3-Aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-ones: a new class of selective A1 adenosine receptor antagonists

Structure-based molecular modeling in SAR analysis and lead optimization

In silico methods like molecular docking and pharmacophore modeling are established strategies in lead identification. Their successful application for finding new active molecules for a target is reported by a plethora of studies. However, once a potential lead is identified, lead optimization, with the focus on improving potency, selectivity, or pharmacokinetic parameters of a parent compound, is a much more complex task. Even though in silico molecular modeling methods could contribute a lot of time and cost-saving by rationally filtering synthetic optimization options, they are employed less widely in this stage of research. In this review, we highlight studies that have successfully used computer-aided SAR analysis in lead optimization and want to showcase sound methodology and easily accessible in silico tools for this purpose.

Exploiting the Indole Scaffold to Design Compounds Binding to Different Pharmacological Targets

Several indole derivatives have been disclosed by our research groups that have been collaborating for nearly 25 years. The results of our investigations led to a variety of molecules binding selectively to different pharmacological targets, specifically the type A γ-aminobutyric acid (GABA_A) chloride channel, the translocator protein (TSPO), the murine double minute 2 (MDM2) protein, the A_2B adenosine receptor (A_2B AR) and the Kelch-like ECH-associated protein 1 (Keap1). Herein, we describe how these works were conceived and carried out thanks to the versatility of indole nucleus to be exploited in the design and synthesis of drug-like molecules.

Novel fluorescent triazinobenzimidazole derivatives as probes for labelling human A1 and A2B adenosine receptor subtypes

The expression levels and the subcellular localization of adenosine receptors (ARs) are affected in several pathological conditions as a consequence of changes in adenosine release and metabolism. In this respect, labelled probes able to monitor the AR expression could be a useful tool to investigate different pathological conditions. Herein, novel ligands for ARs, bearing the fluorescent 7-nitrobenzofurazan (NBD) group linked to the N¹ (1,2) or N¹⁰ (3,4) nitrogen of a triazinobenzimidazole scaffold, were synthesized. The compounds were biologically evaluated as fluorescent probes for labelling A₁ and A_2B AR subtypes in bone marrow-derived mesenchymal stem cells (BM-MSCs) that express both receptor subtypes. The binding affinity of the synthetized compounds towards the different AR subtypes was determined. The probe 3 revealed a higher affinity to A₁ and A_2B ARs, showing interesting spectroscopic properties, and it was selected as the most suitable candidate to label both AR subtypes in undifferentiated MSCs. Fluorescence confocal microscopy showed that compound 3 significantly labelled ARs on cell membranes and the fluorescence signal was decreased by the cell pre-incubation with the A₁ AR and A_2B AR selective agonists, R-PIA and BAY 60-6583, respectively, thus confirming the specificity of the obtained signal. In conclusion, compound 3 could represent a useful tool to investigate the expression pattern of both A₁ and A_2B ARs in different pathological and physiological processes. Furthermore, these results provide an important basis for the design of new and more selective derivatives able to monitor the expression and localization of each different ARs in several tissues and living cells.

Toward PET imaging of A2B adenosine receptors: a carbon-11 labeled triazinobenzimidazole tracer: Synthesis and imaging of a new A2B PET tracer

INTRODUCTION

A2B adenosine receptors (ARs) are commonly defined as "danger" sensors because they are triggered during cell injury when the endogenous molecule, adenosine, increases rapidly. These receptors, together with the other receptor subtypes (A1, A2A and A3), exert a wide variety of immunomodulating and (cyto)protective effects, thus representing a pivotal therapeutic target for different pathologies including diabetes, tumors, cardiovascular diseases, pulmonary fibrosis and others. The limited availability of potent and selective ligands for A2B ARs has prevented this receptor to emerge both as therapeutic and diagnostic target.

METHODS

Recently, a new class of potent A2B ARs antagonists was developed featuring the triazinobenzimidazole scaffold. Starting from this chemotype, we synthesized a new radiotracer, [(11)C]-4 (1-[(11)C]methyl-3-phenyl triazino[4,3-a]benzimidazol-4(1H)-one), and investigated the pharmacokinetics of this compound in vivo to define its potential use in the imaging of A2B AR with positron emission tomography.

RESULTS

[(11)C]-4 showed a very high chemical and blood stability. Results of in vivo and ex vivo experiments underlined the ability of this molecule to bind the A2B AR and correlated with the A2B AR protein and gene expression data.

