Pharmacological characterization of the new histamine H4 receptor agonist VUF 8430

Background and purpose:

We compare the pharmacological profiles of a new histamine H₄ receptor agonist 2-(2-guanidinoethyl)isothiourea (VUF 8430) with that of a previously described H₄ receptor agonist, 4-methylhistamine.

Experimental approach:

Radioligand binding and functional assays were performed using histamine H₄ receptors expressed in mammalian cell lines. Compounds were also evaluated ex vivo in monocyte-derived dendritic cells endogenously expressing H₄ receptors and in vivo in anaesthetized rats for gastric acid secretion activity.

Key results:

Both VUF 8430 and 4-methylhistamine were full agonists at human H₄ receptors with lower affinity at rat and mouse H₄ receptors. Both compounds induced chemotaxis of monocyte-derived dendritic cells. VUF 8430 also showed reasonable affinity and was a full agonist at the H₃ receptor. Agmatine is a metabolite of arginine, structurally related to VUF 8430, and was a H₄ receptor agonist with micromolar affinity. At histamine H₃ receptors, agmatine was a full agonist, whereas 4-methylhistamine was an agonist only at high concentrations. Both VUF 8430 and agmatine were inactive at H₁ and H₂ receptors, whereas 4-methylhistamine is as active as histamine at H₂ receptors. In vivo, VUF 8430 only caused a weak secretion of gastric acid mediated by H₂ receptors, whereas 4-methylhistamine, dimaprit, histamine and amthamine, at equimolar doses, induced 2.5- to 6-fold higher output than VUF 8430.

Conclusions and implications:

Our results suggest complementary use of 4-methylhistamine and VUF 8430 as H₄ receptor agonists. Along with H₄ receptor antagonists, both agonists can serve as useful pharmacological tools in studies of histamine H₄ receptors.     

The role of histamine H4 receptor in immune and inflammatory disorders

Since its discovery at the beginning of the 20th century, histamine has been established to play a pathophysiological regulatory role in cellular events through binding to four types of G-protein-coupled histamine receptors that are differentially expressed in various cell types. The discovery, at the turn of the millennium, that the histamine H₄ receptor is largely expressed in haemopoietic cells as well as its chemotactic properties designate its regulatory role in the immune system. H₄ receptors modulate eosinophil migration and selective recruitment of mast cells leading to amplification of histamine-mediated immune responses and eventually to chronic inflammation. H₄ receptor involvement in dendritic cell activation and T cell differentiation documents its immunomodulatory function. The characterization of the H₄ as the immune system histamine receptor directed growing attention towards its therapeutic exploitation in inflammatory disorders, such as allergy, asthma, chronic pruritus and autoimmune diseases. The efficacy of a number of H₄ receptor ligands has been evaluated in in vivo and in vitro animal models of disease and in human biological samples. However, before reaching decisive conclusions on H₄ receptor pathophysiological functions and therapeutic exploitation, identification of genetic polymorphisms and interspecies differences in its relative actions and pharmacological profile need to be addressed and taken into consideration. Despite certain variations in the reported findings, the available data strongly point to the H₄ receptor as a novel target for the pharmacological modulation of histamine-transferred immune signals and offer an optimistic perspective for the therapeutic exploitation of this promising new drug target in inflammatory disorders.     

Pharmacological characterization of GSK1004723, a novel, long-acting antagonist at histamine H(1) and H(3) receptors

BACKGROUND AND PURPOSE

Preclinical pharmacological characterization of GSK1004723, a novel, dual histamine H₁ and H₃ receptor antagonist.

EXPERIMENTAL APPROACH

GSK1004723 was characterized in vitro and in vivo using methods that included radioligand binding, intracellular calcium mobilization, cAMP production, GTPγS binding, superfused human bronchus and guinea pig whole body plethysmography.

