Characterization of the potency, selectivity, and pharmacokinetic profile for six adenosine A2A receptor antagonists

Adenosine and adenosine receptors in metabolic imbalance-related neurological issues

Metabolic syndromes (e.g., obesity) are characterized by insulin resistance, chronic inflammation, impaired glucose metabolism, and dyslipidemia. Recently, patients with metabolic syndromes have experienced not only metabolic problems but also neuropathological issues, including cognitive impairment. Several studies have reported blood-brain barrier (BBB) disruption and insulin resistance in the brain of patients with obesity and diabetes. Adenosine, a purine nucleoside, is known to regulate various cellular responses (e.g., the neuroinflammatory response) by binding with adenosine receptors in the central nervous system (CNS). Adenosine has four known receptors: A1R, A2AR, A2BR, and A3R. These receptors play distinct roles in various physiological and pathological processes in the brain, including endothelial cell homeostasis, insulin sensitivity, microglial activation, lipid metabolism, immune cell infiltration, and synaptic plasticity. Here, we review the recent findings on the role of adenosine receptor-mediated signaling in neuropathological issues related to metabolic imbalance. We highlight the importance of adenosine signaling in the development of therapeutic solutions for neuropathological issues in patients with metabolic syndromes.

Recording of hippocampal activity on the effect of convulsant doses of caffeine

Seizures occur when there is a hyper-excitation of the outer layer of the brain, with subsequent excessive synchrony in a group of neurons. According to the World Health Organization (WHO), an estimated 50 million people are affected by this disease, a third of whom are resistant to the treatments available on the market. Caffeine (1,3,7-trimethylxanthine), which belongs to the purine alkaloid family, is the most widely consumed psychoactive drug in the world. It is ingested by people through drinks containing this substance, such as coffee, and as an adjuvant in analgesic therapy with non-steroidal antiflammatory drugs. The present study evaluated the electrocorticographic changes observed in the hippocampus of Wistar rats subjected to acute doses of caffeine (150 mg/kg i.p), which represents a toxic dose of caffeine corresponding to an estimated acute intake of more than 12 cups of coffee to record its convulsant activity. Our results showed, for the first time, that the administration of high doses of caffeine (150 mg/kg i.p.) in rats caused an increase in the spectral distribution of power in all frequency bands and suggested the appearance of periods of ictal and interictal peaks in the electrocorticogram (ECog). We have also shown that the anticonvulsants phenytoin, diazepam and phenobarbital have a satisfactory response when associated with caffeine.

Adenosine A2a receptor inhibition increases the anti-tumor efficacy of anti-PD1 treatment in murine hepatobiliary cancers

Backgrounds & Aims

The efficacy of immune checkpoint inhibitor (ICI) therapy for liver cancer remains limited. As the hypoxic liver environment regulates adenosine signaling, we tested the efficacy of adenosine A2a receptor (A2aR) inhibition in combination with ICI treatment in murine models of liver cancer.

Methods

RNA expression related to the adenosine pathway was analyzed from public databases. Peripheral blood mononuclear cells of 13 patients with hepatocellular carcinoma (HCC) were examined by flow cytometry. The following murine cell lines were used: SB-1, RIL175, and Hep55.1c (liver cancer), CT26 (colon cancer), and B16-F10 (melanoma). C57BL/6 and BALB/c mice were used for orthotopic tumor models and were treated with SCH58261, an A2aR inhibitor, in combination with anti-PD1 therapy.

Results

RNA expression of ADORA2A in tumor tissues derived from patients with HCC was higher than in tissues from other cancer types. A2aR+ T cells in peripheral blood from patients with HCC were highly proliferative after immunotherapy. Likewise, in an orthotopic murine model, A2aR expression on T cells increased following anti-PD1 treatment, and the expression of A2aR on T cells increased more in tumor-bearing mice compared with tumor-free mice. The combination of SCH58261 and anti-PD1 led to activation of T cells and reductions in tumor size in orthotopic liver cancer models. In contrast, SCH58261 monotherapy was ineffective in orthotopic liver cancer models and the combination was ineffective in the subcutaneous tumor models tested. CD4+ T-cell depletion attenuated the efficacy of the combination therapy.

Conclusion

A2aR inhibition and anti-PD1 therapy had a synergistic anti-tumor effect in murine liver cancer models.

Impact and implications

Adenosine A2a receptor (A2aR)-expressing T cells in the liver increased in tumor-bearing mice and after anti-PD1 treatment. The combination of an A2aR inhibitor and anti-PD1 treatment had potent anti-tumor effects in two murine models of orthotopic liver cancer. Adenosine A2a receptor blockade promotes immunotherapy efficacy in murine models, highlighting putative clinical benefits for advanced stage liver cancer patients.

The adenosine A (2A) receptor antagonist SCH58261 protects photoreceptors by inhibiting microglial activation and the inflammatory response

Photoreceptor degeneration is a principal event in a variety of human retinal diseases. Progressive apoptosis of photoreceptors leads to impaired vision and blindness, for which there is no curative treatment. Adenosine 2A receptors (A_2AR) are expressed in microglia. Blockade of A_2AR has been shown to protect neurons via suppression of inflammation. However, the therapeutic effects of A_2AR antagonists on photoreceptor degeneration have not been characterized. In this study, adult zebrafish were exposed to short term high-intensity light to induce photoreceptor death. SCH58261, a selective A_2AR antagonist, was immediately injected into the vitreous body. Photoreceptor degeneration and microglia-induced inflammation were evaluated using immunohistochemistry, quantitative real-time polymerase chain reaction, polarization sensitive optical coherence tomography, and optomotor response. Co-culture of BV2 and 661W cells was used to investigate the interaction between microglia and photoreceptors. The results showed that A_2AR was over-expressed during photoreceptor degeneration. Following intraocular SCH58261 injection, microglial activation and release of inflammatory factors were inhibited, and photoreceptor survival increased. Inactivation of microglia prevented apoptosis and autophagy in photoreceptors. Our results showed that SCH58261 intervention at the early stage of photoreceptor degeneration protected photoreceptors through inhibition of the inflammatory response, apoptosis, and autophagy.

Adenosine A2A receptors control synaptic remodeling in the adult brain

The molecular mechanisms underlying circuit re-wiring in the mature brain remains ill-defined. An eloquent example of adult circuit remodelling is the hippocampal mossy fiber (MF) sprouting found in diseases such as temporal lobe epilepsy. The molecular determinants underlying this retrograde re-wiring remain unclear. This may involve signaling system(s) controlling axon specification/growth during neurodevelopment reactivated during epileptogenesis. Since adenosine A_2A receptors (A_2AR) control axon formation/outgrowth and synapse stabilization during development, we now examined the contribution of A_2AR to MF sprouting. A_2AR blockade significantly attenuated status epilepticus(SE)-induced MF sprouting in a rat pilocarpine model. This involves A_2AR located in dentate granule cells since their knockdown selectively in dentate granule cells reduced MF sprouting, most likely through the ability of A_2AR to induce the formation/outgrowth of abnormal secondary axons found in rat hippocampal neurons. These A_2AR should be activated by extracellular ATP-derived adenosine since a similar prevention/attenuation of SE-induced hippocampal MF sprouting was observed in CD73 knockout mice. These findings demonstrate that A_2AR contribute to epilepsy-related MF sprouting, most likely through the reactivation of the ability of A_2AR to control axon formation/outgrowth observed during neurodevelopment. These results frame the CD73-A_2AR axis as a regulator of circuit remodeling in the mature brain.

