1,3-Dialkylxanthine Derivatives Having High Potency as Antagonists at Human A2B Adenosine Receptors

Caffeine and Purine Derivatives: A Comprehensive Review on the Chemistry, Biosynthetic Pathways, Synthesis-Related Reactions, Biomedical Prospectives and Clinical Applications

Caffeine and purine derivatives represent interesting chemical moieties, which show various biological activities. Caffeine is an alkaloid that belongs to the family of methylxanthine alkaloids and it is present in food, beverages, and drugs. Coffee, tea, and some other beverages are a major source of caffeine in the human diet. Caffeine can be extracted from tea or coffee using hot water with dichloromethane or chloroform and the leftover is known as decaffeinated coffee or tea. Caffeine and its derivatives were synthesized via different procedures on small and large scales. It competitively antagonizes the adenosine receptors (ARs), which are G protein-coupled receptors largely distributed in the human body, including the heart, vessels, brain, and kidneys. Recently, many reports showed the effect of caffeine derivatives in the treatment of many diseases such as Alzheimer's, asthma, parkinsonism, and cancer. Also, it is used as an antioxidant, anti-inflammatory, analgesic, and hypocholesterolemic agent. The present review article discusses the synthesis, reactivity, and biological and pharmacological properties of caffeine and its derivatives. The biosynthesis and biotransformation of caffeine in coffee and tea leaves and the human body were summarized in the review.

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

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

A2A Adenosine Receptor Antagonists and their Potential in Neurological Disorders

Endogenous nucleoside adenosine modulates a number of physiological effects through interaction with P1 purinergic receptors. All of them are G protein coupled receptors and, to date, four subtypes have been characterized and named A1, A2A, A2B, and A3. In recent years adenosine receptors, particularly the A2A subtype, have become attractive targets for the treatment of several neurodegenerative disorders, known to involve neuroinflammation, like Parkinson's and Alzheimer's diseases, multiple sclerosis and neuropsychiatric conditions. In fact, it has been demonstrated that inhibition of A2A adenosine receptors exerts neuroprotective effects counteracting neuroinflammatory processes and astroglial and microglial activation. The A2A adenosine receptor antagonist istradefylline, developed by Kyowa Hakko Kirin Inc., was approved in Japan as adjunctive therapy for the treatment of Parkinson's disease and very recently it was approved also by the US Food and Drug Administration. These findings pave the way for new therapeutic opportunities, so, in this review, a summary of the most relevant and promising A2A adenosine receptor antagonists will be presented along with their preclinical and clinical studies in neuroinflammation related diseases.

Recent developments in the synthesis and anti-cancer activity of acridine and xanthine-based molecules

Cancer is the uncontrolled growth and development of abnormal cells which is a major cause of death in both advanced and emerging countries. Although currently chemotherapy is most broadly used among an extensive range of anti-cancer therapies, it includes many demerits, such as highly toxic, side-effects, expensive and partial lack of targeting specificity. So the design and synthesis of new molecules that perform specifically on target proteins in tumor cells is a focus of contemporary research. So many researchers aim for new drugs that will be more efficient, more selective, and less toxic. Because of the interesting structures and significant biological profile, naturally occurring acridines and xanthines as well as their analogues have attracted considerable interest in researchers and technologists. Natural and synthetic acridine derivatives form a significant category of heterocycles having nitrogen that is of considerable interest for organic chemists and biological communities due to their attractive anti-cancer activity. Another important class of therapeutic agents with diverse biological properties including cytotoxic effects is xanthine derivatives which are collectively called xanthines (a group of alkaloids). Among many significant molecules based on the structure of the purine, there is a group of natural xanthines, involving theobromine, caffeine, and theophylline and analogues of xanthine display anti-cancer activity. Hence the present chapter wishes to concentrate the attention on the synthesis and anti-cancer activity of acridine and xanthine-based compounds brilliantly.

Novel curcumin analog (cis-trans curcumin) as ligand to adenosine receptors A2A and A2B: potential for therapeutics

All four of the adenosine receptor (AR) subtypes mediate pain and have been targeted by pharmacologists to generate new therapeutics for chronic pain. The vanilloid phytochemicals, which include curcumin, capsaicin, and gingerol, have been shown to alleviate pain. However, there is little to no literature on the interaction of vanilloid phytochemicals with ARs. In this study, photochemical methods were used to generate a novel isomer of curcumin (cis-trans curcumin or CTCUR), and the interactions of both curcumin and CTCUR with the two Gs-linked AR subtypes were studied. Competitive binding assays, docking analysis, and confocal fluorescence microscopy were performed to measure binding affinity; cell survival assays were used to measure toxicity; and cAMP assays were performed to measure receptor activation. Competitive binding results indicated that CTCUR binds to both AR A2A and AR A2B with Ki values of 5 μM and 7 μM, respectively, which is consistent with our docking results. Fluorescence microscopy data also shows binding for A2B and A2A. Cell survival results show that CTCUR and CUR are nontoxic at the tested concentrations in these cell lines. Overall, our results suggest that vanilloid phytochemicals may be slightly modified to increase interaction with Gs-ARs, and thereby can be further explored to provide a novel class of non-opioid antinociceptives.

Aminophylline for renal protection in neonatal hypoxic-ischemic encephalopathy in the era of therapeutic hypothermia

BACKGROUND

Neonates with hypoxic-ischemic encephalopathy (HIE) frequently develop acute kidney injury (AKI). Aminophylline has been shown to reduce severe renal dysfunction in neonates after perinatal asphyxia. However, the effect of aminophylline on renal function in neonates undergoing hypothermia has not been studied.

METHODS

A single-center, retrospective chart review of neonates cooled for moderate/severe HIE who received aminophylline for AKI was conducted to assess changes in urine output (UOP) and serum creatinine (SCr). Comparisons were also made to control neonates matched for hours of life who were cooled but unexposed to aminophylline.

RESULTS

Sixteen neonates cooled for HIE received aminophylline starting at 25 ± 14 h of life. Within 12 h of starting aminophylline, UOP increased by 2.6 ± 1.9 mL/kg/h. SCr declined by 0.4 ± 0.2 mg/dL in survivors over the first 4 days. When compared to control neonates, UOP increase was greater in the aminophylline group (p < 0.001). SCr declined in survivors in both groups, although baseline SCr was higher in the aminophylline group.

