Characterization of adenosine receptor(s) involved in adenosine-induced bronchoconstriction in an allergic mouse model

Alcohol inhibits alveolar fluid clearance through the epithelial sodium channel via the A2 adenosine receptor in acute lung injury

Chronic alcohol abuse increases the risk of mortality and poor outcomes in patients with acute respiratory distress syndrome. However, the underlying mechanisms remain to be elucidated. The present study aimed to investigate the effects of chronic alcohol consumption on lung injury and clarify the signaling pathways involved in the inhibition of alveolar fluid clearance (AFC). In order to produce rodent models with chronic alcohol consumption, wild‑type C57BL/6 mice were treated with alcohol. A2a adenosine receptor (AR) small interfering (si)RNA or A2bAR siRNA were transfected into the lung tissue of mice and primary rat alveolar type II (ATII) cells. The rate of AFC in lung tissue was measured during exposure to lipopolysaccharide (LPS). Epithelial sodium channel (ENaC) expression was determined to investigate the mechanisms underlying alcohol‑induced regulation of AFC. In the present study, exposure to alcohol reduced AFC, exacerbated pulmonary edema and worsened LPS‑induced lung injury. Alcohol caused a decrease in cyclic adenosine monophosphate (cAMP) levels and inhibited α‑ENaC, β‑ENaC and γ‑ENaC expression levels in the lung tissue of mice and ATII cells. Furthermore, alcohol decreased α‑ENaC, β‑ENaC and γ‑ENaC expression levels via the A2aAR or A2bAR‑cAMP signaling pathways in vitro. In conclusion, the results of the present study demonstrated that chronic alcohol consumption worsened lung injury by aggravating pulmonary edema and impairing AFC. An alcohol‑induced decrease of α‑ENaC, β‑ENaC and γ‑ENaC expression levels by the A2AR‑mediated cAMP pathway may be responsible for the exacerbated effects of chronic alcohol consumption in lung injury.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

Attenuation of Pulmonary Ischemia-Reperfusion Injury by Adenosine A2B Receptor Antagonism

BACKGROUND

Ischemia-reperfusion injury (IRI) is a major source of morbidity and mortality after lung transplantation. We previously demonstrated a proinflammatory role of adenosine A2B receptor (A2BR) in lung IR injury. The current study tests the hypothesis that A2BR antagonism is protective of ischemic lungs after in vivo reperfusion or ex vivo lung perfusion (EVLP).

METHODS

Mice underwent lung IR with or without administration of ATL802, a selective A2BR antagonist. A murine model of EVLP was also used to evaluate rehabilitation of donation after circulatory death (DCD) lungs. DCD lungs underwent ischemia, cold preservation, and EVLP with Steen solution with or without ATL802. A549 human type 2 alveolar epithelial cells were exposed to hypoxia-reoxygenation (HR) (3 hours/1 hour) with or without ATL802 treatment. Cytokines were measured in bronchoalveolar lavage (BAL) fluid and culture media by enzyme-linked immunoassay (ELISA).

RESULTS

After IR, ATL802 treatment significantly improved lung function (increased pulmonary compliance and reduced airway resistance and pulmonary artery pressure) and significantly attenuated proinflammatory cytokine production, neutrophil infiltration, vascular permeability, and edema. ATL802 also significantly improved the function of DCD lungs after EVLP (increased compliance and reduced pulmonary artery pressure). After HR, A549 cells exhibited robust production of interleukin (IL)-8, a potent neutrophil chemokine, which was significantly attenuated by ATL802.

CONCLUSIONS

These results demonstrate that A2BR antagonism attenuates lung IRI and augments reconditioning of DCD lungs by EVLP. The protective effects of ATL802 may involve targeting A2BRs on alveolar epithelial cells to prevent IL-8 production. A2BR may be a novel therapeutic target for mitigating IRI to increase the success of lung transplantation.

Adenosine A1 receptor activation attenuates lung ischemia-reperfusion injury

OBJECTIVES

Ischemia-reperfusion injury contributes significantly to morbidity and mortality in lung transplant patients. Currently, no therapeutic agents are clinically available to prevent ischemia-reperfusion injury, and treatment strategies are limited to maintaining oxygenation and lung function. Adenosine can modulate inflammatory activity and injury by binding to various adenosine receptors; however, the role of the adenosine A1 receptor in ischemia-reperfusion injury and inflammation is not well understood. The present study tested the hypothesis that selective, exogenous activation of the A1 receptor would be anti-inflammatory and attenuate lung ischemia-reperfusion injury.

METHODS

Wild-type and A1 receptor knockout mice underwent 1 hour of left lung ischemia and 2 hours of reperfusion using an in vivo hilar clamp model. An A1 receptor agonist, 2-chloro-N6-cyclopentyladenosine, was administered 5 minutes before ischemia. After reperfusion, lung function was evaluated by measuring airway resistance, pulmonary compliance, and pulmonary artery pressure. The wet/dry weight ratio was used to assess edema. The myeloperoxidase and cytokine levels in bronchoalveolar lavage fluid were measured to determine the presence of neutrophil infiltration and inflammation.

RESULTS

In the wild-type mice, 2-chloro-N6-cyclopentyladenosine significantly improved lung function and attenuated edema, cytokine expression, and myeloperoxidase levels compared with the vehicle-treated mice after ischemia-reperfusion. The incidence of lung ischemia-reperfusion injury was similar in the A1 receptor knockout and wild-type mice; and 2-chloro-N6-cyclopentyladenosine had no effects in the A1 receptor knockout mice. In vitro treatment of neutrophils with 2-chloro-N6-cyclopentyladenosine significantly reduced chemotaxis.

