Agonist derived molecular probes for A2 adenosine receptors

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

Molecular probes for the human adenosine receptors

Adenosine receptors, G protein-coupled receptors (GPCRs) that are activated by the endogenous ligand adenosine, have been considered potential therapeutic targets in several disorders. To date however, only very few adenosine receptor modulators have made it to the market. Increased understanding of these receptors is required to improve the success rate of adenosine receptor drug discovery. To improve our understanding of receptor structure and function, over the past decades, a diverse array of molecular probes has been developed and applied. These probes, including radioactive or fluorescent moieties, have proven invaluable in GPCR research in general. Specifically for adenosine receptors, the development and application of covalent or reversible probes, whether radiolabeled or fluorescent, have been instrumental in the discovery of new chemical entities, the characterization and interrogation of adenosine receptor subtypes, and the study of adenosine receptor behavior in physiological and pathophysiological conditions. This review summarizes these applications, and also serves as an invitation to walk another mile to further improve probe characteristics and develop additional tags that allow the investigation of adenosine receptors and other GPCRs in even finer detail.

Chemical Probes for the Adenosine Receptors

Research on the adenosine receptors has been supported by the continuous discovery of new chemical probes characterized by more and more affinity and selectivity for the single adenosine receptor subtypes (A₁, A_2A, A_2B and A₃ adenosine receptors). Furthermore, the development of new techniques for the detection of G protein-coupled receptors (GPCR) requires new specific probes. In fact, if in the past radioligands were the most important GPCR probes for detection, compound screening and diagnostic purposes, nowadays, increasing importance is given to fluorescent and covalent ligands. In fact, advances in techniques such as fluorescence resonance energy transfer (FRET) and fluorescent polarization, as well as new applications in flow cytometry and different fluorescence-based microscopic techniques, are at the origin of the extensive research of new fluorescent ligands for these receptors. The resurgence of covalent ligands is due in part to a change in the common thinking in the medicinal chemistry community that a covalent drug is necessarily more toxic than a reversible one, and in part to the useful application of covalent ligands in GPCR structural biology. In this review, an updated collection of available chemical probes targeting adenosine receptors is reported.

Fluorescent ligands for adenosine receptors

Interest is increasing in developing fluorescent ligands for characterization of adenosine receptors (ARs), which hold a promise of usefulness in the drug discovery process. The size of a strategically labeled AR ligand can be greatly increased after the attachment of a fluorophore. The choice of dye moiety (e.g. Alexa Fluor 488), attachment point and linker length can alter the selectivity and potency of the parent molecule. Fluorescent derivatives of adenosine agonists and antagonists (e.g. XAC and other heterocyclic antagonist scaffolds) have been synthesized and characterized pharmacologically. Some are useful AR probes for flow cytometry, fluorescence correlation spectroscopy, fluorescence microscopy, fluorescence polarization, fluorescence resonance energy transfer, and scanning confocal microscopy. Thus, the approach of fluorescent labeled GPCR ligands, including those for ARs, is a growing dynamic research field.

[(3)H]XAC (xanthine amine congener) is a radioligand for A(2)-adenosine receptors in rabbit striatum

The intrinsic affinity of 8-phenylxanthine analogs at striatal A(2)-adenosine receptors is highly species dependent. [(3)H]XAC (8-[2-aminoethyl[amino[carbonyl[methyl[oxyphenyl]]]]]-1,3-dipropylxanthine), although A(1)-selective in the rat brain, binds to A(2) receptors in rabbit striatal membranes with sufficiently high affinity to serve as a radioligand. In the presence of 50 nM CPX (8-cyclopentyl-1,3-dipropylxanthine), an A(1)-selective antagonist added to eliminate binding to A(1) receptors, [(3)H]XAC exhibits saturable, specific binding (70% of total) to A(2) sites with a K(d) of 3.8 nM and a B(max) of 1.23 pmol/mg protein. At 24 degrees C, the association and dissociation rate constants were 0.13 min(?1) nM(?1) and 0.36 min(?1), respectively. Binding was performed for 1 h, with non-specific binding defined in the presence of 100 ?M NECA (N-ethylcarboxamidoadenosine). The potency order for antagonists against 1 nM [(3)H]XAC at rabbit A(2)-receptors was XAC ? N(?)-Me-XAC ? CPX = XCC > 1,3-dipropyl-8-p-sulfophenylxanthine > PSPT. The relative potency order for agonists was CGS ? NECA > APEC [= 2-(aminoethylaminocarbonyl-ethylphenylethylamino)-NECA] > PAPA-APEC > ADAC > R-PIA (N(6)-phenylisopropyladenosine) > S-PIA. The ability to characterize central A(2)-adenosine receptors using an antagonist ligand that is chemically functionalized offers the possibility to design affinity labeling probes for this receptor subtype in the brain, similar to those antagonist probes already developed for A(1)-receptors. The results also suggest that affinity columns containing chemically immobilized XAC may be used for isolating central A(2)-adenosine receptors from rabbit striatum.

Evaluation of molecular modeling of agonist binding in light of the crystallographic structure of an agonist-bound A₂A adenosine receptor

Molecular modeling of agonist binding to the human A(2A) adenosine receptor (AR) was assessed and extended in light of crystallographic structures. Heterocyclic adenine nitrogens of cocrystallized agonist overlaid corresponding positions of the heterocyclic base of a bound triazolotriazine antagonist, and ribose moiety was coordinated in a hydrophilic region, as previously predicted based on modeling using the inactive receptor. Automatic agonist docking of 20 known potent nucleoside agonists to agonist-bound A(2A)AR crystallographic structures predicted new stabilizing protein interactions to provide a structural basis for previous empirical structure activity relationships consistent with previous mutagenesis results. We predicted binding of novel C2 terminal amino acid conjugates of A(2A)AR agonist CGS21680 and used these models to interpret effects on binding affinity of newly synthesized agonists. d-Amino acid conjugates were generally more potent than l-stereoisomers and free terminal carboxylates more potent than corresponding methyl esters. Amino acid moieties were coordinated close to extracellular loops 2 and 3. Thus, molecular modeling is useful in probing ligand recognition and rational design of GPCR-targeting compounds with specific pharmacological profiles.

