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.     

Synthesis and antihypertensive activity of pyrimidin-4(3H)-one derivatives as losartan analogue for new angiotensin II receptor type 1 (AT1) antagonists

The discovery, in vitro and in vivo studies of the highly potent AT(1) antagonist 12a (BR-A-657, Fimasartan) antagonists are presented. A series of pyrimidin-4(3H)-one derivatives as losartan analogue were synthesized and evaluated for a novel class of AT(1) receptor antagonists. Among them, 12a containing thioamido moiety displayed both high in vitro functional antagonism and binding affinity [IC(50)=0.42 and 0.13 nM, respectively] and inhibited strongly in vivo AngII-induced pressor response in pithed rats with an ED(50) of 0.018 mg/kg. Moreover, in vivo evaluation in furosemide-treated rat and conscious renal hypertensive rat models and the pharmacokinetic study showed that 12a is a highly potent and orally active AT(1) selective antagonist having stronger in vivo potency than losartan.     

Are the receptors of 1,25-dihydroxyvitamin D3 vitamin K dependent?

Alimentary deficiency of vitamin K in rats causes a decrease in the level of in vivo occupied nuclear 1,25 (OH)2D3 receptors in small intestinal mucosa and an 2-2.5-fold increase in the ability of cytosolic 1,25 (OH)2D3-receptor complexes to bind to heterologous DNA. The 1,25 (OH)2D3 binding by the receptors is thereby unaffected. Preincubation of kidney and intestinal cytosol of rats with the secondary K-avitaminosis induced by vitamin K antagonist with the microsomal vitamin K-dependent gamma-carboxylation system sharply decreases the binding of the 1.25 (OH)2D3-receptor complexes to DNA. In rats treated with the vitamin K antagonist in combination with a low calcium diet, the subsequent maintenance on a high calcium diet does not cause, in contrast with vitamin K-repleted animals, a sharp decrease of the level of the in vivo occupied 1,25 (OH)2D3 receptors. In vitro Ca2+ cations decrease the binding of the 1,25 (OH)2D3-receptor complexes to DNA only in vitamin K-repleted rats (ED50 = 2.5 x 10(-6) M). The existence of a vitamin K-dependent Ca-sensitive mechanism regulating the binding of the 1,25 (OH)2D3 receptor to DNA has been postulated for the first time.     

In vitro activation and DNA binding affinity of human lymphoid (CEM-C7) cytoplasmic receptors labeled with the antiglucocorticoid RU 38486

The data reported here demonstrate that the synthetic steroid RU 38486 functions as an optimal antagonist in the glucocorticoid-sensitive human leukemic cell line CEM-C7. This steroid blocks the ability of the potent agonist triamcinolone acetonide (TA) to induce glutamine synthetase activity and to ultimately cause cell lysis, but when given alone does not exhibit partial agonist activity. Both [3H]RU 38486 and [3H]TA bind with high affinity and specificity to cytosolic glucocorticoid receptors in this cell line. However, under a variety of in vitro conditions (elevated temperature and presence of exogenous ATP), [3H]TA promotes receptor activation more effectively than [3H]RU 38486. This difference in the extent of activation was verified by two independent techniques: DEAE-cellulose chromatography and DNA-cellulose binding. [3H]RU 38486 and [3H]TA dissociate at the same rate from the unactivated receptors but at 25 degrees C (not 0 degree C) [3H]RU 38486 dissociates slightly more rapidly from the activated receptors. The defective receptors in the glucocorticoid-resistant subclone 3R7 appear to be "activation labile" (rapid dissociation of ligand from activated form) using either tritiated steroid. Once activated in vivo, the CEM-C7 [3H]TA- and [3H]RU 38486-receptor complexes undergo similar nuclear translocation and those activated complexes generated in vitro appear to bind to nonspecific DNA-cellulose with the same relative affinities. Thus the precise mechanism(s) by which RU 38486 exerts its potent antiglucocorticoid effect in this human cell line cannot be easily explained in terms of a defect in one of the crucial steps (specific high affinity binding, activation, translocation, DNA binding) required to elicit a physiological response. However, the data presented here do suggest that when comparing an antagonist and agonist which both bind to receptors with the same relative high affinity, the agonist may be more effective in facilitating the conformational change associated with in vitro activation.     

