N6-methoxy-2-alkynyladenosine derivatives as highly potent and selective ligands at the human A3 adenosine receptor

C2-linked alkynyl poly-ethylene glycol(PEG) adenosine conjugates as water-soluble adenosine receptor agonists

A series of 12 novel polyethylene-glycol(PEG)-alkynyl C2-adenosine(ADN) conjugates were synthesized using a robust Sonogashira coupling protocol and characterized by NMR spectroscopy and mass spectrometry analysis. The ADN-PEG conjugates showed null to moderate toxicity in murine macrophages and 12c was active against Mycobacterium aurum growth (MIC = 62.5 mg/L). The conjugates were not active against Mycobacterium bovis BCG. Conjugates 10b and 11b exhibited high water solubility with solubility values of 1.22 and 1.18 mg/ml, respectively, in phosphate buffer solutions at pH 6.8. Further, 10b and 11b induced a significant increase in cAMP accumulation in RAW264.7 cells comparable with that induced by adenosine. Analogues 10c, 11c and 12c were docked to the A₁ , A_2A , A_2B and A₃ adenosine receptors (ARs) using crystal-structures and homology models. ADN-PEG-conjugates bearing chains with up to five ethyleneoxy units could be well accommodated within the binding sites of A₁ , A_2A and A₃ ARs. Docking studies showed that compound 10b and 11b were the best A_2A receptor binders of the series, whereas 12c was the best binder for A₁ AR. In summary, introduction of hydrophilic PEG substituents at the C2 of adenine ring significantly improved water solubility and did not affect AR binding properties of the ADN-PEG conjugates.

A3 Adenosine Receptor Antagonists with Nucleoside Structures and Their Anticancer Activity

The overexpression of the A3 adenosine receptor (AR) in a number of cancer cell types makes it an attractive target for tumor diagnosis and therapy. Hence, in the search for new A3AR ligands, a series of novel 2,N6-disubstituted adenosines (Ados) was synthesized and tested in radioligand binding and functional assays at ARs. Derivatives bearing a 2-phenethylamino group in the N6-position were found to exert higher A3AR affinity and selectivity than the corresponding N6-(2,2-diphenylethyl) analogues. 2-Chloro-N6-phenylethylAdo (15) was found to be a potent full A3AR agonist with a Ki of 0.024 nM and an EC50 of 14 nM, in a cAMP accumulation assay. Unlike 15, the other ligands behaved as A3AR antagonists, which concentration-dependently reduced cell growth and exerted cytostatic activity on the prostate cancer cell line PC3, showing comparable and even more pronounced effects with respect to the ones elicited by the reference full agonist Cl-IB-MECA. In particular, the N6-(2,2-diphenylethyl)-2-phenylethynylAdo (12: GI50 = 14 µM, TGI = 29 µM, and LC50 = 59 µM) showed the highest activity proving to be a potential antitumor agent. The cytostatic effect of both A3AR agonist (Cl-IB-MECA) and antagonists (12 and other newly synthesized compounds) confirm previous observations according to which, in addition to the involvement of A3ARs, other cellular mechanisms are responsible for the anticancer effects of these ligands.

Adenosine Receptor Agonists Increase the Inhibition of Platelet Function by P2Y12 Antagonists in a cAMP- and Calcium-Dependent Manner

