The A3 adenosine receptor: history and perspectives

Structure-Affinity Relationships and Structure-Kinetics Relationships of Pyrido[2,1-f]purine-2,4-dione Derivatives as Human Adenosine A3 Receptor Antagonists

We expanded on a series of pyrido[2,1-f]purine-2,4-dione derivatives as human adenosine A₃ receptor (hA₃R) antagonists to determine their kinetic profiles and affinities. Many compounds showed high affinities and a diverse range of kinetic profiles. We found hA₃R antagonists with very short residence time (RT) at the receptor (2.2 min for 5) and much longer RTs (e.g., 376 min for 27 or 391 min for 31). Two representative antagonists (5 and 27) were tested in [³⁵S]GTPγS binding assays, and their RTs appeared correlated to their (in)surmountable antagonism. From a k_on–k_off–K_D kinetic map, we divided the antagonists into three subgroups, providing a possible direction for the further development of hA₃R antagonists. Additionally, we performed a computational modeling study that sheds light on the crucial receptor interactions, dictating the compounds’ binding kinetics. Knowledge of target binding kinetics appears useful for developing and triaging new hA₃R antagonists in the early phase of drug discovery.

Determination and validation of LJ-2698, a potent human A3 adenosine receptor antagonist, in rat plasma by liquid chromatography-tandem mass spectrometry and its application in pharmacokinetic study

LJ-2698, a highly potent human A₃ adenosine receptor antagonist with nucleoside structure, was designed to have a minimal species dependence. For further pre-clinical studies, analytical method for the detection of LJ-2698 in rat plasma was developed by liquid chromatography-tandem mass. Plasma samples were processed by protein precipitation method with acetonitrile, using losartan as the internal standard (IS). Chromatographic separation was carried out using a Kinetex C18 column (100 × 4.6 mm; 100 Å; 2.6 μ) with acetonitrile/water with 0.2% (v/v) formic acid (65:35, v/v) in the isocratic mode at a flow rate of 0.4 mL/min. Mass spectrometric detection in multiple reaction monitoring mode was performed with positive electrospray ionization. The mass transitions of LJ-2698 and IS were m/z 412.3 → 294.1 and m/z 423.1 → 207.2, respectively. The calibration curves were linear in the range 5.00-5000 ng/mL (r ² ≥ 0.998). The lower limit of quantification was established as 5.00 ng/mL. Within- and between-run precisions were <7.01%, as relative standard deviation; and accuracies were in the range 3.37-3.64%, as relative error. The validated method was successfully applied to its pharmacokinetic evaluation after intravenous and oral administration in rats, and the dose-dependent pharmacokinetic behavior of LJ-2698 was elucidated for the first time.

Interaction between saliva's adenosine and tick parasitism: effects on feeding and reproduction

Background

It has recently been demonstrated that saliva from Rhipicephalus sanguineus ticks contains adenosine (ADO) and prostaglandin E2 (PGE2), two non-protein molecules that have significant immunomodulatory properties. These molecules can inhibit cytokine production by dendritic cells (DCs), while also reducing the expression of CD40 in these cells. However, more studies are needed for a better understanding of their participation in the feeding of ticks in vivo. This work, therefore, evaluated the importance of ADO during tick infestations. Mice were infested with adult ticks (3 couples/mouse), and their skin was collected at the tick-infested site (3rd and 7th day), and mRNA for receptors of ADO was quantified by real-time PCR.

Results

Tick infestation increased by four and two times the expression of the A2b and A3v1 receptors on day 3, respectively, while expression of other ADO receptors was unaltered. In addition, we treated mice (n = 10/group) daily with 8-(p-Sulfophenyl)theophylline, 8-pSPT, 20 mg/kg, i.p.), a non-selective antagonist of ADO receptors, and evaluated the performance of ticks during infestations. Female ticks fed on 8-pSPT-treated mice presented a reduction in their engorgement, weight and hatching rates of egg masses, and survival times of larvae compared to the same parameters presented by ticks in the control group. To investigate if these 8-pSPT-treated mice presented altered immune responses, we performed three tick infestations and collected their lymph node cells to determine the percentages and activation state of DCs and cytokine production by lymphocytes by flow cytometry (Cytometric Bead Array technique, CBA). Our data showed that 8-pSPT-treated mice presented an increase in the percentage of DCs as well as of their stimulatory and co-stimulatory molecules (CD40, CD80 and MHCII). Regarding production of T cell cytokines, we observed a significant increase in the levels of IL-2 and a significant decrease in IL-10, IL-17, TNF-α and IFN-γ cytokines.

Conclusions

These results suggest that ADO produced by ticks helps them feed and reproduce and that this effect may be due to modulation of host DCs and T cells.

Pathological overproduction: the bad side of adenosine

Adenosine is an endogenous ubiquitous purine nucleoside, which is increased by hypoxia, ischaemia and tissue damage and mediates a number of physiopathological effects by interacting with four GPCRs, identified as A1 , A2A , A2B and A3 . Physiological and acutely increased adenosine is mostly associated with beneficial effects that include vasodilatation and a decrease in inflammation. In contrast, chronic overproduction of adenosine occurs in important pathological states, where long-lasting increases in the nucleoside levels are responsible for the bad side of adenosine associated with chronic inflammation, fibrosis and organ damage. In this review, we describe and critically discuss the pathological overproduction of adenosine and analyse when, where and how adenosine exerts its detrimental effects throughout the body.

Exploring the Role of N6-Substituents in Potent Dual Acting 5'-C-Ethyltetrazolyladenosine Derivatives: Synthesis, Binding, Functional Assays, and Antinociceptive Effects in Mice ∇

Structural determinants of affinity of N⁶-substituted-5'-C-(ethyltetrazol-2-yl)adenosine and 2-chloroadenosine derivatives at adenosine receptor (AR) subtypes were studied with binding and molecular modeling. Small N⁶-cycloalkyl and 3-halobenzyl groups furnished potent dual acting A₁AR agonists and A₃AR antagonists. 4 was the most potent dual acting human (h) A₁AR agonist (K_i = 0.45 nM) and A₃AR antagonist (K_i = 0.31 nM) and highly selective versus A_2A; 11 and 26 were most potent at both h and rat (r) A₃AR. All N⁶-substituted-5'-C-(ethyltetrazol-2-yl)adenosine derivatives proved to be antagonists at hA₃AR but agonists at the rA₃AR. Analgesia of 11, 22, and 26 was evaluated in the mouse formalin test (A₃AR antagonist blocked and A₃AR agonist strongly potentiated). N⁶-Methyl-5'-C-(ethyltetrazol-2-yl)adenosine (22) was most potent, inhibiting both phases, as observed combining A₁AR and A₃AR agonists. This study demonstrated for the first time the advantages of a single molecule activating two AR pathways both leading to benefit in this acute pain model.