In vitro and in vivo comparisons of antiandrogenic potencies of two histamine H2-receptor antagonists, cimetidine and etintidine--HCl

The antiandrogenic potency of cimetidine was compared to that of a new histamine H2-receptor antagonist, etintidine-HCl (ORF 16753-02). Although both compounds displaced [3H]dihydrotestosterone from androgen receptors in vitro and inhibited androgen-stimulated growth of the accessory sex organs of male rats in vivo, etintidine-HCl was significantly less antiandrogenic than cimetidine in the analysis of androgen receptor binding and in the inhibition of seminal vesicle weights. Because etintidine-HCl has been shown previously to have more potent gastric antisecretory activity than cimetidine, its lower antiandrogenic activity suggests that etintidine-HCl has a much wider therapeutic ratio and that its use clinically will result in fewer antiandrogenic side effects.     

Minimal role for H1 and H2 histamine receptors in cutaneous thermal hyperemia to local heating in humans.

The precise mechanism(s) underlying the thermal hyperemic response to local heating of human skin are not fully understood. The purpose of this study was to investigate a potential role for H1 and H2 histamine-receptor activation in this response. Two groups of six subjects participated in two separate protocols and were instrumented with three microdialysis fibers on the ventral forearm. In both protocols, sites were randomly assigned to receive one of three treatments. In protocol 1, sites received 1) 500 microM pyrilamine maleate (H1-receptor antagonist), 2) 10 mM L-NAME to inhibit nitric oxide synthase, and 3) 500 microM pyrilamine with 10 mM NG-nitro-L-arginine methyl ester (L-NAME). In protocol 2, sites received 1) 2 mM cimetidine (H2 antagonist), 2) 10 mM L-NAME, and 3) 2 mM cimetidine with 10 mM L-NAME. A fourth site served as a control site (no microdialysis fiber). Skin sites were locally heated from a baseline of 33 to 42 degrees C at a rate of 0.5 degrees C/5 s, and skin blood flow was monitored using laser-Doppler flowmetry (LDF). Cutaneous vascular conductance was calculated as LDF/mean arterial pressure. To normalize skin blood flow to maximal vasodilation, microdialysis sites were perfused with 28 mM sodium nitroprusside, and control sites were heated to 43 degrees C. In both H1 and H2 antagonist studies, no differences in initial peak or secondary plateau phase were observed between control and histamine-receptor antagonist only sites or between L-NAME and L-NAME with histamine receptor antagonist. There were no differences in nadir response between L-NAME and L-NAME with histamine-receptor antagonist. However, the nadir response in H1 antagonist sites was significantly reduced compared with control sites, but there was no effect of H2 antagonist on the nadir response. These data suggest only a modest role for H1-receptor activation in the cutaneous response to local heating as evidenced by a diminished nadir response and no role for H2-receptor activation.     

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.     

Design, synthesis and pharmacological screening of a series of N1-(substituted) aryl-5,7-dimethyl-2-(substituted)pyrido(2,3-d)-pyrimidin-4(3H)-ones as potential histamine H1-receptor antagonists

A series of N1-(substituted)aryl-5,7-dimethyl-2-(substituted)pyrido(2,3-d)pyrimidin-4(3H)-one was designed on the basis of the triangular pharmacophoric requirement of histamine H1-receptor antagonists. The designed series was synthesized by cyclo-condensation of monoaryl thiourea with ethyl cyanoacetate in the presence of dry HCl gas to give N1-(substituted aryl)-2-mercaptopyrimidine-4(3H)-one, which on cyclo-condensation with acetylacetone gave the pyridopyrimidinone. Further methylation of the mercapto group at C-2 with methyl iodide followed by nucleophilic displacement of the methylmercapto group by various amines gave the targeted compounds. All the synthesized compounds were screened for histamine H1-receptor antagonistic activity by the in vitro method of inhibition of the isotonic contraction induced by histamine on isolated guinea pig ileum using cetirizine as a standard drug. All the compounds exhibited potent histamine H1-receptor antagonistic activity with pA2 values from 7.30- 9.75 (cetirizine, pA2 value 9.40). The potent compounds were screened for their in vivo antihistaminic activity by protection of animal from asphyxic shock. The sedative potential of potent compounds was checked on albino mice by photoactometer and they had comparative sedative potential to the standard drug cetirizine. None of the compound exhibited anticholinergic activity in the in vitro rat ileum model.     