Nucleoside conjugates of quantum dots for characterization of G protein-coupled receptors: strategies for immobilizing A2A adenosine receptor agonists

Background

Quantum dots (QDs) are crystalline nanoparticles that are compatible with biological systems to provide a chemically and photochemically stable fluorescent label. New ligand probes with fluorescent reporter groups are needed for detection and characterization of G protein-coupled receptors (GPCRs).

Results

Synthetic strategies for coupling the A_2A adenosine receptor (AR) agonist CGS21680 (2-[4-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine) to functionalized QDs were explored. Conjugates tethered through amide-linked chains and poly(ethyleneglycol) (PEG) displayed low solubility and lacked receptor affinity. The anchor to the dendron was either through two thiol groups of (R)-thioctic acid or through amide formation to a commercial carboxy-derivatized QD. The most effective approach was to use polyamidoamine (PAMAM) D5 dendrons as multivalent spacer groups, grafted on the QD surface through a thioctic acid moiety. In radioligand binding assays, dendron nucleoside conjugate 11 displayed a moderate affinity at the human A_2AAR (K_iapp 1.02 ± 0.15 μM). The QD conjugate of increased water solubility 13, resulting from the anchoring of this dendron derivative, interacted with the receptor with K_iapp of 118 ± 54 nM. The fluorescence emission of 13 occurred at 565 nm, and the presence of the pendant nucleoside did not appreciably quench the fluorescence.

Conclusions

This is a feasibility study to demonstrate a means of conjugating to a QD a small molecular pharmacophore of a GPCR that is relatively hydrophobic. Further enhancement of affinity by altering the pharmacophore or the linking structures will be needed to make useful affinity probes.

Functionalized congener approach to the design of ligands for G protein-coupled receptors (GPCRs)

Functionalized congeners, in which a chemically functionalized chain is incorporated at an insensitive site on a pharmacophore, have been designed from the agonist and antagonist ligands of various G protein-coupled receptors (GPCRs). These chain extensions enable a conjugation strategy for detecting and characterizing GPCR structure and function and pharmacological modulation. The focus in many studies of functionalized congeners has been on two families of GPCRs: those responding to extracellular purines and pyrimidines-i.e., adenosine receptors (ARs) and P2Y nucleotide receptors. Functionalized congeners of small molecule as ligands for other GPCRs and non-G protein coupled receptors have also been designed. For example, among biogenic amine neurotransmitter receptors, muscarinic acetylcholine receptor antagonists and adrenergic receptor ligands have been studied with a functionalized congener approach. Adenosine A(1), A(2A), and A(3) receptor functionalized congeners have yielded macromolecular conjugates, irreversibly binding AR ligands for receptor inactivation and cross-linking, radioactive probes that use prosthetic groups, immobilized ligands for affinity chromatography, and dual-acting ligands that function as binary drugs. Poly(amidoamine) dendrimers have served as nanocarriers for covalently conjugated AR functionalized congeners. Rational methods of ligand design derived from molecular modeling and templates have been included in these studies. Thus, the design of novel ligands, both small molecules and macromolecular conjugates, for studying the chemical and biological properties of GPCRs have been developed with this approach, has provided researchers with a strategy that is more versatile than the classical medicinal chemical approaches.

Application of the functionalized congener approach to dendrimer-based signaling agents acting through A(2A) adenosine receptors

As a continued effort to develop multivalent ligands to enhance the pharmacological effects of monomeric drugs, DITC-APEC, a chemically reactive nucleoside A(2A) adenosine receptor (AR) agonist, was employed to derivatize the surface of third-generation (G3) polyamidoamine (PAMAM) dendrimers. The resulting conjugates carried multiple copies of the agonist attached through a thiourea linkage and differed in the number of attachments and in the presence of a fluorophore or additional surface modification. Computer modeling studies suggested that these DITC-APEC-loaded dendrimers extended the overall diameter of the previously reported PAMAM-CGS21680 dendrimer derivatives (Kim et al., Bioconjugate Chem 2008 19:406-411) by ca. 20 A, potentially increasing the conformational flexibility of the appended ligands to achieve optimal geometry for efficient binding at A(2A) ARs. Increased affinity and selectivity in binding in comparison to the CGS21680 conjugate were envisioned, due to the presence of an extended linker, i.e., a dithioureylenephenyl functionality. In vitro radioligand competition experiments showed effective binding of these PAMAM-DITC-APEC dendrimer conjugates at the human A(2A) and A(3) ARs with submicromolar K (i) values and selectivity in comparison to the human A(1) AR. Furthermore, these nucleoside-loaded dendrimers exhibited an A(2A) AR-mediated inhibitory effect on ADP-induced aggregation of human platelets. The present study demonstrates the potential of applying the functionalized congener concept to engineer dendrimer-based multivalent ligands for G protein-coupled receptors.

