[(3)H]8-Ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]-purin-5-one ([(3)H]PSB-11), a novel high-affinity antagonist radioligand for human A(3) adenosine receptors

Computational Workflow for Refining AlphaFold Models in Drug Design Using Kinetic and Thermodynamic Binding Calculations: A Case Study for the Unresolved Inactive Human Adenosine A3 Receptor

A structure-based drug design pipeline that considers both thermodynamic and kinetic binding data of ligands against a receptor will enable the computational design of improved drug molecules. For unresolved GPCR-ligand complexes, a workflow that can apply both thermodynamic and kinetic binding data in combination with alpha-fold (AF)-derived or other homology models and experimentally resolved binding modes of relevant ligands in GPCR-homologs needs to be tested. Here, as test case, we studied a congeneric set of ligands that bind to a structurally unresolved G protein-coupled receptor (GPCR), the inactive human adenosine A3 receptor (hA3R). We tested three available homology models from which two have been generated from experimental structures of hA1R or hA2AR and one model was a multistate alphafold 2 (AF2)-derived model. We applied alchemical calculations with thermodynamic integration coupled with molecular dynamics (TI/MD) simulations to calculate the experimental relative binding free energies and residence time (τ)-random accelerated MD (τ-RAMD) simulations to calculate the relative residence times (RTs) for antagonists. While the TI/MD calculations produced, for the three homology models, good Pearson correlation coefficients, correspondingly, r = 0.74, 0.62, and 0.67 and mean unsigned error (mue) values of 0.94, 1.31, and 0.81 kcal mol-1, the τ-RAMD method showed r = 0.92 and 0.52 for the first two models but failed to produce accurate results for the multistate AF2-derived model. With subsequent optimization of the AF2-derived model by reorientation of the side chain of R1735.34 located in the extracellular loop 2 (EL2) that blocked ligand's unbinding, the computational model showed r = 0.84 for kinetic data and improved performance for thermodynamic data (r = 0.81, mue = 0.56 kcal mol-1). Overall, after refining the multistate AF2 model with physics-based tools, we were able to show a strong correlation between predicted and experimental ligand relative residence times and affinities, achieving a level of accuracy comparable to an experimental structure. The computational workflow used can be applied to other receptors, helping to rank candidate drugs in a congeneric series and enabling the prioritization of leads with stronger binding affinities and longer residence times.

Xanthine-Dopamine Hybrid Molecules as Multitarget Drugs with Potential for the Treatment of Neurodegenerative Diseases

Multitarget drugs based on a hybrid dopamine-xanthine core were designed as potential drug candidates for the treatment of neurodegenerative diseases. Monoamine oxidase B (MAO-B) inhibitors with significant ancillary A_2A adenosine receptor (A_2AAR) antagonistic properties were further developed to exhibit additional phosphodiesterase-4 and -10 (PDE4/10) inhibition and/or dopamine D₂ receptor (D₂R) agonistic activity. While all of the designed compounds showed MAO-B inhibition in the nanomolar range mostly combined with submicromolar A_2AAR affinity, significant enhancement of PDE-inhibitory and D₂R-agonistic activity was additionally reached for some compounds through various structural modifications. The final multitarget drugs also showed promising antioxidant properties in vitro. In order to evaluate their potential neuroprotective effect, representative ligands were tested in a cellular model of toxin-induced neurotoxicity. As a result, protective effects against oxidative stress in neuroblastoma cells were observed, confirming the utility of the applied strategy. Further evaluation of the newly developed multitarget ligands in preclinical models of Alzheimer's and Parkinson's diseases is warranted.

Dual A1/A3 Adenosine Receptor Antagonists: Binding Kinetics and Structure-Activity Relationship Studies Using Mutagenesis and Alchemical Binding Free Energy Calculations

