The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

Adenosine Receptor Ligands: Coumarin-Chalcone Hybrids as Modulating Agents on the Activity of hARs

Adenosine receptors (ARs) play an important role in neurological and psychiatric disorders such as Alzheimer’s disease, Parkinson’s disease, epilepsy and schizophrenia. The different subtypes of ARs and the knowledge on their densities and status are important for understanding the mechanisms underlying the pathogenesis of diseases and for developing new therapeutics. Looking for new scaffolds for selective AR ligands, coumarin–chalcone hybrids were synthesized (compounds 1–8) and screened in radioligand binding (hA₁, hA_2A and hA₃) and adenylyl cyclase (hA_2B) assays in order to evaluate their affinity for the four human AR subtypes (hARs). Coumarin–chalcone hybrid has been established as a new scaffold suitable for the development of potent and selective ligands for hA₁ or hA₃ subtypes. In general, hydroxy-substituted hybrids showed some affinity for the hA₁, while the methoxy counterparts were selective for the hA₃. The most potent hA₁ ligand was compound 7 (K_i = 17.7 µM), whereas compound 4 was the most potent ligand for hA₃ (K_i = 2.49 µM). In addition, docking studies with hA₁ and hA₃ homology models were established to analyze the structure–function relationships. Results showed that the different residues located on the protein binding pocket could play an important role in ligand selectivity.

An Advanced In Silico Modelling of the Interaction between FSCPX, an Irreversible A1 Adenosine Receptor Antagonist, and NBTI, a Nucleoside Transport Inhibitor, in the Guinea Pig Atrium

In earlier studies, we generated concentration-response (E/c) curves with CPA (N6-cyclopentyladenosine; a selective A1 adenosine receptor agonist) or adenosine, in the presence or absence of S-(2-hydroxy-5-nitrobenzyl)-6-thioinosine (NBTI, a selective nucleoside transport inhibitor), and with or without a pretreatment with 8-cyclopentyl-N3-[3-(4-(fluorosulfonyl)-benzoyloxy)propyl]-N1-propylxanthine (FSCPX, a chemical known as a selective, irreversible A1 adenosine receptor antagonist), in isolated, paced guinea pig left atria. Meanwhile, we observed a paradoxical phenomenon, i.e. the co-treatment with FSCPX and NBTI appeared to enhance the direct negative inotropic response to adenosine. In the present in silico study, we aimed to reproduce eight of these E/c curves. Four models (and two additional variants of the last model) were constructed, each one representing a set of assumptions, in order to find the model exhibiting the best fit to the ex vivo data, and to gain insight into the paradoxical phenomenon in question. We have obtained in silico evidence for an interference between effects of FSCPX and NBTI upon our ex vivo experimental setting. Regarding the mechanism of this interference, in silico evidence has been gained for the assumption that FSCPX inhibits the effect of NBTI on the level of endogenous (but not exogenous) adenosine. As an explanation, it may be hypothesized that FSCPX inhibits an enzyme participating in the interstitial adenosine formation. In addition, our results suggest that NBTI does not stop the inward adenosine flux in the guinea pig atrium completely.

Novel thiazole-thiophene conjugates as adenosine receptor antagonists: synthesis, biological evaluation and docking studies

Here we report novel thiazole-thiophene conjugates as adenosine receptor antagonists. All the molecules were evaluated for their binding affinity for adenosine receptors. Most of the molecules were found to interact with the A1, A2A and A3 adenosine receptor subtypes with good affinity values. The most potent and selective compound 8n showed an A3Ki value of 0.33μM with selectivity ratios of >90 versus the A1 and >30 versus the A2 subtypes. For compound 8n docking studies into the binding site of the A3 adenosine receptor are provided to visualize its binding mode.