A3 Adenosine Receptor Agonist Inhibited Survival of Breast Cancer Stem Cells via GLI-1 and ERK1/2 Pathway

Numerous studies have demonstrated the role of A3 adenosine receptor (A3AR) and signaling pathways in the multiple aspects of the tumor. However, there is a little study about the function of A3AR in the biological processes of cancer stem cells (CSCs). CSCs have a critical role in the maintenance and survival of breast cancer. The aim of current study was to investigate the effect of A3AR agonist on breast cancer stem cells (BCSCs). XTT assay showed antiproliferative effect of A3AR agonist (Cl-IB-MECA) on BCSCs. Our results also demonstrated that A3AR agonist reduces mammosphere formation in a dose-dependent manner. Flow cytometry analysis showed that A3AR agonist induces G1 cell cycle arrest and apoptosis in BCSCs. Western blot assay showed that A3AR agonist inhibits the expression of cell cycle and apoptotic regulatory proteins as well as the expression of ERK1/2 and GLI-1 proteins. Finally, these findings propose that A3AR agonist induces cell cycle arrest and apoptosis in BCSCs by inhibition of ERK1/2 and GLI-1 cascade. J. Cell. Biochem. 118: 2909-2920, 2017. © 2017 Wiley Periodicals, Inc.

Safety issues of compounds acting on adenosinergic signalling

OBJECTIVES

Much research has been performed on the field of identifying the roles of adenosine and adenosinergic signalling, but a relatively low number of marketing authorizations have been granted for adenosine receptor (AdR) ligands. In part, this could be related to their safety issues; therefore, our aim was to examine the toxicological and adverse effects data of different compounds acting on adenosinergic signalling, including different AdR ligands and compounds resembling the structure of adenosine. We also wanted to present recent pharmaceutical developments of experimental compounds that showed promising results in clinical trial setting.

KEY FINDINGS

Safety issues of compounds modulating adenosinergic signalling were investigated, and different mechanisms were presented. Structurally different classes of compounds act on AdRs, the most important being adenosine, adenosine derivatives and other non-nucleoside compounds. Many of them are either not selective enough or are targeting other targets of adenosinergic signalling such as metabolizing enzymes that regulate adenosine levels. Many other targets are also involved that are not part of adenosinergic signalling system such as GABA receptors, different channels, enzymes and others. Some synthetic AdR ligands even showed to be genotoxic.

SUMMARY

Current review presents safety data of adenosine, adenosine derivatives and other non-nucleoside compounds that modulate adenosinergic signalling. We have presented different mechanisms that participate to an adverse effect or toxic outcome. A separate section also deals with possible organ-specific toxic effects on different in-vitro and in-vivo models.

Role and Function of A2A and A₃ Adenosine Receptors in Patients with Ankylosing Spondylitis, Psoriatic Arthritis and Rheumatoid Arthritis

Rheumatoid arthritis (RA), ankylosing spondylitis (AS) and psoriatic arthritis (PsA) are chronic inflammatory rheumatic diseases that affect joints, causing debilitating pain and disability. Adenosine receptors (ARs) play a key role in the mechanism of inflammation, and the activation of A_2A and A₃AR subtypes is often associated with a reduction of the inflammatory status. The aim of this study was to investigate the involvement of ARs in patients suffering from early-RA (ERA), RA, AS and PsA. Messenger RNA (mRNA) analysis and saturation binding experiments indicated an upregulation of A_2A and A₃ARs in lymphocytes obtained from patients when compared with healthy subjects. A_2A and A₃AR agonists inhibited nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) activation and reduced inflammatory cytokines release, such as tumor necrosis factor-α (TNF-α), interleukin (IL)-1β and IL-6. Moreover, A_2A and A₃AR activation mediated a reduction of metalloproteinases (MMP)-1 and MMP-3. The effect of the agonists was abrogated by selective antagonists demonstrating the direct involvement of these receptor subtypes. Taken together, these data confirmed the involvement of ARs in chronic autoimmune rheumatic diseases highlighting the possibility to exploit A_2A and A₃ARs as therapeutic targets, with the aim to limit the inflammatory responses usually associated with RA, AS and PsA.

Hypothermia in mouse is caused by adenosine A1 and A3 receptor agonists and AMP via three distinct mechanisms

Small mammals have the ability to enter torpor, a hypothermic, hypometabolic state, allowing impressive energy conservation. Administration of adenosine or adenosine 5'-monophosphate (AMP) can trigger a hypothermic, torpor-like state. We investigated the mechanisms for hypothermia using telemetric monitoring of body temperature in wild type and receptor knock out (Adora1-/-, Adora3-/-) mice. Confirming prior data, stimulation of the A3 adenosine receptor (AR) induced hypothermia via peripheral mast cell degranulation, histamine release, and activation of central histamine H1 receptors. In contrast, A1AR agonists and AMP both acted centrally to cause hypothermia. Commonly used, selective A1AR agonists, including N6-cyclopentyladenosine (CPA), N6-cyclohexyladenosine (CHA), and MRS5474, caused hypothermia via both A1AR and A3AR when given intraperitoneally. Intracerebroventricular dosing, low peripheral doses of Cl-ENBA [(±)-5'-chloro-5'-deoxy-N6-endo-norbornyladenosine], or using Adora3-/- mice allowed selective stimulation of A1AR. AMP-stimulated hypothermia can occur independently of A1AR, A3AR, and mast cells. A1AR and A3AR agonists and AMP cause regulated hypothermia that was characterized by a drop in total energy expenditure, physical inactivity, and preference for cooler environmental temperatures, indicating a reduced body temperature set point. Neither A1AR nor A3AR was required for fasting-induced torpor. A1AR and A3AR agonists and AMP trigger regulated hypothermia via three distinct mechanisms.