CONCLUSIONS

Aminophylline use in neonates with HIE undergoing hypothermia was associated with an increase in UOP and a decline in SCr. A randomized trial will be needed to establish a potential renal protective role of aminophylline.

IMPACT

The renal protective effect of aminophylline in neonates with HIE has not yet been studied in the context of therapeutic hypothermia. Aminophylline exposure in neonates cooled for HIE was associated with increased UOP and a similar decline in SCr when compared to control infants unexposed to aminophylline. Improved renal function after receiving aminophylline in this observational cohort study suggests the need for future randomized trials to establish the potential benefit of aminophylline in the HIE population undergoing hypothermia.

Design, Synthesis, and Biological Activity Studies of Istradefylline Derivatives Based on Adenine as A2A Receptor Antagonists

Due to its double bond, istradefylline rapidly isomerizes to Z-istradefylline when exposed to normal daylight in dilute solution. To solve the poor photostability of the istradefylline solution, a series of istradefylline derivatives (in total 17 compounds, including II-1 and II-2 series) were designed and synthesized, and their biological activity in inhibiting cAMP was evaluated. The IC50 values of compounds II-1-3, II-2-1, II-2-2, II-2-3, II-2-4, and II-2-6 were 7.71, 6.52, 6.16, 7.23, 7.96, and 9.68 μg/mL, respectively, which had the same order of activity as that of istradefylline (IC50 value was 1.94 μg/mL). The preliminary structure-activity relationship suggested that the 6-amino in adenine played an important role in binding an A2A receptor. The results of photostability experiments showed that the photostability of the target compounds of II-1 and II-2 series was improved when compared with that of istradefylline.

Theophylline dosing and pharmacokinetics for renal protection in neonates with hypoxic-ischemic encephalopathy undergoing therapeutic hypothermia

BACKGROUND

Theophylline, a non-selective adenosine receptor antagonist, improves renal perfusion in the setting of hypoxia-ischemia and may offer therapeutic benefit in neonates with hypoxic-ischemic encephalopathy (HIE) undergoing hypothermia. We evaluated the pharmacokinetics and dose-exposure relationships of theophylline in this population to guide dosing strategies.

METHODS

A population pharmacokinetic analysis was performed in 22 neonates with HIE undergoing hypothermia who were part of a prospective study or retrospective chart review. Aminophylline (intravenous salt form of theophylline) was given per institutional standard of care for low urine output and/or rising serum creatinine (5 mg/kg intravenous (i.v.) load then 1.8 mg/kg i.v. q6h). The ability of different dosing regimens to achieve target concentrations (4-10 mg/L) associated with clinical response was examined.

RESULTS

Birth weight was a significant predictor of theophylline clearance and volume of distribution (p < 0.05). The median half-life was 39.5 h (range 27.2-50.4). An aminophylline loading dose of 7 mg/kg followed by 1.6 mg/kg q12h was predicted to achieve target concentrations in 84% of simulated neonates.

CONCLUSIONS

In neonates with HIE undergoing hypothermia, theophylline clearance was low with a 50% longer half-life compared to full-term normothermic neonates without HIE. Dosing strategies need to consider the unique pharmacokinetic needs of this population.

IMPACT

Theophylline is a potential renal-protective therapy in neonates with HIE undergoing therapeutic hypothermia; however, the pharmacokinetics and dose needs in this population are not known. Theophylline clearance was low in neonates with HIE undergoing therapeutic hypothermia with a 50% longer half-life compared to full-term normothermic neonates without HIE. As theophylline is advanced in clinical development, dosing strategies will need to consider the unique pharmacokinetic needs of neonates with HIE undergoing therapeutic hypothermia.

Tricyclic xanthine derivatives containing a basic substituent: adenosine receptor affinity and drug-related properties

A library of 27 novel amide derivatives of annelated xanthines was designed and synthesized. The new compounds represent 1,3-dipropyl- and 1,3-dibutyl-pyrimido[2,1-f]purinedione-9-ethylphenoxy derivatives including a CH2CONH linker between the (CH2)2-amino group and the phenoxy moiety. A synthetic strategy to obtain the final products was developed involving solvent-free microwave irradiation. The new compounds were evaluated for their adenosine receptor (AR) affinities. The most potent derivatives contained a terminal tertiary amino function. Compounds with nanomolar AR affinities and at the same time high water-solubility were obtained (A1 (Ki = 24-605 nM), A2A (Ki = 242-1250 nM), A2B (Ki = 66-911 nM) and A3 (Ki = 155-1000 nM)). 2-(4-(2-(1,3-Dibutyl-2,4-dioxo-1,2,3,4,7,8-hexahydropyrimido[2,1-f]purin-9(6H)-yl)ethyl)phenoxy)-N-(3-(diethylamino)propyl)acetamide (27) and the corresponding N-(2-(pyrrolidin-1-yl)ethyl)acetamide (36) were found to be the most potent antagonists of the present series. While 27 showed CYP inhibition and moderate metabolic stability, 36 was found to possess suitable properties for in vivo applications. In an attempt to explain the affinity data for the synthesized compounds, molecular modeling and docking studies were performed using homology models of A1 and A2A adenosine receptors. The potent compound 36 was used as an example for discussion of the possible ligand-protein interactions. Moreover, the compounds showed high water-solubility indicating that the approach of introducing a basic side chain was successful for the class of generally poorly soluble AR antagonists.