Biochemical assay of G protein-coupled receptor oligomerization: adenosine A1 and thromboxane A2 receptors form the novel functional hetero-oligomer

G protein-coupled receptors (GPCRs) are classified into a family of seven transmembrane receptors. Receptor oligomerization may be the key to the expression and function of these receptors, for example, ligand binding, desensitization, membrane trafficking, and signaling. The accumulation of evidence that GPCRs form an oligomerization with a functional alternation may change the strategy for the discovery of novel drugs targeting GPCRs. Identification of the oligomer is essential to elucidate GPCR oligomerization. GPCR oligomerizations have been demonstrated using various biochemical approaches, which include the coimmunoprecipitation method, fluorescence resonance energy transfer assay, and bioluminescence RET assay. Thus, various assays are useful for the study of GPCR oligomerization, and we should choose the best method to match the purpose. We previously targeted adenosine A1 and thromboxane A2 (TP) receptors to form a functionally novel hetero-oligomer, since both receptors function in the same cells. This chapter describes the methods used to detect GPCR oligomerization and alterations in the signaling pathways, principally according to our findings on oligomerization between adenosine A1 and TPα receptors.

The second extracellular loop of the adenosine A1 receptor mediates activity of allosteric enhancers

Allosteric enhancers of the adenosine A1 receptor amplify signaling by orthosteric agonists. Allosteric enhancers are appealing drug candidates because their activity requires that the orthosteric site be occupied by an agonist, thereby conferring specificity to stressed or injured tissues that produce adenosine. To explore the mechanism of allosteric enhancer activity, we examined their action on several A1 receptor constructs, including (1) species variants, (2) species chimeras, (3) alanine scanning mutants, and (4) site-specific mutants. These findings were combined with homology modeling of the A1 receptor and in silico screening of an allosteric enhancer library. The binding modes of known docked allosteric enhancers correlated with the known structure-activity relationship, suggesting that these allosteric enhancers bind to a pocket formed by the second extracellular loop, flanked by residues S150 and M162. We propose a model in which this vestibule controls the entry and efflux of agonists from the orthosteric site and agonist binding elicits a conformational change that enables allosteric enhancer binding. This model provides a mechanism for the observations that allosteric enhancers slow the dissociation of orthosteric agonists but not antagonists.

Homodimerization of adenosine A₁ receptors in brain cortex explains the biphasic effects of caffeine

Using bioluminescence resonance energy transfer and proximity ligation assays, we obtained the first direct evidence that adenosine A₁ receptors (A₁Rs) form homomers not only in cell cultures but also in brain cortex. By radioligand binding experiments in the absence or in the presence of the A₁Rs allosteric modulator, adenosine deaminase, and by using the two-state dimer receptor model to fit binding data, we demonstrated that the protomer-protomer interactions in the A₁R homomers account for some of the pharmacological characteristics of agonist and antagonist binding to A₁Rs. These pharmacological properties include the appearance of cooperativity in agonist binding, the change from a biphasic saturation curve to a monophasic curve in self-competition experiments and the molecular cross-talk detected when two different specific molecules bind to the receptor. In this last case, we discovered that caffeine binding to one protomer increases the agonist affinity for the other protomer in the A₁R homomer, a pharmacological characteristic that correlates with the low caffeine concentrations-induced activation of agonist-promoted A₁R signaling. This pharmacological property can explain the biphasic effects reported at low and high concentration of caffeine on locomotor activity.

Structural sweet spot for A1 adenosine receptor activation by truncated (N)-methanocarba nucleosides: receptor docking and potent anticonvulsant activity

