Xanthines as adenosine receptor antagonists

Overview of the role of purinergic signaling and insights into its role in cancer therapy

Innovation of cancer therapy has received a dramatic acceleration over the last fifteen years thanks to the introduction of the novel immune checkpoint inhibitors (ICI). On the other hand, the conspicuous scientific knowledge accumulated in purinergic signaling since the early seventies is finally being transferred to the clinic. Several Phase I/II clinical trials are currently underway to investigate the effect of drugs interfering with purinergic signaling as stand-alone or combination therapy in cancer. This is supporting the novel concept of "purinergic immune checkpoint" (PIC) in cancer therapy. In the present review we will address a) the basic pharmacology and cell biology of the purinergic system; b) principles of its pathophysiology in human diseases; c) implications for cell death, cell proliferation and cancer; d) novel molecular tools to investigate nucleotide homeostasis in the extracellular environment; e) recent developments in the pharmacology of P1, P2 receptors and related ecto-enzymes; f) P1 and P2 ligands as novel diagnostic tools; g) current issues in PIC-based anti-cancer therapy. This review will provide an appraisal of the current status of purinergic signaling in cancer and will help identify future avenues of development.

Evaluation of metabolomics-based urinary biomarker models for recognizing major depression disorder and bipolar disorder

BACKGROUND

Major depressive disorder (MDD) and bipolar disorder (BD) are psychiatric disorders with overlapping symptoms, leading to high rates of misdiagnosis due to the lack of biomarkers for differentiation. This study aimed to identify metabolic biomarkers in urine samples for diagnosing MDD and BD, as well as to establish unbiased differential diagnostic models.

METHODS

We utilized a metabolomics approach employing ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS) to analyze the metabolic profiles of urine samples from individuals with MDD (n = 50), BD (n = 12), and healthy controls (n = 50). The identification of urine metabolites was verified using MS data analysis tools and online metabolite databases.

RESULTS

Two diagnostic panels consisting of a combination of metabolites and clinical indicators were identified-one for MDD and another for BD. The discriminative capacity of these panels was assessed using the area under the receiver operating characteristic (ROC) curve, yielding an area under the curve (AUC) of 0.9084 for MDD and an AUC value of 0.9017 for BD.

CONCLUSIONS

High-resolution mass spectrometry-based assays show promise in identifying urinary biomarkers for depressive disorders. The combination of urine metabolites and clinical indicators is effective in differentiating healthy controls from individuals with MDD and BD. The metabolic pathway indicating oxidative stress is seen to significantly contribute to depressive disorders.

Effect of Caffeine on the Inflammatory-Dependent Changes in the GnRH/LH Secretion in a Female Sheep Model

Caffeine is one of the most widely consumed psychoactive drugs in the world. It easily crosses the blood-brain barrier, and caffeine-interacting adenosine and ryanodine receptors are distributed in various areas of the brain, including the hypothalamus and pituitary. Caffeine intake may have an impact on reproductive and immune function. Therefore, in the present study performed on the ewe model, we decided to investigate the effect of peripheral administration of caffeine (30 mg/kg) on the secretory activity of the hypothalamic-pituitary unit which regulates the reproductive function in females during both a physiological state and an immune/inflammatory challenge induced by lipopolysaccharide (LPS; 400 ng/kg) injection. It was found that caffeine stimulated (p < 0.01) the biosynthesis of gonadotropin-releasing hormone (GnRH) in the hypothalamus of ewe under both physiological and inflammatory conditions. Caffeine also increased (p < 0.05) luteinizing hormone (LH) secretion in ewes in a physiological state; however, a single administration of caffeine failed to completely release the LH secretion from the inhibitory influence of inflammation. This could result from the decreased expression of GnRHR in the pituitary and it may also be associated with the changes in the concentration of neurotransmitters in the median eminence (ME) where GnRH neuron terminals are located. Caffeine and LPS increased (p < 0.05) dopamine in the ME which may explain the inhibition of GnRH release. Caffeine treatment also increased (p < 0.01) cortisol release, and this stimulatory effect was particularly evident in sheep under immunological stress. Our studies suggest that caffeine affects the secretory activity of the hypothalamic-pituitary unit, although its effect appears to be partially dependent on the animal's immune status.

Anti-Inflammatory Activities of 8-Benzylaminoxanthines Showing High Adenosine A2A and Dual A1/A2A Receptor Affinity

Chronic inflammation plays an important role in the development of neurodegenerative diseases, such as Parkinson's disease (PD). In the present study, we synthesized 25 novel xanthine derivatives with variable substituents at the N1-, N3- and C8-position as adenosine receptor antagonists with potential anti-inflammatory activity. The compounds were investigated in radioligand binding studies at all four human adenosine receptor subtypes, A1, A2A, A2B and A3. Compounds showing nanomolar A2A and dual A1/A2A affinities were obtained. Three compounds, 19, 22 and 24, were selected for further studies. Docking and molecular dynamics simulation studies indicated binding poses and interactions within the orthosteric site of adenosine A1 and A2A receptors. In vitro studies confirmed the high metabolic stability of the compounds, and the absence of toxicity at concentrations of up to 12.5 µM in various cell lines (SH-SY5Y, HepG2 and BV2). Compounds 19 and 22 showed anti-inflammatory activity in vitro. In vivo studies in mice investigating carrageenan- and formalin-induced inflammation identified compound 24 as the most potent anti-inflammatory derivative. Future studies are warranted to further optimize the compounds and to explore their therapeutic potential in neurodegenerative diseases.

Drugs targeting adenosine signaling pathways: A current view

Adenosine is an endogenous nucleoside that regulates many physiological and pathological processes. It is derived from either the intracellular or extracellular dephosphorylation of adenosine triphosphate and interacts with cell-surface G-protein-coupled receptors. Adenosine plays a substantial role in protecting against cell damage in areas of increased tissue metabolism and preventing organ dysfunction in pathological states. Targeting adenosine metabolism and receptor signaling may be an effective therapeutic approach for human diseases, including cardiovascular and central nervous system disorders, rheumatoid arthritis, asthma, renal diseases, and cancer. Several lines of evidence have shown that many drugs exert their beneficial effects by modulating adenosine signaling pathways but this knowledge urgently needs to be summarized, and most importantly, actualized. The present review collects pharmaceuticals and pharmacological or diagnostic tools that target adenosine signaling in their primary or secondary mode of action. We overviewed FDA-approved drugs as well as those currently being studied in clinical trials. Among them are already used in clinic A2A adenosine receptor modulators like istradefylline or regadenoson, but also plenty of anti-platelet, anti-inflammatory, or immunosuppressive, and anti-cancer drugs. On the other hand, we investigated dozens of specific adenosine pathway regulators that are tested in clinical trials to treat human infectious and noninfectious diseases. In conclusion, targeting purinergic signaling represents a great therapeutic challenge. The actual knowledge of the involvement of adenosinergic signaling as part of the mechanism of action of old drugs has open a path not only for drug-repurposing but also for new therapeutic strategies.

