Pyrazolo-triazolo-pyrimidine Scaffold as a Molecular Passepartout for the Pan-Recognition of Human Adenosine Receptors.

Adenosine receptors are largely distributed in our organism and are promising therapeutic targets for the treatment of many pathologies. In this perspective, investigating the structural features of the ligands leading to affinity and/or selectivity is of great interest. In this work, we have focused on a small series of pyrazolo-triazolo-pyrimidine antagonists substituted in positions 2, 5, and N8, where bulky acyl moieties at the N5 position and small alkyl groups at the N8 position are associated with affinity and selectivity at the A3 adenosine receptor even if a good affinity toward the A2B adenosine receptor has also been observed. Conversely, a free amino function at the 5 position induces high affinity at the A2A and A1 receptors with selectivity vs. the A3 subtype. A molecular modeling study suggests that differences in affinity toward A1, A2A, and A3 receptors could be ascribed to two residues: one in the EL2, E168 in human A2A/E172 in human A1, that is occupied by the hydrophobic residue V169 in the human A3 receptor; and the other in TM6, occupied by H250/H251 in human A2A and A1 receptors and by a less bulky S247 in the A3 receptor. In the end, these findings could help to design new subtype-selective adenosine receptor ligands.

[1,2,4]Triazolo[1,5-c]pyrimidines as Tools to Investigate A3 Adenosine Receptors in Cancer Cell Lines.

The A3 adenosine receptor is an interesting target whose role in cancer is controversial. In this work, a structural investigation at the 2-position of the [1,2,4]triazolo[1,5-c]pyrimidine nucleus was performed, finding new potent and selective A3 adenosine receptor antagonists such as the ethyl 2-(4-methoxyphenyl)-5-(methylamino)-[1,2,4]triazolo[1,5-c]pyrimidine-8-carboxylate (20, DZ123) that showed a Ki value of 0.47 nM and an exceptional selectivity profile over the other adenosine receptor subtypes. Computational studies were performed to rationalize the affinity and the selectivity profile of the tested compounds at the A3 adenosine receptor and the A1 and A2A adenosine receptors. Compound 20 was tested on both A3 adenosine receptor positive cell lines (CHO-A3 AR transfected, THP1 and HCT16) and on A3 negative cancer cell lines, showing no effect in the latter and a pro-proliferative effect at a low concentration in the former. These interesting results pave the way to further investigation on both the mechanism involved and potential therapeutic applications.

A Versatile Synthetic Affinity Probe Reveals Inhibitory Synapse Ultrastructure and Brain Connectivity.

Visualization of inhibitory synapses requires protocol tailoring for different sample types and imaging techniques, and usually relies on genetic manipulation or the use of antibodies that underperform in tissue immunofluorescence. Starting from an endogenous ligand of gephyrin, a universal marker of the inhibitory synapse, we developed a short peptidic binder and dimerized it, significantly increasing affinity and selectivity. We further tailored fluorophores to the binder, yielding "Sylite"-a probe with outstanding signal-to-background ratio that outperforms antibodies in tissue staining with rapid and efficient penetration, mitigation of staining artifacts, and simplified handling. In super-resolution microscopy Sylite precisely localizes the inhibitory synapse and enables nanoscale measurements. Sylite profiles inhibitory inputs and synapse sizes of excitatory and inhibitory neurons in the midbrain and combined with complimentary tracing techniques reveals the synaptic connectivity.

Potent and selective A3 adenosine receptor antagonists bearing aminoesters as heterobifunctional moieties.