A(1) adenosine receptor (AR) agonists display antiischemic and antiepileptic neuroprotective activity, but peripheral cardiovascular side effects impeded their development. SAR study of N(6)-cycloalkylmethyl 4'-truncated (N)-methanocarba-adenosines identified 10 (MRS5474, N(6)-dicyclopropylmethyl, K(i) = 47.9 nM) as a moderately A(1)AR-selective full agonist. Two stereochemically defined N(6)-methynyl group substituents displayed narrow SAR; groups larger than cyclobutyl greatly reduced AR affinity, and those larger or smaller than cyclopropyl reduced A(1)AR selectivity. Nucleoside docking to A(1)AR homology model characterized distinct hydrophobic cyclopropyl subpockets, the larger "A" forming contacts with Thr270 (7.35), Tyr271 (7.36), Ile274 (7.39), and carbon chains of glutamates (EL2) and the smaller subpocket "B" forming contacts between TM6 and TM7. 10 suppressed minimal clonic seizures (6 Hz mouse model) without typical rotarod impairment of A(1)AR agonists. Truncated nucleosides, an appealing preclinical approach, have more druglike physicochemical properties than other A(1)AR agonists. Thus, we identified highly restricted regions for substitution around N(6) suitable for an A(1)AR agonist with anticonvulsant activity.

Allosteric enhancers for A1 adenosine receptor

Allosteric enhancers at the adenosine A(1) receptor have received attention as anti-arrhythmic cardiac agents, and, more recently, as anti-lipolytic agents. In addition, allosteric modulators at the adenosine A(1) receptor have therapeutic potential as analgesics and neuroprotective agents. In particular, the compounds with improved potency as enhancers and reduced antagonist properties are mentioned.

Probing the binding site of the A1 adenosine receptor reengineered for orthogonal recognition by tailored nucleosides

His272 (7.43) in the seventh transmembrane domain (TM7) of the human A3 adenosine receptor (AR) interacts with the 3' position of nucleosides, based on selective affinity enhancement at a H272E mutant A3 AR (neoceptor) of 3'-ureido, but not 3'-OH, adenosine analogues. Here, mutation of the analogous H278 of the human A1 AR to Ala, Asp, Glu, or Leu enhanced the affinity of novel 2'- and 3'-ureido adenosine analogues, such as 10 (N6-cyclopentyl-3'-ureido-3'-deoxyadenosine), by >100-fold, while decreasing the affinity or potency of adenosine and other 3'-OH adenosine analogues. His278 mutant receptors produced a similar enhancement regardless of the charge character of the substituted residue, implicating steric rather than electrostatic factors in the gain of function, a hypothesis supported by rhodopsin-based molecular modeling. It was also demonstrated that this interaction was orientationally specific; i.e., mutations at the neighboring Thr277 did not enhance the affinity for a series of 2'- and 3'-ureido nucleosides. Additionally, H-bonding groups placed on substituents at the N6 or 5' position demonstrated no enhancement in the mutant receptors. These reengineered human A1 ARs revealed orthogonality similar to that of the A3 but not the A2A AR, in which mutation of the corresponding residue, His278, to Asp did not enhance nucleoside affinity. Functionally, the H278D A1 AR was detectable only in a measure of membrane potential and not in calcium mobilization. This neoceptor approach should be useful for the validation of molecular modeling and the dissection of promiscuous GPCR signaling.

N9-benzyl-substituted 1,3-dimethyl- and 1,3-dipropyl-pyrimido[2,1-f]purinediones: synthesis and structure-activity relationships at adenosine A1 and A2A receptors

Synthesis and physicochemical properties of N-benzyl pyrimido[2,1-f]purinediones are described. These derivatives were synthesized by the cyclization of 7-chloropropylo-8-bromo-1,3-dimethyl- or 1,3-dipropyl xanthine derivatives with corresponding (un)substituted benzylamines. Dipropyl derivatives were obtained under microwave irradiation conditions either. The obtained compounds (1-20) were evaluated for their affinity to adenosine A1 and A2A receptors, selected compounds were additionally investigated for affinity to the A3 receptor subtype. The results of the radioligand binding assays to A1 and A2A adenosine receptors showed that most of the 1,3-dimethyl-9-benzylpyrimidopurinediones exhibited selective affinity to A2A receptors at micromolar or submicromolar concentrations (for example, derivative 9 with o-methoxy substituent displayed a Ki value of 0.699 microM at rat A2A receptor with more than 36-fold selectivity). Contrary to previously described arylpyrimido[2,1-f]purinediones dipropyl derivatives (compounds 15-20) showed affinity to both kinds of receptors increased, however A1 affinity increased to a larger extent, with the result that A2A selectivity was abolished. The best adenosine A1 receptor ligand was m-chlorobenzyl derivative 18 (Ki=0.089 microM and 5-fold A1 selectivity). Structure-activity relationships were discussed with the analysis of lipophilic and spatial properties of the investigated compounds. Pharmacophore model of adenosine A1 receptor antagonist was adopted for this purpose.

