Progress in the development of histamine H3 receptor antagonists/inverse agonists: a patent review (2013-2017)

Introduction: Since years, ligands blocking histamine H₃ receptor (H₃R) activity (antagonists/inverse agonists) are interesting targets in the search for new cures for CNS disorders. Intensive works done by academic and pharmaceutical company researchers have led to many potent and selective H₃R antagonists/inverse agonists. Some of them have reached to clinical trials.

Areas covered: Patent applications from January 2013 to September 2017 and the most important topics connected with H₃R field are analysed. Espacenet, Patentscope, Pubmed, GoogleScholar or Cochrane Library online databases were principially used to collect all the materials.

Expert opinion: The research interest in histamine H₃R field is still high although the number of patent applications has decreased during the past 4 years (around 20 publications). Complexity of histamine H₃R biology e.g. many isoforms, constitutive activity, heteromerization with other receptors (dopamine D₂, D₁, adenosine A_2A) and pharmacology make not easy realization and evaluation of therapeutic potential of anti-H₃R ligands. First results from clinical trials have verified potential utility of histamine H₃R antagonist/inverse agonists in some diseases. However, more studies are necessary for better understanding of an involvement of the histaminergic system in CNS-related disorders and helping more ligands approach to clinical trials and the market.

Lists of abbreviations: hAChEI – human acetylcholinesterase inhibitor; hBuChEI – human butyrylcholinesterase inhibitor; hMAO – human monoamine oxidase; MAO – monoamine oxidase     

The cardiovascular effects of intraventricularly administered histamine in the anaesthetised rat

Summary In urethane-anaesthetised rats intraventricular (i.c.v.) injections of histamine (0.1–10.0 g) elicited dose-related rises in both the resting blood pressure and heart rate. These cardiovascular effects of histamine were antagonised in a dose-dependent manner by i.c.v. pretreatments with the histamine H₁-receptor antagonists mepyramine (10, 50 and 100 g) and diphenylpyraline (100 and 200 g). Pretreatment with the histamine H₂-receptor antagonist metiamide (100 and 200 g i.c.v.) failed to modify either of the responses. A dose-related antagonism of the hypertensive response to histamine i.c.v. was elicited by phentolamine (100 and 200 g i.c.v.) but the positive chronotropic effect was not modified by this pretreatment. The cardiovascular responses to histamine i.c.v. were abolished by mecamylamine (5.0 mg/kg i.v.) and greatly reduced by 6-hydroxydopamine (3×250 g i.c.v.), but only the tachycardia was significantly modified by atropine (100 g i.c.v.) and propranolol (1 mg/kg i.v.). Propranolol (100 g i.c.v.), bilateral vagotomy, or acute bilateral adrenal demedullation failed to modify the cardiovascular responses to histamine i.c.v. The results suggest that histamine is able to modify the resting blood pressure and heart rate by independent central modes of action, which involve central adrenergic and cholinergic mechanisms.Preliminary findings of this study were presented at the Autumn meeting of the British Pharmacological Society (Finch and Hicks, 1975).     

Identification of endothelial H1, vascular H2 and cardiac presynaptic H3 receptors in the pithed rat