Drugs targeting adenosine receptors (AR) can provide treatment for diseases. We report the identification of 7-(phenylamino)-pyrazolo[3,4-c]pyridines L2-L10, A15, and A17 as low-micromolar to low-nanomolar A₁R/A₃R dual antagonists, with 3-phenyl-5-cyano-7-(trimethoxyphenylamino)-pyrazolo[3,4-c]pyridine (A17) displaying the highest affinity at both receptors with a long residence time of binding, as determined using a NanoBRET-based assay. Two binding orientations of A17 produce stable complexes inside the orthosteric binding area of A₁R in molecular dynamics (MD) simulations, and we selected the most plausible orientation based on the agreement with alanine mutagenesis supported by affinity experiments. Interestingly, for drug design purposes, the mutation of L250^6.51 to alanine increased the binding affinity of A17 at A₁R. We explored the structure-activity relationships against A₁R using alchemical binding free energy calculations with the thermodynamic integration coupled with the MD simulation (TI/MD) method, applied on the whole G-protein-coupled receptor-membrane system, which showed a good agreement (r = 0.73) between calculated and experimental relative binding free energies.

The impact of inosine on hippocampal synaptic transmission and plasticity involves the release of adenosine through equilibrative nucleoside transporters rather than the direct activation of adenosine receptors

Inosine has robust neuroprotective effects, but it is unclear if inosine acts as direct ligand of adenosine receptors or if it triggers metabolic effects indirectly modifying the activity of adenosine receptors. We now combined radioligand binding studies with electrophysiological recordings in hippocampal slices to test how inosine controls synaptic transmission and plasticity. Inosine was without effect at 30 μM and decreased field excitatory post-synaptic potentials by 14% and 33% at 100 and 300 μM, respectively. These effects were prevented by the adenosine A₁ receptor antagonist DPCPX. Inosine at 300 (but not 100) μM also decreased the magnitude of long-term potentiation (LTP), an effect prevented by DPCPX and by the adenosine A_2A receptor antagonist SCH58261. Inosine showed low affinity towards human and rat adenosine receptor subtypes with K_i values of > 300 µM; only at the human and rat A₁ receptor slightly higher affinities with K_i values of around 100 µM were observed. Affinity of inosine at the rat A₃ receptor was higher (K_i of 1.37 µM), while it showed no interaction with the human orthologue. Notably, the effects of inosine on synaptic transmission and plasticity were abrogated by adenosine deaminase and by inhibiting equilibrative nucleoside transporters (ENT) with dipyridamole and NBTI. This shows that the impact of inosine on hippocampal synaptic transmission and plasticity is not due to a direct activation of adenosine receptors but is instead due to an indirect modification of the tonic activation of these adenosine receptors through an ENT-mediated modification of the extracellular levels of adenosine.

Agonist-Dependent Coupling of the Promiscuous Adenosine A2B Receptor to Gα Protein Subunits

The adenosine A_2B receptor (A_2BAR) belongs to the rhodopsin-like G protein-coupled receptor (GPCR) family. It is upregulated under hypoxic conditions, in inflammation and cancer. Previous studies indicated the coupling of the A_2BAR to different G proteins, mainly G_s, but in some cases G_q/11 or G_i, depending on the cell type. We have now utilized novel technologies, (i) heterologous expression of individual members of the Gα_q/11 protein family (Gα_q, Gα₁₁, Gα₁₄, and Gα₁₅) in Gα_q/11 knockout cells, and (ii) the TRUPATH platform, allowing the direct observation of Gα protein activation for each of the Gα subunits by bioluminescence resonance energy transfer (BRET) measurements. Three structurally diverse A_2BAR agonists were studied: the cognate agonist adenosine, its metabolically stable analog NECA, and the non-nucleosidic partial agonist BAY 60-6583. Adenosine and NECA activated most members of all four Gα protein families (Gα_s, Gα_q/11, Gα_i, and Gα_12/13). Significant differences in potencies and efficacies were observed; the highest efficacies were determined at the Gα₁₅, Gα_s, and Gα₁₂ proteins, and for NECA additionally at the Gα_i2 protein. In contrast, the partial agonist BAY 60-6583 only activated Gα₁₅, Gα_s, and Gα₁₂ proteins. Adenosine deaminase, an allosteric modulator of ARs, selectively increased the potency and efficacy of NECA and BAY 60-6583 at the Gα₁₅ protein, while it had no effect or decreased efficacy at the other Gα proteins. We conclude that the A_2BAR is preferably coupled to the Gα₁₅, Gα_s, and Gα₁₂ proteins. Upon upregulation of receptor or Gα protein expression, coupling to further Gα proteins likely occurs. Importantly, different agonists can display different activation profiles.

