Novel histamine H3 receptor antagonists: affinities in an H3 receptor binding assay and potencies in two functional H3 receptor models

Regulation of Norepinephrine Release from Isolated Bovine Irides by Histamine

In the present study, we investigated the effect of histamine on sympathetic neurotransmission from isolated, superfused bovine irides. We also studied the pharmacology of prejunctional histamine receptors that regulate the release of norepinephrine (NE) from this tissue. The effect of exogenous histamine and various histamine receptor agonists was examined on the release of [(3)H]-norepinephrine ([(3)H]NE) triggered by electrical field stimulation using the Superfusion Method. Histamine receptor agonists caused a concentration-dependent inhibition of field-stimulated [(3)H]NE overflow with the following rank order of potency: imetit > histamine > R-alpha-methylhistamine. In all cases, the inhibitory action of histamine receptor agonists was attenuated at high concentrations of these compounds. The histamine receptor antagonists, clobenpropit (H(3)-antagonist/H(4)-agonist) and thioperamide (H(3)-antagonist) blocked the inhibitory response elicited by R-alpha-methylhistamine and imetit, respectively. Inhibitory effects of R-alpha-methylhistamine and clonidine were not additive suggesting that prejunctional H(3)- and alpha(2)-adrenoceptors coexist at neurotransmitter release sites. We conclude that histamine produces an inhibitory action on sympathetic neurotransmission in the bovine iris, an effect mimicked by selective H(3)-receptor agonists and blocked by H(3)-antagonists.

Non-imidazole histamine H3 ligands. Part III. New 4-n-propylpiperazines as non-imidazole histamine H3-antagonists

In search for a new lead of non-imidazole histamine H3-receptor antagonists, a series of 1[(2-thiazolopyridine)-4-n-propyl]piperazines, the analogous 1-[(2-oxazolopyridine)-4-npropyl]piperazines, 1-[(2-benzothiazole)-4-n-propyl]piperazine and 1-[(2-benzooxazole)4-n-propyl]piperazine were prepared and in vitro tested as H3-receptor antagonists (the electrically evoked contraction of the guinea-pig jejunum). It appeared that by comparison of homologous pairs the thiazolo derivatives have slightly higher activity than their oxazolo analogues. The most potent compound of these series is the 1-(2-thiazolo[4,5-c]pyridine)-4-n-propylpiperazine (3c) with pA2 = 7.25 (its oxazole analogue (4g) showed pA2 = 6.9). The structure-activity relationships for compounds with various positions of the nitrogen in the benzene ring for the thiazoles compared with oxazoles are discussed.

4-(omega-(alkyloxy)alkyl)-1H-imidazole derivatives as histamine H(3) receptor antagonists/agonists

In an effort to develop new histamine H(3) receptor antagonists usable as pharmacological tools we present here novel unsymmetrical ether derivatives. Etherification of different omega-(1H-imidazol-4-yl)alkyl scaffolds led to compounds containing alkyl chains of increasing lengths either with or without unsaturated termini, cycloalkyl or arylalkyl moieties, or additional heteroatoms. When investigated in an in vitro assay on rat synaptosomes, the majority of compounds displayed potencies in the low nanomolar concentration range at the H(3) receptor, e.g., 4-(3-(3-cyclopentylpropyloxy)propyl)-1H-imidazole (27, K(i) = 7 nM). FUB 465, 4-(3-(ethoxy)propyl)-1H-imidazole (14), a useful tool for the characterization of constitutive activity of H(3) receptors in vivo in rodents, proved to be of high oral in vivo potency in mice (ED(50) = 0.26 mg/kg). Further, the influence of chosen compounds on specific [(35)S]GTPgammaS binding was assayed on HEK293 cell membranes expressing the human histamine H(3) receptor revealing partial agonism of the compounds in this particular model. These distinct responses are further hints for "protean agonism" in this class of compounds. Additionally, selected compounds were functionally investigated in vitro on isolated organs of the guinea-pig at H(3), H(1), and H(2) receptors.

Structural variations of 1-(4-(phenoxymethyl)benzyl)piperidines as nonimidazole histamine H3 receptor antagonists

Recent bioisoteric replacements in histamine H3 receptor ligands with an exchange of the imidazole moiety by a piperidino group as well as of the trimethylene chain in 4-((3-phenoxy)propyl)-lH-imidazole derivatives (proxifan class) by an alpha,alpha'-xylendiyl linker represents the starting point in the development of 1-(4-(phenoxymethyl)benzyl)piperidines as a new class of nonimidazole histamine H3 receptor antagonists. According to different strategies in optimization of imidazole-containing antagonists the central benzyl phenyl ether moiety was replaced by numerous other polar functionalities. Additionally, the ortho- and meta-analogues of the lead were synthesized to determine the influence of the position of the piperidinomethyl substituent. The new compounds were tested in an in vitro binding assay for their affinities for cloned human H3 receptors stably expressed in CHO-K1 cells and for their oral in vivo potencies brain in a functional screening assay in the brain of mice. Additionally, activities of selected compounds were determined in the guinea-pig ileum functional test model. In contrast to the analogues ortho-substituted compounds all other compounds maintained respectable affinities for the human H3 receptor (-log Ki values 6.3-7.5). Despite the results from other classes of compounds the 4-methyl substituted derivatives generally displayed higher affinities than the corresponding 4-chloro substituted compounds. In vivo only the inverse phenyl benzyl ether (3) showed worthwhile antagonist potencies.