KEY RESULTS

In cell membranes over-expressing human recombinant H₁ and H₃ receptors, GSK1004723 displayed high affinity, competitive binding (H₁ pKi = 10.2; H₃ pKi = 10.6). In addition, GSK1004723 demonstrated slow dissociation from both receptors with a t_1/2 of 1.2 and 1.5 h for H₁ and H₃ respectively. GSK1004723 specifically antagonized H₁ receptor mediated increases in intracellular calcium and H₃ receptor mediated increases in GTPγS binding. The antagonism exerted was retained after cell washing, consistent with slow dissociation from H₁ and H₃ receptors. Duration of action was further evaluated using superfused human bronchus preparations. GSK1004723 (100 nmol·L⁻¹) reversed an established contractile response to histamine. When GSK1004723 was removed from the perfusate, only 20% recovery of the histamine response was observed over 10 h. Moreover, 21 h post-exposure to GSK1004723 there remained almost complete antagonism of responses to histamine. In vivo pharmacology was studied in conscious guinea pigs in which nasal congestion induced by intranasal histamine was measured indirectly (plethysmography). GSK1004723 (0.1 and 1 mg·mL⁻¹ intranasal) antagonized the histamine-induced response with a duration of up to 72 h.

CONCLUSIONS AND IMPLICATIONS

GSK1004723 is a potent and selective histamine H₁ and H₃ receptor antagonist with a long duration of action and represents a potential novel therapy for allergic rhinitis.     

Novel histamine H4 receptor ligands and their potential therapeutic applications: an update

Introduction: Histamine H₄ receptor (H₄R) has been shown to be involved in various inflammatory conditions. Ligands acting on H₄R show therapeutic potential in various diseases. For the first time, the positive proof-of-concept clinical trials of the H₄ antagonist JNJ39758979 have demonstrated this potential in histamine-induced pruritus. Besides role of H₄R in inflammatory conditions, preclinical results in cancer, neuropathic pain, vestibular disorders and type 2 diabetes show the diverse signaling network modulated by H₄R. This suggests further potential for H₄ ligands in such diseases.

Areas covered: In this review, we have summarized patent applications and papers of the H₄R field published between 2012 and 2014. Additionally, we have analyzed the quality of the already described H₄ ligands in terms of their ligand efficiency, lipophilic ligand efficiency and lipophilicity-corrected ligand efficiency.

Expert opinion: We demonstrate that the number of published patent applications reached a maximum in 2009 and showed some decrease in the last few years. On the other hand, the field is still very lively, reflected by the numerous publications on novel potential therapeutic applications. The favorable property profile of H₄ ligands in development shows promise for the upcoming human clinical trials.     

Design and pharmacological characterization of VUF14480, a covalent partial agonist that interacts with cysteine 983.36 of the human histamine H4 receptor

Background and Purpose

The recently proposed binding mode of 2-aminopyrimidines to the human (h) histamine H₄ receptor suggests that the 2-amino group of these ligands interacts with glutamic acid residue E182^5.46 in the transmembrane (TM) helix 5 of this receptor. Interestingly, substituents at the 2-position of this pyrimidine are also in close proximity to the cysteine residue C98^3.36 in TM3. We hypothesized that an ethenyl group at this position will form a covalent bond with C98^3.36 by functioning as a Michael acceptor. A covalent pyrimidine analogue will not only prove this proposed binding mode, but will also provide a valuable tool for H₄ receptor research.

Experimental Approach

We designed and synthesized VUF14480, and pharmacologically characterized this compound in hH₄ receptor radioligand binding, G protein activation and β-arrestin2 recruitment experiments. The ability of VUF14480 to act as a covalent binder was assessed both chemically and pharmacologically.

Key Results

VUF14480 was shown to be a partial agonist of hH₄ receptor-mediated G protein signalling and β-arrestin2 recruitment. VUF14480 bound covalently to the hH₄ receptor with submicromolar affinity. Serine substitution of C98^3.36 prevented this covalent interaction.