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

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

Adenosine A2AR/A1R Antagonists Enabling Additional H3R Antagonism for the Treatment of Parkinson's Disease

Adenosine A1/A2A receptors (A1R/A2AR) represent targets in nondopaminergic treatment of motor disorders such as Parkinson's disease (PD). As an innovative strategy, multitargeting ligands (MTLs) were developed to achieve comprehensive PD therapies simultaneously addressing comorbid symptoms such as sleep disruption. Recognizing the wake-promoting capacity of histamine H3 receptor (H3R) antagonists in combination with the "caffeine-like effects" of A1R/A2AR antagonists, we designed A1R/A2AR/H3R MTLs, where a piperidino-/pyrrolidino(propyloxy)phenyl H3R pharmacophore was introduced with overlap into an adenosine antagonist arylindenopyrimidine core. These MTLs showed distinct receptor binding profiles with overall nanomolar H3R affinities (Ki < 55 nM). Compound 4 (ST-2001, Ki (A1R) = 11.5 nM, Ki (A2AR) = 7.25 nM) and 12 (ST-1992, Ki (A1R) = 11.2 nM, Ki (A2AR) = 4.01 nM) were evaluated in vivo. l-DOPA-induced dyskinesia was improved after administration of compound 4 (1 mg kg-1, i.p. rats). Compound 12 (2 mg kg-1, p.o. mice) increased wakefulness representing novel pharmacological tools for PD therapy.

Synthesis of Novel Fluorinated Xanthine Derivatives with High Adenosine A2B Receptor Binding Affinity

The G protein-coupled adenosine A_2B receptor is suggested to be involved in various pathological processes accompanied by increased levels of adenosine as found in inflammation, hypoxia, and cancer. Therefore, the adenosine A_2B receptor is currently in focus as a novel target for cancer therapy as well as for noninvasive molecular imaging via positron emission tomography (PET). Aiming at the development of a radiotracer labeled with the PET radionuclide fluorine-18 for imaging the adenosine A_2B receptor in brain tumors, one of the most potent and selective antagonists, the xanthine derivative PSB-603, was selected as a lead compound. As initial biodistribution studies in mice revealed a negligible brain uptake of [³H]PSB-603 (SUV_3min: 0.2), structural modifications were performed to optimize the physicochemical properties regarding blood–brain barrier penetration. Two novel fluorinated derivatives bearing a 2-fluoropyridine (5) moiety and a 4-fluoro-piperidine (6) moiety were synthesized, and their affinity towards the four adenosine receptor subtypes was determined in competition binding assays. Both compounds showed high affinity towards the adenosine A_2B receptor (K_i (5) = 9.97 ± 0.86 nM; K_i (6) = 12.3 ± 3.6 nM) with moderate selectivity versus the other adenosine receptor subtypes.

Molecular probes for the human adenosine receptors

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

Design, Synthesis and Biological Evaluation of Highly Potent Simplified Archazolids

The archazolids are potent antiproliferative compounds that have recently emerged as a novel class of promising anticancer agents. Their complex macrolide structures and scarce natural supply make the development of more readily available analogues highly important. Herein, we report the design, synthesis and biological evaluation of four simplified and partially saturated archazolid derivatives. We also reveal important structure-activity relationship data as well as insights into the pharmacophore of these complex polyketides.

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

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

Therapeutic Potential of Agonists and Antagonists of A1, A2a, A2b and A3 Adenosine Receptors

Adenosine is a naturally occurring nucleoside and an essential component of the energy production and utilization systems of the body. Adenosine is formed by the degradation of adenosine-triphosphate (ATP) during energy-consuming processes. Adenosine regulates numerous physiological processes through activation of four subtypes of G-protein coupled membrane receptors viz. A1, A2A, A2B and A3. Its physiological importance depends on the affinity of these receptors and the extracellular concentrations reached. ATP acts as a neurotransmitter in both peripheral and central nervous systems. In the peripheral nervous system, ATP is involved in chemical transmission in sensory and autonomic ganglia, whereas in central nervous system, ATP, released from synaptic terminals, induces fast excitatory postsynaptic currents. ATP provides the energetics for all muscle movements, heart beats, nerve signals and chemical reactions inside the body. Adenosine has been traditionally considered an inhibitor of neuronal activity and a regulator of cerebral blood flow. Since adenosine is neuroprotective against excitotoxic and metabolic dysfunctions observed in neurological and ocular diseases, the search for adenosinerelated drugs regulating adenosine transporters and receptors can be important for advancement of therapeutic strategies against these diseases. This review will summarize the therapeutic potential and recent SAR and pharmacology of adenosine and its receptor agonists and antagonists.