Adenosine A(2A) receptor dynamics studied with the novel fluorescent agonist Alexa488-APEC

G protein-coupled receptors, such as the adenosine A(2A) receptor, are dynamic proteins, which undergo agonist-dependent redistribution from the cell surface to intracellular membranous compartments, such as endosomes. In order to study the kinetics of adenosine A(2A) receptor redistribution in living cells, we synthesized a novel fluorescent agonist, Alexa488-APEC. Alexa488-APEC binds to adenosine A(2A) (K(i)=149+/-27 nM) as well as A(3) receptors (K(i)=240+/-160 nM) but not to adenosine A(1) receptors. Further, we characterized the dose-dependent increase in Alexa488-APEC-induced cAMP production as well as cAMP response element binding (CREB) protein phosphorylation, verifying the ligand's functionality at adenosine A(2A) but not A(2B) receptors. In live-cell imaging studies, Alexa488-APEC-induced adenosine A(2A) receptor internalization, which was blocked by the competitive reversible antagonist ZM 241385 and hyperosmolaric sucrose. Further, internalized adenosine A(2A) receptors co-localized with clathrin and Rab5, indicating that agonist stimulation promotes adenosine A(2A) receptor uptake through a clathrin-dependent mechanism to Rab5-positive endosomes. The basic characterization of Alexa488-APEC described here showed that it provides a useful tool for tracing adenosine A(2A) receptors in vitro.

Fluorosulfonyl- and bis-(beta-chloroethyl)amino-phenylamino functionalized pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine derivatives: irreversible antagonists at the human A3 adenosine receptor and molecular modeling studies

A series of pyrazolotriazolopyrimidines was previously reported to be highly potent and selective human A(3) adenosine receptor antagonists (Baraldi et al. J. Med. Chem. 2000, 43, 4768-4780). A derivative having a methyl group at the N(8) pyrazole combined with a 4-methoxyphenylcarbamoyl moiety at N(5) position, displayed a K(i) value at the hA(3) receptor of 0.2 nM. We now describe chemically reactive derivatives which act as irreversible inhibitors of this receptor. Electrophilic groups, specifically sulfonyl fluoride and nitrogen mustard (bis-(beta-chloroethyl)amino) moieties, have been incorporated at the 4-position of the aryl urea group. Membranes containing the recombinant hA(3) receptor were preincubated with the compounds and washed exhaustively. The loss of ability to bind radioligand following this treatment indicated irreversible binding. The most potent compound in irreversibly binding to the receptor was 14, which contained a sulfonyl fluoride moiety and a propyl group at the N(8) pyrazole nitrogen. The bis-(beta-chloroethyl)amino derivatives displayed a much smaller degree of irreversible binding than the sulfonyl fluoride derivatives. A computer-generated model of the human A(3) receptor was built and analyzed to help interpret these results. The model of the A(3) transmembrane region was derived using primary sequence comparison, secondary structure predictions, and three-dimensional homology building, using the recently published crystal structure of rhodopsin as a template. According to our model, sulfonyl fluoride derivatives could dock within the hypothetical TM binding domain, adopting two different energetically favorable conformations. We have identified two amino acids, Ser247 and Cys251, both in TM6, as potential nucleophilic partners of the irreversible binding to the receptor.

Mutagenesis reveals structure-activity parallels between human A2A adenosine receptors and biogenic amine G protein-coupled receptors

Structure-affinity relationships for ligand binding at the human A2A adenosine receptor have been probed using site-directed mutagenesis in the transmembrane helical domains (TMs). The mutant receptors were expressed in COS-7 cells and characterized by binding of the radioligands [3H]CGS21680, [3H]NECA, and [3H]XAC. Three residues, at positions essential for ligand binding in other G protein-coupled receptors, were individually mutated. The residue V(3.32) in the A2A receptor that is homologous to the essential aspartate residue of TM3 in the biogenic amine receptors, i.e., V84(3.32), may be substituted with L (present in the A3 receptor) but not with D (in biogenic amine receptors) or A. H250(6.52), homologous to the critical N507 of rat m3 muscarinic acetylcholine receptors, may be substituted with other aromatic residues or with N but not with A (Kim et al. J. Biol. Chem. 1995, 270, 13987-13997). H278(7.43), homologous to the covalent ligand anchor site in rhodopsin, may not be substituted with either A, K, or N. Both V84L(3.32) and H250N(6.52) mutant receptors were highly variable in their effect on ligand competition depending on the structural class of the ligand. Adenosine-5'-uronamide derivatives were more potent at the H250N(6.52) mutant receptor than at wild type receptors. Xanthines tended to be close in potency (H250N(6.52)) or less potent (V84L(3.32)) than at wild type receptors. The affinity of CGS21680 increased as the pH was lowered to 5.5 in both the wild type and H250N(6.52) mutant receptors. Thus, protonation of H250(6.52) is not involved in this pH dependence. These data are consistent with a molecular model predicting the proximity of bound agonist ligands to TM3, TM5, TM6, and TM7.

Hydrophilic side chains in the third and seventh transmembrane helical domains of human A2A adenosine receptors are required for ligand recognition

Hydrophilic residues of the G protein-coupled human A2A adenosine receptor that are potentially involved in the binding of the ribose moiety of adenosine were targeted for mutagenesis. Residues in a T88QSS91 sequence in the third transmembrane helical domain (TM3) were individually replaced with alanine and other amino acids. Two additional serine residues in TM7 that were previously shown to be involved in ligand binding were mutated to other uncharged, hydrophilic amino acids. The binding affinity of agonists at T88 mutant receptors was greatly diminished, although the receptors were well expressed and bound antagonists similar to the wild-type receptor. Thus, mutations that are specific for diminishing the affinity of ribose-containing ligands (i.e., adenosine agonists) have been identified in both TM3 and TM7. The T88A and T88S mutant receptor fully stimulated adenylyl cyclase, with the dose-response curves to CGS 21680 highly shifted to the right. A Q89A mutant gained affinity for all agonist and antagonist ligands examined in binding and functional assays. Q89 likely plays an indirect role in ligand binding. S90A, S91A, and S277C mutant receptors displayed only moderate changes in ligand affinity. A S281N mutant gained affinity for all adenosine derivatives (agonists), but antagonist affinity was generally diminished, with the exception of a novel tetrahydrobenzothiophenone derivative.

