7-Amino-2-phenylpyrazolo[4,3-d]pyrimidine derivatives: Structural investigations at the 5-position to target human A1 and A2A adenosine receptors. Molecular modeling and pharmacological studies

Adenosine receptor antagonists: Recent advances and therapeutic perspective

Adenosine is an endogenous purine-based nucleoside expressed nearly in all body tissues. It regulates various body functions by activating four G-protein coupled receptors, A₁, A_2A, A_2B, and A₃. These receptors are widely acknowledged as drug targets for treating different neurological, metabolic, and inflammatory diseases. Although numerous adenosine receptor inhibitors have been developed worldwide, achieving target selectivity is still a big hurdle in drug development. However, the identification of specific radioligands-based affinity assay, fluorescent ligands, and MS-based ligand assay have contributed to the development of selective and potent adenosine ligands. In recent years various small heterocyclic-based molecules have shown some promising results. Istradefylline has been approved for treating Parkinson's in Japan, while preladenant, tozadenant, CVT-6883, MRS-1523, and many more are under different phases of clinical development. The present review is focused on the quest to develop potent and selective adenosine inhibitors from 2013 to early 2021 by various research groups. The review also highlights their biological activity, selectivity, structure-activity relationship, molecular docking, and mechanistic studies. A special emphsesis on drug designing strategies has been also given the manuscript. The comprehensive compilation of research work carried out in the field will provide inevitable scope for designing and developing novel adenosine inhibitors with improved selectivity and efficacy.

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

An assessment study of known pyrazolopyrimidines: Chemical methodology and cellular activity

Heterocyclic compounds with nitrogen atom play a key role in the normal life cycle of a cell. Pyrazolopyrimidine is a privileged class of nitrogen containing fused heterocyclic compound contributing to a major portion of all lead molecules in medicinal chemistry. The thumbprint of pyrazolopyrimidine as a pharmacophore is always noticeable due to its analogy with the adenine base in DNA. Pyrazolopyrimidines are divided into five types [I, II, III, IV, V] based on the mechanism of action on the specific target conferring a wide scope of research which has accelerated the interest of researchers to investigate its biological profile. In 1956, the anti-cancer activity of pyrazolopyrimidine was evaluated for the first time with appreciable results. Since then, medicinal chemists centered their work on various methods of synthesis and evaluating the biological profile of pyrazolopyrimidine isomers. This report consists of novel methodologies followed to synthesize pyrazolopyrimidine isomers along with a note on their biological significance. To the best of our knowledge, this review article will be first of its kind to encompass different synthetic procedures along with anti-cancer, kinase inhibition, phosphodiesterase inhibition and receptor blocking activity of pyrazolopyrimidine moieties. IC₅₀ values of potent compounds are added wherever necessary to understand the suitability of pyrazolopyrimidine skeletons for a specific biological activity.

Modifications on the Amino-3,5-dicyanopyridine Core To Obtain Multifaceted Adenosine Receptor Ligands with Antineuropathic Activity

A new series of amino-3,5-dicyanopyridines (1-31) was synthesized and biologically evaluated in order to further investigate the potential of this scaffold to obtain adenosine receptor (AR) ligands. In general, the modifications performed have led to compounds having high to good human (h) A₁AR affinity and an inverse agonist profile. While most of the compounds are hA₁AR-selective, some derivatives behave as mixed hA₁AR inverse agonists/A_2A and A_2B AR antagonists. The latter compounds (9-12) showed that they reduce oxaliplatin-induced neuropathic pain by a mechanism involving the alpha7 subtype of nAchRs, similar to the nonselective AR antagonist caffeine, taken as the reference compound. Along with the pharmacological evaluation, chemical stability of methyl 3-(((6-amino-3,5-dicyano-4-(furan-2-yl)pyridin-2-yl)sulfanyl)methyl)benzoate 10 was assessed in plasma matrices (rat and human), and molecular modeling studies were carried out to better rationalize the available structure-activity relationships.