Adenosine Signaling and Clathrin-Mediated Endocytosis of Glutamate AMPA Receptors in Delayed Hypoxic Injury in Rat Hippocampus: Role of Casein Kinase 2

Chronic adenosine A1R stimulation in hypoxia leads to persistent hippocampal synaptic depression, while unopposed adenosine A2AR receptor stimulation during hypoxia/reperfusion triggers adenosine-induced post-hypoxia synaptic potentiation (APSP) and increased neuronal death. Still, the mechanisms responsible for this adenosine-mediated neuronal damage following hypoxia need to be fully elucidated. We tested the hypothesis that A1R and A2AR regulation by protein kinase casein kinase 2 (CK2) and clathrin-dependent endocytosis of AMPARs both contribute to APSPs and neuronal damage. The APSPs following a 20-min hypoxia recorded from CA1 layer of rat hippocampal slices were abolished by A1R and A2AR antagonists and by broad-spectrum AMPAR antagonists. The inhibitor of GluA2 clathrin-mediated endocytosis Tat-GluA2-3Y peptide and the dynamin-dependent endocytosis inhibitor dynasore both significantly inhibited APSPs. The CK2 antagonist DRB also inhibited APSPs and, like hypoxic treatment, caused opposite regulation of A1R and A2AR surface expression. APSPs were abolished when calcium-permeable AMPAR (CP-AMPAR) antagonist (IEM or philanthotoxin) or non-competitive AMPAR antagonist perampanel was applied 5 min after hypoxia. In contrast, perampanel, but not CP-AMPAR antagonists, abolished APSPs when applied during hypoxia/reperfusion. To test for neuronal viability after hypoxia, propidium iodide staining revealed significant neuroprotection of hippocampal CA1 pyramidal neurons when pretreated with Tat-GluA2-3Y peptide, CK2 inhibitors, dynamin inhibitor, CP-AMPAR antagonists (applied 5 min after hypoxia), and perampanel (either at 5 min hypoxia onset or during APSP). These results suggest that the A1R-CK2-A2AR signaling pathway in hypoxia/reperfusion injury model mediates increased hippocampal synaptic transmission and neuronal damage via calcium-permeable AMPARs that can be targeted by perampanel for neuroprotective stroke therapy.

Tumor Immunotherapy Using A2A Adenosine Receptor Antagonists

The A_2A adenosine receptor (A_2AAR) plays critical roles in human physiology and pathophysiology, which makes it an important drug target. Previous drug-discovery efforts targeting the A_2AAR have been focused on the use of A_2AAR antagonists for the treatment of Parkinson's disease. More recently, the A_2AAR has attracted additional attention for its roles in immuno-oncology, and a number of A_2AAR antagonists are currently used as lead compounds for antitumor drugs in both preclinical models and clinical trials. This review surveys recent advances in the development of A_2AAR antagonists for cancer immunotherapy. The therapeutic potential of representative A_2AAR antagonists is discussed based on both animal efficacy studies and clinical data.

Prolonged caffeine intake decreases alveolar bone damage induced by binge-like ethanol consumption in adolescent female rats

Ethanol consumption has been reported to negatively impact on periodontal disease. In particular, oral cavity disorders occur upon ethanol exposure during adolescence, a life period associated with particular patterns of short and intense ('binge-like') ethanol consumption that is most deleterious to oral health. The hazardous central effects of ethanol have been linked to the overfunction of adenosine receptors, which are antagonized by caffeine, a bioactive substance present in numerous natural nutrients, which can also modify bone metabolism. The aim of this study was to investigate the effects of caffeine on alveolar bone damage induced by an ethanol binge drinking paradigm during adolescence. Female Wistar rats (35 days old; n = 30) were allocated to six groups: control (vehicle), ethanol (3 g/kg/day; 3 days On-4 days Off challenge), caffeine (10 mg/kg/day), caffeine plus ethanol, SCH58261 (0.1 mg/kg/day, an antagonist of A_2A receptors), and SCH58261 plus ethanol. Bone micromorphology and vertical bone loss were analyzed by computed microtomography. Our data showed that ethanol binge drinking reduced alveolar bone quality, with repercussion on alveolar bone size. This ethanol-induced alveolar bone deterioration was abrogated upon treatment with caffeine, but not with SCH58261. This shows that caffeine prevented the periodontal disorder caused by ethanol binge drinking during adolescence, an effect that was not mediated by adenosine A_2A receptor blockade.

Age-related shift in LTD is dependent on neuronal adenosine A2A receptors interplay with mGluR5 and NMDA receptors

Synaptic dysfunction plays a central role in Alzheimer's disease (AD), since it drives the cognitive decline. An association between a polymorphism of the adenosine A2A receptor (A2AR) encoding gene-ADORA2A, and hippocampal volume in AD patients was recently described. In this study, we explore the synaptic function of A2AR in age-related conditions. We report, for the first time, a significant overexpression of A2AR in hippocampal neurons of aged humans, which is aggravated in AD patients. A similar profile of A2AR overexpression in rats was sufficient to drive age-like memory impairments in young animals and to uncover a hippocampal LTD-to-LTP shift. This was accompanied by increased NMDA receptor gating, dependent on mGluR5 and linked to enhanced Ca2+ influx. We confirmed the same plasticity shift in memory-impaired aged rats and APP/PS1 mice modeling AD, which was rescued upon A2AR blockade. This A2AR/mGluR5/NMDAR interaction might prove a suitable alternative for regulating aberrant mGluR5/NMDAR signaling in AD without disrupting their constitutive activity.

Role of Adenosine in Epilepsy and Seizures

Adenosine is an endogenous anticonvulsant and neuroprotectant of the brain. Seizure activity produces large quantities of adenosine, and it is this seizure-induced adenosine surge that normally stops a seizure. However, within the context of epilepsy, adenosine plays a wide spectrum of different roles. It not only controls seizures (ictogenesis), but also plays a major role in processes that turn a normal brain into an epileptic brain (epileptogenesis). It is involved in the control of abnormal synaptic plasticity and neurodegeneration and plays a major role in the expression of comorbid symptoms and complications of epilepsy, such as sudden unexpected death in epilepsy (SUDEP). Given the important role of adenosine in epilepsy, therapeutic strategies are in development with the goal to utilize adenosine augmentation not only for the suppression of seizures but also for disease modification and epilepsy prevention, as well as strategies to block adenosine A2A receptor overfunction associated with neurodegeneration. This review provides a comprehensive overview of the role of adenosine in epilepsy.

Assessment of Pharmacokinetics and Metabolism Profiles of SCH 58261 in Rats Using Liquid Chromatography-Mass Spectrometric Method

5-Amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine (SCH 58261) is one of the new chemical entities that has been developed as an adenosine A_2A receptor antagonist. Although SCH 58261 has been reported to be beneficial, there is little information about SCH 58261 from a drug metabolism or pharmacokinetics perspective. This study describes the metabolism and pharmacokinetic properties of SCH 58261 in order to understand its behaviors in vivo. Rats were used as the in vivo model species. First, an LC-MS/MS method was developed for the determination of SCH 58261 in rat plasma. A GastroPlus™ simulation, in vitro microsomal metabolic stability, and bile duct-cannulated studies were also performed to understand its pharmacokinetic profile. The parameter sensitivity analysis of GastroPlus™ was used to examine the factors that influence exposure when the drug is orally administered. The factors are as follows: permeability, systemic clearance, renal clearance, and liver first-pass effect. In vitro microsomal metabolic stability indicates how much the drug is metabolized. The extrapolated hepatic clearance value of SCH 58261 was 39.97 mL/min/kg, indicating that the drug is greatly affected by hepatic metabolism. In vitro microsomal metabolite identification studies revealed that metabolites produce oxidized and ketone-formed metabolites via metabolic enzymes in the liver. The bile duct-cannulated rat study, after oral administration of SCH 58261, showed that a significant amount of the drug was excreted in feces. These results imply that the drug is not absorbed well in the body after oral administration. Taken together, SCH 58261 showed quite a low bioavailability when administered orally and this was likely due to significantly limited absorption, as well as high metabolism in vivo.

