Problems and prospects for finding new pharmacological agents among adenosine receptor agonists, antagonists, or their allosteric modulators for the treatment of cardiovascular diseases.

A1-adenosine receptors (A1AR) are widely distributed in the human body and mediate many different effects. They are abundantly present in the cardiovascular system, where they control angiogenesis, vascular tone, heart rate, and conduction. This makes the cardiovascular system A1AR an attractive target for the treatment of cardiovascular diseases (CVD). The review summarizes the literature data on the structure and functioning of A1AR, and analyzes their involvement in the formation of myocardial hypertrophy, ischemia-reperfusion damage, various types of heart rhythm disorders, chronic heart failure, and arterial hypertension. Special attention is paid to the role of some allosteric regulators of A1AR as potential agents for the CVD treatment.

A2A adenosine receptor agonists, antagonists, inverse agonists and partial agonists.

The Gs-coupled A2A adenosine receptor (A2AAR) has been explored extensively as a pharmaceutical target, which has led to numerous clinical trials. However, only one selective A2AAR agonist (regadenoson, Lexiscan) and one selective A2AAR antagonist (istradefylline, Nouriast) have been approved by the FDA, as a pharmacological agent for myocardial perfusion imaging (MPI) and as a cotherapy for Parkinson's disease (PD), respectively. Adenosine is widely used in MPI, as Adenoscan. Despite numerous unsuccessful clinical trials, medicinal chemical activity around A2AAR ligands has accelerated recently, particularly through structure-based drug design. New drug-like A2AAR antagonists for PD and cancer immunotherapy have been identified, and many clinical trials have ensued. For example, imaradenant (AZD4635), a compound that was designed computationally, based on A2AAR X-ray structures and biophysical mapping. Mixed A2AAR/A2BAR antagonists are also hopeful for cancer treatment. A2AAR antagonists may also have potential as neuroprotective agents for treatment of Alzheimer's disease.

Torpor-like Hypothermia Induced by A1 Adenosine Receptor Agonist: A Novel Approach to Protect against Neuroinflammation.

Hypothermia is a promising clinical therapy for acute injuries, including neural damage, but it also faces practical limitations due to the complexities of the equipment and procedures required. This study investigates the use of the A1 adenosine receptor (A1AR) agonist N6-cyclohexyladenosine (CHA) as a more accessible method to induce steady, torpor-like hypothermic states. Additionally, this study investigates the protective potential of CHA against LPS-induced sepsis and neuroinflammation. Our results reveal that CHA can successfully induce a hypothermic state by activating a neuronal circuit similar to the one that induces physiological torpor. This state is characterized by maintaining a steady core body temperature below 28 °C. We further found that this torpor-like state effectively mitigates neuroinflammation and preserves the integrity of the blood-brain barrier during sepsis, thereby limiting the infiltration of inflammatory factors into the central nervous system. Instead of being a direct effect of CHA, this protective effect is attributed to inhibiting pro-inflammatory responses in macrophages and reducing oxidative stress damage in endothelial cells under systemic hypothermia. These results suggest that A1AR agonists such as CHA could potentially be potent neuroprotective agents against neuroinflammation. They also shed light on possible future directions for the application of hypothermia-based therapies in the treatment of sepsis and other neuroinflammatory conditions.

A novel genetically modified mouse seizure model for evaluating anticonvulsive and neuroprotective efficacy of an A1 adenosine receptor agonist following soman intoxication.