A3 adenosine receptors were found to have a role in different pathological states, such as glaucoma, renal fibrosis, neuropathic pain and cancer. Consequently, it is important to utilize any molecular tool which could help to study these conditions. In the present study we continue our search for potent A3 adenosine receptor ligands which could be successively conjugated to other molecules with the aim of obtaining more potent (e.g. allosteric ligand conjugation) or detectable ligands (e.g. fluorescent molecule or biotin conjugation). Specifically, different aminoester moieties were introduced at the 5 position of the pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine core. The ester functionalization represents the candidate for the subsequent conjugation. All the reported compounds are potent hA3 adenosine receptor antagonists and some of them exhibited high selectivity against the other adenosine receptors. The main structural terms of ligand recognition and selectivity were disclosed by molecular modelling studies. Molecular docking results led to the characterization of an alternative binding mode for antagonists at the orthosteric binding site of the hA3 adenosine receptor, evaluated and assessed by classical molecular dynamics simulations.

Bronchospasmolytic and Adenosine Binding Activity of 8- (Proline / Pyrazole)- Substituted Xanthine Derivatives.

BACKGROUND: 8-Phenyltheophylline derivatives exhibit prophylactic effects at a specific dose but do not produce the cardiovascular or emetic side effects associated with xanthines, thereby exhibiting unique characteristics of potential therapeutic importance. METHODS: Novel series of 8-(proline/pyrazole)-substituted xanthine analogs have been synthesized. The affinity and selectivity of compounds to adenosine receptors have been assessed by radioligand binding studies. The synthesized compounds also showed good bronchospasmolytic properties (increased onset of bronchospasm; decreased duration of jerks) with 100% survival of animals in comparison to the standard drug. Besides, compound 8f & 9f showed good binding affinity in comparison to other synthesized compounds in the micromolar range. RESULTS: The maximum binding affinity of these compounds was observed for A2B receptors, which was ~ 7 or 10 times higher as compared to A1, A2A and A3 receptors. The newly synthesized derivatives 8f, 9a-f, 17g-m, and 18g-m displayed significant protection against histamine aerosol induced bronchospasm in guinea pigs. CONCLUSION: Newly synthesized proline/pyrazole based xanthines compounds showed a satisfactory binding affinity for adenosine receptor subtypes. Replacement or variation of substituted proline ring with substituted pyrazole scaffold at the 8th-position of xanthine moiety resulted in the reduction of adenosine binding affinity and bronchospasmolytic effects.

A New Series of 1,3-Dimethylxanthine Based Adenosine A2A Receptor Antagonists as a Non-Dopaminergic Treatment of Parkinson's Disease.

BACKGROUND: Adenosine receptors (AR) have emerged as competent and innovative nondopaminergic targets for the development of potential drug candidates and thus constitute an effective and safer treatment approach for Parkinson's disease (PD). Xanthine derivatives are considered as potential candidates for the treatment Parkinson's disease due to their potent A2A AR antagonistic properties. OBJECTIVE: The objectives of the work are to study the impact of substituting N7-position of 8-m/pchloropropoxyphenylxanthine structure on in vitro binding affinity of compounds with various AR subtypes, in vivo antiparkinsonian activity and binding modes of newly synthesized xanthines with A2A AR in molecular docking studies. METHODS: Several new 7-substituted 8-m/p-chloropropoxyphenylxanthine analogues have been prepared. Adenosine receptor binding assays were performed to study the binding interactions with various subtypes and perphenazine induced rat catatonia model was used for antiparkinsonian activity. Molecular docking studies were performed using Schrödinger molecular modeling interface. RESULTS: 8-para-substituted xanthine 9b bearing an N7-propyl substituent displayed the highest affinity towards A2A AR (Ki = 0.75 µM) with moderate selectivity versus other AR subtypes. 7-Propargyl analogue 9d produced significantly long-lasting antiparkinsonian effects and also produced potent and selective binding affinity towards A2A AR. In silico docking studies further highlighted the crucial structural components required to develop xanthine derived potential A2A AR ligands as antiparkinsonian agents. CONCLUSION: A new series of 7-substituted 8-m/p-chloropropoxyphenylxanthines having good affinity for A2A AR and potent antiparkinsonian activity has been developed.

Adenosine Receptor Ligands: Coumarin-Chalcone Hybrids as Modulating Agents on the Activity of hARs.