CD73 Controls Extracellular Adenosine Generation in the Trigeminal Nociceptive Nerves

Purinergic signaling is involved in pain generation and modulation in the nociceptive sensory nervous system. Adenosine triphosphate (ATP) induces pain via activation of ionotropic P2X receptors while adenosine mediates analgesia via activation of metabotropic P1 receptors. These purinergic signaling are determined by ecto-nucleotidases that control ATP degradation and adenosine generation. Using enzymatic histochemistry, we detected ecto-AMPase activity in dental pulp, trigeminal ganglia (TG) neurons, and their nerve fibers. Using immunofluorescence staining, we confirmed the expression of ecto-5'-nucleotidase (CD73) in trigeminal nociceptive neurons and their axonal fibers, including the nociceptive nerve fibers projecting into the brainstem. In addition, we detected the existence of CD73 and ecto-AMPase activity in the nociceptive lamina of the trigeminal subnucleus caudalis (TSNC) in the brainstem. Furthermore, we demonstrated that incubation with specific anti-CD73 serum significantly reduced the ecto-AMPase activity in the nociceptive lamina in the brainstem. Our results indicate that CD73 might participate in nociceptive modulation by affecting extracellular adenosine generation in the trigeminal nociceptive pathway. Disruption of TG neuronal ecto-nucleotidase expression and axonal terminal localization under certain circumstances such as chronic inflammation, oxidant stress, local constriction, and injury in trigeminal nerves may contribute to the pathogenesis of orofacial neuropathic pain.

Adenosine Receptors as a Biological Pathway for the Anti-Inflammatory and Beneficial Effects of Low Frequency Low Energy Pulsed Electromagnetic Fields

Several studies explored the biological effects of low frequency low energy pulsed electromagnetic fields (PEMFs) on human body reporting different functional changes. Much research activity has focused on the mechanisms of interaction between PEMFs and membrane receptors such as the involvement of adenosine receptors (ARs). In particular, PEMF exposure mediates a significant upregulation of A_2A and A₃ARs expressed in various cells or tissues involving a reduction in most of the proinflammatory cytokines. Of particular interest is the observation that PEMFs, acting as modulators of adenosine, are able to increase the functionality of the endogenous agonist. By reviewing the scientific literature on joint cells, a double role for PEMFs could be hypothesized in vitro by stimulating cell proliferation, colonization of the scaffold, and production of tissue matrix. Another effect could be obtained in vivo after surgical implantation of the construct by favoring the anabolic activities of the implanted cells and surrounding tissues and protecting the construct from the catabolic effects of the inflammatory status. Moreover, a protective involvement of PEMFs on hypoxia damage in neuron-like cells and an anti-inflammatory effect in microglial cells have suggested the hypothesis of a positive impact of this noninvasive biophysical stimulus.

Deregulation of Adenosine Receptors in Psoriatic Epidermis: An Option for Therapeutic Treatment

Purinergic signaling is involved in psoriasis, a chronic skin disease characterized by increased epidermis cell growth. In particular, Andrés et al. focus on the keratinocyte biology modulated by adenosine receptors providing evidence that the A2B subtype plays a prominent role in the reduction of keratinocyte proliferation whereas A2A and A2B agonists have antiinflammatory effects independent of adenosine receptors. The authors report that psoriatic epidermis presents a deregulated adenosine receptor expression profile with reduced A2B and increased A2A.

Hippocampal GABAergic transmission: a new target for adenosine control of excitability

Physiological network functioning in the hippocampus is dependent on a balance between glutamatergic cell excitability and the activity of diverse local circuit neurons that release the inhibitory neurotransmitter γ-aminobutyric acid (GABA). Tuners of neuronal communication such as adenosine, an endogenous modulator of synapses, control hippocampal network operations by regulating excitability. Evidence has been recently accumulating on the influence of adenosine on different aspects of GABAergic transmission to shape hippocampal function. This review addresses how adenosine, through its high-affinity A₁ (A₁ R) and A_2A receptors (A_2A R), interferes with different GABA-mediated forms of inhibition in the hippocampus to regulate neuronal excitability. Adenosine-mediated modulation of phasic/tonic inhibitory transmission, of GABA transport mechanisms and its interference with other modulatory systems are discussed together with the putative implications for neuronal function in physiological and pathological conditions. This article is part of a mini review series: 'Synaptic Function and Dysfunction in Brain Diseases'.

A2B adenosine receptors stimulate IL-6 production in primary murine microglia through p38 MAPK kinase pathway

The hallmark of neuroinflammation is the activation of microglia, the immunocompetent cells of the CNS, releasing a number of proinflammatory mediators implicated in the pathogenesis of neuronal diseases. Adenosine is an ubiquitous autacoid regulating several microglia functions through four receptor subtypes named A₁, A_2A, A_2B and A₃ (ARs), that represent good targets to suppress inflammation occurring in CNS. Here we investigated the potential role of ARs in the modulation of IL-6 secretion and cell proliferation in primary microglial cells. The A_2BAR agonist 2-[[6-Amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]-2-pyridinyl]thio]-acetamide (BAY60-6583) stimulated IL-6 increase under normoxia and hypoxia, in a dose- and time-dependent way. In cells incubated with the blockers of phospholipase C (PLC), protein kinase C epsilon (PKC-ε) and PKC delta (PKC-δ) the IL-6 increase due to A_2BAR activation was strongly reduced, whilst it was not affected by the inhibitor of adenylyl cyclase (AC). Investigation of cellular signalling involved in the A_2BAR effect revealed that only the inhibitor of p38 mitogen activated protein kinase (MAPK) was able to block the agonist's effect on IL-6 secretion, whilst inhibitors of pERK1/2, JNK1/2 MAPKs and Akt were not. Stimulation of p38 by BAY60-6583 was A_2BAR-dependent, through a pathway affecting PLC, PKC-ε and PKC-δ but not AC, in both normoxia and hypoxia. Finally, BAY60-6583 increased microglial cell proliferation involving A_2BAR, PLC, PKC-ε, PKC-δ and p38 signalling. In conclusion, A_2BARs activation increased IL-6 secretion and cell proliferation in murine primary microglial cells, through PLC, PKC-ε, PKC-δ and p38 pathways, thus suggesting their involvement in microglial activation and neuroinflammation.