In vivo activities of peptide and pseudo-peptide analogs of the C-terminal octapeptide of cholecystokinin on pancreatic secretion in the rat

Most studies measuring the agonist and antagonist activities of CCK analogs and derivatives on the exocrine pancreas have been done with in vitro models. However, extrapolation to the in vivo situation may be sometimes hazardous, due to the catabolism of the peptides by circulating and tissue peptidases, and to their eventual interaction with various endogenous factors. The present experiments were organized to measure the efficacy and potency on pancreatic secretion of the rat in vivo of a series of CCK 8 analogs whose binding and activity had been previously measured on guinea-pig and rat isolated acini. The molecules tested were derivatives of Boc-(Nle 28-Nle 31)-CCK 26–33 (1), and comprised 2-phenylethylester derivatives, des-Phe derivatives, and a series of pseudo-peptides with a “reduced” bond CH2-NH replacing the peptide bond in position 28–29 to 32–33. They were perfused in anaesthetized rats, and the outputs of sodium, bicarbonate and total protein were measured. All of the derivatives studied had in vivo the same efficacy as (1) on the output of protein, and were 10 to 500 times less potent. For most compounds, the relative order of potencies measured in vivo was similar to that measured in vitro on amylase secretion by rat acini. However, the derivatives with reduced bonds in positions 28–29 and 29–30 were respectively 3 and 2 times less potent in vivo, relative to (1), while derivatives with reduced bonds in positions 30–31, 31–32 and 32–33 were 1.5 to 2.5 times more potent in vivo. The 2-phenylethylester derivatives were 7 and 9 times as potent in vitro as in vivo. The des-Phe derivative, which had in vitro antagonist properties on guinea-pig acini, and acted like a partial agonist on rat acini, was in vivo a complete agonist and was relatively 300 times as potent in vivo as in vitro. These results indicate that the metabolism of the peptides and/or their interaction with endogenous factors may change appreciably the effect of CCK derivatives on pancreatic secretion of the rat in vivo.     

Regulation of Histamine Release and Synthesis in the Brain by Muscarinic Receptors

The cholinergic modulation of histamine release and synthesis was studied in rat brain slices or synaptosomes labeled with L-[3H]histidine. Carbachol in increasing concentrations progressively reduced the K+-induced [3H]histamine release from cortical slices. Pirenzepine, a preferential M1-receptor antagonist, reversed the carbachol effect in an apparently competitive manner and with Ki values of 1-6 X 10(-8) M. 11-[(2-[(Diethylamino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116), considered a preferential M2-receptor antagonist, reversed the carbachol effect with a mean Ki of approximately 2 X 10(-7) M. Oxotremorine behaved as a partial agonist in the modulation of histamine release. Neostigmine, an acetylcholinesterase inhibitor, inhibited the K+-induced release of [3H]histamine from cortical slices, and the effect was largely reversed by pirenzepine, an observation suggesting a modulation by endogenous acetylcholine. The effects of carbachol and pirenzepine were observed with slices of other brain regions known to contain histaminergic nerve terminals or perikarya, as well as with cortical synaptosomes. The two drugs also modified, in opposite directions, [3H]histamine formation in depolarized cortical slices. In vivo oxotremorine inhibited [3H]histamine formation in cerebral cortex, and this effect was reversed by scopolamine. When administered alone, scopolamine failed to enhance significantly the 3H- labeled amine formation, a finding suggesting that muscarinic receptors are not activated by endogenous acetylcholine released under basal conditions. It is concluded that muscarinic heteroreceptors, directly located on histaminergic nerve terminals, control release and synthesis of histamine in the brain. These receptors apparently belong to the broad M1-receptor category and may correspond to a receptor subclass displaying a rather high affinity for AF-DX 116.     