On the stability of 2-aminoselenophene-3-carboxylates: potential dual-acting selenium-containing allosteric enhancers of A1 adenosine receptor binding

Ethyl-2-amino-4,5,6,7-tetrahydro-1-benzoselenophene-3-carboxylate (4), has been prepared as a potential dual-acting selenium-containing allosteric enhancer of adenosine A(1)A receptor binding utilising a modified Gewald reaction. While preliminary testing indicated that 4 is a superior enhancer of A(1)AR binding than its thiophene counterpart, its instability under mildly acidic conditions is cause for concern. X-Ray crystallography, together with DFT calculations, provide evidence that the decomposition of 4 involves the ring-opening of selenophenium ion (12b) followed by the loss of elemental selenium through a radical chain process.

The allosteric enhancer PD81,723 increases chimaeric A1/A2A adenosine receptor coupling with Gs

PD81,723 {(2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluromethyl)-phenyl]methanone} is a selective allosteric enhancer of the G(i)-coupled A1 AR (adenosine receptor) that is without effect on G(s)-coupled A2A ARs. PD81,723 elicits a decrease in the dissociation kinetics of A1 AR agonist radioligands and an increase in functional agonist potency. In the present study, we sought to determine whether enhancer sensitivity is dependent on coupling domains or G-protein specificity of the A1 AR. Using six chimaeric A1/A2A ARs, we show that the allosteric effect of PD81,723 is maintained in a chimaera in which the predominant G-protein-coupling domain of the A1 receptor, the 3ICL (third intracellular loop), is replaced with A2A sequence. These chimaeric receptors are dually coupled with G(s) and G(i), and PD81,723 increases the potency of N6-cyclopentyladenosine to augment cAMP accumulation with or without pretreatment of cells with pertussis toxin. Thus PD81,723 has similar functional effects on chimaeric receptors with A1 transmembrane sequences that couple with G(i) or G(s). This is the first demonstration that an allosteric regulator can function in the context of a switch in G-protein-coupling specificity. There is no enhancement by PD81,723 of G(i)-coupled A2A chimaeric receptors with A1 sequence replacing A2A sequence in the 3ICL. The results suggest that the recognition site for PD81,723 is on the A1 receptor and that the enhancer acts to directly stabilize the receptor to a conformational state capable of coupling with G(i) or G(s).

Pharmacological Chaperone Activity of SR49059 to Functionally Recover Misfolded Mutations of the Vasopressin V1a Receptor

Pharmacological chaperones represent a new class of ligand with the potential to facilitate the delivery of misfolded, but still active, G-protein-coupled receptors to the cell surface. Using transfected HEK 293T cells, treatment with a nonpeptide antagonist, SR49059, dramatically increased ( approximately 60-fold) the surface expression of a misfolded, nonfunctional and intracellularly localized vasopressin V(1a) receptor (V(1a)R) mutant (D148A). This rescue of surface expression (111 +/- 7%) was almost identical to wild type assessed by confocal microscopy and quantitative enzyme-linked immunosorbent assay-based techniques. Recovery was not specific to D148A, since other surface-impaired mutations, D148N and D148E, and wild type were also increased following SR49059 exposure. However, surface delivery was specific to SR49059, since V(1a)R-selective peptide ligands or unrelated ligands were unable to mimic this action, suggesting that SR49059 acts intracellularly. SR49059-mediated surface rescue was time-, mutant-, and concentration-dependent but not directly related to its binding affinity. Maximal recovery was achieved following 12 h of treatment and did not involve de novo receptor synthesis or a consequence of preventing endogenous constitutive activity and/or internalization. Once at the surface, all mutants displayed enhanced signaling ability, and D148A was able to undergo agonist-mediated internalization. SR49059 was not effectively removed from the receptor, since signaling (EC(50)) of both wild type and D148A was reduced approximately 40-fold. This is the first report of a pharmacological chaperone ligand to act on misfolded mutant V(1a) Rs. This work provides an excellent model to understand the mechanistic action of an important new class of drug that may have potential in the treatment of diseases caused by inherited mutations.