Design, Synthesis, and Pharmacological Characterization of 2-(2-Furanyl)thiazolo[5,4-d]pyrimidine-5,7-diamine Derivatives: New Highly Potent A2A Adenosine Receptor Inverse Agonists with Antinociceptive Activity

In this study, we describe the design and synthesis of new N⁵-substituted-2-(2-furanyl) thiazolo[5,4-d]pyrimidine-5,7-diamines (2-18) and their pharmacological characterization as A_2A adenosine receptor (AR) antagonists by using in vitro and in vivo assays. In competition binding experiments two derivatives (13 and 14) emerged as outstanding ligands showing two different affinity values (KH and KL) for the hA_2A receptor with the high affinity KH value in the femtomolar range. The in vitro functional activity assays, performed by using cyclic AMP experiments, assessed that they behave as potent inverse agonists at the hA_2A AR. Compounds 13 and 14 were evaluated for their antinociceptive activity in acute experimental models of pain showing an effect equal to or greater than that of morphine. Overall, these novel inverse agonists might represent potential drug candidates for an alternative approach to the management of pain.

Genome-wide RNAi screening identifies TMIGD3 isoform1 as a suppressor of NF-κB and osteosarcoma progression

The ability of cancer cells to survive and grow in anchorage- and serum-independent conditions is well correlated with their aggressiveness. Here, using a human whole-genome shRNA library, we identify TMIGD3 isoform1 (i1) as a factor that suppresses this ability in osteosarcoma (OS) cells, mainly by inhibiting NF-κB activity. Knockdown of TMIGD3 increases proliferation, tumour formation and metastasis of OS cells. Overexpression of TMIGD3 isoform1 (i1), but not isoform3 (i3) which shares a common C-terminal region, suppresses these malignant properties. Adenosine A3 receptor (A3AR) having an identical N-terminal region shows similar biological profiles to TMIGD3 i1. Protein expression of TMIGD3 and A3AR is lower in human OS tissues than normal tissues. Mechanistically, TMIGD3 i1 and A3AR commonly inhibit the PKA−Akt−NF-κB axis. However, TMIGD3 i1 only partially rescues phenotypes induced by A3AR knockdown, suggesting the presence of distinct pathways. Our findings reveal an unappreciated role for TMIGD3 i1 as a suppressor of NF-κB activity and OS progression.

The ability of cancer cells to survive in anchorage-independent conditions correlates with cancer aggressiveness. Here, by screening a human whole-genome shRNA library for the ability of osteosarcoma cells to form spheres in vitro, the authors identify a role for TMIGD3 isoform 1 in suppressing the metastatic potential of osteosarcoma.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Imidazo[1,2-a]pyrazin-8-amine core for the design of new adenosine receptor antagonists: Structural exploration to target the A3 and A2A subtypes

The imidazo[1,2-a]pyrazine ring system has been chosen as a new decorable core skeleton for the design of novel adenosine receptor (AR) antagonists targeting either the human (h) A₃ or the hA_2A receptor subtype. The N⁸-(hetero)arylcarboxyamido substituted compounds 4-14 and 21-30, bearing a 6-phenyl moiety or not, respectively, show good hA₃ receptor affinity and selectivity versus the other ARs. In contrast, the 8-amino-6-(hetero)aryl substituted derivatives designed for targeting the hA_2A receptor subtype (compounds 31-38) and also the 6-phenyl analogues 18-20 do not bind the hA_2A AR, or show hA₁ or balanced hA₁/hA_2A AR affinity in the micromolar range. Molecular docking of the new hA₃ antagonists was carried out to depict their hypothetical binding mode to our refined model of the hA₃ receptor. Some derivatives were evaluated for their fluorescent potentiality and showed some fluorescent emission properties. One of the most active hA₃ antagonists herein reported, i.e. the 2,6-diphenyl-8-(3-pyridoylamino)imidazo[1,2-a]pyrazine 29, tested in a rat model of cerebral ischemia, delayed the occurrence of anoxic depolarization caused by oxygen and glucose deprivation in the hippocampus and allowed disrupted synaptic activity to recover.

A3 adenosine receptor agonist, CF102, protects against hepatic ischemia/reperfusion injury following partial hepatectomy

Ischemia/reperfusion (IR) injury during clinical hepatic procedures is characterized by inflammatory conditions and the apoptosis of hepatocytes. Nuclear factor‑κB (NF‑κB), nitric oxide and the expression levels of inflammatory cytokines, tumor necrosis factor‑α and interleukin‑1 were observed to increase following IR and mediate the inflammatory response in the liver. CF102 is a highly selective A3 adenosine receptor (A3AR) agonist, and has been identified to induce an anti‑inflammatory and protective effect on the liver via the downregulation of the NF‑κB signaling pathway. The present study aimed to determine the effect of CF102 on protecting the liver against IR injury. The potential protective effect of CF102 (100 µg/kg) was assessed using an IR injury model on 70% of the liver of Wistar rats, which was induced by clamping the hepatic vasculature for 30 min. The regenerative effect of CF102 was assessed by the partial hepatectomy of 70% of the liver during 10 min of IR. CF102 reduced the levels of liver enzymes following IR injury. A higher regeneration rate in the CF102 treatment group was observed compared with the control group, suggesting that CF102 had a positive effect on the proliferation of hepatocytes following hepatectomy. CF102 had a protective effect on the liver of Wistar rats subsequent to IR injury during hepatectomy. This may be due to an anti‑inflammatory and anti‑apoptotic effect mediated by the A3AR.