Conclusion and Implications

VUF14480 is thought to bind covalently to the hH₄ receptor-C98^3.36 residue and partially induce hH₄ receptor-mediated G protein activation and β-arrestin2 recruitment. Moreover, these observations confirm our previously proposed binding mode of 2-aminopyrimidines. VUF14480 will be a useful tool to stabilize the receptor into an active confirmation and further investigate the structure of the active hH₄ receptor.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

Biochemical characterization of desloratadine,a potent antagonist of the human histamine H(1) receptor

We have characterized desloratadine (5H-benzo[5,6]cyclohepta[1,2-b]pyridine, 8-chloro-6,11-dihydro-11-(4-piperidinylidene), CAS 100643-71-8) as a potent antagonist of the human histamine H(1) receptor. [3H]Desloratadine bound to membranes expressing the recombinant human histamine H(1) receptor in Chinese hamster ovary cells (CHO-H(1)) in a specific and saturable manner with a K(d) of 1.1+/-0.2 nM, a B(max) of 7.9+/-2.0 pmol/mg protein, and an association rate constant of 0.011 nM(-1) x min(-1). The K(d) calculated from the kinetic measurements was 1.5 nM. Dissociation of [3H]desloratadine from the human histamine H(1) receptor was slow, with only 37% of the binding reversed at 6 h in the presence of 5 microM unlabeled desloratadine. Seventeen histamine H(1)-receptor antagonists were evaluated in competition-binding studies. Desloratadine had a K(i) of 0.9+/-0.1 nM in these competition studies. In CHO-H(1) cells, histamine stimulation resulted in a concentration-dependent increase in [Ca(2+)](i) with an EC(50) of 170+/-30 nM. After a 90-min preincubation with desloratadine, the histamine-stimulated increase in [Ca(2+)](i) was shifted to the right, with a depression of the maximal response at higher concentrations of antagonist. The apparent K(b) value was 0.2+/-0.14 nM with a slope of 1.6+/-0.1. The slow dissociation from the receptor and noncompetitive antagonism suggests that desloratadine may be a pseudoirreversible antagonist of the human histamine H(1) receptor. The mechanism of desloratadine antagonism of the human histamine H(1) receptor may help to explain the high potency and 24-h duration of action observed in clinical studies.     

Profiling of histamine H4 receptor agonists in native human monocytes

Background and Purpose

Since the identification of the histamine H₄ receptor, several ligands activating this receptor have been described and more compounds are in development. These ligands are well characterized in pharmacological assays, including radioligand competition binding studies, GTPγS and GTPase assays. In most cases, these experiments are performed in transfected cell lines, expressing unnaturally high levels of target receptors and G-protein signalling components. In this study we investigated the specific properties of H₄ receptor ligands in native cells.

Experimental Approach

Histamine and five different H₄ receptor agonists – 4-methylhistamine, UR-PI376, clobenpropit, VUF8430 and ST-1006 – were characterized in freshly isolated human monocytes. The ligands (10 nM–10 μM) were tested as inhibitors of IL-12p70 secretion from human monocytes and the effects of the H₂ receptor antagonist ranitidine and the H₄ receptor antagonist JNJ7777120 on their action was investigated.

Key Results

Histamine and all the tested agonists reduced IL-12p70 secretion into monocyte supernatants by 40–70%. The potencies varied with pEC₅₀ values ranging from 5.7 to 6.9, depending on the agonist used. All potencies were lower than those determined in the original investigations of the compounds. Pretreatment of monocytes with H₂ or H₄ receptor antagonists showed that some H₄ receptor ligands also had low activity at the H₂ receptor.

Conclusions and Implications

Our study demonstrates discrepancies between the potencies obtained from assays in transfected cell lines and assays in native human cells, indicating the importance of evaluating H₄ receptor ligands in native cells.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

Molecular and biochemical pharmacology of the histamine H4 receptor

The elucidation of the human genome has had a major impact on histamine receptor research. The identification of the human H₄ receptor by several groups has been instrumental for a new appreciation of the role of histamine in the modulation of immune function. In this review, we summarize the historical developments and the molecular and biochemical pharmacology of the H₄ receptor.     