We have shown previously that platelet activity can be lowered through the simultaneous inhibition of P2Y₁₂ receptor and activation of adenosine receptors (AR). This work explores this concept by testing the antiplatelet potential of nine AR agonists in combination with P2Y₁₂ receptor antagonists—cangrelor and prasugrel metabolite. A panel of in vitro methods was used to assess platelet viability, P-selectin expression, GPIIb-IIIa activation, fibrinogen binding, calcium ion mobilization, VASP-P level, and cAMP formation, utilizing whole blood or isolated platelets from healthy volunteers. The AR agonists demonstrated anti-platelet effects, but stimulated signaling pathways to varying degrees. AR agonists and P2Y₁₂ antagonists reduced expression of both P-selectin and the activated form of GPIIb-IIIa on platelets; however, the combined systems (AR agonist + P2Y₁₂ antagonist) demonstrated stronger effects. The antiplatelet effects of AR when combined with P2Y₁₂ were more pronounced with regard to exogenous fibrinogen binding and calcium mobilization. The cAMP levels in both resting and ADPactivated platelets were increased by AR agonist treatment, and more so when combined with P2Y₁₂ inhibitor. In conclusion, as AR agonists are fast-acting compounds, the methods detecting early activation events are more suitable for assessing their antiplatelet action. The exogenous fibrinogen binding, calcium mobilisation and cAMP level turned out to be sensitive markers for detecting the inhibition caused by AR agonists alone or in combination with P2Y₁₂ receptor antagonists.

Assessment of biased agonism at the A3 adenosine receptor using β-arrestin and miniGαi recruitment assays

The A₃ adenosine receptor (A₃AR) is a G protein-coupled receptor that is involved in a wide variety of physiological and pathological processes, such as cancer. However, the use of compounds pharmacologically targeting this receptor remains limited in clinical practice, despite extensive efforts for compound synthesis. Moreover, the possible occurrence of biased agonism further complicates the interpretation of the functional characteristics of compounds. Hence the need for simple assays, which are comparable in terms of the used cell lines and read-out technique. We previously established a stable β-arrestin 2 (βarr2) bioassay, employing a simple, luminescent read-out via functional complementation of a split nanoluciferase enzyme. Here, we developed a complementary, new bioassay in which coupling of an engineered miniGα_i protein to activated A₃AR is monitored using a similar approach. Application of both bioassays for the concurrent determination of the potencies and efficacies of a set of 19 N⁶-substituted adenosine analogues not only allowed for the characterization of structure-activity relationships, but also for the quantification of biased agonism. Although a broad distribution in potency and efficacy values was obtained within the test panel, no significant bias was observed toward either the βarr2 or miniGα_i pathway.

Blood Platelet Adenosine Receptors as Potential Targets for Anti-Platelet Therapy

Adenosine receptors are a subfamily of highly-conserved G-protein coupled receptors. They are found in the membranes of various human cells and play many physiological functions. Blood platelets express two (A_2A and A_2B) of the four known adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Agonization of these receptors results in an enhanced intracellular cAMP and the inhibition of platelet activation and aggregation. Therefore, adenosine receptors A_2A and A_2B could be targets for anti-platelet therapy, especially under circumstances when classic therapy based on antagonizing the purinergic receptor P2Y₁₂ is insufficient or problematic. Apart from adenosine, there is a group of synthetic, selective, longer-lasting agonists of A_2A and A_2B receptors reported in the literature. This group includes agonists with good selectivity for A_2A or A_2B receptors, as well as non-selective compounds that activate more than one type of adenosine receptor. Chemically, most A_2A and A_2B adenosine receptor agonists are adenosine analogues, with either adenine or ribose substituted by single or multiple foreign substituents. However, a group of non-adenosine derivative agonists has also been described. This review aims to systematically describe known agonists of A_2A and A_2B receptors and review the available literature data on their effects on platelet function.

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

Stimulation of gastric acid secretion by rabbit parietal cell A2B adenosine receptor activation