CONCLUSIONS

Exogenous A1 receptor activation improves lung function and decreases inflammation, edema, and neutrophil chemotaxis after ischemia and reperfusion. These results suggest a potential therapeutic application for A1 receptor agonists for the prevention of lung ischemia-reperfusion injury after transplantation.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Hemodynamic response, arrhythmic risk, and overall safety of regadenoson as a pharmacologic stress agent for myocardial perfusion imaging in chronic obstructive pulmonary disease and bronchial asthma patients

Regadenoson (REG) is a A2a receptor selective pharmacologic SPECT imaging agent. Its safety in unselected chronic obstructive pulmonary disease (COPD) or asthma (AM) undergoing SPECT imaging has not been well evaluated. We retrospectively identified 228 patients (COPD n = 126 and AM n = 102, Grp 1) undergoing REG SPECT from Jan to Nov 2009 and compared to 1,142 patients without COPD and AM (control, Grp 2). A standard 400 μg REG bolus was used and gated Tc-99 m tetrofosmin SPECT done. Patient demographics, REG SPECT data, side effects, arrhythmia occurrences, and any exacerbation of COPD or AM leading to treatment, hospitalization or death were evaluated. The side effect profile of Grp 1 was also compared to a historical cohort who underwent intravenous dipyridamole thallium-201 imaging and adenosine SPECT. Both groups were comparable with regards to baseline characteristics. There was 0% incidence of clinical exacerbation of COPD or AM after REG. COPD patients had more non-significant arrhythmias (58.3% vs. Grp 2, 43%, P = 0.004). There was 0% incidence of any atrio-ventricular block and only 2 instances of brief supraventricular tachycardia. When compared to the historical cohort of COPD who underwent IV dipyridamole thallium imaging, COPD in Grp 1, had more dyspnea and flushing and when compared to COPD/AM patients who underwent adenosine SPECT, Grp 1 pts had more of flushing and headache (24.9% vs. 2.8%, P = <0.001) but less of bronchospasm (1.3% vs. 6.9%, P = 0.022) and AV block (0% vs. 4.2%, P = 0.014). REG SPECT can be safely performed in COPD and AM population.

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. 

Targeting of adenosine receptors in ischemia-reperfusion injury

IMPORTANCE OF THE FIELD

Ischemia-reperfusion (IR) injury is a common problem after transplantation as well as myocardial infarction and stroke. IR initiates an inflammatory response leading to rapid tissue damage. Adenosine, produced in response to IR, is generally considered a protective signaling molecule and elicits its physiological responses through four distinct adenosine receptors. The short half-life, lack of specificity and rapid metabolism limits the use of adenosine as a therapeutic agent. Thus, intense research efforts have focused on the synthesis and implementation of specific adenosine receptor agonists and antagonists as potential therapeutic agents for a variety of inflammatory conditions including IR injury.

AREAS COVERED IN THIS REVIEW

Current knowledge on IR injury with a focus on lung, heart and kidney and studies that have advanced our understanding of the role of adenosine receptors and the therapeutic potential of adenosine receptor agonists and antagonists for the prevention of IR injury.

WHAT THE READER WILL GAIN

Insight into the role of adenosine receptor signaling in IR injury.

TAKE HOME MESSAGE

No therapies are currently available that specifically target IR injury; however, targeting of specific adenosine receptors may offer therapeutic strategies in this regard.

Adenosine receptors and vascular inflammation

Epidemiological studies have shown a positive correlation between poor lung function and respiratory disorders like asthma and the development of adverse cardiovascular events. Increased adenosine (AD) levels are associated with lung inflammation which could lead to altered vascular responses and systemic inflammation. There is relatively little known about the cardiovascular effects of adenosine in a model of allergy. We have shown that A(1) adenosine receptors (AR) are involved in altered vascular responses and vascular inflammation in allergic mice. Allergic A(1)wild-type mice showed altered vascular reactivity, increased airway responsiveness and systemic inflammation. Our data suggests that A(1) AR is pro-inflammatory systemically in this model of asthma. There are also reports of the A(2B) receptor having anti-inflammatory effects in vascular stress; however its role in allergy with respect to vascular effects has not been fully explored. In this review, we have focused on the role of adenosine receptors in allergic asthma and the cardiovascular system and possible mechanism(s) of action.

Involvement of A1 adenosine receptors in altered vascular responses and inflammation in an allergic mouse model of asthma

Poor lung function and respiratory disorders like asthma have a positive correlation with the development of adverse cardiovascular events. Increased adenosine levels are associated with lung inflammation that could lead to altered vascular responses and systemic inflammation. We hypothesized that asthmatic lung inflammation has systemic effects through A(1) adenosine receptors (A(1)AR) and investigated the effects of aerosolized adenosine on vascular reactivity and inflammation, using A(1)AR knockout (A(1)KO) and corresponding wild-type (A(1)WT) mice that were divided into three experimental groups each: control (CON), allergen sensitized and challenged (SEN), and SEN + aerosolized adenosine (SEN + AD). Animals were sensitized with ragweed (200 microg ip; days 1 and 6), followed by 1% ragweed aerosol challenges (days 11 to 13). On day 14, the SEN + AD groups received one adenosine aerosol challenge (6 mg/ml) for 2 min, and aortae were collected on day 15. 5'-N-ethylcarboxamidoadenosine (NECA; nonselective adenosine analog) induced concentration-dependent aortic relaxation in the A(1)WT CON group, which was impaired in the A(1)WT SEN and SEN + AD groups. All groups of A(1)KO mice showed similar (no significant difference) concentration-dependent relaxation to NECA. The A(1)WT SEN and SEN + AD groups had a significantly higher contraction to selective A(1) agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) compared with the CON group. Western blot data showed that aortic A(1)AR expression was significantly increased in WT SEN and SEN + AD mice compared with CON mice. Gene expression of ICAM-1 and IL-5 was significantly increased in allergic A(1)WT aorta and were undetected in the A(1)KO groups. A(1)WT allergic mice had significantly higher airway hyperresponsiveness (enhanced pause) to NECA, with adenosine aerosol further enhancing it. In conclusion, allergic A(1)WT mice showed altered vascular reactivity, increased airway hyperresponsiveness, and systemic inflammation. These data suggest that A(1)AR is proinflammatory systemically in this model of allergic asthma.

