Synthesis and structure-activity relationships of 3,7-dimethyl-1-propargylxanthine derivatives, A2A-selective adenosine receptor antagonists

On the basis of sex: male vs. female rat adenosine A1/A2A receptor affinity

OBJECTIVE

To ensure reproducibility in biomedical research, the biological variable sex must be reported; yet a reason for using male (instead of female) rodents is seldom given. In our search for novel adenosine receptor ligands, our research group routinely determines a test compound's binding affinities at male Sprague-Dawley rat (r) adenosine A1 and A2A receptors via in vitro radioligand binding studies. This pilot study compared the binding affinities of four adenosine receptor ligands (frequently used as reference standards) at male and female adenosine rA1 and rA2A receptors.

RESULTS

The inhibition constant (Ki) values determined using female rats correspond well to the values obtained using male rats and no markable difference could be observed in affinity and selectivity of reference standards. For example, DPCPX the selective adenosine A1 receptor antagonist: male rA1Ki: 0.5 ± 0.1 nM versus female rA1Ki: 0.5 ± 0.03 nM; male rA2AKi: 149 ± 23 nM versus female rA2AKi: 135 ± 29 nM. From the limited data at hand, we conclude that even when using female rats for in vitro studies without regard for the oestrous cycle, the obtained data did not vary much from their male counterparts.

Synthesis, QSAR modeling, and molecular docking of novel fused 7-deazaxanthine derivatives as adenosine A2A receptor antagonists

Predictive QSAR models for the search of new adenosine A_2A receptor antagonists were developed by using OCHEM platform. The predictive ability of the regression models has coefficient of determination q²  = 0.65-0.71 with cross-validation and independent test set. The inhibition activities of novel fused 7-deazaxanthine compounds were predicted by the developed QSAR models. A preparative method for the synthesis of pyrimido[5',4':4,5]pyrrolo[1,2-a][1,4]diazepine derivatives was developed, and 11 new adenosine A_2A receptor antagonists were obtained. Preliminary investigations into the toxicology of fused 7-deazaxanthine compounds toward commonly used model organism to assess toxicity invertebrate cladoceran D. magna were also described.

Design, synthesis and evaluation of amino-3,5-dicyanopyridines and thieno[2,3-b]pyridines as ligands of adenosine A1 receptors for the potential treatment of epilepsy

Due to the implication of adenosine in seizure suppression, adenosine-based therapies such as adenosine receptor (AR) agonists have been investigated. This study aimed at investigating thieno[2,3-b]pyridine derivatives as non-nucleoside A₁ agonists that could be used in pharmaco-resistant epilepsy (PRE). Compound 7c (thieno[2,3-b]pyridine derivative), displayed good binding affinity to the rA₁ AR (K_i = 61.9 nM). This could be a breakthrough for further investigation of this heterocyclic scaffold as potential ligand. In silico evaluation of this compound raised bioavailability concerns but performed well on drug-likeness tests. The effect of intramolecular cyclisation that occurs during synthesis of thieno[2,3-b]pyridines from the lead compounds, amino-3,5-dicyanopyridine derivatives (6a-s) in relation to AR binding was also evaluated. A significant loss of activity against rA₁/rA_2A ARs with cyclisation was revealed. Amino-3,5-dicyanopyridines exhibited greater affinity towards rA₁ ARs (K_i < 10 nM) than rA_2A. Compound 6c had the best rA₁ affinity (K_i = 0.076 nM). Novel compounds (6d, 6k, 6l, 6m, 6n, 6o, 6p) were highly selective towards rA₁ AR (K_i between 0.179 and 21.0 nM). Based on their high selectivity for A₁ ARs, amino-3,5-dicyanopyridines may be investigated further as AR ligands in PRE with the right structural optimisations and formulations.

Chalcone-inspired rA1 /A2A adenosine receptor ligands: Ring closure as an alternative to a reactive substructure

Over the past few years, great progress has been made in the development of high-affinity adenosine A₁ and/or A_2A receptor antagonists-promising agents for the potential treatment of Parkinson's disease. Unfortunately, many of these compounds raise structure-related concerns. The present study investigated the effect of ring closures on the rA₁ /A_2A affinity of compounds containing a highly reactive α,β-unsaturated carbonyl system, hence providing insight into the potential of heterocycles to address these concerns. A total of 12 heterocyclic compounds were synthesised and evaluated in silico and in vitro. The test compounds performed well upon qualitative assessment of drug-likeness and were generally found to be free from potentially problematic fragments. Most also showed low/weak cytotoxicity. Results from radioligand binding experiments confirm that heterocycles (particularly 2-substituted 3-cyanopyridines) can replace the promiscuous α,β-unsaturated ketone functional group without compromising A₁ /A_2A affinity. Structure-activity relationships highlighted the importance of hydrogen bonds in binding to the receptors of interest. Compounds 3c (rA₁ K_i  = 16 nM; rA_2A K_i  = 65 nM) and 8a (rA₁ K_i  = 102 nM; rA_2A K_i  = 37 nM), which both act as A₁ antagonists, showed significant dual A₁ /A_2A affinity and may, therefore, inspire further investigation into heterocycles as potentially safe and potent adenosine receptor antagonists.