Impact of the cAMP efflux and extracellular cAMP-adenosine pathway on airway smooth muscle relaxation induced by formoterol and phosphodiesterase inhibitors

β2-adrenoceptors agonists and phosphodiesterase (PDE) inhibitors are effective bronchodilators, due to their ability to increase intracellular cyclic AMP (cAMP) levels and induce airway smooth muscle (ASM) relaxation. We have shown that increment of intracellular cAMP induced by β2-adrenoceptors agonist fenoterol is followed by efflux of cAMP, which is converted by ecto-PDE and ecto-5'-nucleotidases (ecto-5'NT) to adenosine, leading to ASM contraction. Here we evaluate whether other classical bronchodilators used to treat asthma and chronic obstructive pulmonary disease (COPD) could induce cAMP efflux and, as consequence, influence the ASM contractility. Our results showed that β2-adrenoceptor agonists formoterol and PDE inhibitors IBMX, aminophylline and roflumilast induced cAMP efflux and a concentration-dependent relaxation of rat trachea precontracted with carbachol. Pretreatment of tracheas with MK-571 (MRP transporter inhibitor), AMP-CP (ecto-5'NT inhibitor) or CGS-15943 (nonselective adenosine receptor antagonist) potentiated the relaxation induced by β2-adrenoceptor agonists but did not change the relaxation induced by PDE inhibitors. These data showed that all bronchodilators tested were able to induce cAMP efflux. However, only β2-adrenoceptor-induced relaxation of tracheal smooth muscle was affected by cAMP efflux and extracellular cAMP-adenosine pathway.

Theophylline reverses oxycodone's but not fentanyl's respiratory depression in mice while caffeine is ineffective against both opioids

RATIONALE

The opioid epidemic remains a pressing public health crisis in the United States. Most of these overdose deaths are a result of lethal respiratory depression. In recent years the increasing incidence of opioid-involved overdose deaths has been driven by fentanyl, which is more resistant to adequate reversal by naloxone (NARCAN ®) than semi-synthetic or classical morphinan predecessors like oxycodone and heroin. For this and other reasons (e.g., precipitating withdrawal) non-opioidergic pharmacotherapies to reverse opioid-depressed respiration are needed. Methylxanthines are a class of stimulant drugs including caffeine and theophylline which exert their effects primarily via adenosine receptor antagonism. Evidence suggests methylxanthines can stimulate respiration by enhancing neural activity in respiratory nuclei in the pons and medulla independent of opioid receptors. This study aimed to determine whether caffeine and theophylline can stimulate respiration in mice when depressed by fentanyl and oxycodone.

METHODS

Whole-body plethysmography was used to characterize fentanyl and oxycodone's effects on respiration and their reversal by naloxone in male Swiss Webster mice. Next, caffeine and theophylline were tested for their effects on basal respiration. Finally, each methylxanthine was evaluated for its ability to reverse similar levels of respiratory depression induced by fentanyl or oxycodone.

RESULTS AND CONCLUSIONS

Oxycodone and fentanyl dose-dependently reduced respiratory minute volume (ml/min; MVb) that was reversible by naloxone. Caffeine and theophylline each significantly increased basal MVb. Theophylline, but not caffeine, completely reversed oxycodone-depressed respiration. In contrast, neither methylxanthine elevated fentanyl-depressed respiration at the doses tested. Despite their limited efficacy for reversing opioid-depressed respiration when administered alone, the methylxanthines safety, duration, and mechanism of action supports further evaluation in combination with naloxone to augment its reversal of opioid-depressed respiration.

Curcumin Stereoisomer, Cis-Trans Curcumin, as a Novel Ligand to A1 and A3 Adenosine Receptors

Adenosine receptors (ARs) are being explored to generate non-opioid pain therapeutics. Vanilloid compounds, curcumin, capsaicin, and vanillin possess antinociceptive properties through their interactions with the transient receptor potential channel family. However, their binding with adenosine receptors has not been well studied. The hypothesis in this study was that a vanilloid compound, cis-trans curcumin (CTCUR), binds to each of the two Gi-linked AR subtypes (A₁AR and A₃AR). CTCUR was synthesized from curcumin (CUR) using the cavitand-mediated photoisomerization technique. The cell lines transfected with the specific receptor (A₁AR or A₃AR) were treated with CTCUR or CUR and the binding was analyzed using competitive assays, confocal microscopy, and docking. The binding assays and molecular docking indicated that CTCUR had Ki values of 306 nM (A₁AR) and 400 nM (A₃AR). These values suggest that CTCUR is selective for Gi-linked ARs (A₁AR or A₃AR) over Gs-linked ARs (A_2AAR or A_2BAR), based on our previous published research. In addition, the docking showed that CTCUR binds to the toggle switch domain of ARs. Curcumin (CUR) did not exhibit binding at any of these receptors. In summary, CTCUR and other modifications of CUR can be developed as novel therapeutic ligands for the Gi-linked ARs (A₁AR and A₃AR) involved with pain and cancer.

Microwave-Assisted Synthesis, Structure, and Preliminary Biological Evaluation of Novel 6-Methoxy-5,6-dihydro-5-azapurines

We herein disclose the microwave-assisted synthesis of previously unreported 6-methoxy-5,6-dihydro-5-azapurines, whose purine-like scaffold is promising for drug discovery. The method is simple, fast, and relies on easily accessible reagents such as trimethyl orthoformate, acetic acid, and aminotriazole-derived N,N'-disubstituted formamidines. The preliminary biological evaluation revealed that selected representatives of synthesized 6-methoxy-5,6-dihydro-5-azapurines dose-dependently reduce the viability of HepG2 and A549 cancer cells having little to no influence on five tested purinergic receptors.

Pharmacological treatment for central sleep apnoea in adults

BACKGROUND

The term central sleep apnoea (CSA) encompasses diverse clinical situations where a dysfunctional drive to breathe leads to recurrent respiratory events, namely apnoea (complete absence of ventilation) and hypopnoea sleep (insufficient ventilation) during sleep. Studies have demonstrated that CSA responds to some extent to pharmacological agents with distinct mechanisms, such as sleep stabilisation and respiratory stimulation. Some therapies for CSA are associated with improved quality of life, although the evidence on this association is uncertain. Moreover, treatment of CSA with non-invasive positive pressure ventilation is not always effective or safe and may result in a residual apnoea-hypopnoea index.

OBJECTIVES

To evaluate the benefits and harms of pharmacological treatment compared with active or inactive controls for central sleep apnoea in adults.

SEARCH METHODS

We used standard, extensive Cochrane search methods. The latest search date was 30 August 2022.

SELECTION CRITERIA

We included parallel and cross-over randomised controlled trials (RCTs) that evaluated any type of pharmacological agent compared with active controls (e.g. other medications) or passive controls (e.g. placebo, no treatment or usual care) in adults with CSA as defined by the International Classification of Sleep Disorders 3rd Edition. We did not exclude studies based on the duration of intervention or follow-up. We excluded studies focusing on CSA due to periodic breathing at high altitudes.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methods. Our primary outcomes were central apnoea-hypopnoea index (cAHI), cardiovascular mortality and serious adverse events. Our secondary outcomes were quality of sleep, quality of life, daytime sleepiness, AHI, all-cause mortality, time to life-saving cardiovascular intervention, and non-serious adverse events. We used GRADE to assess certainty of evidence for each outcome.