Renoprotective Effects of a Highly Selective A3 Adenosine Receptor Antagonist in a Mouse Model of Adriamycin-induced Nephropathy

The concentration of adenosine in the normal kidney increases markedly during renal hypoxia, ischemia, and inflammation. A recent study reported that an A3 adenosine receptor (A3AR) antagonist attenuated the progression of renal fibrosis. The adriamycin (ADX)-induced nephropathy model induces podocyte injury, which results in severe proteinuria and progressive glomerulosclerosis. In this study, we investigated the preventive effect of a highly selective A3AR antagonist (LJ1888) in ADX-induced nephropathy. Three groups of six-week-old Balb/c mice were treated with ADX (11 mg/kg) for four weeks and LJ1888 (10 mg/kg) for two weeks as following: 1) control; 2) ADX; and 3) ADX + LJ1888. ADX treatment decreased body weight without a change in water and food intake, but this was ameliorated by LJ1888 treatment. Interestingly, LJ1888 lowered plasma creatinine level, proteinuria, and albuminuria, which had increased during ADX treatment. Furthermore, LJ1888 inhibited urinary nephrin excretion as a podocyte injury marker, and urine 8-isoprostane and kidney lipid peroxide concentration, which are markers of oxidative stress, increased after injection of ADX. ADX also induced the activation of proinflammatory and profibrotic molecules such as TGF-β1, MCP-1, PAI-1, type IV collagen, NF-κB, NOX4, TLR4, TNFα, IL-1β, and IFN-γ, but they were remarkably suppressed after LJ1888 treatment. In conclusion, our results suggest that LJ1888 has a renoprotective effect in ADX-induced nephropathy, which might be associated with podocyte injury through oxidative stress. Therefore, LJ1888, a selective A3AR antagonist, could be considered as a potential therapeutic agent in renal glomerular diseases which include podocyte injury and proteinuria.

Adenosine production: a common path for mesenchymal stem-cell and regulatory T-cell-mediated immunosuppression

Adenosine is an important molecule that exerts control on the immune system, by signaling through receptors lying on the surface of immune cells. This nucleotide is produced, in part, by the action of the ectoenzymes CD39 and CD73. Interestingly, these proteins are expressed on the cell surface of regulatory T-cells (Tregs) and mesenchymal stromal cells (MSCs)-two cell populations that have emerged as potential therapeutic tools in the field of cell therapy. In fact, the production of adenosine constitutes a mechanism used by both cell types to control the immune response. Recently, great scientific progress was obtained regarding the role of adenosine in the inflammatory environment. In this context, the present review focuses on the advances related to the impact of adenosine production over the immune modulatory activity of Tregs and MSCs, and how this nucleotide controls the biological functions of these cells. Finally, we mention the main challenges and hurdles to bring such molecule to clinical settings.

Genetic Abrogation of Adenosine A3 Receptor Prevents Uninephrectomy and High Salt-Induced Hypertension

Background

Early‐life reduction in nephron number (uninephrectomy [UNX]) and chronic high salt (HS) intake increase the risk of hypertension and chronic kidney disease. Adenosine signaling via its different receptors has been implicated in modulating renal, cardiovascular, and metabolic functions as well as inflammatory processes; however, the specific role of the A₃ receptor in cardiovascular diseases is not clear. In this study, gene‐modified mice were used to investigate the hypothesis that lack of A₃ signaling prevents the development of hypertension and attenuates renal and cardiovascular injuries following UNX in combination with HS (UNX‐HS) in mice.

Methods and Results

Wild‐type (A₃^+/+) mice subjected to UNX‐HS developed hypertension compared with controls (mean arterial pressure 106±3 versus 82±3 mm Hg; P<0.05) and displayed an impaired metabolic phenotype (eg, increased adiposity, reduced glucose tolerance, hyperinsulinemia). These changes were associated with both cardiac hypertrophy and fibrosis together with renal injuries and proteinuria. All of these pathological hallmarks were significantly attenuated in the A₃^−/− mice. Mechanistically, absence of A₃ receptors protected from UNX‐HS–associated increase in renal NADPH oxidase activity and Nox2 expression. In addition, circulating cytokines including interleukins 1β, 6, 12, and 10 were increased in A₃^+/+ following UNX‐HS, but these cytokines were already elevated in naïve A₃^−/− mice and did not change following UNX‐HS.

Conclusions

Reduction in nephron number combined with chronic HS intake is associated with oxidative stress, chronic inflammation, and development of hypertension in mice. Absence of adenosine A₃ receptor signaling was strongly protective in this novel mouse model of renal and cardiovascular disease.

Extracellular purines, purinergic receptors and tumor growth

Virtually, all tumor cells as well as all immune cells express plasma membrane receptors for extracellular nucleosides (adenosine) and nucleotides (ATP, ADP, UTP, UDP and sugar UDP). The tumor microenvironment is characterized by an unusually high concentration of ATP and adenosine. Adenosine is a major determinant of the immunosuppressive tumor milieu. Sequential hydrolysis of extracellular ATP catalyzed by CD39 and CD73 is the main pathway for the generation of adenosine in the tumor interstitium. Extracellular ATP and adenosine mold both host and tumor responses. Depending on the specific receptor activated, extracellular purines mediate immunosuppression or immunostimulation on the host side, and growth stimulation or cytotoxicity on the tumor side. Recent progress in this field is providing the key to decode this complex scenario and to lay the basis to harness the potential benefits for therapy. Preclinical data show that targeting the adenosine-generating pathway (that is, CD73) or adenosinergic receptors (that is, A2A) relieves immunosuppresion and potently inhibits tumor growth. On the other hand, growth of experimental tumors is strongly inhibited by targeting the P2X7 ATP-selective receptor of cancer and immune cells. This review summarizes the recent data on the role played by extracellular purines (purinergic signaling) in host-tumor interaction and highlights novel therapeutic options stemming from recent advances in this field.

