Synthesis and biological evaluation of novel 1-deoxy-1-[6-[((hetero)arylcarbonyl)hydrazino]- 9H-purin-9-yl]-N-ethyl-beta-D-ribofuranuronamide derivatives as useful templates for the development of A2B adenosine receptor agonists

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

Therapeutic Potentials of A2B Adenosine Receptor Ligands: Current Status and Perspectives

BACKGROUND

Adenosine receptors (ARs) are classified as A1, A2A, A2B, and A3 subtypes belong to the superfamily of G-protein coupled receptors (GPCRs). More than 40% of modern medicines act through either activation or inhibition of signaling processes associated with GPCRs. In particular, A2B AR signaling pathways are implicated in asthma, inflammation, cancer, ischemic hyperfusion, diabetes mellitus, cardiovascular diseases, gastrointestinal disorders, and kidney disease.

METHODS

This article reviews different disease segments wherein A2B AR is implicated and discusses the potential role of subtype-selective A2B AR ligands in the management of such diseases or disorders. All the relevant publications on this topic are reviewed and presented scientifically.

RESULTS

This review provides an up-to-date highlight of the recent advances in the development of novel and selective A2B AR ligands and their therapeutic role in treating various disease conditions. A special focus has been given to the therapeutic potentials of selective A2B AR ligands in the management of airway inflammatory conditions and cancer.

CONCLUSIONS

This systematic review demonstrates the current status and perspectives of A2B AR ligands as therapeutically useful agents that would assist medicinal chemists and pharmacologists in discovering novel and subtype-selective A2B AR ligands as potential drug candidates.

Tritium-labeled agonists as tools for studying adenosine A2B receptors

A selective agonist radioligand for A_2B adenosine receptors (A_2BARs) is currently not available. Such a tool would be useful for labeling the active conformation of the receptors. Therefore, we prepared BAY 60-6583, a potent and functionally selective A_2BAR (partial) agonist, in a tritium-labeled form. Despite extensive efforts, however, we have not been able to establish a radioligand binding assay using [³H]BAY 60-6583. This is probably due to its high non-specific binding and its moderate affinity, which had previously been overestimated based on functional data. As an alternative, we evaluated the non-selective A_2BAR agonist [³H]NECA for its potential to label A_2BARs. [³H]NECA showed specific, saturable, and reversible binding to membrane preparations of Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells stably expressing human, rat, or mouse A_2BARs. In competition binding experiments, the AR agonists 2-chloroadenosine (CADO) and NECA displayed significantly higher affinity when tested versus [³H]NECA than versus the A_2B-antagonist radioligand [³H]PSB-603 while structurally diverse AR antagonists showed the opposite effects. Although BAY 60-6583 is an A_2BAR agonist, it displayed higher affinity versus [³H]PSB-603 than versus [³H]NECA. These results indicate that nucleoside and non-nucleoside agonists are binding to very different conformations of the A_2BAR. In conclusion, [³H]NECA is currently the only useful radioligand for determining the affinity of ligands for an active A_2BAR conformation.

The aminopyridine-3,5-dicarbonitrile core for the design of new non-nucleoside-like agonists of the human adenosine A2B receptor

A new series of amino-3,5-dicyanopyridines (3-28) as analogues of the adenosine hA_2B receptor agonist BAY60-6583 (compound 1) was synthesized. All the compounds that interact with the hA_2B adenosine receptor display EC₅₀ values in the range 9-350 nM behaving as partial agonists, with the only exception being the 2-{[4-(4-acetamidophenyl)-6-amino-3,5-dicyanopyridin-2-yl]thio}acetamide (8) which shows a full agonist profile. Moreover, the 2-[(1H-imidazol-2-yl)methylthio)]-6-amino-4-(4-cyclopropylmethoxy-phenyl)pyridine-3,5-dicarbonitrile (15) turns out to be 3-fold more active than 1 although less selective. This result can be considered a real breakthrough due to the currently limited number of non-adenosine hA_2B AR agonists reported in literature. To simulate the binding mode of nucleoside and non-nucleoside agonists at the hA_2B AR, molecular docking studies were performed at homology models of this AR subtype developed by using two crystal structures of agonist-bound A_2A AR as templates. These investigations allowed us to represent a hypothetical binding mode of hA_2B receptor agonists belonging to the amino-3,5-dicyanopyridine series and to rationalize the observed SAR.