CONCLUSIONS

Although further studies are necessary, these data suggest that [(11)C]-4 may represent a good lead compound for the development of novel selective and potent A2B AR radiotracers, and a new option for the clinical investigation of several pathophysiological processes and chronic diseases.

Modulation of A2B adenosine receptor by 1-Benzyl-3-ketoindole derivatives

We have disclosed a series of 1-benzyl-3-ketoindole derivatives acting as either positive or negative modulators of the human A(2B) adenosine receptor (A(2B) AR) depending on small differences in their side chain. The new compounds were designed taking into account structural similarities between AR antagonists and ligands of the GABA(A)/benzodiazepine receptor. All compounds resulted totally inactive at A(2A) and A₃ ARs and showed small (8a,b) or none (7a,b, 8c and 9a,b) affinity for A₁ AR. When tested on A(2B) AR-transfected CHO cells, 7a,b and 8a acted as positive modulators, whereas 8b,c and 9a,b acted as negative modulators, enhancing or weakening the NECA-induced increase of cAMP levels, respectively. Compounds 7-9 might be regarded as useful biological and pharmacological tools to explore the therapeutic potential of A(2B) AR modulators, while their 3-ketoindole scaffold might be taken as a reference to design new analogs.

3-aryl-[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one: a novel template for the design of highly selective A₂B adenosine receptor antagonists

In an effort to identify novel ligands possessing high affinity and selectivity for the A(2B) AR subtype, we further investigated the class of 3-aryl[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-ones V, previously disclosed by us as selective A(1) AR antagonists. Preliminary assays on a number of triazinobenzimidazoles derived from our "in-house" collection revealed that all the derivatives selected showed significant affinity at A(2B) AR, no affinity at A(3) AR, and various degrees of selectivity toward A(1) and A(2A) ARs. Investigation of a new series featuring modified substituents at the 10-position (4'-chlorophenyl or phenylethyl groups), and a chlorine atom at the 7-position (X) of the triazinobenzimidazole nucleus, yielded highly potent and selective A(2B) AR antagonists. The presence of a pendant 3-phenyl ring appears to hamper the interaction with A(2A) AR, conferring high A(2B)/A(2A) AR selectivity. Derivative 13 (X = Cl, R = C(6)H(5)) is the most potent and selective compound, with an IC(50) of 3.10 nM at A(2B) AR and no affinity at A(1), A(2A), and A(3) ARs.

3-(Fur-2-yl)-10-(2-phenylethyl)-[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one, a novel adenosine receptor antagonist with A(2A)-mediated neuroprotective effects

In this study, compound FTBI (3-(2-furyl)-10-(2-phenylethyl)[1,2,4]triazino[4,3-a]benzimidazol-4(10H)-one) was selected from a small library of triazinobenzimidazole derivatives as a potent A(2A) adenosine receptor (AR) antagonist and tested for its neuroprotective effects against two different kinds of dopaminergic neurotoxins, 1-methyl-4-phenylpyridinium (MPP+) and methamphetamine (METH), in rat PC12 and in human neuroblastoma SH-SY5Y cell lines. FTBI, in a concentration range corresponding to its affinity for A(2A) AR subtype, significantly increased the number of viable PC12 cells after their exposure to METH and, to a similar extent, to MPP+, as demonstrated in both trypan blue exclusion assay and in cytological staining. These neuroprotective effects were also observed with a classical A(2A) AR antagonist, ZM241385, and appeared to be completely counteracted by the AR agonist, NECA, supporting A(2A) ARs are directly involved in FTBI-mediated effects. Similarly, in human SH-SY5Y cells, FTBI was able to prevent cell toxicity induced by MPP+ and METH, showing that this A(2A) AR antagonist has a neuroprotective effect independently by the specific cell model. Altogether these results demonstrate that the A(2A) AR blockade mediates cell protection against neurotoxicity induced by dopaminergic neurotoxins in dopamine containing cells, supporting the potential use of A(2A) AR antagonists in dopaminergic degenerative diseases including Parkinson's disease.

Search for new antagonist ligands for adenosine receptors from QSAR point of view. How close are we?

In view of the large libraries of nucleoside analogues that are now being handled in organic synthesis, the identification of drug biological activity is advisable prior to synthesis and this can be achieved by employing predictive biological property methods. In this sense, Quantitative Structure-Activity Relationships (QSAR) or docking approaches have emerged as promising tools. Although a large number of in silico approaches have been described in the literature for the prediction of different biological activities, the use of QSAR applications to develop adenosine receptor (AR) antagonists is not common as for the case of the antibiotics and anticancer compounds for instance. The intention of this review is to summarize the present knowledge concerning computational predictions of new molecules as adenosine receptor antagonists.