Development of a Radiofluorinated Adenosine A2B Receptor Antagonist as Potential Ligand for PET Imaging

The adenosine A_2B receptor has been proposed as a novel therapeutic target in cancer, as its expression is drastically elevated in several tumors and cancer cells. Noninvasive molecular imaging via positron emission tomography (PET) would allow the in vivo quantification of this receptor in pathological processes and most likely enable the identification and clinical monitoring of respective cancer therapies. On the basis of a bicyclic pyridopyrimidine-2,4-dione core structure, the new adenosine A_2B receptor ligand 9 was synthesized, containing a 2-fluoropyridine moiety suitable for labeling with the short-lived PET radionuclide fluorine-18. Compound 9 showed a high binding affinity for the human A_2B receptor (K_i(A_2B) = 2.51 nM), along with high selectivities versus the A₁, A_2A, and A₃ receptor subtypes. Therefore, it was radiofluorinated via nucleophilic aromatic substitution of the corresponding nitro precursor using [¹⁸F]F^-/K_2.2.2./K₂CO₃ in DMSO at 120 °C. Metabolic studies of [¹⁸F]9 in mice revealed about 60% of radiotracer intact in plasma at 30 minutes p.i. A preliminary PET study in healthy mice showed an overall biodistribution of [¹⁸F]9, corresponding to the known ubiquitous but low expression of the A_2B receptor. Consequently, [¹⁸F]9 represents a novel PET radiotracer with high affinity and selectivity toward the adenosine A_2B receptor and a suitable in vivo profile. Subsequent studies are envisaged to investigate the applicability of [¹⁸F]9 to detect alterations in the receptor density in certain cancer-related disease models.

Development of Covalent Ligands for G Protein-Coupled Receptors: A Case for the Human Adenosine A3 Receptor

The development of covalent ligands for G protein-coupled receptors (GPCRs) is not a trivial process. Here, we report a streamlined workflow thereto from synthesis to validation, exemplified by the discovery of a covalent antagonist for the human adenosine A₃ receptor (hA₃AR). Based on the 1 H,3 H-pyrido[2,1- f]purine-2,4-dione scaffold, a series of ligands bearing a fluorosulfonyl warhead and a varying linker was synthesized. This series was subjected to an affinity screen, revealing compound 17b as the most potent antagonist. In addition, a nonreactive methylsulfonyl derivative 19 was developed as a reversible control compound. A series of assays, comprising time-dependent affinity determination, washout experiments, and [³⁵S]GTPγS binding assays, then validated 17b as the covalent antagonist. A combined in silico hA₃AR-homology model and site-directed mutagenesis study was performed to demonstrate that amino acid residue Y265^7.36 was the unique anchor point of the covalent interaction. This workflow might be applied to other GPCRs to guide the discovery of covalent ligands.

A live cell NanoBRET binding assay allows the study of ligand-binding kinetics to the adenosine A3 receptor

There is a growing interest in understanding the binding kinetics of compounds that bind to G protein-coupled receptors prior to progressing a lead compound into clinical trials. The widely expressed adenosine A₃ receptor (A₃AR) has been implicated in a range of diseases including immune conditions, and compounds that aim to selectively target this receptor are currently under development for arthritis. Kinetic studies at the A₃AR have been performed using a radiolabelled antagonist, but due to the kinetics of this probe, they have been carried out at 10 °C in membrane preparations. In this study, we have developed a live cell NanoBRET ligand binding assay using fluorescent A₃AR antagonists to measure kinetic parameters of labelled and unlabelled compounds at the A₃AR at physiological temperatures. The kinetic profiles of four fluorescent antagonists were determined in kinetic association assays, and it was found that XAC-ser-tyr-X-BY630 had the longest residence time (RT = 288 ± 62 min) at the A₃AR. The association and dissociation rate constants of three antagonists PSB-11, compound 5, and LUF7565 were also determined using two fluorescent ligands (XAC-ser-tyr-X-BY630 or AV039, RT = 6.8 ± 0.8 min) as the labelled probe and compared to those obtained using a radiolabelled antagonist ([³H]PSB-11, RT = 44.6 ± 3.9 min). There was close agreement in the kinetic parameters measured with AV039 and [³H]PSB-11 but significant differences to those obtained using XAC-S-ser-S-tyr-X-BY630. These data indicate that selecting a probe with the appropriate kinetics is important to accurately determine the kinetics of unlabelled ligands with markedly different kinetic profiles.

