Dynamic mechanisms of non-classical antagonism by competitive AT(1) receptor antagonists

Pseudoirreversible inhibition elicits persistent efficacy of a sphingosine 1-phosphate receptor 1 antagonist

Sphingosine 1-phosphate receptor 1 (S1PR1), a G protein-coupled receptor, is required for lymphocyte trafficking, and is a promising therapeutic target in inflammatory diseases. Here, we synthesize a competitive S1PR1 antagonist, KSI-6666, that effectively suppresses pathogenic inflammation. Metadynamics simulations suggest that the interaction of KSI-6666 with a methionine residue Met124 in the ligand-binding pocket of S1PR1 may inhibit the dissociation of KSI-6666 from S1PR1. Consistently, in vitro functional and mutational analyses reveal that KSI-6666 causes pseudoirreversible inhibition of S1PR1, dependent on the Met124 of the protein and substituents on the distal benzene ring of KSI-6666. Moreover, in vivo study suggests that this pseudoirreversible inhibition is responsible for the persistent activity of KSI-6666.

3-Quinuclidinyl-α-methoxydiphenylacetate: A multi-targeted ligand with antimuscarinic and antinicotinic effects designed for the treatment of anticholinesterase poisoning

Racemic 3-quinuclidinyl-α-methoxydiphenylacetate (MB266) was synthesised. Its activity at muscarinic acetylcholine receptors (mAChRs), and muscle and neuronal nicotinic acetylcholine receptors (nAChRs), was compared to that of atropine and racemic 3-quinucidinyl benzilate (QNB) using a functional assay based on agonist-induced elevation of intracellular calcium ion concentration in CN21, Chinese Hamster Ovary (CHO) and SHSY5Y human cell lines. MB266 acted as an antagonist at acetylcholine receptors, displaying 18-fold selectivity for mAChR versus nAChR (compared to the 15,200-fold selectivity observed for QNB). Thus O-methylation of QNB reduced the affinity for mAChR antagonism and increased the relative potency at both muscle and neuronal nAChRs. Despite MB266 having a pharmacological profile potentially useful for the treatment of anticholinesterase poisoning, its administration did not improve the neuromuscular function in a soman-poisoned guinea-pig diaphragm preparation pretreated with the organophosphorus nerve agent soman. Consideration should be given to exploring the potential of MB266 for possible anticonvulsant action in vitro as part of a multi-targeted ligand approach.

An insurmountable NPY Y5 receptor antagonist exhibits superior anti-obesity effects in high-fat diet-induced obese mice

Neuropeptide Y (NPY) Y5 receptor plays a key role in the effects of NPY, an important neurotransmitter in the control of energy homeostasis including stimulation of food intake and inhibition of energy expenditure. The NPY-Y5 receptor system has been an attractive drug target for potential use in treating obesity. Here we report the discovery and characterization of two novel Y5 receptor antagonists, S-2367 and S-234462. Both compounds displayed high affinity for the Y5 receptor in the radio-ligand binding assay, while in the cell-based functional assay, S-2367 and S-234462 showed, respectively, surmountable and insurmountable antagonism. In cell-based washout experiments, S-234462 dissociated from the Y5 receptor more slowly than S-2367. In vivo study showed that S-234462 effectively suppressed food intake induced by acute central injection of a selective Y5 receptor agonist. Furthermore, high-fat diet-induced obese (DIO) mice treated with S-234462 for 5 weeks showed a significant decrease in body weight gain and food intake compared to those treated with S-2367. In conclusion, S-234462 exhibits insurmountable antagonism of NPY Y5 receptor in vitro and superior anti-obesity effects to the surmountable NPY Y5 antagonist S-2367 in DIO mice.

Pharmacological characterization of CRTh2 antagonist LAS191859: Long receptor residence time translates into long-lasting in vivo efficacy

The chemoattractant receptor-homologous molecule expressed on T-helper type 2 cells (CRTh2) is a G protein-coupled receptor expressed on the leukocytes most closely associated with asthma and allergy like eosinophils, mast cells, Th2-lymphocytes and basophils. At present it is clear that CRTh2 mediates most prostaglandin D2 (PGD2) pro-inflammatory effects and as a result antagonists for this receptor have reached asthma clinical studies showing a trend of lung function improvement. The challenge remains to identify compounds with improved clinical efficacy when administered once a day. Herein we described the pharmacological profile of LAS191859, a novel, potent and selective CRTh2 antagonist. In vitro evidence in GTPγS binding studies indicate that LAS191859 is a CRTh2 antagonist with activity in the low nanomolar range. This potency is also maintained in cellular assays performed with human eosinophils and whole blood. The main differentiation of LAS191859 vs other CRTh2 antagonists is in its receptor binding kinetics. LAS191859 has a residence time half-life of 21h at CRTh2 that translates into a long-lasting in vivo efficacy that is independent of plasma levels. We believe that the strategy behind this compound will allow optimal efficacy and posology for chronic asthma treatment.

