Human Muscarinic Receptor Subtypes Research Articles

Antihistamines H1 as Potential Anthelmintic Agents against the Zoonotic Parasite Angiostrongylus cantonensis.

Infections caused by parasitic helminths pose significant health concerns for both humans and animals. The limited efficacy of existing drugs underscores the urgent need for novel anthelmintic agents. Given the reported potential of antihistamines against various parasites, including worms, this study conducted a screening of clinically available antihistamines against Angiostrongylus cantonensis-a nematode with widespread implications for vertebrate hosts, including humans. Twenty-one anti-H1 antihistamines were screened against first-stage larvae (L1) of A. cantonensis obtained from the feces of infected rats. Standard anthelmintic drugs ivermectin and albendazole were employed for comparative analysis. The findings revealed four active compounds (promethazine, cinnarizine, desloratadine, and rupatadine), with promethazine demonstrating the highest potency (EC50 = 31.6 μM). Additionally, morphological analysis showed that antihistamines induced significant changes in larvae. To understand the mechanism of action, antimuscarinic activities were reported based on average pK i values for human muscarinic receptor (mAChR) subtypes of the evaluated compounds. Furthermore, an analysis of the physicochemical and pharmacodynamic properties of antihistamines revealed that their anthelmintic activity does not correlate with their activity at H1 receptors. This study marks the first documentation of antihistamines' activity against A. cantonensis, offering a valuable contribution to the quest for novel agents effective against zoonotic helminths.

3-iodothyronamine (T1AM), a novel antagonist of muscarinic receptors

3-iodothyronamine (T1AM) is a trace amine suspected to derive from thyroid hormone metabolism. T1AM was described as a ligand of G-protein coupled monoaminergic receptors, including trace amine associated receptors, suggesting the amine may exert a modulatory role on the monoaminergic transmission. Nothing is known on the possibility that T1AM could also modulate the cholinergic transmission interacting with muscarinic receptors.We evaluated whether T1AM (10nM–100μM) was able to i) displace [3H]-NMS (0.20nM) binding to membrane preparations from CHO cells stably transfected with human muscarinic receptor subtypes (M1-M5); ii) modify basal or acetylcholine induced pERK1/2 levels in CHO expressing the human muscarinic type 3 receptor subtype by Western blot iii) modify basal and carbachol-induced contraction of isolated rat urinary bladder. T1AM fitting within rat muscarinic type 3 receptor was simulated by Docking studies.T1AM recognized all muscarinic receptor subtypes (pKi values in the micromolar range). Interacting at type 3, T1AM reduced acetylcholine-increased pERK1/2 levels. T1AM reduced carbachol-induced contraction of the rat urinary bladder. The fenoxyl residue and the iodide ion were found essential for establishing contacts with the active site of the rat muscarinic type 3 receptor subtype.Our results indicate that T1AM binds at muscarinic receptors behaving as a weak, not selective, antagonist. This finding adds knowledge on the pharmacodynamics features of T1AM and it may prompt investigation on novel pharmacological effects of T1AM at conditions of hyper-activation of the muscarinic tone including the overactive urinary bladder.

Discovery and SAR of muscarinic receptor subtype 1 (M1) allosteric activators from a molecular libraries high throughput screen. Part 1: 2,5-Dibenzyl-2H-pyrazolo[4,3-c]quinolin-3(5H)-ones as positive allosteric modulators

Results from a 2012 high-throughput screen of the NIH Molecular Libraries Small Molecule Repository (MLSMR) against the human muscarinic receptor subtype 1 (M1) for positive allosteric modulators is reported. A content-rich screen utilizing an intracellular calcium mobilization triple-addition protocol allowed for assessment of all three modes of pharmacology at M1, including agonist, positive allosteric modulator, and antagonist activities in a single screening platform. We disclose a dibenzyl-2H-pyrazolo[4,3-c]quinolin-3(5H)-one hit (DBPQ, CID 915409) and examine N-benzyl pharmacophore/SAR relationships versus previously reported quinolin-3(5H)-ones and isatins, including ML137. SAR and consideration of recently reported crystal structures, homology modeling, and structure–function relationships using point mutations suggests a shared binding mode orientation at the putative common allosteric binding site directed by the pendant N-benzyl substructure.