Functional characterization of adenosine A2 receptors in Jurkat cells and PC12 cells using adenosine receptor agonists

The effect of several adenosine analogues on cyclic AMP accumulation was examined in the rat phaeochromocytoma cell PC12 and in the human T-cell leukaemia cell Jurkat, selected as prototypes of cells predominantly expressing adenosine A2A or A2B receptors. Using the reverse transcription-polymerase chain reaction it was, however, demonstrated that the Jurkat cell and the PC12 cell express both A2A and A2B receptor mRNA, albeit in different relative proportions. In PC12 cells the concentration required for half-maximal response (EC50) for the full agonist 5'-N-ethyl-carboxamidoadenosine (NECA) was 30 times lower than in Jurkat cells. There was no significant difference in the pA2 for the antagonist 5-amino-9-chloro-2-(2-furanyl)- 1,2,4-triazolo(1,5-C)quinazolinemonomethanesulphonate (CGS 15943) between the two cell types. In the presence of forskolin (1 microM in PC12 cells; 10 microM in Jurkat cells) the EC50 value for NECA was reduced two-to sixfold. Forskolin also increased the maximal cAMP accumulation twofold in PC12 cells and sevenfold in Jurkat cells. A series of 2-substituted adenosine analogues CV 1808 (2-phenylamino adenosine), CV 1674 [2-(4-methoxyphenyl)adenosine], CGS 21680 ¿2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethyl- carboxamido adenosine¿, and four 2-substituted isoguanosines, SHA 40 [2-(2-phenylethoxy)adenosine; PEA], SHA 91 [2-(2-cyclohexylethoxy)adenosine; CEA], SHA 118 ¿2-[2-(p-methylphenyl)ethoxy]adenosine; MPEA¿, and SHA 125 (2-hexyloxyadenosine; HOA), all raised cAMP accumulation in PC12 cells, but had minimal or no effect in Jurkat cells. In the PC12 cells the addition of forskolin (1 microM) reduced the EC50 by a factor of 2(CV 1808) to 12 (SHA 125). In Jurkat cells all the analogues gave a significant, but submaximal, cAMP response in the presence of forskolin (10 microM), but they were essentially inactive in its absence. The results show that a series of 2-substituted adenosine analogues can be used to discriminate between A2A and A2B receptors. The two receptor subtypes appear to coexist, even in clonal cells selected for typical pharmacology. A2 receptor pharmacology can therefore be complex.

Evidence for high-affinity binding sites for the adenosine A2A receptor agonist [3H] CGS 21680 in the rat hippocampus and cerebral cortex that are different from striatal A2A receptors

The binding of the adenosine A2A receptor selective agonist 2-[4-(2-p-carboxyethyl)phenylamino] -5'-N-ethylcarboxamidoadenosine (CGS 21680) to the rat hippocampal and cerebral cortical membranes was studied and compared with that to striatal membranes. [3H] CGS 21680, in the concentration range tested (0.2-200 nM), bound to a single site with a Kd of 58 nM and a Bmax of 353 fmol/mg protein in the hippocampus, and with a Kd of 58 nM and a Bmax of 264 fmol/mg protein in the cortex; in the striatum, the single high-affinity [3H] CGS 21680 binding site had a Kd of 17 nM and a Bmax of 419 fmol/mg protein. Both guanylylimidodiphosphate (100 microM) and Na+ (100 mM) reduced the affinity of [3H] CGS 21680 binding in the striatum by half and virtually abolished [3H] CGS 21680 binding in the hippocampus and cortex. The displacement curves of [3H] CGS 21680 binding with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), N6-cyclohexyladenosine (CHA), 5'-N-ethylcarboxamidoadenosine (NECA) and 2-chloroadenosine (CADO) were biphasic in the hippocampus and cortex as well as in the striatum. The predominant [3H]CGS 21680 binding site in the striatum (80%) had a pharmacological profile compatible with A2A receptors and was also present in the hippocampus and cortex, representing 10-25% of [3H]CGS 21680 binding. The predominant [3H]CGS 21680 binding site in the hippocampus and cortex had a pharmacological profile distinct from A2A receptors: the relative potency order of adenosine antagonists DPCPX, 1,3-dipropyl- 8-¿4-[(2-aminoethyl)amino]carbonylmethyl- oxyphenyl¿ xanthine (XAC), 8-(3-chlorostyryl)caffeine (CSC), and (E)-1,3-dipropyl-8-(3,4-dimethoxystyryl)- methylxanthine (KF 17,837) as displacers of [3H] CGS 21680 (5 nM) binding in the hippocampus and cerebral cortex was DPCPX > XAC >> CSC approximately KF 17,837, and the relative potency order of adenosine agonists CHA, NECA, CADO, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5'-N- ethylcarboxamidoadenosine (APEC), and 2-phenylaminoadenosine (CV 1808) was CHA approximately NECA > or = CADO > APEC approximately CV1808 > CGS 21680. In the presence of DPCPX (20 nM), [3H] CGS 21680 (0.2-200 nM) bound to a site (A2A-like) with a Kd of 20 nM and a Bmax of 56fmol/mg protein in the hippocampus and with a Kd of 22 nM and a Bmax of 63fmol/mg protein in the cortex. In the presence of CSC (200 nM), [3H]CGS 21680(0.2-200 nM) bound to a second high-affinity site with a Kd of 97 nM and a Bmax of 255 fmol/mg protein in the hippocampus and with a Kd of 112 nM and a Bmax of 221 fmol/mg protein in the cortex. Two pharmacologically distinct [3H]CGS 21680 binding sites were found in synaptosomal membranes of the hippocampus and cortex and in the striatum, one corresponding to A2A receptors and the other to the second high-affinity [3H]CGS 21680 binding site. In contrast, the pharmacology of [3H]CHA binding was similar in synaptosomal membranes of the three brain areas. The present results establish the existence of at least two high-affinity [3H]CGS 21680 binding sites in the CNS and demonstrate that the [3H]CGS 21680 binding site predominant in the hippocampus and cerebral cortex has different binding characteristics from the classic A2A adenosine receptor, which predominates in the striatum.