A Non-imaging High Throughput Approach to Chemical Library Screening at the Unmodified Adenosine-A3 Receptor in Living Cells

Recent advances in fluorescent ligand technology have enabled the study of G protein-coupled receptors in their native environment without the need for genetic modification such as addition of N-terminal fluorescent or bioluminescent tags. Here, we have used a non-imaging plate reader (PHERAstar FS) to monitor the binding of fluorescent ligands to the human adenosine-A3 receptor (A3AR; CA200645 and AV039), stably expressed in CHO-K1 cells. To verify that this method was suitable for the study of other GPCRs, assays at the human adenosine-A1 receptor, and β1 and β2 adrenoceptors (β1AR and β2AR; BODIPY-TMR-CGP-12177) were also carried out. Affinity values determined for the binding of the fluorescent ligands CA200645 and AV039 to A3AR for a range of classical adenosine receptor antagonists were consistent with A3AR pharmacology and correlated well (R2 = 0.94) with equivalent data obtained using a confocal imaging plate reader (ImageXpress Ultra). The binding of BODIPY-TMR-CGP-12177 to the β1AR was potently inhibited by low concentrations of the β1-selective antagonist CGP 20712A (pKi 9.68) but not by the β2-selective antagonist ICI 118551(pKi 7.40). Furthermore, in experiments conducted in CHO K1 cells expressing the β2AR this affinity order was reversed with ICI 118551 showing the highest affinity (pKi 8.73) and CGP20712A (pKi 5.68) the lowest affinity. To determine whether the faster data acquisition of the non-imaging plate reader (~3 min per 96-well plate) was suitable for high throughput screening (HTS), we screened the LOPAC library for inhibitors of the binding of CA200645 to the A3AR. From the initial 1,263 compounds evaluated, 67 hits (defined as those that inhibited the total binding of 25 nM CA200645 by ≥40%) were identified. All compounds within the library that had medium to high affinity for the A3AR (pKi ≥6) were successfully identified. We found three novel compounds in the library that displayed unexpected sub-micromolar affinity for the A3AR. These were K114 (pKi 6.43), retinoic acid p-hydroxyanilide (pKi 6.13) and SU 6556 (pKi 6.17). Molecular docking of these latter three LOPAC library members provided a plausible set of binding poses within the vicinity of the established orthosteric A3AR binding pocket. A plate reader based library screening using an untagged receptor is therefore possible using fluorescent ligand opening the possibility of its use in compound screening at natively expressed receptors.

Design and synthesis of novel xanthine derivatives as potent and selective A2B adenosine receptor antagonists for the treatment of chronic inflammatory airway diseases

Adenosine induces bronchial hyperresponsiveness and inflammation in asthmatics through activation of A_2B adenosine receptor (A_2BAdoR). Selective antagonists have been shown to attenuate airway reactivity and improve inflammatory conditions in pre-clinical studies. Hence, the identification of novel, potent and selective A_2BAdoR antagonist may be beneficial for the potential treatment of asthma and Chronic Obstructive Pulmonary Disease (COPD). Towards this effort, we explored several prop-2-ynylated C8-aryl or heteroaryl substitutions on xanthine chemotype and found that 1-prop-2-ynyl-1H-pyrazol-4-yl moiety was better tolerated at the C8 position. Compound 59, exhibited binding affinity (K_i) of 62 nM but was non-selective for A_2BAdoR over other AdoRs. Incorporation of substituted phenyl on the terminal acetylene increased the binding affinity (K_i) significantly to <10 nM. Various substitutions on terminal phenyl group and different alkyl substitutions on N-1 and N-3 were explored to improve the potency, selectivity for A_2BAdoR and the solubility. In general, compounds with meta-substituted phenyl provided better selectivity for A_2BAdoR compared to that of para-substituted analogs. Substitutions such as basic amines like pyrrolidine, piperidine, piperazine or cycloalkyls with polar group were tried on terminal acetylene, keeping in mind the poor solubility of xanthine analogs in general. However, these substitutions led to a decrease in affinity compared to compound 59. Subsequent SAR optimization resulted in identification of compound 46 with high human A_2BAdoR affinity (K_i = 13 nM), selectivity against other AdoR subtypes and with good pharmacokinetic properties. It was found to be a potent functional A_2BAdoR antagonist with a K_i of 8 nM in cAMP assay in hA_2B-HEK293 cells and an IC₅₀ of 107 nM in IL6 assay in NIH-3T3 cells. Docking study was performed to rationalize the observed affinity data. Structure-activity relationship (SAR) studies also led to identification of compound 36 as a potent A_2BAdoR antagonist with K_i of 1.8 nM in cAMP assay and good aqueous solubility of 529 μM at neutral pH. Compound 46 was further tested for in vivo efficacy and found to be efficacious in ovalbumin-induced allergic asthma model in mice.

A2B adenosine receptor antagonists: Design, synthesis and biological evaluation of novel xanthine derivatives

A_2BAdoR is a low affinity adenosine receptor that functions by Gs mediated elevation of cAMP and subsequent downstream signaling. The receptor has been implicated in lung inflammatory disorders like COPD and asthma. Several potent and selective A_2BAdoR antagonists have been reported in literature, however most of the compounds suffer from poor pharmacokinetic profile. Therefore, with the aim to identify novel, potent and selective A_2BAdoR antagonists with improved pharmacokinetic properties, we first explored more constrained form of MRS-1754 (4). To improve the metabolic stability, several linker modifications were attempted as replacement of amide linker along with different phenyl or other heteroaryls between C8 position of xanthine head group and terminal phenyl ring. SAR optimization resulted in identification of two novel A_2BAdoR antagonists, 8-{1-[5-Oxo-1-(4-trifluoromethyl-phenyl)-pyrrolidin-3-ylmethyl]-1H-pyrazol-4-yl}-1,3-dipropyl-xanthine (31) and 8-(1-{2-Oxo-2-[4-(3-trifluoromethyl-phenyl)-piperazin-1-yl]-ethyl}-1H-pyrazol-4-yl)-1,3-dipropyl-xanthine (65), with high binding affinity (K_i = 1 and 1.5 nM, respectively) and selectivity for A_2BAdoR with very good functional potency of 0.9 nM and 4 nM, respectively. Compound 31 and 65 also displayed good pharmacokinetic properties in mice with 27% and 65% oral bioavailability respectively. When evaluated in in vivo mice model of asthma, compound 65 also inhibited airway inflammation and airway reactivity in ovalbumin induced allergic asthma at 3 mpk dose.