Adenosine signaling and the regulation of chronic lung disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease and interstitial lung disease are characterized by inflammation and tissue remodeling processes that compromise pulmonary function. Adenosine is produced in the inflamed and damaged lung where it plays numerous roles in the regulation of inflammation and tissue remodeling. Extracellular adenosine serves as an autocrine and paracrine signaling molecule by engaging cell surface adenosine receptors. Preclinical and cellular studies suggest that adenosine plays an anti-inflammatory role in processes associated with acute lung disease, where activation of the A(2A)R and A(2B)R has promising implications for the treatment of these disorders. In contrast, there is growing evidence that adenosine signaling through the A(1)R, A(2B)R and A(3)R may serve pro-inflammatory and tissue remodeling functions in chronic lung diseases. This review discusses the current progress of research efforts and clinical trials aimed at understanding the complexities of these signaling pathway as they pertain to the development of treatment strategies for chronic lung diseases.

Attenuation of chronic pulmonary inflammation in A2B adenosine receptor knockout mice

Pharmacologic evidence suggests that activation of A(2B) adenosine receptors results in proinflammatory effects relevant to the progression of asthma, a chronic lung disease associated with elevated interstitial adenosine concentrations in the lung. This concept has been challenged by the finding that genetic removal of A(2B) receptors leads to exaggerated responses in models of acute inflammation. Therefore, the goal of our study was to determine the effects of A(2B) receptor gene ablation in the context of ovalbumin-induced chronic pulmonary inflammation. We found that repetitive airway allergen challenge induced a significant increase in adenosine levels in fluid recovered by bronchoalveolar lavage. Genetic ablation of A(2B) receptors significantly attenuated allergen-induced chronic pulmonary inflammation, as evidenced by a reduction in the number of bronchoalveolar lavage eosinophils and in peribronchial eosinophilic infiltration. The most striking difference in the pulmonary inflammation induced in A(2B) receptor knockout (A(2B)KO) and wild-type mice was the lack of allergen-induced IL-4 release in the airways of A(2B)KO animals, in line with a significant reduction in IL-4 protein and mRNA levels in lung tissue. In addition, attenuation of allergen-induced transforming growth factor-beta release in airways of A(2B)KO mice correlated with reduced airway smooth muscle and goblet cell hyperplasia/hypertrophy. In conclusion, genetic removal of A(2B) adenosine receptors in mice leads to inhibition of allergen-induced chronic pulmonary inflammation and airway remodeling. These findings are in agreement with previous pharmacologic studies suggesting a deleterious role for A(2B) receptor signaling in chronic lung inflammation.

Adenosine signaling contributes to ethanol-induced fatty liver in mice

Fatty liver is commonly associated with alcohol ingestion and abuse. While the molecular pathogenesis of these fatty changes is well understood, the biochemical and pharmacological mechanisms by which ethanol stimulates these molecular changes remain unknown. During ethanol metabolism, adenosine is generated by the enzyme ecto-5'-nucleotidase, and adenosine production and adenosine receptor activation are known to play critical roles in the development of hepatic fibrosis. We therefore investigated whether adenosine and its receptors play a role in the development of alcohol-induced fatty liver. WT mice fed ethanol on the Lieber-DeCarli diet developed hepatic steatosis, including increased hepatic triglyceride content, while mice lacking ecto-5'-nucleotidase or adenosine A1 or A2B receptors were protected from developing fatty liver. Similar protection was also seen in WT mice treated with either an adenosine A1 or A2B receptor antagonist. Steatotic livers demonstrated increased expression of genes involved in fatty acid synthesis, which was prevented by blockade of adenosine A1 receptors, and decreased expression of genes involved in fatty acid metabolism, which was prevented by blockade of adenosine A2B receptors. In vitro studies supported roles for adenosine A1 receptors in promoting fatty acid synthesis and for A2B receptors in decreasing fatty acid metabolism. These results indicate that adenosine generated by ethanol metabolism plays an important role in ethanol-induced hepatic steatosis via both A1 and A2B receptors and suggest that targeting adenosine receptors may be effective in the prevention of alcohol-induced fatty liver.

Adenosine receptors and asthma

The pathophysiological processes underlying respiratory diseases like asthma are complex, resulting in an overwhelming choice of potential targets for the novel treatment of this disease. Despite this complexity, asthmatic subjects are uniquely sensitive to a range of substances like adenosine, thought to act indirectly to evoke changes in respiratory mechanics and in the underlying pathology, and thereby to offer novel insights into the pathophysiology of this disease. Adenosine is of particular interest because this substance is produced endogenously by many cells during hypoxia, stress, allergic stimulation, and exercise. Extracellular adenosine can be measured in significant concentrations within the airways; can be shown to activate adenosine receptor (AR) subtypes on lung resident cells and migrating inflammatory cells, thereby altering their function, and could therefore play a significant role in this disease. Many preclinical in vitro and in vivo studies have documented the roles of the various AR subtypes in regulating cell function and how they might have a beneficial impact in disease models. Agonists and antagonists of some of these receptor subtypes have been developed and have progressed to clinical studies in order to evaluate their potential as novel antiasthma drugs. In this chapter, we will highlight the roles of adenosine and AR subtypes in many of the characteristic features of asthma: airway obstruction, inflammation, bronchial hyperresponsiveness and remodeling. We will also discuss the merit of targeting each receptor subtype in the development of novel antiasthma drugs.

Bench-to-bedside review: adenosine receptors--promising targets in acute lung injury?

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are life-threatening disorders that have substantial adverse effects on outcomes in critically ill patients. ALI/ARDS develops in response to pulmonary or extrapulmonary injury and is characterized by increased leakage from the pulmonary microvasculature and excessive infiltration of polymorphonuclear cells into the lung. Currently, no therapeutic strategies are available to control these fundamental pathophysiological processes in human ALI/ARDS. In a variety of animal models and experimental settings, the purine nucleoside adenosine has been demonstrated to regulate both endothelial barrier integrity and polymorphonuclear cell trafficking in the lung. Adenosine exerts its effects through four G-protein-coupled receptors (A1, A2A, A2B, and A3) that are expressed on leukocytes and nonhematopoietic cells, including endothelial and epithelial cells. Each type of adenosine receptor (AR) is characterized by a unique pharmacological and physiological profile. The development of selective AR agonists and antagonists, as well as the generation of gene-deficient mice, has contributed to a growing understanding of the cellular and molecular processes that are critically involved in the development of ALI/ARDS. Adenosine-dependent pathways are involved in both protective and proinflammatory effects, highlighting the need for a detailed characterization of the distinct pathways. This review summarizes current experimental observations on the role of adenosine signaling in the development of acute lung injury and illustrates that adenosine and ARs are promising targets that may be exploited in the development of innovative therapeutic strategies.

