Multigram-scale syntheses, stability, and photoreactions of A2A adenosine receptor antagonists with 8-styrylxanthine structure: potential drugs for Parkinson's disease

A General Synthetic Strategy toward the Truxillate Natural Products via Solid-State Photocycloadditions

The truxillates constitute a large class of dimeric natural products featuring a central, highly substituted cyclobutane core. In principle, these structures could be efficiently synthesized via [2 + 2] photocycloaddition. However, the difficulty in controlling the high-energy electronically excited reactive intermediates in the solution state can lead to poor regio- and diastereocontrol. This has limited the use of photocycloaddition methodology toward the synthesis of this important class of natural products. Herein, we demonstrate that acid-controlled precipitation of C-acyl imidazoles promotes a highly selective solid-state photocycloaddition, and the products of this reaction can be quickly transformed into truxillate natural products.

Optical Control of Adenosine A2A Receptor Using Istradefylline Photosensitivity

In recent years, there has been growing interest in the potential therapeutic use of inhibitors of adenosine A2A receptors (A2AR) for the treatment of neurodegenerative diseases and cancer. Nevertheless, the widespread expression of A2AR throughout the body emphasizes the importance of temporally and spatially selective ligands. Photopharmacology is an emerging strategy that utilizes photosensitive ligands to attain high spatiotemporal precision and regulate the function of biomolecules using light. In this study, we combined photochemistry and cellular and in vivo photopharmacology to investigate the light sensitivity of the FDA-approved antagonist istradefylline and its potential use as an A2AR photopharmacological tool. Our findings reveal that istradefylline exhibits rapid trans-to-cis isomerization under near-UV light, and prolonged exposure results in the formation of photocycloaddition products. We demonstrate that exposure to UV light triggers a time-dependent decrease in the antagonistic activity of istradefylline in A2AR-expressing cells and enables real-time optical control of A2AR signaling in living cells and zebrafish. Together, these data demonstrate that istradefylline is a photoinactivatable A2AR antagonist and that this property can be utilized to perform photopharmacological experiments in living cells and animals.

Discovery of Potent Agonists for the Predominant Variant of the Orphan MAS-Related G Protein-Coupled Receptor X4 (MRGPRX4)

The MAS-related Gq protein-coupled receptor X4 (MRGPRX4) is poorly investigated. MRGPRX4 has been proposed to be involved in pain transmission, itch, inflammation, wound healing, and cancer. However, so far only a few moderately potent, nonselective MRGPRX4 agonists have been described, most of which appear to preferably activate the minor receptor variant MRGPRX4-83L but not the main variant 83S. In the present study, we discovered a xanthine derivative bearing a phosphate substituent that activates the main variant of MRGPRX4. Optimization resulted in analogs with high potency and metabolic stability. The best compounds of the present series include 8-(m-methoxyphenethyl)-1-propargylxanthine substituted with a butyl linker in the 3-position containing a terminal phosphonate (30d, PSB-22034, EC50 Ca2+ assay/β-arrestin assay, 11.2 nM/32.0 nM) and its N7-methyl derivative 31d (PSB-22040, EC50, 19.2/30.0 nM) showing high selectivity versus all other MRGPRX subtypes. They present promising tool compounds for exploring the potential of MRGPRX4 as a future drug target.

Parkinson's Disease: Advances in Treatment and the Syntheses of Various Classes of Pharmaceutical Drug Substances

An overview of Parkinson's disease (PD) prevalence, diagnosis, and currently available treatment options is provided. A comprehensive list of different classes of marketed pharmaceutical drug products and the syntheses of various drug substances are summarized based on published literature.

Lateral septum adenosine A2A receptors control stress-induced depressive-like behaviors via signaling to the hypothalamus and habenula

Major depressive disorder ranks as a major burden of disease worldwide, yet the current antidepressant medications are limited by frequent non-responsiveness and significant side effects. The lateral septum (LS) is thought to control of depression, however, the cellular and circuit substrates are largely unknown. Here, we identified a subpopulation of LS GABAergic adenosine A_2A receptors (A_2AR)-positive neurons mediating depressive symptoms via direct projects to the lateral habenula (LHb) and the dorsomedial hypothalamus (DMH). Activation of A_2AR in the LS augmented the spiking frequency of A_2AR-positive neurons leading to a decreased activation of surrounding neurons and the bi-directional manipulation of LS-A_2AR activity demonstrated that LS-A_2ARs are necessary and sufficient to trigger depressive phenotypes. Thus, the optogenetic modulation (stimulation or inhibition) of LS-A_2AR-positive neuronal activity or LS-A_2AR-positive neurons projection terminals to the LHb or DMH, phenocopied depressive behaviors. Moreover, A_2AR are upregulated in the LS in two male mouse models of repeated stress-induced depression. This identification that aberrantly increased A_2AR signaling in the LS is a critical upstream regulator of repeated stress-induced depressive-like behaviors provides a neurophysiological and circuit-based justification of the antidepressant potential of A_2AR antagonists, prompting their clinical translation.

