Selective ligands for rat A3 adenosine receptors: structure-activity relationships of 1,3-dialkylxanthine 7-riboside derivatives

Structure-Based Screening of Uncharted Chemical Space for Atypical Adenosine Receptor Agonists

Small molecule screening libraries cover only a small fraction of the astronomical number of possible drug-like compounds, limiting the success of ligand discovery efforts. Computational screening of virtual libraries representing unexplored chemical space could potentially bridge this gap. Drug development for adenosine receptors (ARs) as targets for inflammation and cardiovascular diseases has been hampered by the paucity of agonist scaffolds. To identify novel AR agonists, a virtual library of synthetically tractable nucleosides with alternative bases was generated and structure-based virtual screening guided selection of compounds for synthesis. Pharmacological assays were carried out at three AR subtypes for 13 ribosides. Nine compounds displayed significant activity at the ARs, and several of these represented atypical agonist scaffolds. The discovered ligands also provided insights into receptor activation and revealed unknown interactions of endogenous and clinical compounds with the ARs. The results demonstrate that virtual compound databases provide access to bioactive matter from regions of chemical space that are sparsely populated in commercial libraries, an approach transferrable to numerous drug targets.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Medicinal chemistry of the A3 adenosine receptor: agonists, antagonists, and receptor engineering

A(3) adenosine receptor (A(3)AR) ligands have been modified to optimize their interaction with the A(3)AR. Most of these modifications have been made to the N(6) and C2 positions of adenine as well as the ribose moiety, and using a combination of these substitutions leads to the most efficacious, selective, and potent ligands. A(3)AR agonists such as IB-MECA and Cl-IB-MECA are now advancing into Phase II clinical trials for treatments targeting diseases such as cancer, arthritis, and psoriasis. Also, a wide number of compounds exerting high potency and selectivity in antagonizing the human (h)A(3)AR have been discovered. These molecules are generally characterized by a notable structural diversity, taking into account that aromatic nitrogen-containing monocyclic (thiazoles and thiadiazoles), bicyclic (isoquinoline, quinozalines, (aza)adenines), tricyclic systems (pyrazoloquinolines, triazoloquinoxalines, pyrazolotriazolopyrimidines, triazolopurines, tricyclic xanthines) and nucleoside derivatives have been identified as potent and selective A(3)AR antagonists. Probably due to the "enigmatic" physiological role of A(3)AR, whose activation may produce opposite effects (for example, concerning tissue protection in inflammatory and cancer cells) and may produce effects that are species dependent, only a few molecules have reached preclinical investigation. Indeed, the most advanced A(3)AR antagonists remain in preclinical testing. Among the antagonists described above, compound OT-7999 is expected to enter clinical trials for the treatment of glaucoma, while several thiazole derivatives are in development as antiallergic, antiasthmatic and/or antiinflammatory drugs.

Role of adenosine A3 receptors on CA1 hippocampal neurotransmission during oxygen-glucose deprivation episodes of different duration

The role of adenosine A3 receptors in synaptic transmission under severe (7 min) and shorter (2-5 min) ischemic conditions, obtained by oxygen and glucose deprivation (OGD), was investigated in rat hippocampal slices. The effects of selective A3 agonists or antagonists were examined on field excitatory postsynaptic potentials (fEPSPs) extracellularly recorded at the dendritic level of the CA1 pyramidal region. The novel, selective A3 antagonist LJ1251 ((2R,3R,4S)-2-(2-chloro-6-(3-iodobenzylamino)-9H-purin-9-yl)tetrahydrothiophene-3,4-diol, 0.1-10 nM) protected hippocampal slices from irreversible fEPSP depression induced by severe OGD and prevented or delayed the appearance of anoxic depolarization. Similar results were obtained when severe OGD was carried out with a long, receptor-desensitizing exposure to various selective A3 agonists: 5'-N-methylcarboxamidoadenosine derivatives Cl-IB-MECA (N6-(3-iodobenzyl)-2-chloro), VT72 (N6-methoxy-2-phenylethynyl), VT158 (N6-methoxy-2-phenylethynyl), VT160 (N6-methoxy-2-(2-pyridinyl)-ethynyl), and VT163 (N6-methoxy-2-p-acetylphenylethynyl) and AR132 (N6-methyl-2-phenylethynyladenosine). The selective A3 antagonist MRS1523 (3-propyl-6-ethyl-5-[(ethylthio)carbonyl]-2-phenyl-4-propyl-3-pyridine carboxylate, 100 nM) reduced fEPSP depression evoked by 2-min OGD and induced a faster recovery of fEPSP amplitude after 5-min OGD. Similar results were obtained for 2- or 5-min OGD applied in the presence of each of the A3 agonists tested. Shorter exposure to A3 agonists significantly delayed the recovery of fEPSP amplitude after 5-min OGD. This indicates that A3 receptors, stimulated by selective A3 agonists, undergo desensitization during OGD. It is inferred that CA1 hippocampal A3 receptors stimulated by adenosine released during brief ischemia (2 and 5 min) might exert A1-like protective effects on neurotransmission. Severe ischemia would transform the A3 receptor-mediated effects from protective to injurious.