Adenosine receptors (ARs) play an important role in neurological and psychiatric disorders such as Alzheimer's disease, Parkinson's disease, epilepsy and schizophrenia. The different subtypes of ARs and the knowledge on their densities and status are important for understanding the mechanisms underlying the pathogenesis of diseases and for developing new therapeutics. Looking for new scaffolds for selective AR ligands, coumarin-chalcone hybrids were synthesized (compounds 1-8) and screened in radioligand binding (hA1, hA2A and hA3) and adenylyl cyclase (hA2B) assays in order to evaluate their affinity for the four human AR subtypes (hARs). Coumarin-chalcone hybrid has been established as a new scaffold suitable for the development of potent and selective ligands for hA1 or hA3 subtypes. In general, hydroxy-substituted hybrids showed some affinity for the hA1, while the methoxy counterparts were selective for the hA3. The most potent hA1 ligand was compound 7 (Ki = 17.7 µM), whereas compound 4 was the most potent ligand for hA3 (Ki = 2.49 µM). In addition, docking studies with hA1 and hA3 homology models were established to analyze the structure-function relationships. Results showed that the different residues located on the protein binding pocket could play an important role in ligand selectivity.

Targeting G Protein-Coupled Receptors with Magnetic Carbon Nanotubes: The Case of the A3 Adenosine Receptor.

The A3 adenosine receptor (AR) is a G protein-coupled receptor (GPCR) overexpressed in the membrane of specific cancer cells. Thus, the development of nanosystems targeting this receptor could be a strategy to both treat and diagnose cancer. Iron-filled carbon nanotubes (CNTs) are an optimal platform for theranostic purposes, and the use of a magnetic field can be exploited for cancer magnetic cell sorting and thermal therapy. In this work, we have conjugated an A3 AR ligand on the surface of iron-filled CNTs with the aim of targeting cells overexpressing A3 ARs. In particular, two conjugates bearing PEG linkers of different length were designed. A docking analysis of A3 AR showed that neither CNT nor linker interferes with ligand binding to the receptor; this was confirmed by in vitro preliminary radioligand competition assays on A3 AR. Encouraged by this result, magnetic cell sorting was applied to a mixture of cells overexpressing or not the A3 AR in which our compound displayed indiscriminate binding to all cells. Despite this, it is the first time that a GPCR ligand has been anchored to a magnetic nanosystem, thus it opens the door to new applications for cancer treatment.

Bronchospasmolytic activity and adenosine receptor binding of some newer 1,3-dipropyl-8-phenyl substituted xanthine derivatives.

The aldehyde derivatives of 1,3-dipropyl xanthines as described in this paper, constitutes a new series of selective adenosine ligands displaying bronchospasmolytic activity. The effect of substitution at third- and fourth-position of 8-phenyl xanthine has also been taken into consideration. The synthesized compounds showed varying binding affinities at different adenosine receptor subtypes (A1 , A2A , A2B , and A3 ) and also good in vivo bronchospasmolytic activity against histamine aerosol-induced asthma in guinea pigs. Most of the compounds showed maximum affinity toward the A2A receptor subtype. The monosubstituted 3-aminoalkoxyl 8-phenyl xanthine with a aminodiethyl moiety (compound 12e) was found to be most potent A2A adenosine receptor ligand (Ki  = 0.036 µM) followed by disubstituted 4-aminoalkoxyl-3-methoxy-8-phenyl xanthine (Ki  = 0.050 µM) (compound 10a).

Structure-Based Optimization of Coumarin hA3 Adenosine Receptor Antagonists.

Adenosine receptors participate in many physiological functions. Molecules that may selectively interact with one of the receptors are favorable multifunctional chemical entities to treat or decelerate the evolution of different diseases. 3-Arylcoumarins have already been studied as neuroprotective agents by our group. Here, differently 8-substituted 3-arylcoumarins are complementarily studied as ligands of adenosine receptors, performing radioligand binding assays. Among the synthesized compounds, selective A3 receptor antagonists were found. 3-(4-Bromophenyl)-8-hydroxycoumarin (compound 4) displayed the highest potency and selectivity as A3 receptor antagonist (Ki = 258 nM). An analysis of its X-ray diffraction provided detailed information on its structure. Further evaluation of a selected series of compounds indicated that it is the nature and position of the substituents that determine their activity and selectivity. Theoretical modeling calculations corroborate and explain the experimental data, suggesting this novel scaffold can be involved in the generation of candidates as multitarget drugs.

