Cell-surface receptors are the targets for more than 60% of current drugs. Traditionally, optimizing the interaction of lead molecules with the binding site for the endogenous agonist (orthosteric site) has been viewed as the best means of achieving selectivity of action. However, recent developments have highlighted the fact that drugs can interact with binding sites on the receptor molecule that are distinct from the orthosteric site, known as allosteric sites. Allosteric modulators could offer several advantages over orthosteric ligands, including greater selectivity and saturability of their effect.

http://fulltext.calis.edu.cn/nature/nrd/1/3/nrd746.pdf

Allosteric binding sites on cell-surface receptors: novel targets for drug discovery

Allosteric modulation of G protein-coupled receptors has recently been recognized as an alternative approach for selectivity in drug action. Allosteric modulators that enhance or diminish the effects of (endogenous) agonists or antagonists on a variety of G protein-coupled receptors are described in this review, with emphasis on the latest developments in this research area. Specific examples include allosteric ligands for adenosine A1 and A3 receptors, Ca(2+)-sensing receptors, metabotropic glutamate receptor subtypes, gamma-aminobutyric acid type B and muscarinic receptors. It appears that all three major classes of G protein-coupled receptors (A, B and C) are amenable to allosteric modulation by small molecules. This constitutes an attractive and novel means to identify new leads in the drug discovery process. However, it requires a reengineering of most current assays.

Allosteric modulation of G protein-coupled receptors.

The adenosine A2A receptor is a G-protein-coupled receptor (GPCR) that has been extensively studied during the past few decades because it offers numerous possibilities for therapeutic applications. Herein we describe adenosine A2A receptor distribution, signaling pathways, pharmacology, and molecular structure, followed by a summary and SAR discussion of the most relevant series of adenosine A2A agonists and antagonists. This review also provides an update of the A2A ligands that are undergoing or have undergone clinical studies, including the two currently marketed agonists adenosine and regadenoson.

/pdf/adenosine-a2a-receptor-as-a-drug-discovery-target-2fs6q19dn8.pdf

Adenosine A2A Receptor as a Drug Discovery Target

Background—Adenosine is a potent coronary vasodilator and causes an increase of coronary blood flow by activation of A2A-adenosine receptors (A2A-AdoRs). The purpose of this study was to test the hypothesis that the high potency of adenosine and adenosine analogues to cause coronary vasodilation is explained by the presence of a large A2A-AdoR reserve (“spare receptors”). Methods and Results—A novel, irreversible antagonist of A2A-AdoRs was used to inactivate receptors and reduce the response to agonist. Agonist-induced increases of coronary conductance before and after exposure of hearts to the irreversible antagonist were compared. Three agonists were studied: 2-p-(2-carboxyethyl)-phenethylamino-5′-N-ethylcarboxamidoadenosine (CGS21680), adenosine, and 2-chloro-N6-cyclopentyladenosine (CCPA). Data were analyzed to determine agonist KA (equilibrium dissociation constant) and EC50 values. Values of KA for activation of A2A-AdoRs by CGS21680, adenosine, and CCPA were 105, 1800, and 2630 nmol/L, respectivel...

A2A-Adenosine Receptor Reserve for Coronary Vasodilation

The role of receptor structure in determining adenosine receptor activity

The 2-amino-3-benzoylthiophene derivative PD 81,723 is an allosteric enhancer of agonist binding to brain A1 adenosine receptors. One aim of this study was to characterize and contrast the effects of PD 81,723 on the A1 receptor-mediated negative dromotropic and A2a receptor-mediated vasodilatory actions of adenosine and of a nonmetabolizable and unselective N6-(3-pentyl)adenosine derivative. A second aim was to determine the mechanism of action of PD 81,723. In guinea pig isolated hearts, PD 81,723 potentiated the adenosine and the N6-(3-pentyl)adenosine derivative-induced prolongations of the stimulus-to-His bundle (S-H) interval in a concentration-dependent manner. PD 81,723 (30 mumol/L) decreased the EC50 value for adenosine to prolong the S-H interval by ninefold from 7.4 +/- 1.2 to 0.8 +/- 0.1 mumol/L but did not increase the content of adenosine in cardiac effluent. PD 81,723 (30 mumol/L) increased the specific binding of the A1 agonist [3H]cyclohexyladenosine ([3H]CHA) to human atrial and guinea pig atrial and brain membranes by 38%, 78%, and 300%, respectively. PD 81,723 also increased the fraction of A1 receptors in the high-affinity binding state by an average of 56 +/- 13%. The dissociation rate of [3H]CHA from guinea pig brain membranes was decreased in the presence of PD 81,723 (10 mumol/L) from 0.55 +/- 0.01/min to 0.35 +/- 0.01/min. PD 81,723 did not alter the binding of the A1 antagonist [3H]cyclopentyldipropylxanthine to guinea pig brain membranes. The IC50 values for 5'-guanylylimidodiphosphate to reduce specific binding of [3H]CHA to guinea pig cardiac and brain membranes were increased from 1.5 +/- 0.2 and 2.0 +/- 0.2 mumol/L in the absence of PD 81,723 to 10 +/- 3.3 and 18 +/- 0.5 mumol/L, respectively, in the presence of PD 81,723 (30 mumol/L). PD 81,723 did not potentiate the coronary vasodilatory actions of the N6-(3-pentyl)adenosine derivative. Specific binding of the A2a agonist [3H]CGS 21680 to brain membranes and the nucleoside transporter ligand [3H]nitrobenzylthioinosine to cardiac membranes was unchanged in the presence of PD 81,723. The results suggest that PD 81,723 specifically potentiates the action of adenosine on A1 receptors by stabilizing receptor-G protein interactions in the presence of agonists.