Effect of A2B adenosine receptor gene ablation on proinflammatory adenosine signaling in mast cells

Pharmacological studies suggest that A(2B) adenosine receptors mediate proinflammatory effects of adenosine in human mast cells in part by up-regulating production of Th2 cytokines and angiogenic factors. This concept has been recently challenged by the finding that mast cells cultured from bone marrow-derived mast cells (BMMCs) of A(2B) knockout mice display an enhanced degranulation in response to FcepsilonRI stimulation. This finding was interpreted as evidence of anti-inflammatory functions of A(2B) receptors and it was suggested that antagonists with inverse agonist activity could promote activation of mast cells. In this report, we demonstrate that genetic ablation of the A(2B) receptor protein has two distinct effects on BMMCs, one is the previously reported enhancement of Ag-induced degranulation, which is unrelated to adenosine signaling; the other is the loss of adenosine signaling via this receptor subtype that up-regulates IL-13 and vascular endothelial growth factor secretion. Genetic ablation of A(2B) receptors had no effect on A(3) adenosine receptor-dependent potentiation of Ag-induced degranulation in mouse BMMCs, but abrogated A(2B) adenosine receptor-dependent stimulation of IL-13 and vascular endothelial growth factor secretion. Adenosine receptor antagonists MRS1706 and DPCPX with known inverse agonist activity at the A(2B) subtype inhibited IL-13 secretion induced by the adenosine analog NECA, but did not mimic the enhanced Ag-induced degranulation observed in A(2B) knockout BMMCs. Thus, our study confirmed the proinflammatory role of adenosine signaling via A(2B) receptors and the anti-inflammatory actions of A(2B) antagonists in mouse BMMCs.

Adenosine induces a cholinergic tracheal reflex contraction in guinea pigs in vivo via an adenosine A1 receptor-dependent mechanism

Adenosine induces dyspnea, cough, and airways obstruction in asthma, a phenomenon that also occurs in various sensitized animal models in which a neuronal involvement has been implicated. Although adenosine has been suggested to activate cholinergic nerves, the precise mechanism has not been established. In the present study, the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced a cholinergic reflex, causing tracheal smooth muscle contraction that was significantly inhibited by the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 100 microg/kg) (P < 0.05) in anesthetized animals. Furthermore, the adenosine A(2) agonist 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) induced a small reflex, whereas the A(3) selective agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) was without effect. The tracheal reflex induced by CPA was also inhibited by recurrent nerve ligation or muscarinic receptor blockade (P < 0.001), indicating that a cholinergic neuronal mechanism of action accounted for this response. The cholinergic reflex in response to aerosolized CPA was significantly greater in passively sensitized compared with naive guinea pigs (P < 0.01). Chronic capsaicin treatment, which inhibited sensory nerve function, failed to inhibit CPA-induced reflex tracheal contractions in passively sensitized guinea pigs, although the local anesthetic lidocaine inhibited CPA-induced tracheal contractions. The effects of CPA on the reflex response was not dependent on the release of histamine from tissue mast cells or endogenous prostaglandins as shown by the lack of effect of the histamine H(1) receptor antagonist pyrilamine (1 mg/kg) or the cyclooxygenase inhibitor meclofenamic acid (3 mg/kg), respectively. In conclusion, activation of pulmonary adenosine A(1) receptors can stimulate cholinergic reflexes, and these reflexes are increased in allergic guinea pigs.

Safety of regadenoson, an adenosine A2A receptor agonist for myocardial perfusion imaging, in mild asthma and moderate asthma patients: a randomized, double-blind, placebo-controlled trial

BACKGROUND

Patients with reactive airways are at risk for adenosine-induced bronchoconstriction, mediated via A(2B) and/or A(3) adenosine receptors.

METHODS AND RESULTS

To examine the effects of regadenoson, a selective adenosine A(2A) receptor agonist, on airway resistance, we conducted a randomized, double-blind, placebo-controlled crossover trial in asthmatic patients with a positive adenosine monophosphate challenge test. The mean ratio of the forced expiratory volume in 1 second (FEV(1)) at each tested time point relative to the baseline FEV(1) was significantly higher after treatment with regadenoson compared with placebo from 10 to 60 minutes after treatment. One patient had a substantial but asymptomatic FEV(1) reduction (-36.2%) after regadenoson that reversed spontaneously. The most common adverse events with regadenoson were tachycardia (66%), dizziness (53%), headache (45%), and dyspnea (34%). The mean heart rate significantly increased with regadenoson (maximum of +10.4 beats/min) versus placebo.

CONCLUSIONS

In this pilot safety study of 48 patients with mild or moderate asthma who had bronchial reactivity to adenosine monophosphate, regadenoson was safe and well tolerated.

Adenosine receptors and asthma

The accumulation of evidence implicating a role for adenosine in the pathogenesis of asthma has led to investigations into all adenosine receptor subtypes as potential therapeutic targets for the treatment of asthma. Selective A(1) receptor antagonists are currently in preclinical development since adenosine has been shown experimentally to mediate various features of asthma through this receptor such as bronchoconstriction, mucus secretion and inflammation. The A(2A) receptor is expressed on most inflammatory cells implicated in asthma, and as A(2A) stimulation activates adenylate cyclase and consequently elevates cAMP, selective A(2A) receptor agonists have now reached clinical development. However, initial reports concerning their efficacy are inconclusive. A(2B) receptor antagonists are also under investigation based on the rationale that inhibiting the effects of adenosine on mast cells would be beneficial, in addition to other reported pro-inflammatory effects mediated by the A(2B) receptor on cells such as airway smooth muscle, epithelial cells and fibroblasts. Whilst the effects in pre-clinical models are promising, their efficacy in the clinical setting has also yet to be reported. Finally, adenosine A(3) receptor stimulation has been demonstrated to mediate inhibitory effects on eosinophils since it also elevates cAMP. However, some experimental reports suggest that A(3) antagonists mediate anti-inflammatory effects, thus the rationale for A(3) receptor ligands as therapeutic agents remains to be determined. In conclusion, establishing the precise role of adenosine in the pathogenesis of asthma and developing appropriate subtype selective agonists/antagonists represents an exciting opportunity for the development of novel therapeutics for the treatment of asthma.