Chemoinformatics Profiling of the Chromone Nucleus as a MAO-B/A2AAR Dual Binding Scaffold

Background:

In the context of the current drug discovery efforts to find disease modifying therapies for Parkinson´s disease (PD) the current single target strategy has proved inefficient. Consequently, the search for multi-potent agents is attracting more and more attention due to the multiple pathogenetic factors implicated in PD. Multiple evidences points to the dual inhibition of the monoamine oxidase B (MAO-B), as well as adenosine A2A receptor (A2AAR) blockade, as a promising approach to prevent the neurodegeneration involved in PD. Currently, only two chemical scaffolds has been proposed as potential dual MAO-B inhibitors/A2AAR antagonists (caffeine derivatives and benzothiazinones).

Methods:

In this study, we conduct a series of chemoinformatics analysis in order to evaluate and advance the potential of the chromone nucleus as a MAO-B/A2AAR dual binding scaffold.

Results:

The information provided by SAR data mining analysis based on network similarity graphs and molecular docking studies support the suitability of the chromone nucleus as a potential MAO-B/A2AAR dual binding scaffold. Additionally, a virtual screening tool based on a group fusion similarity search approach was developed for the prioritization of potential MAO-B/A2AAR dual binder candidates. Among several data fusion schemes evaluated, the MEAN-SIM and MIN-RANK GFSS approaches demonstrated to be efficient virtual screening tools. Then, a combinatorial library potentially enriched with MAO-B/A2AAR dual binding chromone derivatives was assembled and sorted by using the MIN-RANK and then the MEAN-SIM GFSS VS approaches.

Conclusion:

The information and tools provided in this work represent valuable decision making elements in the search of novel chromone derivatives with a favorable dual binding profile as MAO-B inhibitors and A2AAR antagonists with the potential to act as a disease-modifying therapeutic for Parkinson´s disease.

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.

DABCO-based ionic liquids: introduction of two metal-free catalysts for one-pot synthesis of 1,2,4-triazolo[4,3-a]pyrimidines and pyrido[2,3-d]pyrimidines

In this study, structurally functionalized 1,2,4-triazolo[4,3-a]pyrimidines and pyrido[2,3-d]pyrimidines were synthesized by two non-metal DABCO-based ionic liquids which were compared in catalytic activity in both reactions.

The adenosine receptor affinities and monoamine oxidase B inhibitory properties of sulfanylphthalimide analogues

Based on a report that sulfanylphthalimides are highly potent monoamine oxidase (MAO) B selective inhibitors, the present study examines the adenosine receptor affinities and MAO-B inhibitory properties of a series of 4- and 5-sulfanylphthalimide analogues. Since adenosine antagonists (A1 and A2A subtypes) and MAO-B inhibitors are considered agents for the therapy of neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease, dual-target-directed drugs that antagonize adenosine receptors and inhibit MAO-B may have enhanced therapeutic value. The results document that the sulfanylphthalimide analogues are selective for the adenosine A1 receptor over the A2A receptor subtype, with a number of compounds also possessing MAO-B inhibitory properties. Among the compounds evaluated, 5-[(4-methoxybenzyl)sulfanyl]phthalimide was found to possess the highest binding affinity to adenosine A1 receptors with a Ki value of 0.369 μM. This compound is reported to also inhibit MAO-B with an IC50 value of 0.020 μM. Such dual-target-directed compounds may act synergistic in the treatment of Parkinson's disease: antagonism of the A1 receptor may facilitate dopamine release, while MAO-B inhibition may reduce dopamine metabolism. Additionally, dual-target-directed compounds may find therapeutic value in Alzheimer's disease: antagonism of the A1 receptor may be beneficial in the treatment of cognitive dysfunction, while MAO-B inhibition may exhibit neuroprotective properties. In neurological diseases, such as Parkinson's disease and Alzheimer's disease, dual-target-directed drugs are expected to be advantageous over single-target treatments.

The role of the adenosinergic system in lung fibrosis

Adenosine (ADO) is a retaliatory metabolite that is expressed in conditions of injury or stress. During these conditions ATP is released at the extracellular level and is metabolized to adenosine. For this reason, adenosine is defined as a "danger signal" for cells and organs, in addition to its important role as homeostatic regulator. Its physiological functions are mediated through interaction with four specific transmembrane receptors called ADORA1, ADORA2A, ADORA2B and ADORA3. In the lungs of mice and humans all four adenosine receptors are expressed with different roles, having pro- and anti-inflammatory roles, determining bronchoconstriction and regulating lung inflammation and airway remodeling. Adenosine receptors can also promote differentiation of lung fibroblasts into myofibroblasts, typical of the fibrotic event. This last function suggests a potential involvement of adenosine in the fibrotic lung disease processes, which are characterized by different degrees of inflammation and fibrosis. Idiopathic pulmonary fibrosis (IPF) is the pathology with the highest degree of fibrosis and is of unknown etiology and burdened by lack of effective treatments in humans.