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 µg·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.     

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

The histamine H3 receptor, first described in 1983 as a histamine autoreceptor and later shown to also function as a heteroreceptor that regulates the release of other neurotransmitters, has been the focus of research by numerous laboratories as it represents an attractive drug target for a number of indications including cognition. The purpose of this review is to acquaint the reader with the current understanding of H3 receptor localization and function as a modulator of neurotransmitter release and its effects on cognitive processes, as well as to provide an update on selected H3 antagonists in various states of preclinical and clinical advancement. Blockade of centrally localized H3 receptors by selective H3 receptor antagonists has been shown to enhance the release of neurotransmitters such as histamine, ACh, dopamine and norepinephrine, among others, which play important roles in cognitive processes. The cognitive-enhancing effects of H3 antagonists across multiple cognitive domains in a wide number of preclinical cognition models also bolster confidence in this therapeutic approach for the treatment of attention deficit hyperactivity disorder, Alzheimer's disease and schizophrenia. However, although a number of clinical studies examining the efficacy of H3 receptor antagonists for a variety of cognitive disorders are currently underway, no clinical proof of concept for an H3 receptor antagonist has been reported to date. The discovery of effective H3 antagonists as therapeutic agents for the novel treatment of cognitive disorders will only be accomplished through continued research efforts that further our insights into the functions of the H3 receptor.     

The histamine H4 receptor is functionally expressed on neurons in the mammalian CNS

Background and purpose:

The histamine H₄ receptor is the most recently identified of the G protein-coupled histamine receptor family and binds several neuroactive drugs, including amitriptyline and clozapine. So far, H₄ receptors have been found only on haematopoietic cells, highlighting its importance in inflammatory conditions. Here we investigated the possibility that H₄ receptors may be expressed in both the human and mouse CNS.

Methods:

Immunological and pharmacological studies were performed using a novel anti-H₄ receptor antibody in both human and mouse brains, and electrophysiological techniques in the mouse brain respectively. Pharmacological tools, selective for the H₄ receptor and patch clamp electrophysiology, were utilized to confirm functional properties of the H₄ receptor in layer IV of the mouse somatosensory cortex.

Results:

Histamine H₄ receptors were prominently expressed in distinct deep laminae, particularly layer VI, in the human cortex, and mouse thalamus, hippocampal CA4 stratum lucidum and layer IV of the cerebral cortex. In layer IV of the mouse somatosensory cortex, the H₄ receptor agonist 4-methyl histamine (20 µmol·L⁻¹) directly hyperpolarized neurons, an effect that was blocked by the selective H₄ receptor antagonist JNJ 10191584, and promoted outwardly rectifying currents in these cells. Monosynaptic thalamocortical CNQX-sensitive excitatory postsynaptic potentials were not altered by 4-methyl histamine (20 µmol·L⁻¹) suggesting that H₄ receptors did not act as hetero-receptors on thalamocortical glutamatergic terminals.

Conclusions and implications:

This is the first demonstration that histamine H₄ receptors are functionally expressed on neurons, which has major implications for the therapeutic potential of these receptors in neurology and psychiatry.     

Novel heterocyclic-substituted benzofuran histamine H3 receptor antagonists: in vitro properties, drug-likeness, and behavioral activity