Ligand binding determinants for angiotensin II type 1 receptor from computer simulations

The ligand binding determinants for the angiotensin II type 1 receptor (AT1R), a G protein-coupled receptor (GPCR), have been characterized by means of computer simulations. As a first step, a pharmacophore model of various known AT1R ligands exhibiting a wide range of binding affinities was generated. Second, a structural model of AT1R was built making use of the growing set of crystal structures of GPCRs, which was further used for the docking of the AT1R ligands based on the devised pharmacophore model. Next, ligand-receptor-lipid bilayer systems were studied by means of molecular dynamics (MD) simulations. Overall, the present study has permitted, combining the pharmacophore model with binding free energy calculations obtained from the MD simulations, to propose the molecular mechanisms by which sartans interact with AT1R.

Isolated dorsal root ganglion neurones inhibit receptor-dependent adenylyl cyclase activity in associated glial cells

BACKGROUND AND PURPOSE

Hyper-nociceptive PGE(2) EP(4) receptors and prostacyclin (IP) receptors are present in adult rat dorsal root ganglion (DRG) neurones and glial cells in culture. The present study has investigated the cell-specific expression of two other G(s) -protein coupled hyper-nociceptive receptor systems: β-adrenoceptors and calcitonin gene-related peptide (CGRP) receptors in isolated DRG cells and has examined the influence of neurone-glial cell interactions in regulating adenylyl cyclase (AC) activity.

EXPERIMENTAL APPROACH

Agonist-stimulated AC activity was determined in mixed DRG cell cultures from adult rats and compared with activity in DRG neurone-enriched cell cultures and pure DRG glial cell cultures.

KEY RESULTS

Pharmacological analysis showed the presence of G(s) -coupled β(2) -adrenoceptors and CGRP receptors, but not β(1) -adrenoceptors, in all three DRG cell preparations. Agonist-stimulated AC activity was weakest in DRG neurone-enriched cell cultures. DRG neurones inhibited IP receptor-stimulated glial cell AC activity by a process dependent on both cell-cell contact and neurone-derived soluble factors, but this is unlikely to involve purine or glutamine receptor activation.

CONCLUSIONS AND IMPLICATIONS

G(s) -coupled hyper-nociceptive receptors are readily expressed on DRG glial cells in isolated cell cultures and the activity of CGRP, EP(4) and IP receptors, but not β(2) -adrenoceptors, in glial cells is inhibited by DRG neurones. Studies using isolated DRG cells should be aware that hyper-nociceptive ligands may stimulate receptors on glial cells in addition to neurones, and that variable numbers of neurones and glial cells will influence absolute measures of AC activity and affect downstream functional responses.

Dual action molecules: bioassays of combined novel antioxidants and angiotensin II receptor antagonists

In this study we have investigated the in vitro angiotensin II receptor antagonist and antioxidant activity of a series of compounds in which the antioxidant pharmacophores (selenium, phenol, benzothiophene, ebselen or nitroxide) have been incorporated into the AT(1) receptor antagonist (sartan) milfasartan. Activity of these compounds was assessed in tissue-based assays. The novel molecules (30nM), nitrasartan or phenol-milfasartan, retained AT(1) receptor antagonist potency in rat isolated right atria. Antioxidant capacity of the substituted sartans was examined in an AAPH (2,2'-azobis (2-amidinopropane) hydrochloride)-induced haemolysis assay (mouse C57/BL6 isolated erythrocytes). Each of the antioxidant pharmacophores (10μM), except benzothiophene, protected against radical-mediated lysis. Of the novel sartans, only analogues incorporating selenium, phenol or nitroxide (nitrasartan) protected against radical-induced haemolysis. In the tissue-based assay using mouse isolated paced left atria, the free radical generator doxorubicin (30μM) resulted in a decrease in left atrial force over 90min. In this assay the phenol, nitroxide or ebselen antioxidant pharmacophores protected against doxorubicin-induced negative inotropy but selenocystine and benzothiophene did not. Nitrasartan (10μM) was the only novel analogue to protect against radical-induced negative inotropy. Nitrasartan also antagonised angiotensin II responses and decreased superoxide production in a concentration-dependent manner in rat isolated carotid arteries and aortae, respectively. In conclusion, nitrasartan is a dual action molecule demonstrating both AT(1) receptor antagonist potency and antioxidant properties in vitro.