Novel nonimidazole histamine H3 receptor antagonists: 1-(4-(phenoxymethyl)benzyl)piperidines and related compounds

In an extension of very recently published studies on successful imidazole replacements in some series of histamine H(3) receptor antagonists, we report on a new class of lipophilic nonimidazole antagonist having an aliphatic tertiary amino moiety connected to a benzyl template substituted in the 4-position by a phenoxymethyl group. The structural modifications were performed with the intention to avoid possible negative side effects reported for other series of antagonists. The novel compounds combine different characteristics of recently developed histamine H(3) receptor antagonists. The compounds were screened for their affinity in a binding assay for the human histamine H(3) receptor stably expressed in CHO-K1 cells and tested for their in vivo potency in the central nervous system of mice after oral administration. Different substitution patterns on the phenoxy group were used to optimize in vitro and/or in vivo potency leading to some compounds with low nanomolar affinity and high oral in vivo potency. Modifications of the basic piperidino moiety were performed by ring expansion, contraction, and opening. Selected compounds exhibited selectivity in functional assays on isolated organs of guinea-pig for H(3) vs H(1) and H(2) receptors. Unexpectedly, some of the novel antagonists also showed a slight preference for the human histamine H(3) receptor compared to their affinities for the guinea-pig H(3) receptor.

Influence of bulky substituents on histamine h(3) receptor agonist/antagonist properties

Novel derivatives of 3-(1H-imidazol-4-yl)propanol were designed on the basis of lead compounds belonging to the carbamate or ether series possessing (partial) agonist properties on screening assays of the histamine H(3) receptor. One pair of enantiomers in the series of alpha-methyl-branched chiral carbamates was stereoselectively prepared in high optical yields. Enantiomeric purity was checked by Mosher amide derivatives of precursors and capillary electrophoresis of the final compounds with trimethyl-beta-cyclodextrin as chiral selector, and was determined to be >/=95%. The novel compounds were investigated in various histamine H(3) receptor assays in vitro and in vivo. Some compounds displayed partial agonist activity on synaptosomes of rat brain cortex, whereas others exhibited antagonist properties only. Selected compounds were investigated in [(125)I]iodoproxyfan binding studies on the human histamine H(3) receptor and showed high affinity in the nanomolar concentration range. Under in vivo conditions after oral administration to mice, some of the compounds exhibited partial or full agonist activity in the brain at low dosages. The (S)-enantiomer of one pair of chiral carbamates (9) proved to be the eutomer; thus, the (S)-enantiomer was selected for further pharmacological studies. In a peripheral in vivo test model in rats, measuring the level of inhibition of capsaicin-induced plasma extravasation, (S)-9 again proved its high oral agonist potency with full intrinsic activity (ED(50) values of 0.07-0.1 mg/kg depending on tissue).

Piperidino-hydrocarbon compounds as novel non-imidazole histamine H(3)-receptor antagonists

In search for novel non-imidazole histamine H(3)-receptor antagonists, piperidino-hydrocarbon compounds were synthesized using the known non-imidazole histamine H(3)-receptor antagonist FUB 637 (3-phenylpropyl 3-piperidinopropyl ether) as lead structure. Piperidino-alkyl derivatives containing highly flexible side chains (2, 4-7) were prepared via N-alkylation. Compounds containing unsaturated alkyl groups were synthesized in order to investigate the impact of rigidifying the side chain (8-16). Terminal alkynes were prepared by alkylation of lithium acetylide-ethylenediamine complex, disubstituted alkynes were synthesized by alkylation of the appropriate acetylene in the presence of n-butyllithium-N,N,N',N'-tetramethylene-ethylene-diamine complex. The novel compounds were investigated in an in vitro functional assay on the guinea-pig ileum, in which N-(7-phenylhept-3-ynyl)piperidine (14) proved to be of good potency in this class (pA(2)=7.21). In an in vivo assay the compounds were additionally screened for their abilities to influence central H(3)-histaminergic neuron activity in mice with regard to their oral availabilities and distribution properties. In this screening, N-pent-4-ynylpiperidine (9) and N-hex-5-ynylpiperidine (10) proved to be highly potent and orally available histamine H(3)-receptor antagonists. The ED(50) values for 9 and 10 were 1.3 and 1.4mg/kg po, respectively, which is in the potency range of the reference antagonist thioperamide.