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

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

Novel multi-target directed ligands based on annelated xanthine scaffold with aromatic substituents acting on adenosine receptor and monoamine oxidase B. Synthesis, in vitro and in silico studies

N9-Benzyl-substituted imidazo-, pyrimido- and 1,3-diazepino[2,1-f]purinediones were designed as dual-target-directed ligands combining A_2A adenosine receptor (AR) antagonistic activity with blockade of monoamine oxidase B (MAO-B). A library of 37 novel compounds was synthesized and biologically evaluated in radioligand binding studies at AR subtypes and for their ability to inhibit MAO-B. A systematic modification of the tricyclic structures based on a xanthine core by enlargement of the third heterocyclic ring or attachment of various substituted benzyl moieties resulted in the development of 9-(2-chloro-6-fluorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrimido[2,1-f]purine-2,4(1H,3H)-dione (9u; K_i human A_2AAR: 189 nM and IC₅₀ human MAO-B: 570 nM) as the most potent dual acting ligand of the series displaying high selectivity versus related targets. Moreover, some potent, selective MAO-B inhibitors were identified in the group of pyrimido- and 1,3-diazepino[2,1-f]purinediones. Compound 10d (10-(3,4-dichlorobenzyl)-1,3-dimethyl-7,8,9,10-tetrahydro-1H-[1,3]diazepino[2,1-f]purine-2,4(3H,6H)-dione) displayed an IC₅₀ value at human MAO-B of 83 nM. Analysis of structure-activity relationships was complemented by molecular docking studies based on previously published X-ray structures of the protein targets. An extended biological profile was determined for selected compounds including in vitro evaluation of potential hepatotoxicity calculated in silico and antioxidant properties as an additional desirable activity. The new molecules acting as dual target drugs may provide symptomatic relief as well as disease-modifying effects for neurodegenerative diseases, in particular Parkinson's disease.

Radiosynthesis and in vivo evaluation of a fluorine-18 labeled pyrazine based radioligand for PET imaging of the adenosine A2B receptor

On the basis of a pyrazine core structure, three new adenosine A_2B receptor ligands (7a-c) were synthesized containing a 2-fluoropyridine moiety suitable for ¹⁸F-labeling. Compound 7a was docked into a homology model of the A_2B receptor based on X-ray structures of the related A_2A receptor, and its interactions with the adenosine binding site were rationalized. Binding affinity data were determined at the four human adenosine receptor subtypes. Despite a rather low selectivity regarding the A₁ receptor, 7a was radiolabeled as the most suitable candidate (K_i(A_2B) = 4.24 nM) in order to perform in vivo studies in mice with the aim to estimate fundamental pharmacokinetic characteristics of the compound class. Organ distribution studies and a single PET study demonstrated brain uptake of [¹⁸F]7a with a standardized uptake value (SUV) of ≈1 at 5 min post injection followed by a fast wash out. Metabolism studies of [¹⁸F]7a in mice revealed the formation of a blood-brain barrier penetrable radiometabolite, which could be structurally identified. The results of this study provide an important basis for the design of new derivatives with improved binding properties and metabolic stability in vivo.

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

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

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

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.