2-Substitution of N6-benzyladenosine-5'-uronamides enhances selectivity for A3 adenosine receptors

Adenosine derivatives bearing an N6-(3-iodobenzyl) group, reported to enhance the affinity of adenosine-5'-uronamide analogues as agonists at A3 adenosine receptors (J. Med. Chem. 1994, 37, 636-646), were synthesized starting from methyl beta-D-ribofuranoside in 10 steps. Binding affinities at A1 and A2a receptors in rat brain membranes and at cloned rat A3 receptors from stably transfected CHO cells were compared. N6-(3-Iodobenzyl)adenosine was 2-fold selective for A3 vs A1 or A2a receptors; thus it is the first monosubstituted adenosine analogue having any A3 selectivity. The effects of 2-substitution in combination with modifications at the N6- and 5'-positions were explored. 2-Chloro-N6-(3-iodobenzyl)adenosine had a Ki value of 1.4 nM and moderate selectivity for A3 receptors. 2-Chloro-N6-(3-iodobenzyl)adenosine- 5'-N-methyluronamide, which displayed a Ki value of 0.33 nM, was selective for A3 vs A1 and A2a receptors by 2500- and 1400-fold, respectively. It was 46,000-fold selective for A3 receptors vs the Na(+)-independent adenosine transporter, as indicated in displacement of [3H]N6-(4- nitrobenzyl)-thioinosine binding in rat brain membranes. In a functional assay in CHO cells, it inhibited adenylate cyclase via rat A3 receptors with an IC50 of 67 nM. 2-(Methylthio)-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide and 2-(methylamino)-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide were less potent, but nearly as selective for A3 receptors. Thus, 2-substitution (both small and sterically bulky) is well-tolerated at A3 receptors, and its A3 affinity-enhancing effects are additive with effects of uronamides at the 5'-position and a 3-iodobenzyl group at the N6-position.

Regulation of agonist binding to A2A adenosine receptors: effects of guanine nucleotides (GDP[S] and GTP[S]) and Mg2+ ion

Adenosine acts as a neuromodulator through at least two receptor subtypes, A1 and A2. A2 receptors have been further divided into A2A (high agonist affinity) and A2B (low agonist affinity) receptors. Both A1 and A2 receptors belong to the superfamily of guanine nucleotide-binding regulatory protein (G protein)-coupled receptors. A Gs protein couples the A2A receptor to the activation of adenylyl cyclase. In order to elucidate the mechanism of coupling between the A2A receptor and Gs, we studied the modulation by guanine nucleotides and divalent cations of agonist binding to the A2A receptor in rat striatal membranes, using [3H]CGS 21680 as a selective high-affinity agonist. We demonstrated that in rat striatal membranes agonist binding to A2A receptors was modulated by guanine nucleotides. Both GDP and GTP inhibited [3H]CGS 21680 binding to rat striatal membranes with about equal potency. The nonhydrolyzable analogs, GDP[S] and GTP[S], were equipotent inhibitors and approx. 100-times more potent than GDP and GTP. Data from competition studies with labeled and unlabeled CGS 21680 when analyzed by nonlinear regression demonstrated the presence of two binding sites in rat striatal membranes with mean values for KD of 5.6 and 343 nM and Bmax of 200 and 942 fmol/mg protein. The high-affinity binding site has the characteristics of the A2A receptor. In the presence both of (0.1 mM) GDP[S] and GTP[S], the KD values for the high-affinity site were increased severalfold, whereas the low-affinity site was no longer detected in filtration assays. Dissociation studies revealed monophasic dissociation curves both in the absence and presence of 0.1 mM GDP[S]. However the K-1 value increased in the presence of guanine nucleotide. We also showed that in bovine striatal membranes agonist binding to A2A receptors was modestly modulated by guanine nucleotides, suggesting differences of receptor Gs-protein-coupling a mechanism in different species. Divalent cations often increase agonist binding to different receptors, whereas Mg2+ ions play a role in regulating the initial steps of G-protein activation. We investigated the effects of divalent cations on [3H]CGS 21680 binding to the A2A receptor and determined the requirement of these cations to obtain the modulation of binding by guanine nucleotides. We found that millimolar concentrations of divalent cations were required to obtain an effective interaction between the A2A receptor and Gs. The high-affinity binding of [3H]CGS 21680 to the A2A receptor in rat striatal membranes was dependent on the presence of Mg2+ ions.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Combinations of Methylxanthines and Adenosine Analogs on Locomotor Activity in Control and Chronic Caffeine-Treated Mice

Chronic caffeine ingestion (CCI) by male NIH Swiss strain mice results in a prolonged reduction in locomotor activity and alterations in response to caffeine, other xanthines, and adenosine analogs. Caffeine, the A1 selective 8-cyclopentyltheophylline (CPT), and the A2-selective 3,7-dimethyl-1-propargylxanthine (DMPX) remain stimulatory and the bell-shaped locomotor dose-response curves are left-shifted after CCI. The depressant effects of methylxanthines that are potent phosphodiesterase inhibitors remain after CCI. After CCI, mice became more sensitive to depressant effects of A1, mixed A1/A2, and A2 agonists. In the presence of caffeine the A1-selective agonist N6-cyclohexyladenosine (CHA), the mixed A1/A2 agonist 5'-N-ethylcarboxamidoadenosine and the A2-selective agonist 2-[(2-aminoethylamino)-carbonylethylphenylethylamino]-5'-N-ethylcarboxamidoadenosine (APEC) all have dose-response curves, appearing to consist of initial depressant effects, then stimulatory effects, and finally pronounced depressant effects. The phasic character is reduced or absent after CCI. Synergistic depressant effects of combinations of CHA and APEC also appear reduced after CCI.