Progress in the proxifan class: heterocyclic congeners as novel potent and selective histamine H(3)-receptor antagonists

Histamine H(3) receptors are critically involved in the pathophysiology of several disorders of the central nervous system (CNS). Among other families of H(3)-receptor ligands, the proxifan class has recently been described to contain numerous potent histamine H(3)-receptor antagonists, e.g. ciproxifan or imoproxifan. In the present study, we report on the design of novel heterocyclic proxifan analogues and their antagonist potencies at histamine H(3) receptors. The new compounds were tested for in vitro and in vivo H(3)-receptor antagonist potencies in different species as well as for H(3)-receptor selectivity vs. H(1) and H(2) receptors. In vitro, all compounds investigated proved to be potent H(3)-receptor antagonists in the rat as well as in the guinea-pig. In addition, they showed good to high oral CNS potency in vivo in mice. Especially, oxadiazole derivatives 24-26 displayed nanomolar antagonist activity in vitro and high potency in vivo (ED(50)=0.47-0.57 mg/kg). The results show that the additional heteroaromatic moieties might act as bioisosteres of the ketone or oxime moieties of ciproxifan or imoproxifan, respectively, and might cause divergent pharmacokinetic properties. Thus, these novel H(3)-receptor antagonists are interesting leads for further development.

Development of a new class of nonimidazole histamine H(3) receptor ligands with combined inhibitory histamine N-methyltransferase activity

In search of novel ways to enhance histaminergic neurotransmission in the central nervous system, a new class of nonimidazole histamine H(3) receptor ligands were developed that simultaneously possess strong inhibitory activity on the main histamine metabolizing enzyme, histamine N-methyltransferase (HMT). The novel compounds contain an aminoquinoline moiety, which is an important structural feature for HMT inhibitory activity, connected by different spacers to a piperidino group (for H(3) receptor antagonism). Variation of the spacer structure provides two different series of compounds. One series, having only an alkylene spacer between the basic centers, led to highly potent HMT inhibitors with moderate to high affinity at human histamine H(3) receptors. The second series possesses a p-phenoxypropyl spacer, which may be extended by another alkylene chain. This latter series also showed strong inhibitory activity on HMT, and in most cases, the H(3) receptor affinity even surpassed that of the first series. One of the most potent compounds with this dual mode of action is 4-(4-(3-piperidinopropoxy)phenylamino)quinoline (34) (hH(3), K(i) = 0.09 nM; HMT, IC(50) = 51 nM). This class of compounds showed high antagonist potency and good H(3) receptor selectivity in functional assays in guinea pig on H(1), H(2), and H(3) receptors. Because of low or missing in vivo activity of two selected compounds, the proof of concept of these valuable pharmacological tools for the supposed superior overall enhancing effect on histaminergic neurotransmission failed to appear hitherto.

Species-related pharmacological heterogeneity of histamine H(3) receptors

We compared radioligand binding and functional data for histamine H(3) receptor ligands across different tissues or species to evaluate the basis for pharmacological evidence of receptor heterogeneity previously reported. Agonist binding affinities showed correlation coefficients near unity in comparing human, dog, rat, and guinea pig cerebral cortical histamine H(3) receptors. Antagonist binding affinities revealed lower correlations for human compared to dog, rat, or guinea pig, suggesting species-based pharmacological differences. The functional potencies of histamine H(3) receptor antagonists in field-stimulated guinea pig ileum were highly correlated to binding affinities for guinea pig, dog, and, to a lesser extent, rat cerebral cortex. However, antagonist binding affinity at human cerebral cortex did not correlate well with guinea pig ileum functional potency. These results suggest significant interspecies histamine H(3) receptor heterogeneity, consistent with recent receptor gene sequence data. Therefore, genetic heterogeneity, rather than peripheral and central histamine H(3) receptor diversity, is responsible for the pharmacological differences observed.

Influence of imidazole replacement in different structural classes of histamine H(3)-receptor antagonists

The reference compounds for histamine H(3)-receptor antagonists carry as a common feature an imidazole moiety substituted in the 4-position. Very recently novel ligands lacking an imidazole ring have been described possessing a N-containing non-aromatic heterocycle instead. In this study we investigated whether imidazole replacement, favourably by a piperidine moiety, is generally applicable to different structural classes of reference compounds, e.g., thioperamide, carboperamide, clobenpropit, FUB 181, ciproxifan, etc. While replacement led to a loss of affinity for many of the compounds, it was successfully applied to some ether derivatives. The piperidine analogues of FUB 181 and ciproxifan, 3-(4-chlorophenyl)propyl 3-piperidinopropyl ether hydrogen oxalate (6) and cyclopropyl 4-(3-piperidinopropyloxy)phenyl methanone hydrogen maleate (7), almost maintained in vitro affinities, pK(i) values of 7.8 and 8.4, respectively, and showed high potency in vivo after p.o. administration (ED(50) values of 1.6 and 0.18 mg/kg, respectively).