2-Amino[1,2,4]triazolo[1,5-c]quinazolines and Derived Novel Heterocycles: Syntheses and Structure-Activity Relationships of Potent Adenosine Receptor Antagonists

2-Amino[1,2,4]triazolo[1,5-c]quinazolines were identified as potent adenosine receptor (AR) antagonists. Synthetic strategies were devised to gain access to a broad range of derivatives including novel polyheterocyclic compounds. Potent and selective A₃ AR antagonists were discovered, including 3,5-diphenyl[1,2,4]triazolo[4,3-c]quinazoline (17, K_i human A₃ AR 1.16 nm) and 5'-phenyl-1,2-dihydro-3'H-spiro[indole-3,2'-[1,2,4]triazolo[1,5-c]quinazolin]-2-one (20, K_i human A₃ AR 6.94 nm). In addition, multitarget antagonists were obtained, such as the dual A₁ /A₃ antagonist 2,5-diphenyl[1,2,4]triazolo[1,5-c]quinazoline (13 b, K_i human A₁ AR 51.6 nm, human A₃ AR 11.1 nm), and the balanced pan-AR antagonists 5-(2-thienyl)[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 c, K_i human A₁ AR 131 nm, A_2A AR 32.7 nm, A_2B AR 150 nm, A₃ AR 47.5 nm) and 9-bromo-5-phenyl[1,2,4]triazolo[1,5-c]quinazolin-2-amine (11 q, K_i human A₁ AR 67.7 nm, A_2A AR 13.6 nm, A_2B AR 75.0 nm, A₃ AR 703 nm). In many cases, significantly different affinities for human and rat receptors were observed, which emphasizes the need for caution in extrapolating conclusions between different species.

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

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

Synthesis of indeno and acenaphtho cores containing dihydroxy indolone, pyrrole, coumarin and uracil fused heterocyclic motifs under sustainable conditions exploring the catalytic role of the SnO2 quantum dot

A tin oxide (SnO₂) quantum dot (QD) catalyzed approach for the synthesis of indeno and acenaphtho cores containing dihydroxy indolone, pyrrole, coumarin and uracil fused derivatives was achieved via multicomponent one-pot approach in aqueous medium.

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.

International Union of Basic and Clinical Pharmacology. LXXXI. Nomenclature and classification of adenosine receptors--an update

In the 10 years since our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors, no developments have led to major changes in the recommendations. However, there have been so many other developments that an update is needed. The fact that the structure of one of the adenosine receptors has recently been solved has already led to new ways of in silico screening of ligands. The evidence that adenosine receptors can form homo- and heteromultimers has accumulated, but the functional significance of such complexes remains unclear. The availability of mice with genetic modification of all the adenosine receptors has led to a clarification of the functional roles of adenosine, and to excellent means to study the specificity of drugs. There are also interesting associations between disease and structural variants in one or more of the adenosine receptors. Several new selective agonists and antagonists have become available. They provide improved possibilities for receptor classification. There are also developments hinting at the usefulness of allosteric modulators. Many drugs targeting adenosine receptors are in clinical trials, but the established therapeutic use is still very limited.

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.

Novel Alexa Fluor-488 labeled antagonist of the A(2A) adenosine receptor: Application to a fluorescence polarization-based receptor binding assay

Fluorescence polarization (FP) assay has many advantages over the traditional radioreceptor binding studies. We developed an A(2A) adenosine receptor (AR) FP assay using a newly synthesized fluorescent antagonist of the A(2A)AR (MRS5346), a pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-amine derivative conjugated to the fluorescent dye Alexa Fluor-488. MRS5346 displayed a K(i) value of 111+/-16nM in radioligand binding using [(3)H]CGS21680 and membranes prepared from HEK293 cells stably expressing the human A(2A)AR. In a cyclic AMP functional assay, MRS5346 was shown to be an A(2A)AR antagonist. MRS5346 did not show any effect on A(1) and A(3) ARs in binding or the A(2B)AR in a cyclic AMP assay at 10microM. Its suitability as a fluorescent tracer was indicated in an initial observation of an FP signal following A(2A)AR binding. The FP signal was optimal with 20nM MRS5346 and 150microg protein/mL HEK293 membranes. The association and dissociation kinetic parameters were readily determined using this FP assay. The K(d) value of MRS5346 calculated from kinetic parameters was 16.5+/-4.7nM. In FP competition binding experiments using MRS5346 as a tracer, K(i) values of known AR agonists and antagonists consistently agreed with K(i) values from radioligand binding. Thus, this FP assay, which eliminates using radioisotopes, appears to be appropriate for both routine receptor binding and high-throughput screening with respect to speed of analysis, displaceable signal and precision. The approach used in the present study could be generally applicable to other GPCRs.

Recent developments in A2B adenosine receptor ligands

A selective, high-affinity A(2B) adenosine receptor (AR) antagonist will be useful as a pharmacological tool to help determine the role of the A(2B)AR in inflammatory diseases and angiogenic diseases. Based on early A(2B)AR-selective ligands with nonoptimal pharmaceutical properties, such as 15 (MRS 1754: K(i)(hA(2B)) = 2 nM; K(i)(hA(1)) = 403 nM; K(i)(hA(2A)) = 503 NM, and K(i)(hA(3)) = 570 nM), several groups have discovered second-generation A(2B)AR ligands that are suitable for development. Scientists at CV Therapeutics have discovered the selective, high-affinity A(2B)AR antagonist 22, a 8-(4-pyrazolyl)-xanthine derivative, (CVT-6883, K(i)(hA(2B)) = 22 nM; K(i)(hA(1)) = 1,940 nM; K(i)(hA(2A)) = 3,280; and K(i)(hA(3)) = 1,070 nM). Compound 22 has demonstrated favorable pharmacokinetic (PK) properties (T(1/2) = 4 h and F > 35% rat), and it is a functional antagonist at the A(2B)AR(K (B) = 6 nM). In a mouse model of asthma, compound 22 demonstrated a dose-dependent efficacy supporting the role of the A(2B)AR in asthma. In two Phase I clinical trails, 22 (CVT-6883) was found to be safe, well tolerated, and suitable for once-daily dosing. Baraldi et al. have independently discovered a selective, high-affinity A(2B)AR antagonist, 30 (MRE2029F20), 8-(5-pyrazolyl)-xanthine (K(i)(hA(2B)) = 5.5 nM; K(i)(hA(1)) = 200 nM; K(i)(hA(2A), A(3)) > 1,000, that has been selected for development in conjunction with King Pharmaceuticals. Compound 30 has been demonstrated to be a functional antagonist of the A(2B)AR, and it has been radiolabeled for use in pharmacological studies. A third compound, 58 (LAS-38096), is a 2-aminopyrimidine derivative (discovered by the Almirall group) that has high A(2B)AR affinity and selectivity (K(i)(hA(2B)) = 17 nM; K(i)(hA(1)) > 1,000 nM; K(i)(hA(2A)) > 2,500; and K(i)(hA(3)) > 1,000 nM), and 58 has been moved into preclinical safety testing. A fourth selective, high-affinity A(2B)AR antagonist, 54 (OSIP339391 K(i))(hA(2B)) = 0.5 nM; K(i))(hA(1)) = 37 nM; K(i))(hA(2A)) = 328; and K(i))(hA(3)) = 450 nm) was discovered by the OSI group. The three highly selective, high-affinity A(2B)AR antagonists that have been selected for development should prove useful in subsequent clinical trials that will establish the role of the A(2B)ARs in various disease states.

