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1.  Structure–Activity Relationships of 1,3-Dialkylxanthine Derivatives at Rat A3 Adenosine Receptors 
Journal of medicinal chemistry  1994;37(20):3373-3382.
1,3-Dialkylxanthine analogues containing carboxylic acid and other charged groups on 8-position substituents were synthesized. These derivatives were examined for affinity in radioligand binding assays at rat brain A3 adenosine receptors stably expressed in CHO cells using the new radioligand [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5′-N-methyluronamide), and at rat brain A1 and A2a receptors using [3H]PIA and [3H]CGS 21680, respectively. A synthetic strategy for introducing multiple carboxylate groups at the 8-position using iminodiacetic acid derivatives was explored. The presence of a sulfonate, a carboxylate, or multiple carboxylate groups did not result in a significant enhancement of affinity at rat A3 receptors, although as previously observed an anionic group tended to diminish potency at A1 and A2a receptors. The rat A3 receptor affinity was not highly dependent on the distance of a carboxylate group from the xanthine pharmacophore. 2-Thio vs 2-oxo substitution favored A3 potency, and 8-alkyl vs 8-aryl substitution favored A3 selectivity, although few derivatives were truly selective for rat A3 receptors. 1,3-Dimethyl-8-(3-carboxypropyl)-2-thioxanthine was 7-fold selective for A3 vs A2a receptors. 1,3,7-Trimethyl-8-(trans-2-carboxyvinyl)xanthine was somewhat selective for A3 vs A1 receptors. For 8-arylxanthines affinity at A3 receptors was enhanced by 1,3-dialkyl substituents, in the order dibutyl > dipropyl > diallyl.
PMCID: PMC3471218  PMID: 7932565
2.  Xanthine Functionalized Congeners as Potent Ligands at A2-Adenosine Receptors†,‡ 
Journal of medicinal chemistry  1987;30(1):211-214.
Amide derivatives of a carboxylic acid congener of 1,3-dialkylxanthine, having a 4-[(carboxymethyl)oxy]phenyl substituent at the 8-position, have been synthesized in order to identify potent antagonists at A2-adenosine receptors stimulatory to adenylate cyclase in platelets. Distal structural features of amide-linked chains and the size of the 1,3-dialkyl groups have been varied. 1,3-Diethyl groups, more than 1,3-dimethyl or 1,3-dipropyl groups, favor A2 potency, even in the presence of extended chains attached at the 8-(p-substituted-phenyl) position. Polar groups, such as amines, on the chain simultaneously enhance water solubility and A2 potency. Among the most potent A2 ligands are an amine congener, 8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]phenyl]-1,3-diethylxanthine, and its D-lysyl conjugate, which have KB values of 21 and 23 nM, respectively, for the antagonism of N-ethyl-adenosine-5′-uronamide-stimulated adenylate cyclase activity in human platelet membranes. Strategies for the selection and tritiation of new radioligands for use in competitive binding assays at A2-adenosine receptors have been considered.
PMCID: PMC3433718  PMID: 3806597
3.  Structure–Activity Relationships of 9-Alkyladenine and Ribose-Modified Adenosine Derivatives at Rat A3 Adenosine Receptors† 
Journal of medicinal chemistry  1995;38(10):1720-1735.
9-Alkyladenine derivatives and ribose-modified N6-benzyladenosine derivatives were synthesized in an effort to identify selective ligands for the rat A3 adenosine receptor and leads for the development of antagonists. The derivatives contained structural features previously determined to be important for A3 selectivity in adenosine derivatives, such as an N6-(3-iodobenzyl) moiety, and were further substituted at the 2-position with halo, amino, or thio groups. Affinity was determined in radioligand binding assays at rat brain A3 receptors stably expressed in Chinese hamster ovary (CHO) cells, using [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)adenosine-5′-(N-methyluronamide)), and at rat brain A1 and A2a receptors using [3H]-N6-PIA ((R)-N6-phenylisopropyladenosine) and [3H]CGS 21680 (2-[[[4-(2-carboxyethyl)-phenyl]ethyl]amino]-5′-(N-ethylcarbamoyl)adenosine), respectively. A series of N6-(3-iodobenzyl) 2-amino derivatives indicated that a small 2-alkylamino group, e.g., methylamino, was favored at A3 receptors. N6-(3-Iodobenzyl)-9-methyl-2-(methylthio)adenine was 61-fold more potent than the corresponding 2-methoxy ether at A3 receptors and of comparable affinity at A1 and A2a receptors, resulting in a 3–6-fold selectivity for A3 receptors. A pair of chiral N6-(3-iodobenzyl) 9-(2,3-dihydroxypropyl) derivatives showed stereoselectivity, with the R-enantiomer favored at A3 receptors by 5.7-fold. 2-Chloro-9-(β-d-erythrofuranosyl)-N6-(3-iodobenzyl)adenine had a Ki value at A3 receptors of 0.28 µM. 2-Chloro-9-[2-amino-2,3-dideoxy-β-d-5-(methylcarbamoyl)-arabinofuranosyl]-N6-(3-iodobenzyl)adenine was moderately selective for A1 and A3 vs A2a receptors. A 3′-deoxy analogue of a highly A3-selective adenosine derivative retained selectivity in binding and was a full agonist in the inhibition of adenylyl cyclase mediated via cloned rat A3 receptors expressed in CHO cells. The 3′-OH and 4′-CH2OH groups of adenosine are not required for activation at A3 receptors. A number of 2′,3′-dideoxyadenosines and 9-acyclic-substituted adenines appear to inhibit adenylyl cyclase at the allosteric “P” site.
PMCID: PMC3445626  PMID: 7752196
4.  Structure–Activity Relationships of N6-Benzyladenosine-5′-uronamides as A3-Selective Adenosine Agonists† 
Journal of medicinal chemistry  1994;37(5):636-646.