Molecular Docking and Prediction of Pharmacokinetic Properties of Dual Mechanism Drugs that Block MAO-B and Adenosine A(2A) Receptors for the Treatment of Parkinson's Disease

Monoamine oxidase B (MAO-B) inhibitory potential of adenosine A(2A) receptor (AA(2A)R) antagonists has raised the possibility of designing dual-target-directed drugs that may provide enhanced symptomatic relief and that may also slow the progression of Parkinson's disease (PD) by protecting against further neurodegeneration. To explain the dual inhibition of MAO-B and AA(2A)R at the molecular level, molecular docking technique was employed. Lamarckian genetic algorithm methodology was used for flexible ligand docking studies. A good correlation (R(2)= 0.524 and 0.627 for MAO-B and AA(2A)R, respectively) was established between docking predicted and experimental K(i) values, which confirms that the molecular docking approach is reliable to study the mechanism of dual interaction of caffeinyl analogs with MAO-B and AA(2A)R. Parameters for Lipinski's "Rule-of-Five" were also calculated to estimate the pharmacokinetic properties of dual-target-directed drugs where both MAO-B inhibition and AA(2A)R antagonism exhibited a positive correlation with calculated LogP having a correlation coefficient R(2) of 0.535 and 0.607, respectively. These results provide some beneficial clues in structural modification for designing new inhibitors as dual-target-directed drugs with desired pharmacokinetic properties for the treatment of PD.

Discovery of Multi-Target Agents for Neurological Diseases via Ligand Design

The incidence of neurological disorders in the developed world is rising in concert with an increase in human life expectancy, due in large part to better nutrition and health care. Even as drug discovery efforts are refocused on these disorders, there has been a dearth in the introduction of new disease-modifying therapies to prevent or delay their onset, or reverse their progression. Mounting evidence points to complex and heterogeneous etiopathologies that underlie these diseases. Therefore, it is unlikely that disorders in this class will be mitigated by any single drug that acts exclusively on a single pathway or target. The rational design of novel drug entities with the ability to simultaneously address multiple drug targets of a complex pathophysiology has recently emerged as a new paradigm in drug discovery. Similarly to the concept of multi-target agents within the psychopharmacology field, ligand design has gained an increasing prominence within the medicinal chemistry community. In this chapter we discuss several examples of select chemical scaffolds (polyamines, alkylxanthines, and propargyl carbamates) wherein these concepts were applied to develop novel drug candidates for Alzheimer's disease and Parkinson's disease.

Recent developments in adenosine receptor ligands and their potential as novel drugs

Medicinal chemical approaches have been applied to all four of the adenosine receptor (AR) subtypes (A(1), A(2A), A(2B), and A(3)) to create selective agonists and antagonists for each. The most recent class of selective AR ligands to be reported is the class of A(2B)AR agonists. The availability of these selective ligands has facilitated research on therapeutic applications of modulating the ARs and in some cases has provided clinical candidates. Prodrug approaches have been developed which improve the bioavailability of the drugs, reduce side-effects, and/or may lead to site-selective effects. The A(2A) agonist regadenoson (Lexiscan®), a diagnostic drug for myocardial perfusion imaging, is the first selective AR agonist to be approved. Other selective agonists and antagonists are or were undergoing clinical trials for a broad range of indications, including capadenoson and tecadenoson (A(1) agonists) for atrial fibrillation, or paroxysmal supraventricular tachycardia, respectively, apadenoson and binodenoson (A(2A) agonists) for myocardial perfusion imaging, preladenant (A(2A) antagonist) for the treatment of Parkinson's disease, and CF101 and CF102 (A(3) agonists) for inflammatory diseases and cancer, respectively.

Xanthines as adenosine receptor antagonists

The natural plant alkaloids caffeine and theophylline were the first adenosine receptor (AR) antagonists described in the literature. They exhibit micromolar affinities and are non-selective. A large number of derivatives and analogues were subsequently synthesized and evaluated as AR antagonists. Very potent antagonists have thus been developed with selectivity for each of the four AR subtypes.