The synthesis of a series of adenosine A3 receptor agonists

A series of 1'-(6-aminopurin-9-yl)-1'-deoxy-N-methyl-β-d-ribofuranuronamides that were characterised by 2-dialkylamino-7-methyloxazolo[4,5-b]pyridin-5-ylmethyl substituents on N6 of interest for screening as selective adenosine A3 receptor agonists, have been synthesised. This work involved the synthesis of 2-dialkylamino-5-aminomethyl-7-methyloxazolo[4,5-b]pyridines and analogues that were coupled with the known 1'-(6-chloropurin-9-yl)-1'-deoxy-N-methyl-β-d-ribofuranuronamide. The oxazolo[4,5-b]pyridines were synthesized by regioselective functionalisation of 2,4-dimethylpyridine N-oxides. The regioselectivities of these reactions were found to depend upon the nature of the heterocycle with 2-dimethylamino-5,7-dimethyloxazolo[4,5-b]pyridine-N-oxide undergoing regioselective functionalisation at the 7-methyl group on reaction with trifluoroacetic anhydride in contrast to the reaction of 4,6-dimethyl-3-hydroxypyridine-N-oxide with acetic anhydride that resulted in functionalisation of the 6-methyl group. To optimise selectivity for the A3 receptor, 5-aminomethyl-7-bromo-2-dimethylamino-4-[(3-methylisoxazol-5-yl)methoxy]benzo[d]oxazole was synthesised and coupled with the 1'-(6-chloropurin-9-yl)-1'-deoxy-N-methyl-β-d-ribofuranuronamide. The products were active as selective adenosine A3 agonists.

Rigid Adenine Nucleoside Derivatives as Novel Modulators of the Human Sodium Symporters for Dopamine and Norepinephrine

Thirty-two congeneric rigid adenine nucleoside derivatives containing a North (N)-methanocarba ribose substitution and a 2-arylethynyl group either enhanced (up to 760% of control) or inhibited [(125)I] methyl (1R,2S,3S)-3-(4-iodophenyl)-8-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate (RTI-55) binding at the human dopamine (DA) transporter (DAT) and inhibited DA uptake. Several nucleosides also enhanced [(3)H]mazindol [(±)-5-(4-chlorophenyl)-3,5-dihydro-2H-imidazo[2,1-a]isoindol-5-ol] binding to the DAT. The combination of binding enhancement and functional inhibition suggests possible allosteric interaction with the tropanes. The structure-activity relationship of this novel class of DAT ligands was explored: small N(6)-substition (methyl or ethyl) was favored, while the N1 of the adenine ring was essential. Effective terminal aryl groups include thien-2-yl (compounds 9 and 16), with EC50 values of 35.1 and 9.1 nM, respectively, in [(125)I]RTI-55 binding enhancement, and 3,4-difluorophenyl as in the most potent DA uptake inhibitor (compound 6) with an IC50 value of 92 nM (3-fold more potent than cocaine), but not nitrogen heterocycles. Several compounds inhibited or enhanced binding at the norepinephrine transporter (NET) and serotonin transporter (SERT) and inhibited function in the micromolar range; truncation at the 4'-position in compound 23 allowed for weak inhibition of the SERT. We have not yet eliminated adenosine receptor affinity from this class of DAT modulators, but we identified modifications that remove DAT inhibition as an off-target effect of potent adenosine receptor agonists. Thus, we have identified a new class of allosteric DAT ligands, rigidified adenosine derivatives, and explored their initial structural requirements. They display a very atypical pharmacological profile, i.e., either enhancement by increasing affinity or inhibition of radioligand binding at the DAT, and in some cases the NET and SERT, and inhibition of neurotransmitter uptake.

Identification of A3 adenosine receptor agonists as novel non-narcotic analgesics

Chronic pain negatively impacts the quality of life in a variety of patient populations. The current therapeutic repertoire is inadequate in managing patient pain and warrants the development of new therapeutics. Adenosine and its four cognate receptors (A1 , A2A , A2B and A3 ) have important roles in physiological and pathophysiological states, including chronic pain. Preclinical and clinical studies have revealed that while adenosine and agonists of the A1 and A2A receptors have antinociceptive properties, their therapeutic utility is limited by adverse cardiovascular side effects. In contrast, our understanding of the A3 receptor is only in its infancy, but exciting preclinical observations of A3 receptor antinociception, which have been bolstered by clinical trials of A3 receptor agonists in other disease states, suggest pain relief without cardiovascular side effects and with sufficient tolerability. Our goal herein is to briefly discuss adenosine and its receptors in the context of pathological pain and to consider the current data regarding A3 receptor-mediated antinociception. We will highlight recent findings regarding the impact of the A3 receptor on pain pathways and examine the current state of selective A3 receptor agonists used for these studies. The adenosine-to-A3 receptor pathway represents an important endogenous system that can be targeted to provide safe, effective pain relief from chronic pain.

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

The importance of adenosine for human health cannot be overstated. Indeed, this ubiquitous nucleoside is an integral component of ATP, and regulates the function of every tissue and organ in the body. Acting via receptor-dependent and -independent mechanisms [the former mediated via four G-protein-coupled receptors (GPCRs), A1, A2A, A2B, and A3,], it has a significant role in protecting against cell damage in areas of increased tissue metabolism, and combating organ dysfunction in numerous pathological states. Accordingly, raised levels of adenosine have been demonstrated in epilepsy, ischaemia, pain, inflammation, and cancer, in which its behaviour can be likened to that of a guardian angel, even though there are instances in which overproduction of adenosine is pathological. In this review, we condense the current body of knowledge on the issue, highlighting when, where, and how adenosine exerts its protective effects in both the brain and the periphery.

Purine (N)-Methanocarba Nucleoside Derivatives Lacking an Exocyclic Amine as Selective A3 Adenosine Receptor Agonists

Purine (N)-methanocarba-5′-N-alkyluronamidoriboside A₃ adenosine receptor (A₃AR) agonists lacking an exocyclic amine resulted from an unexpected reaction during a Sonogashira coupling and subsequent aminolysis. Because the initial C6-Me and C6-styryl derivatives had unexpectedly high A₃AR affinity, other rigid nucleoside analogues lacking an exocyclic amine were prepared. Of these, the C6-Me-(2-phenylethynyl) and C2-(5-chlorothienylethynyl) analogues were particularly potent, with human A₃AR K_i values of 6 and 42 nM, respectively. Additionally, the C2-(5-chlorothienyl)-6-H analogue was potent and selective at A₃AR (MRS7220, K_i 60 nM) and also completely reversed mouse sciatic nerve mechanoallodynia (in vivo, 3 μmol/kg, po). The lack of a C6 H-bond donor while maintaining A₃AR affinity and efficacy could be rationalized by homology modeling and docking of these hypermodified nucleosides. The modeling suggests that a suitable combination of stabilizing features can partially compensate for the lack of an exocyclic amine, an otherwise important contributor to recognition in the A₃AR binding site.