Recently a novel humanized mouse strain has been successfully generated, in which serum carboxylesterase (CES) knock out (KO) mice (Es1-/-) were further genetically modified by knocking in (KI), or adding, the gene that encodes the human form of acetylcholinesterase (AChE). The resulting human AChE KI and serum CES KO (or KIKO) mouse strain should not only exhibit organophosphorus nerve agent (NA) intoxication in a manner more similar to humans, but also display AChE-specific treatment responses more closely mimicking those of humans to facilitate data translation to pre-clinic trials. In this study, we utilized the KIKO mouse to develop a seizure model for NA medical countermeasure investigation, and then applied it to evaluate the anticonvulsant and neuroprotectant (A/N) efficacy of a specific A1 adenosine receptor (A1AR) agonist, N-bicyclo-(2.2.1)hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA), which has been shown in a rat seizure model to be a potent A/N compound. Male mice surgically implanted with cortical electroencephalographic (EEG) electrodes a week earlier were pretreated with HI-6 and challenged with various doses (26 to 47 µg/kg, SC) of soman (GD) to determine a minimum effective dose (MED) that induced sustained status epilepticus (SSE) activity in 100% of animals while causing minimum lethality at 24 h. The GD dose selected was then used to investigate the MED doses of ENBA when given either immediately following SSE initiation (similar to wartime military first aid application) or at 15 min after ongoing SSE seizure activity (applicable to civilian chemical attack emergency triage). The selected GD dose of 33 µg/kg (1.4 x LD50) generated SSE in 100% of KIKO mice and produced only 30% mortality. ENBA at a dose as little as 10 mg/kg, IP, caused isoelectric EEG activity within minutes after administration in naïve un-exposed KIKO mice. The MED doses of ENBA to terminate GD-induced SSE activity were determined to be 10 and 15 mg/kg when treatment was given at the time of SSE onset and when seizure activity was ongoing for 15 min, respectively. These doses were much lower than in the non-genetically modified rat model, which required an ENBA dose of 60 mg/kg to terminate SSE in 100% GD-exposed rats. At MED doses, all mice survived for 24 h, and no neuropathology was observed when the SSE was stopped. The findings confirmed that ENBA is a potent A/N for both immediate and delayed (i.e., dual purposed) therapy to victims of NA exposure and serves as a promising neuroprotective antidotal and adjunctive medical countermeasure candidate for pre-clinical research and development for human application.

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 , A2A , A2B 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 , A2A and A3 ARs. Docking studies showed that compound 10b and 11b were the best A2A 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.

A2A adenosine receptor agonist reduced MMP8 expression in healthy M2-like macrophages but not in macrophages from ankylosing spondylitis patients.

BACKGROUND: Ankylosing spondylitis (AS) is an inflammatory autoimmune disease that mostly affects different joints of the body. Macrophages are the predominant cells that mediate disease progression by secreting several pro-inflammatory mediators. Different receptors are involved in macrophages' function including the adenosine receptors (AR). Our main objective in this study was to assess the effect of applying A2A adenosine receptor agonist (CGS-21,680) on the gene expression of inflammatory mediators including bone morphogenetic proteins (BMP)-2, 4 and matrix metalloproteinases (MMP)-3, 8, 9, and 13 on the macrophages from AS patients compared to healthy macrophages. METHODS: Monocytes were isolated from the whole blood of 28 individuals (AS patients and healthy controls in a 1:1 ratio). Macrophages were differentiated using macrophage colony-stimulating factor (M-CSF), and flow cytometry was performed to confirm surface markers. CGS-21,680 was used to treat cells that had been differentiated. Using SYBR green real-time PCR, relative gene expression was determined. RESULTS: Activating A2AAR diminished MMP8 expression in healthy macrophages while it cannot reduce MMP8 expression in patients' macrophages. The effect of A2AAR activation on the expression of BMP2 and MMP9 reached statistical significance neither in healthy macrophages nor in the patients' group. We also discovered a significant positive connection between MMP8 expression and patient scores on the Bath ankylosing spondylitis functional index (BASFI). CONCLUSION: Due to the disability of A2AAR activation in the reduction of MMP8 expression in patients' macrophages and the correlation of MMP8 expression with BASFI index in patients, these results represent defects and dysregulations in the related signaling pathway in patients' macrophages.

Discovery and Structure-Activity Relationship Studies of Novel Adenosine A1 Receptor-Selective Agonists.

A series of benzyloxy and phenoxy derivatives of the adenosine receptor agonists N6-cyclopentyl adenosine (CPA) and N6-cyclopentyl 5'-N-ethylcarboxamidoadenosine (CP-NECA) were synthesized, and their potency and selectivity were assessed. We observed that the most potent were the compounds with a halogen in the meta position on the aromatic ring of the benzyloxy- or phenoxycyclopentyl substituent. In general, the NECA-based compounds displayed greater A1R selectivity than the adenosine-based compounds, with N6-2-(3-bromobenzyloxy)cyclopentyl-NECA and N6-2-(3-methoxyphenoxy)cyclopentyl-NECA showing ∼1500-fold improved A1R selectivity compared to NECA. In addition, we quantified the compounds' affinity and kinetics of binding at both human and rat A1R using a NanoBRET binding assay and found that the halogen substituent in the benzyloxy- or phenoxycyclopentyl moiety seems to confer high affinity for the A1R. Molecular modeling studies suggested a hydrophobic subpocket as contributing to the A1R selectivity displayed. We believe that the identified selective potent A1R agonists are valuable tool compounds for adenosine receptor research.