Suppression of GABA input by A1 adenosine receptor activation in rat cerebellar granule cells

Synaptic transmission has been shown to be modulated by purinergic receptors. In the cerebellum, spontaneous inhibitory input to Purkinje neurons is enhanced by ATP via P2 receptors, while evoked excitatory input via the granule cell parallel fibers is reduced by presynaptic P1 (A1) adenosine receptors. We have now studied the modulation of the complex GABAergic input to granule cells by the purinergic receptor agonists ATP and adenosine in acute rat cerebellar tissue slices using the whole-cell patch-clamp technique. Our experiments indicate that ATP and adenosine substantially reduce the bicuculline- and gabazine-sensitive GABAergic input to granule cells. Both phasic and tonic inhibitory components were reduced leading to an increased excitability of granule cells. The effect of ATP and adenosine could be blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), but not by other P1 and P2 receptor antagonists, indicating that it was mediated by activation of A1 adenosine receptors. Our results suggest that, in the cerebellar network, A1 receptor activation, known to decrease the excitatory output of granule cells, also increases their excitability by reducing their complex GABAergic input. These findings extend our knowledge on purinergic receptors, mediating multiple modulations at both inhibitory and excitatory input and output sites in the cerebellar network.

Dual-target-directed drugs that block monoamine oxidase B and adenosine A(2A) receptors for Parkinson's disease

Inadequacies of the current pharmacotherapies to treat Parkinson's disease (PD) have prompted efforts to identify novel drug targets. The adenosine A(2A) receptor is one such target. Antagonists of this receptor (A(2A) antagonists) are considered promising agents for the symptomatic treatment of PD. Evidence suggests that A(2A) antagonists may also have neuroprotective properties that may prevent the development of the dyskinesia that often complicates levodopa treatment. Because the therapeutic benefits of A(2A) antagonists are additive to that of dopamine replacement therapy, it may be possible to reduce the dose of the dopaminergic drugs and therefore the occurrence of side effects. Inhibitors of monoamine oxidase (MAO)-B also are considered useful tools for the treatment of PD. When used in combination with levodopa, inhibitors of MAO-B may enhance the elevation of dopamine levels after levodopa treatment, particularly when used in early stages of the disease when dopamine production may not be so severely compromised. Furthermore, MAO-B inhibitors may also possess neuroprotective properties in part by reducing the damaging effect of dopamine turnover in the brain. These effects of MAO-B inhibitors are especially relevant when considering that the brain shows an age-related increase in MAO-B activity. Based on these observations, dual-target-directed drugs, compounds that inhibit MAO-B and antagonize A(2A) receptors, may have value in the management of PD. This review summarizes recent efforts to develop such dual-acting drugs using caffeine as the lead compound.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Recent developments in adenosine A2A receptor ligands

The development of potent and selective agonists and antagonists of adenosine receptors (ARs) has been a target of medicinal chemistry research for several decades, and recently the US Food and Drug Administration has approved Lexiscan, an adenosine derivative substituted at the 2 position, for use as a pharmacologic stress agent in radionuclide myocardial perfusion imaging. Currently, some other adenosine A(2A) receptor (A(2A)AR) agonists and antagonists are undergoing preclinical testing and clinical trials. While agonists are potent antiinflammatory agents also showing hypotensive effects, antagonists are being developed for the treatment of Parkinson's disease.However, since there are still major problems in this field, including side effects, low brain penetration (for the targeting of CNS diseases), short half-life, or lack of in vivo effects, the design and development of new AR ligands is a hot research topic.This review presents an update on the medicinal chemistry of A(2A)AR agonists and antagonists, and stresses the strong need for more selective ligands at the human A(2A)AR subtype, in particular in the case of agonists.

Dual inhibition of monoamine oxidase B and antagonism of the adenosine A(2A) receptor by (E,E)-8-(4-phenylbutadien-1-yl)caffeine analogues

The adenosine A(2A) receptor has emerged as an attractive target for the treatment of Parkinson's disease (PD). Evidence suggests that antagonists of the A(2A) receptor (A(2A) antagonists) may be neuroprotective and may help to alleviate the symptoms of PD. We have reported recently that several members of the (E)-8-styrylcaffeine class of A(2A) antagonists also are potent inhibitors of monoamine oxidase B (MAO-B). Since MAO-B inhibitors are known to possess anti-parkinsonian properties, dual-target-directed drugs that block both MAO-B and A(2A) receptors may have enhanced value in the management of PD. In an attempt to explore this concept further we have prepared three additional classes of C-8 substituted caffeinyl analogues. The 8-phenyl- and 8-benzylcaffeinyl analogues exhibited relatively weak MAO-B inhibition potencies while selected (E,E)-8-(4-phenylbutadien-1-yl)caffeinyl analogues were found to be exceptionally potent reversible MAO-B inhibitors with enzyme-inhibitor dissociation constants (K(i) values) ranging from 17 to 149 nM. Furthermore, these (E,E)-8-(4-phenylbutadien-1-yl)caffeines acted as potent A(2A) antagonists with K(i) values ranging from 59 to 153 nM. We conclude that the (E,E)-8-(4-phenylbutadien-1-yl)caffeines are a promising candidate class of dual-acting compounds.