Adenosine-mediated alteration of vascular reactivity and inflammation in a murine model of asthma

Chronic respiratory disorders such as asthma are believed to be associated with adverse cardiovascular events. We hypothesize that asthmatic inflammation translates into systemic inflammation and alters vascular responses where adenosine (AD) plays an important role. Therefore, this study investigated the effects of aerosolized AD, used to elevate lung AD levels, on vascular reactivity and inflammation in our allergic mouse model of asthma. Balb/c mice were divided into four groups: control (Con), Con + aerosolized AD (Con + AD), allergen sensitized and challenged (Sen), and Sen + aerosolized AD (Sen + AD). The animals were sensitized with ragweed (200 mug ip) on days 1 and 6, followed by 1% ragweed aerosol challenges from days 11 to 13. On day 14, the Con + AD and Sen + AD groups received a single AD aerosol challenge (6 mg/ml) for 2 min, followed by the collection of the aorta and plasma on day 15. Organ bath experiments showed concentration-dependent aortic relaxations to AD in the Con and Con + AD groups, which were impaired in the Sen and Sen + AD groups. Real-time PCR data showed changes in aortic AD receptors (ARs), with the expression of A(1)ARs upregulated, whereas the expression of A(2)ARs and endothelial nitric oxide synthase genes were downregulated, resulting in an impairment of vasorelaxation in the Sen and Sen + AD groups. The A(1)AR antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) reversed the impairment in vasorelaxation observed in the Sen and Sen + AD groups, whereas the A(2B)AR antagonist alloxazine inhibited vasorelaxation in all groups. Allergen challenge caused systemic inflammation in allergic mice, with AD aerosol further enhancing it as determined by the inflammatory cytokines profile in plasma. In conclusion, asthmatic mice showed altered vascular reactivity and systemic inflammation, with AD aerosol further exacerbating these effects.

Involvement of COX-1 in A3 adenosine receptor-mediated contraction through endothelium in mice aorta

We investigated whether A(3) adenosine receptor (A(3)AR) is involved in endothelium-mediated contraction through cyclooxygenases (COXs) with the use of wild-type (WT) and A(3) knockout (A(3)KO) mice aorta. A(3)AR-selective agonist, Cl-IBMECA, produced a concentration-dependent contraction (EC(50): 2.9 +/- 0.2 x 10(-9) M) in WT mouse aorta with intact endothelium (+E) and negligible effects in A(3)KO +E aorta. At 10(-7) M, contractions produced by Cl-IBMECA were 29% in WT +E, while being insignificant in A(3)KO +E aorta. Cl-IBMECA-induced responses were abolished in endothelium-denuded tissues (-E), in both WT and A(3)KO aorta. A(3)AR gene and protein expression were reduced by 74 and 72% (P < 0.05), respectively, in WT -E compared with WT +E aorta, while being undetected in A(3)KO +E/-E aorta. Indomethacin (nonspecific COXs blocker, 10(-5) M), SC-560 (specific COX-1 blocker, 10(-8) M), SQ 29549 (thromboxane prostanoid receptor antagonist, 10(-6) M), and furegrelate (thromboxane synthase inhibitor, 10(-5) M) inhibited Cl-IBMECA-induced contraction significantly. Cl-IBMECA-induced thromboxane B(2) production was also attenuated significantly by indomethacin, SC-560, and furegrelate in WT +E aorta, while having negligible effects in A(3)KO +E aorta. NS-398 (specific COX-2 blocker) produced negligible inhibition of Cl-IBMECA-induced contraction in both WT +E and A(3)KO +E aorta. Cl-IBMECA-induced increase in COX-1 and thromboxane prostanoid receptor expression were significantly inhibited by MRS1523, a specific A(3)AR antagonist in WT +E aorta. Expression of both A(3)AR and COX-1 was located mostly on endothelium of WT and A(3)KO +E aorta. These results demonstrate for the first time the involvement of COX-1 pathway in A(3)AR-mediated contraction via endothelium.

Adenosine regulation of alveolar fluid clearance

Adenosine is a purine nucleoside that regulates cell function through G protein-coupled receptors that activate or inhibit adenylyl cyclase. Based on the understanding that cAMP regulates alveolar epithelial active Na(+) transport, we hypothesized that adenosine and its receptors have the potential to regulate alveolar ion transport and airspace fluid content. Herein, we report that type 1 (A(1)R), 2a (A(2a)R), 2b (A(2b)R), and 3 (A(3)R) adenosine receptors are present in rat and mouse lungs and alveolar type 1 and 2 epithelial cells (AT1 and AT2). Rat AT2 cells generated and produced cAMP in response to adenosine, and micromolar concentrations of adenosine were measured in bronchoalveolar lavage fluid from mice. Ussing chamber studies of rat AT2 cells indicated that adenosine affects ion transport through engagement of A(1)R, A(2a)R, and/or A(3)R through a mechanism that increases CFTR and amiloride-sensitive channel function. Intratracheal instillation of low concentrations of adenosine (< or =10(-8)M) or either A(2a)R- or A(3)R-specific agonists increased alveolar fluid clearance (AFC), whereas physiologic concentrations of adenosine (> or =10(-6)M) reduced AFC in mice and rats via an A(1)R-dependent pathway. Instillation of a CFTR inhibitor (CFTR(inh-172)) attenuated adenosine-mediated down-regulation of AFC, suggesting that adenosine causes Cl(-) efflux by means of CFTR. These studies report a role for adenosine in regulation of alveolar ion transport and fluid clearance. These findings suggest that physiologic concentrations of adenosine allow the alveolar epithelium to counterbalance active Na(+) absorption with Cl(-) efflux through engagement of the A(1)R and raise the possibility that adenosine receptor ligands can be used to treat pulmonary edema.