Adenosine analogues modified at the 5′-position as uronamides and/or as N6-benzyl derivatives were synthesized. These derivatives were examined for affinity in radioligand binding assays at the newly discovered rat brain A3 adenosine receptor and at rat brain A1 and A2a receptors. 5′-Uronamide substituents favored A3 selectivity in the order N-methyl > N-ethyl ∞ unsubstituted carboxamide > N-cyclopropyl. 5′-(N-Methylcarboxamido)-N6-benzyladenosine was 37–56-fold more selective for A3 receptors. Potency at A3 receptors was enhanced upon substitution of the benzyl substituent with nitro and other groups. 5′-N-Methyluronamides and N6-(3-substituted-benzyl)adenosines are optimal for potency and selectivity at A3 receptors. A series of 3-(halobenzyl)-5′-N-ethyluronamide derivatives showed the order of potency at A1 and A2a receptors of I ~ Br > Cl > F. At A3 receptors the 3-F derivative was weaker than the other halo derivatives. 5′-N-Methyl-N6-(3-iodobenzyl)adenosine displayed a Ki value of 1.1 nM at A3 receptors and selectivity versus A1 and A2a receptors of 50-fold. A series of methoxybenzyl derivatives showed that a 4-methoxy group best favored A3 selectivity. A 4-sulfobenzyl derivative was a specific ligand at A3 receptors of moderate potency. An aryl amino derivative was prepared as a probe for radioiodination and receptor cross-linking.
PMCID: PMC4474279  PMID: 8126704
5.  Search for New Purine- and Ribose-Modified Adenosine Analogues as Selective Agonists and Antagonists at Adenosine Receptors† 
Journal of medicinal chemistry  1995;38(7):1174-1188.
The binding affinities at rat A1, A2a, and A3 adenosine receptors of a wide range of derivatives of adenosine have been determined. Sites of modification include the purine moiety (1-, 3-, and 7-deaza; halo, alkyne, and amino substitutions at the 2- and 8-positions; and N6-CH2-ring, -hydrazino, and -hydroxylamino) and the ribose moiety (2′-, 3′-, and 5′-deoxy; 2′- and 3′-O-methyl; 2′-deoxy 2′-fluoro; 6′-thio; 5′-uronamide; carbocyclic; 4′- or 3′-methyl; and inversion of configuration). (−)- and (+)-5′-Noraristeromycin were 48- and 21-fold selective, respectively, for A2a vs A1 receptors. 2-Chloro-6′-thioadenosine displayed a Ki value of 20 nM at A2a receptors (15-fold selective vs A1). 2-Chloroadenin-9-yl(β-L-2′-deoxy-6′-thiolyxofuranoside) displayed a Ki value of 8 μM at A1 receptors and appeared to be an antagonist, on the basis of the absence of a GTP-induced shift in binding vs a radiolabeled antagonist (8-cyclopentyl-1,3-dipropylxanthine). 2-Chloro-2′-deoxyadenosine and 2-chloroadenin-9-yl(β-D-6′-thioarabinoside) were putative partial agonists at A1 receptors, with Ki values of 7.4 and 5.4 μM, respectively. The A2a selective agonist 2-(1-hexynyl)-5′-(N-ethylcarbamoyl)adenosine displayed a Ki value of 26 nM at A3 receptors. The 4′-methyl substitution of adenosine was poorly tolerated, yet when combined with other favorable modifications, potency was restored. Thus, N6-benzyl-4′-methyladenosine-5′-(N-methyluronamide) displayed a Ki value of 604 nM at A3 receptors and was 103- and 88-fold selective vs A1 and A2a receptors, respectively. This compound was a full agonist in the A3-mediated inhibition of adenylate cyclase in transfected CHO cells. The carbocyclic analogue of N6-(3-iodobenzyl)adenosine-5′-(N-methyluronamide) was 2-fold selective for A3 vs A1 receptors and was nearly inactive at A2a receptors.
PMCID: PMC3457658  PMID: 7707320
6.  2-Substituted adenosine derivatives: affinity and efficacy at four subtypes of human adenosine receptors 
Biochemical pharmacology  2004;68(10):1985-1993.
The affinity and efficacy at four subtypes (A1, A2A, A2B and A3) of human adenosine receptors (ARs) of a wide range of 2-substituted adenosine derivatives were evaluated using radioligand binding assays and a cyclic AMP functional assay in intact CHO cells stably expressing these receptors. Similar to previous studies of the N6-position, several 2-substituents were found to be critical structural determinants for the A3AR activation. The following adenosine 2-ethers were moderately potent partial agonists (Ki, nM): benzyl (117), 3-chlorobenzyl (72), 2-(3-chlorophenyl)ethyl (41), and 2-(2-naphthyl)ethyl (130). The following adenosine 2-ethers were A3AR antagonists: 2,2-diphenylethyl, 2-(2-norbornan)ethyl, R- and S-2-phenylbutyl, and 2-(2-chlorophenyl)ethyl. 2-(S-2-Phenylbutyloxy)a-denosine as an A3AR antagonist right-shifted the concentration–response curve for the inhibition by NECA of cyclic AMP accumulation with a KB value of 212 nM, which is similar to its binding affinity (Ki = 175 nM). These 2-substituted adenosine derivatives were generally less potent at the A1AR in comparison to the A3AR, but fully efficacious, with binding Ki values over 100 nM. The 2-phenylethyl moiety resulted in higher A3AR affinity (Ki in nM) when linked to the 2-position of adenosine through an ether group (54), than when linked through an amine (310) or thioether (1960). 2-[2-(l-Naphthyl)ethyloxy]adenosine (Ki = 3.8 nM) was found to be the most potent and selective (>50-fold) A2A agonist in this series. Mixed A2A/A3AR agonists have been identified. Interestingly, although most of these compounds were extremely weak at the A2BAR, 2-[2-(2-naphthyl)ethyloxy]adenosine (EC50 = 1.4 µM) and 2-[2-(2-thienyl)-ethyloxy]adenosine (EC50 = 1.8 (M) were found to be relatively potent A2B agonists, although less potent than NECA (EC50 = 140 nM).
doi:10.1016/j.bcp.2004.06.011
PMCID: PMC3408601  PMID: 15476669
Adenosine receptors; Purines; Nucleosides; GPCR; Efficacy; Structure–activity relationships
7.  Trifunctional Agents as a Design Strategy for Tailoring Ligand Properties: Irreversible Inhibitors of A1 Adenosine Receptors† 
Bioconjugate chemistry  1991;2(2):77-88.