Activation of histamine H3 receptors inhibits renal noradrenergic neurotransmission in anesthetized dogs

To investigate the possible involvement of histamine H(3) receptors in renal noradrenergic neurotransmission, effects of (R)alpha-methylhistamine (R-HA), a selective H3-receptor agonist, and thioperamide (Thiop), a selective H3-receptor antagonist, on renal nerve stimulation (RNS)-induced changes in renal function and norepinephrine (NE) overflow in anesthetized dogs were examined. RNS (0.5-2.0 Hz) produced significant decreases in urine flow and urinary sodium excretion and increases in NE overflow rate (NEOR), without affecting renal hemodynamics. When R-HA (1 microg x kg(-1) x min(-1)) was infused intravenously, mean arterial pressure and heart rate were significantly decreased, and there was a tendency to reduce basal values of urine flow and urinary sodium excretion. During R-HA infusion, RNS-induced antidiuretic action and increases in NEOR were markedly attenuated. Thiop infusion (5 microg x kg(-1) x min(-1)) did not affect basal hemodynamic and excretory parameters. Thiop infusion caused RNS-induced antidiuretic action and increases in NEOR similar to the basal condition. When R-HA was administered concomitantly with Thiop infusion, R-HA failed to attenuate the RNS-induced antidiuretic action and increases in NEOR. However, in the presence of pyrilamine (a selective H1-receptor antagonist) or cimetidine (a selective H2-receptor antagonist) infusion, R-HA attenuated the RNS-induced actions, similarly to the case without these antagonists. Thus functional histamine H3 receptors, possibly located on renal noradrenergic nerve endings, may play the role of inhibitory modulators of renal noradrenergic neurotransmission.     

N-glycosylation plays a role in biosynthesis and internalization of the adenylate cyclase stimulating vasopressin V2-receptor of LLC-PK1 renal epithelial cells: an effect of concanavalin A on binding and expression.

The role of N-glycosylation in the function and biosynthesis of the vasopressin V2-receptor in LLC-PK1 renal epithelial cells was examined using various lectins and inhibitors operating at different steps of the glycosidic pathway. Tunicamycin, which blocks all N-glycosylation, and castanospermine, which inhibits glycosidase I and hence blocks formation of high-mannose-type N-glycosylated intermediates, resembled one another in affecting V2-receptor biosynthesis and internalization in a concentration-dependent manner. In contrast, swainsonine, an inhibitor of mannosidase II and hence of complex-type oligosaccharide formation, had no effect. Interestingly, the alpha-D-mannose/alpha-D-glucose-specific lectin concanavalin A, (Con A), in contrast to the beta-D-galactose-specific lectin ricin, had a marked effect on the V2-receptor in LLC-PK1 cells, increasing both receptor numbers up to twofold in vivo and specific [3H]AVP binding up to 50% in vitro in a concentration-dependent manner. The concentrations inducing half-maximal response were about 0.2 and 20 micrograms/ml for the in vivo and in vitro responses, respectively, implying distinct effects on V2-expression and ligand binding. That the in vitro effect on binding was due to a direct effect on the V2-receptor could be shown by the lack of a Con A effect on [3H]AVP binding in membranes prepared from LLC-PK1 cells down-regulated for the V2-receptor or from cells of the LLC-PK1 V2-receptor deficient mutant M18. All results were consistent with a functional role for N-glycosylation of the V2-receptor in LLC-PK1 cells.     

Discovery of 1,1-dioxo-1,2,6-thiadiazine-5-carboxamide derivatives as cannabinoid-like molecules with agonist and antagonist activity

A series of new 2-substituted 1,1-dioxo-1,2,6-thiadiazine-5-carboxylate derivatives have been prepared from monosubstituted sulfamides in order to obtain N-substituted 1,1-dioxo-1,2,6-thiadiazine-5-carboxamides as novel cannabinoid derivatives, analogues of Rimonabant (SR141716A). Their potential functional activity on cannabinoid receptors has been evaluated in vitro and in vivo in mice, showing that two compounds (37 and 39) behave as cannabinoid agonists in vitro. Their potency is lower than that of the reference compound, WIN 55,212-2, but their efficacy is similar to that of this cannabinoid agonist, although no in vivo activity is observed. Another derivative (38) behaves as a cannabinoid antagonist both in vitro and in vivo, being its efficacy and potency similar to that of the well-known antagonist SR141716A.     