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

Adenosine receptor antagonists are generally based on heterocyclic core structures presenting substituents of various volumes and chemical-physical profiles. Adenine and purine-based adenosine receptor antagonists have been reported in literature. In this work we combined various substituents in the 2, 6, and 8-positions of 9-ethylpurine to depict a structure-affinity relationship analysis at the human adenosine receptors. Compounds were rationally designed trough molecular modeling analysis and then synthesized and evaluated at radioligand binding studies at human adenosine receptors. The new compounds showed affinity for the human adenosine receptors, with some derivatives endowed with low nanomolar Ki data, in particular at the A2AAR subtype. The purine core proves to be a versatile core structure for the development of novel adenosine receptor antagonists with nanomolar affinity for these membrane proteins.

Pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidines to develop functionalized ligands to target adenosine receptors: fluorescent ligands as an example.

A series of adenosine receptor antagonists bearing a reactive linker was developed. Functionalization of these derivatives is useful to easily obtain multi-target ligands, receptor probes, drug delivery systems, and diagnostic or theranostic systems. The pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine scaffold was chosen as a pharmacophore for the adenosine receptors. It was substituted at the 5 position with reactive linkers of different lengths. Then, these compounds were used to synthesise probes for the adenosine receptors by functionalization with a fluorescent moiety. Both series of compounds were evaluated for their binding at the four adenosine receptor subtypes. Different affinity and selectivity profiles were observed towards hA1, hA2A and hA3 adenosine receptors. In particular, fluorescent compounds behave as dual hA2A/hA3 ligands. Computational studies suggested different binding modes for developed compounds at the three receptors. Both molecular docking and supervised molecular dynamics (SuMD) simulations confirmed that the preferred binding mode at the single receptor was driven by the substitution present at the 5 position. Obtained results rationalized the compounds' binding profile at the adenosine receptors and pave the way for the development of more potent conjugable and conjugated ligands targeting these membrane receptors.

7-Amino-2-aryl/hetero-aryl-5-oxo-5,8-dihydro[1,2,4]triazolo[1,5-a]pyridine-6-carbonitriles: Synthesis and adenosine receptor binding studies.

A series of novel 7-amino-5-oxo-2-substituted-aryl/hetero-aryl-5,8-dihydro[1,2,4]triazolo[1,5-a]pyridine-6-carbonitriles (4a-4t) was synthesized, characterized and evaluated for their binding affinity and selectivity towards hA1 , hA2A , hA2B and hA3 adenosine receptors (ARs). Compound 4a with a phenyl ring at 2-position of the triazolo moiety of the scaffold showed high affinity and selectivity for hA1 AR (Ki hA1  = 0.076 µM, hA2A  = 25.6 µM and hA3  > 100 µM). Introduction of various electron donating and withdrawing groups at different positions of the phenyl ring resulted in drastic reduction in affinity and selectivity towards all the ARs, except compound 4b with a 4-hydroxyphenyl group at 2-position. Interestingly, the replacement of the phenyl ring with a smaller heterocyclic thiophene ring (π excessive system) resulted in further improvement of affinity for hA1 AR of compound 4t (Ki hA1  = 0.051 µM, hA2A  = 9.01 µM and hA3  > 13.9 µM) while retaining the significant selectivity against all other AR subtypes similar to compound 4a. The encouraging results for compounds 4a and 4t indicate that substitution at 2-position of the scaffold with π-excessive systems other than thiophene may lead to even more potent and selective hA1 AR antagonists.