Competition radioligand binding assays for the investigation of bispyridinium compound affinities to the human muscarinic acetylcholine receptor subtype 5 (hM5)

Standard treatment of poisoning by organophosphorus (OP) nerve agents with atropine and oximes lacks efficacy with some nerve agents. Promising in vitro and in vivo results were obtained with the bispyridinium compound SAD-128 which was partly attributed to its interaction with nicotinic acetylcholine receptors. Previous studies indicate that bispyridinium compounds interact with muscarinic acetylcholine receptors as well. The muscarinic M(5) receptor is not well investigated compared to other subtypes, but could be important in the search for new drugs for treating nerve agent poisoning. A set of bispyridinium compounds structurally related to SAD-128 were tested in competition binding experiments with recombinant human M(5) muscarinic acetylcholine receptors. Five of the six investigated bispyridinium compounds interacted with the orthosteric binding site, with affinities in the low micromolar range. These data indicate that interaction of bispyridinium compounds with muscarinic receptors may contribute to their therapeutic efficacy.

Binding Profiles of a Series of 2-Arylpropionic Acid Esters on Cloned Human Muscarinic Receptor Subtypes (m1-m5) and Their Relationship to Nootropic Activity

The muscarinic binding profile of a series of 2-arylpropionic acid esters on cloned human muscarinic receptor subtypes (m1-m5) was determined to investigate whether there is a correlation between pharmacological activity and muscarinic receptor subtype selectivity. Among the tested compounds, 1, 7 and 9 showed the highest affinity for the m2 and m4 receptors. Compounds 1, 7 and 9 show good affinity for m4 receptors (pKi = 7.87; 7.73 and 7.10, respectively) and are able to discriminate 10-60 fold between m4/m1, m4/m3, and m4/m5 subtypes. Conversely, these compounds are able only to weakly discriminate between m4/m2. Compounds 1 (50-300 micrograms kg-1 i.p.) and 7 (1-10 micrograms kg-1 i.p.), injected 20 min before the training session, are able to prevent the amnesia induced by dicyclomine (2 mg kg-1 i.p.) in the mouse passive-avoidance test. Compounds 1 and 7, at the highest antiamnesic doses, do not modify motor coordination and spontaneous motility as evaluated by the rota-rod test and Animex apparatus experiments.

Effects of Imidafenacin (KRP-197/ONO-8025), a New Anti-cholinergic Agent, on Muscarinic Acetylcholine Receptors

Imidafenacin (CAS 170105-16-5, KRP-197, ONO-8025) is an antagonist for the muscarinic acetylcholine (ACh) receptor currently under development for the treatment of overactive bladder. Affinities of imidafenacin and other drugs for muscarinic ACh receptor subtypes were investigated by examining inhibitory effects on ACh release in the rat urinary bladder and K+ efflux in the rat salivary gland in functional and binding assays. In the functional assay, imidafenacin had higher affnities for M3 and M1 receptors than for the M2 receptor. In contrast, metabolites of imidafenacin (M-2, M-4 and M-9) had low affinities for muscarinic ACh receptor subtypes. Darifenacin had selectivity for the M3 receptor, while propiverine, tolterodine and oxybutynin had no selectivity for muscarinic ACh receptors. In carbamylcholine (CCh)-induced contraction in the urinary bladder, imidafenacin, propiverine, tolterodine and oxybutynin had affinities similar to those for the M3 receptor in the ileum. In the binding assay for human muscarinic ACh receptor subtypes, imidafenacin had higher affinities for m3 and m1 receptors than for m2 receptor, but tolterodine had no selectivity for m1, m2 and m3 receptors. In ACh release in the urinary bladder, inhibitory effects of imidafenacin, tolterodine, oxybutynin and darifenacin seemed to be partially mediated by the M1 receptor. In ACh-induced and electrical stimulation-induced K+ efflux from the salivary gland, inhibitory effects (IC50) of imidafenacin, propiverine, tolterodine, oxybutynin and darifenacin might be closely related to those for the M3 receptor in the ileum. These results suggest that imidafenacin more strongly antagonizes cholinomimetics on M3 and M1 receptors than on the M2 receptor. Moreover, imidafenacin seems to inhibit the contraction of the bladder smooth muscle by mediating antagonism to the M3 receptor and to regulate ACh release by mediating prejunctional facilitatory M1 receptor. Imidafenacin also inhibited K+ efflux from the salivary gland mainly by mediating the M3 receptor. Therefore, imidafenacin will have higher affinities for M3 and M1 receptors and higher selectivity for the urinary bladder than for the salivary gland.