A SURVEY OF NONXANTHINE DERIVATIVES AS ADENOSINE RECEPTOR LIGANDS

The binding affinities at rat A₁, A_2a, and A₃ adenosine receptors of a wide range of heterocyclic derivatives have been determined. Mono-, bi-, tricyclic and macrocyclic compounds were screened in binding assays, using either [³H]PIA or [³H]CGS 21680 in rat brain membranes or [¹²⁵I]AB-MECA in CHO cells stably transfected with rat A₃ receptors. Several new classes of adenosine antagonists (e.g. 5-oxoimidazopyrimidines and a pyrazoloquinazoline) were identified. Various sulfonylpiperazines, 11-hydroxytetrahydrocarbazolenine, 4H-pyrido[1,2-a]pyrimidinone, folic acid, and cytochalasin H and J bound to A₃ receptors selectively. Moreover, cytochalasin A, which bound to A₁ adenosine receptors with K_i value of 1.9 μM, inhibited adenylyl cyclase in rat adipocytes, but not via reversible A₁ receptor binding.

2,9-disubstituted-N6-(arylcarbamoyl)-8-azaadenines as new selective A3 adenosine receptor antagonists: synthesis, biochemical and molecular modelling studies

A number of N6-(N-arylcarbamoyl)-2-substituted-9-benzyl-8-azaadenines, obtained by a modification of the synthetic scheme used to prepare selective A1 ligands, by only three or two steps, are described. At first we prepared a series of 2-phenyl-9-benzyl-8-azaadenines having as N6 substituent a variously substituted N-phenylcarbamoyl group. Some of these derivatives demonstrated good affinity towards the A3 subtype but low selectivity. Compounds having p-CF3, p-F and p-OCH3, as substituents on the phenylcarbamoyl group were selected as lead compounds for the second part of this study. Without modifying the N6 substituent, which would assure A3 affinity, we varied the 9 and 2 positions on these molecules to enhance selectivity. Some compounds having a p-methyl group on the 2-phenyl substituent showed a very good affinity and selectivity for the A3 subtype, revealing the first class of A3 adenosine receptor selective antagonists with a bicyclic structure strictly correlated to the adenine nucleus. The molecular modelling work, carried out using the DOCK program, supplied two models which may be useful for a better understanding of the binding modes. Both models highlighted the preferred interacting tautomeric forms of the antagonists for human A1 and A3 receptors.

Partial agonists for A(3) adenosine receptors

Selective agonists for A(3) 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 A(3)AR activation have recently been identified, including the N(6)-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 A(3) 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 N(6)-methyl and N(6)-benzyl groups may contribute to the A(3) affinity and selectivity; however, an N(6)-benzyl group but not an N(6)-methyl group diminishes A(3)AR efficacy. N(6)-benzyl substituted adenosine derivatives have similar potency for human and rat A(3)ARs while N(6)-methyl substitution was preferable for the human A(3)AR. The combination of 2-chloro and N(6)-benzyl substitutions appeared to reduce efficacy further than either modification alone. The A(2A)AR agonist DPMA was shown to be an antagonist for the human A(3)AR. Thus, the efficacy of adenosine derivatives at the A(3)AR appears to be more sensitive to small structural changes than at other subtypes. Potent and selective partial agonists for the A(3)AR could be identified by screening known adenosine derivatives and by modifying adenosine and the adenosine derivatives.