8-(3-Chlorostyryl)caffeine (CSC) is a selective A2-adenosine antagonist in vitro and in vivo

An adenosine antagonist, 8-(3-chlorostyryl)caffeine (CSC), was shown previously to be 520-fold selective for A2a-adenosine receptors in radioligand binding assays in the rat brain. In reversing agonist effects on adenylate cyclase, CSC was 22-fold selective for A2a receptors in rat phenochromocytoma cells (Kb 60 nM) vs. A1 receptors in rat adipocytes (Kb 1.3 microM). Administered i.p. in NIH mice at a dose of 1 mg/kg, CSC shifted the curve for locomotor depression elicited by the A2a-selective agonist APEC to the right (ED50 value for APEC shifted from 20 micrograms/kg i.p. to 190 micrograms/kg). CSC had no effect on locomotor depression elicited by an ED50 dose of the A1-selective agonist CHA. CSC alone at a dose of 5 mg/kg stimulated locomotor activity by 22% over control values. Coadministration of CSC and the A1-selective antagonist CPX, both at non-stimulatory doses, increased activity by 37% (P < 0.001) over CSC alone, suggesting a behavioral synergism of A1- and A2-antagonist effects in the CNS.

Covalent binding of a selective agonist irreversibly activates guinea pig coronary artery A2 adenosine receptors

Experiments employing guinea pig heart Langendorff preparations compared the coronary vasoactivity of a functionalized congener of adenosine, 2-[(2-aminoethyl-aminocarbonylethyl)phenylethylamino]-5'-N-e thyl- carboxamidoadenosine, APEC, with the vasoactivity of the product of the reaction of APEC with 1,4-phenylene-diisothiocyanate, 4-isothiocyanatophenylaminothiocarbonyl-APEC (DITC-APEC). Previous experiments showed that whereas APEC binds reversibly to the A2A adenosine receptor of brain striatum, DITC-APEC binds irreversibly. APEC caused concentration-dependent coronary vasodilation that persisted unchanged when agonist administration continued for up to 165 min, but promptly faded when the agent was withdrawn. The unselective adenosine receptor antagonist 8-(4-sulfophenyl)theophyline (8-SPT) antagonized the vasoactivity of APEC. By contrast, DITC-APEC (0.125-1.0 nM) caused progressive, concentration-independent vasodilation that persisted unchanged for as long as 120 min after the agent was stopped and that was insensitive to antagonism by subsequently applied 8-SPT. However, perfusion of the heart with buffer containing 0.1 mM 8-SPT strongly antagonized the coronary vasodilatory action of DITC-APEC given subsequently. Such observations indicate that the covalent binding of DITC-APEC causes irreversible activation of the guinea pig coronary artery A2A adenosine receptor. Neither APEC nor DITC-APEC appeared to desensitize the coronary adenosine receptor during two or more hours of exposure to either agonist.

Locomotor activity in mice during chronic treatment with caffeine and withdrawal

Chronic ingestion of caffeine by mice caused a marked reduction in locomotor exploratory activity. At least 4 days of withdrawal were required to restore activity to normal levels. Stimulatory effects of injected caffeine were lower in chronically treated mice and the biphasic dose-response (stimulatory followed by depressant) curve for injected caffeine was left shifted. Seven days of withdrawal were required before the dose-response curve to caffeine was identical to that of control mice. The depressant effects of a potent xanthine phosphodiesterase inhibitor, 1,3-dipropyl-7-methylxanthine, were blunted in caffeine-treated mice. The depressant effects of A1- and A2-selective adenosine analogs were enhanced after chronic caffeine. There was little or no effect of chronic caffeine on the stimulatory effects of dopaminergic agents (amphetamine, caffeine), while both depressant and stimulatory effects of cholinergic agents (nicotine, oxotremorine, scopolamine) were reduced. The results indicate that chronic caffeine affects functions of adenosine and cholinergic receptors related to regulation of locomotor exploratory activity.

2-[2-[4-[2-[2-[ 1,3-Dihydro- 1,1-bis (4-hydroxyphenyl)-3-oxo-5-isobenzofuranthioureidyl]ethylaminocarbonyl]ethyl]phenyl] ethylamino]-5'-N-ethylcarboxamidoadenosine (FITC-APEC): A Fluorescent Ligand For A2a-Adenosine Receptors

The fluorescein conjugate, FITC-APEC (2-[2-[4-[2-[2-[1,3-dihydro-l,l-bis(4-hydroxyphenyl)-3-oxo-5-isobenzofuranthioureidyl]ethylaminocarbonyl]ethyl]phenyl]ethylamino]-5'-N-ethylcarboxamidoadenosine), is a novel ligand derived from a series of functionalized congeners that act as selective A2a-adenosine receptor agonists. The binding of FITC-APEC to bovine striatal A2a,-adenosine receptors measured by fluorescence techniques was saturable and of a high affinity, with a Bmax, of 2.3 ± 0.3 pmol/mg protein and KD of 57 ± 2 nM. The KD value estimated by fluorescence was consistent with the Ki (11 ± 0.3 nM) obtained by competition studies with [3H]CGS 21680. Additionally, the Bmax, value found by FITC-APEC measurement was in agreement with Bmax, values obtained using radioligand binding. FITC-APEC exhibited rapid and reversible binding to bovine striatum. The potencies of chemically diverse A2a-adenosine receptor ligands estimated by inhibition of FITC-APEC binding were in good agreement with their potencies determined using radioligand binding techniques (r = 0.97, P = 0.0003). FITC-APEC binding was not altered by purine derivatives that do not recognize A2a-adenosine receptors. These findings demonstrate that the novel fluorescent ligand FITC-APEC can be used in the quantitative characterization of ligand binding to A2a-adenosine receptors.