Preclinical Evaluation of Long-Acting Muscarinic Antagonists: Comparison of Tiotropium and Investigational Drugs

Chronic obstructive pulmonary disease (COPD) is characterized by progressive airflow limitation caused by persistent inflammatory processes in the airways. An increased cholinergic tone mediates different pathophysiological features of COPD, such as bronchoconstriction and mucus hypersecretion, mostly through activation of the human muscarinic M(3) receptor (hM(3)) subtype. Tiotropium bromide (Spiriva) is a well established muscarinic antagonist in the pharmacological management of COPD with a once-daily posology. The rationale behind the sustained bronchodilation obtained with tiotropium consists in its slow dissociation from hM(3) receptors. In this study, we performed a comprehensive preclinical comparison of tiotropium with other long-acting muscarinic antagonists (LAMAs) currently in clinical development, namely aclidinium bromide and glycopyrrolate. The different muscarinic antagonists were characterized for their 1) affinity toward the different human muscarinic receptor subtypes expressed in Chinese hamster ovary cells and kinetics of receptor dissociation, 2) potency in inhibiting the agonist-induced activation of muscarinic receptors through measurement of second messengers, and 3) efficacy and duration of bronchoprotection, as tested in a model of acetylcholine-induced bronchoconstriction in anesthetized dogs over a period of 24 h. All of the tested LAMAs showed high affinity and potency toward the hM(3) receptor (tiotropium, pA(2) = 10.4; aclidinium, pA(2) = 9.6; and glycopyrrolate, pA(2) = 9.7). However, dissociation half-lives of the LAMAs from the hM(3) receptor differed significantly (tiotropium, t((1/2)) = 27 h; aclidinium, t((1/2)) = 10.7 h; and glycopyrrolate, t((1/2)) = 6.1 h). In line with their kinetic properties at the hM(3), the tested LAMAs provided different levels of bronchoprotection in the in vivo setting 24 h after administration (tiotropium = 35%, aclidinium = 21%, and glycopyrrolate = 0% at 24 h) when applied at equieffective doses.

Comparison of functional non-glycosylated GPCRs expression in Pichia pastoris

N-linked glycosylation is the most common post-translational modification of G-protein-coupled receptors (GPCRs) and is correlated to the localization and function of the receptors depending on each receptor. However, heterogeneity of glycosylation can interfere with protein crystallization. The removal of N-linked glycosylation from membrane proteins improves the ability to crystallize these proteins. We screened 25 non-glycosylated GPCRs for functional receptor production in the methylotrophic yeast Pichia pastoris using specific ligand–receptor binding assays. We found that five clones were expressed at greater than 10 pmol/mg, 9 clones at 1–10 pmol/mg and 11 clones at less than 1 pmol/mg of membrane protein. Further optimization of culture parameters including culture scale, induction time, pH and temperature enabled us to achieve expression of a functional human muscarinic acetylcholine receptor subtype 2 (CHRM2) with a B max value of 51.2 pmol/mg of membrane protein. Approximately 1.9 mg of the human CHRM2 was produced from a 1-L culture.