Summary In pithed and vagotomized rats the effects of the H₃ receptor agonist R-(–)--methylhistamine, the H₁ receptor agonist 2-(2-thiazolyl)ethylamine and the H₂ receptor agonist dimaprit on basal diastolic blood pressure, basal heart rate and the electrically induced rise in heart rate were examined.Basal diastolic blood pressure was not altered by low, but increased by high doses of R-(–)--methylhistamine; the latter effect was not affected by selective H₁, H₂ or H₃ receptor antagonists and by prazosin, but was attenuated by rauwolscine. Rauwolscine also unmasked a vasodepressor response to R-(–)--methylhistamine not affected by the H₃ receptor antagonist thioperamide, but counteracted by the H₁ receptor antagonist dimetindene or the H₂ receptor antagonist ranitidine. The vasodepressor responses to 2-(2-thiazolyl)ethylamine and dimaprit were antagonized by dimetindene and ranitidine, respectively. The vasodepressor response to 2-(2-thiazolyl)ethylamine was not altered by indomethacin, but reduced by an inhibitor of endothelial nitric oxide synthase, N-nitro-L-arginine methyl ester (which, by itself, markedly increased blood pressure). Both drug tools did not alter the effect of dimaprit. Basal heart rate was not affected by 2-(2-thiazolyl)ethylamine (examined after administration of propranolol), dimaprit and R-(–)--methylhistamine. The electrically induced increase in heart rate (studied in animals which had received rauwolscine) was decreased by R-(–)--methylhistamine but not affected by 2-(2-thiazolyl)ethylamine and dimaprit. The effect of R-(–)--methylhistamine was abolished by thioperamide. R-(–)--methylhistamine did not influence the increase in heart rate produced by isoprenaline.In conclusion, the pithed rat offers the opportunity to study cardiac presynaptic H₃ receptors, endothelial H₁ receptors and vascular H₂ receptors in the same experimental model. Cardiac presynaptic H₁ and H₂ receptors as well as postsynaptic H₃ receptors in the heart and in the resistance vessels were not found. R-(–)--methylhistamine is a weak agonist at ₂, H₁ and H₂ receptors.Correspondence to E. Schlicker at the above address     

Effect of some new histamine H2-receptor antagonists on the guinea-pig papillary muscle

Summary Several histamine H₂-receptor antagonists (cimetidine, ranitidine, oxmetidine and tiotidine) were tested for their activity on the papillary muscle of the guinea pig stimulated by histamine. All of the compounds exerted a competitive antagonism against histamine the order of potency being tiotidine > oxmetidine > ranitidine > cimetidine. Oxmetidine was the only drug which at high concentrations (10^–6 M) decreased the maximum response of histamine probably because of non specific effects of the molecule already described in the literature. As it was expected, the H₁-receptor antagonist, mepyramine, exerted a non-competitive antagonism.     

Pre-synaptic histamine H3 receptors regulate glutamate, but not GABA release in rat thalamus

We have investigated the presence of histamine H(3) receptors (H(3)Rs) on rat thalamic isolated nerve terminals (synaptosomes) and the effect of their activation on glutamate and GABA release. N-alpha-[methyl-(3)H]histamine ([(3)H]-NMHA) bound specifically to synaptosomal membranes with dissociation constant (K(d)) 0.78+/-0.20 nM and maximum binding (B(max)) 141+/-12fmol/mg protein. Inhibition of [(3)H]-NMHA binding by histamine and the H(3)R agonist immepip fit better to a two-site model, whereas for the H(3)R antagonist clobenpropit the best fit was to the one-site model. GTPgammaS (30 microM) decreased [(3)H]-NMHA binding by 55+/-4% and made the histamine inhibition fit better to the one-site model. Immepip (30 nM) induced a modest, but significant increase (113+/-2% of basal) in [(35)S]-GTPgammaS binding to synaptosomal membranes, an effect prevented by clobenpropit (1 microM) and by pre-treatment with pertussis toxin. In thalamus synaptosomes depolarisation-induced, Ca(2+)-dependent glutamate release was inhibited by histamine (1 microM, 25+/-4% inhibition) and immepip (30 nM, 38+/-5% reduction). These effects were reversed by clobenpropit (1microM). Conversely, immepip (up to 1 microM) had no effect on depolarisation-evoked [(3)H]-GABA release. Extracellular synaptic responses were recorded in the thalamus ventrobasal complex by stimulating corticothalamic afferents. H(3)R activation reduced by 38+/-7% the glutamate receptor-mediated field potentials (FPs), but increased the FP2/FP1 ratio (from 0.86+/-0.03 to 1.38+/-0.05) in a paired-pulse paradigm. Taken together, our results confirm the presence of H(3)Rs on thalamic nerve terminals and show that their activation modulates pre-synaptically glutamatergic, but not GABAergic neurotransmission.     