Three novel heterocyclic benzofurans A-688057 (1), A-687136 (2), and A-698418 (3) were profiled for their in vitro and in vivo properties as a new series of histamine H(3) receptor antagonists. The compounds were all found to have nanomolar potency in vitro at histamine H(3) receptors, and when profiled in vivo for CNS activity, all were found active in an animal behavioral model of attention. The compound with the most benign profile versus CNS side effects was selected for greater scrutiny of its in vitro properties and overall drug-likeness. This compound, A-688057, in addition to its potent and robust efficacy in two rodent behavioral models at blood levels ranging 0.2-19 nM, possessed other favorable features, including high selectivity for H(3) receptors (H(3), K(i)=1.5 nM) versus off-target receptors and channels (including the hERG K(+) channel, K(i)>9000 nM), low molecular weight (295), high solubility, moderate lipophilicity (logD(pH7.4)=2.05), and good CNS penetration (blood/brain 3.4x). In vitro toxicological tests indicated low potential for phospholipidosis, genotoxicity, and CYP(450) inhibition. Even though pharmacokinetic testing uncovered only moderate to poor oral bioavailability in rat (26%), dog (30%), and monkey (8%), and only moderate blood half-lives after i.v. administration (t(1/2) in rat of 2.9h, 1.7h in dog, 1.8h in monkey), suggesting poor human pharmacokinetics, the data overall indicated that A-688057 has an excellent profile for use as a pharmacological tool compound.     

Therapeutic potential of histamine H4 receptor agonists in triple-negative human breast cancer experimental model

Background and Purpose

The presence of the histamine H₄ receptor (H₄R) was previously reported in benign and malignant lesions and cell lines derived from the human mammary gland. The aim of this work was to evaluate the effects of H₄R ligands on the survival, tumour growth rate and metastatic capacity of breast cancer in an experimental model.

Experimental Approach

Xenograft tumours of the highly invasive human breast cancer cell line MDA-MB-231 were established in immune deficient nude mice. The following H₄R agonists were employed: histamine (5 mg kg⁻¹), clozapine (1 mg kg⁻¹) and the experimental compound JNJ28610244 (10 mg kg⁻¹).

Results

Data indicate that developed tumours were highly undifferentiated, expressed H₄R and exhibited high levels of histamine content and proliferation marker (PCNA) while displaying low apoptosis. Mice of the untreated group displayed a median survival of 60 days and a tumour doubling time of 7.4 ± 0.6 days. A significant decrease in tumour growth evidenced by an augment of the tumour doubling time was observed in the H₄R agonist groups (13.1 ± 1.2, P < 0.01 in histamine group; 15.1 ± 1.1, P < 0.001 in clozapine group; 10.8 ± 0.7, P < 0.01 in JNJ28610244 group). This effect was associated with a decrease in the PCNA expression levels, and also reduced intratumoural vessels in histamine and clozapine treated mice. Histamine significantly increased median survival (78 days; Log rank Mantel-Cox Test, P = 0.0025; Gehan-Breslow-Wilcoxon Test, P = 0.0158) and tumoural apoptosis.

Conclusions and Implications

Histamine through the H₄R exhibits a crucial role in tumour progression. Therefore, H₄R ligands offer a novel therapeutic potential as adjuvants for breast cancer treatment.

Linked Articles

This article is part of a themed issue on Histamine Pharmacology Update. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2013.170.issue-1     

In vitro and in vivo characterization of A-796260: a selective cannabinoid CB2 receptor agonist exhibiting analgesic activity in rodent pain models

BACKGROUND AND PURPOSE: Selective cannabinoid CB2 receptor agonists have demonstrated analgesic activity across multiple preclinical pain models. AM1241 is an indole derivative that exhibits high affinity and selectivity for the CB2 binding site and broad spectrum analgesic activity in rodent models, but is not an antagonist of CB2 in vitro functional assays. Additionally, its analgesic effects are mu-opioid receptor-dependent. Herein, we describe the in vitro and in vivo pharmacological properties of A-796260, a novel CB2 agonist. EXPERIMENTAL APPROACH: A-796260 was characterized in radioligand binding and in vitro functional assays at rat and human CB1 and CB2 receptors. The behavioural profile of A-796260 was assessed in models of inflammatory, post-operative, neuropathic, and osteoarthritic (OA) pain, as well as its effects on motor activity. The receptor specificity was confirmed using selective CB1, CB2 and mu-opioid receptor antagonists. KEY RESULTS: A-796260 exhibited high affinity and agonist efficacy at human and rat CB2 receptors, and was selective for the CB2 vs CB1 subtype. Efficacy in models of inflammatory, post-operative, neuropathic and OA pain was demonstrated, and these activities were selectively blocked by CB2, but not CB1 or mu-opioid receptor-selective antagonists. Efficacy was achieved at doses that had no significant effects on motor activity. CONCLUSIONS AND IMPLICATIONS: These results further confirm the therapeutic potential of CB2 receptor-selective agonists for the treatment of pain. In addition, they demonstrate that A-796260 may be a useful new pharmacological compound for further studying CB2 receptor pharmacology and for evaluating its role in the modulation of pain.     