Discovery of a Novel Chemical Class of mGlu(5) Allosteric Ligands with Distinct Modes of Pharmacology

We previously discovered a positive allosteric modulator (PAM) of the metabotropic glutamate receptor subtype 5 (mGlu(5)) termed 4 N-{4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl]phenyl}-2-hydroxybenzamide (CPPHA) that elicits receptor activation through a novel allosteric site on mGlu(5), distinct from the classical mGlu(5) negative allosteric modulator (NAM) MPEP allosteric site. However, a shallow structure-activity relationship (SAR), poor physiochemical properties, and weak PAM activity at rat mGlu(5) limited the utility of CPPHA to explore allosteric activation of mGlu(5) at a non-MPEP site. Thus, we performed a functional high-throughput screen (HTS) and identified a novel mGlu(5) PAM benzamide scaffold, exemplified by VU0001850 (EC(50) = 1.3 μM, 106% Glu(max)) and VU0040237 (EC(50) = 350 nM, 84% Glu Max). An iterative parallel synthesis approach delivered 22 analogues, optimized mGlu(5) PAM activity to afford VU0357121 (EC(50) = 33 nM, 92% Glu(max)), and also revealed the first non-MPEP site neutral allosteric ligand (VU0365396). Like CPPHA, PAMs within this class do not appear to bind at the MPEP allosteric site based on radioligand binding studies. Moreover, mutagenesis studies indicate that VU0357121 and related analogues bind to a yet uncharacterized allosteric site on mGlu(5), distinct from CPPHA, yet share a functional interaction with the MPEP site.

Long-lasting target binding and rebinding as mechanisms to prolong in vivo drug action

An increasing number of examples in the literature suggest that the in vivo duration of drug action not only depends on macroscopic pharmacokinetic properties like plasma half-life and the time needed to equilibrate between the plasma and the effect compartments, but is also influenced by long-lasting target binding and rebinding. The present review combines information from different research areas and simulations to explore the nature of these mechanisms and the conditions in which they are most prevalent. Simulations reveal that these latter phenomena become especially influential when there is no longer sufficient free drug around to maintain high levels of receptor occupancy. There is not always a direct link between slow dissociation and long-lasting in vivo target protection, as the rate of free drug elimination from the effect compartment is also a key influencing factor. Local phenomena that hinder the diffusion of free drug molecules away from their target may allow them to consecutively bind to the same target and/or targets nearby (denoted as 'rebinding') even when their concentration in the bulk phase has already dropped to insignificant levels. The micro-anatomic properties of many effect compartments are likely to intensify this phenomenon. By mimicking the complexity of tissues, intact cells offer the opportunity to investigate both mechanisms under the same, physiologically relevant conditions.

SB265610 is an allosteric, inverse agonist at the human CXCR2 receptor

BACKGROUND AND PURPOSE

In several previous studies, the C-X-C chemokine receptor (CXCR)2 antagonist 1-(2-bromo-phenyl)-3-(7-cyano-3H-benzotriazol-4-yl)-urea (SB265610) has been described as binding competitively with the endogenous agonist. This is in contrast to many other chemokine receptor antagonists, where the mechanism of antagonism has been described as allosteric.

EXPERIMENTAL APPROACH

To determine whether it displays a unique mechanism among the chemokine receptor antagonists, the mode of action of SB265610 was investigated at the CXCR2 receptor using radioligand and [(35)S]-GTPgammaS binding approaches in addition to chemotaxis of human neutrophils.