The 1,3-phenylene diisothiocyanate conjugate of XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]-oxy]phenyl]-l,3-dipropylxanthine, a potent A1 selective adenosine antagonist) has been characterized as an irreversible inhibitor of A1 adenosine receptors. To further extend this work, a series of analogues were prepared containing a third substituent in the phenyl isothiocyanate ring, incorporated to modify the physiochemical or spectroscopic properties of the conjugate. Symmetrical trifunctional cross-linking reagents bearing two isothiocyanate groups were prepared as general intermediates for cross-linking functionalized congeners and receptors. Xanthine isothiocyanate derivatives containing hydrophilic, fluorescent, or reactive substituents, linked via an amide, thiourea, or methylene group in the 5-position, were synthesized and found to be irreversible inhibitors of A1 adenosine receptors. The effects of the 5-substituent on water solubility and on the A1/A2 selectivity ratio derived from binding assays in rat brain membranes were examined. Inhibition of binding of [3H]-N6-(2-phenylisopropyl)-adenosine and [3H]CGS21680 (2-[[2-[4-(2-carboxyethyl)phenyl]ethyl]amino]adenosine-5′-N-ethylcarboxamide) at central A1 and A2 adenosine receptors, respectively, was measured. A conjugate of XAC and 1,3,5-triisothiocyanatobenzene was 894-fold selective for A1 receptors. Reporter groups, such as fluorescent dyes and a spin-label, were included as chain substituents in the irreversibly binding analogues, which were designed for spectroscopic assays, histochemical characterization, and biochemical characterization of the receptor protein.
PMCID: PMC3427756  PMID: 1868116
8.  Characterization of the adenosine receptors of the rat superior cervical ganglion. 
British Journal of Pharmacology  1993;110(2):854-860.
1. Adenosine analogues caused hyperpolarization and inhibition of the depolarizing response to muscarine of the rat isolated superior cervical ganglion (SCG) measured by a 'grease gap' recording technique. The receptors mediating these responses have been characterized by use of a range of selective adenosine analogues and adenosine receptor antagonists. 2. In decreasing order of potency N6-cyclopentyladenosine (CPA), 2-chloroadenosine (2CA), adenosine, 2-phenylaminoadenosine (PAA), caused concentration-dependent hyperpolarizations whilst N6-(9-fluorenylmethyl)adenosine (PD 117,413) was inactive at up to 100 microM. 3. The order of potency of adenosine analogues in depressing depolarization caused by a submaximal concentration of muscarine (100 nM) was: CPA > R-PIA = 2CA > NECA > S-PIA > BZA > adenosine > PAA, where R- and S-PIA = R(-)- and S(+)-N6-(2-phenylisopropyl)adenosine, NECA = 5'N-ethylcarboxamidoadenosine and BZA = N6-benzyladenosine. PD 117,413 was inactive at concentrations up to 100 microM. The maximum inhibitions of the muscarine-induced depolarization by CPA, 2CA, NECA and BZA were similar. R-PIA, S-PIA and PAA produced similar maximal inhibitions which were significantly smaller than those produced by CPA. 4. Hyperpolarizations caused by adenosine were antagonized by the P1-purinoceptor selective antagonist 1,3-dimethyl-8-phenylxanthine (8PT) and by the selective A1-adenosine receptor antagonist, 1,3-dipropyl-8-(4-((2-aminoethyl)amino)carbonylmethyloxyphenyl++ +)xanthine (XAC). Hyperpolarizations caused by CPA, adenosine and PAA were antagonized by the A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2-selective antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX).(ABSTRACT TRUNCATED AT 250 WORDS)
PMCID: PMC2175924  PMID: 8242261
9.  Structure–Activity Relationships and Molecular Modeling of 3,5-Diacyl-2,4-dialkylpyridine Derivatives as Selective A3 Adenosine Receptor Antagonists 
Journal of medicinal chemistry  1998;41(17):3186-3201.
The structure-activity relationships of 6-phenyl-1,4-dihydropyridine derivatives as selective antagonists at human A3 adenosine receptors have been explored (Jiang et al. J. Med. Chem. 1997, 39, 4667-4675). In the present study, related pyridine derivatives have been synthesized and tested for affinity at adenosine receptors in radioligand binding assays. Ki values in the nanomolar range were observed for certain 3,5-diacyl-2,4-dialkyl-6-phenylpyridine derivatives in displacement of [125I]AB-MECA (N6-(4-amino-3-iodobenzyl)-5′-N-methylcarbamoyladenosine) at recombinant human A3 adenosine receptors. Selectivity for A3 adenosine receptors was determined vs radioligand binding at rat brain A1 and A2A receptors. Structure–activity relationships at various positions of the pyridine ring (the 3- and 5-acyl substituents and the 2- and 4-alkyl substituents) were probed. A 4-phenylethynyl group did not enhance A3 selectivity of pyridine derivatives, as it did for the 4-substituted dihydropyridines. At the 2-and 4-positions ethyl was favored over methyl. Also, unlike the dihydropyridines, a thioester group at the 3-position was favored over an ester for affinity at A3 adenosine receptors, and a 5-position benzyl ester decreased affinity. Small cycloalkyl groups at the 6-position of 4-phenylethynyl-1,4-dihydropyridines were favorable for high affinity at human A3 adenosine receptors, while in the pyridine series a 6-cyclopentyl group decreased affinity. 5-Ethyl 2,4-diethyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate, 38, was highly potent at human A3 receptors, with a Ki value of 20 nM. A 4-propyl derivative, 39b, was selective and highly potent at both human and rat A3 receptors, with Ki values of 18.9 and 113 nM, respectively. A 6-(3-chlorophenyl) derivative, 44, displayed a Ki value of 7.94 nM at human A3 receptors and selectivity of 5200-fold. Molecular modeling, based on the steric and electrostatic alignment (SEAL) method, defined common pharmacophore elements for pyridine and dihydropyridine structures, e.g., the two ester groups and the 6-phenyl group. Moreover, a relationship between affinity and hydrophobicity was found for the pyridines.
doi:10.1021/jm980093j
PMCID: PMC3474377  PMID: 9703464
10.  Functionalized Congeners of 1,3-Dialkylxanthines: Preparation of Analogues with High Affinity for Adenosine Receptors 
Journal of medicinal chemistry  1985;28(9):1334-1340.