Direct effects of serotonin, and 5-HT2, alpha 1 and H1 receptor-antagonists on embryonic chick skin in vitro: a morphological and functional study

Ketanserin (K), a 5-HT2, alpha 1 and H1 receptor antagonist, at a concentration of 5 micrograms/ml inhibits keratinisation, but increases DNA-, collagen and glycosaminoglycan (GAG) synthesis in embryonic chick skin in vitro. On the other hand, serotonin (S) at a concentration of 10 micrograms/ml stimulates keratinisation and does not enhance DNA, collagen and GAG synthesis. 5 nM doses of ketanserin and serotonin give analogous results. The effects of other 5-HT2-receptor antagonists (e.g. BW 501 and cinanserin) are comparable to those of K, whereas prazosin (an alpha 1 receptor antagonist) and pyrilamin (an H1 receptor antagonist) do not show any effect on embryonic chick skin in vitro.     

Synthesis and stability in biological media of 1H-imidazole-1-carboxylates of ROS203, an antagonist of the histamine H3 receptor

A series of carbamate derivatives of the H(3) antagonist ROS203 (1) were prepared, and their lipophilicity and steric hindrance were modulated by introducing linear or branched alkyl chains of various lengths. In vitro stability studies were conducted to evaluate how structural modulations affect the intrinsic reactivity of the carbamoyl moiety and its recognition by metabolic enzymes. Linear alkyl carbamates were the most susceptible to enzymatic hydrolysis, with bioconversion rates being higher in rat liver and plasma. Chain ramification significantly enhanced the enzymatic stability of the set, with two derivatives (1g and 1h) being more stable by a factor of 8-40 than the ethyl carbamate 1a. Incubation with bovine serum albumin (BSA) showed a protective role of proteins on chemical and porcine-liver esterase (PLE)-catalyzed hydrolysis. Ex vivo binding data after i.v. administration of 1h revealed prolonged displacement of the labeled ligand [(3)H]-(R)-alpha-methylhistamine ([(3)H]RAMHA) from rat-brain cortical membranes, when compared to 1. However, the high rates of bioconversion in liver, as well as the chemical instability of 1h, suggest that further work is needed to optimize the enzymatic and chemical stability of these compounds.     

Radiofluorinated histamine H₃ receptor antagonist as a potential probe for in vivo PET imaging: radiosynthesis and pharmacological evaluation

The histamine H(3) receptor (H(3)R) plays a role in cognition and memory processes and is implicated in different neurological disorders, including Alzheimer's disease, schizophrenia, and narcolepsy. In vivo studies of the H(3)R occupancy using a radiolabeled PET tracer would be very useful for CNS drug discovery and development. We report here the radiosynthesis, in vitro and in vivo evaluation of a novel (18)F-labeled high-affinity H(3)R antagonist (18)F-ST889. The radiosynthesis was accomplished via nucleophilic substitution of the mesylate leaving group with a radiochemical yield of 8-20%, radiochemical purity >99%, and specific radioactivity > 65 GBq/μmol. (18)F-ST889 exhibited high in vivo stability and rather low lipophilicity (logD(7.4)=0.35 ± 0.09). In vitro autoradiography showed specific binding in H(3)R-rich brain regions such as striatum and cortex. However, in vivo PET imaging of the rat brain with (18)F-ST889 was not successful. Possible reasons are discussed.     