Enhanced airway reactivity and inflammation in A2A adenosine receptor-deficient allergic mice

A(2A) adenosine receptor (A(2A)AR) has potent anti-inflammatory properties, which may be important in the regulation of airway reactivity and inflammation. Inflammatory cells that possess A(2A)AR also produce nitrosative stress, which is associated with pathophysiology of asthma, so we hypothesized that A(2A)AR deficiency may lead to increased airway reactivity and inflammation through nitrosative stress. Thus the present study was carried out to investigate the role of A(2A)AR on airway reactivity, inflammation, NF-kappaB signaling, and nitrosative stress in A(2A)AR knockout (KO) and wild-type (WT) mice using our murine model of asthma. Animals were sensitized intraperitoneally on days 1 and 6 with 200 microg of ragweed, followed by aerosolized challenges with 0.5% ragweed on days 11, 12, and 13, twice a day. On day 14, airway reactivity to methacholine was assessed as enhanced pause (Penh) using whole body plethysmography followed by bronchoalveolar lavage (BAL) and lung collection for various analyses. Allergen challenge caused a significant decrease in expression of A(2A)AR in A(2A) WT sensitized mice, with A(2A)AR expression being undetected in A(2A) KO sensitized group leading to decreased lung cAMP levels in both groups. A(2A)AR deletion/downregulation led to an increase in Penh to methacholine and influx of total cells, eosinophils, lymphocytes, and neutrophils in BAL with highest values in A(2A) KO sensitized group. A(2A) KO sensitized group further had increased NF-kappaB expression and nitrosative stress compared with WT sensitized group. These data suggest that A(2A)AR deficiency leads to airway inflammation and airway hyperresponsiveness, possibly via involvement of nitrosative stress in this model of asthma.

Synthesis of New 2-Phenyladenines and 2-Phenylpteridines and Biological Evaluation as Adenosine Receptor Ligands

Synthesis and biological assays of a series of 2-phenylpteridine derivatives are described to compare their affinities to adenosine receptors with those of the corresponding adenines, purposely prepared, and 8-azaadenines previously described. This study demonstrates that the enlargement of the five-membered ring of the adenine nucleus to a six-membered one is a modification that does not allow the molecules to maintain high activity towards adenosine receptors; in fact, pteridine derivatives did not show themselves to be good adenosine receptor ligands. On the contrary, N(6)-cycloalkyl- or N(6)-alkyl-2-phenyladenines showed a very high affinity and selectivity for A(1) adenosine receptors. We demonstrate also that the 9-benzyl substituent is crucial for conferring high affinity for A(3) receptors to molecules having a 2-phenyladenine-like nucleus.

A2B adenosine receptors induce IL-19 from bronchial epithelial cells, resulting in TNF-alpha increase

Adenosine is a signaling nucleoside that has been proposed to contribute to the pathogenesis of asthma and chronic obstructive pulmonary disease. Previous studies suggest that adenosine might play an important role in modulating levels of inflammatory mediators in the lung. Because airway epithelium is an important cellular source of inflammatory mediators, the objective of the present study was to determine whether adenosine affects the expression and release of inflammatory cytokines from human bronchial epithelial cells (HBECs). Among the four subtypes of adenosine receptors, the A(2B) receptor was expressed at the highest level. 5'-(N-ethylcarboxamido)-adenosine (NECA), a stable analog of adenosine, increased the release of IL-19 by 4.6- +/- 1.1-fold. A selective antagonist of the A(2B) receptor, CVT-6694, attenuated this effect of NECA. The amount of IL-19 released from HBEC was sufficient to activate a human monocytic cell line (THP-1) and increase the release of TNF-alpha. Furthermore, TNF-alpha was found to upregulate A(2B) receptor expression in HBECs by 3.1- +/- 0.3-fold. Hence, these data indicate that NECA increases the release of IL-19 from HBECs via activation of A(2B) receptors, and IL-19 in turn activates human monocytes to release TNF-alpha, which upregulates A(2B) receptor expression in HBECs. The results of this study suggest that there is a novel pathway whereby adenosine can initiate and amplify an inflammatory response which might be important in pathogenesis of inflammatory lung diseases.

Tecadenoson: a novel, selective A1 adenosine receptor agonist

Tecadenoson is a novel selective A1 adenosine receptor agonist that is currently being evaluated for the conversion of paroxysmal supraventricular tachycardia (PSVT) to sinus rhythm. By selectively targeting the A1 receptor, tecadenoson may be associated with fewer adverse effects such as flushing, dyspnea, chest discomfort, and hypotension than adenosine, which is a nonselective agonist of all 4 adenosine receptors. Based on the results of phase I and phase II clinical trials, tecadenoson appears to be an effective agent for producing rapid and sustained conversion of PSVT to sinus rhythm. Additionally, the adverse effects that are typically attributed to adenosine's nonselective stimulation of the A2A, A2B, and A3 receptors appear to occur less frequently with the use of tecadenoson. Tecadenoson also appears to be associated with a lower incidence of atrial fibrillation following conversion of PSVT compared with the rates that have been associated with adenosine in the literature. A randomized, prospective trial will need to be conducted in the future to appropriately compare the safety and efficacy of tecadenoson and adenosine.

Adenosine A1 receptors mediate mobilization of calcium in human bronchial smooth muscle cells

Adenosine stimulates contraction of airway smooth muscle, but the mechanism is widely considered indirect, depending on release of contractile agonists from mast cells and nerves. The goal was to determine whether adenosine, by itself, directly regulates calcium signaling in human bronchial smooth muscle cells (HBSMC). Primary cultures of HBSMC from normal subjects were loaded with fura 2-AM, and cytosolic calcium concentrations ([Ca(2+)](i)) were determined ratiometrically by imaging single cells. The nonselective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA), and the adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine (CPA), both stimulated rapid, transient increases in [Ca(2+)](i). In contrast, there were no calcium responses to 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine (100 nM) or N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (100 nM), selective agonists at adenosine A(2A) receptors and adenosine A(3) receptors, respectively. Calcium responses to NECA and CPA were inhibited by 8-cyclopentyl-1,3-dipropylxanthine, an adenosine A(1) receptor antagonist, and by pertussis toxin (PTX). In other experiments, NECA stimulated calcium transients in the absence of extracellular calcium, but not when cells were preincubated in cyclopiazonic acid or thapsigargin to empty intracellular calcium stores. Calcium responses were attenuated by xestospongin C and 2-aminoethoxydiphenylborane, inhibitors of inositol trisphosphate (IP(3)) receptors, and by U73122, an inhibitor of phospholipase C. It was concluded that stimulation of adenosine A(1) receptors on HBSMC rapidly mobilizes intracellular calcium stores by a mechanism dependent on PTX-sensitive G proteins, and IP(3) signaling. These findings suggest that, in addition to its well-established indirect effects on HBSMC, adenosine also has direct effects on contractile signaling pathways.