A series of functionalized congeners of 1,3-dialkylxanthines has been prepared as adenosine receptor antagonists. On the basis of the high potency of 8-(p-hydroxyphenyl)-1,3-dialkylxanthines, the parent compounds were 8-[4-[(carboxymethyl)oxy]phenyl] derivatives of theophylline and 1,3-dipropylxanthine. A series of analogues including esters of ethanol and N-hydroxysuccinimide, amides, a hydrazide, an acylurea, and anilides were prepared. The potency in blocking A1-adenosine receptors (inhibition of binding of N6-[3H]cyclohexyladenosine to brain membranes) and A2-adenosine receptors (inhibition of 2-chloroadenosine-elicited accumulations of cyclic AMP in brain slices) was markedly affected by structural changes distal to the primary pharmacophore (8-phenyl-1,3-dialkylxanthine). Potencies in the dipropyl series at the A1 receptor ranged from K1 values of 1.2 nM for a congener with a terminal amidoethyleneamine moiety to a K1 value of 58 nM for the parent carboxylic acid to a K1 of 96 nM for the bulky ureido congener. Certain congeners were up to 145-fold more active at A1 receptors than at A2 receptors. Various derivatives of the congeners should be useful as receptor probes and for radioidodination, avidin binding, and preparation of affinity columns.
PMCID: PMC3468300  PMID: 2993622
11.  N6-Substituted adenosine derivatives: selectivity, efficacy, and species differences at A3 adenosine receptors 
Biochemical pharmacology  2003;65(10):1675-1684.
The activation of the human A3 adenosine receptor (AR) by a wide range of N6-substituted adenosine derivatives was studied in intact CHO cells stably expressing this receptor. Selectivity of binding at rat and human ARs was also determined. Among N6-alkyl substitutions, small N6-alkyl groups were associated with selectivity for human A3ARs vs. rat A3ARs, and multiple points of branching were associated with decreased hA3AR efficacy. N6-Cycloalkyl-substituted adenosines were full (≤5 carbons) or partial (≥6 carbons) hA3AR agonists. N6-(endo-Norbornyl)adenosine 13 was the most selective for both rat and human A1ARs. Numerous N6-arylmethyl analogues, including substituted benzyl, tended to be more potent in binding to A1 and A3 vs. A2AARs (with variable degrees of partial to full A3AR agonisms). A chloro substituent decreased the efficacy depending on its position on the benzyl ring. The A3AR affinity and efficacy of N6-arylethyl adenosines depended highly on stereochemistry, steric bulk, and ring constraints. Stereoselectivity of binding was demonstrated for N6-(R-1-phenylethyl)adenosine vs. N6-(S-1-phenylethyl)adenosine, as well as for the N6-(1-phenyl-2-pentyl)adenosine, at the rat, but not human A3AR. Interestingly, DPMA, a potent agonist for the A2AAR (Ki = 4 nM), was demonstrated to be a moderately potent antagonist for the human A3AR (Ki = 106 nM). N6-[(1S,2R)-2-Phenyl-1-cyclopropyl]adenosine 48 was 1100-fold more potent in binding to human (Ki = 0.63 nM) than rat A3ARs. Dual acting A1/A3 agonists (N6-3-chlorobenzyl- 29, N6-(S-1-phenylethyl)- 39, and 2-chloro-N6-(R-phenylisopropyl)adenosine 53) might be useful for cardioprotection.
PMCID: PMC3142561  PMID: 12754103
Purines; Nucleosides; GPCR; Cyclic AMP; Receptor binding; Structure–activity relationships
12.  Characterization of P1-purinoceptors on rat isolated duodenum longitudinal muscle and muscularis mucosae. 
British Journal of Pharmacology  1996;117(1):170-174.
1. P1-purinoceptors mediating relaxation of the rat duodenum longitudinal muscle and contraction of the rat duodenum muscularis mucosae were characterized by the use of adenosine and its analogues, 5'-N-ethylcarboxamidoadenosine (NECA), N6-cyclopentyl-adenosine (CPA), N6-(phenylisopropyl)adenosine (R-PIA), 2-chloroadenosine (2-CADO) and 2-p-((carboxyethyl)phenethylamino)-5'-carboxamidoadenosine (CGS21680), as well as the P1-purinoceptor antagonist 8-phenyltheophylline (8-PT) and the A1-selective antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). 2. In the rat duodenum longitudinal muscle, the order of potency of the adenosine agonists was CPA > NECA > adenosine > CGS21680. DPCPX antagonized responses to CPA and NECA at a concentration of 1 nM suggesting that they are acting at A1 receptors. A Schild plot versus CPA gave a slope near to unity (slope = 0.955) and a pA2 of 9.8 confirming that CPA was acting via A1 receptors. Schild analysis for DPCPX versus NECA, however, gave a slope of 0.674 suggesting that NECA was acting on both A1 and A2 receptors. CGS21680, a selective A2a agonist, was much less potent than adenosine suggesting that the A2 receptors are of the A2b subtype. 3. In the rat duodenum muscularis mucosae, the order of potency of the adenosine agonists was NECA > or = R-PIA = CPA > 2-CADO > adenosine, and DPCPX antagonized responses to CPA and NECA at a concentration of 1 microM. CGS21680, at a concentration of 10 microM, had no effect on this tissue. This suggests the presence of A2 receptors in this tissue and that they are of the A2b subtype. 4. These results are in agreement with previous studies in the whole duodenum showing the presence of A1 and A2b receptors causing relaxation, and this shows that the longitudinal muscle dominates the response of the whole tissue. In addition, a contractile A2b receptor has been revealed on the muscularis mucosae, the first time this subtype has been reported to elicit an excitatory response in a smooth muscle preparation.
PMCID: PMC1909363  PMID: 8825359
13.  Adenosine mediates relaxation of human small resistance-like coronary arteries via A2B receptors 
British Journal of Pharmacology  1999;126(8):1796-1800.