Novel histamine H(3)-receptor antagonists and partial agonists with a non-aminergic structure

We determined the affinities of eight novel histamine H(3)-receptor ligands (ethers and carbamates) for H(3)-receptor binding sites and their agonistic/antagonistic effects in two functional H(3)-receptor models. The compounds differ from histamine in that the ethylamine chain is replaced by a propyloxy chain; in the three ethers mentioned below (FUB 335, 373 and 407), R is n-pentyl, 3-methylbutyl and 3,3-dimethylbutyl, respectively. The compounds monophasically inhibited [(3)H]-N(alpha)-methylhistamine binding to mouse cerebral cortex membranes (pK(i) 7.51 - 9.53). The concentration-response curve of histamine for its inhibitory effect on the electrically evoked [(3)H]-noradrenaline overflow from mouse cortex slices was shifted to the right by these compounds (apparent pA(2) 6.61 - 8.00). Only FUB 373 and 407 inhibited the evoked overflow by themselves (intrinsic activities 0.3 and 0.4); these effects were counteracted by the H(3)-receptor antagonist clobenpropit. [(35)S]-GTPgammaS binding to mouse cortex membranes was stimulated by the H(3)-receptor agonist (R)-alpha-methylhistamine in a manner sensitive to clobenpropit. Among the novel compounds only FUB 373 and 407 stimulated [(35)S]-GTPgammaS binding (intrinsic activities 0.6 and 0.4). In conclusion, the novel compounds are partial H(3)-receptor agonists (FUB 373 and 407) or H(3)-receptor antagonists; comparison with FUB 335 shows that the transition from antagonist to agonist is caused by a slight structural change. A protonated N atom in the side chain is not necessary for agonism at H(3) receptors, proposing a receptor-ligand interaction different from that of classical agonists.

New histamine H(3)-receptor ligands of the proxifan series: imoproxifan and other selective antagonists with high oral in vivo potency

Histamine H(3)-receptor antagonists of the proxifan series are described. The novel compounds possess a 4-(3-(phenoxy)propyl)-1H-imidazole structure and various functional groups, e.g., an oxime moiety, on the phenyl ring. Synthesis of the novel compounds and X-ray crystallography of one highly potent oxime derivative, named imoproxifan (4-(3-(1H-imidazol-4-yl)propyloxy)phenylethanone oxime), are described. Most of the title compounds possess high antagonist potency in histamine H(3)-receptor assays in vitro as well as in vivo in mouse CNS following po administration. Structure-activity relationships are discussed. Imoproxifan displays subnanomolar potency on a functional assay on synaptosomes of rat cerebral cortex (K(i) = 0.26 nM). In vivo, imoproxifan increases the central N(tau)-methylhistamine level with an ED(50) of 0.034 mg/kg po. A receptor profile on several functional in vitro assays was determined for imoproxifan, demonstrating high selectivity toward the histamine H(3) receptor for this promising candidate for further development.

(Partial) agonist/antagonist properties of novel diarylalkyl carbamates on histamine H3 receptors

In the search for new ligands of the histamine H3 receptor, novel diarylalkyl carbamates (1-19) were synthesized as derivatives of 3-(1H-imidazol-4-yl)propanol and -ethanol. Carbamates were built up via isocyanates either from corresponding amines by reaction with diphosgene or from related carboxylic acid/diphenylphosphoryl azide and the alcoholic component. Sterically hindered amines were prepared in a two-step reaction sequence from corresponding ketones. Some of the title compounds showed (partial) agonist activity at the histamine H3 receptor in vitro and in vivo. Diphenylmethyl carbamate 2 was identified as a new lead structure (ED50 = 5.3 +/- 2.6 mg/kg po, alpha = 1.0). Aromatic substitution in ortho- or para-positions of 2 led to a loss of agonist activity. meta-Substitution was tolerated to some extent. These effects seemed to be caused by steric rather than electronic properties of the substituents. An investigation of exchange of one or both phenyl rings of 2 by heterocyclic rings led to the highly active and selective thienyl derivative 18 (ED50 3.4 +/- 1.4 mg/kg p.o., alpha = 1.0). These new (partial) agonists of the histamine H3 receptor might serve as pharmacological tools for investigating molecular aspects of the H3 receptor or as possible centrally acting therapeutic agents with oral bioavailability.

Novel partial agonists for the histamine H(3) receptor with high in vitro and in vivo activity

Novel branched N-alkylcarbamates and aliphatic ethers derived from 3-(1H-imidazol-4-yl)propanol were prepared. The compounds were investigated on two functional histamine H(3)-receptor assays. Some compounds displayed partial agonist activity on synaptosomes of rat brain cortex but behaved as pure competitive antagonists on the guinea pig ileum. Under in vivo conditions after po application to mice, some of the compounds showed partial or full agonist activity. Highest in vivo potency was found for the 3,3-dimethylbutyl ether 10 (ED(50) = 0.29 mg/kg, full intrinsic activity). These novel agonists are structurally diverse from classical aminergic histamine H(3)-receptor agonists (e.g., (R)-alpha-methylhistamine, imetit) as they lack a basic moiety in the side chain, which is supposed to be important for the activation of the receptor protein. The selectivity for the histamine H(3) receptor was proven by determination of H(1)- and H(2)-receptor activity on functional assays of the guinea pig.