Computational studies of the binding modes of A 2A adenosine receptor antagonists

A molecular docking study was performed on several structurally diverse A(2A) AR antagonists, including xanthines, and non-xanthine type antagonists to investigate their binding modes with A(2A) adenosine receptor (AR), one of the four subtypes of AR, which is currently of great interest as a target for therapeutic intervention, in particular for Parkinson's disease. The high-affinity binding site was found to be a hydrophobic pocket with the involvement of hydrogen bonding interactions as well as pi-pi stacking interactions with the ligands. The detailed binding modes for both xanthine and non-xanthine type A(2A) antagonists were compared and the essential features were extracted and converted to database searchable queries for virtual screening study of novel A(2A) AR antagonists. Findings from this study are helpful for elucidating the binding pattern of A(2A) AR antagonists and for the design of novel active ligands.

Highlights on the development of A(2A) adenosine receptor agonists and antagonists

Although significant progress has been made in the past few decades demonstrating that adenosine modulates a variety of physiological and pathophysiological processes through the interaction with four subtypes of a family of cell-surface G-protein-coupled receptors, clinical evaluation of some adenosine receptor ligands has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors, low brain penetration (which is important for the targeting of CNS diseases), short half-life of compounds, or a lack of effects, in some cases perhaps due to receptor desensitization or to low receptor density in the targeted tissue. Currently, three A(2A) adenosine receptor agonists have begun phase III studies. Two of them are therapeutically evaluated as pharmacologic stress agents and the third proved to be effective in the treatment of acute spinal cord injury (SCI), while avoiding the adverse effects of steroid agents. On the other hand, the great interest in the field of A(2A) adenosine receptor antagonists is related to their application in neurodegenerative disorders, in particular, Parkinson's disease, and some of them are currently in various stages of evaluation. This review presents an update of medicinal chemistry and molecular recognition of A(2A) adenosine receptor agonists and antagonists, and stresses the strong need for more selective ligands at the A(2A) human subtype.

Microwave-assisted ring closure reactions: synthesis of 8-substituted xanthine derivatives and related pyrimido- and diazepinopurinediones

BACKGROUND

Poly-substituted xanthine derivatives are an important class of compounds in medicinal chemistry. Substitution at the 8-position of the purine ring is generally accessible by ring closure of a carboxamido-substituted uracil precursor. Although several procedures to accomplish this synthetic step have been suggested, it still remains difficult in many cases.

RESULTS

Ring closure reaction with hexamethyldisilazane was studied under microwave conditions. Reaction times were dramatically reduced by the application of microwaves in the syntheses of the 8-styrylxanthine derivative istradefylline, and in the preparation of 2-substituted pyrimido [1,2,3-cd]purines. Furthermore, the new procedure allowed the preparation of a previously unaccessible diazepino [1,2,3-cd]purine. Yields were generally improved by the new method. The addition of THF as a co-solvent proved to be crucial.

CONCLUSION

A new, fast, and efficient ring closure method for the imidazole ring of xanthine derivatives and related tricyclic compounds has been developed. Apart from improving the syntheses of known compounds, some of which are important pharmacological tools or in development as novel drugs, it allows the preparation of 2-substituted diazepino [1,2,3-cd]purines--a new class of tricyclic purine derivatives.

Improving Potency, Selectivity, and Water Solubility of Adenosine A1 Receptor Antagonists: Xanthines Modified at Position 3 and Related Pyrimido[1,2,3-cd]purinediones

The structure-activity relationships of xanthine derivatives related to the adenosine A(1) receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 1,3-dipropyl-8-(3-noradamantyl)xanthine (KW3902) were investigated by focusing on variations of the 3-substituent. Aromatic residues were well tolerated by the A(1) receptor in that position. A moderate effect of stereochemistry was found for the 3-(1-phenylethyl)-substituted analogue of DPCPX (S>R) at A(1) and A(3) receptors, whereas the opposite stereoselectivity was observed at the A(2) receptor subtypes. A 3-hydroxypropyl substituent was found to be optimal for high A(1) affinity and selectivity. The most potent compound of the present series was 1-butyl-3-(3-hydroxypropyl)-8-(3-noradamantyl)xanthine (10 c), which exhibits a K(i) value of 0.124 nM at rat, and 0.7 nM at human adenosine A(1) receptors, combined with high selectivity (>>200-fold) versus the other receptor subtypes. The similarly potent 8-cyclopentyl-3-(3-hydroxypropyl)-1-propylxanthine was converted into a water-soluble phosphate prodrug, which may become a useful pharmacological tool for in vivo studies. 8-Alkyl-2-(3-noradamantyl)pyrimido[1,2,3-cd]purine-8,10-diones, which can be envisaged as xanthine analogues with a fixed 3-propyl substituent, were identified as a new class of potent, selective adenosine A(1) receptor antagonists. For example, compound 14 (8-butyl-substituted) exhibits a K(i) value of 13.8 nM at human A(1) receptors. A selection of the most potent compounds was investigated in [(35)S]GTPgammaS binding assays and showed inverse agonistic activity. Their efficacy was generally lower than that of the full inverse agonist DPCPX, and depended on subtle structural changes. Some of the new compounds belong to the most potent and selective A(1) antagonists described to date.