Hippocampal synaptic dysfunction in the SOD1G93A mouse model of Amyotrophic Lateral Sclerosis: Reversal by adenosine A2AR blockade

Amyotrophic Lateral Sclerosis (ALS) mostly affects motor neurons, but non-motor neural and cognitive alterations have been reported in ALS mouse models and patients. Here, we evaluated if time-dependent biphasic changes in synaptic transmission and plasticity occur in hippocampal synapses of ALS SOD1^G93A mice. Recordings were performed in hippocampal slices of SOD1^G93A and age-matched WT mice, in the pre-symptomatic and symptomatic stages. We found an enhancement of pre-synaptic function and increased adenosine A_2A receptor levels in the hippocampus of pre-symptomatic mice. In contrast, in symptomatic mice, there was an impairment of long-term potentiation (LTP) and a decrease in NMDA receptor-mediated synaptic currents, with A_2AR levels also being increased. Chronic treatment with the A_2AR antagonist KW-6002, rescued LTP and A_2AR values. Altogether, these findings suggest an increase in synaptic function during the pre-symptomatic stage, followed by a decrease in synaptic plasticity in the symptomatic stage, which involves over-activation of A_2AR from early disease stages.

Keep an eye on adenosine: Its role in retinal inflammation

Adenosine is an endogenous purine nucleoside ubiquitously distributed throughout the body that interacts with G protein-coupled receptors, classified in four subtypes: A₁R, A_2AR, A_2BR and A₃R. Among the plethora of functions of adenosine, it has been increasingly recognized as a key mediator of the immune response. Neuroinflammation is a feature of chronic neurodegenerative diseases and contributes to the pathophysiology of several retinal degenerative diseases. Animal models of retinal diseases are helping to elucidate the regulatory roles of adenosine receptors in the development and progression of those diseases. Mounting evidence demonstrates that the adenosinergic system is altered in the retina during pathological conditions, compromising retinal physiology. This review focuses on the roles played by adenosine and the elements of the adenosinergic system (receptors, enzymes, transporters) in the neuroinflammatory processes occurring in the retina. An improved understanding of the molecular and cellular mechanisms of the signalling pathways mediated by adenosine underlying the onset and progression of retinal diseases will pave the way towards the identification of new therapeutic approaches.

Caffeine Exacerbates Postictal Hypoxia

A stroke-like event follows seizures which may be responsible for the postictal state and a contributing factor to the development of seizure-induced brain abnormalities and behavioral dysfunction associated with epilepsy. Caffeine is the world's most popular drug with ∼85% of people in the USA consuming it daily. Thus, persons with epilepsy are likely to have caffeine in their body and brain during seizures. This preclinical study investigated the effects of acute caffeine on local hippocampal tissue oxygenation pre and post seizure. We continuously measured local oxygen levels in the CA1 region of the hippocampus and utilized the electrical kindling model in rats. Rats were acutely administered either caffeine, or one of its metabolites, or agonists and antagonists at adenosine sub-receptor types or ryanodine receptors prior to the elicitation of seizures. Acute caffeine administration caused a significant drop in pre-seizure hippocampal pO₂. Following a seizure, caffeine, as well as two of its metabolites paraxanthine, and theophylline, increased the time below the severe hypoxic threshold (10 mmHg). Likewise, the specific A_2A receptor antagonist, SCH-58261, mimicked caffeine by causing a significant drop in pre-seizure pO₂ and the area and time below the severe hypoxic threshold. Moreover, the A_2A receptor agonist, CGS-21680 was able to prevent the effect of both caffeine and SCH-58261 adding further evidence that caffeine is likely acting through the A_2A receptor. Clinical tracking and investigations are needed to determine the effect of caffeine on postictal symptomology and blood flow in persons with epilepsy.

Adenosine 2A receptor inhibition protects phrenic motor neurons from cell death induced by protein synthesis inhibition

Respiratory motor neuron survival is critical for maintenance of adequate ventilation and airway clearance, preventing dependence to mechanical ventilation and respiratory tract infections. Phrenic motor neurons are highly vulnerable in rodent models of motor neuron disease versus accessory inspiratory motor pools (e.g. intercostals, scalenus). Thus, strategies that promote phrenic motor neuron survival when faced with disease and/or toxic insults are needed to help preserve breathing ability, airway defense and ventilator independence. Adenosine 2A receptors (A2A) are emerging as a potential target to promote neuroprotection, although their activation can have both beneficial and pathogenic effects. Since the role of A2A receptors in the phrenic motor neuron survival/death is not known, we tested the hypothesis that A2A receptor antagonism promotes phrenic motor neuron survival and preserves diaphragm function when faced with toxic, neurodegenerative insults that lead to phrenic motor neuron death. We utilized a novel neurotoxic model of respiratory motor neuron death recently developed in our laboratory: intrapleural injections of cholera toxin B subunit (CtB) conjugated to the ribosomal toxin, saporin (CtB-Saporin). We demonstrate that intrapleural CtB-Saporin causes: 1) profound phrenic motor neuron death (~5% survival); 2) ~7-fold increase in phrenic motor neuron A2A receptor expression prior to cell death; and 3) diaphragm muscle paralysis (inactive in most rats; ~7% residual diaphragm EMG amplitude during room air breathing). The A2A receptor antagonist istradefylline given after CtB-Saporin: 1) reduced phrenic motor neuron death (~20% survival) and 2) preserved diaphragm EMG activity (~46%). Thus, A2A receptors contribute to neurotoxic phrenic motor neuron death, an effect mitigated by A2A receptor antagonism.

Incarvillateine produces antinociceptive and motor suppressive effects via adenosine receptor activation

(-)-Incarvillateine (INCA) is a natural product that has garnered attention due to its purported analgesic effects and historical use as a pain reliever in China. α-Truxillic acid monoesters (TAMEs) constitute a class of inhibitors targeting fatty acid binding protein 5 (FABP5), whose inhibition produces analgesia in animal models. The structural similarity between INCA and TAMEs motivated us to assess whether INCA exerts its antinociceptive effects via FABP inhibition. We found that, in contrast to TAMEs, INCA did not exhibit meaningful binding affinities toward four human FABP isoforms (FABP3, FABP4, FABP5 and FABP7) in vitro. INCA-TAME, a putative monoester metabolite of INCA that closely resembles TAMEs also lacked affinity for FABPs. Administration of INCA to mice produced potent antinociceptive effects while INCA-TAME was without effect. Surprisingly, INCA also potently suppressed locomotor activity at the same dose that produces antinociception. The motor suppressive effects of INCA were reversed by the adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine. Collectively, our results indicate that INCA and INCA-TAME do not inhibit FABPs and that INCA exerts potent antinociceptive and motor suppressive effects at equivalent doses. Therefore, the observed antinociceptive effects of INCA should be interpreted with caution.

Endothelial Adora2a Activation Promotes Blood-Brain Barrier Breakdown and Cognitive Impairment in Mice with Diet-Induced Insulin Resistance

Obesity and insulin resistance elicit blood-brain barrier (BBB) breakdown in humans and animal models, but the relative contributions of the two pathologies remain poorly understood. These studies initially addressed the temporal progression of cerebrovascular dysfunction relative to dietary obesity or diet-induced insulin resistance in male mice. Obesity increased BBB permeability to the low molecular weight fluorophore sodium fluorescein (NaFl), whereas diet-induced insulin resistance increased permeability to both NaFl and Evans blue, which forms a high molecular weight complex with serum albumin. Serial section transmission electron microscopy analysis of hippocampal capillaries revealed that diabetes promotes involution of tight junctions, fenestration of endothelial cells, and pericyte regression. Chronic activation of adenosine receptor 2a (Adora2a) erodes tight junctions between endothelial cells of the cerebral vasculature in other models of chronic neuropathology, and we observed that acute Adora2a antagonism normalized BBB permeability in wild-type mice with diet-induced insulin resistance. Experiments in mice with inducible deletion of Adora2a in endothelial cells revealed protection against BBB breakdown with diet-induced insulin resistance, despite comparable metabolic dysfunction relative to nontransgenic littermates. Protection against BBB breakdown was associated with decreased vascular inflammation, recovery of hippocampal synaptic plasticity, and restoration of hippocampus-dependent memory. These findings indicate that Adora2a-mediated signaling in vascular endothelial cells disrupts the BBB in dietary obesity, and implicate cerebrovascular dysfunction as the underlying mechanism for deficits in synaptic plasticity and cognition with obesity and insulin resistance.SIGNIFICANCE STATEMENT The blood-brain barrier (BBB) restricts the entry of circulating factors into the brain, but obesity promotes BBB breakdown in humans and animal models. We used transgenic mice with resistance to BBB breakdown to investigate the role of neurovascular dysfunction in high-fat diet (HFD)-induced cognitive impairment. Transgenic mice with inducible ablation of Adora2a in endothelial cells were protected against BBB breakdown on HFD, despite comparable metabolic impairments relative to normal mice. Transgenic mice were also resistant to HFD-induced cognitive dysfunction and were protected against deficits in hippocampal synaptic plasticity. These findings indicate that Adora2a-mediated signaling in endothelial cells mediates obesity-induced BBB breakdown, and implicate cerebrovascular dysfunction as the mechanism for deficits in synaptic plasticity and cognition with obesity and diabetes.