Fluorescent-Labeled Selective Adenosine A2B Receptor Antagonist Enables Competition Binding Assay by Flow Cytometry

Fluorescent ligands represent powerful tools for biological studies and are considered attractive alternatives to radioligands. In this study, we developed fluorescent antagonists for A_2B adenosine receptors (A_2BARs), which are targeted by antiasthmatic xanthines and were proposed as novel targets in immuno-oncology. Our approach was to merge a small borondipyrromethene (BODIPY) derivative with the pharmacophore of 8-substituted xanthine derivatives. On the basis of the design, synthesis, and evaluation of model compounds, several fluorescent ligands were synthesized. Compound 29 (PSB-12105), which displayed high affinity for human, rat, and mouse A_2BARs ( K_i = 0.2-2 nM) and high selectivity for this AR subtype, was selected for further studies. A homology model of the human A_2BAR was generated, and docking studies were performed. Moreover, 29 allowed us to establish a homogeneous receptor-ligand binding assay using flow cytometry. These compounds constitute the first potent, selective fluorescent A_2BAR ligands and are anticipated to be useful for a variety of applications.

Structure-Affinity Relationships and Structure-Kinetics Relationships of Pyrido[2,1-f]purine-2,4-dione Derivatives as Human Adenosine A3 Receptor Antagonists

We expanded on a series of pyrido[2,1-f]purine-2,4-dione derivatives as human adenosine A₃ receptor (hA₃R) antagonists to determine their kinetic profiles and affinities. Many compounds showed high affinities and a diverse range of kinetic profiles. We found hA₃R antagonists with very short residence time (RT) at the receptor (2.2 min for 5) and much longer RTs (e.g., 376 min for 27 or 391 min for 31). Two representative antagonists (5 and 27) were tested in [³⁵S]GTPγS binding assays, and their RTs appeared correlated to their (in)surmountable antagonism. From a k_on–k_off–K_D kinetic map, we divided the antagonists into three subgroups, providing a possible direction for the further development of hA₃R antagonists. Additionally, we performed a computational modeling study that sheds light on the crucial receptor interactions, dictating the compounds’ binding kinetics. Knowledge of target binding kinetics appears useful for developing and triaging new hA₃R antagonists in the early phase of drug discovery.

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.

Agonists for the adenosine A1 receptor with tunable residence time. A Case for nonribose 4-amino-6-aryl-5-cyano-2-thiopyrimidines

We report the synthesis and evaluation of previously unreported 4-amino-6-aryl-5-cyano-2-thiopyrimidines as selective human adenosine A1 receptor (hA1AR) agonists with tunable binding kinetics, this without affecting their nanomolar affinity for the target receptor. They show a very diverse range of kinetic profiles (from 1 min (compound 52) to 1 h (compound 43)), and their structure-affinity relationships (SAR) and structure-kinetics relationships (SKR) were established. When put in perspective with the increasing importance of binding kinetics in drug discovery, these results bring new evidence of the consequences of affinity-only driven selection of drug candidates, that is, the potential elimination of slightly less active compounds that may display preferable binding kinetics.

1,3-Dialkyl-substituted tetrahydropyrimido[1,2-f]purine-2,4-diones as multiple target drugs for the potential treatment of neurodegenerative diseases

Adenosine receptors and monoamine oxidases are drug targets for neurodegenerative diseases such as Parkinson's and Alzheimer's disease. In the present study we prepared a library of 55 mostly novel tetrahydropyrimido[2,1-f]purinediones with various substituents in the 1- and 3-position (1,3-dimethyl, 1,3-diethyl, 1,3-dipropyl, 1-methyl-3-propargyl) and broad variation in the 9-position. A synthetic strategy to obtain 3-propargyl-substituted tetrahydropyrimido[2,1-f]purinedione derivatives was developed. The new compounds were evaluated for their interaction with all four adenosine receptor subtypes and for their ability to inhibit monoamine oxidases (MAO). Introduction of mono- or di-chloro-substituted phenyl, benzyl or phenethyl residues at N9 of the 1,3-dimethyl series led to the discovery of a novel class of potent MAO-B inhibitors, the most potent compound being 9-(3,4-dichlorobenzyl)-1,3-dimethyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)-dione (21g, IC(50) human MAO-B: 0.0629 μM), which displayed high selectivity versus the other investigated targets. Potent dually active A1/A2A adenosine receptor antagonists were identified, for example, 9-benzyl-1-methyl-3-propargyl-6,7,8,9-tetrahydropyrimido[1,2-f]purine-2,4(1H,3H)dione (19f, Ki, human receptors, A1: 0.249 μM, A2A: 0.253 μM). Several compounds showed triple-target inhibition, the best compound being 9-(2-methoxybenzyl)-1-methyl-3-(prop-2-ynyl)-6,7,8,9-tetrahydro pyrimido [1,2-f]purine-2,4(1H,3H)-dione (19g, Ki A1: 0.605 μM, Ki A2A: 0.417 μM, IC(50) MAO-B: 1.80 μM). Compounds inhibiting several different targets involved in neurodegeneration may exhibit additive or even synergistic effects in vivo.