Inhibition of noradrenaline release in the rat brain cortex via presynaptic H3 receptors

Summary The effects of histamine and related drugs on the evoked tritium overflow from superfused rat brain cortex slices preincubated with³H-noradrenaline were determined. Tritium overflow was stimulated electrically (3 Hz; slices superfused with normal physiological salt solution) or by introduction of CaCl₂ 1.3 mmol/l (slices superfused with Ca²⁺-free medium containing K⁺ 20 mmol/l).Histamine slightly decreased the electrically evoked^H overflow in slices superfused in the presence of desipramine. The degree of inhibition obtained with histamine was doubled when both desipramine and phentolamine were present in the superfusion medium (pIC₁₅ 6.46). Under the latter condition, the evoked overflow was inhibited by the H₃ receptor agonist R-(–)--methylhistamine and its S-(+) enantiomer (pIC₁₅ 7.36 and 5.09, respectively), but was not affected by the H₂ receptor agonist dimaprit and the H₁ receptoragonist 2-thiazolylethylamine (both at up to 32 µmol/l). The concentration-response curve of histamine was shifted to the right by the H3 receptor antagonists thioperamide, impromidine and burimamide (apparent pA₂ 8.37, 6.86 and 7.05, respectively), by the H₂ receptor antagonist ranitidine (apparent pA₂ 4.27) and was not affected by the H₁ receptor antagonist dimetindene (32 µmol/l). The inhibitory effect of R-(–)--methylhistamine on the evoked overflow was also counteracted by thioperamide. Given alone, none of the five histamine receptor antagonists affected the evoked overflow. In the absence of desipramine plus phentolamine, impromidine and burimamide facilitated the electrically evoked³H overflow whereas thioperamide had no effect. The facilitatory effects of impromidine and burimamide were abolished by phentolamine, but not affected by desipramine. The concentration-response curve of noradrenaline for its inhibitory effect on the evoked overflow was shifted to the right by impromidine and burimamide, but not influenced by thioperamide (apparent pA₂ 5.24, 5.04 and <6.5, respectively; experiments carried out in the presence of desipramine). In slices superfused with Ca²⁺-free K⁺-rich medium containing tetrodotoxin, desipramine plus phentolamine, the tritium overflow evoked by introduction of Ca²⁺ was inhibited by histamine; the concentration-response curve of histamine was shifted to the right by thioperamide.The present study shows that the inhibitory effect of histamine on noradrenaline release in the rat brain cortex involves presynaptic H₃ receptors and that the degree of inhibition is increased in the presence of phentolamine. The H₃ receptor antagonists impromidine and burimamide are weak ₂-adrenoceptor antagonists. Send offprint requests to E. Schlicker at the above address     

Behavioural effects of histamine and its antagonists: a review

This review focusses on the behavioural effects of histamine and drugs which affect histaminergic function, particularly the H₁- and H₂-receptors antagonists. Research in this area has assumed considerable importance with increasing interest in the role of brain histamine, the clinical use of both H₁ and H₂ antagonists and evidence of nonmedical use of H₁ antagonists. Results from a number of studies show that H₁ and H₂ antagonists have clear, but distinct subjective effects and that H₁ antagonists have discriminative effects in animals. While H₁ antagonists are reinforcers in certain conditions, histamine itself is a punisher. Moderate doses of H₁ antagonists affect psychomotor performance in some situations, but the results are variable. The exceptions are terfenadine and astemizole, which do not seem to penetrate the blood-brain barrier readily. In studies of schedule-controlled behaviour, marked changes in response rate have been observed following administration of H₁ antagonists, with the magnitude and direction dependent on the dose and the baseline behaviour. Histamine reduces avoidance responding, an effect mediated via H₁-receptors. Changes in drinking and aggressive behaviour have also been observed following histamine administration and distinct roles for H₁- and H₂-receptors have been delineated. Separate H₁- and H₂-receptor mechanisms have also been suggested to account for changes in activity level. While the H₂ antagonists do not always have strong behavioural effects when administered peripherally, there is evidence that cimetidine has a depressant effect on sexual function. These and other findings reveal an important role for histaminergic systems in a wide range of behaviour.     