Synthesis and pharmacological evaluation of novel 1- and 8-substituted-3-furfuryl xanthines as adenosine receptor antagonists

The synthesis of an important set of 3-furfurylxanthine derivatives is described. Binding affinities were determined for rat A(1) and human A(2A), A(2B) and A(3) receptors. Several of the 3-furfuryl-7-methylxanthine derivatives showed moderate-to-high affinity at human A(2B) receptors, the most active compound (10d) having a K(i) of 7.4 nM for hA(2B) receptors, with selectivities over rA(1) and hA(2A) receptors up to 14-fold and 11-fold, respectively. Affinities for hA(3) receptors were very low for all members of the set.

Synthesis of novel 1-alkyl-8-substituted-3-(3-methoxypropyl) xanthines as putative A(2B) receptor antagonists

In order to identify a high-affinity, selective antagonist for the A(2B) subtype adenosine receptor, more than 40 1,8-disubstituted-3-(3-methoxypropyl) xanthines were prepared and evaluated for their binding affinity at recombinant human adenosine receptors, mainly of the A(2A) and A(2B) subtypes. Some of the 1-ethyl-3-(3-methoxypropyl)-8-aryl substituted derivatives 15(a-m) showed moderate-to-high affinity at human A(2B) receptors, with compound 15d showing A(2B) selectivity over the other A receptors assayed (A(1), A(2A), A(3)) of 34-fold or over.

Progress in the discovery of selective, high affinity A(2B) adenosine receptor antagonists as clinical candidates

The selective, high affinity A(2B) adenosine receptor (AdoR) antagonists that were synthesized by several research groups should aid in determining the role of the A(2B) AdoR in inflammatory diseases like asthma or rheumatoid arthritis (RA) and angiogenic diseases like diabetic retinopathy or cancer. CV Therapeutics scientists discovered the selective, high affinity A(2B) AdoR antagonist 10, a 8-(4-pyrazolyl)-xanthine derivative [CVT-6883, K(i)(hA(2B)) = 22 nM; K(i)(hA(1)) = 1,940 nM; K(i)(hA(2A)) = 3,280; and K(i)(hA(3)) = 1,070 nM] that has favorable pharmacokinetic (PK) properties (t (1/2) = 4 h and F > 35% rat). Compound 10 demonstrated functional antagonism at the A(2B) AdoR (K(B) = 6 nM) and efficacy in a mouse model of asthma. In two phase 1 clinical trials, CVT-6883 was found to be safe, well tolerated, and suitable for once daily dosing. A second compound 20, 8-(5-pyrazolyl)-xanthine, has been nominated for development from Baraldi's group in conjunction with King Pharmaceuticals that has favorable A(2B) AdoR affinity and selectivity [K(i)(hA(2B)) = 5.5 nM; K(i)(hA(1)) > 1,000 nM; K(i)(hA(2A)) > 1,000; and K(i)(hA(3)) > 1,000 nM], and it has been demonstrated to be a functional antagonist. A third compound 32, a 2-aminopyrimidine, from the Almirall group has high A(2B) AdoR affinity and selectivity [K(i)(hA(2B)) = 17 nM; K(i)(hA(1)) > 1,000 nM; K(i)(hA(2A)) > 2,500; and K(i)(hA(3)) > 1,000 nM], and 32 has been moved into preclinical safety testing. Since three highly selective, high affinity A(2B) AdoR antagonists have been nominated for development with 10 (CVT-6883) being the furthest along in the development process, the role of the A(2B) AdoR in various disease states will soon be established.

Structure-affinity relationships of adenosine A2B receptor ligands

Many selective and high affinity agonists and antagonists have been developed for the adenosine A(1), A(2A), and A(3) receptors. Very recently such compounds have been identified for the adenosine A(2B) receptors. This review presents an overview of the structure-affinity relationships of antagonists and agonists for this receptor subtype as published in the scientific and patent literature. To date the most selective >370-fold, high affinity adenosine A(2B) receptor antagonist is the xanthine analog, compound 16 (8-(1-(3-phenyl-1,2,4-oxadiazol-5-yl)methyl)-1H-pyrazol-4-yl)-1,3-dipropyl-1H-purine-2,6(3H,7H)-dione). The pyrrolopyrimidine analog OSIP339391 (73) is slightly less selective, 70-fold, but has a higher affinity 0.41 nM compared to 1 nM for compound 16. Other promising classes of compounds with selectivities ranging from 10- to 160-fold and affinities ranging from 3 to 112 nM include triazolo, aminothiazole, quinazoline, and pyrimidin-2-amine analogs. Progress has also been achieved concerning the development of selective high affinity agonists for the adenosine A(2B) receptor. For years the most potent, albeit non-selective adenosine A(2B) receptor agonist was (S)PHPNECA (88). Last year, a new class of non-ribose ligands was reported. Several compounds displayed selectivity with respect to adenosine A(2A) and A(3) receptors. In addition, full and partial agonists for the adenosine A(2B) receptor were identified with EC(50) values of 10 nM (LUF5835, 103) and 9 nM (LUF5845, 105), respectively.