Adenosine modulates different functional activities in many cells of the gastrointestinal tract; some of them are believed to be mediated by interaction with its four G protein-coupled receptors. The renewed interest in the adenosine A2B receptor (A2BR) subtype can be traced by studies in which the introduction of new genetic and chemical tools has widened the pharmacological and structural knowledge of this receptor as well as its potential therapeutic use in cancer and inflammation- or hypoxia-related pathologies. In the acid-secreting parietal cells of the gastric mucosa, the use of various radioligands for adenosine receptors suggested the presence of the A2 adenosine receptor subtype(s) on the cell surface. Recently, we confirmed A2BR expression in native, nontransformed parietal cells at rest by using flow cytometry and confocal microscopy. In this study, we show that A2BR is functional in primary rabbit gastric parietal cells, as indicated by the fact that agonist binding to A2BR increased adenylate cyclase activity and acid production. In addition, both acid production and radioligand binding of adenosine analogs to isolated cell membranes were potently blocked by selective A2BR antagonists, whereas ligands for A1, A2A, and A3 adenosine receptors failed to abolish activation. We conclude that rabbit gastric parietal cells possess functional A2BR proteins that are coupled to Gs and stimulate HCl production upon activation. Whether adenosine- and A2BR-mediated functional responses play a role in human gastric pathophysiology is yet to be elucidated.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

1,2,4-triazolo[1,5-a]quinoxaline derivatives and their simplified analogues as adenosine A₃ receptor antagonists. Synthesis, structure-affinity relationships and molecular modeling studies

The 1,2,4-triazolo[1,5-a]quinoxaline (TQX) scaffold was extensively investigated in our previously reported studies and recently, our attention was focused at position 5 of the tricyclic nucleus where different acyl and carboxylate moieties were introduced (compounds 2-15). This study produced some interesting compounds endowed with good hA3 receptor affinity and selectivity. In addition, to find new insights about the structural requirements for hA3 receptor-ligand interaction, the tricyclic TQX ring was destroyed yielding some 1,2,4-triazole derivatives (compounds 16-23). These simplified compounds, though maintaining the crucial structural requirements for adenosine receptor-ligand interaction, have a very low hA3 adenosine receptor affinity, the only exception being compound 23 (1-[3-(4-methoxyphenyl)-1-phenyl-1H-1,2,4-triazol-5-yl]-3-phenylurea) endowed with a Ki value in the micro-molar range and high hA3 selectivity versus both hA1 and hA2A AR subtypes. Evaluation of the side products obtained in the herein reported synthetic pathways led to the identification of some new triazolo[1,5-a]quinoxalines as hA3AR antagonists (compounds 24-27). These derivatives, though lacking the classical structural requirements for the anchoring at the hA3 receptor site, show high hA3 affinity and in some case selectivity versus hA1 and hA2A subtypes. Molecular docking of the herein reported tricyclic and simplified derivatives was carried out to depict their hypothetical binding mode to our model of hA3 receptor.

Different efficacy of adenosine and NECA derivatives at the human A3 adenosine receptor: insight into the receptor activation switch

A3 Adenosine receptors are promising drug targets for a number of diseases and intense efforts are dedicated to develop selective agonists and antagonists of these receptors. A series of adenosine derivatives with 2-(ar)-alkynyl chains, with high affinity and different degrees of selectivity for human A3 adenosine receptors was tested for the ability to inhibit forskolin-stimulated adenylyl cyclase. All these derivatives are partial agonists at A3 adenosine receptors; their efficacy is not significantly modified by the introduction of small alkyl substituents in the N(6)-position. In contrast, the adenosine-5'-N-ethyluronamide (NECA) analogs of 2-(ar)-alkynyladenosine derivatives are full A3 agonists. Molecular modeling analyses were performed considering both the conformational behavior of the ligands and the impact of 2- and 5'-substituents on ligand-target interaction. The results suggest an explanation for the different agonistic behavior of adenosine and NECA derivatives, respectively. A sub-pocket of the binding site was analyzed as a crucial interaction domain for receptor activation.