Characterization of the binding of [3H]-clobenpropit to histamine H3-receptors in guinea-pig cerebral cortex membranes

1 We have investigated the binding of a novel histamine H3-receptor antagonist radioligand, [3H]- clobenpropit ([3H]-VUF9153), to guinea-pig cerebral cortex membranes. 2 Saturation isotherms for [3H]-clobenpropit appeared biphasic. Scatchard plots were curvilinear and Hill plot slopes were significantly less than unity (0.63+/-0.03; n = 12+/-s.e.mean). The radioligand appeared to label two sites in guinea-pig cerebral cortex membranes with apparent affinities (pKD') of 10.91+/-0.12 (Bmax = 5.34+/-0.85 fmol mg(-1) original wet weight) and 9.17+/-0.16 (Bmax = 23.20+/-6.70 fmol mg(-1)). 3 In the presence of metyrapone (3 mM) or sodium chloride (100 mM), [3H]-clobenpropit appeared to label a homogeneous receptor population (Bmax=3.41+/-0.46 fmol mg-1 and 3.49+/-0.44 fmol mg(-1), pKD' = 10.59+/-0.17 and 10.77+/-0.02, respectively). Scatchard plots were linear and Hill slopes were not significantly different from unity (0.91+/-0.04 and 0.99+/-0.02, respectively). Granisetron (1 microM), rilmenidine (3 microM), idazoxan (0.3 microM), pentazocine (3 microM) and 1,3-di-(2-tolyl)guanidine (0.3 microM) had no effect on the binding of [3H]-clobenpropit. 4 The specific binding of [3H]-clobenpropit appeared to reach equilibrium after 25 min at 21+/-3 degrees C and remained constant for >180 min. The estimated pKD' (10.27+/-0.27; n = 3+/-s.e.mean) was not significantly different from that estimated by saturation analysis in the presence of metyrapone. 5 A series of histamine H3-receptor ligands expressed affinity values for sites labelled with [3H]-clobenpropit which were not significantly different from those estimated when [3H]-R-alpha-MH was used to label histamine H3-receptors in guinea-pig cerebral cortex membranes.

Development of chiral N-alkylcarbamates as new leads for potent and selective H3-receptor antagonists: synthesis, capillary electrophoresis, and in vitro and oral in vivo activity

Novel carbamates as derivatives of 3-(1H-imidazol-4-yl)propanol with an N-alkyl chain were prepared as histamine H3-receptor antagonists. Branching of the N-alkyl side chain with methyl groups led to chiral compounds which were synthesized stereospecifically by a Mitsunobu protocol adapted Gabriel synthesis. The optical purity of some of the chiral compounds was determined (ee > 95%) by capillary electrophoresis (CE). The investigated compounds showed pronounced to high antagonist activity (Ki values of 4.1-316 nM) in a functional test for histamine H3 receptors on rat cerebral cortex synaptosomes. Similar H3-receptor antagonist activities were observed in a peripheral model on guinea pig ileum. No stereoselective discrimination for the H3 receptor for the chiral antagonists was found with the in vitro assays. All compounds were also screened for central H3-receptor antagonist activity in vivo in mice after po administration. Most compounds were potent agents of the H3-receptor-mediated enhancement of brain Ntau-methylhistamine levels. The enantiomers of the N-2-heptylcarbamate showed a stereoselective differentiation in their pharmacological effect in vivo (ED50 of 0.39 mg/kg for the (S)-derivative vs 1.5 mg/kg for the (R)-derivative) most probably caused by differences in pharmacokinetic parameters. H1- and H2-receptor activities were determined for some of the novel carbamates, demonstrating that they have a highly selective action at the histamine H3 receptor.

Pharmacological characterization of histamine H3 receptors in human saphenous vein and guinea pig ileum

Studies were performed to assess the functional activity of histamine H3 receptors on neurogenic sympathetic end organ responses in cryopreserved human saphenous vein. (R)-alpha-methylhistamine inhibited electrical field stimulation-evoked contractile responses in a dose dependent manner (pD2 = 8.20). Prazosin (1 microM) and tetrodotoxin (1 microM) blocked the electrical field stimulation-evoked contractile responses in human saphenous vein indicating a sympathetic neural origin of these contractions. The histamine H3 antagonists thioperamide (pA2 = 8.41) and clobenpropit (pA2 = 10.10) produced parallel rightward shifts in the concentration response curve to (R)-alpha-methylhistamine in human saphenous vein and guinea pig ileum (pA2 = 8.59 and 9.83, respectively). Pretreatment with (R)-alpha-methylhistamine (1 microM) did not alter contractions to exogenous norepinephrine in human saphenous vein. In addition, clonidine (pD2 = 10.28) inhibited electrical field stimulation-evoked contractile responses in human saphenous vein which were blocked by yohimbine (30 nM, pA2 = 9.92) but did not alter the (R)-alpha-methylhistamine dose response curve. These results demonstrate the presence of functional presynaptic histamine H3 heteroreceptors on cryopreserved human saphenous vein sympathetic nerves that, upon activation, attenuate electrical field stimulation-evoked contractile responses in this vessel.