Development of subtype-selective covalent ligands for the adenosine A2B receptor by tuning the reactive group

Signalling through the adenosine receptors (ARs), in particular through the adenosine A2B receptor (A2BAR), has been shown to play a role in a variety of pathological conditions, ranging from immune disorders to cancer. Covalent ligands for the A2BAR have the potential to irreversibly block the receptor, as well as inhibit all A2BAR-induced signalling pathways. This will allow a thorough investigation of the pathophysiological role of the receptor. In this study, we synthesized and evaluated a set of potential covalent ligands for the A2BAR. The ligands all contain a core scaffold consisting of a substituted xanthine, varying in type and orientation of electrophilic group (warhead). Here, we find that the right combination of these variables is necessary for a high affinity, irreversible mode of binding and selectivity towards the A2BAR. Altogether, this is the case for sulfonyl fluoride 24 (LUF7982), a covalent ligand that allows for novel ways to interrogate the A2BAR.

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

Microwave-assisted reactions: Efficient and versatile one-step synthesis of 8-substituted xanthines and substituted pyrimidopteridine-2,4,6,8-tetraones under controlled microwave heating

We report herein a simple and efficient one-step synthesis of 8-substituted xanthines and substituted pyrimidopteridine-2,4,6,8-tetraones via reaction of 1,3-dimethyl-5,6-diaminouracil 1 with activated double bond systems 2 assisted by controlled microwave irradiation. The obtained heterocycles are privileged biologically relevant scaffolds.

Molecular probes for the human adenosine receptors

Adenosine receptors, G protein-coupled receptors (GPCRs) that are activated by the endogenous ligand adenosine, have been considered potential therapeutic targets in several disorders. To date however, only very few adenosine receptor modulators have made it to the market. Increased understanding of these receptors is required to improve the success rate of adenosine receptor drug discovery. To improve our understanding of receptor structure and function, over the past decades, a diverse array of molecular probes has been developed and applied. These probes, including radioactive or fluorescent moieties, have proven invaluable in GPCR research in general. Specifically for adenosine receptors, the development and application of covalent or reversible probes, whether radiolabeled or fluorescent, have been instrumental in the discovery of new chemical entities, the characterization and interrogation of adenosine receptor subtypes, and the study of adenosine receptor behavior in physiological and pathophysiological conditions. This review summarizes these applications, and also serves as an invitation to walk another mile to further improve probe characteristics and develop additional tags that allow the investigation of adenosine receptors and other GPCRs in even finer detail.

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.

Adenosine A2A Receptor Antagonists for Cancer Immunotherapy

Currently, the most promising therapeutic modality for cancer treatment is the blockade of immune checkpoint pathways, which has revolutionized cancer therapy in the past 15 years. Strategies targeting and modulating adenosine A_2A receptor (A_2AR), an emerging alternative immune checkpoint, have shown the potential to produce significant therapeutic effects. In this review, we describe the immunosuppressive activities of A_2AR and A_2BR in the tumor microenvironment (TME), followed by a summary and discussion of the structure-activity relationship (SAR) of the A_2AR (and dual A_2AR/A_2BR) antagonists that have been experimentally confirmed to exert oncoimmunological effects. This review also provides an update on the compounds under clinical evaluation and insights into the ligand binding modes of the receptor.

8-Benzylaminoxanthine scaffold variations for selective ligands acting on adenosine A2A receptors. Design, synthesis and biological evaluation