Chemical modification and irreversible inhibition of striatal A2a adenosine receptors

The ligand recognition site of A2a-adenosine receptors in rabbit striatal membranes was probed using non-site-directed labeling reagents and specific affinity labels. Exposure of membranes to diethylpyrocarbonate at a concentration of 2.5 mM, followed by washing, was found to inhibit the binding of [3H]CGS 21680 and [3H]xanthine amine congener to A2a receptors, by 86 and 30%, respectively. Protection from diethylpyrocarbonate inactivation by an adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine, and an antagonist, theophylline, suggested the presence of two histidyl residues on the receptor, one associated with agonist binding and the other with antagonist binding. Binding of [3H]CGS 21680 or [3H]xanthine amine congener was partially restored after incubation with 250 mM hydroxylamine, further supporting histidine as the modification site. Preincubation with disulfide-reactive reagents, dithiothreitol or sodium dithionite, at greater than 5 mM inhibited radioligand binding, indicating the presence of essential disulfide bridges in A2a receptors, whereas the concentration of mercaptoethanol required to inhibit binding was greater than 50 mM. A number of isothiocyanate-bearing affinity labels derived from the A2a-selective agonist 2-[(2-aminoethylamino) carbonylethylphenylethylamino]-5'-N- ethylcarboxamidoadenosine (APEC) were synthesized and found to inhibit A2a receptor binding in rabbit and bovine striatal membranes. Binding to rabbit A1 receptors was not inhibited. Preincubation with the affinity label 4-isothiocyanatophenylaminothiocarbonyl-APEC (100 nM) diminished the Bmax for [3H]CGS 21680 binding by 71%, and the Kd was unaffected, suggesting a direct modification of the ligand binding site. Reversal of 4-isothiocyanatophenylaminothiocarbonyl-APEC inhibition of [3H]CGS 21680 binding with hydroxylamine suggested that the site of modification by the isothiocyanate is a cysteine residue. A bromoacetyl derivative of APEC was ineffective as an affinity label at submicromolar concentrations.

A2A adenosine receptors from rat striatum and rat pheochromocytoma PC12 cells: characterization with radioligand binding and by activation of adenylate cyclase

Binding assays and assays of activation of adenylate cyclase with the agonists 5'-N-ethylcarboxyamidoadenosine (NECA) and CGS21680 have been used to compare adenosine receptors in rat pheochromocytoma PC12 cells and in rat striatum. The [3H]NECA binding showed two components, whereas [3H]CGS21680 bound to one component in both tissues. The Kd value for the high affinity site labeled with [3H]NECA in PC12 cell membranes (2.3 nM) was lower than that in striatum (6.5 nM). The [3H]CGS21680 binding site showed a Kd value of 6.7 nM and 11.3 nM in PC12 cells and striatum, respectively. In the presence of GTP the KD values of [3H]NECA and [3H]CGS21680 for the high affinity site were increased severalfold, whereas the low affinity sites for [3H]NECA were no longer detected with filtration assays. A comparison of the ability of a series of agonists and antagonists to inhibit high affinity binding of [3H]NECA to A2 receptors in PC12 cell and striatal membranes indicated that agonists had higher affinities and antagonists had lower affinities in PC12 cells, compared with affinities in striatal membranes. Analysis of activation of adenylate cyclase in PC12 cell membranes suggested that the dose-dependent stimulation by NECA involved two components, whereas CGS21680 stimulated via one component. The maximal stimulation by NECA significantly exceeded that caused by CGS21680. In intact PC12 cells, NECA caused a greater accumulation of AMP than did CGS21680, as was the case in membranes. In striatal membranes, NECA and CGS21680 showed similar maximal stimulations of adenylate cyclase. Both NECA and CGS21680 were more potent in PC12 cell membranes than in striatal membranes, in agreement with binding data. However, in contrast to binding data, antagonists were not less potent versus stimulation of adenylate cyclase by NECA or CGS21680 in PC12 cell membranes, compared with striatal membranes. In toto, the results suggest that A2A receptors in striatum are virtually identical to the A2A receptors in PC12 cells. But, in addition to an A2A receptor, it appears that a lower affinity functional receptor, probably an A2B receptor, is present in PC12 cells and PC12 cell membranes, whereas such a functional low affinity receptor is not detectable in striatal membrane.

Characterization of human striatal A2-adenosine receptors using radioligand binding and photoaffinity labeling