N alpha-alkylated derivatives of 2-phenylhistamines: synthesis and in vitro activity of potent histamine H1-receptor agonists

New potent N alpha-alkylated histamine H1-receptor agonists have been prepared and functionally evaluated for partial agonist potency and selectivity. N alpha-Methyl-2-(3-trifluoromethylphenyl)histamine contracts ileal segments and aortic rings of guinea-pig with a relative potency of 174% (95% confid. lim. 161-188%) and 217% (164-287%), respectively (histamine: 100%) and is the most potent H1 receptor agonist described so far.

Histamine H3 receptor desensitization in the guinea-pig ileum

Histamine H3 receptor ligands are usually tested in guinea-pig intestine preparations. A possible desensitization of agonist-induced twitch inhibition was studied in longitudinal muscle-myenteric plexus from ileal segments. A cumulative concentration-response curve for R-alpha-methylhistamine was made; when a second curve was made 30 min afterwards, a marked decrease of pD2 and a more modest decrease of Emax were observed without changes in tissue sensitivity to electrical stimulation or morphine inhibition. At 120 min, pD2 and Emax were not different from those for the first curve. Receptor desensitization seems homologous and reversible and could interfere with repetitive testing of histamine H3 receptor ligands.

H3-receptor antagonists: synthesis and structure-activity relationships of para- and meta-substituted 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles

We report the synthesis, octanol/water partition coefficient (log P), dissociation constants (pKa), H3-receptor affinity (pKi in rat brain membranes, [3H]-N alpha-methylhistamine), and H3-antagonist potency (pA2 in guinea ileum, (R)-alpha-methylhistamine) of novel H3-receptor antagonists obtained by introducing a para or meta substituent on the phenyl ring of the lead compound 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazole (3a). The substituents were chosen to obtain broad and uncorrelated variation in their lipophilic, electronic, and steric properties. The log P values of the neutral species cover almost 3 orders of magnitude (from 1.40 to 4.11). The pKa,2 values (protonation of the 2-thioimidazole fragment) vary from 3.13 to 4.34, indicating that this fragment, which incorporates the so-called polar group common to many H3-receptor antagonists, is neutral at physiological pH. The compounds had pKi values in a range too narrow (from 7.28 to 8.03) to derive QSAR equations. In one case (3g), a biphasic displacement curve was observed (pKi,1 = 8.53; pKi,2 = 6.90). The pA2 values ranged 2 orders of magnitude (from 6.83 to 8.87) and yielded a QSAR model (PLS) indicating that antagonist potency depends parabolically on lipophilicity and is decreased by bulky para substituents. The compounds of this series, therefore, maintain a fair-to-good affinity for rat brain H3-receptor and a fair-to-good H3-antagonist potency on guinea pig ileum, although varying markedly in their lipophilicity. The series thus appears as a good candidate for pharmacokinetic optimization leading to brain-penetrating H3-receptor antagonists.

Novel histamine H3-receptor antagonists with benzyl ether structure or related moieties: synthesis and structure-activity relationships

In search of new histamine H3-receptor ligands sixteen ether derivatives of 3-(1H-imidazol-4-yl)propanol with benzylic partial structure or related moieties were prepared and investigated as H3-receptor antagonists. The new compounds belong to a general construction pattern developed by other histamine H3-receptor antagonists. Structural modifications were introduced in an attempt to optimize in vitro as well as in vivo activity. Structure-activity relationships of the new histamine H3-receptor antagonists are discussed. All ether derivatives showed in vitro activities in the nanomolar concentration range, but only compounds with bulky lipophilic residues were also active under in vivo conditions. The most active compound within this series was 3-(1H-imidazol-4-yl)propyl 1-naphthylmethyl ether (4n) presenting an ED50 of 3.2 +/- 1.9 mg/kg regarding enhancement of endogenous histamine in brain after p.o. administration to mice. Furthermore, comparison of the H3-receptor activities measured on synaptosomes of rat cerebral cortex and on guinea pig ileum gave a good correlation indicating homogeneity of central and peripheral H3-receptor test models. The most interesting compounds were also evaluated in functional in vitro assays with regard to their activities at histamine H1-, H2-, and muscarinic M3-receptors. The tested compounds showed very weak activities at these receptor subtypes demonstrating their H3-receptor selectivity.