Structure-guided design of A(3) adenosine receptor-selective nucleosides: combination of 2-arylethynyl and bicyclo[3.1.0]hexane substitutions

(N)-Methanocarba adenosine 5'-methyluronamides containing known A(3) AR (adenosine receptor)-enhancing modifications, i.e., 2-(arylethynyl)adenine and N(6)-methyl or N(6)-(3-substituted-benzyl), were nanomolar full agonists of human (h) A(3)AR and highly selective (K(i) ∼0.6 nM, N(6)-methyl 2-(halophenylethynyl) analogues 13 and 14). Combined 2-arylethynyl-N(6)-3-chlorobenzyl substitutions preserved A(3)AR affinity/selectivity in the (N)-methanocarba series (e.g., 3,4-difluoro full agonist MRS5698 31, K(i) 3 nM, human and mouse A(3)) better than that for ribosides. Polyaromatic 2-ethynyl N(6)-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (K(i) hA(3) 3.1 nM; A(1), A(2A), inactive) and fluorescent 1-pyrene adduct MRS5704 35 (K(i) hA(3) 68.3 nM), were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A(2A)AR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, the X-ray structure of related A(2A)AR is useful in guiding the design of new A(3)AR agonists.

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. 

Azide-tetrazole equilibrium of C-6 azidopurine nucleosides and their ligation reactions with alkynes

Facile syntheses of C-6 azidopurine ribonucleosides and 2'-deoxyribonucleosides have been developed. For silyl- and acetyl-protected as well as unprotected nucleosides, access to the azido derivatives could be readily attained via displacement of BtO(-) from the O(6)-(benzotriazol-1-yl)inosine nucleosides by azide anion. Use of diphenylphosphoryl azide/DBU as a simple route to the acetyl-protected azido nucleosides was also evaluated, but this proved to be inferior. Since these azido nucleosides can exist in an azide.tetrazole equilibrium, the effect of solvent polarity on this equilibrium was investigated. Subsequently, a detailed analysis of Cu-mediated azide-alkyne ("click") ligation was undertaken. Biphasic CH(2)Cl(2)/H(2)O medium proved to be best for the ligation reactions, suppressing the undesired azide reduction that was competing. Interestingly, although the tetrazolyl isomer predominates (ca. 80%) in CD(2)Cl(2) and in CD(2)Cl(2)/D(2)O, the Cu-catalyzed click reactions proceed smoothly with the silyl-protected ribo- and 2'-deoxyribonucleosides, leading to the C-6 triazolyl products in good to excellent yields. Thus, depletion of the azido form from the reaction mixture shifts the azide.tetrazole equilibrium, eventually resulting in complete consumption of azide and tetrazole. In several cases, major and minor azide-alkyne ligation products were observed, and characterization data are provided for both. In order to confirm the regiochemistry leading to the major isomer, one product was crystallized and evaluated by X-ray crystallography. The Cu-catalyzed azide-alkyne ligation is clearly efficient and significantly superior to thermal reactions, which were slow. Biological evaluation showed low cytotoxicities for the agents, suggesting their usefulness as biological probes.

Synthesis and biological evaluation of 2-alkynyl-N6-methyl-5'-N-methylcarboxamidoadenosine derivatives as potent and highly selective agonists for the human adenosine A3 receptor

A new series of 2-aralkynyl-N(6)-methyl-MECAs 10-13 were synthesized and evaluated in radioligand binding studies and in a new Eu-GTP functional assay that provides a powerful alternative to radioisotope use. The new compounds possess high affinity and selectivity for the AA(3)R with N(6)-methyl-2-phenylethynylMECA (10) showing a subnanomolar affinity and about 100000-fold selectivity vs AA(1)R and AA(2A)R. Furthermore, the new nucleosides showed to be full agonists, the N(6)-methyl-2-(2-pyridinyl)ethynylMECA (13) being the most potent in the series.

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.

Synthesis of N6 ,N6-Dialkyl Adenine Nucleosides With In Situ Formed Hexaalkylphosphorus Triamides

Reactions between secondary amines and phosphorus trichloride (PCl₃) leads to the formation of the corresponding tris(dialkylamino)phosphines or hexaalkylphosphorus triamides [HAPT: (R₂N)₃P]. Reaction of silyl-protected 2'-deoxyinosine and acetyl-protected inosine with the in situ formed HAPT and iodine (I₂) leads to the formation of N⁶,N⁶-dialkyl adenosine and 2'-deoxyadenosine. In some cases the stoichiometry of the amine is important as also the use of a tertiary amine base. The effect of amine stoichiometry on the reaction of HAPT with I₂ has been studied by ³¹P{¹H} NMR.