BAY60-6583 acts as a partial agonist at adenosine A2B receptors

BAY60-6583 [2-({6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-yl}sulfanyl)acetamide] is the most potent and selective adenosine A2B receptor (A2B AR) agonist known to date. Therefore, it has been widely used for in vitro and in vivo experiments. In the present study, we investigated the binding and functional properties of BAY60-6583 in various native and recombinant cell lines with different A2B AR expression levels. In cAMP accumulation and calcium mobilization assays, BAY60-6583 was found to be significantly less efficacious than adenosine or the adenosine derivative NECA. When it was tested in human embryonic kidney (HEK)293 cells, its efficacy correlated with the A2B expression level of the cells. In Jurkat T cells, BAY60-6583 antagonized the agonistic effect of NECA and adenosine as determined in cAMP accumulation assays. On the basis of these results, we conclude that BAY60-6583 acts as a partial agonist at adenosine A2B receptors. At high levels of the physiologic agonist adenosine, BAY60-6583 may act as an antagonist and block the effects of adenosine at A2B receptors. This has to be considered when applying the A2B-selective "agonist" BAY60-6583 in pharmacological studies, and previous research results may have to be reinterpreted.

Simulation and comparative analysis of binding modes of nucleoside and non-nucleoside agonists at the A2B adenosine receptor

PURPOSE

A2B receptor agonists are studied as possible therapeutic tools for a variety of pathological conditions. Unfortunately, medicinal chemistry efforts have led to the development of a limited number of potent agonists of this receptor, in most cases with a low or no selectivity versus the other adenosine receptor subtypes. Among the developed molecules, two structural families of compounds have been identified based on nucleoside and non-nucleoside (pyridine) scaffolds. The aim of this work is to analyse the binding mode of these molecules at 3D models of the human A2B receptor to identify possible common interaction features and the key receptor residues involved in ligand interaction.

METHODS

The A2B receptor models are built by using two recently published crystal structures of the human A2A receptor in complex with two different agonists. The developed models are used as targets for molecular docking studies of nucleoside and non-nucleoside agonists. The generated docking conformations are subjected to energy minimization and rescoring by using three different scoring functions. Further analysis of top-score conformations are performed with a tool evaluating the interaction energy between the ligand and the binding site residues.

RESULTS

Results suggest a set of common interaction points between the two structural families of agonists and the receptor binding site, as evidenced by the superimposition of docking conformations and by analysis of interaction energy with the receptor residues.

CONCLUSIONS

The obtained results show that there is a conserved pattern of interaction between the A2B receptor and its agonists. These information and can provide useful data to support the design and the development of A2B receptor agonists belonging to nucleoside or non-nucleoside structural families.

Ligand-specific binding and activation of the human adenosine A(2B) receptor

Adenosine A(2B) receptors, which play a role in inflammation and cancer, are of considerable interest as novel drug targets. To gain deeper insights into ligand binding and receptor activation, we exchanged amino acids predicted to be close to the binding pocket. The alanine mutants were stably expressed in CHO cells and characterized by radioligand binding and cAMP assays using three structural classes of ligands: xanthine (antagonist), adenosine, and aminopyridine derivatives (agonists). Asn282(7.45) and His280(7.43) were found to stabilize the binding site by intramolecular hydrogen bond formation as in the related A(2A) receptor subtype. Trp247(6.48), Val250(6.51), and particularly Ser279(7.42) were shown to be important for binding of nucleosidic agonists. Leu81(3.28), Asn186(5.42), and Val250(6.51) were discovered to be crucial for binding of the xanthine-derived antagonist PSB-603. Leu81(3.28), which is not conserved among adenosine receptor subtypes, may be important for the high selectivity of PSB-603. The N186(5.42)A mutant resulted in an increased potency for agonists. The interactions of the non-nucleosidic agonist BAY60-6583 were different from those of the nucleosides: while BAY60-6583 appeared not to interact with Ser279(7.42), its interactions with Trp247(6.48) and Val250(6.51) were significantly weaker compared to those of NECA. Moreover, our results discount the hypothesis of Trp247(6.48) serving as a "toogle switch" because BAY60-6583 was able to activate the corresponding mutant. This study reveals distinct interactions of structurally diverse ligands with the human A(2B) receptor and differences between closely related receptor subtypes (A(2B) and A(2A)). It will contribute to the understanding of G protein-coupled receptor function and advance A(2B) receptor ligand design.

Therapeutic potential of adenosine analogues and conjugates

This review summarizes current knowledge of adenosine analogues and conjugates with promising therapeutic properties. Adenosine is a signaling molecule that triggers numerous physiological responses. It acts through the adenosine receptors (ARs), belonging to the family of G-protein-coupled receptors and widely distributed throughout the body. Moreover, adenosine is involved in key biochemical processes as a part of ATP, the universal energy currency. Thus, compounds that are analogues of adenosine and its conjugates have been extensively studied as potential therapeutics. Many inhibitors of ARs are in clinical trials as promising agents in treatment of inflammation, type 2 diabetes, arrhythmia and as vasodilators used in the myocardial perfusion imaging (MPI) stress test. Furthermore, adenosine analogues revealed high efficacy as enzyme inhibitors, tested for antitrypanosomal action and as bivalent ligands and adenosine-oligoarginine conjugates as inhibitors of protein kinases.