A library of 34 novel compounds based on a xanthine scaffold was explored in biological studies for interaction with adenosine receptors (ARs). Structural modifications of the xanthine core were introduced in the 8-position (benzylamino and benzyloxy substitution) as well as at N1, N3, and N7 (small alkyl residues), thereby improving affinity and selectivity for the A_2A AR. The compounds were characterized by radioligand binding assays, and our study resulted in the development of the potent A_2A AR ligands including 8-((6-chloro-2-fluoro-3-methoxybenzyl)amino)-1-ethyl-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione (12d; K_i human A_2AAR: 68.5 nM) and 8-((2-chlorobenzyl)amino)-1-ethyl-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione (12h; K_i human A_2AAR: 71.1 nM). Moreover, dual A₁/A_2AAR ligands were identified in the group of 1,3-diethyl-7-methylxanthine derivatives. Compound 14b displayed K_i values of 52.2 nM for the A₁AR and 167 nM for the A_2AAR. Selected A_2AAR ligands were further evaluated as inactive for inhibition of monoamine oxidase A, B and isoforms of phosphodiesterase-4B1, -10A, which represent classical targets for xanthine derivatives. Therefore, the developed 8-benzylaminoxanthine scaffold seems to be highly selective for AR activity and relevant for potent and selective A_2A ligands. Compound 12d with high selectivity for ARs, especially for the A_2AAR subtype, evaluated in animal models of inflammation has shown anti-inflammatory activity. Investigated compounds were found to display high selectivity and may therefore be of high interest for further development as drugs for treating cancer or neurodegenerative diseases.

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.

Agonists and Antagonists for Purinergic Receptors

Membrane receptors that are activated by the purine nucleoside adenosine (adenosine receptors) or by purine or pyrimidine nucleotides (P2Y and P2X receptors) transduce extracellular signals to the cytosol. They play important roles in physiology and disease. The G protein-coupled adenosine receptors comprise four subtypes: A₁, A_2A, A_2B, and A₃. The G-protein-coupled P2Y receptors are subdivided into eight subtypes: P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, while the P2X receptors represent ATP-gated homomeric or heteromeric ion channels consisting of three subunits; the most important subunits are P2X1, P2X2, P2X3, P2X4, and P2X7. This chapter provides guidance for selecting suitable tool compounds for studying these large and important purine receptor families.

Structural and Conformational Studies on Carboxamides of 5,6-Diaminouracils-Precursors of Biologically Active Xanthine Derivatives

8-Arylethynylxanthine derivatives are potent, selective adenosine A_2A receptor antagonists, which represent (potential) therapeutics for Parkinson's disease, Alzheimer's dementia, and the immunotherapy of cancer. 6-Amino-5-amidouracil derivatives are important precursors for the synthesis of such xanthines. We noticed an unexpected duplication of NMR signals in many of these uracil derivatives. Here, we present a detailed analytical study of structurally diverse 6-amino-5-carboxamidouracils employing dynamic and two-dimensional NMR spectroscopy, density functional theory calculations, and X-ray analysis to explain the unexpected properties of these valuable drug intermediates.

Fast, Efficient, and Versatile Synthesis of 6-amino-5-carboxamidouracils as Precursors for 8-Substituted Xanthines

Substituted xanthine derivatives are important bioactive molecules. Herein we report on a new, practical synthesis of 6-amino-5-carboxamidouracils, the main building blocks for the preparation of 8-substituted xanthines, by condensation of 5,6-diaminouracil derivatives and various carboxylic acids using the recently developed non-hazardous coupling reagent COMU (1-[(1-(cyano-2-ethoxy-2-oxoethylideneaminooxy)dimethylaminomorpholinomethylene)]methanaminium hexafluorophosphate). Optimized reaction conditions led to the precipitation of pure products after only 5 to 10 min of reaction time. The method tolerates a variety of substituted 5,6-diaminouracil and carboxylic acid derivatives as starting compounds resulting in most cases in more than 80% isolated yield. Regioselectivity of the reaction yielding only the 5-carboxamido-, but not the 6-carboxamidouracil derivatives, was unambiguously confirmed by single X-ray crystallography and multidimensional NMR experiments. The described method represents a convenient, fast access to direct precursors of 8-substituted xanthines under mild conditions without the necessity of hazardous coupling or chlorinating reagents.

Docking Screens for Dual Inhibitors of Disparate Drug Targets for Parkinson's Disease

Modulation of multiple biological targets with a single drug can lead to synergistic therapeutic effects and has been demonstrated to be essential for efficient treatment of CNS disorders. However, rational design of compounds that interact with several targets is very challenging. Here, we demonstrate that structure-based virtual screening can guide the discovery of multi-target ligands of unrelated proteins relevant for Parkinson's disease. A library with 5.4 million molecules was docked to crystal structures of the A_2A adenosine receptor (A_2AAR) and monoamine oxidase B (MAO-B). Twenty-four compounds that were among the highest ranked for both binding sites were evaluated experimentally, resulting in the discovery of four dual-target ligands. The most potent compound was an A_2AAR antagonist with nanomolar affinity ( K_i = 19 nM) and inhibited MAO-B with an IC₅₀ of 100 nM. Optimization guided by the predicted binding modes led to the identification of a second potent dual-target scaffold. The two discovered scaffolds were shown to counteract 6-hydroxydopamine-induced neurotoxicity in dopaminergic neuronal-like SH-SY5Y cells. Structure-based screening can hence be used to identify ligands with specific polypharmacological profiles, providing new avenues for drug development against complex diseases.

