Pharmacology of Adenosine A2A Receptors and Therapeutic Applications (original) (raw)

Adenosine physiology and pharmacology: How about A2 receptors?

Pharmacology & Therapeutics, 1996

Adenosine participates in the physiology of central and peripheral tissues through several subtypes of G-protein-coupled receptors. Positively linked to adenylate cyclase, A1 receptors have been subdivided into AL, and AZ,, sites on the basis of their molecular, biochemical and pharmacological properties. They exhibit selective distribution, and are implicated in the modulation of psychomotor activity, circulation, respiration, and metabolism. Recent data support the evidence that adenosine A, receptor properties may prove useful in future drug development, and selective manipulation of receptor-associated biologic effects might be relevant in the treatment of various disorders, including psychiatric diseases, hypoxia/ischemia, inflammation or erythrocytosis.

Antinociceptive Effects of Novel A2B Adenosine Receptor Antagonists

Journal of Pharmacology and Experimental Therapeutics, 2003

Caffeine, an adenosine A 1 , A 2A , and A 2B receptor antagonist, is frequently used as an adjuvant analgesic in combination with nonsteroidal anti-inflammatory drugs or opioids. In this study, we have examined the effects of novel specific adenosine receptor antagonists in an acute animal model of nociception. Several A 2B-selective compounds showed antinociceptive effects in the hot-plate test. In contrast, A 1-and A 2A-selective compounds did not alter pain thresholds, and an A 3 adenosine receptor antagonist produced thermal hyperalgesia. Evaluation of psychostimulant effects of these compounds in the open field showed only small effects of some antagonists at high doses. Coadministration of low, subeffective doses of A 2Bselective antagonists with a low dose of morphine enhanced the efficacy of morphine. Our results indicate that analgesic effects of caffeine are mediated, at least in part, by A 2B adenosine receptors. Caffeine has intrinsic antinociceptive properties and is frequently used as an adjuvant analgesic drug (Malec and Michalska, 1988; Sawynok and Yaksh, 1993). Although it is thought that caffeine analgesia is produced, at least in part, through adenosine receptor antagonism, it is unclear which receptor subtypes are involved. The adenosine receptor family comprises four subtypes: A 1 , A 2A , A 2B , and A 3 (Fredholm et al., 2001). They are widely distributed in the CNS and peripheral tissues. The A 1 receptors are found in high density in the brain (cortex, cerebellum, and hippocampus) and the dorsal horn of the spinal cord, and at lower levels in other brain regions and in peripheral tissues (Rivkees et al., 1995; Fredholm et al., 2001). The A 2A receptors show a more restricted expression pattern in the CNS with high levels in the striatum, nucleus accumbens, and olfactory tubercle (Ongini and Fredholm, 1996). In the periphery, A 2A receptors are highly expressed in spleen, thymus, leukocytes, and blood platelets (Ongini and Fredholm, 1996). A 2B and A 3 receptors are widely distributed, but have a low density in the CNS (Dixon et al., 1996). In the periphery, A 2B receptors are highly expressed in the large intestine, on mast cells and hematopoietic cells (Feoktistov and Biaggioni, 1995). A 3 receptors show a species-dependent distribution: in rats, testis and mast cells express A 3 receptors in high density (Salvatore et al., 1993), whereas humans exhibit high A 3 receptor expression in the lung and in cells of the immune system (Salvatore et al., 1993). A 1 receptors can couple to G i , thus inhibiting the formation of cAMP, whereas stimulation of A 2 receptors, which bind to G s leads to an increase in adenylate cyclase activity (Fredholm et al., 2001). A 1 receptors also activate phospholipase C and phospholipase D (Fredholm et al., 2001). A 2 receptors are O.M.A.-S. was on leave from the Faculty of Pharmacy (Al-Azhar University) with a scholarship of the Egyptian government. A.M.H. was on leave from the Faculty of Pharmacy (Assiut University) with a scholarship of the Egyptian government. A.Z. was supported by Deutsche Forschungsgemeinschaft (FOR425), the State of North-Rhine-Westfalia (Innovationsprogramm Forschung), and the BONFOR Program. C.E.M. was supported by the Deutsche Forschungsgemeinschaft (FOR425). Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

Cloning and expression of the A2a adenosine receptor from guinea pig brain

Neurochemical Research, 1994

A full-length complementary DNA (cDNA) clone encoding the guinea pig brain A 2 adenosine receptor has been isolated by polymerase chain reaction (PCR) and low-stringency-hybridization screening of a guinea pig brain cDNA library. This eDNA contains a long open reading frame encoding a 409-amino acid-residue protein which is highly homologous to the A2 adenosine receptors previously cloned from other species. Hydrophobicity analysis of the deduced protein sequence reveals seven hydrophobic regions, characteristic of a member of the G-protein-coupled receptor superfamily. Radioligand binding assay and functional (GTPase and cAMP) assays of the receptor, transiently expressed in mammalian cells, demonstrate typical characteristics of the A2 type adenosine receptor. The messenger RNA (mRNA) of this Aa receptor is found in the brain, heart, kidney and spleen. Receptor autoradiography with [3H]CGS21680, a specific A2 agonist, and in situ hybridization with A 2 cRNA probe in guinea pig brain indicate that the receptor is expressed exclusively in the caudate nucleus. The pharmacological profile and anatomical distribution of this receptor indicate that it is of the A2. subtype. This work represents the first cloning of an A2~ receptor in a rodent species, offers a complete pharmacological characterization of the receptor and provides an anatomical comparison between binding profile and gene expression of the receptor.