A3 adenosine receptors in human astrocytoma cells: agonist-mediated desensitization, internalization, and down-regulation

A(3) adenosine receptor activation has been previously demonstrated to result in both neuroprotective and neurodegenerative effects, depending upon specific pathophysiological conditions. This dual effect may depend on receptor regulation mechanisms that are able to change receptor availability and/or function. In the present study, we investigated desensitization, internalization, and down-regulation of native A(3) adenosine receptors in human astrocytoma cells after exposure to the agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (Cl-IBMECA). Cl-IBMECA induced a concentration-dependent inhibition of adenylyl cyclase activity with an EC(50) value of 2.9 +/- 0.1 nM. The effect was suggested to be mediated by A(3) adenosine receptor subtype by the use of selective adenosine receptor antagonists. Cell treatment with pertussis toxin abolished Cl-IBMECA-mediated inhibition of adenylyl cyclase activity, evidencing an A(3) receptor coupling to inhibitory G protein. Short-term exposure to the agonist Cl-IBMECA (100 nM) caused rapid receptor desensitization, within 15 min. Agonist-induced desensitization was accompanied by receptor internalization: A(3) adenosine receptor internalized with rapid kinetics, within 30 min, after cell exposure to 100 nM Cl-IBMECA. The localization of A(3) adenosine receptors on the plasma membrane and in intracellular compartments was directly revealed by immunogold electron microscopy. After desensitization, the removal of agonist led to the restoration of A(3) adenosine receptor functioning through receptor recycling to the cell surface within 120 min. Prolonged agonist exposure (1-24 h) resulted in a marked down-regulation of A(3) adenosine receptors that reached 21.9 +/- 2.88% of control value after 24 h. After down-regulation, the recovery of receptor functioning was slow (24 h) and associated with the restoration of receptor levels close to control values. In conclusion, our results demonstrated that A(3) receptors, in astrocytoma cells, are regulated after short- and long-term agonist exposure.

Synthesis of 2-alkyl-substituted-N6-methyladenine derivatives as potential adenosine receptor ligand

2-(1-Hexynyl), 2-((E)-1-hexenyl) and 2-(n-hexyl)-N6-methyladenines were synthesized, starting from 2-amino-6-chloropurine using palladium-catalyzed coupling as a key step as potential adenosine receptor ligand.

Synthesis and purine receptor affinity of 6-oxopurine nucleosides and nucleotides containing (N)-methanocarba-pseudoribose rings

6-Oxopurine derivatives containing a northern (N) methanocarba modification (i.e., fused cyclopropane and cyclopentane rings in place of the ribose) were synthesized and the adenosine receptor affinity measured. Guanine or hypoxanthine was coupled at the 7-position, or 1,3-dibutylxanthine was coupled at the 9-position. The pseudoribose ring was also substituted at the 5'-position with an N-methyluronamide or with phosphate groups.

A(3) adenosine receptor ligands: history and perspectives

Adenosine regulates many physiological functions through specific cell membrane receptors. On the basis of pharmacological studies and molecular cloning, four different adenosine receptors have been identified and classified as A(1), A(2A), A(2B), and A(3). These adenosine receptors are members of the G-protein-coupled receptor family. While adenosine A(1) and A(2A) receptor subtypes have been pharmacologically characterized through the use of selective ligands, the A(3) adenosine receptor subtype is presently under study in order to better understand its physio-pathological functions. Activation of adenosine A(3) receptors has been shown to stimulate phospholipase C and D and to inhibit adenylate cyclase. Activation of A(3) adenosine receptors also causes the release of inflammatory mediators such as histamine from mast cells. These mediators are responsible for processes such as inflammation and hypotension. It has also been suggested that the A(3) receptor plays an important role in brain ischemia, immunosuppression, and bronchospasm in several animal models. Based on these results, highly selective A(3) adenosine receptor agonists and/or antagonists have been indicated as potential drugs for the treatment of asthma and inflammation, while highly selective agonists have been shown to possess cardioprotective effects. The updated material related to this field of research has been rationalized and arranged in order to offer an overview of the topic.

A3 Adenosine Receptors: Protective vs. Damaging Effects Identified Using Novel Agonists and Antagonists