MAIN RESULTS

We included four cross-over RCTs and one parallel RCT, involving a total of 68 participants. Mean age ranged from 66 to 71.3 years and most participants were men. Four trials recruited people with CSA associated with heart failure, and one study included people with primary CSA. Types of pharmacological agents were acetazolamide (carbonic anhydrase inhibitor), buspirone (anxiolytic), theophylline (methylxanthine derivative) and triazolam (hypnotic), which were given for between three days and one week. Only the study on buspirone reported a formal evaluation of adverse events. These events were rare and mild. No studies reported serious adverse events, quality of sleep, quality of life, all-cause mortality, or time to life-saving cardiovascular intervention. Carbonic anhydrase inhibitors versus inactive control Results were from two studies of acetazolamide versus placebo (n = 12) and acetazolamide versus no acetazolamide (n = 18) for CSA associated with heart failure. One study reported short-term outcomes and the other reported intermediate-term outcomes. We are uncertain whether carbonic anhydrase inhibitors compared to inactive control reduce cAHI in the short term (mean difference (MD) -26.00 events per hour, 95% CI -43.84 to -8.16; 1 study, 12 participants; very low certainty). Similarly, we are uncertain whether carbonic anhydrase inhibitors compared to inactive control reduce AHI in the short term (MD -23.00 events per hour, 95% CI -37.70 to 8.30; 1 study, 12 participants; very low certainty) or in the intermediate term (MD -6.98 events per hour, 95% CI -10.66 to -3.30; 1 study, 18 participants; very low certainty). The effect of carbonic anhydrase inhibitors on cardiovascular mortality in the intermediate term was also uncertain (odds ratio (OR) 0.21, 95% CI 0.02 to 2.48; 1 study, 18 participants; very low certainty). Anxiolytics versus inactive control Results were based on one study of buspirone versus placebo for CSA associated with heart failure (n = 16). The median difference between groups for cAHI was -5.00 events per hour (IQR -8.00 to -0.50), the median difference for AHI was -6.00 events per hour (IQR -8.80 to -1.80), and the median difference on the Epworth Sleepiness Scale for daytime sleepiness was 0 points (IQR -1.0 to 0.00). Methylxanthine derivatives versus inactive control Results were based on one study of theophylline versus placebo for CSA associated with heart failure (n = 15). We are uncertain whether methylxanthine derivatives compared to inactive control reduce cAHI (MD -20.00 events per hour, 95% CI -32.15 to -7.85; 15 participants; very low certainty) or AHI (MD -19.00 events per hour, 95% CI -30.27 to -7.73; 15 participants; very low certainty). Hypnotics versus inactive control Results were based on one trial of triazolam versus placebo for primary CSA (n = 5). Due to very serious methodological limitations and insufficient reporting of outcome measures, we were unable to draw any conclusions regarding the effects of this intervention.

AUTHORS' CONCLUSIONS

There is insufficient evidence to support the use of pharmacological therapy in the treatment of CSA. Although small studies have reported positive effects of certain agents for CSA associated with heart failure in reducing the number of respiratory events during sleep, we were unable to assess whether this reduction may impact the quality of life of people with CSA, owing to scarce reporting of important clinical outcomes such as sleep quality or subjective impression of daytime sleepiness. Furthermore, the trials mostly had short-term follow-up. There is a need for high-quality trials that evaluate longer-term effects of pharmacological interventions.

In silico and in vitro ADME-Tox analysis and in vivo pharmacokinetic study of representative pan-PDE inhibitors from the group of 7,8-disubstituted derivatives of 1,3-dimethyl-7H-purine-2,6-dione

Phosphodiesterase (PDE) inhibitors represent a wide class of chemically different compounds that have been extensively studied in recent years. Their anti-inflammatory and anti-fibrotic effects are particularly desirable in the treatment of chronic respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD). Due to diversified expression of individual PDEs within cells and/or tissues as well as PDE signaling compartmentalization, pan-PDE inhibitors (compounds capable of simultaneously blocking various PDE subtypes) are of particular interest. Recently, a large group of 7,8-disubstituted derivatives of 1,3-dimethyl-7H-purine-2,6-dione (theophylline) was designed and synthesized. These compounds were characterized as potent pan-PDE inhibitors and their prominent anti-inflammatory and anti-fibrotic activity in vitro has been proved. Herein, we investigated a general in vitro safety profile and pharmacokinetic characteristics of two leading compounds from this group: a representative compound with N'-benzylidenebutanehydrazide moiety (38) and a representative derivative containing N-phenylbutanamide fragment (145). Both tested pan-PDE inhibitors revealed no cytotoxic, mutagenic, and genotoxic activity in vitro, showed moderate metabolic stability in mouse and human liver microsomes, as well as fell into the low or medium permeation category. Additionally, 38 and 145 revealed a lack of interaction with adenosine receptors, including A1, A2A, and A2B. Pharmacokinetic analysis revealed that both tested 7,8-disubstituted derivatives of 1,3-dimethyl-7H-purine-2,6-dione were effectively absorbed from the peritoneal cavity. Simultaneously, they were extensively distributed to mouse lungs and after intraperitoneal (i.p.) administration were detected in bronchoalveolar lavage fluid. These findings provide evidence that investigated compounds represent a new drug candidates with a favorable in vitro safety profile and satisfactory pharmacokinetic properties after a single i.p. administration. As the next step, further pharmacokinetic studies after multiple i.p. and p.o. doses will be conducted to ensure effective 38 and 145 serum and lung concentrations for a longer period of time. In summary, 7,8-disubstituted derivatives of 1,3-dimethyl-7H-purine-2,6-dione represent a promising compounds worth testing in animal models of chronic respiratory diseases, the etiology of which involves various PDE subtypes.

Preliminary Evidence of the Potent and Selective Adenosine A2B Receptor Antagonist PSB-603 in Reducing Obesity and Some of Its Associated Metabolic Disorders in Mice

The adenosine A2A and A2B receptors are promising therapeutic targets in the treatment of obesity and diabetes since the agonists and antagonists of these receptors have the potential to positively affect metabolic disorders. The present study investigated the link between body weight reduction, glucose homeostasis, and anti-inflammatory activity induced by a highly potent and specific adenosine A2B receptor antagonist, compound PSB-603. Mice were fed a high-fat diet for 14 weeks, and after 12 weeks, they were treated for 14 days intraperitoneally with the test compound. The A1/A2A/A2B receptor antagonist theophylline was used as a reference. Following two weeks of treatment, different biochemical parameters were determined, including total cholesterol, triglycerides, glucose, TNF-α, and IL-6 blood levels, as well as glucose and insulin tolerance. To avoid false positive results, mouse locomotor and spontaneous activities were assessed. Both theophylline and PSB-603 significantly reduced body weight in obese mice. Both compounds had no effects on glucose levels in the obese state; however, PSB-603, contrary to theophylline, significantly reduced triglycerides and total cholesterol blood levels. Thus, our observations showed that selective A2B adenosine receptor blockade has a more favourable effect on the lipid profile than nonselective inhibition.

Design and synthesis of five-membered heterocyclic derivatives of istradefylline with comparable pharmacological activity

Parkinson's disease (PD) is a common degenerative disease of the central nervous system among the elderly. Istradefylline, an FDA-approved adenosine A_2A receptor antagonist (anti-PD drug), has good efficacy. However, it has been reported that the double bond of istradefylline is easily converted into cis-configuration when exposed to an indoor environment or direct light in a dilute solution. In order to find more stable adenosine A_2A receptor antagonists with similar pharmacological efficacy to istradefylline, the compounds series I-1 (12 compounds) was designed by maintaining the xanthine skeleton of istradefylline unchanged and replacing the trans-double bond with thiazole or benzothiazole and other biologically active heterocyclic compounds. These compounds were synthesized via multi-step experiment and successfully confirmed through different characterization techniques for their ability to inhibit cAMP formation in A_2A AR overexpressing cells. The thiazole derivative of istradefylline (Compound I-1-11, I-1-12) exhibited significant activity (IC₅₀  = 16.74 ± 4.11 μM, 10.36 ± 3.09 μM), as compared to istradefylline (IC₅₀  = 5.05 ± 1.32 μM). In addition, the molecular docking of benzothiazole derivatives I-1-11 and thiazole derivatives I-1-12 with higher inhibition rate were carried out and compared with istradefylline. The molecular docking results showed that I-1-11 and I-1-12 anchored in the same site as that of XAC (3REY) with predicted affinity binding energy -6.63 kcal/mol and - 6.75 kcal/mol, respectively. Validation through dynamics simulation also showed stable interactions, with fluctuations <3 Å and MM/GBSA energy <-20 kcal/mol. Hence, this study could provide a basis for the rational design of adenosine A_2A receptor antagonists with better potency.