Histamine Affects Release and Biosynthesis of Opioid Peptides Primarily via H1-Receptors in Bovine Chromaffin Cells

Histamine is a potent secretagogue for opioid pentapeptides (Met- and Leu-enkephalin) in adrenal chromaffin cells in vitro. This effect is dependent on extracellular Ca2+ and is reduced by Ca2+ channel blockers such as Co2+, D 600, and nifedipine. Moreover, histamine also produced a profound compensatory increase in cellular peptide content after 48 h of exposure, most likely caused by a four- to fivefold increase in the mRNA levels coding for the proenkephalin A precursor. All the histamine-induced effects (acute release, changes in peptide cell content, proenkephalin A mRNA levels) are antagonized by the H1-receptor antagonist, clemastine, whereas the H2-receptor antagonists, ranitidine and cimetidine, were less effective (approximately 20% inhibition).     

New findings on nuclear gangliosides: overview on metabolism and function

Dopamine (DA) is an important transmitter in both motor and limbic pathways. We sought to investigate the role of D(1)-receptor activation in axonal DA release regulation in dorsal striatum using a D(1)-receptor antagonist, SKF-83566. Evoked DA release was monitored in rat striatal slices using fast-scan cyclic voltammetry. SKF-83566 caused a concentration-dependent increase in peak single-pulse evoked extracellular DA concentration, with a maximum increase of ～ 65% in 5 μM SKF-83566. This was accompanied by a concentration-dependent increase in extracellular DA concentration clearance time. Both effects were occluded by nomifensine (1 μM), a dopamine transporter (DAT) inhibitor, suggesting that SKF-83566 acted via the DAT. We tested this by examining [(3)H]DA uptake into LLc-PK cells expressing rat DAT, and confirmed that SKF-83566 is a competitive DAT inhibitor with an IC(50) of 5.7 μM. Binding studies with [(3)H]CFT, a cocaine analog, showed even more potent action of SKF-83566 at the DAT cocaine binding site (IC(50) = 0.51 μM). Thus, data obtained using SKF-83566 as a D(1) DA-receptor antagonist may be confounded by concurrent DAT inhibition. More positively, however, SKF-83566 might be a candidate to attenuate cocaine effects in vivo because of the greater potency of this drug at the cocaine versus DA binding site of the DAT.     

Thioperamide, a selective histamine H3 receptor antagonist, protects against PTZ-induced seizures in mice

The effect of selective histamine H3-receptor antagonist thioperamide was studied on PTZ-induced seizures in mice. Thioperamide significantly protected clonic seizures induced by PTZ in a dose-dependent manner. The effect of thioperamide was completely countered by pretreatment with R (alpha)-methylhistamine (RAMH), a selective H3-receptor agonist suggesting that the observed effect of thioperamide was elicited by histamine H3-receptors. RAMH alone did not significantly modify PTZ seizures. The findings are consistent with a role for the histaminergic neuronal system in seizures and suggest that H3-receptors may play an important role in modulating clonic seizures induced by PTZ in mice.     

Brain pharmacokinetics of non-imidazole biphenyl H3 receptor antagonists: a liquid chromatography/electrospray-mass spectrometry and ex vivo binding study in rats

In the present article, we report on the kinetics of brain penetration in rats of the H3R antagonist 1,1'-[1,1'-biphenyl-4,4'-diylbis(methylene)]bis-[piperidine] (1), which had shown a favorable in vitro pharmacological profile and in vivo potency in preventing scopolamine-induced amnesia. Two different approaches were employed: high-performance liquid chromatography/electrospray-mass spectrometry (HPLC/ESI-MS) and ex vivo binding against the labeled agonist [(3)H]-(R)-α-methylhistamine ([(3)H]RAMHA). Starting from the structure of 1, the rigid piperidine ring was replaced by a flexible dipropylamino group (see 2) or by a morpholino ring (see 3), endowed with lower basicity. The effect of replacement on rat plasma and brain disposition in the 24 h after administration was analyzed. High (μM) and persistent concentrations of 1 were found in rat plasma, while plasma levels were significantly lower (range: 0-200 nM) for the other two derivatives. This could be explained, among other factors, by the higher stability, observed for 1, to liver metabolic cleavage. The applied chemical modulation had an important effect on in vivo brain disposition, as, despite the comparable physico-chemical properties, 2 did not show the tendency to accumulate within the brain, as stated by its brain vs. plasma concentration ratios, if compared to 1. These structure?property relationships should be taken into account in the pharmacokinetic optimization of new series of H3 receptor antagonists.