KEY RESULTS

In equilibrium saturation binding studies, SB265610 depressed the maximal binding of [(125)I]-interleukin-8 ([(125)I]-IL-8) without affecting the K(d). In contrast, IL-8 was unable to prevent binding of [(3)H]-SB265610. Kinetic binding experiments demonstrated that this was not an artefact of irreversible or slowly reversible binding. In functional experiments, SB265610 caused a rightward shift of the concentration-response curves to IL-8 and growth-related oncogene alpha, but also a reduction in maximal response elicited by each agonist. Fitting these data to an operational allosteric ternary complex model suggested that, once bound, SB265610 completely blocks receptor activation. SB265610 also inhibited basal [(35)S]-GTPgammaS binding in this preparation.

CONCLUSIONS AND IMPLICATIONS

Taken together, these data suggest that SB265610 behaves as an allosteric inverse agonist at the CXCR2 receptor, binding at a region distinct from the agonist binding site to prevent receptor activation, possibly by interfering with G protein coupling.

An update on non-peptide angiotensin receptor antagonists and related RAAS modulators

The renin-angiotensin-aldosterone-system (RAAS) is an important regulator of blood pressure and fluid-electrolyte homeostasis. RAAS has been implicated in pathogenesis of hypertension, congestive heart failure, and chronic renal failure. Aliskiren is the first non-peptide orally active renin inhibitor approved by FDA. Angiotensin Converting Enzyme (ACE) Inhibitors are associated with frequent side effects such as cough and angio-oedema. Recently, the role of ACE2 and neutral endopeptidase (NEP) in the formation of an important active metabolite/mediator of RAAS, ang 1-7, has initiated attempts towards development of ACE2 inhibitors and combined ACE/NEP inhibitors. Furukawa and colleagues developed a series of low molecular weight nonpeptide imidazole analogues that possess weak but selective, competitive AT1 receptor blocking property. Till date, many compounds have exhibited promising AT1 blocking activity which cause a more complete RAAS blockade than ACE inhibitors. Many have reached the market for alternative treatment of hypertension, heart failure and diabetic nephropathy in ACE inhibitor intolerant patients and still more are waiting in the queue. But, the hallmark of this area of drug research is marked by a progress in understanding molecular interaction of these blockers at the AT1 receptor and unraveling the enigmatic influence of AT2 receptors on growth/anti-growth, differentiation and the regeneration of neuronal tissue. Different modeling strategies are underway to develop tailor made molecules with the best of properties like Dual Action (Angiotensin And Endothelin) Receptor Antagonists (DARA), ACE/NEP inhibitors, triple inhibitors, AT2 agonists, AT1/TxA2 antagonists, balanced AT1/AT2 antagonists, and nonpeptide renin inhibitors. This abstract gives an overview of these various angiotensin receptor antagonists.

Trapping of a Nonpeptide Ligand by the Extracellular Domains of the Gonadotropin-Releasing Hormone Receptor Results in Insurmountable Antagonism

Drugs that exhibit insurmountable antagonism are proposed to provide improved clinical efficacy through extended receptor blockade. Long-term suppression of the gonadotropin-releasing hormone receptor (GnRHR) is an important therapeutic approach for a number of sex hormone-dependent diseases. In this study, we describe the mechanism and structural components required for insurmountable activity of a GnRHR antagonist. TAK-013 behaves as an insurmountable antagonist at the human receptor (hGnRHR) but as a surmountable antagonist at the macaque receptor (mGnRHR). Mutation of the eight residues that differ between hGnRHR and mGnRHR identified Ser-203 and Leu-300 in extracellular loops (ECL) 2 and 3 of hGnRHR as essential for the insurmountability of TAK-013. Substitution of the corresponding residues in mGnRHR with Ser and Leu (mGnRHR-P203S/V300L) converts TAK-013 to an insurmountable antagonist. In addition, mutation of Met-24 to Leu in the amino terminus of hGnRHR also ablates the insurmountable antagonism of TAK-013. The mechanism of insurmountability of TAK-013 was determined to be governed by its rate of dissociation from the receptor. Although the association rates of TAK-013 to hGnRHR, mGnRHR, and mGnRHR-P203S/V300L do not differ, the dissociation rate half-life correlates closely with the degree of insurmountability observed (169, 9, and 55 min, respectively). Taken together, these data suggest a model of the GnRHR in which ECL2, ECL3, and the amino terminus engage with TAK-013 upon its binding to the transmembrane region of the receptor. These additional interactions form a "trap door" above TAK-013, restricting its dissociation and thus resulting in its insurmountability.