Novel 8-amino-1,2,4-triazolo[4,3-a]pyrazin-3-one derivatives as potent human adenosine A1 and A2A receptor antagonists. Evaluation of their protective effect against β-amyloid-induced neurotoxicity in SH-SY5Y cells

In this work, an enlarged series of 1,2,4-triazolo[4,3-a]pyrazin-3-ones was designed to target the human (h) A_2A adenosine receptor (AR) or both hA₁ and hA_2A ARs. The novel 8-amino-1,2,4-triazolopyrazin-3-one derivatives 1-25 featured a phenyl or a benzyl pendant at position 2 while different aryl/heteroaryl substituents were placed at position 6. Two compounds (8 and 10) endowed with high affinity (K_i = 7.2 and 10.6 nM) and a complete selectivity for the hA_2A AR were identified. Moreover, several derivatives possessed nanomolar affinity for both hA₁ and hA_2A ARs (both K_i < 20 nM) and different degrees of selectivity versus the hA₃ AR. Two selected compounds (10 and 25) demonstrated ability in preventing β-amyloid peptide (25-35)-induced neurotoxicity in SH-SY5Y cells. Results of docking studies at the hA_2A and hA₁ AR crystal structures helped us to rationalize the observed affinity data and to highlight that the steric hindrance of the substituents at the 2- and 6-position of the bicyclic core affects the binding mode in the receptor cavity.

An appraisal on synthetic and pharmaceutical perspectives of pyrazolo[4,3-d]pyrimidine scaffold

Pyrazolo[4,3-d]pyrimidine, a fused heterocycle bearing pyrazole and pyrimidine portions has gained a significant attention in the field of bioorganic and medicinal chemistry. Pyrazolo[4,3-d]pyrimidine derivatives have demonstrated numerous pharmacological activities particularly, anti-cancer, anti-infectious, phosphodiesterase inhibitors, adenosine antagonists and cytokinin antagonists etc. This review extensively unveils the synthetic and pharmacological diversity with special emphasis on structural variations around pyrazolo[4,3-d]pyrimidine scaffold. This endeavour has thus uncovered the medicinal worthiness of pyrazolo[4,3-d]pyrimidine framework. To the best of our knowledge this review is the first compilation on synthetic, medicinal and structure activity relationship (SAR) aspects of pyrazolo[4,3-d]pyrimidines since 1956.

The role of 5-arylalkylamino- and 5-piperazino- moieties on the 7-aminopyrazolo[4,3-d]pyrimidine core in affecting adenosine A1 and A2A receptor affinity and selectivity profiles

New 7-amino-2-phenylpyrazolo[4,3-d]pyrimidine derivatives, substituted at the 5-position with aryl(alkyl)amino- and 4-substituted-piperazin-1-yl- moieties, were synthesized with the aim of targeting human (h) adenosine A₁ and/or A_2A receptor subtypes. On the whole, the novel derivatives 1–24 shared scarce or no affinities for the off-target hA_2B and hA₃ ARs. The 5-(4-hydroxyphenethylamino)- derivative 12 showed both good affinity (K_i = 150 nM) and the best selectivity for the hA_2A AR while the 5-benzylamino-substituted 5 displayed the best combined hA_2A (K_i = 123 nM) and A₁ AR affinity (K_i = 25 nM). The 5-phenethylamino moiety (compound 6) achieved nanomolar affinity (K_i = 11 nM) and good selectivity for the hA₁ AR. The 5-(N⁴-substituted-piperazin-1-yl) derivatives 15–24 bind the hA₁ AR subtype with affinities falling in the high nanomolar range. A structure-based molecular modeling study was conducted to rationalize the experimental binding data from a molecular point of view using both molecular docking studies and Interaction Energy Fingerprints (IEFs) analysis.

Design, Synthesis, and Pharmacological Characterization of 2-(2-Furanyl)thiazolo[5,4-d]pyrimidine-5,7-diamine Derivatives: New Highly Potent A2A Adenosine Receptor Inverse Agonists with Antinociceptive Activity

In this study, we describe the design and synthesis of new N⁵-substituted-2-(2-furanyl) thiazolo[5,4-d]pyrimidine-5,7-diamines (2-18) and their pharmacological characterization as A_2A adenosine receptor (AR) antagonists by using in vitro and in vivo assays. In competition binding experiments two derivatives (13 and 14) emerged as outstanding ligands showing two different affinity values (KH and KL) for the hA_2A receptor with the high affinity KH value in the femtomolar range. The in vitro functional activity assays, performed by using cyclic AMP experiments, assessed that they behave as potent inverse agonists at the hA_2A AR. Compounds 13 and 14 were evaluated for their antinociceptive activity in acute experimental models of pain showing an effect equal to or greater than that of morphine. Overall, these novel inverse agonists might represent potential drug candidates for an alternative approach to the management of pain.