2-(Benzimidazol-2-yl)quinoxalines: a novel class of selective antagonists at human A(1) and A(3) adenosine receptors designed by 3D database searching

The Cambridge Structural Database (CSD) was searched through two 3D queries based on substructures shared by well-known antagonists at the A(1) and A(3) adenosine receptors (ARs). Among the resulting 557 hits found in the CSD, we selected five compounds to purchase, synthesize, or translate synthetically into analogues better tailored to interact with the biological targets. Binding experiments using human ARs showed that four out of five tested compounds turned out to be antagonists at the A(1)AR or A(3)AR with K(i) values between 50 and 440 nM. Lead optimizations of 2-(benzimidazol-2-yl)quinoxalines (BIQs, 3) gave the best results in terms of potency and selectivity at the A(1) and A(3) ARs. Particularly, 2-(4-ethylthiobenzimidazol-2-yl)quinoxaline (3e) exhibited K(i) values at the A(1)AR, A(2A)AR, and A(3)AR of 0.5, 3440, and 955 nM, respectively, whereas 2-(4-methylbenzimidazol-2-yl)quinoxaline (3b) displayed at the same ARs K(i) values of 8000, 833, and 26 nM, respectively.

Role of the second transmembrane domain of rat adenosine A1 receptor in ligand-receptor interaction

Initial mutagenesis studies exploring the ligand recognition model of A1 adenosine receptor (A1R) mainly focused on the residues in the 5th-7th transmembrane domains (TMs5-7). Little is known about the role of residues in TM2. To explore the importance of reserved hydrophobic region in TM2 of A1R, we mutated the hydrophobic residues at positions 65 and 69 to hydrophilic residues (L65T, Leu-65 to Thr-65; I69T, Ile-69 to Thr-69; I69S, Ile-69 to Ser-69) to change the hydrophobicity at the outer end of TM2. Binding assays showed that the affinities of mutant receptors were significantly decreased for ribose group-containing agonists (2-chloro-N6-cyclopentyladenosine (CCPA) and 5'-N-ethyl-carboxamidoadenosine (NECA)) but not for antagonists, N6-cyclopentyl-9-methyladenine (N-0840), an adenine derivative lacking ribose group, and 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), a xanthine derivative. This observation suggests that the hydrophobic region at the outer end of TM2 may mediate the recognition of the ribose group of CCPA and NECA.

Synthesis and 3D QSAR of New Pyrazolo[3,4-b]pyridines: Potent and Selective Inhibitors of A1 Adenosine Receptors

A number of 4-aminopyrazolo[3,4-b]pyridines 5-carboxylic acid esters (2-8) were synthesized and evaluated for their binding affinity at the A1, A2A, and A3 adenosine receptors (AR), in bovine cortical membranes, as well as for their affinity toward human A1AR (hA1AR). Some of the new compounds were characterized by a high affinity and selectivity toward the A1 receptor subtype, showing a significant improvement in comparison with other pyrazolo-pyridines previously reported in the literature. In particular the methyl ester 2h as well as the isopropyl ester 5h, both of them bearing a p-methoxyphenylethylamino side chain at the position 4, presented Ki values of 6 and 7 nM, respectively. To rationalize the relationships between structure and affinity of the novel compounds, a 3D QSAR model was also generated starting from compounds belonging to different classes of known A1AR antagonists.