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.

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.

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.

Design, Synthesis of Novel, Potent, Selective, Orally Bioavailable Adenosine A2A Receptor Antagonists and Their Biological Evaluation

Our initial structure-activity relationship studies on 7-methoxy-4-morpholino-benzothiazole derivatives featured by aryloxy-2-methylpropanamide moieties at the 2-position led to identification of compound 25 as a potent and selective A_2A adenosine receptor (A_2AAdoR) antagonist with reasonable ADME and pharmacokinetic properties. However, poor intrinsic solubility and low to moderate oral bioavailability made this series unsuitable for further development. Further optimization using structure-based drug design approach resulted in discovery of potent and selective adenosine A_2A receptor antagonists bearing substituted 1-methylcyclohexyl-carboxamide groups at position 2 of the benzothiazole scaffold and endowed with better solubility and oral bioavailability. Compounds 41 and 49 demonstrated a number of positive attributes with respect to in vitro ADME properties. Both compounds displayed good pharmacokinetic properties with 63% and 61% oral bioavailability, respectively, in rat. Further, compound 49 displayed oral efficacy in 6-OHDA lesioned rat model of Parkinson diseases.

Effort-related motivational effects of the pro-inflammatory cytokine interleukin-6: pharmacological and neurochemical characterization

RATIONALE

Motivational dysfunctions such as anergia, fatigue, and reduced effort expenditure are common in patients with depression and other disorders. Pro-inflammatory cytokines are implicated in depression, and cytokine administration induces motivational deficits in humans.

OBJECTIVES

These studies focused on the effects of the cytokine interleukin-6 (IL-6) on effort-related decision-making.

METHODS

Rats were assessed using the concurrent fixed ratio 5-lever pressing/chow feeding choice procedure, which measures the tendency of rats to work for a preferred food (high carbohydrate pellets) in the presence of a concurrently available but less preferred substitute (lab chow).

RESULTS

IL-6 (2.0-8.0 μg/kg IP) shifted choice behavior, significantly decreasing lever pressing and increasing chow intake. Further experiments showed that the adenosine A2A antagonist MSX-3 and the stimulant methylphenidate attenuated the effort-related impairments produced by IL-6, increasing lever pressing and decreasing chow intake in IL-6 treated rats. The same doses of IL-6 did not alter food intake or preference in parallel free-feeding choice studies, demonstrating that these low doses were not altering preference for the high carbohydrate pellets or generally suppressing appetite. Also, IL-6 did not affect body temperature. Microdialysis studies showed that 8.0 μg/kg IL-6 significantly decreased extracellular dopamine in nucleus accumbens core.

CONCLUSIONS

In summary, IL-6 reduces the tendency to work for food, even at low doses that do not produce fever or loss of appetite. Dopaminergic mechanisms may be involved in these effort-related effects. This research has implications for the involvement of cytokines in motivational dysfunctions such as anergia and fatigue.

Inhibiting androgen receptor nuclear entry in castration-resistant prostate cancer

Clinical resistance to the second-generation antiandrogen enzalutamide in castration-resistant prostate cancer (CRPC), despite persistent androgen receptor (AR) activity in tumors, highlights an unmet medical need for next-generation antagonists. We have identified and characterized tetra-aryl cyclobutanes (CBs) as a new class of competitive AR antagonists that exhibit a unique mechanism of action. These CBs are structurally distinct from current antiandrogens (hydroxyflutamide, bicalutamide, and enzalutamide) and inhibit AR-mediated gene expression, cell proliferation, and tumor growth in several models of CRPC. Conformational profiling revealed that CBs stabilize an AR conformation resembling an unliganded receptor. Using a variety of techniques, it was determined that the AR-CB complex was not recruited to AR-regulated promoters and, like apo AR, remains sequestered in the cytoplasm, bound to heat shock proteins. Thus, we have identified third-generation AR antagonists whose unique mechanism of action suggests that they may have therapeutic potential in CRPC.

Focused screening to identify new adenosine kinase inhibitors

Adenosine kinase (AdK) is a key player in controlling intra- and extracellular concentrations of the signaling molecule adenosine. Extensive evidence points to an important role of AdK in several diseases, and suggests that AdK inhibition might be a promising therapeutic strategy. The development of a new AdK assay and subsequent screening of part of our focused compound library led to the identification of 12 hit compounds (hit rate of 6%) representing six new classes of non-nucleoside human AdK inhibitors. The most potent inhibitor 1 displayed a K_i value of 184nM. Compound screening with a newly developed assay was useful and efficient for discovering novel AdK inhibitors which may serve as lead structures for developing drugs for adenosine augmentation therapy.