Agomelatine and tianeptine antidepressant activity in mice behavioral despair tests is enhanced by DMPX, a selective adenosine A2A receptor antagonist, but not DPCPX, a selective adenosine A1 receptor antagonist

BACKGROUND

Adenosine, an endogenous nucleoside, modulates the release of monoamines, e.g., noradrenaline, serotonin, and dopamine in the brain. Both nonselective and selective stimulation of adenosine receptors produce symptoms of depression in some animal models. Therefore, the main objective of our study was to assess the influence of a selective adenosine A₁ receptor antagonist (DPCPX) and a selective adenosine A_2A receptor antagonist (DMPX) on the activity of agomelatine and tianeptine.

METHODS

The forced swim test (FST) and tail suspension test (TST) were performed to assess the effects of DPCPX and DMPX on the antidepressant-like activity of agomelatine and tianeptine. Drug serum and brain levels were analyzed using HPLC.

RESULTS

Co-administration of agomelatine (20 mg/kg) or tianeptine (15 mg/kg) with DMPX (3 mg/kg), but not with DPCPX (1 mg/kg), significantly reduced the immobility time both in the FST and TST in mice. These effects were not associated with an enhancement in animals' spontaneous locomotor activity. The observed changes in the mouse behavior after concomitant injection of DMPX and the tested antidepressant agents were associated with elevated brain concentration of agomelatine and tianeptine.

CONCLUSION

Our study shows a synergistic action of the selective A_2A receptor antagonist and the studied antidepressant drugs, and a lack of such interaction in the case of the selective A₁ receptor antagonist. The interaction between DMPX and agomelatine/tianeptine at least partly occurs in the pharmacokinetic phase. A combination of a selective A_2A receptor antagonist and an antidepressant may be a new strategy for treating depression.

Blockade of microglial adenosine A2A receptor suppresses elevated pressure-induced inflammation, oxidative stress, and cell death in retinal cells

Glaucoma is a retinal degenerative disease characterized by the loss of retinal ganglion cells and damage of the optic nerve. Recently, we demonstrated that antagonists of adenosine A_2A receptor (A_2A R) control retinal inflammation and afford protection to rat retinal cells in glaucoma models. However, the precise contribution of microglia to retinal injury was not addressed, as well as the effect of A_2A R blockade directly in microglia. Here we show that blocking microglial A_2A R prevents microglial cell response to elevated pressure and it is sufficient to protect retinal cells from elevated pressure-induced death. The A_2A R antagonist SCH 58261 or the knockdown of A_2A R expression with siRNA in microglial cells prevented the increase in microglia response to elevated hydrostatic pressure. Furthermore, in retinal neural cell cultures, the A_2A R antagonist decreased microglia proliferation, as well as the expression and release of pro-inflammatory mediators. Microglia ablation prevented neural cell death triggered by elevated pressure. The A_2A R blockade recapitulated the effects of microglia depletion, suggesting that blocking A_2A R in microglia is able to control neurodegeneration in glaucoma-like conditions. Importantly, in human organotypic retinal cultures, A_2A R blockade prevented the increase in reactive oxygen species and the morphological alterations in microglia triggered by elevated pressure. These findings place microglia as the main contributors for retinal cell death during elevated pressure and identify microglial A_2A R as a therapeutic target to control retinal neuroinflammation and prevent neural apoptosis elicited by elevated pressure.

Antidepressant-Like Activity of Typical Antidepressant Drugs in the Forced Swim Test and Tail Suspension Test in Mice Is Augmented by DMPX, an Adenosine A2A Receptor Antagonist

Unsatisfactory therapeutic effects of currently used antidepressants force to search for new pharmacological treatment strategies. Recent research points to the relationship between depressive disorders and the adenosinergic system. Therefore, the main goal of our studies was to evaluate the effects of DMPX (3 mg/kg, i.p.), which possesses selectivity for adenosine A_2A receptors versus A₁ receptors, on the activity of imipramine (15 mg/kg, i.p.), escitalopram (2.5 mg/kg, i.p.), and reboxetine (2 mg/kg, i.p.) given in subtherapeutic doses. The studies carried out using the forced swim and tail suspension tests in mice showed that DMPX at a dose of 6 and 12 mg/kg exerts antidepressant-like effect and does not affect the locomotor activity. Co-administration of DMPX at a dose of 3 mg/kg with the studied antidepressant drugs caused the reduction of immobility time in both behavioral tests. The observed effect was not associated with an increase in the locomotor activity. To evaluate whether the observed effects were due to a pharmacokinetic/pharmacodynamic interaction, the levels of the antidepressants in blood and brain were measured using high-performance liquid chromatography. It can be assumed that the interaction between DMPX and imipramine was exclusively pharmacodynamic in nature, whereas an increased antidepressant activity of escitalopram and reboxetine was at least partly related to its pharmacokinetic interaction with DMPX.

Istradefylline reduces memory deficits in aging mice with amyloid pathology

Adenosine A2A receptors are putative therapeutic targets for neurological disorders. The adenosine A2A receptor antagonist istradefylline is approved in Japan for Parkinson's disease and is being tested in clinical trials for this condition elsewhere. A2A receptors on neurons and astrocytes may contribute to Alzheimer's disease (AD) by impairing memory. However, it is not known whether istradefylline enhances cognitive function in aging animals with AD-like amyloid plaque pathology. Here, we show that elevated levels of Aβ, C-terminal fragments of the amyloid precursor protein (APP), or amyloid plaques, but not overexpression of APP per se, increase astrocytic A2A receptor levels in the hippocampus and neocortex of aging mice. Moreover, in amyloid plaque-bearing mice, low-dose istradefylline treatment enhanced spatial memory and habituation, supporting the conclusion that, within a well-defined dose range, A2A receptor blockers might help counteract memory problems in patients with Alzheimer's disease.

Effects of intra-accumbal or intra-prefrontal cortex microinjections of adenosine 2A receptor ligands on responses to cocaine reward and seeking in rats

RATIONALE AND OBJECTIVES

Many studies indicated that adenosine via its A_2A receptors influences the behavioral effects of cocaine by modulating dopamine neurotransmission. The hypothesis was tested that A_2A receptors in the nucleus accumbens (NAc) or the prefrontral cortex (PFc) may modulate cocaine reward and/or cocaine seeking behavior in rats.

METHODS

The effects of local bilateral microinjections of the selective A_2A receptor agonist CGS 21680 or the A_2A receptor antagonists KW 6002 and SCH 58261 were investigated on cocaine self-administration on reinstatement of cocaine seeking.

RESULTS

The intra-NAc shell, but not intra-infralimbic PFc, administration of CGS 21680 significantly reduced the number of active lever presses and the number of cocaine (0.25 mg/kg) infusions. However, tonic activation of A_2A receptors located in the NAc or PFc did not play a role in modulating the rewarding actions of cocaine since neither KW 6002 nor SCH 58261 microinjections altered the cocaine (0.5 mg/kg) infusions. The intra-NAc but not intra-PFc microinjections of CGS 21680 dose- dependently attenuated the reinstatement of active lever presses induced by cocaine (10 mg/kg, i.p.) and the drug-associated combined conditioned stimuli using the subthreshold dose of cocaine (2.5 mg/kg, i.p.). On the other hand, the intra-NAc pretreatment with SCH 58261, but not with KW 6002, given alone evoked reinstatement of cocaine seeking behavior.