Histamine H4 antagonists

Two applications claim novel bicyclic azole carboxamide derivatives as histamine H₄ receptor antagonists. Benzoimidazole, benzoxazole, indoles, benzothiazole, furopyrroles and thienopyrrole carboxamide derivatives, mostly as piperazine amides, are claimed. These amides are claimed to be useful in the treatment of diseases such as asthma and allergic rhinitis.     

The expression and function of histamine H3 receptors in pancreatic beta cells

BACKGROUND AND PURPOSE

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

EXPERIMENTAL APPROACH

The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H₃ receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H₃ receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting.

KEY RESULTS

Histamine H₃ receptors were expressed in pancreatic beta cells. A selective H₃ receptor agonist, imetit, and a selective inverse H₃ receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H₃ receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H₃ receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Histamine H₃ receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.     

Unexpectedly high affinity of a novel histamine H3 receptor antagonist,GSK239512, in vivo in human brain,determined using PET

BACKGROUND AND PURPOSE

This study aimed to investigate the relationship between the plasma concentration (PK) of the novel histamine H₃ receptor antagonist, GSK239512, and the brain occupancy of H₃ receptors (RO) in healthy human volunteers.

EXPERIMENTAL APPROACH

PET scans were obtained after i.v. administration of the H₃-specific radioligand [¹¹C]GSK189254. Each subject was scanned before and after single oral doses of GSK239512, at 4 and 24 h after dose. PET data were analysed by compartmental analysis, and regional RO estimates were obtained by graphical analysis of changes in the total volumes of distribution of the radioligand, followed by a correction for occupancy by the high affinity radioligand. The PK/RO relationship was analysed by a population-modelling approach, using the average PK of GSK239512 during each scan.

KEY RESULTS

Following administration of GSK239512, there was a reduction in the brain uptake of [¹¹C]GSK189254 in all regions, including cerebellum. RO at 4 h was higher than at 24 h, and the PK/RO model estimated a PK associated with 50% of RO of 0.0068 ng·mL⁻¹. This corresponds to a free concentration of 4.50 × 10⁻¹²M (pK = 11.3).

CONCLUSIONS AND IMPLICATIONS

The affinity of GSK239512 for brain H₃ receptors in humans in vivo is much higher than that expected from studies in vitro, and higher than that observed in PET studies in pigs. The study illustrates the utility of carrying out PET studies in humans early in drug development, providing accurate quantification of GSK239512 RO in vivo as a function of time and dose.     

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.     

Molecular aspects of the histamine H3 receptor

The cloning of the histamine H(3) receptor (H(3)R) cDNA in 1999 by Lovenberg et al. [10] allowed detailed studies of its molecular aspects and indicated that the H(3)R can activate several signal transduction pathways including G(i/o)-dependent inhibition of adenylyl cyclase, activation of phospholipase A(2), Akt and the mitogen activated kinase as well as the inhibition of the Na(+)/H(+) exchanger and inhibition of K(+)-induced Ca(2+) mobilization. Moreover, cloning of the H(3)R has led to the discovery several H(3)R isoforms generated through alternative splicing of the H(3)R mRNA. The H(3)R has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders like obesity, myocardial ischemia, migraine, inflammatory diseases and several CNS disorders like Alzheimer's disease, attention-deficit hyperactivity disorder and schizophrenia. In this paper, we review various molecular aspects of the hH(3)R including its signal transduction, dimerization and the occurrence of different H(3)R isoforms.