Molecular modeling and pharmacological analysis of species-related histamine H(3) receptor heterogeneity

The challenge of drug discovery of a GPCR target: Analysis of preclinical pharmacology of histamine H3 antagonists/inverse agonists

Although the histamine H(3) receptor was identified pharmacologically in 1983, and despite widespread pharmaceutical interest in the target, no compound interacting specifically with this site has undergone successful clinical examination to develop the necessary proof-of-concept data. Therefore, clinical knowledge of the therapeutic potential of H(3) receptor antagonists in neuropsychiatric diseases, in metabolic diseases or in sleep disorders has yet to determine if the preclinical data that show broad efficacy in animal models of the aforementioned states are relevant to current unmet medical needs. H(3) receptors are complex, with species-related sequence differences that impact pharmacological responses. The receptors have a complex gene organization that provides opportunity for multiple slice isoforms, most of which remain poorly characterized even within a species. H(3) receptors are constitutively active, although the extent of this could vary either between species and/or receptor splice isoforms, both of which may provide opportunity for preferential coupling to different G-proteins. Thus, it is not surprising that the pharmacological effects of known H(3) ligands are complex and diverse, since these agents may act both as agonists and antagonists in different systems. Moreover, other compounds show inverse agonism in some models but neutral antagonist activity in others. Some of this diversity may be related to different ligand-dependent receptor activation states or to the effects of key amino acids important for ligand recognition. This commentary provides an overview of these complexities as applied to the H(3) receptor and the challenges these intricacies create for drug discovery.

Use of an inverse agonist radioligand [3H]A-317920 reveals distinct pharmacological profiles of the rat histamine H3 receptor

Selective radioligands for histamine H(3) receptors have been used to characterize H(3) receptor pharmacology by radioligand binding assays and to determine H(3) receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [(3)H]A-317920 (furan-2-carboxylic acid(2-[4-[3-([3,5-(3)H]4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H(3) receptor. The binding of [(3)H]A-317920 to rat cortical and cloned H(3) receptors revealed fast on- and slower off-rate kinetics with calculated K(d) values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [(3)H]A-317920 with the agonist [(3)H](N)-alpha-methylhistamine ([(3)H]NalphaMH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [(3)H]NalphaMH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [(3)H]A-317920 displacement by agonists was best fit by two-site binding models, and the potencies of the high affinity, GDP-sensitive sites correlated with the potencies defined in [(3)H]NalphaMH binding. Unlike [(125)I]iodoproxyfan, [(3)H]A-317920 exhibits potent and selective binding to rat H(3) receptors with low binding to non-H(3) sites, including cytochrome P450. These findings show that [(3)H]A-317920 is a potent rat H(3) receptor antagonist radioligand and has utility for studying H(3) receptor pharmacology.

Validation of a histamine H3 receptor model through structure-activity relationships for classical H3 antagonists

Histamine H(3) receptor is a G protein-coupled receptor whose activation inhibits the synthesis and release of histamine and other neurotransmitters from nerve endings and is involved in the modulation of different central nervous system functions. H(3) antagonists have been proposed for their potential usefulness in diseases characterized by impaired neurotransmission and they have demonstrated beneficial effects on learning and food intake in animal models. In the present work, a 3D model of the rat histamine H(3) receptor, built by comparative modeling from the crystallographic coordinates of bovine rhodopsin, is presented with the discussion of its ability to predict the potency of known and new H(3) antagonists. A putative binding site for classical, imidazole-derived H(3) antagonists was identified by molecular docking. Comparison with a known pharmacophore model and the binding affinity of a new rigid H(3) antagonist (compound 1, pK(i)=8.02) allowed the characterization of a binding scheme which could also account for the different affinities observed in a recently reported series of potent H(3) antagonists, characterized by a 2-aminobenzimidazole moiety. Molecular dynamics simulations were employed to assess the stability and reliability of the proposed binding mode. Two new conformationally constrained benzimidazole derivatives were prepared and their binding affinity was tested on rat brain membranes; compound 9, designed to reproduce the conformation of a known potent H(3) antagonist, showed higher potency than compound 8, as expected from the binding scheme hypothesized.