Pharmacological characterization of a novel investigational antimuscarinic drug, fesoterodine, in vitro and in vivo

To investigate the primary pharmacology of fesoterodine (a novel antimuscarinic drug developed for treating overactive bladder) and SPM 7605 (its active metabolite, considered to be the main pharmacologically active principle of fesoterodine in man) against human muscarinic receptor subtypes, and to investigate in vitro and in vivo functional activity of these agents on the rat bladder compared with existing standard agents. The displacement of radioligand binding by fesoterodine, SPM 7605 and standard agents in membrane preparations of Chinese hamster ovary (CHO) cells expressing the different human muscarinic receptors (M1-M5) was characterized. Agonistic and antagonistic activities were studied using different CHO cell lines stably expressing the human recombinant muscarinic receptor subtypes. The effects of fesoterodine and SPM 7605 on isolated bladder strips contracted by carbachol or electrical field stimulation (EFS) were investigated. In vivo the effects of fesoterodine and SPM 7605 on micturition variables were assessed using continuous cystometry in conscious female Sprague-Dawley rats, and compared to those of oxybutynin and atropine. In vitro SPM 7605 potently inhibited radioligand binding at all five human muscarinic receptor subtypes with equal affinity across all five. Fesoterodine had a similar balanced selectivity profile but was less potent than SPM 7605. Both substances were competitive antagonists of cholinergic agonist-stimulated responses in human M1-M5 cell lines and had a similar potency and selectivity profile to the radioligand-binding studies. In rat bladder strips, fesoterodine and SPM 7605 caused a rightward shift of the concentration-response curve for carbachol with no depression of the maximum, and concentration-dependently reduced contractions induced by EFS. The potency of both drugs was similar to that of atropine and oxybutynin. In the presence of the esterase inhibitor neostigmine, the concentration-response curve of fesoterodine was shifted to the right, suggesting that part of the activity was caused by metabolism to SPM 7605 by tissue enzymes. In vivo, low doses (0.01 mg/kg) of fesoterodine and SPM 7605 reduced micturition pressure and increased intercontraction intervals and bladder capacity, but did not affect residual volume. Fesoterodine and its active metabolite, SPM 7605, are nonsubtype selective, competitive antagonists of human muscarinic receptors, but SPM 7605 has greater potency than the parent compound. Pharmacodynamic studies in the rat bladder in vitro confirm the competitive muscarinic antagonist profile of these agents in a native tissue preparation, and in vivo studies in the rat showed effects on bladder function consistent with a muscarinic antagonist profile.

Propiverine and metabolites: differences in binding to muscarinic receptors and in functional models of detrusor contraction

Propiverine is a commonly used antimuscarinic drug used as therapy for symptoms of an overactive bladder. Propiverine is extensively biotransformed into several metabolites that could contribute to its spasmolytic action. In fact, three propiverine metabolites (M-5, M-6 and M-14) have been shown to affect various detrusor functions, including contractile responses and L-type calcium-currents, in humans, pigs and mice, albeit with different potency. The aim of our study was to provide experimental evidence for the relationship between the binding of propiverine and its metabolites to human muscarinic receptor subtypes (hM(1)-hM(5)) expressed in chinese hamster ovary cells, and to examine the effects of these compounds on muscarinic receptor-mediated detrusor function. Propiverine, M-5, M-6 and M-14 bound to hM(1)-hM(5) receptors with the same order of affinity for all five subtypes: M-6 > propiverine > M-14 > M-5. In HEK-293 cells expressing hM(3), carbachol-induced release of intracellular Ca(2+) ([Ca(2+)](i)) was suppressed by propiverine and its metabolites; the respective concentration-response curves for carbachol-induced Ca(2+)-responses were shifted to the right. At higher concentrations, propiverine and M-14, but not M-5 and M-6, directly elevated [Ca(2+)](i). These results were confirmed for propiverine in human detrusor smooth muscle cells (hDSMC). Propiverine and the three metabolites decreased detrusor contractions evoked by electric field stimulation in a concentration-dependent manner, the order of potency being the same as the order of binding affinity. We conclude that, in comparison with the parent compound, loss of the aliphatic side chain in propiverine metabolites is associated with higher binding affinity to hM(1)-hM(5) receptors and higher functional potency. Change from a tertiary to a secondary amine (M-14) results in lower binding affinity and reduced potency. Oxidation of the nitrogen (M-5) further lowers binding affinity as well as functional potency.