Investigation of the physiologic role of the A3 adenosine receptor has been facilitated by the availability of selective agonists and antagonists. Selective agonists include IB-MECA and the 2-chloro derivative Cl-IB-MECA. Selective antagonists have been identified and designed with the aid of molecular modeling among various nonpurine classes of heterocycles: flavonoids, 1,4-dihydropyridine derivatives, triazoloquinazolines, isoquinolines, and a triazolonaphthyridine. The dihydropyridine 3-ethyl 5-benzyl 2-methyl-6-phenyl-4-phenylethynyl-1,4-(±)-dihydropyridine-3,5-dicarboxylate (MRS 1191) is 1,300-fold selective for human A3 (Ki of 31 nM) vs. A1/A2A adenosine receptors and also 28-fold A3 selective in rat tissue (Ki of 1.42 mM). 9-Chloro-2-(2-furyl)-5-phenylacetylamino[1,2,4]-triazolo[1,5-c]quinazoline (MRS 1220) is useful as an A3 selective antagonist only in human tissue, with a Ki value of 0.65 nM. The pyridine derivative 5-propyl 2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate (MRS 1523) is a selective antagonist of both rat and human A3 receptors, with Ki values of 113 and 19 nM, respectively. Paradoxical effects of A3 agonists in the brain, heart and other tissues indicate that acute activation of A3 receptors at greater than 10 mM concentrations acts as a lethal input to cells, whereas low, nanomolar concentrations of A3 receptor agonists protect against apoptosis or ischemic damage. Adenosine A3 receptor agonists, antagonists, or both, may be useful in treating inflammatory conditions.

Activation and Desensitization of Rat A3-Adenosine Receptors by Selective Adenosine Derivatives and Xanthine-7-Ribosides

Strategy, Management and Health PolicyVenture Capital Enabling TechnologyPreclinical ResearchPreclinical Development Toxicology, Formulation Drug Delivery, PharmacokineticsClinical Development Phases I-III Regulatory, Quality, ManufacturingPostmarketing Phase IV Xanthine and adenosine derivatives, known to bind to recombinant rat A3 adenosine receptors stably expressed in Chinese hamster ovary cells, were characterized in a functional assay consisting of activation of A3 receptor-stimulated binding of [35S]GTPγS in rat RBL-2H3 cell membranes. 1,3-Dibutylxanthine-7-riboside-5'-N-methylcarboxamide (DBXRM, 7b), previously shown to inhibit adenylyl cyclase via rat A3 receptors with full efficacy, appeared to be a partial agonist at the rat A3 receptor of RBL-2H3 cells. Full agonists, such as Cl-IB-MECA or I-AB-MECA, were more potent and effective than the partial agonist DBXRM in causing desensitization of rat A3 receptors, as indicated by loss of [35S]GTPγS binding. At A1 receptors, antagonism of agonist-elicited inhibition of rat adipocyte adenylyl cyclase was observed for several xanthine-7-riboside derivatives that had been shown to be full agonists at rat A3 receptors. A new xanthine riboside (3'-deoxyDBXRM, 7c) was synthesized and found to be a partial agonist at rat A3 receptors and an antagonist at rat A1 receptors. Thus, it is possible for the same compound to stimulate one adenosine receptor subtype (A3) and block another subtype (A1) within the same species.

Adenosine receptors: new opportunities for future drugs

This review summarises current knowledge on adenosine receptors, an important G protein-coupled receptor. The four known adenosine receptor subtypes A1, A2A, A2B, and A3 are discussed with special reference to the opportunities for drug development.

Synthesis and adenosine receptor affinity of 7-beta-D-ribofuranosylxanthine

7-beta-D-Ribofuranosylxanthine, a previously unreported isomer of xanthosine, was prepared in four steps from 7-benzylxanthine. The procedure, which involves the use of pivaloyloxymethyl groups to protect the xanthine ring, was also applied to preparation of some 1-N-alkyl derivatives of 7-ribosylxanthine. Adenosine receptor affinity for these compounds was determined. 7-beta-D-Ribofuranosylxanthine was found to have higher affinity and greater selectivity for the A1 receptor than previously reported xanthine nucleosides, and to be a partial agonist.

Recent Developments in Selective Agonists and Antagonists Acting at Purine and Pyrimidine Receptors

The SAR at adenosine (P1) and ATP (P2) receptors is reviewed, with emphasis on recently developed selective agonists and antagonists. These include partial (e.g., N6-ethyl-8-cyclopentylaminoadenosine) and full A1 agonists (e.g., NNC 21-0136, 2-chloro-N6-[(R)-(benzothiazolylthio-2-propyl]adenosine), A2 antagonists (e.g., the non-xanthines: SCH58261, 5-amino-7-(phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine and ZM241385, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3a][1,3,5]triazinyl-amino]ethyl)-phenol; and the 1-propargyl-8-styrylxanthines), and A3 agonists (e.g., CI-IB-MECA, 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluron-amide). Novel adenosine receptor antagonists (e.g., BTH4, ethyl 3-benzylthio-4,5,6,7-tetrahydro-benzo[c]thiophen-4-one-1-carboxylate) have been discovered through screening libraries of natural products and heterocyclic derivatives. The first A3 selective antagonists to be identified include derivatives of flavones (MRS 1067), 1,4-dihydropyridines (MRS 1097), triazolonaphthyridine (L-249313), and thiazolopyrimidine (L-268605). Potent P2 receptor agonists are known. For example, 2-HexylthioAMP is a highly potent agonist at the yet uncloned P2Y receptor in C6 glioma cells. Suramin is a weak and non-selective P2 blocker, while a truncated derivative, NF023, appears to be selective for P2X receptors. More selective P2 antagonists are under development, with the cloning of these receptors. [35S]ATP-γS has been used as a radioligand for the direct labeling of several subtypes of cloned P2X receptors (P2X1-P2X4).