Mast cell involvement in the adenosine mediated airway hyper-reactivity in a murine model of ovalbumin-induced lung inflammation

Airway hyper-reactivity to inhaled adenosine, mediated via mast cell activation, is a cardinal feature of asthma. Animal models have been developed in several species to mimic this phenomenon, but only in the rat has a mast cell involvement been clearly defined. In this study, a model of ovalbumin-induced adenosine hyper-reactivity was developed in BALB/c mice to determine whether mast cells are involved in this phenomenon. Sensitised mice were challenged one, two or three times, on a daily basis, and airway responses to the stable adenosine analogue NECA (5'-N-ethylcarboxamido adenosine) determined 4 and 24 h after each challenge. Airway hyper-reactivity was observed in ovalbumin-challenged mice 4 h after a single challenge and to a minor extent 24 h after a single challenge and 4 h after two challenges. Cromolyn (20 mg ml(-1)), given by aerosol an hour before the NECA provocation, fully inhibited the airway hyper-reactivity observed 4 h after a single allergen challenge, suggesting a role for mast cells in this response. The airway space cellular inflammation was not affected by cromolyn. As observed in human asthma, an acute treatment with steroid (budesonide 3 mg kg(-1), given an hour before the allergen challenge) inhibited the NECA airway hyper-reactivity and significantly inhibited the airway space cellular inflammation. These data suggest that the ovalbumin-challenged BALB/c mice can be considered as a suitable model to study the adenosine-induced airway hyper-reactivity phenomenon observed in human asthma.

Effect of adenosine A2A receptor activation in murine models of respiratory disorders

Activation of the adenosine A(2A) receptor has been postulated as a possible treatment for lung inflammatory diseases such as asthma and chronic obstructive pulmonary disease (COPD). In this report, we have studied the anti-inflammatory properties of the reference A(2A) agonist CGS-21680, given intranasally at doses of 10 and 100 microg/kg, in a variety of murine models of asthma and COPD. After an acute ovalbumin challenge of sensitized mice, prophylactic administration of CGS-21680 inhibited the bronchoalveolar lavage fluid inflammatory cell influx but not the airway hyperreactivity to aerosolized methacholine. After repeated ovalbumin challenges, CGS-21680 given therapeutically inhibited the bronchoalveolar lavage fluid inflammatory cell influx but had no effect on the allergen-induced bronchoconstriction, the airway hyperreactivity, or the bronchoalveolar lavage fluid mucin levels. As a comparator, budesonide given intranasally at doses of 0.1-1 mg/kg fully inhibited all the parameters measured in the latter model. In a lipopolysaccharide-driven model, CGS-21680 had no effect on the bronchoalveolar lavage fluid inflammatory cell influx or TNF-alpha, keratinocyte chemoattractant, and macrophage inflammatory protein-2 levels, but potently inhibited neutrophil activation, as measured by bronchoalveolar lavage fluid elastase levels. With the use of a cigarette smoke model of lung inflammation, CGS-21680 did not significantly inhibit bronchoalveolar lavage fluid neutrophil infiltration but reversed the cigarette smoke-induced decrease in macrophage number. Together, these results suggest that activation of the A(2A) receptor would have a beneficial effect by inhibiting inflammatory cell influx and downregulating inflammatory cell activation in asthma and COPD, respectively.

Pharmacological characterisation of the adenosine receptor mediating increased ion transport in the mouse isolated trachea and the effect of allergen challenge

The effect of adenosine on transepithelial ion transport was investigated in isolated preparations of murine trachea mounted in Ussing chambers. The possible regulation of adenosine receptors in an established model of allergic airway inflammation was also investigated. Mucosally applied adenosine caused increases in short-circuit current (I(SC)) that corresponded to approximately 50% of the response to the most efficacious secretogogue, ATP (delta I(SC) 69.5 +/- 6.7 microA cm2). In contrast, submucosally applied adenosine caused only small (<20%) increases in I(SC), which were not investigated further. The A1-selective (N6-cyclopentyladenosine, CPA, 1 nM-10 microM), A2A-selective (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxoamido adenosine; CGS 21680; 0.1-100 microM) and A3-selective (1-deoxy-1-[6-[[(3-iodophenyl)-methyl]amino]-9H-purin-9-yl]-N-methyl-beta-D-ribofuranuronamide; IB-MECA; 30 nM-100 microM) adenosine receptor agonists were either equipotent or less potent than adenosine, suggesting that these receptors do not mediate the response to adenosine. The A1 receptor selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 nM-1 microM) caused a rightward shift of the adenosine concentration-effect curve only at 1 microM. The mixed A2A/A2B receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) also caused rightward shift of the adenosine concentration-effect curve, again only at micromolar concentrations, suggestive of the involvement of A2B receptors. In preparations from animals sensitised to ovalbumin and challenged over 3 days with aerosol ovalbumin, a decrease in baseline I(SC) was observed and responses to ATP were diminished. Similarly, the amplitude of responses to adenosine were attenuated although there was no change in potency. These results suggest that the A2B receptor mediates the I(SC) response to adenosine in the mouse trachea. This receptor does not appear to be regulated in a standard asthma model.

The Quintiles Prize Lecture 2004. The identification of the adenosine A2B receptor as a novel therapeutic target in asthma

Adenosine is a powerful bronchoconstrictor of asthmatic, but not normal, airways. In vitro studies on isolated human mast cells and basophils revealed that adenosine and selective analogues augmented inflammatory mediator release from mast cells by stimulating A(2) receptors. Pharmacological blockade of mast cell mediator release in vivo also attenuated adenosine-induced bronchoconstriction, as did theophylline, by adenosine A(2) receptor antagonism. Further in vitro studies revealed that the asthmatic response to adenosine is likely to be mediated via the A(2B) subtype which is selectively antagonised by enprofylline. Studies in animal models, especially mice, have shown a close synergistic interaction between adenosine, Th2 and airway remodelling responses. The recent description of A(2B) receptors on human airway smooth muscle cells that mediate cytokine and chemokine release and induce differentiation of fibroblasts into myofibroblasts strengthens the view that adenosine maybe more than an inflammatory mediator in asthma but also participates in airway wall remodelling in this disease. These data have provided a firm basis for developing adenosine A(2B) receptor antagonists as a new therapeutic approach to this disease.