Synthesis, biological activity and molecular modelling studies of tricyclic alkylimidazo-, pyrimido- and diazepinopurinediones

Syntheses and biological activities of imidazo-, pyrimido- and diazepino[2,1-f]purinediones containing N-alkyl substituents (with straight, branched or unsaturated chains) are described. Tricyclic derivatives were synthesized by the cyclization of 8-bromo-substituted 7-(2-bromoethyl)-, 7-(3-chloropropyl)- or 7-(4-bromobutyl)-theophylline with primary amines under various conditions. Compound 22 with an ethenyl substituent was synthesized by dehydrohalogenation of 9-(2-bromoethyl)-1,3-dimethyltetrahydropyrimido[2,1-f]purinedione. The obtained derivatives (5-35) were initially evaluated for their affinity at rat A1 and A2A adenosine receptors (AR), showing moderate affinity for both adenosine receptor subtypes. The best ligands were diazepinopurinedione 28 (K i = 0.28 μM) with fivefold A2A selectivity and the non-selective A1/A2A AR ligand pyrimidopurinedione 35 (K i A1 = 0.28 μM and K i A2A = 0.30 μM). The compounds were also evaluated for their affinity at human A1, A2A, A2B and A3 ARs. All of the obtained compounds were docked to the A2A AR X-ray structure in complex with the xanthine-based, potent adenosine receptor antagonist-XAC. The likely interactions of imidazo-, pyrimido- and diazepino[2,1-f]purinediones with the residues forming the A2A binding pocket were discussed. Furthermore, the new compounds were tested in vivo as anticonvulsants in maximal electroshock, subcutaneous pentylenetetrazole (ScMet) and TOX tests in mice (i.p.). Pyrimidopurinediones showed anticonvulsant activity mainly in the ScMet test. The best derivative was compound 11, showing 100 % protection at a dose of 100 mg/kg without symptoms of neurotoxicity. Compounds 6, 7, 8 and 14 with short substituents showed neurotoxicity and caused death. In rat tests (p.o.), 9 was characterized by a high protection index (>13.3). AR affinity did not apparently correlate with the antiepileptic potency of the compounds.

Dual targeting of adenosine A(2A) receptors and monoamine oxidase B by 4H-3,1-benzothiazin-4-ones

Blockade of A2A adenosine receptors (A2AARs) and inhibition of monoamine oxidase B (MAO-B) in the brain are considered attractive strategies for the treatment of neurodegenerative diseases such as Parkinson's disease (PD). In the present study, benzothiazinones, e.g., 2-(3-chlorophenoxy)-N-(4-oxo-4H-3,1-benzothiazin-2-yl)acetamide (13), were identified as a novel class of potent MAO-B inhibitors (IC50 human MAO-B: 1.63 nM). Benzothiazinones with large substituents in the 2-position, e.g., methoxycinnamoylamino, phenylbutyrylamino, or chlorobenzylpiperazinylbenzamido residues (14, 17, 27, and 28), showed high affinity and selectivity for A2AARs (Ki human A2AAR: 39.5-69.5 nM). By optimizing benzothiazinones for both targets, the first potent, dual-acting A2AAR/MAO-B inhibitors with a nonxanthine structure were developed. The best derivative was N-(4-oxo-4H-3,1-benzothiazin-2-yl)-4-phenylbutanamide (17, Ki human A2A, 39.5 nM; IC50 human MAO-B, 34.9 nM; selective versus other AR subtypes and MAO-A), which inhibited A2AAR-induced cAMP accumulation and showed competitive, reversible MAO-B inhibition. The new compounds may be useful tools for validating the A2AAR/MAO-B dual target approach in PD.