A1 Adenosine Receptor Activation Inhibits P2X3 Receptor-Mediated ATP Currents in Rat Dorsal Root Ganglion Neurons.

Purinergic signaling is involved in multiple pain processes. P2X3 receptor is a key target in pain therapeutics, while A1 adenosine receptor signaling plays a role in analgesia. However, it remains unclear whether there is a link between them in pain. The present results showed that the A1 adenosine receptor agonist N6-cyclopentyladenosine (CPA) concentration dependently suppressed P2X3 receptor-mediated and α,ß-methylene-ATP (α,ß-meATP)-evoked inward currents in rat dorsal root ganglion (DRG) neurons. CPA significantly decreased the maximal current response to α,ß-meATP, as shown a downward shift of the concentration-response curve for α,ß-meATP. CPA suppressed ATP currents in a voltage-independent manner. Inhibition of ATP currents by CPA was completely prevented by the A1 adenosine receptor antagonist KW-3902, and disappeared after the intracellular dialysis of either the Gi/o protein inhibitor pertussis toxin, the adenylate cyclase activator forskolin, or the cAMP analog 8-Br-cAMP. Moreover, CPA suppressed the membrane potential depolarization and action potential bursts, which were induced by α,ß-meATP in DRG neurons. Finally, CPA relieved α,ß-meATP-induced nociceptive behaviors in rats by activating peripheral A1 adenosine receptors. These results indicated that CPA inhibited the activity of P2X3 receptors in rat primary sensory neurons by activating A1 adenosine receptors and its downstream cAMP signaling pathway, revealing a novel peripheral mechanism underlying its analgesic effect.

Synthesis of 2-chloropurine ribosides with chiral amino acid amides at C6 and their evaluation as A1 adenosine receptor agonists.

A series of purine ribonucleosides bearing chiral amino acid amides at the C6 position of 2-chloropurine was synthesized. Molecular docking of the synthesized analogs of 2-chloroadenosine by their affinity for A1 adenosine receptors (A1ARs) was conducted. The investigation of A1AR stimulating activity of synthesized nucleosides was carried out in a model of an isolated mouse atrium. We have shown that derivatives with tyrosine, valine, and serine residues exhibit the properties of A1AR partial agonists. Animal experiments in the open field test have shown that these compounds have different profiles of psychoactive action. These nucleosides have an ophthalmic hypotensive effect and reduce intraocular pressure in a manner slightly inferior to that of timolol and brimonidine. The synthesized nucleosides can be the basis for further design and synthesis of new A1AR agonists.

Examining the Role of the Linker in Bitopic N6-Substituted Adenosine Derivatives Acting as Biased Adenosine A1 Receptor Agonists.

The adenosine A1 receptor is a therapeutic target based on its ability to provide cardioprotection during episodes of myocardial ischemia and reperfusion injury. However, the clinical translation of A1R agonists has been hindered by dose-limiting adverse effects (bradycardia and hypotension). Previously, we demonstrated that the bitopic agonist VCP746 (1), consisting of an adenosine pharmacophore linked to an allosteric moiety, can stimulate cardioprotective A1R signaling effects in the absence of unwanted bradycardia. This study maps the structure-activity relationships of 1 through modifications to the linker moiety. Derivatives differing in the flexibility, length, and nature of the linker were assessed, which revealed that the linker is tolerant of several modifications including added rigidity. Ligands featuring 1,4-disubstituted 1,2,3-triazoles were the most biased of the novel analogues but also displayed sub-nanomolar potency in a cAMP accumulation assay at the A2BR. To our knowledge, 10 is the most potent A2BR agonist published to date.

Adenosine receptor 2a agonists target mouse CD11c+T-bet+ B cells in infection and autoimmunity.