The caffeine-binding adenosine A2A receptor induces age-like HPA-axis dysfunction by targeting glucocorticoid receptor function

Caffeine is associated with procognitive effects in humans by counteracting overactivation of the adenosine A_2A receptor (A_2AR), which is upregulated in the human forebrain of aged and Alzheimer’s disease (AD) patients. We have previously shown that an anti-A_2AR therapy reverts age-like memory deficits, by reestablishment of the hypothalamic-pituitary-adrenal (HPA) axis feedback and corticosterone circadian levels. These observations suggest that A_2AR over-activation and glucocorticoid dysfunction are key events in age-related hippocampal deficits; but their direct connection has never been explored. We now show that inducing A_2AR overexpression in an aging-like profile is sufficient to trigger HPA-axis dysfunction, namely loss of plasmatic corticosterone circadian oscillation, and promotes reduction of GR hippocampal levels. The synaptic plasticity and memory deficits triggered by GR in the hippocampus are amplified by A_2AR over-activation and were rescued by anti-A_2AR therapy; finally, we demonstrate that A_2AR act on GR nuclear translocation and GR-dependent transcriptional regulation. We provide the first demonstration that A_2AR is a major regulator of GR function and that this functional interconnection may be a trigger to age-related memory deficits. This supports the idea that the procognitive effects of A_2AR antagonists, namely caffeine, on Alzheimer’s and age-related cognitive impairments may rely on its ability to modulate GR actions.

A2A adenosine receptor deletion is protective in a mouse model of Tauopathy

Consumption of caffeine, a non-selective adenosine A2A receptor (A2AR) antagonist, reduces the risk of developing Alzheimer's disease (AD) in humans and mitigates both amyloid and Tau burden in transgenic mouse models. However, the impact of selective A2AR blockade on the progressive development of AD-related lesions and associated memory impairments has not been investigated. In the present study, we removed the gene encoding A2AR from THY-Tau22 mice and analysed the subsequent effects on both pathological (Tau phosphorylation and aggregation, neuro-inflammation) and functional impairments (spatial learning and memory, hippocampal plasticity, neurotransmitter profile). We found that deleting A2ARs protect from Tau pathology-induced deficits in terms of spatial memory and hippocampal long-term depression. These effects were concomitant with a normalization of the hippocampal glutamate/gamma-amino butyric acid ratio, together with a global reduction in neuro-inflammatory markers and a decrease in Tau hyperphosphorylation. Additionally, oral therapy using a specific A2AR antagonist (MSX-3) significantly improved memory and reduced Tau hyperphosphorylation in THY-Tau22 mice. By showing that A2AR genetic or pharmacological blockade improves the pathological phenotype in a Tau transgenic mouse model, the present data highlight A2A receptors as important molecular targets to consider against AD and Tauopathies.

The Prodrug Approach: A Successful Tool for Improving Drug Solubility

Prodrug design is a widely known molecular modification strategy that aims to optimize the physicochemical and pharmacological properties of drugs to improve their solubility and pharmacokinetic features and decrease their toxicity. A lack of solubility is one of the main obstacles to drug development. This review aims to describe recent advances in the improvement of solubility via the prodrug approach. The main chemical carriers and examples of successful strategies will be discussed, highlighting the advances of this field in the last ten years.

Selectivity is species-dependent: Characterization of standard agonists and antagonists at human, rat, and mouse adenosine receptors

Adenosine receptors (ARs) have emerged as new drug targets. The majority of data on affinity/potency and selectivity of AR ligands described in the literature has been obtained for the human species. However, preclinical studies are mostly performed in mouse or rat, and standard AR agonists and antagonists are frequently used for studies in rodents without knowing their selectivity in the investigated species. In the present study, we selected a set of frequently used standard AR ligands, 8 agonists and 16 antagonists, and investigated them in radioligand binding studies at all four AR subtypes, A1, A2A, A2B, and A3, of three species, human, rat, and mouse. Recommended, selective agonists include CCPA (for A1AR of rat and mouse), CGS-21680 (for A2A AR of rat), and Cl-IB-MECA (for A3AR of all three species). The functionally selective partial A2B agonist BAY60-6583 was found to additionally bind to A1 and A3AR and act as an antagonist at both receptor subtypes. The antagonists PSB-36 (A1), preladenant (A2A), and PSB-603 (A2B) displayed high selectivity in all three investigated species. MRS-1523 acts as a selective A3AR antagonist in human and rat, but is only moderately selective in mouse. The comprehensive data presented herein provide a solid basis for selecting suitable AR ligands for biological studies.