The adenosine agonist [3H]CGS21680 (2-[4-[[2-carboxyethyl]phenyl]ethylamino]-5'- N-ethylcarboxamidoadenosine) bound to A2 receptors in human striatal membranes with a Kd of 17.8 +/- 1.1 nM and a Bmax of 313 +/- 10 fmol/mg protein. The addition of 100 microM GTP diminished both the affinity of agonist radioligand for A2 adenosine binding sites and the total binding, resulting in Kd and Bmax values of 28.6 +/- 1.0 nM and 185 +/- 22 fmol/mg of protein. Adenosine ligands competed for [3H]CGS21680 with the expected potency order. The adenosine antagonist [3H]XAC (8-[4-[[[[(2-aminoethyl)-amino]carbonyl]methyl] oxy]phenyl]-1,3-dipropylxanthine), although A1-selective in the rat, binds to human striatal A2 receptors with high affinity. 25 nM CPX (8-cyclopentyl-1,3-dipropylxanthine), an A1-selective antagonist, was added to the incubation medium and effectively eliminated 91% of [3H]XAC (1 nM) binding to human A1 receptors, yet preserved 90% of binding to A2 receptors. [3H]XAC exhibited saturable, specific binding (50% of total) to A2 sites with a Kd of 2.98 +/- 0.54 nM and a Bmax of 0.71 +/- 0.23 pmol/mg protein (25 degrees C, non-specific binding defined with 100 microM NECA). The potency order for antagonists against 1 nM [3H]XAC was CGS15943A greater than XAC approximately PD115,119 greater than PAPA-XAC greater than CPX greater than HTQZ approximately XCC approximately CP-66,713 greater than theophylline approximately caffeine, indicative of an A2-type binding site. A2a-receptors were found to be present in the human cortex, albeit at a much lower density than in the striatum. Photoaffinity labeling using 125I-PAPA-APEC revealed a molecular weight of 45K, but proteolytic cleavage was observed, resulting in fragments of MW 43K and 37K. In the absence of proteolytic inhibitors the 37K fragment, which still bound 125I-PAPA-APEC, was predominant.

Behavioral effects of A1- and A2-selective adenosine agonists and antagonists: evidence for synergism and antagonism

The locomotor effects in mice of selective A1 and A2 adenosine agonists, antagonists and combinations of agonists were investigated using a computerized activity monitor. The A2-selective agonist 2-[(2-aminoethylamino)carbonylethylphenylethylamino[-5'-N- ethylcarboxamidoadenosine (APEC), an amine derivative of 2-(carboxyethylphenylethylamino)adenosine-5'-carboxamide, was a more potent locomotor depressant than its amide conjugates. The rank order of potency after i.p. injection for adenosine agonists was 5'-N-ethylcarboxamidoadenosine (NECA) (ED50, 5.8 nmol/kg) greater than APEC (ED50, 25 nmol/kg) greater than N6-cyclohexyladenosine (CHA) (ED50, 270 nmol/kg). An A1-selective, centrally acting, adenosine antagonist, 8-cyclopentyltheophylline (10 mg/kg), completely reversed the locomotor depressant effects of CHA (A1-selective) and NECA (nonselective) at doses of agonists as high as twice the ED50, and shifted the dose-response curves to the right, suggesting a primary involvement of A1 receptors. 8-cyclopentyltheophylline did not affect the depressant effects of APEC at the ED50, consistent with the A2-selectivity of APEC. The locomotor effects of APEC and CHA were completely reversed by theophylline, but not by the peripherally active 8-p-sulfophenyltheophylline, indicating central action of the adenosine agonists. The depressant effects of APEC, but not of NECA or CHA, were reversed significantly by an A2-selective adenosine receptor antagonist, 4-amino-8-chloro-1-phenyl-[1,2,4]triazol[4,3-a]quinoxaline. Low or subthreshold doses of CHA potentiated the depressant effects of APEC. A subthreshold dose of CHA did not alter the depressant effect of NECA, whereas a subthreshold dose of APEC increased the depressant effects of low doses of NECA. Thus, it appears that A1- and A2-selective adenosine agonists have separate central depressant effects, which can be potentiative.(ABSTRACT TRUNCATED AT 250 WORDS)

The A2 adenosine receptor: guanine nucleotide modulation of agonist binding is enhanced by proteolysis

Agonist binding to the A2 adenosine receptor (A2AR) and its regulation by guanine nucleotides was studied using the newly developed radioligand 125I-2-[4-(2-[2-[(4-aminophenyl)methylcarbonylamino] ethylaminnocarbonyl]ethyl)phenyl]ethylamino-5'-N- ethylcarboxamidoadenosine (125I-PAPA-APEC) and its photoaffinity analog 125I-azido-PAPA-APEC. A single protein of Mr 45,000, displaying the appropriate A2AR pharmacology, is labeled in membranes from bovine striatum, PC12 cells, and frog erythrocytes. In DDT1 MF2 cells the labeled protein has a slightly lower molecular weight. Incorporation of 125I-azido-PAPA-APEC into membranes from rabbit striatum, however, reveals two specifically labeled peptides (Mr approximately 47,000 and 38,000), both of which display A2AR pharmacology. Inhibition of protease activity leads to a decrease in the amount of the Mr 38,000 protein, with only the Mr 47,000 protein remaining. This suggests that the Mr 38,000 peptide is a proteolytic product of the Mr 47,000 A2AR protein. In membranes containing the intact undigested A2AR protein, guanine nucleotides induce a small to insignificant decrease in agonist binding, which is atypical of stimulatory GS-coupled receptors. This minimal effect is observed in rabbit striatal membranes prepared in the presence of protease inhibitors, as well as in the other tissues studied. Binding to rabbit striatal membranes that possess the partially digested receptor protein, however, reveals a 50% reduction in maximal specific agonist binding upon addition of guanine nucleotides. Inhibition of proteolysis in rabbit striatum, on the other hand, results in a diminished ability of guanine nucleotides to regulate agonist binding. Thus, the enhanced effectiveness of guanine nucleotides in rabbit striatal membranes is associated with the generation of the Mr 38,000 peptide fragment. Guanosine 5'-(beta,gamma-imido)triphosphate reduces photoaffinity labeling by 55% in the Mr 38,000 protein, whereas the labeling is decreased by only 28% in the Mr 47,000 receptor protein. Our data, therefore, suggest that, unless proteolysis occurs, the A2AR in all tissues studied is tightly associated with the GS protein and displays minimal guanine nucleotide modulation of agonist binding, which makes the A2AR an atypical stimulatory receptor.