CD11c+T-bet+ B cells are recognized as an important component of humoral immunity and autoimmunity. These cells can be distinguished from other B cells by their higher expression of the adenosine receptor 2a. Here we address whether A2A receptor activation can affect CD11c+T-bet+ B cells. We show that administration of the A2A receptor agonist CGS-21680 depletes established CD11c+T-bet+ B cells in ehrlichial-infected mice, in a B cell-intrinsic manner. Agonist treatment similarly depletes CD11c+T-bet+ B cells and CD138+ B cells and reduces anti-nuclear antibodies in lupus-prone mice. Agonist treatment is also associated with reduced kidney pathology and lymphadenopathy. Moreover, A2A receptor stimulation depletes pathogenic lymphocytes and ameliorates disease even after disease onset, highlighting the therapeutic potential of this treatment. This study suggests that targeting the adenosine signaling pathway may provide a method for the treatment of lupus and other autoimmune diseases mediated by T-bet+ B cells.

Discovery of Highly Potent Adenosine A1 Receptor Agonists: Targeting Positron Emission Tomography Probes.

Adenosine receptor (AR) radiotracers for positron emission tomography (PET) have provided knowledge on the in vivo biodistribution of ARs in the central nervous system (CNS), which is of therapeutic interest for various neuropsychiatric disorders. Additionally, radioligands that can image changes in endogenous adenosine levels in different physiological and pathological conditions are still lacking. The binding of known antagonist adenosine A1 receptor (A1R) radiotracer, [11C]MDPX, failed to be inhibited by elevated endogenous adenosine in a rodent PET study. Since most of the known AR PET radiotracers were antagonists, we propose that an A1R agonist radioligand may possess higher sensitivity to measure changes in endogenous adenosine concentration. Herein, we report our latest findings toward the development of a full agonist adenosine A1 radioligand for PET. Based on a 3,5-dicyanopyridine template, 16 new derivatives were designed and synthesized to optimize both binding affinity and functional activity, resulting in two full agonists (compounds 27 and 29) with single-digit nanomolar affinities and good subtype selectivity (A1/A2A selectivity of ∼1000-fold for compound 27 and 29-fold for compound 29). Rapid O-[11C]methylation provided [11C]27 and [11C]29 in high radiochemical yields and radiochemical purity. However, subsequent brain PET imaging in rodents showed poor brain permeability for both radioligands. An in vivo PET study using knockout mice for MDR 1a/a, BCRP, and MRP1 indicated that these compounds might be substrates for brain efflux pumps. In addition, in silico evaluation using multiparameter optimization identified high molecular weight and high polar surface area as the main molecular descriptors responsible for low brain penetration. These results will provide further insight toward development of full agonist adenosine A1 radioligands and also highly potent CNS A1AR drugs.

Adenosine Receptor Agonist HE-NECA Enhances Antithrombotic Activities of Cangrelor and Prasugrel in vivo by Decreasing of Fibrinogen Density in Thrombus.

Blood platelets' adenosine receptors (AR) are considered to be a new target for the anti-platelet therapy. This idea is based on in vitro studies which show that signaling mediated by these receptors leads to a decreased platelet response to activating stimuli. In vivo evidence for the antithrombotic activity of AR agonists published to date were limited, however, to the usage of relatively high doses given in bolus. The present study was aimed at verifying if these substances used in lower doses in combination with inhibitors of P2Y12 could serve as components of dual anti-platelet therapy. We have found that a selective A2A agonist 2-hexynyl-5'-N-ethylcarboxamidoadenosine (HE-NECA) improved the anti-thrombotic properties of either cangrelor or prasugrel in the model of ferric chloride-induced experimental thrombosis in mice. Importantly, HE-NECA was effective not only when applied in bolus as other AR agonists in the up-to-date published studies, but also when given chronically. In vitro thrombus formation under flow conditions revealed that HE-NECA enhanced the ability of P2Y12 inhibitors to decrease fibrinogen content in thrombi, possibly resulting in their lower stability. Adenosine receptor agonists possess a certain hypotensive effect and an ability to increase the blood-brain barrier permeability. Therefore, the effects of anti-thrombotic doses of HE-NECA on blood pressure and the blood-brain barrier permeability in mice were tested. HE-NECA applied in bolus caused a significant hypotension in mice, but the effect was much lower when the substance was given in doses corresponding to that obtained by chronic administration. At the same time, no significant effect of HE-NECA was observed on the blood-brain barrier. We conclude that chronic administration of the A2A agonist can be considered a potential component of a dual antithrombotic therapy. However, due to the hypotensive effect of the substances, dosage and administration must be elaborated to minimize the side-effects. The total number of animals used in the experiments was 146.

A nonolfactory shark adenosine receptor activates CFTR with unique pharmacology and structural features.