The receptor subtype and mechanisms underlying relaxation to adenosine were examined in human isolated small coronary arteries contracted with the thromboxane A2 mimetic, 1,5,5-hydroxy-11α, 9α-(epoxymethano)prosta-5Z, 13E-dienoic acid (U46619) to approximately 50% of their maximum contraction to K+ (125 mM) depolarization (Fmax). Relaxations were normalized as percentages of the 50% Fmax contraction.Adenosine caused concentration-dependent relaxations (pEC50, 5.95±0.20; maximum relaxation (Rmax), 96.7±1.4%) that were unaffected by either combined treatment with the nitric oxide inhibitors, NG-nitro-L-arginine (L-NOARG; 100 μM) and oxyhaemoglobin (HbO; 20 μM) or the ATP-dependent K+ channel (KATP) inhibitor, glibenclamide (10 μM). The pEC50 but not Rmax to adenosine was significantly reduced by high extracellular K+ (30 mM). Relaxations to the adenylate cyclase activator, forskolin, however, were unaffected by high K+ (30 mM).Adenosine and a range of adenosine analogues, adenosine, 2-chloroadenosine (2-CADO), 5′-N-ethyl-carboxamidoadenosine (NECA), R(−)-N6-(2-phenylisopropyl)-adenosine (R-PIA), S(+)-N6-(2-phenylisopropyl)-adenosine (S-PIA), N6-cyclopentyladenosine (CPA), 1-deoxy-1-[6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-N-methyl-β-D-ribofuranuronamide (IB-MECA), 2-p-(2-carboxyethyl)phenethylamino-5′-N-ethylcarboxamido adenosine hydrochloride (CGS 21680), relaxed arteries with a rank order of potency of NECA=2-CADO>adenosine=IB-MECA=R-PIA= CPA>S-PIA)>CGS 21680.Sensitivity but not Rmax to adenosine was significantly reduced approximately 80 and 20 fold by the non-selective adenosine receptor antagonist, 8-(p-sulphophenyl)theophylline (8-SPT) and the A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX). By contrast, the A1-selective antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) had no effect on pEC50 or Rmax to adenosine.These results suggest that A2B receptors mediate relaxation to adenosine in human small coronary arteries which is independent of NO but dependent in part on a K+-sensitive mechanism.
doi:10.1038/sj.bjp.0702462
PMCID: PMC1581442  PMID: 10372822
Adenosine; A2B-purinoceptors; human coronary artery
14.  Partial Agonists for A3 Adenosine Receptors 
Selective agonists for A3 adenosine receptors (ARs) could potentially be therapeutic agents for a variety of disorders, including brain and heart ischemic conditions, while partial agonists may have advantages over full agonists as a result of an increased selectivity of action. A number of structural determinants for A3AR activation have recently been identified, including the N6-benzyl group, methanocarba substitution of ribose, 2-chloro and 2-fluoro substituents, various 2’- and 3’-substitutions and 4’-thio substitution of oxygen. The 2-chloro substitution of CPA and R-PIA led to A3 antagonism (CCPA) and partial agonism (Cl-R-PIA). 2-Chloroadenosine was a full agonist, while 2-fluoroadenosine was a partial agonist. Both 2’- and 3’- substitutions have a pronounced effect on its efficacy, although the effect of 2’-substitution was more dramatic. The 4-thio substitution of oxygen may also diminish efficacy, depending on other substitutions. Both N6-methyl and N6-benzyl groups may contribute to the A3 affinity and selectivity; however, an N6-benzyl group but not an N6-methyl group diminishes A3AR efficacy. N6-benzyl substituted adenosine derivatives have similar potency for human and rat A3ARS while N6-methyl substitution was preferable for the human A3AR. The combination of 2-chloro and N6-benzyl substitutions appeared to reduce efficacy further than either modification alone. The A2AAR agonist DPMA was shown to be an antagonist for the human A3AR. Thus, the efficacy of adenosine derivatives at the A3AR appears to be more sensitive to small structural changes than at other subtypes. Potent and selective partial agonists for the A3AR could be identified by screening known adenosine derivatives and by modifying adenosine and the adenosine derivatives.
PMCID: PMC3425644  PMID: 15078216
15.  A2-purinoceptor-mediated relaxation in the guinea-pig coronary vasculature: a role for nitric oxide. 
British Journal of Pharmacology  1993;109(2):424-429.
1. The Langendorff heart preparation was used to investigate the mechanism of action of the endothelium-dependent vasodilatation evoked by adenosine and its analogues in the guinea-pig coronary vasculature. 2. The relative order of potency of adenosine and its analogues in causing a reduction in perfusion pressure was D-5'-(N-ethylcarboxamide)adenosine (NECA) = 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N- ethylcarboxamidoadenosine (CGS 21680)> R-N6-(2-phenylisopropyl)adenosine (R-PIA) = adenosine = 2-chloroadenosine (2-CA) > S-N6-(2-phenylisopropyl)adenosine (S-PIA) = N6-cyclopentyl-adenosine (CPA); thus suggesting the presence of A2-purinoceptors in this preparation. 3. 8-(p-Sulphophenyl)theophylline (8-PSPT; 3 x 10(-5) M) significantly reduced both the maximum amplitude and area of the vasodilatation produced in response to adenosine (5 x 10(-10) -5 x 10(-8) mol) without having any effect on the response to the P2-purinoceptor agonist, 2-methylthioATP. The relaxation induced by adenosine (5 x 10(-12) -5 x 10(-8) mol) was unaffected by the selective A1-purinoceptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 10(-8) M). This antagonist profile suggests that only A2-purinoceptors are present in the guinea-pig coronary vasculature. 4. The areas of the vasodilator response to adenosine (5 x 10(-10) -5 x 10(-7 mol), NECA (5 x 10(-12) -5 x 10(-7) mol) and CGS 21680 (5 x 10(-12) -5 x 10(-10) mol) were significantly reduced by NG-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)
PMCID: PMC2175698  PMID: 8358543
16.  Functionalized Congeners of 1,4-Dihydropyridines as Antagonist Molecular Probes for A3 Adenosine Receptors 
Bioconjugate chemistry  1999;10(4):667-677.