Synthesis and biological activity of tricyclic aryloimidazo-, pyrimido-, and diazepinopurinediones

Syntheses and physicochemical properties of N-aryl-substituted imidazo-, pyrimido-, and 1,3-diazepino[2,1-f]purinediones are described. These derivatives were synthesized by the cyclization of 7-haloalkyl-8-bromo-1,3-dimethyl- or 1,3-dipropyl-xanthine derivatives with corresponding arylamines. The obtained compounds (1-40), which can be envisaged as sterically fixed and configurationally stable analogs of 8-styrylxanthines, were evaluated for their affinity to adenosine A(1) and A(2A) receptors, the receptor subtypes that are predominant in the brain. Selected compounds were additionally investigated for affinity to the A(2B) and A(3) receptor subtypes. Many of the compounds showed adenosine A(2A) receptor affinity at micromolar or submicromolar concentrations and were A(2A)-selective, for example, compound 23 with p-fluoro substituent displayed K(i) value of 0.147 microM at the rat A(2A) receptor and more than 170-fold-A(2A) selectivity, compound 17 with naphthyl substituent had K(i) value of 0.219 microM and a more than 114-fold-A(2A) selectivity. The compounds were somewhat weaker and less selective at the human receptor subtypes. Elongation of the dimethyl substituent to dipropyl in xanthine moiety improved affinity but reduced selectivity. 1,3-Dimethylimidazo-, pyrimido-, and diazepinopurinediones were evaluated in vivo as anticonvulsants in MES, ScMet, TTE tests and examined for neurotoxicity in mice (ip). Substances with pyrimido ring displayed protective activity in ScMet or in MES and ScMet tests, showing also neurotoxicity. The pyrimidine annelated ring is beneficial for both receptor affinity and anticonvulsant activity.

Regiospecific one-pot synthesis of pyrimido[4,5-d]pyrimidine derivatives in the solid state under microwave irradiations

Electron rich 6-[(dimethylamino)methylene]amino uracil 1, undergoes [4+2] cycloaddition reactions with various in situ generated glyoxylate imine and imine oxides 6 to provide novel pyrimido[4,5-d]pyrimidine derivatives of biological significance, after elimination of dimethylamine from the (1:1) cycloadducts and oxidative aromatisation. This procedure provides a convenient method for the direct synthesis of pyrimido[4,5-d]pyrimidines in excellent yields when carried out in the solid state and under microwave irradiations.

Presynaptic control of striatal glutamatergic neurotransmission by adenosine A1-A2A receptor heteromers

The functional role of heteromers of G-protein-coupled receptors is a matter of debate. In the present study, we demonstrate that heteromerization of adenosine A1 receptors (A1Rs) and A2A receptors (A2ARs) allows adenosine to exert a fine-tuning modulation of glutamatergic neurotransmission. By means of coimmunoprecipitation, bioluminescence and time-resolved fluorescence resonance energy transfer techniques, we showed the existence of A1R-A2AR heteromers in the cell surface of cotransfected cells. Immunogold detection and coimmunoprecipitation experiments indicated that A1R and A2AR are colocalized in the same striatal glutamatergic nerve terminals. Radioligand-binding experiments in cotransfected cells and rat striatum showed that a main biochemical characteristic of the A1R-A2AR heteromer is the ability of A2AR activation to reduce the affinity of the A1R for agonists. This provides a switch mechanism by which low and high concentrations of adenosine inhibit and stimulate, respectively, glutamate release. Furthermore, it is also shown that A1R-A2AR heteromers constitute a unique target for caffeine and that chronic caffeine treatment leads to modifications in the function of the A1R-A2AR heteromer that could underlie the strong tolerance to the psychomotor effects of caffeine.

Progress in the pursuit of therapeutic adenosine receptor antagonists

Ever since the discovery of the hypotensive and bradycardiac effects of adenosine, adenosine receptors continue to represent promising drug targets. First, this is due to the fact that the receptors are expressed in a large variety of tissues. In particular, the actions of adenosine (or methylxanthine antagonists) in the central nervous system, in the circulation, on immune cells, and on other tissues can be beneficial in certain disorders. Second, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes (A1, A2A, A2B, and A3) have been cloned and pharmacologically characterized, all of which are G protein-coupled receptors. Adenosine receptors can be distinguished according to their preferred mechanism of signal transduction: A1 and A3 receptors interact with pertussis toxin-sensitive G proteins of the Gi and Go family; the canonical signaling mechanism of the A2A and of the A2B receptors is stimulation of adenylyl cyclase via Gs proteins. In addition to the coupling to adenylyl cyclase, all four subtypes may positively couple to phospholipase C via different G protein subunits. The development of new ligands, in particular, potent and selective antagonists, for all subtypes of adenosine receptors has so far been directed by traditional medicinal chemistry. The availability of genetic information promises to facilitate understanding of the drug-receptor interaction leading to the rational design of a potentially therapeutically important class of drugs. Moreover, molecular modeling may further rationalize observed interactions between the receptors and their ligands. In this review, we will summarize the most relevant progress in developing new therapeutic adenosine receptor antagonists.