Adenosine receptors (ADORs) are G protein-coupled purinoceptors that have several functions including regulation of chloride secretion via cystic fibrosis transmembrane conductance regulator (CFTR) in human airway and kidney. We cloned an ADOR from Squalus acanthias (shark) that likely regulates CFTR in the rectal gland. Phylogenic and expression analyses indicate that elasmobranch ADORs are nonolfactory and appear to represent extant predecessors of mammalian ADORs. We therefore designate the shark ADOR as the A0 receptor. We coexpressed A0 with CFTR in Xenopus laevis oocytes and characterized the coupling of A0 to the chloride channel. Two-electrode voltage clamping was performed, and current-voltage (I-V) responses were recorded to monitor CFTR status. Only in A0- and CFTR-coinjected oocytes did adenosine analogs produce a significant concentration-dependent activation of CFTR consistent with its electrophysiological signature. A pharmacological profile for A0 was obtained for ADOR agonists and antagonists that differed markedly from all mammalian ADOR subtypes [agonists: R-phenyl-isopropyl adenosine (R-PIA) > S-phenyl-isopropyl adenosine (S-PIA) > CGS21680 > N6-cyclopentyladenosine (CPA) > 2-chloroadenosine (2ClAdo) > CV1808 = N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA) > N-ethyl-carboxyl adenosine (NECA); and antagonists: 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) > PD115199 > 1,3-dimethyl-8-phenylxanthine (8PT) > CGS15943]. Structures of human ADORs permitted a high-confidence homology model of the shark A0 core that revealed unique structural features of ancestral receptors. We conclude that 1) A0 is a novel and unique adenosine receptor ancestor by functional and structural criteria; 2) A0 likely activates CFTR in vivo, and this receptor activates CFTR in oocytes, indicating an evolutionary coupling between ADORs and chloride secretion; and 3) A0 appears to be a nonolfactory evolutionary ancestor of all four mammalian ADOR subtypes.

Targeted Regulation of Blood-Brain Barrier for Enhanced Therapeutic Efficiency of Hypoxia-Modifier Nanoparticles and Immune Checkpoint Blockade Antibodies for Glioblastoma.

Glioblastoma is the most destructive type of brain cancer. The blood-brain barrier (BBB) is a tremendous obstacle that hinders therapeutic agents, such as chemical drugs and antibodies, from reaching glioblastoma tissues. Meanwhile, the abnormal microenvironment of glioblastoma extremely restricts the expected therapeutic effects of accumulated drugs. Therefore, in the present study, BBB-regulating nanovesicles (BRN) are developed to achieve targeted and controlled BBB regulation, carrying adenosine 2A receptor (A2AR) agonists and perfluorocarbon (PF). The red-blood-cell membrane (RBCM) is included on the outside to avoid the premature release of therapeutic agents. In the presence of ultrasonication (US), A2AR agonists are released and induce effects on both F-actin and tight junctions of endothelial cells. Subsequently, BBB permeability is temporarily increased and enables small molecules and nanoparticles to enter brain parenchymal tissues. The high affinity between manganese dioxide and temozolomide (TMZ) is utilized to form multifunctional nanoparticles to ameliorate the hypoxic microenvironment, which yields improved glioblastoma inhibition combined with radiotherapy. Moreover, with the aid of targeted BBB regulation, programmed death ligand-1 (PD-L1) antibody induces a tumor-specific immune response. Taken together, the findings suggest that synergistic combination may have the potential in amplifying the therapeutic efficacies of clinical drugs and immune checkpoint blockade antibodies to overcome the therapeutic resistance of glioblastoma.

Molecular modeling approaches of selective adenosine receptor type 2A agonists as potential anti-inflammatory drugs.

Adenosine A2A receptor (A2AR) is the predominant receptor in immune cells, where its activation triggers cAMP-mediated immunosuppressive signaling and the underlying inhibition of T cells activation and T cells-induced effects mediated by cAMP-dependent kinase proteins mechanisms. In this study, were used ADME/Tox, molecular docking and molecular dynamics simulations to investigate selective adenosine A2AR agonists as potential anti-inflammatory drugs. As a result, we obtained two promising compounds (A and B) that have satisfactory pharmacokinetic and toxicological properties and were able to interact with important residues of the A2AR binding cavity and during the molecular dynamics simulations were able to keep the enzyme complexed.Communicated by Ramaswamy H. Sarma.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example.