Human Muscarinic Receptor Binding Characteristics of Antimuscarinic Agents to Treat Overactive Bladder

We characterized the binding affinities of several antimuscarinic agents in human muscarinic receptors. Competitive inhibitory effects of antimuscarinic agents on specific NMS [H] (PerkinElmer Life Sciences, Boston, Massachusetts) binding were examined in human tissue homogenates and in CHO-K1 cell membranes expressing human muscarinic receptor subtypes. Oxybutynin, propiverine, tolterodine, the respective metabolites DEOB, DPr-P-4(N-->O) and 5-HM, and darifenacin inhibited in concentration dependent fashion specific [(3)H]NMS binding in homogenates of the human bladder and parotid gland as well as in membranes of CHO-K1 cell lines expressing human muscarinic M(1) to M(5) receptor subtypes. Based on inhibition constant values the inhibitory effects of tolterodine, 5-HM and DPr-P-4(N-->O) were 1.4 to 1.7 times greater in the bladder than in the parotid gland, whereas the inhibitory effects of oxybutynin, DEOB, propiverine and darifenacin were 2 to 10 times greater in the parotid gland. Consequently tolterodine, 5-HM and DPr-P-4(N-->O) compared with oxybutynin, DEOB, propiverine and darifenacin were found to show 3 to 4 times greater affinity to muscarinic receptors in the human bladder than in the parotid gland. Tolterodine and 5-HM were 2-fold more potent for inhibiting specific [(3)H]NMS binding at cell membranes expressing the M(2) vs the M(3) subtype. Conversely oxybutynin, DEOB, propiverine, DPr-P-4(N-->O) and darifenacin showed 2 to 22 times higher affinity to the M(3) than to the M(2) subtype. Compared with oxybutynin, tolterodine, 5-HM and DPr-P-4(N-->O) may bind more selectively to muscarinic receptors in the human bladder than in the parotid gland.

Thiochrome Enhances Acetylcholine Affinity at Muscarinic M4Receptors: Receptor Subtype Selectivity via Cooperativity Rather than Affinity

Thiochrome (2,7-dimethyl-5H-thiachromine-8-ethanol), an oxidation product and metabolite of thiamine, has little effect on the equilibrium binding of l-[3H]N-methyl scopolamine ([3H]NMS) to the five human muscarinic receptor subtypes (M1-M5) at concentrations up to 0.3 mM. In contrast, it inhibits [3H]NMS dissociation from M1 to M4 receptors at submillimolar concentrations and from M5 receptors at 1 mM. These results suggest that thiochrome binds allosterically to muscarinic receptors and has approximately neutral cooperativity with [3H]NMS at M1 to M4 and possibly M5 receptors. Thiochrome increases the affinity of acetylcholine (ACh) 3- to 5-fold for inhibiting [3H]NMS binding to M4 receptors but has no effect on ACh affinity at M1 to M3 or M5 receptors. Thiochrome (0.1 mM) also increases the direct binding of [3H]ACh to M4 receptors but decreases it slightly at M2 receptors. In agreement with the binding data, thiochrome does not affect the potency of ACh for stimulating the binding of guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) to membranes containing M1 to M3 receptors, but it increases ACh potency 3.5-fold at M4 receptors. It also selectively reduces the release of [3H]ACh from potassium-stimulated slices of rat striatum, which contain autoinhibitory presynaptic M4 receptors, but not from hippocampal slices, which contain presynaptic M2 receptors. We conclude that thiochrome is a selective M4 muscarinic receptor enhancer of ACh affinity and has neutral cooperativity with ACh at M1 to M3 receptors; it therefore demonstrates a powerful new form of selectivity, "absolute subtype selectivity", which is derived from cooperativity rather than from affinity.

Homo- or hetero-dimerization of muscarinic receptor subtypes is not mediated by direct protein-protein interaction through intracellular and extracellular regions.