Recent developments in adenosine receptor ligands and their potential as novel drugs

Medicinal chemical approaches have been applied to all four of the adenosine receptor (AR) subtypes (A(1), A(2A), A(2B), and A(3)) to create selective agonists and antagonists for each. The most recent class of selective AR ligands to be reported is the class of A(2B)AR agonists. The availability of these selective ligands has facilitated research on therapeutic applications of modulating the ARs and in some cases has provided clinical candidates. Prodrug approaches have been developed which improve the bioavailability of the drugs, reduce side-effects, and/or may lead to site-selective effects. The A(2A) agonist regadenoson (Lexiscan®), a diagnostic drug for myocardial perfusion imaging, is the first selective AR agonist to be approved. Other selective agonists and antagonists are or were undergoing clinical trials for a broad range of indications, including capadenoson and tecadenoson (A(1) agonists) for atrial fibrillation, or paroxysmal supraventricular tachycardia, respectively, apadenoson and binodenoson (A(2A) agonists) for myocardial perfusion imaging, preladenant (A(2A) antagonist) for the treatment of Parkinson's disease, and CF101 and CF102 (A(3) agonists) for inflammatory diseases and cancer, respectively.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

Structure based prediction of subtype-selectivity for adenosine receptor antagonists

One of the major hurdles in the development of safe and effective drugs targeting G-protein coupled receptors (GPCRs) is finding ligands that are highly selective for a specific receptor subtype. Structural understanding of subtype-specific binding pocket variations and ligand-receptor interactions may greatly facilitate design of selective ligands. To gain insights into the structural basis of ligand subtype selectivity within the family of adenosine receptors (AR: A(1), A(2A), A(2B), and A(3)) we generated 3D models of all four subtypes using the recently determined crystal structure of the A(A2)AR as a template, and employing the methodology of ligand-guided receptor optimization for refinement. This approach produced 3D conformational models of AR subtypes that effectively explain binding modes and subtype selectivity for a diverse set of known AR antagonists. Analysis of the subtype-specific ligand-receptor interactions allowed identification of the major determinants of ligand selectivity, which may facilitate discovery of more efficient drug candidates.

Homology modelling of the human adenosine A2B receptor based on X-ray structures of bovine rhodopsin, the beta2-adrenergic receptor and the human adenosine A2A receptor

A three-dimensional model of the human adenosine A2B receptor was generated by means of homology modelling, using the crystal structures of bovine rhodopsin, the beta2-adrenergic receptor, and the human adenosine A2A receptor as templates. In order to compare the three resulting models, the binding modes of the adenosine A2B receptor antagonists theophylline, ZM241385, MRS1706, and PSB601 were investigated. The A2A-based model was much better able to stabilize the ligands in the binding site than the other models reflecting the high degree of similarity between A2A and A2B receptors: while the A2B receptor shares about 21% of the residues with rhodopsin, and 31% with the beta2-adrenergic receptor, it is 56% identical to the adenosine A2A receptor. The A2A-based model was used for further studies. The model included the transmembrane domains, the extracellular and the intracellular hydrophilic loops as well as the terminal domains. In order to validate the usefulness of this model, a docking analysis of several selective and nonselective agonists and antagonists was carried out including a study of binding affinities and selectivities of these ligands with respect to the adenosine A2A and A2B receptors. A common binding site is proposed for antagonists and agonists based on homology modelling combined with site-directed mutagenesis and a comparison between experimental and calculated affinity data. The new, validated A2B receptor model may serve as a basis for developing more potent and selective drugs.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.