Characteristics of recombinantly expressed rat and human histamine H3 receptors

Human and rat histamine H(3) receptors were recombinantly expressed and characterized using receptor binding and a functional cAMP assay. Seven of nine agonists had similar affinities and potencies at the rat and human histamine H(3) receptor. S-alpha-methylhistamine had a significantly higher affinity and potency at the human than rat receptor, and for 4-[(1R*,2R*)-2-(5,5-dimethyl-1-hexynyl)cyclopropyl]-1H-imidazole (Perceptin) the preference was the reverse. Only two of six antagonists had similar affinities and potencies at the human and the rat histamine H(3) receptor. Ciproxifan, thioperamide and (1R*,2R*)-trans-2-imidazol-4 ylcyclopropyl) (cyclohexylmethoxy) carboxamide (GT2394) had significantly higher affinities and potencies at the rat than at the human histamine H(3) receptor, while for N-(4-chlorobenzyl)-N-(7-pyrrolodin-1-ylheptyl)guanidine (JB98064) the preference was the reverse. All antagonists also showed potent inverse agonism properties. Iodoproxyfan, Perceptin, proxyfan and GR175737, compounds previously described as histamine H(3) receptor antagonists, acted as full or partial agonists at both the rat and the human histamine H(3) receptor.     

Regulation of gastric acid secretion by histamine H3 receptors in the dog: an investigation into the site of action

The involvement of histamine H₃ receptors in the regulation of gastric acid secretion was investigated in the conscious dog with gastric fistula, by the use of the selective agonist (R)-methylhistamine and the selective antagonist thioperamide. (R)-methylhistamine (0.3–1.2 mol/kg/h) induced a dose-related inhibition of the acid secretion induced by pentagastrin and by bombesin, maximum inhibition not exceeding 60–65%. The inhibitory effect of the H₃ agonist (0.6 mol/kg/h) was inhibitited by thioperamide (0.1 mol/kg/h), suggesting that the effect was entirely mediated by H₃ receptors. Thioperamide was also able to enhance the acid response to submaximal doses of pentagastrin and bombesin. The acid secretion induced by histamine was not modified by (R)a-methylhistamine (0.3–1.2 mol/kg/h) but it was significantly enhanced by thioperamide (0.1 mol/kg/h). Neither (R)-methylhistamine nor thioperamide significantly modified the increase in plasma gastrin levels induced by bombesin. In conclusion these data demonstrate that histamine H₃ receptors may represent an effective mechanism for the negative control of stimulated gastric acid secretion in the dog; however, since the inhibition was mainly evident against stimuli which involve the release of histamine, a location of H₃ receptors in paracrine cells of the gastric mucosa rather than in gastrin producing cells or parietal cells seems more likely. Correspondence to: G. Bertaccini at the above address     

Effects of activation of central nervous histamine receptors in cardiovascular regulation; studies in H<Subscript>1</Subscript> and H<Subscript>2</Subscript> receptor gene knockout mice

To elucidate the central roles of histamine receptors in cardiovascular regulatory system, systolic, mean, and diastolic blood pressures (BPs) and heart rate (HR) were examined in conscious H₁ receptor gene knockout (H₁KO) mice, H₂ receptor gene knockout (H₂KO) mice, H₁ and H₂ receptor gene double knockout (DKO) mice, and their respective control mice by the tail-cuff system. Histamine, histamine-trifluoromethyl-toluidine derivative (HTMT, an H₁ agonist), dimaprit (an H₂ agonist), and immepip (an H₃ agonist) were intrathecally administered to these KO mice and control mice. Basal BPs and HR were not different among these three KO mice and their control or wild-type mice. Intrathecal administration of histamine significantly increased BPs and decreased HR in control mice. The increases in BPs were produced by histamine in H₁KO and H₂KO mice and by HTMT and dimaprit in C57BL mice. The pressor responses by HTMT and dimaprit in C57BL mice were greater than those by histamine in H₁KO and H₂KO mice, although the same decreases in HR were induced by histamine in C57BL and H₁KO mice and by dimaprit in C57BL mice. The selective stimulation of H₃ receptors by immepip produced a consistent decrease in BPs in control mice. These results obtained with the exogenous selective agonists of three histamine receptors suggest that the pressor responses to histamine are mediated through the stimulation of both H₁ and H₂ receptors, whereas the atropine-sensitive decrease in heart rate is mainly due to H₂ receptors which activate the vagal output to the heart.     