Quantitation of the A2A Adenosine Receptor Density in the Striatum of Mice and Pigs with [18F]FLUDA by Positron Emission Tomography

The cerebral expression of the A_2A adenosine receptor (A_2AAR) is altered in neurodegenerative diseases such as Parkinson’s (PD) and Huntington’s (HD) diseases, making these receptors an attractive diagnostic and therapeutic target. We aimed to further investigate the pharmacokinetic properties in the brain of our recently developed A_2AAR–specific antagonist radiotracer [¹⁸F]FLUDA. For this purpose, we retrospectively analysed dynamic PET studies of healthy mice and rotenone–treated mice, and conducted dynamic PET studies with healthy pigs. We performed analysis of mouse brain time–activity curves to calculate the mean residence time (MRT) by non–compartmental analysis, and the binding potential (BP_ND) of [¹⁸F]FLUDA using the simplified reference tissue model (SRTM). For the pig studies, we performed a Logan graphical analysis to calculate the radiotracer distribution volume (V_T) at baseline and under blocking conditions with tozadenant. The MRT of [¹⁸F]FLUDA in the striatum of mice was decreased by 30% after treatment with the A_2AAR antagonist istradefylline. Mouse results showed the highest BP_ND (3.9 to 5.9) in the striatum. SRTM analysis showed a 20% lower A_2AAR availability in the rotenone–treated mice compared to the control–aged group. Tozadenant treatment significantly decreased the V_T (14.6 vs. 8.5 mL · g⁻¹) and BP_ND values (1.3 vs. 0.3) in pig striatum. This study confirms the target specificity and a high BP_ND of [¹⁸F]FLUDA in the striatum. We conclude that [¹⁸F]FLUDA is a suitable tool for the non–invasive quantitation of altered A_2AAR expression in neurodegenerative diseases such as PD and HD, by PET.

A2A Adenosine Receptor Antagonists: Are Triazolotriazine and Purine Scaffolds Interchangeable?

The A_2A adenosine receptor (A_2AAR) is one of the four subtypes activated by nucleoside adenosine, and the molecules able to selectively counteract its action are attractive tools for neurodegenerative disorders. In order to find novel A_2AAR ligands, two series of compounds based on purine and triazolotriazine scaffolds were synthesized and tested at ARs. Compound 13 was also tested in an in vitro model of neuroinflammation. Some compounds were found to possess high affinity for A_2AAR, and it was observed that compound 13 exerted anti-inflammatory properties in microglial cells. Molecular modeling studies results were in good agreement with the binding affinity data and underlined that triazolotriazine and purine scaffolds are interchangeable only when 5- and 2-positions of the triazolotriazine moiety (corresponding to the purine 2- and 8-positions) are substituted.

Caffeine Supplementation Strategies Among Endurance Athletes

Caffeine is widely accepted as an endurance-performance enhancing supplement. Most scientific research studies use doses of 3–6 mg/kg of caffeine 60 min prior to exercise based on pharmacokinetics. It is not well understood whether endurance athletes employ similar supplementation strategies in practice. The purpose of this study was to investigate caffeine supplementation protocols among endurance athletes. A survey conducted on Qualtrics returned responses regarding caffeine supplementation from 254 endurance athletes (f = 134, m =120; age = 39.4 ± 13.9 y; pro = 11, current collegiate athlete = 37, recreational = 206; running = 98, triathlon = 83, cycling = 54, other = 19; training days per week = 5.4 ± 1.3). Most participants reported habitual caffeine consumption (85.0%; 41.2% multiple times daily). However, only 24.0% used caffeine supplements. A greater proportion of men (31.7%) used caffeine supplements compared with women (17.2%; p = 0.007). Caffeine use was also more prevalent among professional (45.5%) and recreational athletes (25.1%) than in collegiate athletes (9.4%). Type of sport (p = 0.641), household income (p = 0.263), education (p = 0.570) or working with a coach (p = 0.612) did not have an impact on caffeine supplementation prevalence. Of those reporting specific timing of caffeine supplementation, 49.1% and 34.9% reported consuming caffeine within 30 min of training and races respectively; 38.6 and 36.5% used caffeine 30–60 min before training and races. Recreational athletes reported consuming smaller amounts of caffeine before training (1.6 ± 1.0 mg/kg) and races (2.0 ± 1.2 mg/kg) compared with collegiate (TRG: 2.1 ± 1.2 mg/kg; RACE: 3.6 ± 0.2 mg/kg) and professional (TRG: 2.4 ± 1.1 mg/kg; RACE: 3.5 ± 0.6 mg/kg) athletes. Overall, participants reported minor to moderate perceived effectiveness of caffeine supplementation (2.31 ± 0.9 on a four-point Likert-type scale) with greatest effectiveness during longer sessions (2.8 ± 1.1). It appears that recreational athletes use lower caffeine amounts than what has been established as ergogenic in laboratory protocols; further, they consume caffeine closer to exercise compared with typical research protocols. Thus, better education of recreational athletes and additional research into alternative supplementation strategies are warranted.

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.

Antiinflammatory and antiinfective effect of caffeine in a mouse model of disseminated salmonellosis

Caffeine has been reported for its antiinflammatory properties by stimulating phagocytosis. In this study, we investigated the antiinflammatory and antiinfective potential of caffeine in murine macrophage cell cultures and Swiss mice infected with virulent Salmonella enterica serotype typhimurium. Peritoneal macrophages (pMØ) were treated with caffeine on 96-well plates for 24 hr and then infected with Salmonella for 4 hr. In another experiment, the pMØ were first infected with the bacterium for 4 hr and then treated with caffeine for 24 hr. In addition, Swiss mice were inoculated, intraperitoneally, with S. typhimurium and then received caffeine intravenously. Control groups received phosphate-buffered saline (PBS) or dexamethasone. We found that treatments with caffeine increased the macrophage cell viability and reduced the intracellular bacterial load. The administration of caffeine to Swiss mice reduced the infiltration of leukocytes into the peritoneal cavity after the bacterial challenge. Furthermore, the bacterial burdens in the peritoneal fluid, bloodstream, spleen, and liver were decreased by caffeine treatment. The expression levels of tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), IL-6, and inducible nitric oxide synthase (iNOs) were down-regulated after infection in caffeine-treated mice. We can conclude that caffeine has both antiinflammatory and antiinfective properties that can be useful for management of bacterial infections along with antibiotics.

Adenosine receptor antagonists: Recent advances and therapeutic perspective

Adenosine is an endogenous purine-based nucleoside expressed nearly in all body tissues. It regulates various body functions by activating four G-protein coupled receptors, A₁, A_2A, A_2B, and A₃. These receptors are widely acknowledged as drug targets for treating different neurological, metabolic, and inflammatory diseases. Although numerous adenosine receptor inhibitors have been developed worldwide, achieving target selectivity is still a big hurdle in drug development. However, the identification of specific radioligands-based affinity assay, fluorescent ligands, and MS-based ligand assay have contributed to the development of selective and potent adenosine ligands. In recent years various small heterocyclic-based molecules have shown some promising results. Istradefylline has been approved for treating Parkinson's in Japan, while preladenant, tozadenant, CVT-6883, MRS-1523, and many more are under different phases of clinical development. The present review is focused on the quest to develop potent and selective adenosine inhibitors from 2013 to early 2021 by various research groups. The review also highlights their biological activity, selectivity, structure-activity relationship, molecular docking, and mechanistic studies. A special emphsesis on drug designing strategies has been also given the manuscript. The comprehensive compilation of research work carried out in the field will provide inevitable scope for designing and developing novel adenosine inhibitors with improved selectivity and efficacy.