Kinetics of nonpeptide antagonist binding to the human gonadotropin-releasing hormone receptor: Implications for structure–activity relationships and insurmountable antagonism

Numerous nonpeptide ligands have been developed for the human gonadotropin-releasing hormone (GnRH) receptor as potential agents for treatment of disorders of the reproductive-endocrine axis. While the equilibrium binding of these ligands has been studied in detail, little is known of the kinetics of their receptor interaction. In this study we evaluated the kinetic structure-activity relationships (SAR) of uracil-series antagonists by measuring their association and dissociation rate constants. These constants were measured directly using a novel radioligand, [3H] NBI 42902, and indirectly for unlabeled ligands. Receptor association and dissociation of [3H] NBI 42902 was monophasic, with an association rate constant of 93+/-10 microM(-1) min(-1) and a dissociation rate constant of 0.16+/-0.02 h(-1) (t(1/2) of 4.3 h). Four unlabeled compounds were tested with varying substituents at the 2-position of the benzyl group at position 1 of the uracil (-F, -SO(CH3), -SO2(CH3) and -CF3). The nature of the substituent did not appreciably affect the association rate constant but varied the dissociation rate constant >50-fold (t(1/2) ranging from 52 min for -SO(CH3) to >43 h for -CF3). This SAR was poorly resolved in standard competition assays due to lack of equilibration. The functional consequences of the varying dissociation rate were investigated by measuring antagonism of GnRH-stimulated [3H] inositol phosphates accumulation. Slowly dissociating ligands displayed insurmountable antagonism (decrease of the GnRH E(max)) while antagonism by more rapidly dissociating ligands was surmountable (without effect on the GnRH E(max)). Therefore, evaluating the receptor binding kinetics of nonpeptide antagonists revealed SAR, not evident in standard competition assays, that defined at least in part the mode of functional antagonism by the ligands. These findings are of importance for the future definition of nonpeptide ligand SAR and for the identification of potentially useful slowly dissociating antagonists for the GnRH receptor.

Determining the potency and molecular mechanism of action of insurmountable antagonists

Insurmountable antagonism (maximal response to the agonist depressed) can result from a temporal inequilibrium involving a slow offset orthosteric antagonist or be the result of an allosteric modulation of the receptor. The former mechanism is operative when the antagonist, agonist, and receptors cannot come to proper equilibrium during the time allotted for collection of agonist response (hemi-equilibrium conditions). Allosteric effects (changes in the conformation of the receptor through binding of the allosteric modulator to a separate site) can preclude the agonist-induced production of response, leading to depression of maximal responses. In these cases, the effects on receptor affinity can be observed as well. The first premise of this article is that system-independent estimates of insurmountable antagonist potency can be made with no prior knowledge of molecular mechanism through the use of pA(2) (-log molar concentration of antagonist producing a 2-fold shift of the concentration response curve) measurements The relationship between the pA(2) and antagonist pK(B) (-log equilibrium dissociation constant of the antagonist-receptor complex) is described; the former is an extremely close approximation of the latter in most cases. The second premise is that specially designed experiments are required to differentiate orthosteric versus allosteric mechanisms; simply fitting of data to orthosteric or allosteric theoretical models can lead to ambiguous results. A strategy to determine whether the observed antagonism is orthosteric (agonist and antagonist competing for the same binding site on the receptor) or allosteric in nature is described that involves the detection of the hallmarks of allosteric response, namely saturation and probe dependence of effect.

Functional selectivity and classical concepts of quantitative pharmacology

The concept of intrinsic efficacy has been enshrined in pharmacology for half of a century, yet recent data have revealed that many ligands can differentially activate signaling pathways mediated via a single G protein-coupled receptor in a manner that challenges the traditional definition of intrinsic efficacy. Some terms for this phenomenon include functional selectivity, agonist-directed trafficking, and biased agonism. At the extreme, functionally selective ligands may be both agonists and antagonists at different functions mediated by the same receptor. Data illustrating this phenomenon are presented from serotonin, opioid, dopamine, vasopressin, and adrenergic receptor systems. A variety of mechanisms may influence this apparently ubiquitous phenomenon. It may be initiated by differences in ligand-induced intermediate conformational states, as shown for the beta(2)-adrenergic receptor. Subsequent mechanisms that may play a role include diversity of G proteins, scaffolding and signaling partners, and receptor oligomers. Clearly, expanded research is needed to elucidate the proximal (e.g., how functionally selective ligands cause conformational changes that initiate differential signaling), intermediate (mechanisms that translate conformation changes into differential signaling), and distal mechanisms (differential effects on target tissue or organism). Besides the heuristically interesting nature of functional selectivity, there is a clear impact on drug discovery, because this mechanism raises the possibility of selecting or designing novel ligands that differentially activate only a subset of functions of a single receptor, thereby optimizing therapeutic action. It also may be timely to revise classic concepts in quantitative pharmacology and relevant pharmacological conventions to incorporate these new concepts.