CONCLUSION

The results strongly support the involvement of accumbal shell A_2A receptors as a target, the activation of which exerts an inhibitory control over cocaine reward and cocaine seeking.

Targeting Adenosine Signaling in Parkinson's Disease: From Pharmacological to Non-pharmacological Approaches

Parkinson's disease (PD) is one of the most prevalent neurodegenerative disease displaying negative impacts on both the health and social ability of patients and considerable economical costs. The classical anti-parkinsonian drugs based in dopaminergic replacement are the standard treatment, but several motor side effects emerge during long-term use. This mini-review presents the rationale to several efforts from pre-clinical and clinical studies using adenosine receptor antagonists as a non-dopaminergic therapy. As several studies have indicated that the monotherapy with adenosine receptor antagonists reaches limited efficacy, the usage as a co-adjuvant appeared to be a promising strategy. The formulation of multi-targeted drugs, using adenosine receptor antagonists and other neurotransmitter systems than the dopaminergic one as targets, have been receiving attention since Parkinson's disease presents a complex biological impact. While pharmacological approaches to cure or ameliorate the conditions of PD are the leading strategy in this area, emerging positive aspects have arisen from non-pharmacological approaches and adenosine function inhibition appears to improve both strategies.

Treatment with A2A receptor antagonist KW6002 and caffeine intake regulate microglia reactivity and protect retina against transient ischemic damage

Transient retinal ischemia is a major complication of retinal degenerative diseases and contributes to visual impairment and blindness. Evidences indicate that microglia-mediated neuroinflammation has a key role in the neurodegenerative process, prompting the hypothesis that the control of microglia reactivity may afford neuroprotection to the retina against the damage induced by ischemia–reperfusion (I–R). The available therapeutic strategies for retinal degenerative diseases have limited potential, but the blockade of adenosine A_2A receptor (A_2AR) emerges as candidate strategy. Therefore, we evaluated the therapeutic potential of a selective A_2AR antagonist (KW6002) against the damage elicited by I–R. The administration of KW6002 after I–R injury reduced microglia reactivity and inflammatory response and afforded protection to the retina. Moreover, we tested the ability of caffeine, an adenosine receptor antagonist, in mediating protection to the retina in the I–R injury model. We demonstrated that caffeine administration dually regulated microglia reactivity and cell death in the transient retinal ischemic model, depending on the reperfusion time. At 24 h of reperfusion, caffeine increased microglial reactivity, inflammatory response and cell death elicited by I–R. However, at 7 days of reperfusion, caffeine administration decreased microglia reactivity and reduced the levels of proinflammatory cytokines and cell death. Together, these results provide a novel evidence for the use of adenosine A_2AR antagonists as potential therapy for retinal ischemic diseases and demonstrate the effect of caffeine on the regulation of microglia-mediated neuroinflammation in the transient ischemic model.

α-synuclein interacts with PrPC to induce cognitive impairment through mGluR5 and NMDAR2B

Synucleinopathies, such as Parkinson's disease and dementia with Lewy bodies, are neurodegenerative disorders that are characterized by the accumulation of α-synuclein (aSyn) in intracellular inclusions known as Lewy bodies. Prefibrillar soluble aSyn oligomers, rather than larger inclusions, are currently considered to be crucial species underlying synaptic dysfunction. We identified the cellular prion protein (PrP^C) as a key mediator in aSyn-induced synaptic impairment. The aSyn-associated impairment of long-term potentiation was blocked in Prnp null mice and rescued following PrP^C blockade. We found that extracellular aSyn oligomers formed a complex with PrP^C that induced the phosphorylation of Fyn kinase via metabotropic glutamate receptors 5 (mGluR5). aSyn engagement of PrP^C and Fyn activated NMDA receptor (NMDAR) and altered calcium homeostasis. Blockade of mGluR5-evoked phosphorylation of NMDAR in aSyn transgenic mice rescued synaptic and cognitive deficits, supporting the hypothesis that a receptor-mediated mechanism, independent of pore formation and membrane leakage, is sufficient to trigger early synaptic damage induced by extracellular aSyn.

Altered purinergic signaling in uridine adenosine tetraphosphate-induced coronary relaxation in swine with metabolic derangement

We previously demonstrated that uridine adenosine tetraphosphate (Up4A) induces potent and partially endothelium-dependent relaxation in the healthy porcine coronary microvasculature. We subsequently showed that Up4A-induced porcine coronary relaxation was impaired via downregulation of P1 receptors after myocardial infarction. In view of the deleterious effect of metabolic derangement on vascular function, we hypothesized that the coronary vasodilator response to Up4A is impaired in metabolic derangement, and that the involvement of purinergic receptor subtypes and endothelium-derived vasoactive factors (EDVFs) is altered. Coronary small arteries, dissected from the apex of healthy swine and swine 6 months after induction of diabetes with streptozotocin and fed a high-fat diet, were mounted on wire myographs. Up4A (10-9-10-5 M)-induced coronary relaxation was maintained in swine with metabolic derangement compared to normal swine, despite impaired endothelium-dependent relaxation to bradykinin and despite blunted P2X7 receptor and NO-mediated vasodilator influences of Up4A. Moreover, a thromboxane-mediated vasoconstrictor influence was unmasked. In contrast, an increased Up4A-mediated vasodilator influence via P2Y1 receptors was observed, while, in response to Up4A, cytochrome P450 2C9 switched from producing vasoconstrictor to vasodilator metabolites in swine with metabolic derangement. Coronary vascular expression of A2A and P2X7 receptors as well as eNOS, as assessed with real-time PCR, was reduced in swine with metabolic derangement. In conclusion, although the overall coronary vasodilator response to Up4A was maintained in swine with metabolic derangement, the involvement of purinergic receptor subtypes and EDVF was markedly altered, revealing compensatory mechanisms among signaling pathways in Up4A-mediated coronary vasomotor influence in the early phase of metabolic derangement. Future studies are warranted to investigate the effects of severe metabolic derangement on coronary responses to Up4A.

Adenosine A2a Receptor Blockade Diminishes Wnt/β-Catenin Signaling in a Murine Model of Bleomycin-Induced Dermal Fibrosis

Adenosine A_2a receptor (A_2aR) stimulation promotes the synthesis of collagens I and III, and we have recently demonstrated that there is crosstalk between the A_2aR and WNT/β-catenin signaling pathway. In in vitro studies, A_2aR signaling for collagen III expression was mediated by WNT/β-catenin signaling in human dermal fibroblasts; we further verified whether the crosstalk between A_2aR and Wnt/β-catenin signaling was involved in diffuse dermal fibrosis in vivo. Wnt-signaling reporter mice (Tcf/Lef:H2B-GFP) were challenged with bleomycin and treated with the selective A_2aR antagonist istradefylline (KW6002) or vehicle. Dermal fibrosis was quantitated and nuclear translocation of β-catenin in fibroblasts was assessed by double-staining for Green fluorescent protein or dephosphorylated β-catenin or β-catenin phosphorylated at Ser552, and vimentin. KW6002 significantly reduced skin thickness, skinfold thickness, breaking tension, dermal hydroxyproline content, myofibroblast accumulation, and collagen alignment in bleomycin-induced dermal fibrosis. Also, there was increased expression of Tcf/Lef:H2B-GFP reporter in bleomycin-induced dermal fibrosis, an effect that was diminished by treatment with KW6002. Moreover, KW6002 significantly inhibited nuclear translocation of Tcf/Lef:H2B-GFP reporter, as well as dephosphorylated β-catenin and β-catenin phosphorylated at Ser552. Our work supports the hypothesis that pharmacologic blockade of A_2aR inhibits the WNT/β-catenin signaling pathway, contributing to its capacity to inhibit dermal fibrosis in diseases such as scleroderma.

