists/inverse agonists, ciproxifan and A-304121, on rat body weight, food and water intake, core temperature and locomotor activity, as well as H(3) receptor density and gene expression levels. Methylphenidate, used clinically for the treatment of ADHD, was included as an additional comparator. Ciproxifan, an imidazole-based compound, decreased food intake over the first 10 days and locomotor activity acutely, but these effects were lost after further repeated administration. The ex vivo binding studies revealed increased H(3) receptor density in rats following repeated administration of ciproxifan for 10 or 15 days; however, H(3) receptor gene expression was not changed. In contrast, rats treated with the non-imidazole, A-304121, did not differ from controls on any measure during the observation period, while rats treated with methylphenidate exhibited hyperthermia and hyperactivity. The implications for potential long-term treatment with H(3) receptor antagonists in CNS disorders such as ADHD are discussed.

Detailed pharmacological characterization of GT-2331 for the rat histamine H3 receptor

Histamine H(3) receptor antagonists are potential therapeutic agents for cognitive dysfunction, epilepsy, hypersomnia and obesity. GT-2331 (4-[(R,R)-2-(5,5-dimethyl-1-hexynyl)cyclopropyl]-1H-imidazole) was originally identified as a potent histamine H(3) receptor antagonist. However, recent reports demonstrated a complex pharmacology for GT-2331. To further understand the pharmacological profile of GT-2331, we characterized GT-2331 using various in vitro and in vivo assays. In vitro, GT-2331 behaved as a full agonist on adenylyl cyclase inhibition and as a partial agonist on [(35)S]GTPgammaS binding at the recombinant rat histamine H(3) receptor. In contrast, in vivo, GT-2331 had no effect on brain histamine turnover while the histamine H(3) receptor agonist R-alpha-methylhistamine significantly decreased histamine turnover. Furthermore, GT-2331 completely blocked R-alpha-methylhistamine-induced water intake, suggesting that GT-2331 behaves as a full antagonist. Thus, GT-2331 displayed the spectrum of pharmacological activities from full agonism to full antagonism, these observations suggest that histamine H(3) receptor ligands need to be carefully evaluated in various paradigms.

The H3 receptor protean agonist proxyfan enhances the expression of fear memory in the rat

Consolidation of fear memory requires neural changes to occur in the basolateral amygdala (BLA), including modulation of histaminergic neurotransmission. We previously demonstrated that local blockade or activation of histamine H3 receptors in the BLA impaired or ameliorated, respectively, retention of fear memory. The histamine H3 receptor is a G-protein-coupled receptor (GPCR) displaying high constitutive activity that regulates histamine neurons in the brain. Proxyfan is a high-affinity histamine H3 receptor protean agonist exhibiting the full spectrum of pharmacological activities, from full agonist to full inverse agonist depending on the competition between constitutively active and quiescent H3 receptors in a given tissue or brain region. Therefore, protean agonists are powerful tools to investigate receptor conformation and may be useful in designing specific compounds selective for the various receptor conformations. In the present study we examined the effect of post-training, systemic or intra-BLA injections of proxyfan on contextual fear memory. Rats receiving intra-BLA, bilateral injections of 1.66 ng proxyfan immediately after fear conditioning showed stronger memory for the context-footshock association, as demonstrated by longer freezing assessed at retention performed 72 hr later compared to controls. Comparable results were obtained when doses as low as 0.04 mg/kg of proxyfan were injected systemically. Hence, our results suggest that proxyfan behaves as an H3 receptor agonist with a low level of constitutive activity of the H3 receptor in the rat BLA.

Pharmacological Properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile]: II. Neurophysiological Characterization and Broad Preclinical Efficacy in Cognition and Schizophrenia of a Potent and Selective Histamine H3 Receptor Antagonist