On the Mechanism of Interaction of Potent Surmountable and Insurmountable Antagonists with the Prostaglandin D2 Receptor CRTH2

Chemoattractant receptor-homologous molecule expressed on T helper 2 cells (CRTH2) has attracted interest as a potential therapeutic target in inflammatory diseases. Ramatroban, a thromboxane A2 receptor antagonist with clinical efficacy in allergic rhinitis, was recently found to also display potent CRTH2 antagonistic activity. Here, we present the pharmacological profile of three ramatroban analogs that differ chemically from ramatroban by either a single additional methyl group (TM30642), or an acetic acid instead of a propionic acid side chain (TM30643), or both modifications (TM30089). All three compounds bound to human CRTH2 stably expressed in human embryonic kidney 293 cells with nanomolar affinity. [3H]Prostaglandin D2 (PGD2) saturation analysis reveals that ramatroban and TM30642 decrease PGD2 affinity, whereas TM30643 and TM30089 exclusively depress ligand binding capacity (Bmax). Each of the three compounds acted as potent CRTH2 antagonists, yet the nature of their antagonism differed markedly. In functional assays measuring inhibition of PGD2-mediated 1) guanosine 5'-O-(3-thio)triphosphate binding, 2) beta-arrestin translocation, and 3) shape change of human eosinophils endogenously expressing CRTH2, ramatroban, and TM30642 produced surmountable antagonism and parallel rightward shifts of the PGD2 concentration-response curves. For TM30643 and TM30089, this shift was accompanied by a progressive reduction of maximal response. Binding analyses indicated that the functional insurmountability of TM30643 and TM30089 was probably related to long-lasting CRTH2 inhibition mediated via the orthosteric site of the receptor. A mechanistic understanding of insurmountability of CRTH2 antagonists could be fundamental for development of this novel class of anti-inflammatory drugs.

Isolation and characterization at cholinergic nicotinic receptors of a neurotoxin from the venom of the Acanthophis sp. Seram death adder

The present study describes the isolation of the first neurotoxin (acantoxin IVa) from Acanthophis sp. Seram death adder venom and an examination of its activity at nicotinic acetylcholine receptor (nAChR) subtypes. Acantoxin IVa (MW 6815; 0.1-1.0 microM) caused concentration-dependent inhibition of indirect twitches (0.1 Hz, 0.2 ms, supramaximal V) and inhibited contractile responses to exogenous nicotinic agonists in the chick biventer cervicis nerve-muscle, confirming that this toxin is a postsynaptic neurotoxin. Acantoxin IVa (1-10 nM) caused pseudo-irreversible antagonism at skeletal muscle nAChR with an estimated pA2 of 8.36+/-0.17. Acantoxin IVa was approximately two-fold less potent than the long-chain (Type II) neurotoxin, alpha-bungarotoxin. With a pKi value of 4.48, acantoxin IVa was approximately 25,000 times less potent than alpha-bungarotoxin at alpha7-type neuronal nAChR. However, in contrast to alpha-bungarotoxin, acantoxin IVa completely inhibited specific [3H]-methyllycaconitine (MLA) binding in rat hippocampus homogenate. Acantoxin IVa had no activity at ganglionic nAChR, alpha4beta2 subtype neuronal nAChR or cytisine-resistant [3H]-epibatidine binding sites. While long-chain neurotoxin resistant [3H]-MLA binding in hippocampus homogenate requires further investigation, we have shown that a short-chain (Type I) neurotoxin is capable of fully inhibiting specific [3H]-MLA binding.