Chronic and acute adenosine A2A receptor blockade prevents long-term episodic memory disruption caused by acute cannabinoid CB1 receptor activation

Cannabinoid-mediated memory impairment is a concern in cannabinoid-based therapies. Caffeine exacerbates cannabinoid CB₁ receptor (CB₁R)-induced memory deficits through an adenosine A₁ receptor-mediated mechanism. We now evaluated how chronic or acute blockade of adenosine A_2A receptors (A_2ARs) affects long-term episodic memory deficits induced by a single injection of a selective CB₁R agonist. Long-term episodic memory was assessed by the novel object recognition (NOR) test. Mice received an intraperitoneal (i.p.) injection of the CB₁/CB₂ receptor agonist WIN 55,212-2 (1 mg/kg) immediately after the NOR training, being tested for novelty recognition 24 h later. Anxiety levels were assessed by the Elevated Plus Maze test, immediately after the NOR. Mice were also tested for exploratory behaviour at the Open Field. For chronic A_2AR blockade, KW-6002 (istradefylline) (3 mg/kg/day) was administered orally for 30 days; acute blockade of A_2ARs was assessed by i.p. injection of SCH 58261 (1 mg/kg) administered either together with WIN 55,212-2 or only 30 min before the NOR test phase. The involvement of CB₁Rs was assessed by using the CB₁R antagonist, AM251 (3 mg/kg, i.p.). WIN 55,212-2 caused a disruption in NOR, an action absent in mice also receiving AM251, KW-6002 or SCH 58261 during the encoding/consolidation phase; SCH 58251 was ineffective if present during retrieval only. No effects were detected in the Elevated Plus maze or Open Field Test. The finding that CB₁R-mediated memory disruption is prevented by antagonism of adenosine A_2ARs, highlights a possibility to prevent cognitive side effects when therapeutic application of CB₁R drugs is desired.

Enhanced recovery of breathing capacity from combined adenosine 2A receptor inhibition and daily acute intermittent hypoxia after chronic cervical spinal injury

Daily acute intermittent hypoxia (dAIH) improves breathing capacity after C2 spinal hemisection (C2HS) in rats. Since C2HS disrupts spinal serotonergic innervation below the injury, adenosine-dependent mechanisms underlie dAIH-induced functional recovery 2weeks post-injury. We hypothesized that dAIH-induced functional recovery converts from an adenosine-dependent to a serotonin-dependent, adenosine-constrained mechanism with chronic injury. Eight weeks post-C2HS, rats began dAIH (10, 5-min episodes, 10.5% O2; 5-min intervals; 7days) followed by AIH 3× per week (3×wAIH) for 8 additional weeks with/without systemic A2A receptor inhibition (KW6002) on each AIH exposure day. Tidal volume (VT) and bilateral diaphragm (Dia) and T2 external intercostal motor activity were assessed in unanesthetized rats breathing air and during maximum chemoreflex stimulation (MCS: 7% CO2, 10.5% O2). Nine weeks post-C2HS, dAIH increased VT versus time controls (p<0.05), an effect enhanced by KW6002 (p<0.05). dAIH increased bilateral Dia activity (p<0.05), and KW6002 enhanced this effect in contralateral (p<0.05) and ipsilateral Dia activity (p<0.001), but not T2 inspiratory activity. Functional benefits of combined AIH plus systemic A2A receptor inhibition were maintained for 4weeks. Thus, in rats with chronic injuries: 1) dAIH improves VT and bilateral diaphragm activity; 2) VT recovery is enhanced by A2A receptor inhibition; and 3) functional recovery with A2A receptor inhibition and AIH "reminders" last 4weeks. Combined dAIH and A2A receptor inhibition may be a simple, safe, and effective strategy to accelerate/enhance functional recovery of breathing capacity in patients with respiratory impairment from chronic spinal injury.

A2A Adenosine Receptor Antagonism Reverts the Blood-Brain Barrier Dysfunction Induced by Sleep Restriction

Chronic sleep restriction induces blood-brain barrier disruption and increases pro-inflammatory mediators in rodents. Those inflammatory mediators may modulate the blood-brain barrier and constitute a link between sleep loss and blood-brain barrier physiology. We propose that adenosine action on its A_2A receptor may be modulating the blood-brain barrier dynamics in sleep-restricted rats. We administrated a selective A_2A adenosine receptor antagonist (SCH58261) in sleep-restricted rats at the 10^th day of sleep restriction and evaluated the blood-brain barrier permeability to dextrans coupled to fluorescein (FITC-dextrans) and Evans blue. In addition, we evaluated by western blot the expression of tight junction proteins (claudin-5, occludin, ZO-1), adherens junction protein (E-cadherin), A_2A adenosine receptor, adenosine-synthesizing enzyme (CD73), and neuroinflammatory markers (Iba-1 and GFAP) in the cerebral cortex, hippocampus, basal nuclei and cerebellar vermis. Sleep restriction increased blood-brain barrier permeability to FITC-dextrans and Evans blue, and the effect was reverted by the administration of SCH58261 in almost all brain regions, excluding the cerebellum. Sleep restriction increased the expression of A_2A adenosine receptor only in the hippocampus and basal nuclei without changing the expression of CD73 in all brain regions. Sleep restriction reduced the expression of tight junction proteins in all brain regions, except in the cerebellum; and SCH58261 restored the levels of tight junction proteins in the cortex, hippocampus and basal nuclei. Finally, sleep restriction induced GFAP and Iba-1 overexpression that was attenuated with the administration of SCH58261. These data suggest that the action of adenosine on its A_2A receptor may have a crucial role in blood-brain barrier dysfunction during sleep loss probably by direct modulation of brain endothelial cell permeability or through a mechanism that involves gliosis with subsequent inflammation and increased blood-brain barrier permeability.

Impact on monoclonal antibody production in murine hybridoma cell cultures of adenosine receptor antagonists and phosphodiesterase inhibitors

The effects of different adenosine receptor antagonists and cyclic nucleotide phosphodiesterase (PDE) inhibitors on monoclonal antibody (mAb) titer and cell viability of murine hybridoma cells in culture were measured as part of our investigations to discover additives that enhance mAb production. Specific adenosine receptor antagonists and PDE inhibitors were found to enhance or decrease the titer of immunoglobulin G1 (IgG1) mAbs relative to negative controls, depending on the specific compound and cell line employed. The observed enhancements or decreases in IgG1 mAb titer appeared to be mainly due to an increase or decrease in specific productivity rates (ngmAb/cell), respectively. The different effects of the selective adenosine antagonists suggest that antagonism at the level of the adenosine A2A and A1 or the adenosine A3 receptors result in either enhancement or suppression of IgG1 mAb production by hybridoma cells. Overall, these studies have identified hitherto unknown activities of specific adenosine antagonists and PDE inhibitors which indicate they may have valuable roles as cell culture additives in industrial biomanufacturing processes designed to enhance the yields of mAbs or other recombinant proteins produced by mammalian cell culture procedures.

Antinociceptive effects of incarvillateine, a monoterpene alkaloid from Incarvillea sinensis, and possible involvement of the adenosine system

Incarvillea sinensis is a Bignoniaceae plant used to treat rheumatism and relieve pain in traditional Chinese medicine. As a major component of I. sinensis, incarvillateine has shown analgesic activity in mice formalin tests. Using a series of animal models, this study further evaluated the effects of incarvillateine against acute, inflammatory, and neuropathic pain. Incarvillateine (10 or 20 mg/kg, i.p.) dose-dependently attenuated acetic acid-induced writhing, but did not affect thermal threshold in the hot plate test. In a Complete Freund’s Adjuvant model, incarvillateine inhibited both thermal hyperalgesia and paw edema, and increased interleukin-1β levels. Additionally, incarvillateine attenuated mechanical allodynia induced by spared nerve injury or paclitaxel, whereas normal mechanical sensation was not affected. Incarvillateine did not affect locomotor activity and time on the rotarod at analgesic doses, and no tolerance was observed after 7 consecutive daily doses. Moreover, incarvillateine-induced antinociception was attenuated by theophylline, 1,3-dipropyl-8-cyclopentylxanthine, and 3,7-dimethyl-1-propargylxanthine, but not naloxone, indicating that the effects of incarvillateine on chronic pain were related to the adenosine system, but not opioid system. These results indicate that incarvillateine is a novel analgesic compound that is effective against inflammatory and neuropathic pain, and that its effects are associated with activation of the adenosine system.