Inhibition of monoamine oxidase B by analogues of the adenosine A2A receptor antagonist (E)-8-(3-chlorostyryl)caffeine (CSC)

The adenosine A2A receptor has emerged as a possible target for the treatment of Parkinson's disease (PD). Evidence suggests that antagonism of the A2A receptor not only improves the symptoms of the disease but may also protect against the underlying degenerative processes. We have recently reported that several known adenosine A2A receptor antagonists (A2A antagonists) also are moderate to very potent inhibitors of monoamine oxidase B (MAO-B). The most potent among these was (E)-8-(3-chlorostyryl)caffeine (CSC), a compound frequently used when examining the in vivo pharmacological effects of A2A antagonists. Since MAO-B inhibitors are also thought to possess antiparkinsonian properties, dual targeting drugs that block both MAO-B and A2A receptors may have enhanced therapeutic potential in the treatment of PD. In this study, we prepared selected analogues of CSC in an attempt to examine specific structural features that may be important for potent MAO-B inhibition. The results of a SAR study established that the potency of MAO-B inhibition by (E)-8-styrylcaffeinyl analogues depends upon the van der Waals volume (V(w)), lipophilicity (pi), and the Hammett constant (sigma(m)) of the substituents attached to C-3 of the phenyl ring of the styryl moiety. Potency also varies with substituents attached to C-4 with bulkiness (V(w)) and lipophilicity (pi) being the principal substituent descriptors.

Synthesis and preliminary evaluation of new 1- and 3-[1-(2-hydroxy-3-phenoxypropyl)]xanthines from 2-amino-2-oxazolines as potential A1 and A2A adenosine receptor antagonists

The development of potent and selective adenosine receptor ligands as potential drugs is an active area of research. Xanthines are one of the most important classes of adenosine receptor antagonists and have been widely developed in terms of affinity and selectivity for adenosine receptors. We recently developed new original pathways for the synthesis of xanthine analogues starting from 5-substituted-2-amino-2-oxazoline 5 as a synthon. These procedures allowed us to selectively introduce a large, functionalized and beta-adrenergic 2-hydroxy-3-phenoxypropyl pharmacophore at the 1- and 3-position of the xanthine moiety which allowed further structural modifications. In this study, we present a new synthetic access to racemic xanthine derivatives 1-4 from 5, and their evaluation as adenosine A1, A2A and A3 receptor ligands in radioligand binding studies. The 2-hydroxy-3-phenoxypropyl moiety was well tolerated in the 3-position of the xanthine core, while its introduction in the 1-position of the xanthine moiety led to a large decrease in adenosine receptor affinity. 1,7-Dimethyl-3-[1-(2-chloro-3-phenoxypropyl)]-8-(3,4,5-trimethoxystyryl)xanthine (2n) was the most potent and selective A2A antagonist of the present series (Ki=44 nM, >>200-fold selective vs A1). 1-Propyl-3-[1-(2-hydroxy-3-phenoxypropyl)]-8-noradamantylxanthine (3f) was identified as a potent (KiA1=21 nM) and highly selective (>>350-fold vs A2A and A3 receptor) adenosine A1 receptor antagonist.

Dihydropyrido[2,3-d]pyrimidines as a new class of antileishmanial agents

A series of dihydropyrido[2,3-d]pyrimidines have been synthesized and screened for its in vitro antileishmanial activity profile in promastigote and amastigote models. Compounds 2a-2l have shown 83-100% inhibition against promastigotes and 79-100% inhibition against amastigotes at a concentration of 50 microg/mL.

New 8-substituted xanthiene derivatives as potent bronchodilators

The synthesis and structure determination of 8-aryl /alkyl aryl 1, 3-dimethyl-3, 7-dihydropurin-2, 6-dione derivatives (1-13), was carried out in this study. Bronchodilator activity is investigated using isolated guinea-pig tracheal strips, pre-contracted by acetylcholine and histamine. Spasmolytic activity of the compounds was compared to theophylline. Synthesized compounds (1-13) did not inhibit the acetylcholine-induced pre-contractions except compound (8) at 10(-5) M concentration. In contrast, some of the compounds, especially (7), (11), (12) at 10(-5) M and (3), (4), (9) and (11) in 10(-4) M displayed inhibitory activity on the tracheal strips pre-contracted by histamine. The potency of the compounds at human adenosine receptors was evaluated using radioligand binding assay and a cyclic AMP functional assay in CHO cells expressing these receptors. Compound (11) displayed the greatest activity against radioligand binding of specific agonists to A2A and A2B receptors. The compounds were relatively selective for both A2A and A2B compared with A1 and A3 receptors. All compounds were also tested for the inhibition of NECA-induced cAMP accumulation mediated by the A2B adenosine receptor and compound (11) was found to be the most effective. Our results showed that these compounds are acting as selective adenosine antagonists, especially for adenosine A2B receptors, and are promising as potent anti-inflammatory agents rather than bronchodilator for the treatment of asthma.