Inhibition of noradrenaline release from the sympathetic nerves of the human saphenous vein by presynaptic histamine H3 receptors

Summary The human saphenous vein was used to examine whether presynaptic histamine receptors can modulate noradrenaline release and, if so, to determine their pharmacological characteristics. Strips of this blood vessel were incubated with [³H]noradrenaline and subsequently superfused with physiological salt solution containing desipramine and corticosterone. Electrically (2 Hz) evoked ³H overflow was inhibited by histamine and the H₃ receptor agonist R-(–)--methylhistamine. Histamine-induced inhibition of electrically evoked tritium overflow was not affected by ₂-adrenoceptor blockade by rauwolscine. S-(+)--methylhistamine (up to 10 mol/l) as well as the histamine H₁ and H₂ receptor agonists 2-(2-thiazolyl)ethylamine (up to 3 mol/l) and dimaprit (up to 30 mol/l), respectively, were ineffective. The selective histamine H₃ receptor antagonist thioperamide abolished the inhibitory effect of histamine. The histamine H₂ and H₁ receptor antagonists ranitidine and pheniramine, respectively, did not affect the histamine-induced inhibition of evoked tritium overflow. The present results are compatible with the suggestion that the sympathetic nerves of the human saphenous vein are endowed with inhibitory presynaptic histamine receptors of the H₃ class. Send offprint requests to M. Gothert at the above address     

In vivo release by histamine agonists and antagonists of endogenous catecholamines in the cat hypothalamus

Summary The posterior hypothalamus of anaesthetized cats was superfused through a push-pull cannula with histamine agonists and antagonists and the release of endogenous catecholamines was determined in the superfusate. Hypothalamic superfusion with histamine, 2-methylhistamine (H₁-agonist), dimaprit (H₂-agonist) or metiamide (H₂-antagonist) enhanced the release of the catecholamines dopamine, noradrenaline and adrenaline. The releasing effects of these substances depended on the presence of calcium ions. Superfusion with 2-pyridylethylamine (H₁-agonist) was virtually ineffective, while superfusion with 2-thiazolethylamine (H₁-agonist) enhanced the rate of release of noradrenaline and adrenaline without influencing the release of dopamine. Superfusion with mepyramine (H₁-antagonist) inhibited the release of noradrenaline and adrenaline without affecting the release of dopamine. Hypothalamic superfusion with a concentration of procaine which was equi-anaesthetic to that of mepyramine was ineffective. Ranitidine (H₂-antagonist) did not alter the rates of release of the catecholamines. The releasing effect of histamine was inhibited on hypothalamic superfusion with mepyramine and ranitidine. Ranitidine also inhibited the releasing effects of dimaprit and 2-methylhistamine thus indicating that the releasing action of the latter compound was mainly due to stimulation of H₂-receptors. These data suggest that blockade of H₁-receptors of the posterior hypothalamus reduces the release of noradrenaline and adrenaline, while stimulation of H₁-receptors seems to increase the rates of release of these two catecholamines. Stimulation of H₂-receptors enhances the release of all three catecholamines. Thus, dopaminergic neurones of the hypothalamus seem to possess H₂-receptors, while noradrenergic and adrenergic neurones possess H₁- and H₂-receptors.This work was supported by the Deutsche Forschungsgemeinschaft     

Interaction of agonists and selective antagonists with gastric smooth muscle muscarinic receptors

Summary The interaction of cholinergic agonists and antagonists with smooth muscle muscarinic receptors has been investigated by measurement of displacement of the muscarinic antagonist [³H]QNB (quinuclidinyl benzilate) in membranes prepared from toad stomach. The binding of [³H]QNB was saturable, reversible and of high affinity (K _D = 423 pM). The muscarinic receptor subtypes present in gastric smooth muscle were classified by determining the relative affinities for the selective antagonists pirenzepine (M₁), AF-DX 116 (M₂) and 4-DAMP (M₃). The results from these studies indicate the presence of a heterogeneous population of muscarinic receptor subtypes, with a majority (88%) exhibiting characteristics of M₃ receptors and a much smaller population (12%) exhibiting characteristics of M₂ receptors. The binding curve for the displacement of [³H]QNB binding by the agonist oxotremorine was complex and was consistent with presence of two affinity states: 24% of the receptors had a high affinity (K _D = 4.7 nM) for oxotremorine and 76% displayed nearly a 1,000-fold lower affinity (K _D = 4.4 M). When oxotremorine displacement of [³H]QNB binding was determined in the presence GTPS, high affinity binding was abolished, indicating that high affinity agonist binding may represent receptors coupled to G proteins. Moreover, pertussis toxin pretreatment of membranes also abolished high affinity agonist binding, indicating that the muscarinic receptors are coupled to pertussis toxin-sensitive G proteins. Reaction of smooth muscle membranes with pertussis toxin in the presence [³²P]NAD caused the [³²P]-labelling of a 40 kD protein that may represent the subunit(s) of G proteins that are known to be NAD-ribosylated by the toxin. We conclude that both M₃ and M₂ receptors may be coupled to G proteins in a pertussis-sensitive manner. Send offprint requests to T. W. Honeyman at the above address     