Caffeine acts through neuronal adenosine A2A receptors to prevent mood and memory dysfunction triggered by chronic stress

The consumption of caffeine (an adenosine receptor antagonist) correlates inversely with depression and memory deterioration, and adenosine A2A receptor (A2AR) antagonists emerge as candidate therapeutic targets because they control aberrant synaptic plasticity and afford neuroprotection. Therefore we tested the ability of A2AR to control the behavioral, electrophysiological, and neurochemical modifications caused by chronic unpredictable stress (CUS), which alters hippocampal circuits, dampens mood and memory performance, and enhances susceptibility to depression. CUS for 3 wk in adult mice induced anxiogenic and helpless-like behavior and decreased memory performance. These behavioral changes were accompanied by synaptic alterations, typified by a decrease in synaptic plasticity and a reduced density of synaptic proteins (synaptosomal-associated protein 25, syntaxin, and vesicular glutamate transporter type 1), together with an increased density of A2AR in glutamatergic terminals in the hippocampus. Except for anxiety, for which results were mixed, CUS-induced behavioral and synaptic alterations were prevented by (i) caffeine (1 g/L in the drinking water, starting 3 wk before and continued throughout CUS); (ii) the selective A2AR antagonist KW6002 (3 mg/kg, p.o.); (iii) global A2AR deletion; and (iv) selective A2AR deletion in forebrain neurons. Notably, A2AR blockade was not only prophylactic but also therapeutically efficacious, because a 3-wk treatment with the A2AR antagonist SCH58261 (0.1 mg/kg, i.p.) reversed the mood and synaptic dysfunction caused by CUS. These results herald a key role for synaptic A2AR in the control of chronic stress-induced modifications and suggest A2AR as candidate targets to alleviate the consequences of chronic stress on brain function.

Inactivation of adenosine A2A receptors reverses working memory deficits at early stages of Huntington's disease models

Cognitive impairments in Huntington's disease (HD) are attributed to a dysfunction of the cortico-striatal pathway and significantly affect the quality of life of the patients, but this has not been a therapeutic focus in HD to date. We postulated that adenosine A(2A) receptors (A(2A)R), located at pre- and post-synaptic elements of the cortico-striatal pathways, modulate striatal neurotransmission and synaptic plasticity and cognitive behaviors. To critically evaluate the ability of A(2A)R inactivation to prevent cognitive deficits in early HD, we cross-bred A(2A)R knockout (KO) mice with two R6/2 transgenic lines of HD (CAG120 and CAG240) to generate two double transgenic R6/2-CAG120-A(2A)R KO and R6/2-CAG240-A(2A)R KO mice and their corresponding wild-type (WT) littermates. Genetic inactivation of A(2A)R prevented working memory deficits induced by R6/2-CAG120 at post-natal week 6 and by R6/2-CAG240 at post-natal month 2 and post-natal month 3, without modifying motor deficits. Similarly the A2(A)R antagonist KW6002 selectively reverted working memory deficits in R6/2-CAG240 mice at post-natal month 3. The search for possible mechanisms indicated that the genetic inactivation of A(2A)R did not affect ubiquitin-positive neuronal inclusions, astrogliosis or Thr-75 phosphorylation of DARPP-32 in the striatum. Importantly, A(2A)R blockade preferentially controlled long-term depression at cortico-striatal synapses in R6/2-CAG240 at post-natal week 6. The reported reversal of working memory deficits in R6/2 mice by the genetic and pharmacological inactivation of A(2A)R provides a proof-of-principle for A(2A)R as novel targets to reverse cognitive deficits in HD, likely by controlling LTD deregulation.

The VMAT-2 inhibitor tetrabenazine alters effort-related decision making as measured by the T-maze barrier choice task: reversal with the adenosine A2A antagonist MSX-3 and the catecholamine uptake blocker bupropion

RATIONALE

Depressed people show effort-related motivational symptoms, such as anergia, retardation, lassitude, and fatigue. Animal tests can model these motivational symptoms, and the present studies characterized the effort-related effects of the vesicular monoamine transport (VMAT-2) inhibitor tetrabenazine. Tetrabenazine produces depressive symptoms in humans and, at low doses, preferentially depletes dopamine.

OBJECTIVES

The current studies investigated the effects of tetrabenazine on effort-based decision making using the T-maze barrier task.