Acute pharmacological blockade of central histamine H3 receptors (H3Rs) enhances arousal/attention in rodents. However, there is little information available for other behavioral domains or for repeated administration using selective compounds. ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] exemplifies such a selective, nonimidazole H3R antagonist with high affinity for rat (pK(i) = 8.9) and human (pK(i) = 9.5) H3Rs. Acute functional blockade of central H3Rs was demonstrated by blocking the dipsogenia response to the selective H3R agonist (R)-alpha-methylhistamine in mice. In cognition studies, acquisition of a five-trial, inhibitory avoidance test in rat pups was improved with ABT-239 (0.1-1.0 mg/kg), a 10- to 150-fold gain in potency, with similar efficacy, over previous antagonists such as thioperamide, ciproxifan, A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)(cyclopropyl) methanone], A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl) phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxoethyl)-2-furamide], and A-349821 [(4'-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone]. Efficacy in this model was maintained for 3 to 6 h and following repeated dosing with ABT-239. Social memory was also improved in adult (0.01-0.3 mg/kg) and aged (0.3-1.0 mg/kg) rats. In schizophrenia models, ABT-239 improved gating deficits in DBA/2 mice using prepulse inhibition of startle (1.0-3.0 mg/kg) and N40 (1.0-10.0 mg/kg). Furthermore, ABT-239 (1.0 mg/kg) attenuated methamphetamine-induced hyperactivity in mice. In freely moving rat microdialysis studies, ABT-239 enhanced acetylcholine release (0.1-3.0 mg/kg) in adult rat frontal cortex and hippocampus and enhanced dopamine release in frontal cortex (3.0 mg/kg), but not striatum. In summary, broad efficacy was observed with ABT-239 across animal models such that potential clinical efficacy may extend beyond disorders such as ADHD to include Alzheimer's disease and schizophrenia.

Antiobesity effects of A-331440, a novel non-imidazole histamine H3 receptor antagonist

Histamine affects homeostatic mechanisms, including food and water consumption, by acting on central nervous system (CNS) receptors. Presynaptic histamine H(3) receptors regulate release of histamine and other neurotransmitters, and histamine H(3) receptor antagonists enhance neurotransmitter release. A-331440 [4'-[3-(3(R)-(dimethylamino)-pyrrolidin-1-yl)-propoxy]-biphenyl-4-carbonitrile] is a histamine H(3) receptor antagonist which binds potently and selectively to both human and rat histamine H(3) receptors (K(i)<==25 nM). Mice were stabilized on a high-fat diet (45 kcal % lard) prior to 28-day oral b.i.d. dosing for measurement of obesity-related parameters. A-331440 administered at 0.5 mg/kg had no significant effect on weight, whereas 5 mg/kg decreased weight comparably to dexfenfluramine (10 mg/kg). A-331440 administered at 15 mg/kg reduced weight to a level comparable to mice on the low-fat diet. The two higher doses reduced body fat and the highest dose also normalized an insulin tolerance test. These data show that the histamine H(3) receptor antagonist, A-331440, has potential as an antiobesity agent.

Pharmacological and behavioral properties of A-349821, a selective and potent human histamine H3 receptor antagonist

Histamine H3 receptors regulate the release of a variety of central neurotransmitters involved in cognitive processes. A-349821 ((4'-(3-((R,R)2,5-dimethyl-pyrrolidin-1-yl)-propoxy)-biphenyl-4-yl)-morpholin-4-yl-methanone) is a novel, non-imidazole H3 receptor ligand, displaying high affinity for recombinant rat and human H3 receptors, with pKi values of 9.4 and 8.8, respectively, and high selectivity for the H3 receptor versus H1, H2, and H4 histamine receptors. A-349821 is a potent H3 receptor antagonist in a variety of models using recombinant human and rat receptors, reversing agonist induced changes in cyclic AMP formation (pKb= 8.2 and pKb= 8.1, respectively), [35S]-GTPgammaS binding (pKb= 9.3 and pKb= 8.6, respectively) and calcium levels (human pKb= 8.3). In native systems, A-349821 competitively reversed agonist induced inhibition of electric field stimulated guinea-pig ileum (pA2= 9.5) and histamine-mediated inhibition of [3H]-histamine release from rat brain cortical synaptosomes (pKb= 9.2). Additionally, A-349821 inhibited constitutive GTPgammaS binding at both rat and human H3 receptors with respective pEC50 values of 9.1 and 8.6, demonstrating potent inverse agonist properties. In behavioral studies, A-349821 (0.4 mg/kg-4 mg/kg) potently blocked (R)-alpha-methylhistamine-induced dipsogenia in mice. The compound also enhanced cognitive activity in a five-trial inhibitory avoidance model in spontaneously hypertensive rat (SHR) pups at doses of 1-10mg/kg, with the 1mg/kg dose showing comparable efficacy to a fully efficacious dose of ciproxifan (3mg/kg). These doses of A-349821 were without effect on spontaneous locomotor activity. Thus, A-349821 is a novel, selective non-imidazole H3 antagonist/inverse agonist with balanced high potency across species and favorable cognition enhancing effects in rats.