Partners for adenosine A1 receptors

G protein-coupled receptors (GPCRs) are targets for therapy in a variety of neurological diseases. Using adenosine A1 receptors (A1Rs) as paradigm of GPCRs, this review focuses on how protein-protein interactions, from monomers to heteromers, can contribute to hormone/neurotransmitter/neuromodulator regulation. The interaction of A1Rs with other membrane receptors, enzymes, and adaptor and scaffolding proteins is relevant for receptor traffic, internalization, and desensitization, and A1Rs are extremely important in driving signaling through different intracellular pathways. There is even the possibility of linking together GPCR heteromeric complexes with ion channel receptors in a receptor mosaic that might have special integrative value and might constitute the molecular basis for learning and memory.

Allosteric modulation of the adenosine family of receptors

Allosteric modulators for adenosine receptors (ARs) are of an increasing interest and may have potential therapeutic advantage over orthosteric ligands. Benzoylthiophene derivatives (including PD 81,723), 2-aminothiazolium salts, and related allosteric modulators of the A(1) AR have been studied. The benzoylthiophene derivatives were demonstrated to be selective enhancers for the A(1) AR, with little or no effect on other subtypes of ARs. Allosteric modulation of the A(2A) AR has also been reported. A(3) allosteric enhancers may be predicted to be useful against ischemic conditions. We have recently characterized two classes of A(3) AR allosteric modulators: 3-(2-pyridinyl)isoquinolines (e.g. VUF5455) and 1H-imidazo-[4,5-c]quinolin-4-amines (e.g. DU124183), which selectively decreased the agonist dissociation rate at the human A(3)AR but not at A(1) and A(2A) ARs. DU124183 left-shifted the agonist conc.-response curve for inhibition of forskolin-stimulated cAMP accumulation in intact cells expressing the human A(3)AR with up to 30% potentiation of the maximal efficacy. The increased potency of A(3) agonists was evident only in the presence of an A(3) antagonist, since VUF5455 and DU124183 also antagonized, i.e. displaced binding at the orthosteric site, with K(i) values of 1.68 and 0.82 microM, respectively. A(3)AR mutagenesis studies implicated F182(5.43) and N274(7.45) in the action of the enhancers and was interpreted using a rhodopsin-based A(3)AR molecular model, suggesting multiple binding modes. Amiloride analogues, SCH-202676 (N-(2,3-diphenyl-1,2,4-thiadiazol-5(2H)-ylidene)methanamine), and sodium ions were demonstrated to be common allosteric modulators for at least three subtypes (A(1), A(2A), and A(3)) of ARs.

Partial A(1) adenosine receptor agonists from a molecular perspective and their potential use as chronic ventricular rate control agents during atrial fibrillation (AF)

This review provides a molecular perspective of partial agonism at the A(1) adenosine receptor. The structure-activity relationships (SAR) for affinity and intrinsic efficacy of analogues of the full agonist N6-cyclopentyladenosine (CPA) are emphasized. Both general models of activation of G protein-coupled receptors and specific molecular models of the A(1)-adenosine receptor are used to interpret the results of efforts to synthesize and assay effects of partial agonists. The SAR of affinity and intrinsic efficacy of the 2', 3', and especially the 5'-deoxy derivatives of CPA is presented. From this analysis, the nature of the interactions of specific atoms and substituents of the CPA molecule with the A(1)-adenosine receptor are deduced and presented pictorially. As an example of the therapeutic potential of partial agonists, the design and testing of analogues of CPA to provide chronic ventricular rate control during atrial fibrillation is described. The challenges associated with designing a partial A(1)-adenosine receptor agonist for providing chronic ventricular rate control during atrial fibrillation are many. To meet these challenges, further medicinal chemistry efforts in the area of partial A(1)-adenosine receptor agonism are still needed.