New high affinity H3 receptor agonists without a basic side chain

In this study, we replaced the basic amine function of the known histamine H(3) receptor agonists imbutamine or immepip with non-basic alcohol or hydrocarbon moieties. All compounds in this study show a moderate to high affinity for the cloned human H(3) receptor and, unexpectedly, almost all of them act as potent agonists. Moreover, in the alcohol series, we consistently observed an increased selectivity for the human H(3) receptor over the human H(4) receptor, but none of the compounds in this series possess increased affinity and functional activity compared to their alkylamine congeners. In this new series of compounds VUF5657, 5-(1H-imidazol-4-yl)-pentan-1-ol, is the most potent histamine H(3) receptor agonist (pK(i) = 8.0 and pEC(50) = 8.1) with a 320-fold selectivity at the human H(3) receptor over the human H(4) receptor.

Synthesis and SAR of highly potent dual 5-HT1A and 5-HT1B antagonists as potential antidepressant drugs

Novel 5-HT(1) autoreceptor ligands based on the N-4-aryl-piperazinyl-N'-ethyl-5,6,7,8-tetrahydropyrido[4', 3':4,5]thieno[2,3-d]pyrimidin-4(3H)-one core are described. Aiming at antidepressants with a novel mode of action our objective was to identify potent antagonists showing balanced affinities and high selectivity for the 5-HT(1A) and 5-HT(1B) receptors. Strategies for the development of dual 5-HT(1A) and 5-HT(1B) antagonists based on 1 and 2 as leads and the corresponding results are discussed. Isoquinoline analogue 33 displayed high affinity and an antagonistic mode of action for the 5-HT(1A) and the 5-HT(1B) receptors and was characterized further with respect to selectivity, electrically stimulated [(3)H]5-HT release and in vivo efficacy.

Species comparison and pharmacological characterization of rat and human CB2 cannabinoid receptors

Pharmacological effects of cannabinoid ligands are thought to be mediated through cannabinoid CB1 and CB2 receptor subtypes. Sequence analysis revealed that rat and human cannabinoid CB2 receptors are divergent and share 81% amino acid homology. Pharmacological analysis of the possible species differences between rat and human cannabinoid CB2 receptors was performed using radioligand binding and functional assays. Pronounced species selectivity at the rat cannabinoid CB2 receptor (50- to 140-fold) was observed with AM-1710 (3-(1,1-Dimethyl-heptyl)-1-hydroxy-9-methoxy-benzo[c]chromen-6-one) and AM-1714 (3-(1,1-Dimethyl-heptyl)-1-9-dihydroxy-benzo[c]chromen-6-one). In contrast, JWH-015 ((2-Methyl-1-propyl-1H-indol-3-yl)-napthalen-1-yl-methanone) was 3- to 10-fold selective at the human cannabinoid CB2 receptor. Endocannabinoid ligands were more human receptor selective. Cannabinoid CB2 receptor antagonist, AM-630 ((6-Iodo-2-methyl-1-(2-morpholin-4-yl-ethyl)-1H-indol-3-yl)-(4-methoxy-phenyl)-methanone) was more potent at the rat receptor in radioligand binding and functional assays than that of the human receptor. The findings of the pharmacological differences between the human and rat cannabinoid CB2 receptors in this study provide critical information for characterizing cannabinoid ligands in in vivo rodent models for drug discovery purpose.

Assessment of pharmacology and potential anti-obesity properties of H3receptor antagonists/inverse agonists

Histamine is a key neurotransmitter that alters central nervous system functions in both behavioural and homeostatic contexts through its actions on the histamine (H) subreceptors H(1), H(2) and H(3) G-protein-coupled receptors. H(3)receptors have a diverse central nervous system distribution where they function as both homo- and hetero-receptors to modulate the synthesis and/or release of several neurotransmitters. H(3) receptors are constitutively active, which implies that antagonists of H(3) receptors may also function as inverse agonists to alter the basal state of the receptor and uncouple constitutive receptor-G-protein interactions. Reference H(3) antagonists such as thioperamide and ciproxifan, administered either centrally or systemically, have been shown to cause changes in food consumption and/or body weight in proof-of-concept studies. More recently, several non-imidazole-based H(3) antagonists/inverse agonists have also been described with efficacy in at least one animal model of human obesity. Considerable preclinical effort remains necessary before such compounds achieve therapeutic success or failure. Moreover, ongoing research in a number of laboratories has shed new insights into the effects of H(3) ligands in the control of feeding, appetite and body weight, which offer different results and conclusions. The goal of this review is to appraise these findings and forecast whether any H(3) antagonists/inverse agonists will provide clinical utility to treat human obesity.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Pharmacological properties of ABT-239 [4-(2-{2-[(2R)-2-Methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile]: I. Potent and selective histamine H3 receptor antagonist with drug-like properties