Involvement of adenosine A1 receptors in the discriminative-stimulus effects of caffeine in rats

RATIONALE

Caffeine is a non-selective adenosine receptor antagonist in vitro, but involvement of different adenosine receptor subtypes, particularly adenosine A1 and A 2A receptors, in the central effects of caffeine remains a matter of debate.

OBJECTIVE

Investigate the role of adenosine A1 and A 2A receptors in the discriminative-stimulus effects of caffeine.

METHODS

Rats were trained to discriminate an injection of 30 mg/kg (i.p.) caffeine from saline. The selective A1 receptor antagonist CPT, the selective A 2A receptor antagonist MSX-3 and the non-selective adenosine receptor antagonist DMPX were assessed for their ability to produce caffeine-like discriminative effects. The ability of CPT, MSX-3, the A1 receptor agonist CPA and the A 2A receptor agonist CGS21680 to reduce the discriminative effects of caffeine was also tested. Radioligand binding experiments with membrane preparations from rat striatum and transfected mammalian cell lines were performed to characterize binding affinity profiles of the different adenosine antagonists used in the present study (caffeine, DMPX, CPT and MSX-3) in relation to all known adenosine receptors (A1, A 2A, A 2B, A3).

RESULTS

DMPX and CPT, but not MSX-3, produced significant caffeine-like discriminative effects. MSX-3, but not CPT, markedly reduced the discriminative effects of caffeine and the caffeine-like discriminative effects of CPT. Furthermore, the A1 receptor agonist CPA, but not the A 2A agonist CGS21680, reduced caffeine's discriminative effects.

CONCLUSIONS

Adenosine A1 receptor blockade is involved in the discriminative-stimulus effects of behaviorally relevant doses of caffeine; A 2A receptor blockade does not play a central role in caffeine's discriminative effects and counteracts the A1 receptor-mediated discriminative-stimulus effects of caffeine.

Inhibition of monoamine oxidase B by selective adenosine A2A receptor antagonists

Adenosine receptor antagonists that are selective for the A(2A) receptor subtype (A(2A) antagonists) are under investigation as possible therapeutic agents for the symptomatic treatment of the motor deficits associated with Parkinson's disease (PD). Results of recent studies in the MPTP mouse model of PD suggest that A(2A) antagonists may possess neuroprotective properties. Since monoamine oxidase B (MAO-B) inhibitors also enhance motor function and reduce MPTP neurotoxicity, we have examined the MAO-B inhibiting properties of several A(2A) antagonists and structurally related compounds in an effort to determine if inhibition of MAO-B may contribute to the observed neuroprotection. The results of these studies have established that all of the (E)-8-styrylxanthinyl derived A(2A) antagonists examined display significant MAO-B inhibitory properties in vitro with K(i) values in the low micro M to nM range. Included in this series is (E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methylxanthine (KW-6002), a potent A(2A) antagonist and neuroprotective agent that is in clinical trials. The results of these studies suggest that MAO-B inhibition may contribute to the neuroprotective potential of A(2A) receptor antagonists such as KW-6002 and open the possibility of designing dual targeting drugs that may have enhanced therapeutic potential in the treatment of PD.

Recent developments in the field of A2A and A3 adenosine receptor antagonists

In the last years adenosine receptors have been extensively studied, and mainly at present we understand the importance of A(2A) and A(3) adenosine receptors. A(2A) selective adenosine receptors antagonists are promising new drugs for the treatment of Parkinson's disease, while A(3) selective adenosine receptors antagonists have been postulated as novel anti-inflammatory and antiallergic agents; recent studies also indicated a possible employment of these derivatives as antitumour agents. Lately different classes of compounds have been identified as potent A(2A) and A(3) antagonists. In this article we report the past and present efforts which led to development of more potent and selective A(2A) and A(3) antagonists. Our group has mainly worked on the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine nucleus both as A(2A) and A(3) antagonists, aiming to improve the affinity, selectivity and the hydrophilic profile. In fact, we have synthesised several compounds endowed with high affinity and selectivity versus A(2A) adenosine receptors, as 2, 2a-c (K(i)A(2A)=0.12-0.19 nM), or A(3) adenosine receptors, as 4p (K(i)A(3)=0.01 nM) and 4q (K(i)A(3)=0.04 nM).