Potencies of antagonists chemically related to iodoproxyfan at histamine H3 receptors in mouse brain cortex and guinea-pig ileum: evidence for H3 receptor heterogeneity?

We determined the affinities of 16 newly synthesized H3 receptor antagonists in an H3 receptor binding assay and the potencies of 12 of these compounds at functional H3 receptors in the mouse brain cortex and guinea-pig ileum. The compounds differ from histamine in that the C-C-N side chain is replaced by a chain of the structure C-C-C-O. The two major aims of the study were (1) to investigate whether the two functional H3 receptors are pharmacologically different and (2) to derive structure-activity relationships. The specific binding of 3H-Na-methylhistamine to rat brain cortex membranes was monophasically displaced by each of the 16 compounds at pKi values ranging from 7.30 to 9.48. In superfused mouse brain cortex slices preincubated with 3H-noradrenaline, the electrically evoked tritium overflow was slightly decreased by iodoproxyfan and its deiodo analogue; this effect was counteracted by the H3 receptor antagonist clobenpropit. The other compounds did not affect the evoked tritium overflow by themselves. The concentration-response curve of histamine for its inhibitory effect on the electrically evoked tritium overflow was shifted to the right by the 12 compounds with apparent pA2 values ranging from 7.02 to 9.00. The 12 compounds also shifted to the right the concentration-response curve of R-a-methylhistamine for its inhibitory effect on the electrically induced contraction in guinea-pig ileum strips; the apparent pA2 values ranged from 5.97 to 9.00. Iodoproxyfan decreased the electrically induced contraction by itself and this effect was counteracted by the H3 receptor antagonist thioperamide. The apparent pA2 values in the two functional H3 receptor models showed a highly significant correlation (r = 0.882; P < 0.001). Highly significant correlations were also obtained when the pKi values of the compounds in the binding assay were compared to their apparent pA2 values in the mouse brain (r = 0.799; P < 0.004) and in the guinea-pig ileum (r = 0.851; P < 0.001). In each of the three experimental models, iodoproxyfan was the most potent compound; its deiodo analogue was less potent by more than 1.1 log units. The present results show that the compounds under study possess moderate to high affinity and/or (partial) H3 receptor antagonist potency. The two functional H3 receptors in the mouse brain cortex and the guinea-pig ileum may be slightly different; further studies are necessary to clarify whether this difference is due to H3 receptor heterogeneity, species variants or differences in the efficiency of receptor coupling. The marked difference in the affinity/potency between iodoproxyfan and its deiodo analogue may suggest that a highly lipophilic residue in that part of the molecule favours a high affinity/antagonistic potency at H3 receptors.

[125I]iodoproxyfan and related compounds: a reversible radioligand and novel classes of antagonists with high affinity and selectivity for the histamine H3 receptor

The synthesis and biological evaluation of new histamine H3 receptor antagonists with an iodinated aryl partial structure are described as part of an extensive research program to find model compounds for the development of a new radioligand with high H3 receptor affinity and specific activity. All compounds were tested for their H3 receptor antagonist activity in a [3H]-histamine-release assay with synaptosomes from rat cerebral cortex. The new leads with potent H3 receptor antagonist activity belong to a series of derivatives of 3-(1H-imidazol-4-yl)propanol with carbamate (4-7), ester (8-16), and ether (17-22) as functional groups. Structure-activity relationships are discussed. The most active compound in the functional test (-log Ki = 8.3) and in binding studies with [3H]-(R)-alpha-methylhistamine on rat cerebral cortex (-log Ki = 9.0) in vitro was 3-(1H-imidazol-4-yl)propyl (4-iodophenyl)methyl ether (iodoproxyfan, 19) exhibiting no central H3 receptor antagonist activity in vivo. The potency of iodoproxyfan is more than 300 times lower at H1, H2, alpha1, alpha2, beta1, 5-HT2A, 5-HT3, and M3 receptors than at histamine H3 receptors. Because of the high potency and selectivity of 19, this compound has also been prepared in the [125I]-iodinated form by a nucleophilic halogen exchange reaction using the corresponding bromo derivative 22 as a precursor. The newly prepared [125I]iodoproxyfan (23) possesses advantageous pharmacological properties and fulfills all criteria of a useful radioligand.

Novel carbamates as potent histamine H3 receptor antagonists with high in vitro and oral in vivo activity

The known histamine H3 receptor antagonists burimamide, thioperamide, clobenpropit, and a related homohistamine thioamide derivative were taken as templates in search for new leads. Novel histamine H3 receptor antagonists structurally described as carbamate derivatives of 3-(1H-imidazol-4-yl)propanol were prepared in high yields by treatment of the alcohol with corresponding isocyanates or carbamoyl chlorides and investigated for their H3 receptor antagonist activity. Different chain lengths and various substituents possessing different electronic and steric parameters were introduced and structure-activity relationships established. In different functional tests, the new antagonists showed high H3 receptor antagonist potencies in vitro (-log Ki values of 6.4-8.4) at synaptosomes of rat cerebral cortex and low activities at histamine H1 and H2 receptor subtypes. They were also screened for their central in vivo activity in mice after peroral administration. The most promising compounds (2, 16, 19) showed ED(50) values of about 1-2 mg/kg and thus are potential drugs for the therapy of H3 receptor dependent diseases. Some of the novel carbamate derivatives are H3 receptor selective compounds with high in vitro and in vivo activity.