Activation of histamine H3 receptors in human nasal mucosa inhibits sympathetic vasoconstriction

The peripheral histamine H3 receptor is a presynaptic heterologous receptor located on postganglionic sympathetic nerve fibers innervating sympathetic effector systems such as blood vessels and the heart. An extensive body of evidence shows that activation of the histamine H3 receptor attenuates sympathetic tone by presynaptic inhibition of noradrenaline release. It is proposed that this sympathoinhibitory action, in vivo, leads to reduced vasoconstriction, thereby eliciting a vasodilatory effect. In humans, the peripheral histamine H3 receptor has also been shown to exert a sympathoinhibitory function on specific peripheral autonomic effector systems. For example, human saphenous vein and heart possess functional presynaptic histamine H3 receptors on the sympathetic nerve terminals that upon activation decrease the sympathetic tone to these respective organs. The present studies were conducted to define the role of histamine H3 receptors on neurogenic sympathetic vasoconstrictor responses in human nasal turbinate mucosa. Contractility studies were conducted to evaluate the effect of histamine H3 receptor activation on sympathetic vasoconstriction in surgically isolated human nasal turbinate mucosa. We found that the histamine H3 receptor agonist, (R)-alpha-methylhistamine (30 and 300 nM), inhibited electrical field stimulation-induced (neurogenic) sympathetic vasoconstriction in a concentration-dependent fashion. Pretreatment with the selective histamine H3 receptor antagonist, clobenpropit (100 nM), blocked the sympathoinhibitory effect of (R)-alpha-methylhistamine on the neurogenic sympathetic vasoconstriction. In addition, analysis of Taqman mRNA expression studies showed a specific, high level of distribution of the histamine H3 receptor localized in the human nasal mucosa.Taken together, these studies indicate that histamine H3 receptors modulate vascular contractile responses in human nasal mucosa most likely by inhibiting noradrenaline release from sympathetic nerve terminals in nasal mucosa. It is further suggested that histamine H3 receptors may play a role in the regulation of vascular tone and nasal patency in histamine-dependent allergic nasal congestive disease.     

Effects of centrally administered H2 antagonists in the behavioral despair test

Injection of the histamine-2 (H₂) receptor antagonist cimetidine into the lateral ventricles of mice produced a dose-related reduction in swimming in the behavioral despair test. This response can be attenuated by intracerebroventricular (ICV) injection of the histamine-1 (H₁) receptor antagonist chlorpheniramine, or the H₂ receptor agonist impromidine, given simultaneously with cimetidine. At doses which blocked cimetidine, neither chlorpheniramine nor impromidine alone had effects on swimming. A similar decrease in swimming behavior was also seen after ICV injections of the non-imidazole H₂ antagonist, BMY 25,368. This effect of BMY 25,368 was also attenuated by chlorpheniramine and impromidine. These results suggest that H₁ and H₂ receptors in the brain may mediate opposing behavioral effects.     

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.     

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.     