Selective histamine H3 receptor antagonists for treatment of cognitive deficiencies and other disorders of the central nervous system

Evidence exists to implicate the monoamine histamine in the control of arousal and cognitive functions. Antagonists of H(3) receptors are postsynaptic and presynaptic modulators of neural transmission in a variety of neuronal circuits relevant to cognition. Accumulating neuroanatomical, neurochemical, pharmacological, and behavioral data support the idea that H(3) receptor antagonists may function to improve cognitive performances in disease states (e.g., Alzheimer's disease and mild cognitive impairment states). Thus, H(3) receptor antagonists have been shown to increase performance in attention and memory tests in nonhuman experiments and prevent the degradation in performances produced by scopolamine, MK-801, or age. In contrast, agonists of the H(3) receptor generally produce cognitive impairing effects in animal models. The role of H(3) receptors in these behavioral effects is substantiated by data indicating a central origin for their effects, the selectivity of some of the H(3) receptor antagonists studied, and the pharmacological modification of effects of H(3) receptor antagonists by selective H(3) receptor agonists. Data and issues that challenge the potential role for H(3) receptor antagonists in cognitive processes are also critically reviewed. H(3) receptor antagonists may also have therapeutic value in the management of obesity, pain, sleep disorders, schizophrenia, and attention deficit hyperactivity disorder.

Structure–activity relationships of non-imidazole H3 receptor ligands. Part 3: 5-Substituted 3-phenyl-1,2,4-oxadiazoles as potent antagonists

Further SAR studies on novel histamine H(3) receptor antagonists are presented. Compound 14bb is a potent antagonist of both the rat cortical and human clone receptors, and is demonstrated to act functionally as an antagonist in an in vivo mouse dipsogenia model.

An Efficient Multigram Synthesis of the Potent Histamine H3 Antagonist GT-2331 and the Reassessment of the Absolute Configuration

GT-2331 is a potent histamine H(3) antagonist which has entered clinical trials. Efficient multigram syntheses of this compound and its enantiomer are described. The literature reports that GT-2331 is the dextrorotatory (+), more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole with the absolute configuration of (1R,2R)-1. However, we found that the dextrorotatory, more potent, enantiomer of 4-[2-(5,5-dimethylhex-1-ynyl)cyclopropyl]-1H-imidazole has the (1S,2S) absolute configuration. We suggest a reconsideration of the absolute configuration of GT-2331.

Two novel and selective nonimidazole H3 receptor antagonists A-304121 and A-317920: II. In vivo behavioral and neurophysiological characterization

Pharmacological blockade of central histamine H3 receptors (H3Rs) enhances cognition in rodents and offers promise for the clinical treatment of neurological disorders. However, many previously characterized H3R antagonists are either not selective for H3Rs or have potentially significant tolerability issues. Here, we present in vivo behavioral and neurophysiological data for two novel and selective H3R antagonists with improved safety indices. Functional blockade of central H3Rs was first demonstrated for A-304121 [(4-(3-(4-((2R)-2-aminopropanoyl)-1-piperazinyl)propoxy)phenyl)cyclopropylmethanone] (1 mg/kg) and A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl)phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxo-ethyl)-2-furamide] (0.45 mg/kg) by significantly attenuating an acute dipsogenia response to the selective H3R agonist (R)-alpha-methylhistamine [(R)-alpha-MeHA]. Cognitive performance was improved in a five-trial rat pup avoidance test following administration of A-304121 (10 mg/kg) or A-317920 (3 mg/kg), with efficacy comparable with previously published observations for reference H3R antagonists thioperamide (10 mg/kg), ciproxifan (3 mg/kg), and GT-2331 [(1R,2R)-4-(2-(5,5-dimethylhex-1-ynyl)cyclopropyl)imidazole] (1 mg/kg). Social memory was also significantly enhanced in the adult rat with A-304121 (3, 10 mg/kg) and A-317920 (1, 3 mg/kg) at doses that produced no significant change in electroencephalogram slow-wave amplitude activity. Relative therapeutic indices (TIs) of 30 and 42 were estimated for A-304121 and A-317920, respectively, by comparing doses producing adverse effects in general observation studies with potency in inhibitory avoidance, which were superior to TIs of 8, 10, and 18 observed for the reference antagonists thioperamide, ciproxifan, and GT-2331, respectively. A-304121 and A-317920 represent a series of novel, H3R-selective piperazine amides that enhance cognition in vivo, which could offer advantages over existing H3R antagonists or cognition-enhancing agents.

Synthesis and SAR of aminoalkoxy-biaryl-4-carboxamides: novel and selective histamine H3 receptor antagonists

Novel 4'-[(NR1R2-1-yl)]-propoxy-biaryl-4-carboxamides were designed and synthesized. All compounds were tested for affinity at histamine H(3)receptors. Most compounds were highly potent and selective for human and rat H(3) receptors and selected examples such as A-349821 showed functional antagonism of H(3) receptors in vitro and in a mouse dipsogenia model.