N9-benzyl-substituted 1,3-dimethyl- and 1,3-dipropyl-pyrimido[2,1-f]purinediones: synthesis and structure-activity relationships at adenosine A1 and A2A receptors

Synthesis and physicochemical properties of N-benzyl pyrimido[2,1-f]purinediones are described. These derivatives were synthesized by the cyclization of 7-chloropropylo-8-bromo-1,3-dimethyl- or 1,3-dipropyl xanthine derivatives with corresponding (un)substituted benzylamines. Dipropyl derivatives were obtained under microwave irradiation conditions either. The obtained compounds (1-20) were evaluated for their affinity to adenosine A1 and A2A receptors, selected compounds were additionally investigated for affinity to the A3 receptor subtype. The results of the radioligand binding assays to A1 and A2A adenosine receptors showed that most of the 1,3-dimethyl-9-benzylpyrimidopurinediones exhibited selective affinity to A2A receptors at micromolar or submicromolar concentrations (for example, derivative 9 with o-methoxy substituent displayed a Ki value of 0.699 microM at rat A2A receptor with more than 36-fold selectivity). Contrary to previously described arylpyrimido[2,1-f]purinediones dipropyl derivatives (compounds 15-20) showed affinity to both kinds of receptors increased, however A1 affinity increased to a larger extent, with the result that A2A selectivity was abolished. The best adenosine A1 receptor ligand was m-chlorobenzyl derivative 18 (Ki=0.089 microM and 5-fold A1 selectivity). Structure-activity relationships were discussed with the analysis of lipophilic and spatial properties of the investigated compounds. Pharmacophore model of adenosine A1 receptor antagonist was adopted for this purpose.

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.

Synthesis and pharmacology of pyrido[2,3-d]pyrimidinediones bearing polar substituents as adenosine receptor antagonists

Amino-substituted pyrido[2,3-d]pyrimidinediones have previously been found to bind to adenosine A1 and A2A receptors in micromolar concentrations. The present study was aimed at studying the structure-activity relationships of this class of compounds in more detail. Most of the investigated compounds were provided with polar substituents, such as ethoxycarbonyl groups and basic amino functions, in order to improve their water-solubility. The compounds were synthesized starting from 6-amino-1,3-dimethyluracil via different reaction sequences involving (cyano)acetylation, Vilsmeier formylation, or reaction with diethyl ethoxymethylenemalonate (EMME). The most potent and selective compound of the present series was 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-5-(2-naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (11c) with a Ki value of 5 nM at rat and 25 nM at human A1 receptors. The compound was more than 60-fold selective versus A3 and more than 300-fold selective versus A2A receptors. It showed an over 300-fold improvement with respect to the lead compound. In GTPgammaS binding studies at membranes of Chinese hamster ovary cells recombinantly expressing the human adenosine A1 receptor, 11c behaved as an antagonist with inverse agonistic activity. A regioisomer of 11c, 6-carbethoxy-1,2,3,4-tetrahydro-1,3-dimethyl-7-(2- naphthylmethyl)aminopyrido[2,3-d]pyrimidine-2,4-dione (7a) in which the 2-naphthylmethylamino substituent at position 5 of 11c was moved to the 7-position, was a relatively potent (Ki=226 nM) and selective (>20-fold) A3 ligand. In the series of compounds lacking an electron-withdrawing ethoxycarbonyl or cyano substituent in the 6-position, compounds with high affinity for adenosine A2A receptors were identified, such as 1,2,3,4-tetrahydro-1,3-dimethyl-5-(1-naphthyl)aminopyrido[2,3-d]pyrimidine-2,4-dione 16b (Ki human A2A=81.3 nM, Ki human A1=153 nM, and Ki human A3>10,000 nM).

New pyrazolo[3,4-b]pyridones as selective A(1) adenosine receptor antagonists: synthesis, biological evaluation and molecular modelling studies

A series of ethyl 4-amino-1-(2-chloro-2-phenylethyl)-6-oxo-6,7-dihydro-1H-pyrazolo[3,4-b]pyridine-5-carboxylates () has been synthesized as potential A(1) adenosine receptor (A(1) AR) ligands. Binding affinities of the new compounds were determined for adenosine A(1), A(2A) and A(3) receptors. Compounds and showed good affinity (K(i)= 299 nM and 517 nM, respectively) and selectivity towards A(1) AR, whereas showed good affinity for A(2A) AR (K(i)= 290 nM), higher than towards A(1) AR (K(i)= 1000 nM). The only arylamino derivative of the series displayed high affinity (K(i)= 4.6 nM) and selectivity for A(3) AR. Molecular modelling and 3D-QSAR (CoMFA) studies carried out on the most active compounds gave further support to the pharmacological results.