Synthesis, biological evaluation and molecular modelling studies of 1,3,7,8-tetrasubstituted xanthines as potent and selective A2A AR ligands with in vivo efficacy against animal model of Parkinson's disease.

In the present study, an attempt has been made to develop a new series of 1,3,7,8-tetrasubstituted xanthine based potent and selective AR ligands for the treatment of Parkinson's disease. Antagonistic interactions between dopamine and A2A adenosine receptors serve as the basis for the development of AR antagonists as potential drug candidates for PD. All the synthesized compounds have been evaluated for their affinity toward AR subtypes using in vitro radioligand binding assays. 1,3-Dipropylxanthine 7a with a methyl substituent at N-7 position represents the most potent compound of the series and displayed highest affinity (A2A, Ki = 0.108 µM), however incorporation of a propargyl group at 7-positon of the xanthine nucleus seems to be the most appropriate substitution to improve selectivity towards the A2A subtype along with reasonable potency. Antiparkinsonian activity has been evaluated using perphenazine induced catatonia in rats. Most of the synthesized xanthines significantly lowered the catatonic score as compared to control and displayed antiparkinsonian effects comparable to standard drug. All the synthesized compounds were subjected to grid-based molecular docking studies to understand the key structural requirements for the development of new molecules well-endowed with intrinsic efficacy and selectivity as adenosine receptor ligands. In silico studies carried out on newly synthesized xanthines provided further support to the pharmacological results.

Discovery of 2-aminoimidazole and 2-amino imidazolyl-thiazoles as non-xanthine human adenosine A3 receptor antagonists: SAR and molecular modeling studies.

A small-molecule combinatorial library of 24 compounds with 2-aminoimidazole and 2-aminoimidazolyl-thiazole derivatives was synthesized using a 2-chloro trityl resin. The generated compound library was tested against all the human adenosine receptors subtypes. The 2-aminoimidazole derivatives (6a-6l) showed weak to moderate affinity towards the human adenosine receptors. Further modification to 2-aminoimidazolyl-thiazole derivatives (12a-12l) resulted in an improvement of affinity at adenosine A1, A2A and A3 receptor subtypes. Compound 12b was the most potent and selective non-xanthine human adenosine A3 receptor antagonist of this series. A receptor-based modeling study was performed to explore the possible binding mode of these novel 2-aminoimidazole and 2-aminoimidazolyl-thiazole derivatives into human adenosine A1, A2A and A3 receptor subtypes.

[1,2,4]Triazolo[1,5-c]pyrimidines as adenosine receptor antagonists: Modifications at the 8 position to reach selectivity towards A3 adenosine receptor subtype.

[1,2,4]Triazolo[1,5-c]pyrimidine is a promising platform to develop adenosine receptor antagonists. Here, we tried to investigate the effect of the substituent at the 8 position of [1,2,4]triazolo[1,5-c]pyrimidine derivatives on affinity and selectivity at the human A3 adenosine receptor subtype. In particular, we have introduced both esters and amides, principally with a benzylic nature. In addition, a small series of 5-substituted [1,2,4]triazolo[1,5-c]pyrimidines was designed in order to complete the structure-activity relationship analysis. Several of these new compounds showed affinity towards human A3 adenosine receptor in the low nanomolar range, with the most potent derivative of the series bringing a 4-ethylbenzylester at the 8 position (compound 18, hA3AR Ki = 1.21 nM). Docking studies performed on the synthesized compounds inside models of human A1, A2A and A3 adenosine receptors showed similar binding modes, comparable with the typical crystallographic binding mode of the inverse agonist ZM-241,385.

New potent and selective A1 adenosine receptor antagonists as potential tools for the treatment of gastrointestinal diseases.