“Network Leaning” as a Mechanism of Insurmountable Antagonism of the Angiotensin II Type 1 Receptor by Non-peptide Antagonists

A mechanistic understanding of the insurmountable antagonism of the angiotensin II type 1 (AT(1)) receptor could be fundamental in the quest for discovery and improvement of drugs. Candesartan and EXP3174 are competitive, reversible insurmountable antagonists of the AT(1) receptor. They contain di-acidic substitutions, whereas the surmountable antagonist, losartan, contains only one acidic group. We tested the hypothesis that these two classes of ligands interact with the AT(1) receptor through similar but not identical bonds and that the differences in the acid-base group contacts are critical for insurmountable antagonism. By pharmacological characterization of site-directed AT(1) receptor mutants expressed in COS1 cells we show that specific interactions with Gln(257) in transmembrane 6 distinguishes insurmountable antagonists and that abolishing these interactions transforms insurmountable to surmountable antagonism. In the Q257A mutant, the dissociation rate of [(3)H]candesartan is 2.8-fold more than the rate observed with wild type, and the association rate was reduced 4-fold lower than the wild type. The pattern of antagonism of angiotensin II concentration-response in the Q257A mutant pretreated with EXP3174 and candesartan is surmountable. We propose that leaning ability of insurmountable antagonists on Gln(257) in the wild-type receptor is the basis of an antagonist-mediated conformational transition, which is responsible for both slow dissociation and inhibition of maximal IP response.

The two-state model of antagonist-AT1 receptor interaction: an hypothesis defended but not tested

A correlation between insurmountable antagonism and slow dissociation has been observed for the non-peptidic AT1 receptor antagonists. This commentary examines the validity of conclusions regarding a two stage binding mechanism that has been proposed in order to account for both the slow dissociation and insurmountable antagonism. Support for that hypothetical mechanism is in the form of the goodness of fit between experimental data and modelled data in a number of papers from the same laboratory. We challenge the idea that a simple match of model and data is an adequate test of an hypothesis by showing that a simpler model matches the data equally well. We conclude that two stage binding is not necessary to explain the behaviour of AT1 receptor antagonists.

Pharmacokinetics and the Cardiovascular Effects of Irbesartan in Aortic Coarctated Rats

A pharmacokinetic-pharmacodynamic study of irbesartan (IRB) was performed in anesthetized sham-operated (SO) and aortic coarctated (ACo) rats. Anesthetized Wistar rats were used 7 days after ACo procedure or SO. A vascular shunt probe was inserted into the carotid artery for the study of plasma pharmacokinetics. In a separate experiment, a concentric probe was placed into the anterior hypothalamus for the study of the IRB distribution in the central nervous system. IRB (10 mg.kg(-1) i.v.) induced a rapid decrease of the heart rate (HR) in the ACo animals (Delta HR -19.2 +/- 2.0 bpm, n = 6; p < 0.05 vs. basal HR), but not in the SO rats (Delta HR -6.7 +/- 5.1 bpm, n = 6). Moreover, IRB reduced the mean arterial blood pressure in the animals of both experimental groups, but the hypotensive effect lasted longer in ACo rats than in SO animals. Analysis of blood samples showed a lower constant of elimination of IRB in ACo rats (Ke 0.67 +/- 0.28 h(-1), n = 5; p < 0.05) than in SO rats (Ke 1.72 +/- 0.30 h(-1), n = 6). Also, a greater distribution of IRB in the anterior hypothalamus was seen in the ACo rats (area under the curve 32 +/- 4 ng.ml(-1).h(-1), n = 6; p < 0.05) than in the SO rats (area under the curve 12 +/- 1 ng.ml(-1).h(-1)). The protein binding of IRB was similar in both experimental groups (SO rats 7.1 +/- 1.2%, n = 6; ACo rats 7.7 +/- 1.5%, n = 6). In conclusion, ACo reduces the plasma elimination of IRB, increasing the distribution in the central nervous system. The longer hypotensive effect of IRB observed in ACo animals could be explained by the slowest elimination of the drug in ACo rats. On the other hand, the effect of IRB on the HR suggested that angiotensin II modulates this parameter in ACo animals at an early stage of hypertension.