Histamine H3 receptor antagonists are being developed to treat a variety of neurological and cognitive disorders that may be ameliorated by enhancement of central neurotransmitter release. Here, we present the in vitro pharmacological and in vivo pharmacokinetic profiles for the nonimidazole, benzofuran ligand ABT-239 [4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl)benzonitrile] and compare it with several previously described imidazole and nonimidazole H3 receptor antagonists. ABT-239 binds to recombinant human and rat H3 receptors with high affinity, with pK(i) values of 9.4 and 8.9, respectively, and is over 1000-fold selective versus human H1, H2, and H4 histamine receptors. ABT-239 is a potent H3 receptor antagonist at recombinant human and rat receptors, reversing agonist-induced changes in cAMP formation (pK(b) = 7.9 and 7.6, respectively), guanosine 5'-O-(3-[35S]thio) triphosphate ([35S]GTPgammaS) binding (pK(b) = 9.0 and 8.3, respectively), and calcium mobilization (human pK(b) = 7.9). ABT-239 also competitively reversed histamine-mediated inhibition of [3H]histamine release from rat brain cortical synaptosomes (pK(b) = 7.7) and agonist-induced inhibition of contractile responses in electric field stimulated guinea pig ileal segments (pA2 = 8.7). Additionally, ABT-239 is a potent inverse agonist, inhibiting constitutive [35S]GTPgammaS binding at both rat and human H3 receptors with respective pEC50 values of 8.9 and 8.2. ABT-239 demonstrates good pharmacokinetic characteristics in rat, dog, and monkey with t1/2 values ranging from 4 to 29 h, corresponding with clearance values and metabolic turnover in liver microsomes from these species, and good oral bioavailability ranging from 52 to 89%. Thus, ABT-239 is a selective, nonimidazole H3 receptor antagonist/inverse agonist with similar high potency in both human and rat and favorable drug-like properties.

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.

In vitro Optimization of Structure Activity Relationships of Analogues of A-331440 Combining Radioligand Receptor Binding Assays and Micronucleus Assays of Potential Antiobesity Histamine H3 Receptor Antagonists

A-331440 [4'-[3-(3(R)-(dimethylamino)-pyrrolidin-1-yl)-propoxy]-biphenyl-4-carbonitrile], a potent and selective antagonist of histamine H3 receptors, yielded positive results in an in vitro micronucleus assay, predictive of genotoxicity in vivo. Because this compound has highly favourable properties and potential as an antiobesity agent, new compounds of this general chemical class were sought that would retain or improve upon the high potency and selectivity of A-331440 for H3 receptors, but would lack the potential for genotoxicity obtained with that compound. Our working hypothesis was that the biphenyl rings in A-331440 might contribute to interactions with DNA and thereby predispose toward genotoxicity. Toward this end, several analogues were prepared, with substituents introduced onto the biaryl ring to alter the orientation, electronegativity, and polarity of this moiety, and were tested for their radioligand binding potency and selectivity and their propensity to induce genotoxicity in the in vitro micronucleus assay. Using this strategy, novel compounds were discovered that retained or improved upon the potency and selectivity of A-331440 for H3 receptors and were devoid of genotoxicity in vitro. Of these, the simple mono- and di-fluorinated analogues (A-417022 [4'-[3-[(3R)-3-(dimethylamino)-1-pyrrolidinyl]propoxy]-3'-fluoro-1,1'-biphenyl-4-carbonitrile] and A-423579 [4'-[3-[(3R)-3-(dimethylamino)-1-pyrrolidinyl]-propoxy]-3',5'-difluoro-1,1'-biphenyl-4-carbonitrile], respectively) were found to bind to H3 receptors at least as potently as A-331440, while lacking genotoxicity in the micronucleus assay. The reason of the lack of genotoxicity of the fluorinated analogues is unclear, but is especially noteworthy in light of the general principle that fluorine and hydrogen are very similar in size. Therefore, these fluorinated analogues of A-331440 represented the most potent and potentially safest compounds for further evaluation as antiobesity leads. Preliminary findings with one of these examples, A-417022, in a mouse model of obesity are presented.