Peripheral Adenosine A3 Receptor Activation Causes Regulated Hypothermia in Mice That Is Dependent on Central Histamine H1 Receptors

Adenosine can induce hypothermia, as previously demonstrated for adenosine A1 receptor (A1AR) agonists. Here we use the potent, specific A3AR agonists MRS5698, MRS5841, and MRS5980 to show that adenosine also induces hypothermia via the A3AR. The hypothermic effect of A3AR agonists is independent of A1AR activation, as the effect was fully intact in mice lacking A1AR but abolished in mice lacking A3AR. A3AR agonist-induced hypothermia was attenuated by mast cell granule depletion, demonstrating that the A3AR hypothermia is mediated, at least in part, via mast cells. Central agonist dosing had no clear hypothermic effect, whereas peripheral dosing of a non-brain-penetrant agonist caused hypothermia, suggesting that peripheral A3AR-expressing cells drive the hypothermia. Mast cells release histamine, and blocking central histamine H1 (but not H2 or H4) receptors prevented the hypothermia. The hypothermia was preceded by hypometabolism and mice with hypothermia preferred a cooler environmental temperature, demonstrating that the hypothermic state is a coordinated physiologic response with a reduced body temperature set point. Importantly, hypothermia is not required for the analgesic effects of A3AR agonists, which occur with lower agonist doses. These results support a mechanistic model for hypothermia in which A3AR agonists act on peripheral mast cells, causing histamine release, which stimulates central histamine H1 receptors to induce hypothermia. This mechanism suggests that A3AR agonists will probably not be useful for clinical induction of hypothermia.

Progress in the development of small molecules as new human A3 adenosine receptor ligands based on the 3-thiophenylcoumarin core

3-Thiophenylcoumarins are described as adenosine receptor ligands. Synthesis, in vitro pharmacological assays and docking studies were performed.

The Influence of the 1-(3-Trifluoromethyl-Benzyl)-1H-Pyrazole-4-yl Moiety on the Adenosine Receptors Affinity Profile of Pyrazolo[4,3-e][1,2,4]Triazolo[1,5-c]Pyrimidine Derivatives

A new series of pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine (PTP) derivatives has been developed in order to explore their affinity and selectivity profile at the four adenosine receptor subtypes. In particular, the PTP scaffold was conjugated at the C2 position with the 1-(3-trifluoromethyl-benzyl)-1H-pyrazole, a group believed to confer potency and selectivity toward the human (h) A_2B adenosine receptor (AR) to the xanthine ligand 8-(1-(3-(trifluoromethyl)benzyl)-1H-pyrazol-4-yl)-1,3-dimethyl-1H-purine-2,6(3H,7H)-dione (CVT 6975). Interestingly, the synthesized compounds turned out to be inactive at the hA_2B AR but they displayed affinity at the hA₃ AR in the nanomolar range. The best compound of the series (6) shows both high affinity (hA₃ AR K_i = 11 nM) and selectivity (A₁/A₃ and A_2A/A₃ > 9090; A_2B/A₃ > 909) at the hA₃ AR. To better rationalize these results, a molecular docking study on the four AR subtypes was performed for all the synthesized compounds. In addition, CTV 6975 and two close analogues have been subjected to the same molecular docking protocol to investigate the role of the 1-(3-trifluoromethyl-benzyl)-1H-pyrazole on the binding at the four ARs.

Structural refinement of pyrazolo[4,3-d]pyrimidine derivatives to obtain highly potent and selective antagonists for the human A3 adenosine receptor

In previous research, we identified some 7-oxo- and 7-acylamino-substituted pyrazolo[4,3-d]pyrimidine derivatives as potent and selective human (h) A3 adenosine receptor (AR) antagonists. Herein we report on the structural refinement of this class of antagonists aimed at achieving improved receptor-ligand recognition. Hence, substituents with different steric bulk, flexibility and lipophilicity (Me, Ar, heteroaryl, CH2Ph) were introduced at the 5- and 2-positions of the bicyclic scaffold of both the 7-oxo and 7-amino derivatives, and acyl residues were appended on the 7-amino group of the latter. All the 2-phenylpyrazolo[4,3-d]pyrimidin-7-amines and 7-acylamines bearing a 4-methoxyphenyl- or a 2-thienyl group at the 5-position showed high hA3 affinity and selectivity. In particular, the 2-phenyl-5-(2-thienyl)-pyrazolo[4,3-d]pyrimidin-7-(4-methoxybenzoyl)amine 25 (Ki = 0.027 nM) is one of the most potent and selective hA3 antagonists reported so far. By using an in silico receptor-driven approach the obtained binding data were rationalized and the molecular bases of the observed hA3 AR affinities were critically described.

Structure-Based Design, Synthesis by Click Chemistry and in Vivo Activity of Highly Selective A3 Adenosine Receptor Agonists

2-Arylethynyl derivatives of (N)-methanocarba adenosine 5'-uronamides are selective A3AR (adenosine receptor) agonists. Here we substitute a 1,2,3-triazol-1-yl linker in place of the rigid, linear ethynyl group to eliminate its potential metabolic liability. Docking of nucleosides containing possible short linker moieties at the adenine C2 position using a hybrid molecular model of the A3AR (based on the A2AAR agonist-bound structure) correctly predicted that a triazole would maintain the A3AR selectivity, due to its ability to fit a narrow cleft in the receptor. The analogues with various N6 and C2-aryltriazolyl substitution were synthesized and characterized in binding (Ki at hA3AR 0.3 - 12 nM) and in vivo to demonstrate efficacy in controlling chronic neuropathic pain (chronic constriction injury). Among N6-methyl derivatives, a terminal pyrimidin-2-yl group in 9 (MRS7116) increased duration of action (36% pain protection at 3 h) in vivo. N6-Ethyl 5-chlorothien-2-yl analogue 15 (MRS7126) preserved in vivo efficacy (85% protection at 1 h) with short duration. Larger N6 groups, e.g. 17 (MRS7138, >90% protection at 1 and 3 h), greatly enhanced in vivo activity. Thus, we have combined structure-based methods and phenotypic screening to identify nucleoside derivatives having translational potential.