METHODS

Rats were tested in a T-maze in which the choice arms of the maze contain different reinforcement densities, and under some conditions, a vertical barrier was placed in the high-density arm to provide an effort-related challenge. The first experiment assessed the effects of tetrabenazine under different maze conditions: a barrier in the arm with 4 food pellets and 2 pellets in the no barrier arm (4-2 barrier), 4 pellets in one arm and 2 pellets in the other with no barrier in either arm (no barrier), and 4 pellets in the barrier arm with no pellets in the other (4-0 barrier).

RESULTS

Tetrabenazine (0.25-0.75 mg/kg IP) decreased selection of the high cost/high reward arm when the barrier was present, but had no effect on choice under the no barrier and 4-0 barrier conditions. The effects of tetrabenazine on barrier climbing in the 4-2 condition were reversed by the adenosine A2A antagonist MSX-3 and the catecholamine uptake inhibitor and antidepressant bupropion.

CONCLUSIONS

These studies have implications for the development of animal models of the motivational symptoms of depression and other disorders.

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.

Cardiac myocyte-secreted cAMP exerts paracrine action via adenosine receptor activation

Acute stimulation of cardiac β-adrenoceptors is crucial to increasing cardiac function under stress; however, sustained β-adrenergic stimulation has been implicated in pathological myocardial remodeling and heart failure. Here, we have demonstrated that export of cAMP from cardiac myocytes is an intrinsic cardioprotective mechanism in response to cardiac stress. We report that infusion of cAMP into mice averted myocardial hypertrophy and fibrosis in a disease model of cardiac pressure overload. The protective effect of exogenous cAMP required adenosine receptor signaling. This observation led to the identification of a potent paracrine mechanism that is dependent on secreted cAMP. Specifically, FRET-based imaging of cAMP formation in primary cells and in myocardial tissue from murine hearts revealed that cardiomyocytes depend on the transporter ABCC4 to export cAMP as an extracellular signal. Extracellular cAMP, through its metabolite adenosine, reduced cardiomyocyte cAMP formation and hypertrophy by activating A1 adenosine receptors while delivering an antifibrotic signal to cardiac fibroblasts by A2 adenosine receptor activation. Together, our data reveal a paracrine role for secreted cAMP in intercellular signaling in the myocardium, and we postulate that secreted cAMP may also constitute an important signal in other tissues.

Neuroprotective potential of adenosine A2A and cannabinoid CB1 receptor antagonists in an animal model of Parkinson disease

The development of nondopaminergic therapeutic strategies that may improve motor and nonmotor deficits, while possibly slowing down the neurodegenerative process and associated neuroinflammation,is a primary goal of Parkinson disease (PD) research. We investigated the neuroprotective and anti-inflammatory potential of combined and single treatment with adenosine A2A and cannabinoid CB1 receptor antagonists MSX-3 and rimonabant, respectively, in a rodent model of PD. Rats bearing a unilateral intrastriatal 6-hydroxydopamine lesion were treated chronically with MSX-3 (0.5or 1 mg/kg/d) and rimonabant (0.1 mg/kg/d) given as monotherapy or combined. The effects of the treatments to counteract dopaminergic cell death and neuroinflammation were assessed by immunohistochemistry for tyrosine hydroxylase and glial cell markers, respectively. Both rimonabant and MSX-3 (1 mg/kg/d) promoted dopaminergic neuron survival in the substantia nigra pars compacta (SNc) when given alone; this effect was weakened when the compounds were combined. Glial activation was not significantly affected by MSX-3 (1 mg/kg/d), whereas rimonabant seemed to increase astrocyte cell density in the SNc. Our findings demonstrate the neuroprotective potential of single treatments and suggest that glial cells might be involved in this protective effect. The results also indicate that the neuroprotective potential of combined therapy may not necessarily reflect or promote single-drug effects and point out that special care should be taken when considering multidrug therapies in PD.

Effort-related motivational effects of the pro-inflammatory cytokine interleukin 1-beta: studies with the concurrent fixed ratio 5/ chow feeding choice task

RATIONALE

Effort-related motivational symptoms such as anergia and fatigue are common in patients with depression and other disorders. Research implicates pro-inflammatory cytokines in depression, and administration of cytokines can induce effort-related motivational symptoms in humans.

OBJECTIVES

The present experiments focused on the effects of the pro-inflammatory cytokine interleukin 1-beta (IL-1β) on effort-related choice behavior.

METHODS

Rats were tested on a concurrent fixed ratio 5 lever pressing/chow feeding choice procedure, which assesses the tendency of rats to work for a preferred food (high carbohydrate pellets) in the presence of a concurrently available but less preferred substitute (laboratory chow).