Coronary Microvascular Dysfunction: A Practical Approach to Diagnosis and Management

PURPOSE OF REVIEW

The purpose of this review is to provide an overview of diagnostic and treatment considerations in patients with coronary microvascular dysfunction (CMD) in the absence of obstructive coronary artery disease (CAD).

RECENT FINDINGS

The prevalence of obstructive CAD in unselected patient populations referred for evaluation of angina is less than 10%. A significant proportion of patients with angina and no obstructive CAD have CMD, a condition associated with impaired cardiovascular prognosis. Non-invasive and invasive evaluation of coronary microvascular function is feasible and widely available, yet CMD is underdiagnosed and undertreated. A patient-tailored treatment approach guided by coronary microvascular testing shows promising results for patient-reported outcomes of symptom burden and quality of life. Coronary microvascular testing should be considered in angina patients with no obstructive CAD, before other causes of chest pain are explored. A patient-tailored treatment approach guided by a complete evaluation of epicardial anatomy and macro-and microvascular function may help optimize treatment strategy and prevent unnecessary medical interventions. More research is needed to establish the long-term effect of patient-tailored therapies on risk reduction in CMD.

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

Lighting Up the Plasma Membrane: Development and Applications of Fluorescent Ligands for Transmembrane Proteins

Despite the fact that transmembrane proteins represent the main therapeutic targets for decades, complete and in-depth knowledge about their biochemical and pharmacological profiling is not fully available. In this regard, target-tailored small-molecule fluorescent ligands are a viable approach to fill in the missing pieces of the puzzle. Such tools, coupled with the ability of high-precision optical techniques to image with an unprecedented resolution at a single-molecule level, helped unraveling many of the conundrums related to plasma proteins' life-cycle and druggability. Herein, we review the recent progress made during the last two decades in fluorescent ligand design and potential applications in fluorescence microscopy of voltage-gated ion channels, ligand-gated ion channels and G-coupled protein receptors.

The Medicinal Chemistry of Caffeine

The purine alkaloid caffeine is the most widely consumed psychostimulant drug in the world and has multiple beneficial pharmacological activities, for example, in neurodegenerative diseases. However, despite being an extensively studied bioactive natural product, the mechanistic understanding of caffeine's pharmacological effects is incomplete. While several molecular targets of caffeine such as adenosine receptors and phosphodiesterases have been known for decades and inspired numerous medicinal chemistry programs, new protein interactions of the xanthine are continuously discovered providing potentially improved pharmacological understanding and a molecular basis for future medicinal chemistry. In this Perspective, we gather knowledge on the confirmed protein interactions, structure activity relationship, and chemical biology of caffeine on well-known and upcoming targets. The diversity of caffeine's molecular activities on receptors and enzymes, many of which are abundant in the CNS, indicates a complex interplay of several mechanisms contributing to neuroprotective effects and highlights new targets as attractive subjects for drug discovery.

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

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

New framework for a non-animal approach adequately assures the safety of cosmetic ingredients - A case study on caffeine

This case study on the model substance caffeine demonstrates the viability of a 10-step read-across (RAX) framework in practice. New approach methodologies (NAM), including RAX and physiologically-based kinetic (PBK) modelling were used to assess the consumer safety of caffeine. Appropriate animal systemic toxicity data were used from the most relevant RAX analogue while assuming that no suitable animal toxicity data were available for caffeine. Based on structural similarities, three primary metabolites of the target chemical caffeine (theophylline, theobromine and paraxanthine) were selected as its most relevant analogues, to estimate a point of departure in order to support a next generation risk assessment (NGRA). On the basis of the pivotal mode of action (MOA) of caffeine and other methylxanthines, theophylline appeared to be the most potent and suitable analogue. A worst-case aggregate exposure assessment determined consumer exposure to caffeine from different sources, such as cosmetics and food/drinks. Using a PBK model to estimate human blood concentrations following exposure to caffeine, an acceptable Margin of Internal Exposure (MOIE) of 27-fold was derived on the basis of a RAX using theophylline animal data, which suggests that the NGRA approach for caffeine is sufficiently conservative to protect human health.

Transthoracic Assessment of Coronary Flow Velocity Reserve: A Practical Approach to Diagnostic Testing in Patients with Angina and No Obstructive Coronary Artery Disease

More than half of the patients with symptoms suggestive of myocardial ischemia presenting at invasive angiography have no obstructive coronary artery disease (CAD). A large proportion of these patients have ischemia caused by coronary microvascular dysfunction, a condition associated with adverse cardiovascular prognosis. Measurement of coronary flow velocity reserve by transthoracic Doppler echocardiography is a feasible and reproducible method for the evaluation of coronary microvascular function. This review provides a practical overview of the method in a clinical setting of angina and noobstructive CAD, including technical details and prognostic significance.

Unique Role of Caffeine Compared to Other Methylxanthines (Theobromine, Theophylline, Pentoxifylline, Propentofylline) in Regulation of AD Relevant Genes in Neuroblastoma SH-SY5Y Wild Type Cells

Methylxanthines are a group of substances derived from the purine base xanthine with a methyl group at the nitrogen on position 3 and different residues at the nitrogen on position 1 and 7. They are widely consumed in nutrition and used as pharmaceuticals. Here we investigate the transcriptional regulation of 83 genes linked to Alzheimer’s disease in the presence of five methylxanthines, including the most prominent naturally occurring methylxanthines—caffeine, theophylline and theobromine—and the synthetic methylxanthines pentoxifylline and propentofylline. Methylxanthine-regulated genes were found in pathways involved in processes including oxidative stress, lipid homeostasis, signal transduction, transcriptional regulation, as well as pathways involved in neuronal function. Interestingly, multivariate analysis revealed different or inverse effects on gene regulation for caffeine compared to the other methylxanthines, which was further substantiated by multiple comparison analysis, pointing out a distinct role for caffeine in gene regulation. Our results not only underline the beneficial effects of methylxanthines in the regulation of genes in neuroblastoma wild-type cells linked to neurodegenerative diseases in general, but also demonstrate that individual methylxanthines like caffeine mediate unique or inverse expression patterns. This suggests that the replacement of single methylxanthines by others could result in unexpected effects, which could not be anticipated by the comparison to other substances in this substance class.

Adenosine A2A and A3 Receptors as Targets for the Treatment of Hypertensive-Diabetic Nephropathy

Diabetic nephropathy (DN) and hypertension are prime causes for end-stage renal disease (ESRD) that often coexist in patients, but are seldom studied in combination. Kidney adenosine levels are markedly increased in diabetes, and the expression and function of renal adenosine receptors are altered in experimental diabetes. The aim of this work is to explore the impact of endogenous and exogenous adenosine on the expression/distribution profile of its receptors along the nephron of hypertensive rats with experimentally-induced diabetes. Using spontaneously hypertensive (SHR) rats rendered diabetic with streptozotocin (STZ), we show that treatment of SHR-STZ rats with an agonist of adenosine receptors increases A_2A immunoreactivity in superficial glomeruli (SG), proximal tubule (PCT), and distal tubule (DCT). Differently, treatment of SHR-STZ rats with a xanthinic antagonist of adenosine receptors decreases adenosine A₃ immunoreactivity in SG, PCT, DCT, and collecting duct. There is no difference in the immunoreactivity against the adenosine A₁ and A_2B receptors between the experimental groups. The agonist of adenosine receptors ameliorates renal fibrosis, probably via A_2A receptors, while the antagonist exacerbates it, most likely due to tonic activation of A₃ receptors. The reduction in adenosine A₃ immunoreactivity might be due to receptor downregulation in response to prolonged activation. Altogether, these results suggest an opposite regulation exerted by endogenous and exogenous adenosine upon the expression of its A_2A and A₃ receptors along the nephron of hypertensive diabetic rats, which has a functional impact and should be taken into account when considering novel therapeutic targets for hypertensive-diabetic nephropathy.