Synthesis and pharmacological characterization of [(125)I]MRS1898, a high-affinity, selective radioligand for the rat A(3) adenosine receptor

A known selective agonist of the A₃ adenosine receptors (AR), MRS1898 [(1′R,2′R,3′S,4′R,5′S)-4-{2-chloro-6-[(3-iodophenylmethyl)amino]purin-9-yl}-1-(methylaminocarbonyl)bicyclo[3.1.0]hexane-2,3-diol], was synthesized in radioactive form and characterized pharmacologically. This agonist ligand series, based on nucleoside analogues containing a rigid, bicyclic ring system in place of the ribose moiety, was selected for radiolabeling due to its high A₃AR affinity across species, with nanomolar binding at both rat and human A₃ARs. The radioiodination of MRS1898 on its N⁶–3-iodobenzyl substituent was accomplished in 76% radiochemical yield by iododestannylation of a 3-(trimethylstannyl)benzyl precursor. [¹²⁵I]MRS1898 bound to the rat A₃AR with a K_d value of 0.17 ± 0.04 nM and a B_max value of 0.66 ± 0.15 pmol/mg protein. The competition binding profiles for other agonists and antagonists obtained with this radioligand are similar to those previously obtained with other radioligands. The advantages of [¹²⁵I]MRS1898 compared with previously used radioligands are primarily its high selectivity and affinity for the rat A₃AR and also its facile synthesis and radiochemical stability; however, a relatively high level of nonspecific binding presents a limitation. Thus, we have introduced the first selective radioligand for the rat A₃AR.

Design of (N)-methanocarba adenosine 5'-uronamides as species-independent A3 receptor-selective agonists

2-Chloro-5'-N-methylcarboxamidoadenosine analogues containing the (N)-methanocarba (bicyclo[3.1.0]hexane) ring system as a ribose substitute display increased selectivity as agonists of the human A(3) adenosine receptor (AR). However, the selectivity in mouse was greatly reduced due to an increased tolerance of this ring system at the mouse A(1)AR. Therefore, we varied substituents at the N(6) and C2 positions in search of compounds that have improved A(3)AR selectivity and are species independent. An N(6)-methyl analogue was balanced in affinity at mouse A(1)/A(3)ARs, with high selectivity in comparison to the A(2A)AR. Substitution of the 2-chloro atom with larger and more hydrophobic substituents, such as iodo and alkynyl groups, tended to increase the A(3)AR selectivity (up to 430-fold) in mouse and preserve it in human. Extended and chemically functionalized alkynyl chains attached at the C2 position of the purine moiety preserved A(3)AR selectivity more effectively than similar chains attached at the 3-position of the N(6)-benzyl group.

New 2,6,9-trisubstituted adenines as adenosine receptor antagonists: a preliminary SAR profile

A new series of 2,6,9-trisubstituted adenines (5-14) have been prepared and evaluated in radioligand binding studies for their affinity at the human A(1), A(2A) and A(3) adenosine receptors and in adenylyl cyclase experiments for their potency at the human A(2B) subtype. From this preliminary study the conclusion can be drawn that introduction of bulky chains at the N (6) position of 9-propyladenine significantly increased binding affinity at the human A(1) and A(3) adenosine receptors, while the presence of a chlorine atom at the 2 position resulted in a not univocal effect, depending on the receptor subtype and/or on the substituent present in the N (6) position. However, in all cases, the presence in the 2 position of a chlorine atom favoured the interaction with the A(2A) subtype. These results demonstrated that, although the synthesized compounds were found to be quite inactive at the human A(2B) subtype, adenine is a useful template for further development of simplified adenosine receptor antagonists with distinct receptor selectivity profiles.

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.