3D-Pharmacophore Models for Selective A2A and A2B Adenosine Receptor Antagonists

Three-dimensional pharmacophore models were generated for A2A and A2B adenosine receptors (ARs) based on highly selective A2A and A2B antagonists using the Catalyst program. The best pharmacophore model for selective A2A antagonists (Hypo-A2A) was obtained through a careful validation process. Four features contained in Hypo-A2A (one ring aromatic feature (R), one positively ionizable feature (P), one hydrogen bond acceptor lipid feature (L), and one hydrophobic feature (H)) seem to be essential for antagonists in terms of binding activity and A2A AR selectivity. The best pharmacophore model for selective A2B antagonists (Hypo-A2B) was elaborated by modifying the Catalyst common features (HipHop) hypotheses generated from the selective A2B antagonists training set. Hypo-A2B also consists of four features: one ring aromatic feature (R), one hydrophobic aliphatic feature (Z), and two hydrogen bond acceptor lipid features (L). All features play an important role in A2B AR binding affinity and are essential for A2B selectivity. Both A2A and A2B pharmacophore models have been validated toward a wide set of test molecules containing structurally diverse selective antagonists of all AR subtypes. They are capable of identifying correspondingly high potent antagonists and differentiating antagonists between subtypes. The results of our study will act as a valuable tool for retrieving structurally diverse compounds with desired biological activities and designing novel selective adenosine receptor ligands.

A new synthesis of sulfonamides by aminolysis of p-nitrophenylsulfonates yielding potent and selective adenosine A2B receptor antagonists

1-Propyl- and 1,3-dimethyl-8-p-sulfophenylxanthine (PSB-1115 and SPT) were used as starting compounds for the development of adenosine A(2B) receptor antagonists with a sulfonamide structure. Since standard reactions for sulfonamide formation failed or resulted in very low yields, we developed a new method for the preparation of sulfonamides. p-Nitrophenoxide was used as a suitable leaving group with well balanced stability-reactivity properties. A large variety of amines, including aniline, benzylamine, phenethylamine, propylamine, butylamine, 2-hydroxyethylamine, aminoacetic acid, and N-benzylpiperazine reacted with p-nitrophenoxysulfonylphenylxanthine derivatives yielding the desired sulfonamides in satisfying to very good yields. The obtained sulfonamides were much more potent at A(2B) receptors than the parent sulfonates. The most active compound of the present series was 8-[4-(4-benzylpiperazide-1-sulfonyl)phenyl]-1-propylxanthine (11, PSB-601) exhibiting a K(i) value of 3.6 nM for the human A(2B) receptor combined with high selectivity versus the other human adenosine receptor subtypes (575-fold versus A(1), 134-fold versus A(2A), and >278-fold versus A(3)).

Novel 1,3-disubstituted 8-(1-benzyl-1H-pyrazol-4-yl) xanthines: high affinity and selective A2B adenosine receptor antagonists

Adenosine has been suggested to induce bronchial hyperresponsiveness in asthmatics, which is believed to be an A(2B) adenosine receptor (AdoR) mediated pathway. We hypothesize that a selective, high-affinity A(2B) AdoR antagonist may provide therapeutic benefit in the treatment of asthma. In an attempt to identify a high-affinity, selective antagonist for the A(2B) AdoR, we synthesized 8-(C-4-pyrazolyl) xanthines. Compound 22, 8-(1H-pyrazol-4-yl)-1,3-dipropyl xanthine, is a N-1 unsubstituted pyrazole derivative that has favorable binding affinity (K(i) = 9 nM) for the A(2B) AdoR, but it is only 2-fold selective versus the A(1) AdoR. Introduction of a benzyl group at the N-1-pyrazole position of 22 resulted in 19, which had moderate selectivity. The initial focus of the SAR study was on the preparation of substituted benzyl derivatives of 19 because the corresponding phenyl, phenethyl, and phenpropyl derivatives showed a decrease in A(2B) AdoR affinity and selectivity relative to 19. The preferred substitution on the phenyl ring of 19 contains an electron-withdrawing group, specifically F or CF(3) at the m-position, as in 33 and 36 respectively, increases the selectivity while retaining the affinity for the A(2B) AdoR. Exploring disubstitutions on the phenyl ring of derivatives 33 and36 led to the 2-chloro-5-trifluoromethylphenyl derivative 50, which retained the A(2B) AdoR affinity but enhanced the selectivity relative to 36. After optimization of the substitution on the 8-pyrazole xanthine, 1,3-disubstitution of the xanthine core was explored with methyl, ethyl, butyl, and isobutyl groups. In comparison to the corresponding dipropyl analogues, the smaller 1,3-dialkyl groups (methyl and ethyl) increased the A(2B) AdoR binding selectivity of the xanthine derivatives while retaining the affinity. However, the larger 1,3-dialkyl groups (isobutyl and butyl) resulted in a decrease in both A(2B) AdoR affinity and selectivity. This final SAR optimization led to the discovery of 1,3-dimethyl derivative 60, 8-(1-(3-(trifluoromethyl) benzyl)-1H-pyrazol-4-yl)-1,3-dimethyl xanthine, a high-affinity (K(i) = 1 nM) A(2B) AdoR antagonist with high selectivity (990-, 690-, and 1,000-) for the human A(1), A(2A,) and A(3) AdoRs.

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.

Design, synthesis, and structure-activity relationships of 1-,3-,8-, and 9-substituted-9-deazaxanthines at the human A2B adenosine receptor

Over two hundred 1-, 3-, 8-, and 9-substituted-9-deazaxanthines were prepared and evaluated for their binding affinity at the recombinant human adenosine receptors, in particular at the hA(2B) and hA(2A) subtypes. Several ligands endowed with sub-micromolar to low nanomolar binding affinity at hA(2B) receptors, good selectivity over hA(2A) and hA(3), but a relatively poor selectivity over hA(1) were obtained. Good antagonistic potencies and efficacies, with pA(2) values close to the corresponding pK(i)s, were observed in functional assays in vitro performed on a selected series of compounds. 1,3-Dimethyl-8-phenoxy-(N-p-halogenophenyl)-acetamido-9-deazaxanthine derivatives appeared as the most interesting leads, some of them showing outstanding hA(2B) affinities, high selectivity over hA(2A) and hA(3), but low selectivity over hA(1). Structure-affinity relationships suggested that the binding potency at the hA(2B) receptor was mainly modulated by the steric (lipophilic) properties of the substituents at positions 1 and 3 and by the electronic and lipophilic characteristics of the substituents at position 8. A comparison among affinity and selectivity profiles of 9-deazaxanthines with the corresponding xanthines suggested some possible differences in their binding mode.