Therapeutic Perspectives of Adenosine Deaminase Inhibition in Cardiovascular Diseases

Adenosine deaminase (ADA) is an enzyme of purine metabolism that irreversibly converts adenosine to inosine or 2'deoxyadenosine to 2'deoxyinosine. ADA is active both inside the cell and on the cell surface where it was found to interact with membrane proteins, such as CD26 and adenosine receptors, forming ecto-ADA (eADA). In addition to adenosine uptake, the activity of eADA is an essential mechanism that terminates adenosine signaling. This is particularly important in cardiovascular system, where adenosine protects against endothelial dysfunction, vascular inflammation, or thrombosis. Besides enzymatic function, ADA protein mediates cell-to-cell interactions involved in lymphocyte co-stimulation or endothelial activation. Furthermore, alteration in ADA activity was demonstrated in many cardiovascular pathologies such as atherosclerosis, myocardial ischemia-reperfusion injury, hypertension, thrombosis, or diabetes. Modulation of ADA activity could be an important therapeutic target. This work provides a systematic review of ADA activity and anchoring inhibitors as well as summarizes the perspectives of their therapeutic use in cardiovascular pathologies associated with increased activity of ADA.

Adenosine Signaling in Autoimmune Disorders

The molecular components of the purinergic system (i.e., receptors, metabolizing enzymes and membrane transporters) are widely expressed in the cells of the immune system. Additionally, high concentrations of adenosine are generated from the hydrolysis of ATP in any "danger" condition, when oxygen and energy availability dramatically drops. Therefore, adenosine acts as a retaliatory metabolite to counteract the nucleotide-mediated boost of the immune reaction. Based on this observation, it can be foreseen that the recruitment with selective agonists of the receptors involved in the immunomodulatory effect of adenosine might represent an innovative anti-inflammatory approach with potential exploitation in autoimmune disorders. Quite surprisingly, pro-inflammatory activity exerted by some adenosine receptors has been also identified, thus paving the way for the hypothesis that at least some autoimmune disorders may be caused by a derailment of adenosine signaling. In this review article, we provide a general overview of the roles played by adenosine on immune cells with a specific focus on the development of adenosine-based therapies for autoimmune disorders, as demonstrated by the exciting data from concluded and ongoing clinical trials.

A Novel, Pan-PDE Inhibitor Exerts Anti-Fibrotic Effects in Human Lung Fibroblasts via Inhibition of TGF-β Signaling and Activation of cAMP/PKA Signaling

Phosphodiesterase (PDE) inhibitors are currently a widespread and extensively studied group of anti-inflammatory and anti-fibrotic compounds which may find use in the treatment of numerous lung diseases, including asthma and chronic obstructive pulmonary disease. Several PDE inhibitors are currently in clinical development, and some of them, e.g., roflumilast, are already recommended for clinical use. Due to numerous reports indicating that elevated intracellular cAMP levels may contribute to the alleviation of inflammation and airway fibrosis, new and effective PDE inhibitors are constantly being sought. Recently, a group of 7,8-disubstituted purine-2,6-dione derivatives, representing a novel and prominent pan-PDE inhibitors has been synthesized. Some of them were reported to modulate transient receptor potential ankyrin 1 (TRPA1) ion channels as well. In this study, we investigated the effect of selected derivatives (832—a pan-PDE inhibitor, 869—a TRPA1 modulator, and 145—a pan-PDE inhibitor and a weak TRPA1 modulator) on cellular responses related to airway remodeling using MRC-5 human lung fibroblasts. Compound 145 exerted the most considerable effect in limiting fibroblast to myofibroblasts transition (FMT) as well as proliferation, migration, and contraction. The effect of this compound appeared to depend mainly on its strong PDE inhibitory properties, and not on its effects on TRPA1 modulation. The strong anti-remodeling effects of 145 required activation of the cAMP/protein kinase A (PKA)/cAMP response element-binding protein (CREB) pathway leading to inhibition of transforming growth factor type β₁ (TGF-β₁) and Smad-dependent signaling in MRC-5 cells. These data suggest that the TGF-β pathway is a major target for PDE inhibitors leading to inhibitory effects on cell responses involved in airway remodeling. These potent, pan-PDE inhibitors from the group of 7,8-disubstituted purine-2,6-dione derivatives, thus represent promising anti-remodeling drug candidates for further research.

New Insights into Key Determinants for Adenosine 1 Receptor Antagonists Selectivity Using Supervised Molecular Dynamics Simulations

Adenosine receptors (ARs), like many otherGprotein-coupledreceptors (GPCRs), are targets of primary interest indrug design. However, one of the main limits for the development of drugs for this class of GPCRs is the complex selectivity profile usually displayed by ligands. Numerous efforts have been madefor clarifying the selectivity of ARs, leading to the development of many ligand-based models. The structure of the AR subtype A₁ (A₁AR) has been recently solved, providing important structural insights. In the present work, we rationalized the selectivity profile of two selective A₁AR and A_2AAR antagonists, investigating their recognition trajectories obtained by Supervised Molecular Dynamics from an unbound state and monitoring the role of the water molecules in the binding site.

Conflicting Effects of Coffee Consumption on Cardiovascular Diseases: Does Coffee Consumption Aggravate Pre-existing Risk Factors?

Coffee is one of the most popular beverages worldwide. Its effect on health is generally regarded as beneficial in many studies. However, there are growing concerns about the adverse effect of coffee consumption on cardiovascular disease (CVD) due to the potential aggravating impact on the cardiovascular system attributed to various compounds within coffee. This review is focused on deteriorative effects of coffee consumption on CVDs with possible mechanisms. Patients with risk factors of CVDs should prudently consider heavy consumption of coffee as it may exacerbate hypertension, dyslipidemia, and atherosclerosis, and increase the odds of cardiovascular events. J-shaped or U-shaped dose-response graphs of coffee consumption and CVD parameters partially explain the inconsistency of conclusions between coffee studies on CVD, highlighting a moderate intake of coffee. Moreover, there are discrepancies in results from clinical studies elucidating considerable influences of confounding factors including gender and smoking status on outcomes of those conducted to reveal the actual impact of coffee consumption on CVDs. Physical features of subjects including genetic variations and body mass index (BMI) make it difficult to determine moderate intake of coffee for individuals in terms of caffeine metabolism. Further epidemiological studies with consideration about characteristics of the study population are needed to determine the exact effect of coffee consumption on CVD.

Caffeine and its main targets of colorectal cancer

Caffeine is a purine alkaloid and is widely consumed in coffee, soda, tea, chocolate and energy drinks. To date, a growing number of studies have indicated that caffeine is associated with many diseases including colorectal cancer. Caffeine exerts its biological activity through binding to adenosine receptors, inhibiting phosphodiesterases, sensitizing calcium channels, antagonizing gamma-aminobutyric acid receptors and stimulating adrenal hormones. Some studies have indicated that caffeine can interact with signaling pathways such as transforming growth factor β, phosphoinositide-3-kinase/AKT/mammalian target of rapamycin and mitogen-activated protein kinase pathways through which caffeine can play an important role in colorectal cancer pathogenesis, metastasis and prognosis. Moreover, caffeine can act as a general antioxidant that protects cells from oxidative stress and also as a regulatory factor of the cell cycle that modulates the DNA repair system. Additionally, as for intestinal homeostasis, through the interaction with receptors and cytokines, caffeine can modulate the immune system mediating its effects on T lymphocytes, B lymphocytes, natural killer cells and macrophages. Furthermore, caffeine can not only directly inhibit species in the gut microbiome, such as Escherichia coli and Candida albicans but also can indirectly exert inhibition by increasing the effects of other antimicrobial drugs. This review summarizes the association between colorectal cancer and caffeine that is being currently studied.