Allosteric enhancer PD 81,723 acts by novel mechanism to potentiate cardiac actions of adenosine.

This study demonstrates quantification of A2A adenosine receptors (A2AAdoRs) in membranes prepared from porcine coronary arteries, porcine striatum, and PC12 cells. Radioligand binding assays were performed using the new selective A2AAdoR antagonist radioligand [3H]-5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo [4,3-epsilon]-1,2,4-triazolo[1,5-c)pyrimidine ([3H]SCH58261). Binding of the radioligand to membranes was rapid, reversible, and saturable. The densities of A2AAdoRs in membranes prepared from porcine coronary arteries, porcine striatum, and PC12 cells were 900 +/- 61, 892 +/- 35, and 959 +/- 76 fmol/mg protein, respectively. Equilibrium dissociation constants (Kd values) calculated from results of saturation binding assays were 2.19, 1.20, and 0.81 nmol/L, and Kd values calculated from results of association and dissociation assays were 2.42, 1.01, and 0.40 nmol/L for [3H]SCH58261 binding to membranes prepared from porcine coronary arteries, porcine striatum, and PC12 cells, respectively. The specific binding of [3H]SCH58261 as a percentage of total binding at a radioligand concentration equal to the Kd value was 65% to 90% in the three membrane preparations. The order of ligand potencies determined by assay of competition binding to sites in porcine coronary membranes using [3H]SCH58261, unlabeled antagonists (SCH58261, 8-(3-chlorostyryl)caffeine [CSC], and xanthine amine congener [XAC]), and unlabeled agonists ([3H]2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoaden osine [CGS 21680], 2-hexynyl-5'-N-ethylcarboxamidoadenosine [HE-NECA], [3H]5'-N-ethylcarboxamidoadenosine [NECA], and R(-)N6-(2-phenylisopropyl)adenosine [R-PIA]) was SCH58261 > HE-NECA = CSC = CGS 21680 = XAC > NECA = R-PIA. The Hill coefficients of displacement by A2AAdoR ligands of [3H]SCH58261 binding were not significantly different from unity, indicating that [3H]SCH58261 bound to a group of homogeneous noninteracting sites in all membrane preparations. The order of ligand potencies to compete for [3H]SCH58261 binding sites in porcine striatal and PC12 cell membranes was, in part, different from that for porcine coronary arterial membranes. The different rank orders of potencies for agonists and antagonists at A2A receptors of porcine coronary arteries, striatum, and PC12 cells and significant differences in absolute values of potency of ligands in the three preparations may indicate the existence of different subtypes of A2AAdoRs. The antagonist radio-ligand [3H]SCH58261 should be of value for pharmacological characterization of A2A adenosine receptors in other preparations.

Binding of the Novel Nonxanthine A2A Adenosine Receptor Antagonist [3H]SCH58261 to Coronary Artery Membranes

We determined the effects of 8-cyclopentyl-3-[3-[[4-(fluorosulfonyl)benzoyl]oxy]propyl]-1-propylxanth ine (FSCPX), a putative irreversible antagonist of the A1-adenosine receptor, on cardiac A1-adenosine receptor-mediated responses and on the specific binding of [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX) to guinea pig cardiac and brain membranes. FSCPX (5 microM) completely reversed the increase in K+ current of guinea pig atrial myocytes caused by 100 microM adenosine (259 +/- 30 to 20 +/- 7 pA) but had no significant effect on K+ currents caused by either 0.5 microM carbachol or 100 microM GTP gamma S. The attenuation of K+ current by FSCPX was both time and concentration dependent and persisted after washout of the antagonist. Pretreatment of atrial myocytes with FSCPX (50 nM) markedly attenuated the activation of K+ current and the inhibition of isoproterenol-stimulated I(Ca,L) caused by adenosine by 90.1% and 84.2%, respectively, but did not alter the responses of atrial myocytes to carbachol. FSCPX (1 microM) irreversibly antagonized the A1-adenosine receptor-mediated increase in atrioventricular nodal conduction time of isolated perfused guinea pig hearts from 10.5 +/- 0.5 to 0.7 +/- 0.6 msec but did not significantly alter the A2-adenosine receptor-mediated decrease in coronary resistance. Preincubation of guinea pig cardiac membranes with 0.1, 1.0, or 3.0 microM FSCPX for 30 min reduced the Bmax of [3H]CPX binding by 41 +/- 10%, 67 +/- 6%, and 80 +/- 1% (mean +/- standard error, three experiments), respectively, with no significant change in the Kd. Similarly, 0.1 and 1.0 microM FSCPX irreversibly reduced the binding of [3H]CPX to guinea pig forebrain membranes by 65 +/- 5% and 83 +/- 2% (four experiments), respectively, but did not reduce the binding of [3H]CGS 21680, an A2a-adenosine receptor agonist, to striatal membranes. FSCPX did not affect the potency of 59-guanylylimidodiphosphate to inhibit the binding of [3H]CCPA, an A1-adenosine receptor agonist, to brain membranes. The results indicate that FSCPX is a specific, irreversible, A1-adenosine subtype-selective receptor antagonist.

A novel irreversible antagonist of the A1-adenosine receptor.

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Allosteric enhancer PD 81,723 acts by novel mechanism to potentiate cardiac actions of adenosine.

Binding of the Novel Nonxanthine A2A Adenosine Receptor Antagonist [3H]SCH58261 to Coronary Artery Membranes

A novel irreversible antagonist of the A1-adenosine receptor.