N6-cycloalkyl-2-phenyl-3-deaza-8-azaadenines: a new class of A1 adenosine receptor ligands. A comparison with the corresponding adenines and 8-azaadenines

Several 9-benzyl-N6-cycloalkyl-2-phenyladenines, 9-benzyl-N6-cycloalkyl-2-phenyl-8-azaadenines and 4-cycloalkylamino-1-benzyl-6-phenyl-1H-1,2,3-triazolo[4,5-c]pyridines were prepared and assayed as A1 adenosine receptor ligands. The 1H-1,2,3-triazolo[4,5-c]pyridines were obtained starting from N,N-diethyl-1-benzyl-4-carboxyamido-5-methyl-1H-1,2,3-triazole by lithiation in anhydrous tetrahydrofurane in the presence of benzonitrile. The usual work up afforded the isolation of 1-benzyl-6-phenyl-1H-1,2,3-triazolo[4,5-c]pyridin-4-one which was treated with phosphorous oxychloride and cycloalkylamines. Some compounds showed high affinity and selectivity and the trend of Ki values corresponds to the series of 9-benzyl-N6-cycloalkyl-2-phenyladenines and 9-benzyl-N6-cycloalkyl-2-phenyl-8-azaadenines, therefore they can be considered bioisosteres. The affinity data permitted us to ascertain the role and the importance of the N3 in the adenine or 8-azaadenine moiety in the receptor binding and to study the dimension of the receptor lipophilic pocket which is filled by the N6 substituent of adenosine derivatives.

A1 Adenosine Receptor Agonists: Medicinal Chemistry and Therapeutic Potential

Adenosine receptors are widely distributed in the body and modulate numerous physiological processes. Four receptor subtypes (termed A(1), A(2A), A(2B) and A(3)) have been identified based on their pharmacological profile and cloning. Activation of the A(1) adenosine receptors produces a number of effects including a reduction in heart rate and atrial contractility, the attenuation of the stimulatory actions of catecholamines on the heart as well as a reduction of lipolysis in adipose tissue. As a result, A(1)AR agonists have been targeted as anti-arrhythmic and cardioprotective agents. This review discusses the synthesis, structure-activity relationships and therapeutic potential of A(1)AR agonists.

A TOPS-MODE approach to predict affinity for A1 adenosine receptors. 2-(Arylamino)adenosine analogues

The TOPological Sub-Structural Molecular Design (TOPS-MODE) approach has been applied to the study of the affinity of A(1) adenosine receptor of different 2-(arylamino)adenosine analogues. A model able to describe closed to 79% of the variance in the values for binding experiments of 32 analogues of these compounds through multilinear regression analysis (MRA) was developed with the use of the mentioned approach. In contrast, no one of seven different approaches, including the use of Constitutional, Topological, Molecular walk counts, BCUT, Randic Molecular profiles, Geometrical, and RDF descriptors was able to explain more than 70% of the variance in the mentioned property with the same number of descriptors. In addition, the TOPS-MODE approach permitted to find the contribution of different fragments to the biological property giving to the model a straightforward structural interpretability.

Comparative Analysis of Putative Agonist-Binding Modes in the Human A1 Adenosine Receptor

A recent study reported a model of the human A(1) adenosine receptor and its agonist binding site, proposing two putative binding modes in the same binding site for the natural agonist, adenosine. The present work investigates the flexibility of this binding site by exhaustive exploration with the natural agonist and with three other adenosine derivatives: N6-cyclopentyladenosine (CPA), 2-chloro-N6-cyclopentyladenosine (CCPA), and 5'-N-ethylcarboxamidoadenosine (NECA). Our aim was to find a common binding mode for agonists that would explain the role in the binding process of the different substitutions allowed at the 2, N6, and 5' positions of adenosine. This problem was addressed through docking simulations, molecular dynamics studies, and estimations of the ligand-binding free energy with both the AUTODOCK scoring function and the linear interaction energy (LIE) approach. The results point to a single receptor-binding position that explains the effects of the different chemical modifications on the adenosine derivatives considered here.