The synthesis of 9-alkyl substituted adenine derivatives presenting aromatic groups and cycloalkyl rings in 8- and N6-position, respectively, is reported. The compounds were tested with radioligand binding studies showing, in some cases, a low nanomolar A1 adenosine receptor affinity and a very good selectivity versus the other adenosine receptor subtypes. Functional assays at human adenosine receptors and at a mouse ileum tissue preparation clearly demonstrate the antagonist profile of these molecules, with inhibitory potency at nanomolar level. A molecular modeling study, consisting in docking analysis at the recently reported A1 adenosine receptor crystal structure, was performed for the interpretation of the obtained pharmacological results. The N6-cyclopentyl-9-methyl-8-phenyladenine (17), resulting the most active derivative of the series (Ki = 2.8 nM and IC50 = 14 nM), was also very efficacious in counteracting the effect of the agonist CCPA on mouse ileum contractility. This new compound represents a tool for the development of new agents for the treatment of intestinal diseases as constipation and postoperative ileus.

Synthesis, adenosine receptor binding and molecular modelling studies of novel thieno[2,3-d]pyrimidine derivatives.

A series of new molecules containing a thieno[2,3-d]pyrimidine scaffold was synthesized and characterized by adopting an efficient synthetic scheme. The effect of a free or substituted amino group at 2-position as well as an oxo-group, imidazole or 1,2,4-triazole ring at 4-position of the scaffold on the affinity and selectivity towards adenosine receptors (ARs) was evaluated. Compounds 17-19 with a free amino group at 2-position along with the presence of an imidazole/1,2,4-triazole ring at 4-position of the scaffold showed selective binding affinities for hA2A AR, whereas carbamoylation of the amino group at 2-position (in the presence of an oxo-group at 4-position of the scaffold) increased the affinity and selectivity of certain compounds (7-10) for hA3 AR. Molecular dynamic simulation study of one of the most active compound 8 (Ki hA1  > 30 µm, hA2A  = 0.65 µm, and hA3  = 0.124 µm) revealed the role of important amino acid residues for imparting good affinity towards hA3 and hA2A ARs. Molecular docking studies were carried out for other compounds using the crystal structure of hA2A AR and a homology model of hA3 AR to rationalize their structure-activity relationships. The molecular docking results were in agreement with the experimental binding affinity data of ARs.

Structure-Based Design, Synthesis, and In Vivo Antinociceptive Effects of Selective A1 Adenosine Receptor Agonists.

Our previous work discovered that combining the appropriate 5'- and N6-substitution in adenosine derivatives leads to the highly selective human A1 adenosine receptor (hA1AR) agonists or highly potent dual hA1AR agonists and hA3AR antagonists. In order to explore novel dual adenosine receptor ligands, a series of N6-substituted-5'-pyrazolyl-adenosine and 2-chloro-adenosine derivatives were synthesized and assayed in vitro at all ARs. The N6-(±)-endo-norbornyl derivative 12 was the most potent and selective at A1AR and effective as an analgesic in formalin test in mice, but none of the 5'-pyrazolyl series compounds showed a dual behavior at hA1 and hA3AR. Molecular modeling studies rationalized the structure-activity relationships and the selectivity profiles of the new series of A1AR agonists. Interestingly, an unexpected inverted binding mode of the N6-tetrahydrofuranyl derivative 14 was hypothesized to explain its low affinity at A1AR.

Coumarins and adenosine receptors: New perceptions in structure-affinity relationships.

Adenosine receptor (AR) subtypes are involved in several physiological and pharmacological processes. Ligands that are able to selectively modulate one receptor subtype can delay or slow down the progression of diverse diseases. In this context, our research group focused its investigation into the discovery and development of novel, potent and selective AR ligands based on coumarin scaffold. Therefore, a series of 3-phenylcarboxamidocoumarins were synthesized and their affinity for the human AR subtypes was screened by radioligand binding assays for A1 , A2A and A3 receptors and for A2B by adenylyl cyclase assay. Compound 26 was found to be the most remarkable, with a hA1 /hA3 and hA2A /hA3 selectivity of 42, for the A3 AR (Ki  = 2.4 µm). Receptor-driven molecular modelling studies have provided valuable information on the binding/selectivity data of compound 26 and for the following optimization process. Moreover, compound 26 presents drug-like properties according to the general guidelines linked to the concept.