The oligomerization of G-proteincoupled receptors (GPCRs) has been shown to occur by various mechanisms, such as via disulfide covalent linkages, noncovalent (ionic, hydrophobic) interactions of the N-terminal, and/or transmembrane and/or intracellular domains. Interactions between GPCRs could involve an association between identical proteins (homomers) or non-identical proteins (heteromers), or between two monomers (to form dimers) or multiple monomers (to form oligomers). It is believed that muscarinic receptors may also be arranged into dimeric or oigomeric complexes, but no systematic experimental evidence exists concerning the direct physical interaction between receptor proteins as its mechanism. We undertook this study to determine whether muscarinic receptors form homomers or a heteromers by direct protein-protein interaction within the same or within different subtypes using a yeast two-hybrid system. Intracellular loops (i1, i2 and i3) and the C-terminal cytoplasmic tails (C) of human muscarinic (Hm) receptor subtypes, Hm1, Hm2 and Hm3, were cloned into the vectors (pB42AD and pLexA) of a two-hybrid system and examined for heteromeric or homodimeric interactions between the cytoplasmic domains. No physical interaction was observed between the intracellular domains of any of the Hm/Hm receptor sets tested. The results of our study suggest that the Hm1, Hm2 and Hm3 receptors do not form dimers or oligomers by interacting directly through either the hydrophilic intracellular domains or the C-terminal tail domains. To further investigate extracellular domain interactions, the N-terminus (N) and extracellular loops (o1 and o2) were also cloned into the two-hybrid vectors. Interactions of Hm2N with Hm2N, Hm2o1, Hm2o2, Hm3N, Hm3o1 or Hm3o2 were examined. The N-terminal domain of Hm2 was found to have no direct interaction with any extracellular domain. From our results, we excluded the possibility of a direct interaction between the muscarinic receptor subtypes (Hm1, Hm2 and Hm3) as a mechanism for homo- or hetero-meric dimerization/oligomerization. On the other hand, it remains a possibility that interaction may occur indirectly or require proper conformation or subunit formation or hydrophobic region involvement.

The C-terminal Tail of the M3-muscarinic Receptor Possesses Anti-apoptotic Properties

This study investigates the mechanisms by which the muscarinic receptor gene family can protect against apoptosis. Chinese hamster ovary cells transfected with human muscarinic receptor subtypes underwent apoptotic cell death following treatment with the DNA-damaging agent etoposide. Apoptosis was significantly reduced following muscarinic receptor stimulation of cells that were transfected with receptor subtypes that couple to the Gq/11/phospholipase C pathway, namely M1, M3, and M5. No protection was detected in cells transfected with the Gi-coupled M2 and M4 receptors. Further analysis of the Gq/11-coupled M3 receptor revealed that truncation of the carboxyl-tail (Delta 565-M3 mutant) removed the ability of the receptor to protect against etoposide-induced cell death. This mutation did not affect the ability of the receptor to signal through the phospholipase C pathway. Furthermore, activation of the Delta 565-M3 receptor resulted in robust activation of the extracellular-regulated kinase (ERK) and c-Jun kinase (JNK). The Delta 565-M3 receptor mutant also underwent agonist-driven phosphorylation in a similar manner to the wild-type receptor indicating that the anti-apoptotic effect of the M3 receptor is independent of receptor phosphorylation. Consistent with this was the fact that two M3-muscarinic receptor mutants deficient in agonist-induced receptor phosphorylation were capable of producing a full anti-apoptotic response. We conclude that the anti-apoptotic response of the muscarinic receptor family was confined to the Gq/11-coupled members of this family. The direct involvement of Gq/11/phospholipase C signaling and the ERK-1/2 and JNK pathways together with receptor phosphorylation in the anti-apoptotic response were eliminated. Mutation of a poly-basic region within the short C-terminal tail of the M3-muscarinic receptor inhibited the ability of the receptor to induce an anti-apoptotic response. We conclude that the conserved poly-basic region in the C-terminal tail of the M1, M3, and M5 receptors contributes to the ability of these receptors to mediate protection against apoptotic cell death.

Structure-activity relationships of dimethindene derivatives as new M2-selective muscarinic receptor antagonists.