RESULTS

IL-1β (1.0-4.0 μg/kg IP) shifted choice behavior, significantly decreasing lever pressing and increasing intake of the freely available chow. The second experiment assessed the ability of the adenosine A2A antagonist (E)-phosphoric acid mono-[3-[8-[2-(3-methoxyphenyl)vinyl]-7-methyl-2,6-dioxo-1-prop-2-ynyl-1,2,6,7-tetrahydropurin-3-yl] propyl] ester disodium salt (MSX-3) to reverse the behavioral effects of IL-1β. MSX-3 attenuated the effort-related impairments produced by IL-1β, increasing lever pressing and also decreasing chow intake. In the same dose range that shifted effort-related choice behavior, IL-1β did not alter food intake or preference in parallel free-feeding choice studies, indicating that these low doses were not generally suppressing appetite or altering preference for the high carbohydrate pellets. In addition, IL-1β did not affect core body temperature.

CONCLUSIONS

These results indicate that IL-1β can reduce the tendency to work for food, even at low doses that do not produce a general sickness, malaise, or loss of appetite. This research has implications for the involvement of cytokines in motivational symptoms such as anergia and fatigue.

L-DOPA disrupts adenosine A(2A)-cannabinoid CB(1)-dopamine D(2) receptor heteromer cross-talk in the striatum of hemiparkinsonian rats: biochemical and behavioral studies

Long-term therapy with L-3,4-dihydroxyphenylalanine (L-DOPA), still the most effective treatment in Parkinson's disease (PD), is associated with severe motor complications such as dyskinesia. Experimental and clinical data have indicated that adenosine A2A receptor antagonists can provide symptomatic improvement by potentiating L-DOPA efficacy and minimizing its side effects. It is known that the G-protein-coupled adenosine A2A, cannabinoid CB1 and dopamine D2 receptors may interact and form functional A2A-CB1-D2 receptor heteromers in co-transfected cells as well as in rat striatum. These data suggest that treatment with a combination of drugs or a single compound selectively acting on A2A-CB1-D2 heteromers may represent an alternative therapeutic treatment of PD. We investigated the expression of A2A-CB1-D2 receptor heteromers in the striatum of both naïve and hemiparkinsonian rats (HPD-rats) bearing a unilateral 6-hydroxydopamine (6-OHDA) lesion, and assessed how receptor heteromer expression and biochemical properties were affected by L-DOPA treatment. Radioligand binding data showed that A2A-CB1-D2 receptor heteromers are present in the striatum of both naïve and HPD-rats. However, behavioral results indicated that the combined administration of A2A (MSX-3 or SCH58261) and CB1 (rimonabant) receptor antagonists, in the presence of L-DOPA does not produce a response different from administration of the A2A receptor antagonist alone. These behavioral results prompted identification of heteromers in L-DOPA-treated animals. Interestingly, the radioligand binding results in samples from lesioned animals suggest that the heteromer is lost following acute or chronic treatment with L-DOPA.

Effort-related motivational effects of the VMAT-2 inhibitor tetrabenazine: implications for animal models of the motivational symptoms of depression

Motivated behaviors are often characterized by a high degree of behavioral activation, and work output and organisms frequently make effort-related decisions based upon cost/benefit analyses. Moreover, people with major depression and other disorders often show effort-related motivational symptoms such as anergia, psychomotor retardation, and fatigue. It has been suggested that tasks measuring effort-related choice behavior could be used as animal models of the motivational symptoms of depression, and the present studies characterized the effort-related effects of the vesicular monoamine transport (VMAT) inhibitor tetrabenazine. Tetrabenazine produces depressive symptoms in humans and, because of its selective inhibition of VMAT-2, it preferentially depletes dopamine (DA). Rats were assessed using a concurrent fixed-ratio 5/chow feeding choice task that is known to be sensitive to dopaminergic manipulations. Tetrabenazine shifted response choice in rats, producing a dose-related decrease in lever pressing and a concomitant increase in chow intake. However, it did not alter food intake or preference in parallel free-feeding choice studies. The effects of tetrabenazine on effort-related choice were reversed by the adenosine A2A antagonist MSX-3 and the antidepressant bupropion. A behaviorally active dose of tetrabenazine decreased extracellular DA in nucleus accumbens and increased expression of DARPP-32 in accumbens medium spiny neurons in a pattern indicative of reduced transmission at both D1 and D2 DA receptors. These experiments demonstrate that tetrabenazine, which is used in animal models to produce depression-like effects, can alter effort-related choice behavior. These studies have implications for the development of animal models of the motivational symptoms of depression and related disorders.