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Medicinal chemistry of P2 and adenosine receptors: Common scaffolds adapted for multiple targets.

Prof. Geoffrey Burnstock originated the concept of purinergic signaling. He demonstrated the interactions and biological roles of ionotropic P2X and metabotropic P2Y receptors. This review paper traces the historical origins of many currently used antagonists and agonists for P2 receptors, as well as adenosine receptors, in early attempts to identify ligands for these receptors - prior to the use of chemical libraries for screening. Rather than presenting a general review of current purinergic ligands, we focus on common chemical scaffolds (privileged scaffolds) that can be adapted for multiple receptor targets. By carefully analyzing the structure activity relationships, one can direct the selectivity of these scaffolds toward different receptor subtypes. For example, the weak and non-selective P2 antagonist reactive blue 2 (RB-2) was derivatized using combinatorial synthetic approaches, leading to the identification of selective P2Y2, P2Y4, P2Y12 or P2X2 receptor antagonists. A P2X4 antagonist NC-2600 is in a clinical trial, and A3 adenosine agonists show promise, for chronic pain. P2X7 antagonists have been in clinical trials for depression (JNJ-54175446), inflammatory bowel disease (IBD), Crohn's disease, rheumatoid arthritis, inflammatory pain and chronic obstructive pulmonary disease (COPD). P2X3 antagonists are in clinical trials for chronic cough, and an antagonist named after Burnstock, gefapixant, is expected to be the first P2X3 antagonist filed for approval. We are seeing that the vision of Prof. Burnstock to use purinergic signaling modulators, most recently at P2XRs, for treating disease is coming to fruition.

Adenosine A1 and A2a receptors modulate the nitrergic system in cell culture from dorsomedial medulla oblongata.

Adenosine and nitric oxide act on the fine-tuning regulation of neural cardiovascular control in the nucleus tractus solitarius (NTS). Although the interaction between adenosine and NO is well known in the periphery, the mechanisms by which adenosine interferes in the dynamics of nitrergic neurotransmission, related to neural control of circulation, are not completely understood and might be relevant for individuals predisposed to hypertension. In this study we evaluate the interaction between adenosinergic and nitrergic systems in cell culture from the dorsomedial medulla oblongata of Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). Using quantification of nitrite levels, RT-PCR analysis and RNA interference we demonstrate that adenosine A1 (A1R) and A2a receptor (A2aR) agonists induce a concentration-dependent decrease and increase of nitrite and nNOS mRNA levels in cultured cells from WKY and SHR, respectively. These effects in nitrite levels are attenuated by the administration of A1R and A2aR selective antagonists, CPT and ZM 241385. Furthermore, knockdown of A1R and A2aR show an increase and decrease of nNOS mRNA levels, respectively. Pretreatment with the nonselective inhibitor of NOS, L-NAME, abolishes nitrite-increased levels triggered by CGS 21680 in WKY and SHR cells. Finally, it is shown that the cAMP-PKA pathway is involved in A1R and A2aR-mediated decrease and increase in nitrite levels in SHR and WKY cells. Our results highlight the influence of adenosine on nitric oxide levels in cultured cells from dorsal medulla oblongata of neonate WKY and SHR rats. In part, the modulatory profile is different in the SHR strain.

Targeting Adenosine Receptors: A Potential Pharmacological Avenue for Acute and Chronic Pain.

Adenosine is a purine nucleoside, responsible for the regulation of multiple physiological and pathological cellular and tissue functions by activation of four G protein-coupled receptors (GPCR), namely A1, A2A, A2B, and A3 adenosine receptors (ARs). In recent years, extensive progress has been made to elucidate the role of adenosine in pain regulation. Most of the antinociceptive effects of adenosine are dependent upon A1AR activation located at peripheral, spinal, and supraspinal sites. The role of A2AAR and A2BAR is more controversial since their activation has both pro- and anti-nociceptive effects. A3AR agonists are emerging as promising candidates for neuropathic pain. Although their therapeutic potential has been demonstrated in diverse preclinical studies, no AR ligands have so far reached the market. To date, novel pharmacological approaches such as adenosine regulating agents and allosteric modulators have been proposed to improve efficacy and limit side effects enhancing the effect of endogenous adenosine. This review aims to provide an overview of the therapeutic potential of ligands interacting with ARs and the adenosinergic system for the treatment of acute and chronic pain.