Activation of Phosphoinositide Breakdown and Elevation of Intracellular Calcium in a Rat RBL-2H3 Mast Cell Line by Adenosine Analogs: Involvement of A(3)-Adenosine Receptors?

A variety of adenosine analogs activate phosphoinositide breakdown in a rat RBL-2H3 mast cell line. It is presumed that an A(3)-adenosine receptor is involved, since the phosphoinositide response is insensitive to xanthines. However, the very potent A(3)- receptor agonist 2-chloro-N(6)-iodobenzyl-N-methylcarboxamidoadenosine (2-CI-IBMECA) with an EC(50) of 4.1 µM is about twofold less potent (and less efficacious) than N-ethylcarboxamidoadenosine (NECA) with an EC(50) of 2.1 µM. The other agents consisting of N(6)-p-aminophenylethyladenosine (APNEA), N(6)-iodobenzylMECA (IB-MECA), N(6)-R- phenylisopropyladenosine (R-PIA), 2-chloroadenosine, N(6)-benzyladenosine, N(6)- cyclohexyladenosine (CHA), N(6)-cyclohexylNECA (CHNECA), 2-(p- carboxyethylphenyl-ethylaminoNECA (CGS 21680), 1,3-dibutylxanthine 7-riboside-5'-N-methylcarboxamide (DBXRM), adenosine, and 8-bromoadenosine are all nearly equipotent with EC(50) values of 5.5-13.9 µM. The rank order of potencies of the analogs in causing an elevation of intracellular calcium is quite different. The potent A(3) receptor agonists 2-CI-IBMECA and IB-MECA with EC(50) values of 0.07 and 0.11 µM, respectively, are about fourfold more potent than N(6)-cyclohexylNECA and about 15-fold more potent than NECA. The other analogs are comparable or somewhat less potent than NECA, some are less efficacious, and 8-bromoadenosine is inactive. The results suggest that stimulation of phosphoinositide breakdown by adenosine analogs in RBL-2H3 cells as measured by IP(1) accumulation is not predictive of IP(3)-mediated elevations of intracellular calcium. Rank order of potency for the calcium response is consonant with intermediacy of A(3)-adenosine receptors, while the former, as measured by [(3)H]IP(1)-formation, probably reflects contributions from both an A(3)-mediated response and some other mechanism. Combinations of subthreshold concentrations of 2-CI-IBMECA with either the A(1)-selective agonist CHA or the A(2A)-selective agonist CGS 21680 caused a marked stimulation of phosphoinositide breakdown, providing further evidence for dual mechanisms. The selective A(3)-adenosine receptor antagonist 3,6-dichloro-2'-(isopropyloxy)-4'-methylflavone (MRS 1067) inhibits 2-CI-IBMECA- and NECA-elicited elevation of calcium levels, and had differential effects on phosphoinositide breakdown, blocking [(3)H]IP(3) accumulation and either blocking (NECA) or having no effect (2-CI-IBMECA) on [(3)H]IP(1) accumulation.

Calf spleen purine nucleoside phosphorylase: purification, sequence and crystal structure of its complex with an N(7)-acycloguanosine inhibitor

Calf spleen purine nucleoside phosphorylase was purified to homogeneity and its amino acid sequence was determined. The complex of the enzyme with an N(7)-acycloguanosine inhibitor crystallized in the cubic space group P2(1)3, with unit cell dimension a = 94.02 A and one monomer in the asymmetric crystal unit. The biologically active trimer is formed by the crystallographic three-fold axis. The structure was solved by molecular replacement methods, using the model of the human erythrocyte enzyme, and refined at a resolution of 2.9 A to an R-factor of 0.21. The orientation of the inhibitor at the active site is examined in relation to the catalytic activity of the enzyme in the phosphorolysis of N(7)-beta-D-purine nucleosides.