Metabolic mapping of A3 adenosine receptor agonist MRS5980

(1S,2R,3S,4R,5S)-4-(2-((5-Chlorothiophen-2-yl)ethynyl)-6-(methylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo[3.1.0]hexane-1-carboxamide (MRS5980) is an A3AR selective agonist containing multiple receptor affinity- and selectivity-enhancing modifications and a therapeutic candidate drug for many inflammatory diseases. Metabolism-related poor pharmacokinetic behavior and toxicities are a major reason for drug R&D failure. Metabolomics with UPLC-MS was employed to profile the metabolism of MRS5980 and MRS5980-induced disruption of endogenous compounds. Recombinant drug-metabolizing enzymes screening experiment were used to determine the enzymes involved in MRS5980 metabolism. Analysis of lipid metabolism-related genes was performed to investigate the reason for MRS5980-induced lipid metabolic disorders. Unsupervised principal components analysis separated the control and MRS5980 treatment groups in feces, urine, and liver samples, but not in bile and serum. The major ions mainly contributing to the separation of feces and urine were oxidized MRS5980, glutathione (GSH) conjugates and cysteine conjugate (degradation product of the GSH conjugates) of MRS5980. The major ions contributing to the group separation of liver samples were phosphatidylcholines. In vitro incubation experiments showed the involvement of CYP3A enzymes in the oxidative metabolism of MRS5980 and direct GSH reactivity of MRS5980. The electrophilic attack by MRS5980 is a minor pathway and did not alter GSH levels in liver or liver histology, and thus may be of minor clinical consequence. Gene expression analysis further showed decreased expression of PC biosynthetic genes choline kinase a and b, which further accelerated conversion of lysophosphatidylcholine to phosphatidylcholines through increasing the expression of lysophosphatidylcholine acyltransferase 3. These data will be useful to guide rational design of drugs targeting A3AR, considering efficacy, metabolic elimination, and electrophilic reactivity.

Rigidified A3 Adenosine Receptor Agonists: 1-Deazaadenine Modification Maintains High in Vivo Efficacy

Substitution of rigidified A3 adenosine receptor (AR) agonists with a 2-((5-chlorothiophen-2-yl)ethynyl) or a 2-(4-(5-chlorothiophen-2-yl)-1H-1,2,3-triazol-1-yl) group provides prolonged protection in a model of chronic neuropathic pain. These agonists contain a bicyclo[3.1.0]hexane ((N)-methanocarba) ring system in place of ribose, which adopts a receptor-preferred conformation. N (6)-Small alkyl derivatives were newly optimized for A3AR affinity and the effects of a 1-deaza-adenine modification probed. 1-Deaza-N (6)-ethyl alkyne 20 (MRS7144, K i 1.7 nM) and 1-aza N (6)-propyl alkyne 12 (MRS7154, K i 1.1 nM) were highly efficacious in vivo. Thus, the presence of N1 is not required for nanomolar binding affinity or potent, long-lasting functional activity. Docking of 1-deaza compounds to a receptor homology model confirmed a similar binding mode as previously reported 1-aza derivatives. This is the first demonstration in nonribose adenosine analogues that the 1-deaza modification can maintain high A3AR affinity, selectivity, and efficacy.

One-pot reaction to obtain N,N'-disubstituted guanidines of pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine scaffold as human A3 adenosine receptor antagonists

In this paper we describe an extension SAR study of pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine nucleus as A3AR antagonist. Our initial aim was to replace the phenylcarbamoyl moiety at the 5 position of PTP nucleus with a thiourea functionality to evaluate the contribution of new structural modification against the A3AR. The synthesized 12-25 were not characterized by the predicted side chain but by a 1,3-disubstituted guanidine and are shown to be interesting A3AR antagonists.

Engagement of the GABA to KCC2 signaling pathway contributes to the analgesic effects of A3AR agonists in neuropathic pain

More than 1.5 billion people worldwide suffer from chronic pain, yet current treatment strategies often lack efficacy or have deleterious side effects in patients. Adenosine is an inhibitory neuromodulator that was previously thought to mediate antinociception through the A1 and A2A receptor subtypes. We have since demonstrated that A3AR agonists have potent analgesic actions in preclinical rodent models of neuropathic pain and that A3AR analgesia is independent of adenosine A1 or A2A unwanted effects. Herein, we explored the contribution of the GABA inhibitory system to A3AR-mediated analgesia using well-characterized mouse and rat models of chronic constriction injury (CCI)-induced neuropathic pain. The deregulation of GABA signaling in pathophysiological pain states is well established: GABA signaling can be hampered by a reduction in extracellular GABA synthesis by GAD65 and enhanced extracellular GABA reuptake via the GABA transporter, GAT-1. In neuropathic pain, GABAAR-mediated signaling can be further disrupted by the loss of the KCC2 chloride anion gradient. Here, we demonstrate that A3AR agonists (IB-MECA and MRS5698) reverse neuropathic pain via a spinal mechanism of action that modulates GABA activity. Spinal administration of the GABAA antagonist, bicuculline, disrupted A3AR-mediated analgesia. Furthermore, A3AR-mediated analgesia was associated with reductions in CCI-related GAD65 and GAT-1 serine dephosphorylation as well as an enhancement of KCC2 serine phosphorylation and activity. Our results suggest that A3AR-mediated reversal of neuropathic pain increases modulation of GABA inhibitory neurotransmission both directly and indirectly through protection of KCC2 function, underscoring the unique utility of A3AR agonists in chronic pain.

Navigating in chromone chemical space: discovery of novel and distinct A3 adenosine receptor ligands

One of the major hurdles in the development of effective drugs targeting GPCRs is finding ligands selective for a specific receptor subtype. Here we describe a potent and selective hormone-based hA₃ AR ligand (K_i of 167 nM) with a remarkable selectivity.