Development of Polar Adenosine A2A Receptor Agonists for Inflammatory Bowel Disease: Synergism with A2B Antagonists

Adenosine A2A receptor agonists for the local treatment of inflammatory bowel disease (IBS) were designed and synthesized. Polar groups were introduced to prevent peroral absorption and subsequent systemic, e.g., hypotensive, side effects. 4-(2-{6-Amino-9-[(2R,3R,4S,5R)-3,4-dihydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl]-9H-purin-2-ylthio}ethyl)benzenesulfonic acid (7, PSB-0777) was selected for further evaluation in rat ileum/jejunum preparations in ex vivo experiments. Compound 7 significantly improved impaired acetylcholine-induced contractions induced by 2,4,6-trinitrobenzenesulfonic acid and showed synergism with an A2B-selective antagonist. Thus, nonabsorbable, locally active A2A agonists, as a monotherapy or in combination with an A2B antagonist, may be an efficient novel treatment for IBS, preventing the severe systemic side effects of known A2A agonists.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

New 2-heterocyclyl-imidazo[2,1-i]purin-5-one derivatives as potent and selective human A3 adenosine receptor antagonists

A series of 4-allyl/benzyl-7,8-dihydro-8-methyl/ethyl-2-[(substituted)isoxazol/pyrazol-3/5-yl]-1H-imidazo[2,1-i]purin-5(4H)-ones has been synthesized and evaluated in radioligand binding assays to determine their affinities at the human A(1), A(2A), and A(3) adenosine receptors. Efficacy at the hA(2B) AR and antagonism of selected ligands at the hA(3) AR were also assessed through cAMP experiments. All of the synthesized molecules exhibited high affinity at the hA(3) AR (K(i) values ranging from 1.46 to 44.8 nM), as well as remarkable selectivity versus A(1), A(2A), and A(2B) AR subtypes. Compound (R)-4-allyl-8-ethyl-7,8-dihydro-2-(3-methoxy-1-methyl-1H-pyrazol-5-yl)-1H-imidazo[2,1-i]purin-5(4H)-one (R-33) was found to be the most potent and selective ligand of the series (K(i) hA(3) = 1.46 nM, K(i) hA(2A)/K(i) hA(3) > 3425; IC(50) hA(2B)/K(i) hA(3) > 3425; K(i) hA(1)/K(i) hA(3) = 1729). Molecular modeling studies were helpful in rationalizing the available structure-activity relationships along with the selectivity profiles of the new series of ligands.

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.

Nucleoside-5'-monophosphates as prodrugs of adenosine A2A receptor agonists activated by ecto-5'-nucleotidase

Prodrugs of adenosine A(2A) receptor agonists were developed that are activated by ecto-5'-nucleotidase (ecto-5'-NT, CD73). Because ecto-5'-NT is upregulated in inflamed tissue, the A(2A) agonists are expected to be released from their prodrug form at the sites of inflammation. 2-(Ar)alkyl-substituted AMP derivatives were synthesized and investigated. Certain 2-substituted AMP derivatives, including 2-hexylthio-AMP, 2-cyclopentylthio-AMP, 2-cyclohexylmethylthio-AMP, and 2-cyclohexylethylthio-AMP were accepted as substrates by ecto-5'-NT and readily converted to the corresponding 2-substituted adenosine derivatives. The 2-cyclohexylethylthio substitution was a good compromise between the requirements of the ecto-5'-NT and the adenosine A(2A) receptor. The corresponding AMP derivative (12g) was a similarly good substrate as AMP itself, while the resulting adenosine derivative (11g) was a relatively potent A(2A) agonist (radioligand binding to rat brain striatal membranes: K(i) = 372 nM; inhibition of anti-CD3/anti-CD28-induced IFN-gamma release in mouse CD4+ cells: EC(50) = 50 nM). Compound 11g was released from 12g by incubation with CD4+ cells isolated from wild-type mice but only to a much smaller extent by cells from ecto-5'-NT knockout mice. Compound 12g will be a new lead structure for the development of more potent and selective ecto-5'-NT-activated prodrugs of selective anti-inflammatory A(2A) receptor agonists.