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.

Selective small molecule PARG inhibitor causes replication fork stalling and cancer cell death

Poly(ADP-ribose)ylation (PARylation) by PAR polymerase 1 (PARP1) and PARylation removal by poly(ADP-ribose) glycohydrolase (PARG) critically regulate DNA damage responses; yet, conflicting reports obscure PARG biology and its impact on cancer cell resistance to PARP1 inhibitors. Here, we found that PARG expression is upregulated in many cancers. We employed chemical library screening to identify and optimize methylxanthine derivatives as selective bioavailable PARG inhibitors. Multiple crystal structures reveal how substituent positions on the methylxanthine core dictate binding modes and inducible-complementarity with a PARG-specific tyrosine clasp and arginine switch, supporting inhibitor specificity and a competitive inhibition mechanism. Cell-based assays show selective PARG inhibition and PARP1 hyperPARylation. Moreover, our PARG inhibitor sensitizes cells to radiation-induced DNA damage, suppresses replication fork progression and impedes cancer cell survival. In PARP inhibitor-resistant A172 glioblastoma cells, our PARG inhibitor shows comparable killing to Nedaplatin, providing further proof-of-concept that selectively inhibiting PARG can impair cancer cell survival.

Novel phosphodiesterases inhibitors from the group of purine-2,6-dione derivatives as potent modulators of airway smooth muscle cell remodelling

Airway remodelling (AR) is an important pathological feature of chronic asthma and chronic obstructive pulmonary disease. The etiology of AR is complex and involves both lung structural and immune cells. One of the main contributors to airway remodelling is the airway smooth muscle (ASM), which is thickened by asthma, becomes more contractile and produces more extracellular matrix. As a second messenger, adenosine 3',5'-cyclic monophosphate (cAMP) has been shown to contribute to ASM cell (ASMC) relaxation as well as to anti-remodelling effects in ASMC. Phosphodiesterase (PDE) inhibitors have drawn attention as an interesting new group of potential anti-inflammatory and anti-remodelling drugs. Recently, new hydrazide and amide purine-2,6-dione derivatives with anti-inflammatory properties have been synthesized by our team (compounds 1 and 2). We expanded our study of their PDE selectivity profile, ability to increase intracellular cAMP levels, metabolic stability and, above all, their capacity to modulate cell responses associated with ASMC remodelling. The results show that both compounds have subtype specificity for several PDE isoforms (including inhibition of PDE1, PDE3, PDE4 and PDE7). Interestingly, such combined PDE subtype inhibition exerts improved anti-remodelling efficacies against several ASMC-induced responses such as proliferation, contractility, extracellular matrix (ECM) protein expression and migration when compared to other non-selective and selective PDE inhibitors. Our findings open novel perspectives in the search for new chemical entities with dual anti-inflammatory and anti-remodelling profiles in the group of purine-2,6-dione derivatives as broad-spectrum PDE inhibitors.

Effect of Caffeine and Other Methylxanthines on Aβ-Homeostasis in SH-SY5Y Cells

Methylxanthines (MTX) are alkaloids derived from the purine-base xanthine. Whereas especially caffeine, the most prominent known MTX, has been formerly assessed to be detrimental, this point of view has changed substantially. MTXs are discussed to have beneficial properties in neurodegenerative diseases, however, the mechanisms of action are not completely understood. Here we investigate the effect of the naturally occurring caffeine, theobromine and theophylline and the synthetic propentofylline and pentoxifylline on processes involved in Alzheimer's disease (AD). All MTXs decreased amyloid-β (Aβ) level by shifting the amyloid precursor protein (APP) processing from the Aβ-producing amyloidogenic to the non-amyloidogenic pathway. The α-secretase activity was elevated whereas β-secretase activity was decreased. Breaking down the molecular mechanism, caffeine increased protein stability of the major α-secretase ADAM10, downregulated BACE1 expression and directly decreased β-secretase activity. Additionally, APP expression was reduced. In line with literature, MTXs reduced oxidative stress, decreased cholesterol and a decreased in Aβ1-42 aggregation. In conclusion, all MTXs act via the pleiotropic mechanism resulting in decreased Aβ and show beneficial properties with respect to AD in neuroblastoma cells. However, the observed effect strength was moderate, suggesting that MTXs should be integrated in a healthy diet rather than be used exclusively to treat or prevent AD.

New A2A adenosine receptor antagonists: a structure-based upside-down interaction in the receptor cavity

Adenosine receptor antagonists are generally based on heterocyclic core structures presenting substituents of various volumes and chemical-physical profiles. Adenine and purine-based adenosine receptor antagonists have been reported in literature. In this work we combined various substituents in the 2, 6, and 8-positions of 9-ethylpurine to depict a structure-affinity relationship analysis at the human adenosine receptors. Compounds were rationally designed trough molecular modeling analysis and then synthesized and evaluated at radioligand binding studies at human adenosine receptors. The new compounds showed affinity for the human adenosine receptors, with some derivatives endowed with low nanomolar K_i data, in particular at the A_2AAR subtype. The purine core proves to be a versatile core structure for the development of novel adenosine receptor antagonists with nanomolar affinity for these membrane proteins.

Stimulatory Effects of KPR-A148 on Osteoblast Differentiation and Bone Regeneration

Background

Xanthine derivatives have been used to treat a variety of medical conditions including respiratory disease and neural degeneration. However, few studies have reported their effects on bone regeneration. Therefore, we investigated the effects of KPR-A148, a synthetic xanthine derivative on osteoblast differentiation in vitro and bone regeneration in vivo.

Methods

The cytotoxicity of KPR-A148 was evaluated using MC3T3-E1 cells by the 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltertrazolium bromide assay. The effects of KPR-A148 on osteoblast differentiation were examined by alkaline phosphatase staining, Alizarin red S staining, and real-time PCR of osteoblast differentiation marker genes. To investigate the effects of KPR-A148 on in vivo bone regeneration, a KPR-A148-containing collagen sponge was implanted into a mouse calvarial defect and KPR-A148 was injected twice, weekly. Bone regeneration was evaluated quantitatively by micro-CT and qualitatively by hematoxylin and eosin, as well as Masson's Trichrome staining.

Results

KPR-A148 did not show toxicity in the MC3T3-E1 cells and promoted osteoblast differentiation in a concentration-dependent manner. 10 μM of KPR-A148 showed the most significant effect on alkaline phospatase staining and matrix mineralization. KPR-A148 increased the expression of osteoblast marker genes in both the early and late stages of differentiation. In addition, KPR-A148 significantly induced new bone formation in the calvarial defect model.

Conclusion

These results demonstrate that KPR-A148 strongly induces osteoblast differentiation and new bone formation. Therefore, it could be used as a potential therapeutic agent for regenerating bone following its destruction by disease or trauma.