Interactions between histamine H3 and dopamine D2 receptors and the implications for striatal function

The striatum contains a high density of histamine H(3) receptors, but their role in striatal function is poorly understood. Previous studies have demonstrated antagonistic interactions between striatal H(3) and dopamine D(1) receptors at the biochemical level, while contradictory results have been reported about interactions between striatal H(3) and dopamine D(2) receptors. In this study, by using reserpinized mice, we demonstrate the existence of behaviorally significant antagonistic postsynaptic interactions between H(3) and D(1) and also between H(3) and dopamine D(2) receptors. The selective H(3) receptor agonist imetit inhibited, while the H(3) receptor antagonist thioperamide potentiated locomotor activation induced by either the D(1) receptor agonist SKF 38393 or the D(2) receptor agonist quinpirole. High scores of locomotor activity were obtained with H(3) receptor blockade plus D(1) and D(2) receptor co-activation, i.e., when thioperamide was co-administered with both SKF 38393 and quinpirole. Radioligand binding experiments in striatal membrane preparations showed the existence of a strong and selective H(3)-D(2) receptor interaction at the membrane level. In agonist/antagonist competition experiments, stimulation of H(3) receptors with several H(3) receptor agonists significantly decreased the affinity of D(2) receptors for the agonist. This kind of intramembrane receptor-receptor interactions are a common biochemical property of receptor heteromers. In fact, by using Bioluminescence Resonance Energy Transfer techniques in co-transfected HEK-293 cells, H(3) (but not H(4)) receptors were found to form heteromers with D(2) receptors. This study demonstrates an important role of postsynaptic H(3) receptors in the modulation of dopaminergic transmission by means of a negative modulation of D(2) receptor function.     

Cardiovascular effects of the novel histamine H2 receptor agonist amthamine: interaction with the adrenergic system

The cardiovascular effects of the new histamine H₂ receptor agonist amthamine were studied in the anaesthetized rat, with particular reference to a possible interaction with the adrenergic system. Amthamine (0.03–3 mol/kg i.v.) caused vasodepressor responses which were antagonized by famotidine (3 mol/kg i.v.). At higher doses (30–100 mol/kg i.v.), amthamine induced a modest increase in the mean arterial pressure, which was significantly enhanced by the blockade of H₂ receptors and significantly reduced by the ₂ adrenoceptor antagonist yohimbine (1 mol/kg i.v.). The vasopressor response to amthamine was not modified in rats pre-treated with reserpine or 6-hydroxydopamine, and was only minimally modified in adrenalectomized animals, thus suggesting a predominant interaction with postjunctional ₂ adrenoceptors in the vascular muscle. The H₂ receptor agonist dimaprit (0.3–100 mol/kg i.v.) caused a reduction in arterial pressure, which was antagonized by famotidine, no pressor response being unmasked.Dimaprit (0.1–30 mol/kg i.v.) did not modify heart rate but caused a modest bradycardia at 100 mol/kg i.v. Amthamine (1-100 mol/kg i.v.) induced a dose-dependent tachycardia, which was only partially (approximately 20%) reduced by famotidine and was totally blocked by propranolol (0.3 mg/kg i.v.). This effect was significantly reduced in rats pre-treated with reserpine or 6-hydroxydopamine and was further reduced by cocaine, thus suggesting a tyramine-like action of amthamine.In conclusion, these data demonstrate that the H₂ receptor agonist amthamine can also interact with the adrenergic system when used at doses higher than those necessary to activate H2 receptors. Whereas the increase in blood pressure induced by amthamine seems to be mainly mediated by a direct activation of postjunctional ₂ adrenoceptors, the increase in heart rate is predominantly due to neuronal release of catecholamines. These effects should be considered when using amthamine in cardiovascular or other studies when high doses are employed.     

Cardiovascular effects of flesinoxan in anaesthetized and conscious dogs

Summary Stimulation of 5-HT_1A receptors is known to decrease the arterial blood pressure in anaesthetized rats, cats and dogs. We investigated the hypotensive activity of flesinoxan (0.1 + 0.2 + 0.7 mol/kg), a 5-HT_1A-receptor agonist, in dogs anaesthetized with either morphine and pentobarbital or enflurane and also in the conscious state. Flesinoxan led to a decrease in arterial blood pressure in anaesthetized, but not in conscious dogs. In the conscious state the marked increase in heart rate, which can be taken as an indicator of sympathetic tone, may have masked the consequences of vasodilatation. These different haemodynamic responses to flesinoxan may be dependent on side effects of the drug in the conscious dogs, in particular hyperventilation and salivation combined with anxiety, and on the magnitude of the decrease in baroreceptor reflex activity during anaesthesia with morphine and pentobarbital on the one hand and enflurane on the other hand.