Molecular modeling and molecular dynamics simulation of the human A2B adenosine receptor. The study of the possible binding modes of the A2B receptor antagonists

A molecular model of the human A(2B) adenosine receptor containing seven transmembrane alpha helices connected by three intracellular and three extracellular hydrophilic loops had been constructed. A molecular docking of seven structurally diverse xanthine antagonists of the A(2B) receptor was performed, and the differences in their binding modes were investigated. The 1 ns molecular dynamics (MD) simulations of several obtained ligand-receptor complexes inserted into the phospholipid bilayer were carried out. The conformational changes of the A(2B) receptor occurring during MD simulations were explored, and the stable binding modes of the studied antagonists were determined. According to the models presented in this work, the involvement of the His251, Asn282, Ser92, Thr89, and some aromatic residues in ligand recognition was determined. The obtained binding modes of the A(2B) antagonists demonstrate good agreement with the site-directed mutagenesis data.

New heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2

Extracellular adenosine and adenine nucleotides induce various cellular responses through activation of P1 and P2 receptors. P1 receptors preferentially recognize adenosine and four different G protein-coupled receptors (A(1), A(2A), A(2B), and A(3) subtypes) have been identified. On the other hand, P2 receptors are activated by adenine and/or uridine nucleotides and classified into two families: ionotropic P2X and G protein-coupled P2Y receptors. In this article, we summarize our studies which led to development of new potent and selective heterocyclic ligands for the adenosine receptors P1 and for the ATP receptors P2X(7).

Exploring QSAR of thiazole and thiadiazole derivatives as potent and selective human adenosine A3 receptor antagonists using FA and GFA techniques

Binding affinity data of thiazole and thiadiazole derivatives (n=30) for human adenosine A3 receptor subtype have been subjected to Quantitative Structure-Activity Relationship (QSAR) analysis using quantum chemical and hydrophobicity parameters. Wang-Ford charges of the common atoms of the compounds [calculated from molecular electrostatic potential surface of energy minimized geometry using Austin Model 1 (AM1) technique] were used as independent variables apart from partition coefficient (logP) and suitable dummy parameters. The variables for the multiple regression analyses were selected based on principal component factor analysis (FA), and generated equations were statistically validated using leave-one-out technique. The best equation thus obtained explained and predicted 74.4% and 68.9% respectively of the variance of the binding affinity. The results suggested importance of Wang-Ford charges of atoms C2, C5 and C7. Furthermore, the A3 binding affinity increases with decrease of lipophilicity of the compounds and in the presence of methyl or ethyl substituent at R position. Again, the binding affinity decreases in the presence of tert-butyloxy group at R position. When factor scores were used as predictor variables in principal component regression analysis, the resulted model showed 87.0% predicted variance and 89.5% explained variance. The data set was also modeled using genetic function approximation (GFA) technique. The best two equations derived from GFA show better predicted variance values (0.753 and 0.739) than that found in case of the best equation derived from FA. However, considerable intercorrelation was found between two predictor variables in case of GFA derived equations. GFA derived equations show importance of Wang-Ford charges of different atoms of the thiazole/thiadiazole nucleus and phenyl ring (S9, X8 and C2, the effects of the first two being predominant) along with similar impact of lipophilicity and R group on the binding affinity as found in case of the FA derived relation.

The receptor mechanism mediating the contractile response to adenosine on lung parenchymal strips from actively sensitised, allergen-challenged Brown Norway rats

Parenchymal strips prepared from lungs removed from actively sensitised Brown Norway rats challenged with allergen show hyperresponsiveness to adenosine. The response is mast cell mediated and a preliminary pharmacological analysis suggested the involvement of a receptor (or receptors) that could not be classified as any of the known adenosine receptor subtypes. We present a further analysis of the response. Male Brown Norway (BN) rats, actively sensitised to ovalbumin (OA), were challenged intratracheally with OA and killed 3 h later to provide parenchymal strip preparations. The augmented contractile responses to adenosine were partially blocked by the 5-HT receptor antagonist, methysergide, or the A(1) receptor antagonist, DPCPX, and abolished in the presence of both antagonists. Responses to high concentrations of the A(1) receptor agonist, CPA were, like those to adenosine, augmented on tissues from allergen-challenged animals and blocked by a combination of methysergide and DPCPX. The A(3) receptor agonist, Cl-IB-MECA, did not contract the tissue, but partially blocked the response to adenosine. A combination of Cl-IB-MECA and methysergide induced a similar degree of blockade to that seen with either drug given alone. Combination of Cl-IB-MECA and/or methysergide with DPCPX abolished the response to adenosine. The effects of the A(3) receptor agonist, inosine, were augmented on tissues from allergen-challenged animals and markedly inhibited by disodium cromoglycate, methysergide or Cl-IB-MECA. Responses to adenosine were abolished when parenchymal strips were taken from rats pretreated 48 h previously with pertussis toxin. 8-SPT, CGS 15943, XAC, MRS 1754, DPCPX and theophylline, at concentrations which inhibit the A(1) A(2A) and/or A(2B) receptors but have negligible affinity for the rat A(3) receptor, inhibited responses to adenosine, but high concentrations were required and blockade was incomplete. MRS 1523 and MRS 1191, which are antagonists at the rat A(3) receptor, had no effect on the response to adenosine. The present results support and clarify our earlier conclusion that an atypical receptor mechanism mediates contraction of the parenchymal strip prepared from the lungs of actively sensitised BN rats challenged with allergen to adenosine. The response arises from a combined effect of adenosine on the A(1) receptor and a receptor with similarities to the A(3) receptor, but where Cl-IB-MECA behaves as an antagonist and MRS 1523 and MRS 1191 are inactive at concentrations that substantially exceed their affinities for the rat A(3) receptor.

8-Substituted-9-deazaxanthines as adenosine receptor ligands: design, synthesis and structure-affinity relationships at A2B

A number of 8-substituted-9-deazaxanthine derivatives (1,3-dialkyl-6-substituted-1H-pyrrolo[3,2-d]pyrimidine-2,4(3H,5H)-diones) were prepared and tested for their antagonistic activity at the recombinant human adenosine receptors, in particular at the A(2B) and A(2A) receptor subtypes. Compounds endowed with micromolar to nanomolar binding affinities, but with poor A(2B)/A(2A) selectivity, were obtained. Preliminary quantitative structure-affinity relationships suggested that the binding potency at the A(2B) receptor is mainly modulated by the electronic and lipophilic properties of the ligands.