Incorporating Molecular Shape into the Alignment-free GRid-INdependent Descriptors

The recently introduced GRid-INdependent Descriptors (GRIND) were designed to provide a suitable description of a series of ligands for 3D-QSAR studies not requiring the spatial superimposition of their structures. Despite the proven usefulness of the method, it was recognized that the original GRIND failed to describe appropriately the shape of the ligand molecules, which in some cases plays a major role in ligand-receptor binding. For this reason, the original descriptors have been enhanced with the addition of a molecular shape description based on the local curvature of the molecular surface. The integration of this description into the GRIND allows the generation of 3D-QSAR models able to identify both favorable and unfavorable shape complementarity in a simple and alignment-independent way. The usefulness of the new GRIND-shape description in 3D-QSAR is illustrated using two structure-activity studies: one performed on a set of xanthine-like antagonists of the A(1) adenosine receptor; another performed on a series of Plasmodium falciparum plasmepsin II inhibitors.

Novel approaches for modeling of the A1adenosine receptor and its agonist binding site

The present work describes the building of a human A(1) adenosine receptor (hA(1)AR) model, based on the X-ray crystal structure of bovine rhodopsin, and its use as a basis for the investigation of some important structural characteristics of the receptor. One of the issues investigated was the protonation position of two histidine residues known to influence ligand binding, with protonation of His251 (6.52) in epsilon position and His278 (7.43) in delta position showing the best agreement with experimental evidence. The model was also used to study the position and structural role of water molecules present in the helical bundle. Finally, the binding site location and the ligand docking were investigated using an objective strategy. A suitable site for the binding of the ribose moiety of adenosine was first postulated and further confirmed by means of a novel chemometric strategy based on GRIND descriptors. Using this position as an anchor point, the binding of adenosine was studied by docking and molecular dynamics simulations obtaining two putative binding positions in good agreement with experimental data.

Cytoskeletal protein 4.1G binds to the third intracellular loop of the A1 adenosine receptor and inhibits receptor action

To identify binding partners of the A1AR (A1 adenosine receptor), yeast two-hybrid screening of a rat embryonic cDNA library was performed. This procedure led to the identification of erythrocyte membrane cytoskeletal protein (represented as 4.1G) as an A1AR-binding partner. Truncation studies revealed that the C-terminal domain of 4.1G was essential for binding to A1ARs and that the C-terminal domain of 4.1G and the third intracellular loop of A1ARs interacted. A1AR-4.1G interaction was also confirmed in studies using brain tissue. Studies in HEK-293 (human embryonic kidney 293) cells and Chinese-hamster ovary cells showed that 4.1G interfered with A1AR signal transduction, as 4.1G reduced A1AR-mediated inhibition of cAMP accumulation and intracellular calcium release. 4.1G also altered cell-surface A1AR expression. These observations identify 4.1G as a novel A1AR-binding partner that can regulate adenosine action.

Exploring the molecular basis of selectivity in A1 adenosine receptors agonists: a case study

Adenosine is a naturally occurring purine nucleoside that has a wide variety of well-documented regulatory functions and physiological roles. Selective activation of the adenosine A1 receptor has drawn attention in drug discovery for the therapeutic effects on neural and cardiovascular disorders. We have developed a model of the human A1 adenosine receptor using bovine rhodopsin as a template. A flexible docking approach has been subsequently carried out for evaluating the molecular interactions of twenty-one selective A1 agonists with the receptor model. The results of these studies are consistent with mutational and biochemical data. In particular, they highlight a wide hydrogen-bonding network between the nucleoside portion of the ligands and the A1 receptor as well as key amino acids for hydrophobic interactions with the different N6-groups of the agonists. The models presented here provide a detailed molecular map for the selective stimulation of the adenosine A1 receptor subtype and a steady basis for the rational design of new A1 selective ligands.