4-Phenylethynyl-6-phenyl-1,4-dihydropyridine derivatives are selective antagonists at human A3 adenosine receptors, with Ki values in a radioligand binding assay vs [125I]AB-MECA [N6-(4-amino-3-iodobenzyl)-5′-N-methylcarbamoyl-adenosine] in the submicromolar range. In this study, functionalized congeners of 1,4-dihydropyridines were designed as chemically reactive adenosine A3 antagonists, for the purpose of synthesizing molecular probes for this receptor subtype. Selectivity of the new analogues for cloned human A3 adenosine receptors was determined in radioligand binding in comparison to binding at rat brain A1 and A2A receptors. Benzyl ester groups at the 3- and/or 5-positions and phenyl groups at the 2- and/or 6-positions were introduced as potential sites for chain attachment. Structure–activity analysis at A3 adenosine receptors indicated that 3,5-dibenzyl esters, but not 2,6-diphenyl groups, are tolerated in binding. Ring substitution of the 5-benzyl ester with a 4-fluorosulfonyl group provided enhanced A3 receptor affinity resulting in a Ki value of 2.42 nM; however, a long-chain derivative containing terminal amine functionalization at the 4-position of the 5-benzyl ester showed only moderate affinity. This sulfonyl fluoride derivative appeared to bind irreversibly to the human A3 receptor (1 h incubation at 100 nM resulting in the loss of 56% of the specific radioligand binding sites), while the binding of other potent dihydropyridines and other antagonists was generally reversible. At the 3-position of the dihydropyridine ring, an amine-functionalized chain attached at the 4-position of a benzyl ester provided higher A3 receptor affinity than the corresponding 5-position isomer. This amine congener was also used as an intermediate in the synthesis of a biotin conjugate, which bound to A3 receptors with a Ki value of 0.60 μM.
doi:10.1021/bc9900136
PMCID: PMC3446815  PMID: 10411465
17.  8-SUBSTITUTED XANTHINES AS ANTAGONISTS AT A1- AND A2-ADENOSINE RECEPTORS 
Biochemical pharmacology  1988;37(19):3653-3661.
Two classes of 8-substituted analogs of theophylline (1,3-dialkylxanthines), having 8-cycloalkyl, 8-cycloalkenyl or 8-(para-substituted aryl) groups, were shown to be potent and, in some cases, receptor subtype selective antagonists at A1- and A2-adenosine receptors. New analogs based on a functionalized cogener approach and on classical medicinal chemical approaches were prepared. Affinity at A1-adenosine receptors was evaluated by inhibition of binding of [3H)N6-phenylisopropyladenosine to rat brain membranes. Activity at A2A-adenosine receptors was measured by the reversal of 5′-N-ethylcarboxamidoadenosine (NECA)-stimulated production of cyclic AMP in membranes from rat pheochromocytoma PC12 cells. Cycloalkenyl analogs containing rigid olefinic bonds differed greatly in potency from the saturated analogs. The selectivity of phenylsulfonamide analogs depended on distal structural features. Novel xanthine analogs include diamino-, thiol-, aldehyde, and halogen-substituted derivatives, peptide conjugates of 8-[4-[2-aminoethylaminocarbonylmethyloxy]phenyl]1,3-dipropylxanthine (XAC), and a hydroxyethylamide analog of XAC.
PMCID: PMC3469272  PMID: 3178879
18.  Improvement of Cold Tolerance by Selective A1 Adenosine Receptor Antagonists in Rats 
Previously we have shown that the improvement of cold tolerance by theophylline is due to antagonism at adenosine receptors rather than inhibition of phosphodiesterase. Since theophylline is a nonselective adenosine receptor antagonist for both A1 and A2 receptors, the present study investigated the adenosine receptor subtype involved in theophylline’s action. Acute systemic injection of selective A1 receptor antagonists (1,3-dialkyl-8-aryl or 1,3-dialkyl-8-cyclopentyl xanthine derivatives) significantly increased both the total and maximal heat production as well as cold tolerance. In contrast, injection of a relatively selective A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (compound No. 19), failed to significantly alter the thermogenic response of the rat under cold exposure. Further, the relative effectiveness of these compounds in increasing total thermogenesis was positively correlated with their potency in blocking the A1 adenosine receptor (r= .52, p<0.01), but not in A2 adenosine receptor (r= .20, p<0.2). It is likely that the thermally beneficial effects of adenosine A1 antagonists are due to their attenuation of the inhibitory effects of endogenously released adenosine on lipolysis and glucose utilization, resulting in increased substrate mobilization and utilization for enhanced thermogenesis.
PMCID: PMC4516057  PMID: 2263650
Adenosine Thermogenesis; Cold tolerance; Adenosine receptors; 1,3,7-Substituted xanthine derivatives; 1,3,8-Substituted xanthine derivatives
19.  Validation of Furchgott's method to determine agonist-dependent A1-adenosine receptor reserve in guinea-pig atrium 
British Journal of Pharmacology  1998;123(7):1425-1433.
The ubiquitous distribution of A1-adenosine receptors (A1AdoR) represents an impediment to achieve organ and/or response selectivity of A1AdoR agonists. Differential receptor reserve may be exploited to overcome this problem. We hypothesize that A1AdoR reserve is agonist-dependent and can be accurately estimated with Furchgott's method.Concentration-response curves were constructed from measurement of the atrial monophasic action potential duration in guinea-pig, isolated hearts treated with R(−) N6-(2-phenylisopropyl)adenosine (R-PIA) or 2-chloro-N6-cyclopentyl-adenosine (CCPA) before and after treatment with the selective, irreversible A1AdoR antagonist 8-cyclopentyl-3-[3-[[4-(fluorosulphonyl)benzoyl]oxy]propyl]-1-propyl-xanthine (FSCPX). Using Furchgott's method, we determined the equilibrium dissociation constant (KA) of R-PIA and CCPA, and the fraction of non-inactivated A1AdoRs remaining after FSCPX treatment (qfunctional). Values of qfunctional were correlated to the fraction of specific binding sites after FSCPX treatment labelled by [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-CPX) derived from saturation binding normalized to control (qbinding).Both R-PIA and CCPA are full A1AdoR agonists, but have significantly different potencies (pD2 [EC50]=6.84±0.04 [145 nM] vs 7.36±0.04 [44 nM], respectively), receptor affinities (pKA [KA]= 6.54±0.10 [288 nM] vs 6.13±0.03 [734 nM]), and pharmacological shift ratios defined as KA/EC50 (2.2±0.6 vs 15.9±1.5). Values for qfunctional and qbinding were highly correlated (r2=0.96). The ratio between the intrinsic efficacies of CCPA and R-PIA derived from Furchgott's analysis was 5.9, a value similar to the ratio of 6.2–6.6 calculated from previously obtained binding data.Radioligand binding studies validated the use of Furchgott's method to estimate A1AdoR reserve. A1AdoR reserve was agonist-dependent. CCPA was shown to be a high intrinsic efficacy, low affinity agonist, whereas R-PIA was found to be a low intrinsic efficacy, high affinity agonist.
doi:10.1038/sj.bjp.0701747
PMCID: PMC1565302  PMID: 9579739
Adenosine; A1-adenosine receptor; CCPA; Furchgott's method; receptor reserve; R-PIA
20.  Species differences in brain adenosine A1 receptor pharmacology revealed by use of xanthine and pyrazolopyridine based antagonists 
British Journal of Pharmacology  1997;122(6):1202-1208.