Role of adenosine in airway inflammation in an allergic mouse model of asthma

In the present study, we examined dynamic changes in cellular profile of bronchoalveolar lavage (BAL) fluid after adenosine challenge in ragweed sensitized and challenged mice. Mice systemically sensitized and airway challenged with ragweed showed marked airway inflammation manifesting increased eosinophils, lymphocytes, neutrophils and activated macrophages in BAL. Adenosine challenge further enhanced influx of inflammatory cells into BAL, notably neutrophils from 1 to 72 h and eosinophils from 1 to 48 h time-points (p<0.05), which sharply rose at 6-h time-point following adenosine challenge. Greater infiltration of lymphocytes into BAL was observed at 1 and 72 h and macrophages from 6 to 72 h (p<0.05) after adenosine challenge. Accordingly, markers of eosinophils, neutrophils and mast cells were analyzed at 6-h time-point after adenosine challenge. Adenosine challenge significantly increased the levels of eosinophil peroxidase, neutrophil myeloperoxidase and beta-hexosaminidase in BAL. There were more significant effects of adenosine challenge on the degranulation of mast cells in the lung than that in blood. The chemoattractant, eotaxin, was detected in BAL, which increased after adenosine challenge. Theophylline, a non-specific adenosine receptor antagonist, prevented adenosine-enhanced infiltration of inflammatory cells and their respective markers. Our findings suggest that adenosine plays an important role in airway inflammation in an allergic mouse model.

Trial to evaluate the management of paroxysmal supraventricular tachycardia during an electrophysiology study with tecadenoson

BACKGROUND

Tecadenoson is a potent selective A1-adenosine receptor agonist with a dose-dependent negative dromotropic effect on the AV node. Tecadenoson terminates induced paroxysmal supraventricular tachycardia (PSVT) without the clinically significant side effects caused by stimulation of other adenosine receptors. This trial was designed to determine a safe and effective tecadenoson bolus for termination of electrophysiologically induced PSVT.

METHODS AND RESULTS

Patients with a history of symptomatic PSVT and inducible PSVT at the time of a clinically indicated electrophysiology study were randomized into a multicenter, double-blind, placebo-controlled trial. Five 2-dose tecadenoson bolus regimens were evaluated versus placebo (75/150, 150/300, 300/600, 450/900, 900 microg/900 microg). The second bolus was administered only if PSVT persisted for 1 minute after the first bolus. Each tecadenoson regimen resulted in a significant therapeutic conversion rate compared with placebo (range, 50.0% to 90.3%, analysis of all patients dosed; n=181; P<0.0005). Conversion by the first bolus was dose related (range: placebo, 3.3% to 86.7% for 900 microg/900 microg). Time to conversion was dose dependent, with a median time of <1 minute for the 3 highest dose regimens. Postconversion arrhythmias were transient, requiring no additional treatment in 4 regimens (including placebo). Transient second- and third-degree heart block occurred at higher doses (300/600, 450/900, 900 microg/900 microg) and was supported with backup pacing when needed. No effect on blood pressure was observed. Ten patients with a history of asthma or chronic obstructive pulmonary disease tolerated tecadenoson without bronchospasm.

CONCLUSIONS

We identified an optimal tecadenoson regimen (300 microg/600 microg) that effectively and rapidly converted 90% (28 of 31) of PSVT patients to normal sinus rhythm with no significant adverse effects.

Involvement of mast cells in adenosine-mediated bronchoconstriction and inflammation in an allergic mouse model

In allergen-induced asthma, activation of lung mast cells leads to bronchial constriction, increased mucus secretion, and an increase in the localization of inflammatory cells to the airways. The purpose of this study was to explore the role of mast cells in adenosine-mediated airway reactivity and inflammation using the mast cell degranulating agent, compound 48/80 (C48/80). Mice were sensitized and challenged with ragweed (or 0.9% saline) followed by C48/80 administration twice a day in increasing doses for 5 days. Dose-responsiveness to the nonspecific adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) was established, and lung lavage was performed 24 h later for cell differential analysis to evaluate inflammation. At a dose of 375 microg/ml (aerosolized NECA), C48/80 pretreatment resulted in a significant attenuation in airway reactivity when compared with sensitized control mice (330.07 versus 581.57%, respectively). Lung lavage from the C48/80 treated mice showed a decrease in eosinophils (17.7 versus 60.9%, respectively) and an increase in macrophages when compared with the sensitized control group (76.4 versus 30.8%, respectively). These results support the conclusion that mast cell degranulation plays an important role in adenosine receptor-mediated airway hyperresponsiveness and inflammation.

Structure-activity relationships of thiazole and thiadiazole derivatives as potent and selective human adenosine A3 receptor antagonists

4-(4-Methoxyphenyl)-2-aminothiazole and 3-(4-methoxyphenyl)-5-aminothiadiazole derivatives have been synthesized and evaluated as selective antagonists for human adenosine A3 receptors. A methoxy group in the 4-position of the phenyl ring and N-acetyl or propionyl substitutions of the aminothiazole and aminothiadiazole templates displayed great increases of binding affinity and selectivity for human adenosine A3 receptors. The most potent A3 antagonist of the present series, N-[3-(4-methoxy-phenyl)-[1,2,4]thiadiazol-5-yl]-acetamide (39) exhibiting a Ki value of 0.79 nM at human adenosine A3 receptors, showed antagonistic property in a functional assay of cAMP biosynthesis involved in one of the signal transduction pathways of adenosine A3 receptors. Molecular modeling study of conformation search and receptor docking experiments to investigate the dramatic differences of binding affinities between two regioisomers of thiadiazole analogues, (39) and (42), suggested possible binding mechanisms in the binding pockets of adenosine receptors.