Adenosine A(2A) receptors are necessary and sufficient to trigger memory impairment in adult mice

BACKGROUND AND PURPOSE

Caffeine (a non-selective adenosine receptor antagonist) prevents memory deficits in aging and Alzheimer's disease, an effect mimicked by adenosine A2 A receptor, but not A1 receptor, antagonists. Hence, we investigated the effects of adenosine receptor agonists and antagonists on memory performance and scopolamine-induced memory impairment in mice.

EXPERIMENTAL APPROACH

We determined whether A2 A receptors are necessary for the emergence of memory impairments induced by scopolamine and whether A2 A receptor activation triggers memory deficits in naïve mice, using three tests to assess short-term memory, namely the object recognition task, inhibitory avoidance and modified Y-maze.

KEY RESULTS

Scopolamine (1.0 mg·kg(-1) , i.p.) impaired short-term memory performance in all three tests and this scopolamine-induced amnesia was prevented by the A2 A receptor antagonist (SCH 58261, 0.1-1.0 mg·kg(-1) , i.p.) and by the A1 receptor antagonist (DPCPX, 0.2-5.0 mg·kg(-1) , i.p.), except in the modified Y-maze where only SCH58261 was effective. Both antagonists were devoid of effects on memory or locomotion in naïve rats. Notably, the activation of A2 A receptors with CGS 21680 (0.1-0.5 mg·kg(-1) , i.p.) before the training session was sufficient to trigger memory impairment in the three tests in naïve mice, and this effect was prevented by SCH 58261 (1.0 mg·kg(-1) , i.p.). Furthermore, i.c.v. administration of CGS 21680 (50 nmol) also impaired recognition memory in the object recognition task.

CONCLUSIONS AND IMPLICATIONS

These results show that A2 A receptors are necessary and sufficient to trigger memory impairment and further suggest that A1 receptors might also be selectively engaged to control the cholinergic-driven memory impairment.

Daily acute intermittent hypoxia elicits functional recovery of diaphragm and inspiratory intercostal muscle activity after acute cervical spinal injury

A major cause of mortality after spinal cord injury is respiratory failure. In normal rats, acute intermittent hypoxia (AIH) induces respiratory motor plasticity, expressed as diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF). Dia (not T2 EIC) LTF is enhanced by systemic adenosine 2A (A2A) receptor inhibition in normal rats. We investigated the respective contributions of Dia and T2 EIC to daily AIH-induced functional recovery of breathing capacity with/without A2A receptor antagonist (KW6002, i.p.) following C2 hemisection (C2HS). Rats received daily AIH (dAIH: 10, 5-min episodes, 10.5% O2; 5-min normoxic intervals; 7 successive days beginning 7days post-C2HS) or daily normoxia (dNx) with/without KW6002, followed by weekly (reminder) presentations for 8weeks. Ventilation and EMGs from bilateral diaphragm and T2 EIC muscles were measured with room air breathing (21% O2) and maximum chemoreceptor stimulation (

MCS

7% CO2, 10.5% O2). dAIH increased tidal volume (VT) in C2HS rats breathing room air (dAIH+vehicle: 0.47±0.02, dNx+vehicle: 0.40±0.01ml/100g; p<0.05) and MCS (dAIH+vehicle: 0.83±0.01, dNx+vehicle: 0.73±0.01ml/100g; p<0.001); KW6002 had no significant effect. dAIH enhanced contralateral (uninjured) diaphragm EMG activity, an effect attenuated by KW6002, during room air breathing and MCS (p<0.05). Although dAIH enhanced contralateral T2 EIC EMG activity during room air breathing, KW6002 had no effect. dAIH had no statistically significant effects on diaphragm or T2 EIC EMG activity ipsilateral to injury. Thus, two weeks post-C2HS: 1) dAIH enhances breathing capacity by effects on contralateral diaphragm and T2 EIC activity; and 2) dAIH-induced recovery is A2A dependent in diaphragm, but not T2 EIC. Daily AIH may be a useful in promoting functional recovery of breathing capacity after cervical spinal injury, but A2A receptor antagonists (e.g. caffeine) may undermine its effectiveness shortly after injury.

History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A1 , A2A , A2B , and A3 adenosine receptors (ARs). At the present time, the role of A2A ARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A2A AR signal transduction pathways, together with an analysis of the structure-activity relationships of A2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A1 /A2A ARs and monoamine oxidase-B (MAO-B), both closely implicated in the pathogenesis of PD.

Adenosine 2A receptor inhibition enhances intermittent hypoxia-induced diaphragm but not intercostal long-term facilitation

Acute intermittent hypoxia (AIH) elicits diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF) in normal unanesthetized rats. Although AIH-induced phrenic LTF is serotonin dependent, adenosine constrained in anesthetized rats, this has not been tested in unanesthetized animals. Cervical (C2) spinal hemisection (C2HS) abolishes phrenic LTF because of loss of serotonergic inputs 2 weeks post-injury, but LTF returns 8 weeks post-injury. We tested three hypotheses in unanesthetized rats: (1) systemic adenosine 2aA (A2A) receptor inhibition with intraperitoneal (IP) KW6002 enhances Dia and T2 EIC LTF in normal rats; (2) Dia and T2 EIC LTF are expressed after chronic (8 weeks), but not acute (1 week) C2HS; and (3) KW6002 enhances Dia and T2 EIC LTF after chronic (not acute) C2HS. Electromyography radiotelemetry was used to record Dia and T2 EIC activity during normoxia (21% O2), before and after AIH (10, 5-min 10.5% O2, 5-min intervals). In normal rats, KW6002 enhanced DiaLTF versus AIH alone (33.1±4.6% vs. 22.1±6.4% baseline, respectively; p<0.001), but had no effect on T2 EIC LTF (p>0.05). Although Dia and T2 EIC LTF were not observed 2 weeks post-C2HS, LTF was observed in contralateral (uninjured) Dia and T2 EIC 8 weeks post-C2HS (18.7±2.7% and 34.9±4.9% baseline, respectively; p<0.05), with variable ipsilateral expression. KW6002 had no significant effects on contralateral Dia (p=0.447) or T2 EIC LTF (p=0.796). We conclude that moderate AIH induces Dia and T2 EIC LTF after chronic, but not acute cervical spinal injuries. A single A2A receptor antagonist dose enhances AIH-induced Dia LTF in normal rats, but this effect is not significant in chronic (8 weeks) C2HS unanesthetized rats.

Design and evaluation of xanthine based adenosine receptor antagonists: potential hypoxia targeted immunotherapies

Molecular modeling techniques were applied to the design, synthesis and optimization of a new series of xanthine based adenosine A(2A) receptor antagonists. The optimized lead compound was converted to a PEG derivative and a functional in vitro bioassay used to confirm efficacy. Additionally, the PEGylated version showed enhanced aqueous solubility and was inert to photoisomerization, a known limitation of existing antagonists of this class.

8-(Furan-2-yl)-3-phenethylthiazolo[5,4-e][1,2,4]triazolo[1,5-c]pyrimidine-2(3H)-thione as novel, selective and potent adenosine A(2A) receptor antagonist

Antagonism of the human A2A receptor has been implicated to alleviate the symptoms associated with Parkinson's disease. The present finding reveals the potential of PTTP (8-(furan-2-yl)-3-phenethylthiazolo[1,2,4]triazolo[1,5-c]pyrimidine-2(3H)-thione) as novel and potent A2AR antagonist. In radioligand binding assay, PTTP showed significantly high binding affinity (Ki 6.3 nM) and selectivity with A2AR (A1R/A2AR=4603) which was comparable to the results of docking analysis (Ki=1.6 nM, ΔG=-14.52 Kcal/mol). PTTP antagonized (0.46 pmol/ml) the effect of NECA-induced increase in cAMP concentration (0.65 pmol/ml) better than SCH58261 (0.55 pmol/ml) in HEK293T cells. Haloperidol and NECA-induced mice pre-treated with PTTP at 10mg/kg showed attenuation in catalepsy and akinesia without significant neurotoxicity in rotarod test at 20mg/kg. Essentially, novel compound demonstrated remarkable potential as A2AR antagonist in the therapy of PD.