Pharmacological characterization of adenosine A 2B receptors

Biochemical Pharmacology, 1998

Adenosine induces airways obstruction in subjects with asthma, but the receptor subtype responsible remains unknown. The objectives of this study were to determine the pharmacological profile of adenosine receptor subtypes mediating contraction and to investigate the mechanism in normal and passively sensitized human airway tissues. Contraction of bronchial rings isolated from resected lung tissue of patients with lung carcinoma was measured in response to nonselective adenosine receptor agonists, 5-AMP and 5'-(N-Ethylcarboxamido)adenosine, and A 1 receptor agonist, N 6 -cyclopentyladenosine, in the absence and presence of selective adenosine receptor antagonists. Pharmacological antagonists, chemical ablation of airway sensory nerves using capsaicin, and passive sensitization of tissue with serum from subjects with atopy and asthma was used to investigate the mechanism of contraction. Human bronchial tissue contracted in a concentration-dependent manner to adenosine agonists that showed a rank order of activity of A 1 . A 2B .. A2 A ¼ A3. The maximum contractile response to N 6 -cyclopentyladenosine (231.0 6 23.8 mg) was significantly reduced in tissues chemically treated with capsaicin to desensitize sensory nerves (desensitized: 101.6 6 15.2 mg; P , 0.05). Passive sensitization significantly augmented the contraction induced by adenosine A 1 receptor activation (sensitized: 389.7 6 52.8 mg versus nonsensitized; P , 0.05), which was linked to the release of leukotrienes, and not histamine (MK571: 25.5 6 1.7 mg; epinastine 260.0 6 22.2 mg versus control; P , 0.05). This study provides evidence for a role for adenosine A 1 receptors in eliciting human airway smooth muscle constriction, which, in part, is mediated by the action of capsaicin sensitive sensory nerves.

Cloning, Expression and Pharmacological Characterization of Rabbit Adenosine A 1 and A 3 Receptors

The role of adenosine A 1 and A 3 receptors in mediating cardioprotection has been studied predominantly in rabbits, yet the pharmacological characteristics of rabbit adenosine A 1 and A 3 receptor subtypes are unknown. Thus, the rabbit adenosine A 3 receptor was cloned and expressed, and its pharmacology was compared with that of cloned adenosine A 1 receptors. Stable transfection of rabbit A 1 or A 3 cDNAs in Chinese hamster ovary-K1 cells resulted in high levels of expression of each of the receptors, as demonstrated by high-affinity binding of the A 1 /A 3 adenosine receptor agonist N 6 -(4-amino-3-[ 125 I]iodobenzyl)adenosine ( 125 I-ABA). For both receptors, binding of 125 I-ABA was inhibited by the GTP analog 5Ј-guanylimidodiphosphate, and forskolin-stimulated cyclic AMP accumulation was inhibited by the adenosine receptor agonist (R)-phenylisopropyladenosine. The rank orders of potency of adenosine receptor agonists for inhibition of 125 I-ABA binding were as follows: rabbit A 1 , N 6 -cyclopentyladenosine ϭ (R)-phenylisopropyladenosine Ͼ N-ethylcarboxamidoadenosine Ն I-ABA Ն N 6 -2-(4-

Synthesis and Biological Activity of New Potential Agonists for the Human Adenosine A 2A Receptor

Journal of Medicinal Chemistry, 2004

New adenosine derivatives have been synthesized and tested as putative agonists of adenosine receptors. Compounds 2-6 derive from the introduction of several types of substituents (electron donating, electron withdrawing, and halogens) in the para-position of the phenyl ring of the parent compound 1, and compound 7 lacks the hydroxyl group of amino alcohol 1. In radioligand binding assays using recombinant human A 1 , A 2A , A 2B , and A 3 receptors, all compounds showed very low or negligible affinity for A 1 and A 2B receptors but compounds 3, 5, and 7 displayed a remarkably potent affinity for the A 2A receptor with K i values of 1-5 nM. Bromo derivative 3 displayed a selectivity A 1 /A 2A ) 62 and A 3 /A 2A ) 16 whereas the presence of a hydroxyl group (compound 5) improved the selectivity of A 1 /A 2A and A 3 /A 2A to 120-and 28-fold, respectively. When the methoxy derivative 4 lacks the hydroxyl group on the side chain (compound 7), the binding affinity for A 2A is increased to 1 nM, improving selectivity ratios to 356-and 100-fold against A 1 and A 3 , respectively. In Chinese hamster ovary cells transfected with human A 2A and A 2B receptors, most compounds showed a remarkable activity for the A 2A receptor, except chloro derivative 2, with EC 50 values ranging from 1.4 to 8.8 nM. The compounds behaved as good A 2A agonists, and all were more selective than 5′-(N-ethylcarboxamino)adenosine (NECA), with A 2B /A 2A ratios of cAMP accumulation ranging from 48 for compound 2 to 666 for compound 7 while the corresponding A 2B /A 2A ratio for NECA was only 9. Compounds 1, 3, 5, and 7 also displayed higher selectivities than NECA up to 100-fold in isolated aortas of rat and guinea pig. In guinea pig tracheal rings precontracted by carbachol, compounds 2 and 4 were more potent than adenosine (100-fold) and NECA (10-fold), whereas compounds 1 and 7 displayed similar effects to NECA. Pretreatment of the tracheal rings with A 2 , A 2A , and A 2B receptor antagonists 3,7-dimethyl-L-propargylxanthine, 8-(3-chlorostyryl)caffeine, and alloxazine produced a marked inhibition of the tracheal relaxations induced by compounds 1, 2, and 4, but none of the compounds showed selectivity toward any of the adenosine receptors.