Recent improvements in the development of A(2B) adenosine receptor agonists

Adenosine is known to exert most of its physiological functions by acting as local modulator at four receptor subtypes named A(1), A(2A), A(2B) and A(3) (ARs). Principally as a result of the difficulty in identifying potent and selective agonists, the A(2B) AR is the least extensively characterised of the adenosine receptors family. Despite these limitations, growing understanding of the physiological meaning of this target indicates promising therapeutic perspectives for specific ligands. As A(2B) AR signalling seems to be associated with pre/postconditioning cardioprotective and anti-inflammatory mechanisms, selective agonists may represent a new therapeutic group for patients suffering from coronary artery disease. Herein we present an overview of the recent advancements in identifying potent and selective A(2B) AR agonists reported in scientific and patent literature. These compounds can be classified into adenosine-like and nonadenosine ligands. Nucleoside-based agonists are the result of modifying adenosine by substitution at the N (6)-, C(2)-positions of the purine heterocycle and/or at the 5'-position of the ribose moiety or combinations of these substitutions. Compounds 1-deoxy-1-{6-[N'-(furan-2-carbonyl)-hydrazino]-9H-purin-9-yl}-N-ethyl-beta-D-ribofuranuronamide (19, hA(1) K (i) = 1050 nM, hA(2A) K (i) = 1550 nM, hA(2B) EC(50) = 82 nM, hA(3) K (i) > 5 muM) and its 2-chloro analogue 23 (hA(1) K (i) = 3500 nM, hA(2A) K (i) = 4950 nM, hA(2B) EC(50) = 210 nM, hA(3) K (i) > 5 muM) were confirmed to be potent and selective full agonists in a cyclic adenosine monophosphate (cAMP) functional assay in Chinese hamster ovary (CHO) cells expressing hA(2B) AR. Nonribose ligands are represented by conveniently substituted dicarbonitrilepyridines, among which 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY-60-6583, hA(1), hA(2A), hA(3) EC(50) > 10 muM; hA(2B) EC(50) = 3 nM) is currently under preclinical-phase investigation for treating coronary artery disorders and atherosclerosis.

Probing distal regions of the A2B adenosine receptor by quantitative structure-activity relationship modeling of known and novel agonists

The binding modes at the A 2B adenosine receptor (AR) of 72 derivatives of adenosine and its 5'- N-methyluronamide with diverse substitutions at the 2 and N (6) positions were studied using a molecular modeling approach. The compounds in their receptor-docked conformations were used to build CoMFA and CoMSIA quantitative structure-activity relationship models. Various parameters, including different types of atomic charges, were examined. The best statistical parameters were obtained with a joint CoMFA and CoMSIA model: R (2) = 0.960, Q (2) = 0.676, SEE = 0.175, F = 158, and R (2) test = 0.782 for an independent test set containing 18 compounds. On the basis of the modeling results, four novel adenosine analogues, having elongated or bulky substitutions at N (6) position and/or 2 position, were synthesized and evaluated biologically. All of the proposed compounds were potent, full agonists at the A 2B AR in adenylate cyclase studies. Thus, in support of the modeling, bulky substitutions at both positions did not prevent A 2B AR activation, which predicts separate regions for docking of these moieties.

Emerging adenosine receptor agonists

Adenosine receptors (ARs) are a four-member subfamily of G protein-coupled receptors and are major targets of caffeine and theophylline. There are four subtypes of ARs, designated as A1, A2A, A2B and A3. Selective agonists are now available for all four subtypes. Over a dozen of these selective agonists are now in clinical trials for various conditions, although none has received regulatory approval except for the endogenous AR agonist adenosine itself. A1AR agonists are in clinical trials for cardiac arrhythmias and neuropathic pain. A2AAR agonists are now in trials for myocardial perfusion imaging and as anti-inflammatory agents. A2BAR agonists are under preclinical scrutiny for potential treatment of cardiac ischemia. A3AR agonists are in clinical trials for the treatment of rheumatoid arthritis and colorectal cancer. The present review will mainly cover the agonists that are presently in clinical trials for various conditions and only a brief introduction will be given to major chemical classes of AR agonists presently under investigation.

N(6)-[(hetero)aryl/(cyclo)alkyl-carbamoyl-methoxy-phenyl]-(2-chloro)-5'-N-ethylcarboxamido-adenosines: the first example of adenosine-related structures with potent agonist activity at the human A(2B) adenosine receptor

A new series of N(6)-[(hetero)aryl/(cyclo)alkyl-carbamoyl-methoxy-phenyl]-(2-chloro)-5'-N-ethylcarboxamido-adenosines (24-43) has been synthesised and tested in binding assays at hA(1), hA(2A) and hA(3) adenosine receptors, and in a functional assay at the hA(2B) subtype. The examined compounds displayed high potency in activating A(2B) receptors with good selectivity versus A(2A) subtypes. The introduction of an unsubstituted 4-[(phenylcarbamoyl)-methoxy]-phenyl chain at the N(6) position of 5'-N-ethylcarboxamido-adenosine led us to the recognition of compound 24 as a full agonist displaying the highest efficacy of the series (EC(50) hA(2B)=7.3 nM). These compounds represent the first report about adenosine-related structures capable of activating hA(2B) subtype in the low nanomolar range.