Design, synthesis, and biological evaluation of novel 4-alkylamino-1-hydroxymethylimidazo[1,2-a]quinoxalines as adenosine A1 receptor antagonists

A series of 4-alkylamino-1-hydroxymethylimidazo[1,2-a]quinoxalines have been synthesized and evaluated for their adenosine A(1) receptor inhibitory activity in the radioligand binding assays. The compounds were tested for the inhibition percent (IP) and the affinity toward A(1)AR (K(i)) that IP were more than 90% in the nanomolar range. 4-Cyclopentylamino-7,8-dichloro-1-hydroxymethylimidazo[1,2-a]quinoxaline 18 is the most potent compound in this series, having K(i)=7nM, which is remarkably higher than that of IRFI-165 (K(i)=48). 1-Hydroxymethyl groups of the tricyclic heteroarmatic compounds displayed the potent affinities toward A(1)AR.

Synthesis of 1-(2-chloro-2-phenylethyl)-6-methylthio-1H-pyrazolo[3,4-d]pyrimidines 4-amino substituted and their biological evaluation

A new series of 4-amino-6-methylthio-1H-pyrazolo[3,4-d]pyrimidines (2a-m) bearing the 2-chloro-2-phenylethyl chain at the N1 position, has been synthesized. The affinity of these compounds for A1 adenosine receptor (A1AR) was measured. The compounds showed poor affinity. A more interesting result was obtained by 2a, 2d, 2g, which demonstrated inhibitory activity on cell proliferation of the A-431 cell line stimulated by epithelial growth factor (EGF) and on EGF receptor tyrosine kinase (EGFR-TK) phosphorylation.

1,2,4-Triazolo[1,5-a]quinoxaline derivatives: synthesis and biological evaluation as adenosine receptor antagonists

Since most of the reported adenosine receptor antagonists are 2-(hetero)aryl-substituted tricyclic heteroaromatic derivatives, in the present study we report the synthesis and the biological evaluation of a new set of 4-amino-1,2,4-triazolo[1,5-a]quinoxalines containing at position-2 an ethyl carboxylate group or a hydrogen atom. The structure-activity relationships on these compounds were in accordance with those of a previously reported series of analogous size and shape, thus suggesting a similar A(1)-binding mode. In particular, the binding data indicate that alkylation of the 4-amino group of these derivatives lead to potent A(1)-receptor antagonists. Moreover, as new results, this study has pointed out that the ethyl 2-carboxylate group can advantageously replace the 2-(hetero)aryl ring of previously reported triazoloquinoxaline derivatives, affording an ameliorated interaction with the A(1)-receptor subtype.

Novel approaches for modeling of the A1adenosine receptor and its agonist binding site

The present work describes the building of a human A(1) adenosine receptor (hA(1)AR) model, based on the X-ray crystal structure of bovine rhodopsin, and its use as a basis for the investigation of some important structural characteristics of the receptor. One of the issues investigated was the protonation position of two histidine residues known to influence ligand binding, with protonation of His251 (6.52) in epsilon position and His278 (7.43) in delta position showing the best agreement with experimental evidence. The model was also used to study the position and structural role of water molecules present in the helical bundle. Finally, the binding site location and the ligand docking were investigated using an objective strategy. A suitable site for the binding of the ribose moiety of adenosine was first postulated and further confirmed by means of a novel chemometric strategy based on GRIND descriptors. Using this position as an anchor point, the binding of adenosine was studied by docking and molecular dynamics simulations obtaining two putative binding positions in good agreement with experimental data.

Synthesis and biological data of 4-amino-1-(2-chloro-2-phenylethyl)-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid ethyl esters, a new series of A1-adenosine receptor (A1AR) ligands

The synthesis of a new family of A1-adenosine receptor (A1AR) ligands 3a-n has been performed in a straightforward way. Affinity data at A1AR, A2AAR and A3AR in bovine membranes show that these new compounds bind the A1AR in a selective way over A2AAR and A3AR and one of them (3j) presents a very high affinity, probably due to the phenethylamine substituent at C-4.