A series of 2,3-disubstituted indenes, which are analogues of the widely used histamine H(1) receptor antagonist dimethindene, have been synthesized and studied as muscarinic and histamine receptor antagonists. The affinities of these compounds for the five human muscarinic receptor subtypes (M(1)-M(5)) and for human histamine H(1) receptors were determined in radioligand binding studies using membranes from transfected Chinese hamster ovary (CHO) cells and [(3)H]N-methylscopolamine ([(3)H]NMS). The results demonstrate that the diisopropyl analogue 19 has a similar high affinity as (S)-dimethindene at M(2) receptors ((S)-dimethindene: pK(i) = 7.52; (-)-19: pK(i) = 7.37) with an improved selectivity pattern ((S)-dimethindene: M(2)/M(1) = 6-fold, M(2)/M(3) = 5-fold, M(2)/M(4) = 10-fold, M(2)/M(5) = 25-fold; (-)-19: M(2)/M(1) = 36-fold, M(2)/M(3) = 96-fold, M(2)/M(4) = 42-fold, M(2)/M(5) = 275-fold). In addition, compound (-)-19 showed 35-fold lower affinity at histamine H(1) receptors (pK(i) = 5.61) than (S)-dimethindene (pK(i) = 7.16). Another interesting compound is the fluoroethyl derivative 20 (pK(i)/M(2) = 7.49), which also exhibits a higher M(2) selectivity (M(2)/M(1) = 19-fold; M(2)/M(3) = 22-fold; M(2)/M(4) = 13-fold; M(2)/M(5) = 62-fold) than (S)-dimethindene. Unfortunately, compound 20 also shows a high affinity for histamine H(1) receptors (pK(i) = 8.14). The compound with the highest affinity for M(2) receptors (pK(i) = 7.91), the dimethylaminomethylene analogue 31, displayed only a small preference for M(2) receptors. In conclusion, compound (-)-19 might be useful to test the hypothesis that blockade of muscarinic M(2) receptors in the brain is a viable mechanism by which to produce improved cognition. This second-generation dimethindene analogue might also be the starting point for the development of M(2)-selective muscarinic antagonists useful for quantifying M(2) receptors in the central nervous system with positron emission tomography imaging.

Receptor binding studies of soft anticholinergic agents.

Receptor binding studies were performed on 24 soft anticholinergic agents and 5 conventional anticholinergic agents using 4 cloned human muscarinic receptor subtypes. The measured pK(i) values of the soft anticholinergic agents ranged from 6.5 to 9.5, with the majority being in the range of 7.5 to 8.5. Strong correlation was observed between the pK(i) s determined here and the pA 2 values measured earlier in guinea pig ileum contraction assays. The corresponding correlation coefficients (r2) were 0.80, 0.73, 0.81, and 0.78 for pK(i) (m1), pK(i) (m2), pK(i) (m3), and pK(i) (m4), respectively. Quantitative structure-activity relationship (QSAR) studies were also performed, and good characterization could be obtained for the soft anticholinergics containing at least 1 tropine moiety in their structure. For these compounds, the potency as measured by the pK i values was found to be related to geometric, electronic, and lipophilicity descriptors. A linear regression equation using ovality (O(e)), dipole moment (D), and a calculated log octanol-water partition coefficient (QLogP) gave reasonably good descriptions (r = 0.88) for the pK(i) (m3) values.

Protein tyrosine phosphatase CD148-mediated inhibition of T-cell receptor signal transduction is associated with reduced LAT and phospholipase Cgamma1 phosphorylation.

In this study, we investigate the role of the receptor-like protein tyrosine phosphatase CD148 in T-cell activation. Overexpression of CD148 in the Jurkat T-cell line inhibited activation of the transcription factor nuclear factor of activated T cells following T-cell receptor (TCR) stimulation but not following stimulation through a heterologously expressed G protein-coupled receptor, the human muscarinic receptor subtype 1. Using a tetracycline-inducible expression system, we show that the TCR-mediated activation of both the Ras and calcium pathways was inhibited by expression of CD148 at levels that approximate those found in activated primary T cells. These effects were dependent on the phosphatase activity of CD148. Analysis of TCR-induced protein tyrosine phosphorylation demonstrated that most phosphoproteins were unaffected by CD148 expression. However, phospholipase Cgamma1 (PLCgamma1) and LAT were strikingly hypophosphorylated in CD148-expressing cells following TCR stimulation, whereas the phosphorylation levels of Slp-76 and Itk were modestly reduced. Based on these results, we propose that CD148 negatively regulates TCR signaling by interfering with the phosphorylation and function of PLCgamma1 and LAT.