Different pharmacological properties of two equipotent antagonists (clozapine and rauwolscine) for 5-HT2B receptors in rat stomach fundus

On the basis of the previously demonstrated constitutive activity in natural systems and the possibility of specific ligand-induced conformations, the aims of this study were: (i) to characterize the effects of two competitive antagonists (rauwolscine, RAU and clozapine, CLO) with very similar potencies for 5-HT(2B) receptors in a natural system (rat stomach fundus), and (ii) to evaluate a new method for detecting ligand-specific generated conformations through the study of the effects of RAU and CLO in 5-HT efficacy and in the time course of the response to the agonists. RAU and CLO behaved as competitive antagonists and showed similar potencies (pA(2) 7.56+/-0.25 and 7.50+/-0.30, respectively). However, RAU displayed greater efficacy than CLO in relaxing basal tension (10 microM CLO represented 64+/-6% of 10 microM RAU-induced relaxation). CLO partially reverted RAU-induced relaxation and RAU promoted an additional relaxation of maximal CLO-induced relaxation. This may indicate different degrees of inverse agonism. RAU also was more effective in generating insurmountable antagonism after long-term incubation (>3 hr) and modified the time course of the 5-HT(2B) response to 5-HT; conversely, CLO did not affect the time course of this response. This suggests that classical competitive antagonists may generate different specific conformational states and differential effects on receptor system regulation.

Pharmacological analysis of calcium responses mediated by the human A3 adenosine receptor in monocyte-derived dendritic cells and recombinant cells

Extensive characterization of adenosine receptors expressed by human monocyte-derived dendritic cells (MDDCs) was performed with quantitative polymerase chain reaction, radioligand binding, and calcium signaling. Transcript for the A3 adenosine receptor was elevated more than 100-fold in immature MDDCs compared with monocyte precursors. A3 receptor transcript was substantially diminished, and A2A receptor transcript increased, by lipopolysaccharide maturation of MDDCs. Saturation binding of N(6)-(3-[(125)I]iodo-4-aminobenzyl)-adenosine-5'-N-methyluronamide ([(125)I]AB-MECA) to membranes from immature MDDCs yielded B(max) of 298 fmol/mg of protein and K(D) of 0.7 nM. Competition against [(125)I]AB-MECA binding confirmed the site to be the A3 receptor. Adenosine elicited pertussis toxin-sensitive calcium responses with EC(50) values ranging as low as 2 nM. The order of potency for related agonists was N(6)-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (IB-MECA) >/= I-AB-MECA > 2Cl-IB-MECA >/= adenosine > 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxyamidoadenosine (CGS21680). The order of efficacy was adenosine >/= CGS21680 > IB-MECA >/= I-AB-MECA > 2Cl-IB-MECA. Calcium responses to 2Cl-IB-MECA and CGS21680, and the lower range of adenosine concentrations, were completely blocked by 10 nM N-(2-methoxyphenyl)-N-[2-(3-pyridyl)quinazolin-4-yl]urea (VUF5574) but not by 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261) or 8-cyclopentyl-1,3-dipropylxanthine. Pretreatment with 100 nM 2Cl-IB-MECA eliminated responses to CGS21680 but not to monocyte inhibitory protein-1alpha. For comparison, dose-response functions were obtained from double-recombinant human embryonic kidney 293 cells expressing the human A3 receptor and a chimeric Galphaq-i3 protein, which was required to establish A3-mediated calcium signaling. The pharmacological profile of calcium signaling elicited by adenosine-related agonists in the double-recombinant cells was essentially identical to that obtained from immature MDDCs. Our results provide an extensive analysis of A3-mediated calcium signaling and unequivocally identify immature MDDCs as native expressers of the human A3 receptor.

Models and methods for studying insurmountable antagonism

Insurmountable antagonists depress the concentration-response curves of subsequently added agonists. The longevity of the antagonist-receptor complex and the existence of allosteric binding sites are the most frequent explanations for this phenomenon. Yet, observed antagonist behaviour often depends on the tissue, the animal species, the duration of the measured response and the study design. Intact cell studies allow greater flexibility and tighter control of the experimental conditions and therefore have the potential to offer a better insight into the molecular basis of insurmountable antagonism.

New insights in insurmountable antagonism

Antagonists that produce parallel rightward shifts of agonist dose-response curves with no alteration of the maximal response are traditionally classified as surmountable, while insurmountable antagonists also depress the maximal response. Although the longevity of the antagonist-receptor complex is quoted in many studies to explain insurmountable antagonism, slowly interconverting receptor conformations, allosteric binding sites, and receptor internalization have been evoked as alternative explanations. To complicate matters even further, insurmountable antagonism is not only drug-related; it may also depend on the tissue, species and experimental design. For the sake of drug development, it is important to elucidate the molecular mechanisms of insurmountable antagonism. New experimental approaches, such as intact cell studies and the use of computer-assisted simulations based on dynamic receptor models, herald the advent of better insight in the future.