Pharmacological characterization of histamine H3 receptors in isolated guinea pig pulmonary artery and ileum

We characterized the histamine H3 receptors involved in the modulation of electrical field stimulated neurogenic contraction of guinea pig pulmonary artery sympathetic, and guinea pig ileum parasympathetic preparations. Simultaneous measures of electrical field stimulation-evoked 3H overflow and tension in [3H]norepinephrine-loaded pulmonary artery were sensitive to tetrodotoxin (300 nM) and insensitive to hexamethonium (100 microM). Only the contractile response was inhibited by prazosin (100 nM). (R)-alpha-Methylhistamine's inhibition of the pulmonary artery contraction and 3H overflow were dose-dependently antagonized by thioperamide (30-100 nM). (R)-alpha-Methylhistamine also inhibited the neurogenic contractions of the isolated ileum (pD2 = 8.2). In the pulmonary artery, the relative potency of the histamine H3 receptor antagonists vs. (R)-alpha-methylhistamine inhibition of neurogenic contractions (pD2 = 7.1) was thioperamide (pA2 = 8.6 +/- 0.1) > burimamide (pA2 = 7.6 +/- 0.2) > impromidine (pA2 = 6.9 +/- 0.02). Similarly, the relative potency of histamine H3 receptor antagonists in the isolated ileum was thioperamide > burimamide > or = impromidine, with pA2 estimates of 8.7 +/- 0.1, 7.3 +/- 0.1 and 7.1 +/- 0.1, respectively. Antagonist potencies suggest a predominant histamine H3A-like receptor population on postganglionic sympathetic neurons innervating the pulmonary artery and parasympathetic neurons innervating the ileum longitudinal muscle.

An update on histamine H3 receptors and gastrointestinal functions

The distribution and functions of histamine H3 receptors in the gastrointestinal tract is reviewed with particular reference to the effects on gastric acid secretion, mucosal protection, and intestinal motility. Histamine H3 receptor activation has negative effects on acid secretion induced by indirect secretagogues in cats, dogs, and rabbits; less clear effects were found in rats. An inhibitory effect on histamine release induced by different stimuli was observed in rats, rabbits, and dogs after H3 receptor agonists, thus supporting the idea that H3 receptors occur in ECL cells. (R)-alpha-methylhistamine has a marked protective effect against gastric lesions induced by ethanol in rats, being slightly less effective against aspirin and stress. H3 receptor activation decreases the intestinal motility induced by electrical stimulation in a variety of gut preparations, reducing both cholinergic and NANC neurotransmitter release. In this tissue the inhibitory effects mediated by histamine H3 receptors seem to be coupled, via a G protein, to a restriction of Ca2+ access into the nerve terminal; other mechanisms, however, have been suggested in the gastric mucosa. Histamine H3 receptors have already been subdivided into two receptor subtypes, H3A and H3B, the former being the subtype predominant in the gastrointestinal tissue. The increasing availability of selective agonists and antagonists of H3 receptors will unravel possible novel actions and physiological roles of histamine.

Intermediate affinity and potency of clozapine and low affinity of other neuroleptics and of antidepressants at H3 receptors

It was the aim of the present study to determine the affinities of four neuroleptics and five antidepressants for histamine H3 receptors. In rat brain cortex membranes, the specifically bound [3H]-N alpha-methylhistamine was monophasically displaced by clozapine (pKi 6.15). The other drugs did not completely displace the radioligand even at 100 microM; the pKi values were: haloperidol (4.91); sulpiride (4.73); amitriptyline (4.56); desipramine (4.15); levomepromazine (4.14); fluovoxamine (4.13); maprotiline (4.09); moclobemide (< 4.0). The effect of clozapine was further examined in a functional H3 receptor model, i.e., in superfused mouse brain cortex slices preincubated with [3H]-noradrenaline. The electrically evoked tritium overflow was not affected by clozapine 0.5-32 microM. However, clozapine shifted the concentration-response curve of histamine for its inhibitory effect on the evoked overflow to the right, but did not affect the maximum effect of histamine. The Schild plot yielded a pA2 value of 6.33. In conclusion, clozapine shows an intermediate affinity and potency (as a competitive antagonist) at H3 receptors. The Ki value of clozapine at H3 receptors resembles its Ki value at D2 receptors (the target of the classical neuroleptics), but is higher than its Ki values at D4, 5-HT2 or muscarinic acetylcholine receptors, which according to current hypotheses, might be involved in the atypical profile of clozapine.