J-104129, a novel muscarinic M3 receptor antagonist with high selectivity for M3 over M2 receptors

A new class of 4-acetamidopiperidine derivatives has been synthesized and investigated for human muscarinic receptor subtype selectivity. Introduction of a hydrocarbon chain of appropriate length into the piperidine nitrogen of the racemic N-(piperidin-4-yl)-2-cyclobutyl-2-hydroxy-2-phenylacetamide platform conferred up to 70-fold selectivity for human muscarinic M3 receptors over M2 receptors. Subsequent synthetic derivatizations resulted in highly potent M3 receptor antagonists with selectivity greater than two orders of magnitude for M3 over M2 receptors, from which the analogue 4r was selected. Preparation of both enantiomers of 4r led to the identification of (2R)-N-[1-(4-methyl-3-pentenyl)piperidin-4-yl]-2-cyclopentyl-2-hydroxy-2-phenylacetamide (J-104129, (R)-4r), which exhibited 120-fold selectivity for M3 receptors (Ki=4.2 nM) over M2 receptors (Ki=490 nM). In isolated rat trachea, (R)-4r potently and specifically antagonized acetylcholine (ACh)-induced responses with a KB value of 3.3 nM. The highly subtype-selective profile was also seen in isolated rat tissue assays (50-fold) and in anesthetized rats (>250-fold). Oral administration of J-104129 ((R)-4r) antagonized ACh-induced bronchoconstriction with an ED50 value of 0.58 mg/kg in rats. Thus, J-104129 ((R)-4r) may effectively facilitate bronchodilation in the treatment of obstructive airway disease.

Affinity profiles of various muscarinic antagonists for cloned human muscarinic acetylcholine receptor (mAChR) subtypes and mAChRs in rat heart and submandibular gland

A family of five subtypes of muscarinic acetylcholine receptors (mAChR) has been identified based on their molecular structures and second signal transduction pathways. In the present study, we examined the antagonist binding profiles of 9 muscarinic antagonists (atropine, 4-DAMP, pirenzepine, oxybutynin, tiquizium, timepidium, propiverine, darifenacin and zamifenacin) for human muscarinic acetylcholine receptor subtypes (m1, m2, m3, m4 and m5) produced by using a baculovirus infection system in Sf9 insect cells and rat tissue membrane preparations (heart and submandibular gland). In a scopolamine methyl chloride [N-methyl- 3H]- ([ 3H]NMS) binding assay, pirenzepine and timepidium displayed the highest affinities for the m1 and m2 subtypes, respectively, and both zamifenacin and darifenacin had the highest affinities for the m3 subtype, although the selectivities among the five subtypes were less than 10-fold. Propiverine showed a slightly higher affinity for the m5 subtype, whereas none of the drugs used in this study was uniquely selective for the m4 subtype. The binding affinities of muscarinic antagonists for rat heart and submandibular gland strong correlated with those for human cloned m2 and m3 subtypes, respectively. These data suggest that [ 3H]NMS binding studies using rat heart and submandibular gland might be useful methods which predict the affinities of test drugs for human muscarinic M2 and M3 receptor subtypes.

Synthesis and muscarinic receptor pharmacology of a series of 4,5,6,7-tetrahydroisothiazolo[4,5- c]pyridine bioisosteres of arecoline

A series of O- and ring-alkylated derivatives of 4,5,6,7-tetrahydroisothiazolo[4,5- c]pyridin-3-ol was synthesized via treatment of appropriately substituted 4-benzylamino-1,2,5,6-tetrahydropyridine-3-carboxamides with hydrogen sulfide and subsequent ring closure by oxidation with bromine. The muscarinic receptor affinity as well as estimated relative efficacy and subtype selectivity of this series of bicyclic arecoline bioisosteres were determined using rat brain membranes and a number of tritiated muscarinic receptor ligands. The effects at the five cloned human muscarinic receptor subtypes of a selected series of chiral analogues, with established absolute stereochemistry, were studied using receptor selection and amplification technology (R-SAT ™). The potency, relative efficacy, and receptor subtype selectivity of these compounds were related to the structure of the O-substituents and the position and stereochemical orientation of the piperidine ring methyl substituents.