The pharmacological profile of adenosine A1 receptors in human, guinea-pig, rat and mouse brain membranes was characterized in a radioligand binding assay by use of the receptor selective antagonist, [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]-DPCPX).The affinity of [3H]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes than in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pKD values (M) were 9.55, 9.44, 8.85, 8.94, 8.67, 9.39 and 8.67, respectively. The binding site density (Bmax) was lower in rat cortical membranes than in guinea-pig or human cortical membranes.The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5′-N-ethylcarboxamidoadenosine (NECA), agonist affinity was 3.5–26.2 fold higher in rat cortical membranes than in human and guinea-pig brain membranes; affinity in rat and mouse brain membranes was similar. While NECA exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP analogue, Gpp(NH)p (100 μM) reduced 2-chloro-N6-cyclopentyladenosine (CCPA) affinity 7–13.9 fold, whereas the affinity of DPCPX was unaffected.The affinity of six xanthine-based adenosine receptor antagonists was 2.2–15.9 fold higher in rat cortical membranes compared with human or guinea-pig membranes. The rank order of potency was species-independent. In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl) acryloyl]-piperidin-2-yl acetic acid (FK352) and 6-oxo-3-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-1(6H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pKi values (M) for [3H]-DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7.92, respectively.Drug affinity for adenosine A2A receptors was determined in a [3H]-2-[4-(2-carboxyethyl)phenethylamino]-5′-N-ethylcarboxamidoadenosine ([3H]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyridine derivatives, FK453, FK838 and FK352 exhibited pKi values (M) of 5.90, 5.92 and 4.31, respectively, compared with pKi values of 9.31, 8.18 and 7.57 determined in the [3H]-DPCPX binding assay in rat cortical membranes. These novel pyrazolopyridine derivatives therefore represent high affinity, adenosine A1 receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity for [3H]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.
doi:10.1038/sj.bjp.0701465
PMCID: PMC1565029  PMID: 9401787
Adenosine receptors; [3H]-DPCPX; [3H]-CGS 21680; radioligand binding; FK453; brain
21.  A Functionalized Congener Approach to Adenosine Receptor Antagonists: Amino Acid Conjugates of 1,3-Dipropylxanthine 
Molecular pharmacology  1986;29(2):126-133.
SUMMARY
1,3-Dipropyl-8-phenylxanthine, a synthetic analog of theophylline and a potent antagonist of adenosine at A1 and A2-adenosine receptors, has been attached covalently through a functionalized chain to amino acids and oligopeptides. The xanthine conjugates have been studied as competitive inhibitors of the specific binding of [3H]N6-cyclohexyladenosine to A1-receptors of rat cerebral cortical membranes and for inhibition of cyclic AMP accumulation elicited by 2-chloroadenosine in guinea pig brain slices through A2-receptors. A free amino group on the extended chain generally resulted in high potency at A1-receptors. The potency (in some cases extending into the subnanomolar range) and selectivity for A1-receptors (up to 200-fold) suggest that this approach can yield a versatile class of “functionalized congeners” of adenosine receptor antagonists in which distal modifications of the attached moiety (“carrier”) can serve also to improve pharmacodynamic and pharmacokinetic parameters. The water solubility in many of the more potent analogs has been enhanced by two orders of magnitude over that of simple, uncharged 8-phenyl xanthine derivatives. Analogs in which the carrier contains d-tyrosine have potential for development of iodinated radioligands for adenosine receptors. The functionalized congener approach is potentially applicable to other drugs and for development of prodrugs.
PMCID: PMC3459325  PMID: 3005825
22.  Characterization of the locomotor depression produced by an A2-selective adenosine agonist 
FEBS letters  1990;261(1):67-70.
Adenosine analogs, such as N6-cyclohexyladenosine (CHA) that are selective for A1-adenosine receptors, and analogs, such as 5′-N-ethylcarboxamidoadenosine (NECA) that are active at both A1 and A2 receptors, cause a profound depression of locomotor activity in mice via a central mechanism. The depression is effectively reversed by non-selective adenosine antagonists such as theophylline. We report that 2-[(2-aminoethyl-amino)carbonylethylphenylethylamino]-5′-N-ethylcarboxamidoadenosine (APEC), an amine derivative of the A2-selective agonist, CGS21680, is a potent locomotor depressant in mice. The in vivo pharmacology is consistent with A2-selectivity at a central site of action. Two parameters indicative of locomotor activity, horizontal activity and total distance travelled, were measured using a computerized activity monitor. From dose-response curves it was found that APEC (ED50 16 μg/kg) is more potent than CHA (ED50 60 μg/kg) and less potent than NECA (ED50 2 μg/kg). The locomotor depression by APEC was reversible by theophylline, but not by the A1-selective antagonists 8-cyclopentyltheophylline (CPT) and 8-cyclopentyl-1,3-dipropyl-2-thioxanthine, nor by the peripheral antagonists 8-p-sulfophenyltheophylline (8-PST) and 1,3-dipropyl-8-p-sulfophenylxanthine. The locomotor activity depression elicited by NECA and CHA was reversed by A1-selective antagonists. These results suggest that the effects of APEC are due to stimulation of A2 adenosine receptors in the brain.
PMCID: PMC3469261  PMID: 2307237
Adenosine analog; Locomotor depression; Adenosine receptor
23.  Cyclic AMP-dependent inhibition of human neutrophil oxidative activity by substituted 2-propynylcyclohexyl adenosine A2A receptor agonists 
British Journal of Pharmacology  2001;132(5):1017-1026.