Adenosine 2A receptor antagonist prevented and reversed liver fibrosis in a mouse model of ethanol-exacerbated liver fibrosis

The effect of moderate alcohol consumption on liver fibrosis is not well understood, but evidence suggests that adenosine may play a role in mediating the effects of moderate ethanol on tissue injury. Ethanol increases the concentration of adenosine in the liver. Adenosine 2A receptor (A2AR) activation is known to enhance hepatic stellate cell (HSC) activation and A2AR deficient mice are protected from fibrosis in mice. Making use of a novel mouse model of moderate ethanol consumption in which female C57BL/6J mice were allowed continued access to 2% (vol/vol) ethanol (11% calories) or pair-fed control diets for 2 days, 2 weeks or 5 weeks and superimposed with exposure to CCl4, we tested the hypothesis that moderate ethanol consumption increases fibrosis in response to carbon tetrachloride (CCl4) and that treatment of mice with an A2AR antagonist prevents and/or reverses this ethanol-induced increase in liver fibrosis. Neither the expression or activity of CYP2E1, required for bio-activation of CCl4, nor AST and ALT activity in the plasma were affected by ethanol, indicating that moderate ethanol did not increase the direct hepatotoxicity of CCl4. However, ethanol feeding enhanced HSC activation and exacerbated liver fibrosis upon exposure to CCl4. This was associated with an increased sinusoidal angiogenic response in the liver. Treatment with A2AR antagonist both prevented and reversed the ability of ethanol to exacerbate liver fibrosis.

CONCLUSION

Moderate ethanol consumption exacerbates hepatic fibrosis upon exposure to CCl4. A2AR antagonism may be a potential pharmaceutical intervention to decrease hepatic fibrosis in response to ethanol.

Adenosine A(2A) receptor blockade reverts hippocampal stress-induced deficits and restores corticosterone circadian oscillation

Maternal separation (MS) is an early life stress model that induces permanent changes in the central nervous system, impairing hippocampal long-term potentiation (LTP) and spatial working memory. There are compelling evidences for a role of hippocampal adenosine A(2A) receptors in stress-induced modifications related to cognition, thus opening a potential window for therapeutic intervention. Here, we submitted rats to MS and evaluated the long-lasting molecular, electrophysiological and behavioral impairments in adulthood. We then assessed the therapeutic potential of KW6002, a blocker of A(2A) receptors, in stress-impaired animals. We report that the blockade of A(2A) receptors was efficient in reverting the behavior, electrophysiological and morphological impairments induced by MS. In addition, this effect is associated with restoration of the hypothalamic-pituitary-adrenal axis (HPA-axis) activity, as both the plasma corticosterone levels and hippocampal glucocorticoid receptor expression pattern returned to physiological-like status after the treatment. These results reveal the involvement of A(2A) receptors in the stress-associated impairments and directly in the stress response system by showing that the dysfunction of the HPA-axis as well as the long-lasting synaptic and behavioral effects of MS can be reverted by targeting adenosine A(2A) receptors. These findings provide a novel evidence for the use of adenosine A(2A) receptor antagonists as potential therapy against psychopathologies.

Adenosine A₁ receptors do not play a major role in the regulation of lipogenic gene expression in hepatocytes

Activation of adenosine A₁ receptors was reported to promote fatty acid synthesis in AML-12 cells, by increasing the expression of SREBP-(1c) (sterol regulatory binding protein 1c) and FAS (fatty acid synthase). Since these findings have important therapeutic implications for the discovery of adenosine A₁ receptor agonists, further studies were undertaken to determine the expression and functional relevance of adenosine A₁ receptor in the liver. To that end, we used two classes of distinct adenosine A₁ receptor agonists: CPA (N⁶-cyclopentyl-adenosine), a full agonist and GS-9667 (2-{6-[((1R,2R)-2-hydroxycyclopentyl)-amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]-oxolane-3,4-diol), a partial agonist. Treatment of AML-12 cells, HepG2 cells and primary human hepatocytes with either CPA or GS-9667 did not increase the gene expression of SREBP-(1c) or FAS. Furthermore, in AML-12 and HepG2 cells, CPA did not antagonize forskolin-stimulated cAMP production, a characteristic of adenosine A₁ receptor activation, indicating that these cells lack adenosine A₁ receptor function. Consistent with this finding, adenosine A₁ receptor gene expression was found to be very low and adenosine A₁ receptor protein levels were hardly detectable by radioligand binding assays in hepatic cell lines such as AML-12 and HepG2 as well as in both mouse and human liver tissues. Finally, acute treatment with adenosine A₁ receptor agonist GS-9667 had no significant effect on gene expression of both SREBP-(1c) and FAS in livers of Sprague Dawley rats. Taken together, our data suggest that the expression of adenosine A₁ receptor is too low to play a major role in the regulation of lipogenic gene expression in hepatocytes.

Preladenant in patients with Parkinson's disease and motor fluctuations: a phase 2, double-blind, randomised trial

BACKGROUND

Preladenant is an adenosine 2A (A₂(A)) receptor antagonist. In animal models of Parkinson's disease, preladenant monotherapy improves motor function without causing dyskinesia and, as an adjunct to levodopa, it improves motor function without worsening dyskinesia. We aimed to assess the efficacy and safety of preladenant in patients with Parkinson's disease and motor fluctuations who were receiving levodopa and other antiparkinsonian drugs.

METHODS

In this phase 2, dose-finding trial, patients with Parkinson's disease who were receiving levodopa were enrolled and treated at 44 sites in 15 countries between December, 2006, and November, 2008. Assignment to treatment was done centrally with an interactive voice response system, according to a block randomisation schedule that was computer generated by the sponsor. Patients were assigned to receive 1, 2, 5, or 10 mg oral preladenant twice daily, or matching placebo for 12 weeks. Patients, study staff, investigators, and all sponsor personnel were masked to treatment assignment. The primary outcome was change in mean daily off time from baseline to week 12, as assessed by home diaries. Efficacy analysis included all patients who received at least one dose of study drug and had data for assessments after baseline. This trial is registered with ClinicalTrials.gov, number NCT00406029.

FINDINGS

253 patients were randomised to receive preladenant (1 mg [n=49], 2 mg [n=49], 5 mg [n=49], 10 mg [n=57]) or placebo (n=49), of whom 234 on preladenant (1 mg [n=47], 2 mg [n=48], 5 mg [n=45], 10 mg [n=49]) and placebo (n=45) were eligible for the efficacy analysis. Mean daily off time from baseline to week 12 was reduced versus placebo in patients on 5 mg preladenant (difference -1·0 h, 95% CI -2·1 to 0·0; p=0·0486) and 10 mg preladenant (-1·2 h, -2·2 to -0·2; p=0·019). Changes in mean daily off time versus placebo were not significant for 1 mg preladenant (0·2 h, -0·9 to 1·2; p=0·753) or 2 mg preladenant (-0·7 h, -1·7 to 0·3; p=0·162). The most common adverse events in the combined preladenant group versus placebo were worsening of Parkinson's disease (22 [11%] vs 4 [9%]), somnolence (20 [10%] vs 3 [6%]), dyskinesia (18 [9%] vs 6 [13%]), nausea (17 [9%] vs 5 [11%]), constipation (15 [8%] vs 1 [2%]), and insomnia (15 [8%] vs 4 [9%]).

INTERPRETATION

5 and 10 mg preladenant twice daily might be clinically useful to reduce off time in patients with Parkinson's disease and motor fluctuations.

FUNDING

Schering-Plough, a subsidiary of Merck.