[3H]-MRE 2029-F20, a selective antagonist radioligand for the human A2B adenosine receptors

MRE 2029-F20 [N-benzo[1,3]dioxol-5-yl-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]-acetamide] is a selective antagonist ligand of A2B adenosine receptors. For use as a radioligand, 1,3-diallyl-xanthine, the precursor of [3H]-MRE 2029-F20, was synthesized, and tritiated on the allyl groups. [3H]-MRE 2029-F20 bound to human A2B receptors expressed in CHO cells showed a KD value of 1.65+/-0.10 nM and Bmax value of 36+/-4 fmol/mg protein. [3H]-MRE2029-F20 represents a useful tool for the pharmacological characterization of human A2B adenosine receptor subtype.

Design, synthesis, and biological evaluation of new 8-heterocyclic xanthine derivatives as highly potent and selective human A2B adenosine receptor antagonists

Here we report the synthesis of 8-heterocycle-substituted xanthines as potent and selective A(2B) adenosine receptor antagonists. The structure-activity relationships (SAR) of the xanthines synthesized in binding to recombinant human A(2B) adenosine receptors (ARs) in HEK-293 cells (HEK-A(2B)) and at other AR subtypes were explored. The synthesized compounds showed A(2B) adenosine receptor affinity in the nanomolar range and good levels of selectivity evaluated in radioligand binding assays at human (h) A(1), A(2A), A(2B), and A(3) ARs. We introduced several heterocycles, such as pyrazole, isoxazole, pyridine, and pyridazine, at the 8-position of the xanthine nucleus and we have also investigated different spacers (substituted acetamide, oxyacetamide, and urea moieties) on the heterocycle introduced. Various groups at the 3- and 4-positions of phenylacetamide moiety were studied. This study allowed us to identify the derivatives 2-(3,4-dimethoxyphenyl)-N-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yl]acetamide (29b, MRE2028F20) [K(i)(hA(2B)) = 38 nM, K(i)(hA(1),hA(2A),hA(3)) >1000 nM], N-benzo[1,3]dioxol-5-yl-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]acetamide (62b, MRE2029F20) [K(i)(hA(2B)) = 5.5 nM, K(i)(hA(1),hA(2A),hA(3)) > 1000 nM], and N-(3,4-dimethoxyphenyl)-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetrahydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]acetamide (72b, MRE2030F20) [K(i)(hA(2B) = 12 nM, K(i)(hA(1),hA(2A), hA(3)) > 1000 nM], which showed high affinity at the A(2B) receptor subtype and very good selectivity vs the other ARs. Substitution of the acetamide with an urea moiety afforded bioisosteric xanthines with good affinity and selectivity comparable to the acetamide derivatives. Substitution at the para-position of a 4-benzyloxy group of the phenylacetamido chain enhanced affinity at the A(2B) receptor [compound 30b (K(i)(hA(2B)) = 13 nM) vs compound 21b (K(i)(hA(2B) = 56 nM)] but did not favor selectivity. The derivatives with higher affinity at human A(2B) AR proved to be antagonists, in the cyclic AMP assay, capable of inhibiting the stimulatory effect of NECA (100 nM) with IC(50) values in the nanomolar range, a trend similar to that observed in the binding assay (62b, IC(50) = 38 nM; 72b, IC(50) = 46 nM). In conclusion, the 8-pyrazolo-1,3-dipropyl-1H-purine-2,6-dione derivatives described herein represent a new family of selective antagonists for the adenosine A(2B) receptor.

Pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine derivatives as adenosine receptor antagonists. Influence of the N5 substituent on the affinity at the human A 3 and A 2B adenosine receptor subtypes: a molecular modeling investigation

A new series of pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines bearing various substituents at both the N5-pyrimidinyl and N8-pyrazolyl positions have been synthesized, and their binding affinities at the four human adenosine receptor subtypes (hA(1), hA(2A), hA(2B), and hA(3)) have been evaluated. All the described compounds contain arylacetyl moieties at the N5 position and arylalkyl substituents at the N8 position. Surprisingly, all the compounds present their most potent affinities at the hA(2B) adenosine receptor with a range of selectivities against the other subtypes. When bulky groups are present simultaneously at the N5 and N8 positions (e.g., compound 9), the best selectivity for the hA(2B) receptor was observed (K(i)(hA(1)) = 1100 nM; K(i)(hA(2A)) = 800 nM; K(i)(hA(2B)) = 20 nM; K(i)(hA(3)) = 300 nM, K(i)(hA(1)/A(2B)) = 55, K(i)(hA(2A)/A(2B)) = 40, K(i)(hA(3)/hA(2B)) = 15). To understand the molecular significance of these results, we compared the putative TM (transmembrane) binding motif of compound 9 on both hA(2B) and hA(3) receptors. From our docking studies, compound 9 fits neatly inside the TM region of the hA(2B) receptor but not in the corresponding hA(3) region, illustrating significant differences between the two subtypes. The study herein presented permits an understanding of why the bioisosteric replacement of an -NH, present in previously reported hA(3) receptor antagonists, with a -CH(2) group at the N5 position induces such large differences in hA(2B)/hA(3) affinity. In the molecular structure of the hA(3) receptor, two residues, Ser243 (TM6) and Ser271 (TM7), create a hydrophilic region, which seems to permit a better accommodation of the phenylurea series into this putative hA(3) binding site than the phenylacetyl series.

Impact of the aryl substituent kind and distance from pyrimido[2,1-f]purindiones on the adenosine receptor selectivity and antagonistic properties

Adenosine receptor (AR) antagonists belong to two major groups of compounds: xanthines and non-xanthines. Recently several annelated xanthine derivatives have been described as selective A(1), A(2A), A(2B) and A(3) ARs antagonists. Contrary to dipropyl derivatives, in the group of dimethyl (un)substituted arylalkyl pyrimido[2,1-f]purindiones selective mainly adenosine A(2A) receptor antagonists were identified. Their activity depended on aryl substitution and its distance from pyrimido[2,1-f]purindione.