Novel 2-propynylcyclohexyl-5′-N-ehtylcarboxamidoadenosines, trans-substituted in the 4-position of the cyclohexyl ring, were evaluated in binding assays to the four subtypes of adenosine receptors (ARs). Two esters, 4-{3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl}-cyclohexanecarboxylic acid methyl ester (ATL146e) and acetic acid 4-{3-[6-amino-9-(5-ethylcarbamoyl-3, 4-dihydroxy-tetrahydro-furan -2-yl)-9H-purin-2-yl] -prop-2-ynyl}-cyclohexylmethyl ester (ATL193) were >50×more potent than 2-[4-(2-carboxyethyl)phenethylamino]-5′-N-ethylcarboxamidoadenosine (CGS21680) for human A2A AR binding. Human A2A AR affinity for substituted cyclohexyl-propynyladenosine analogues was inversely correlated with the polarity of the cyclohexyl side chain. There was a comparable order of potency for A2A AR agonist stimulation of human neutrophil [cyclic AMP]i, and inhibition of the neutrophil oxidative burst. ATL146e and CGS21680 were ∼equipotent agonists of human A3 ARs.We measured the effects of selective AR antagonists on agonist stimulated neutrophil [cyclic AMP]i and the effect of PKA inhibition on A2A AR agonist activity. ATL193-stimulated neutrophil [cyclic AMP]i was blocked by antagonists with the potency order: ZM241385 (A2A-selective)>MRS1220 (A3-selective)>>N-(4-Cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754; A2B-selective) ≈amp; 8-(N-methylisopropyl)amino-N6-(5′-endohydroxy-endonorbornyl)-9-methyladenine (WRC0571; A1-selective). The type IV phosphodiesterase inhibitor, rolipram (100 nM) potentiated ATL193 inhibition of the oxidative burst, and inhibition by ATL193 was counteracted by the PKA inhibitor H-89.The data indicate that activation of A2AARs inhibits neutrophil oxidative activity by activating [cyclic AMP]i/PKA.
doi:10.1038/sj.bjp.0703893
PMCID: PMC1572638  PMID: 11226132
A2A adenosine receptors; neutrophil; oxidative burst; cyclic AMP-dependent protein kinase
24.  Predominant role of A1 adenosine receptors in mediating adenosine induced vasodilatation of rat diaphragmatic arterioles: involvement of nitric oxide and the ATP-dependent K+ channels 
British Journal of Pharmacology  1997;121(7):1355-1363.
We investigated, by intravital microscopy in rats, the role of the subtypes of adenosine receptors A1 (A1/AR) and A2 (A2AR) in mediating adenosine-induced vasodilatation of second and third order arterioles of the diaphragm.Adenosine, and the A1AR selective agonists R(−)-N6-(2-phenylisopropyl)-adenosine (R-PIA) and N6-cyclo-pentyl-adenosine (CPA) induced a similar concentration-dependent dilatation of diaphragmatic arterioles. The non selective A2AR subtype agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl) ethyl]adenosine (DPMA) also dilated diaphragmatic arterioles but induced a significantly smaller dilatation than adenosine. By contrast the selective A2aAR subtype agonist 2-[p-(2-carboxyethyl)phenyl amino]-5′-N-ethyl carboxamido adenosine (CGS 21680) did not modify diaphragmatic arteriolar diameter.The non selective adenosine receptor antagonist 1,3-dipropyl-8-p-sulphophenylxanthine (SPX, 100 μM) and the selective A1AR antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX, 50 nM) significantly attenuated adenosine-induced dilatation of diaphragmatic arterioles. By contrast, adenosine significantly dilated diaphragmatic arterioles in the presence of A2AR antagonist 3,7-dimethyl-1-propargylxanthine (DMPX, 10 μM).The dilatation induced by adenosine was unchanged by the mast cell stabilizing agent sodium cromoglycate (cromolyn, 10 μM).The nitric oxide (NO) synthase inhibitor Nω-nitro-L-arginine (L-NOARG, 300 μM) attenuated the dilatation induced by adenosine, and by the A1AR and A2AR agonists.The ATP-dependent K+ channel blocker glibenclamide (3 μM) significantly attenuated diaphragmatic arteriolar dilatation induced by adenosine and by the A1AR agonists R-PIA and CPA. By contrast, glibenclamide did not significantly modify arteriolar dilatation induced by the A2AR agonist DPMA.These findings suggest that adenosine-induced dilatation of diaphragmatic arterioles in the rat is predominantly mediated by the A1AR, via the release of NO and activation of the ATP-dependent K+ channels.
doi:10.1038/sj.bjp.0701247
PMCID: PMC1564813  PMID: 9257914
Diaphragm; arterioles; adenosine receptors; R-PIA; DPMA; DPCPX; DMPX; DPSPX; nitric oxide; glibenclamide
25.  Structural determinants of efficacy at A3 adenosine receptors: modification of the ribose moiety 
Biochemical pharmacology  2004;67(5):893-901.
We have found previously that structural features of adenosine derivatives, particularly at the N6- and 2-positions of adenine, determine the intrinsic efficacy as A3 adenosine receptor (AR) agonists. Here, we have probed this phenomenon with respect to the ribose moiety using a series of ribose-modified adenosine derivatives, examining binding affinity and activation of the human A3 AR expressed in CHO cells. Both 2′- and 3′-hydroxyl groups in the ribose moiety contribute to A3 AR binding and activation, with 2′-OH being more essential. Thus, the 2′-fluoro substitution eliminated both binding and activation, while a 3′-fluoro substitution led to only a partial reduction of potency and efficacy at the A3 AR. A 5′-uronamide group, known to restore full efficacy in other derivatives, failed to fully overcome the diminished efficacy of 3′-fluoro derivatives. The 4′-thio substitution, which generally enhanced A3 AR potency and selectivity, resulted in 5′-CH2OH analogues (10 and 12) which were partial agonists of the A3 AR. Interestingly, the shifting of the N6-(3-iodobenzyl)adenine moiety from the 1′- to 4′-position had a minor influence on A3 AR selectivity, but transformed 15 into a potent antagonist (16) (Ki = 4.3 nM). Compound 16 antagonized human A3 AR agonist-induced inhibition of cyclic AMP with a KB value of 3.0 nM. A novel apio analogue (20) of neplanocin A, was a full A3 AR agonist. The affinities of selected, novel analogues at rat ARs were examined, revealing species differences. In summary, critical structural determinants for human A3 AR activation have been identified, which should prove useful for further understanding the mechanism of receptor activation and development of more potent and selective full agonists, partial agonists and antagonists for A3 ARs.
PMCID: PMC3150582  PMID: 15104242
Nucleosides; A3 adenosine receptor agonist; A3 adenosine receptor antagonist; Adenylyl cyclase; Phospholipase C; Partial agonist

Results 1-25 (1127162)