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.

Cloning and pharmacological characterization of the monkey histamine H3 receptor

Species differences have been described previously for histamine H(3) receptor pharmacology. Rat selective histamine H(3) receptor ligands such as ciproxifan and A-304121 (2-amino-1-[4-[3-(4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-propan-1-one) show over 100-fold selectivity for the rat receptor compared to the human receptor. To date, however, the pharmacology of the cloned monkey histamine H(3) receptor has not been examined. In this study, we cloned the monkey histamine H(3) receptor gene (H(3)R) and evaluated the receptor pharmacology in binding and functional assays. The monkey histamine H(3) receptor is highly homologous to the human receptor with 438 identities in their 445 amino acid sequences, but less homologous to the rat receptor. However, unlike the human or rat, we found no evidence for additional splicing for the monkey H(3)R. Pharmacological analysis indicated that the monkey receptor exhibited similar pharmacological profiles to those of the human receptor, providing critical information for characterizing histamine H(3) receptor ligands in monkey behavioral models.

Genetic and pharmacological aspects of histamine H3 receptor heterogeneity

Histaminergic H3 receptors modulate the release of neurotransmitters within the CNS and periphery. Ligands for these receptors have potential clinical utility in a variety of disease states. However, the pharmacological characteristics of these receptors have been enigmatic for more than a decade because of the diversity of pharmacological effects observed with the limited number of heretofore-available compounds. Recent cloning of the H3 receptor has revealed interspecies differences in the protein sequences in key regions, the existence of splice variants that differ in composition between species, and potential differences in signal transduction processes between either different tissues and/or species. This review attempts to summarize these findings within the context of the molecular biological and pharmacological data accumulated to date. Also, we suggest a nomenclature strategy to reduce potential confusion that has arisen from different naming systems used by various investigators. While some facets of this genetic and pharmacological diversity help to rationalize various aspects of H3 receptor heterogeneity, there remains an insufficient repertoire of selective ligands, assays, or other measures to completely resolve all components of this diversity. The promise of newly available tools to further explore H3 receptor function may provide the insight to bring the promised clinical potential of H3 receptor ligands to realization.

Two novel and selective nonimidazole histamine H3 receptor antagonists A-304121 and A-317920: I. In vitro pharmacological effects

Histamine H3 receptor (H3R) antagonists enhance neurotransmitter release and are being developed for the treatment of a variety of neurological and cognitive disorders. Many potent histamine H3R antagonists contain an imidazole moiety that limits receptor selectivity and the tolerability of this class of compounds. Here we present the in vitro pharmacological data for two novel piperazine amide ligands, A-304121 [4-(3-((2R)-2-aminopropanoyl-1-piperazinyl)propoxy)phenyl)cyclopropylmethanone] and A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl)phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxo-ethyl-)-2-furamide], and compare them with the imidazole H3R antagonists ciproxifan, clobenpropit, and thioperamide. Both A-304121 and A-317920 bind potently to recombinant full-length rat H3R(pKi values = 8.6 and 9.2, respectively) but have lower potencies for binding the full-length human H3R (pKi values = 6.1 and 7.0, respectively). A-304121 and A-317920 are potent antagonists at rat H3R in reversing R-alpha-methylhistamine [(R)-alpha-MeHA] inhibition of forskolin-stimulated cAMP formation (pKb values = 8.0 and 9.1) but weak antagonists at human H3Rs in cyclase (pKb values = 6.0 and 6.3) and calcium mobilization (pKb values = 6.0 and 7.3) assays in cells co-expressing Galphaqi5-protein. Both compounds potently antagonize native H3Rs by blocking histamine inhibition of potassium-evoked [3H]histamine release from rat brain cortical synaptosomes (pKb values = 8.6 and 9.3) and (R)-alpha-MeHA reversal of electric field-stimulated guinea pig ileum contractions (pA2 values = 7.1 and 8.3). A-304121 and A-317920 are also more efficacious inverse agonists in reversing basal guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding at the human H3R (pEC50 values = 5.7 and 7.0) than are the imidazole antagonists. These novel and selective piperazine amides represent useful leads for the development of H3R antagonist therapeutic agents.