Lipotoxicity-associated inflammation is prevented by guarana (Paullinia cupana) in a model of hyperlipidemia

Hyperlipidemia causes lipotoxicity which prompts an inflammatory response linked to the development of cardiovascular diseases. Natural compounds have been receiving special attention for its potential to treat diseases, inexpensiveness, and safety. Guarana (Paullinia cupana) has demonstrated notable anti-inflammatory and antioxidant effects, which may prevent chronic diseases caused by changes in lipid profile. Thus, this study aims to evaluate the effect of guarana powder (Paullinia cupana) in the purine metabolism and inflammatory profile in lymphocytes and serum of rats with Poloxamer-407-induced hyperlipidemia. Pretreatment with guarana 12.5, 25, and 50 mg/kg/day or caffeine (0.2 mg/kg/day) by gavage was applied to adult male Wistar rats for a period of 30 days. As a comparative standard, we used simvastatin (0.04 mg/kg) post-induction. Hyperlipidemia was acutely induced with intraperitoneally injection of Poloxamer-407 (500 mg/kg). Guarana powder and caffeine increased the activity of the E-NTPDase (ecto-apyrase), and all pretreatments decreased the E-ADA (ecto-adenosine deaminase) activity, reducing the inflammatory process caused by lipotoxicity. In hyperlipidemic rats, ATP levels were increased while adenosine levels were decreased, guarana and caffeine reverted these changes. Guarana powder, caffeine, and simvastatin also prevented the increase in INF-γ and potentiated the increase in IL-4 levels, promoting an anti-inflammatory profile. Guarana promoted a more robust effect than caffeine. Our results show that guarana powder and caffeine have an anti-inflammatory as seen by the shift from a proinflammatory to an anti-inflammatory profile. The effects of guarana were more pronounced, suggesting that guarana powder may be used as a complementary therapy to improve the lipotoxicity-associated inflammation.

Relevance of In Vitro Metabolism Models to PET Radiotracer Development: Prediction of In Vivo Clearance in Rats from Microsomal Stability Data

The prediction of in vivo clearance from in vitro metabolism models such as liver microsomes is an established procedure in drug discovery. The potentials and limitations of this approach have been extensively evaluated in the pharmaceutical sector; however, this is not the case for the field of positron emission tomography (PET) radiotracer development. The application of PET radiotracers and classical drugs differs greatly with regard to the amount of substance administered. In typical PET imaging sessions, subnanomolar quantities of the radiotracer are injected, resulting in body concentrations that cannot be readily simulated in analytical assays. This raises concerns regarding the predictability of radiotracer clearance from in vitro data. We assessed the accuracy of clearance prediction for three prototypical PET radiotracers developed for imaging the A₁ adenosine receptor (A₁AR). Using the half-life (t_1/2) approach and physiologically based scaling, in vivo clearance in the rat model was predicted from microsomal stability data. Actual clearance could be accurately predicted with an average fold error (AFE) of 0.78 and a root mean square error (RMSE) of 1.6. The observed slight underprediction (1.3-fold) is in accordance with the prediction accuracy reported for classical drugs. This result indicates that the prediction of radiotracer clearance is possible despite concentration differences of more than three orders of magnitude between in vitro and in vivo conditions. Consequently, in vitro metabolism models represent a valuable tool for PET radiotracer development.

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.

Influence of incubation conditions on microsomal metabolism of xanthine-derived A1 adenosine receptor ligands

INTRODUCTION

In vitro metabolism models such as liver microsomes represent an important tool for the development of novel radioligands. Comparability and physiological relevance of in vitro metabolism data critically depend on the careful evaluation and optimization of assay protocols. We therefore investigated the influence of incubation conditions on the microsomal stability of xanthine-derived A₁ adenosine receptor (A₁AR) ligands which have been developed for positron emission tomography (PET).

METHODS

Substrate depletion assays using rat liver microsomes (RLM) were performed for three analogous compounds which differ with regard to the metabolically vulnerable substituent at the xanthine C8 position. Incubation conditions were varied systematically. Additionally, the stability of the cofactor NADPH during incubation was investigated.

RESULTS

Microsomal metabolism was strongly influenced by buffer pH, organic solvents and preincubation time. Substrate depletion values varied up to 5-fold depending on incubation matrix composition, however, the rank order of metabolic stability remained unchanged. Prolonged incubation periods led to drastic loss in enzyme activity which could not be prevented by addition of metal chelators or antioxidants. Cofactor NADPH was rapidly oxidized in microsomal matrix, even in the absence of cytochrome P450 substrates.

DISCUSSION

In summary, short incubation times, precise pH control and minimal concentrations of organic solvents are mandatory to obtain reliable microsomal stability data. Furthermore, in vitro metabolic stability of the tested A₁AR ligands varied largely depending on the particular C8 substituent. Consequently, structural modifications at the xanthine C8 position appear to be a promising strategy for the improvement of A₁AR PET radioligands.

DPCPX, a selective adenosine A1 receptor antagonist, enhances the antidepressant-like effects of imipramine, escitalopram, and reboxetine in mice behavioral tests

The main goal of the present study was to evaluate the influence of the adenosine A1 receptor (A1R) antagonist — DPCPX — on depressive-like behavior in mice, as well as the effect of DPCPX on the activity of imipramine, escitalopram, and reboxetine, each at non-effective doses. The influence of DPCPX on behavior and its influence on the activity of selected antidepressants was evaluated in the forced swim test (FST) and the tail suspension test (TST) in mice. Locomotor activity was measured to verify and exclude false-positive data obtained in the FST and TST. Moreover, serum and brain concentrations of tested antidepressants were determined using HPLC. DPCPX, at doses of 2 and 4 mg/kg, exhibited antidepressant activity in the FST and TST, which was not related to changes in the spontaneous locomotor activity. Co-administration of DPCPX with imipramine, escitalopram, or reboxetine, each at non-active doses, significantly reduced the immobilization period in the FST and TST in mice, which was not due to the increase in locomotor activity. Both antagonists of 5-HT receptors (WAY 100635 and ritanserin) completely antagonized the effect elicited by DPCPX in the behavioral tests. Results of assessment of the nature of the interaction between DPCPX and test drugs show that in the case of DPCPX and imipramine or reboxetine, there were pharmacodynamic interactions, whereas the DPCPX-escitalopram interaction is at least partially pharmacokinetic in nature. Presented outcomes indicate that an inhibition of A1Rs and an increase of monoaminergic transduction in the CNS may offer a novel strategy for the development of antidepressant drugs.

Probing Substituents in the 1- and 3-Position: Tetrahydropyrazino-Annelated Water-Soluble Xanthine Derivatives as Multi-Target Drugs With Potent Adenosine Receptor Antagonistic Activity

Tetrahydropyrazino-annelated theophylline (1,3-dimethylxanthine) derivatives have previously been shown to display increased water-solubility as compared to the parent xanthines due to their basic character. In the present study, we modified this promising scaffold by replacing the 1,3-dimethyl residues by a variety of alkyl groups including combinations of different substituents in both positions. Substituted benzyl or phenethyl residues were attached to the N8 of the resulting 1,3-dialkyl-tetrahydropyrazino[2,1-f ]purinediones with the aim to obtain multi-target drugs that block human A₁ and A_2A adenosine receptors (ARs) and monoaminoxidase B (MAO-B). 1,3-Diethyl-substituted derivatives showed high affinity for A₁ ARs, e.g., 15d (PSB-18339, 8-m-bromobenzyl-substituted) displayed a K_i value of 13.6 nM combined with high selectivity. 1-Ethyl-3-propargyl-substituted derivatives exhibited increased A_2A AR affinity. The 8-phenethyl derivative 20h was selective for the A_2A AR (K_i 149 nM), while the corresponding 8-benzyl-substituted compound 20e (PSB-1869) blocked A₁ and A_2A ARs with equal potency (K_i A₁, 180 nM; A_2A, 282 nM). The 1-ethyl-3-methyl-substituted derivative 16a (PSB-18405) bearing a m,p-dichlorobenzyl residue at N8 blocked all three targets, A₁ ARs (K_i 396 nM), A_2A ARs (K_i 1,620 nM), and MAO-B (IC₅₀ 106 nM) with high selectivity vs. the other subtypes (A_2B and A₃ ARs, MAO-A), and can thus be considered as a multi-target drug. Our findings were rationalized by molecular docking studies based on previously published X-ray structures of the protein targets. The new drugs have potential for the treatment of neurodegenerative diseases, in particular Parkinson's disease.