Imidazo[1,2-a]pyrazin-8-amine core for the design of new adenosine receptor antagonists: Structural exploration to target the A3 and A2A subtypes

The imidazo[1,2-a]pyrazine ring system has been chosen as a new decorable core skeleton for the design of novel adenosine receptor (AR) antagonists targeting either the human (h) A₃ or the hA_2A receptor subtype. The N⁸-(hetero)arylcarboxyamido substituted compounds 4-14 and 21-30, bearing a 6-phenyl moiety or not, respectively, show good hA₃ receptor affinity and selectivity versus the other ARs. In contrast, the 8-amino-6-(hetero)aryl substituted derivatives designed for targeting the hA_2A receptor subtype (compounds 31-38) and also the 6-phenyl analogues 18-20 do not bind the hA_2A AR, or show hA₁ or balanced hA₁/hA_2A AR affinity in the micromolar range. Molecular docking of the new hA₃ antagonists was carried out to depict their hypothetical binding mode to our refined model of the hA₃ receptor. Some derivatives were evaluated for their fluorescent potentiality and showed some fluorescent emission properties. One of the most active hA₃ antagonists herein reported, i.e. the 2,6-diphenyl-8-(3-pyridoylamino)imidazo[1,2-a]pyrazine 29, tested in a rat model of cerebral ischemia, delayed the occurrence of anoxic depolarization caused by oxygen and glucose deprivation in the hippocampus and allowed disrupted synaptic activity to recover.

Exploring the 2- and 5-positions of the pyrazolo[4,3-d]pyrimidin-7-amino scaffold to target human A1 and A2A adenosine receptors

A new series of 7-aminopyrazolo[4,3-d]pyrimidine derivatives (1-31) were synthesized to evaluate some structural modifications at the 2- and 5-positions aimed at shifting affinity towards the human (h) A2A adenosine receptor (AR) or both hA2A and hA1 ARs. The most active compounds were those featured by a 2-furyl or 5-methylfuran-2-yl moiety at position 5, combined with a benzyl or a substituted-benzyl group at position 2. Several of these derivatives (22-31) displayed nanomolar affinity for the hA2A AR (Ki=3.62-57nM) and slightly lower for the hA1 ARs, thus showing different degrees (3-22 fold) of hA2A versus hA1 selectivity. In particular, the 2-(2-methoxybenzyl)-5-(5-methylfuran-2-yl) derivative 25 possessed the highest hA2A and hA1 AR affinities (Ki=3.62nM and 18nM, respectively) and behaved as potent antagonist at both these receptors (cAMP assays). Its 2-(2-hydroxybenzyl) analog 26 also showed a high affinity for the hA2A AR (Ki=5.26nM) and was 22-fold selective versus the hA1 subtype. Molecular docking investigations performed at the hA2A AR crystal structure and at a homology model of the hA1 AR allowed us to represent the hypothetical binding mode of our derivatives and to rationalize the observed SARs.

Structural refinement of pyrazolo[4,3-d]pyrimidine derivatives to obtain highly potent and selective antagonists for the human A3 adenosine receptor

In previous research, we identified some 7-oxo- and 7-acylamino-substituted pyrazolo[4,3-d]pyrimidine derivatives as potent and selective human (h) A3 adenosine receptor (AR) antagonists. Herein we report on the structural refinement of this class of antagonists aimed at achieving improved receptor-ligand recognition. Hence, substituents with different steric bulk, flexibility and lipophilicity (Me, Ar, heteroaryl, CH2Ph) were introduced at the 5- and 2-positions of the bicyclic scaffold of both the 7-oxo and 7-amino derivatives, and acyl residues were appended on the 7-amino group of the latter. All the 2-phenylpyrazolo[4,3-d]pyrimidin-7-amines and 7-acylamines bearing a 4-methoxyphenyl- or a 2-thienyl group at the 5-position showed high hA3 affinity and selectivity. In particular, the 2-phenyl-5-(2-thienyl)-pyrazolo[4,3-d]pyrimidin-7-(4-methoxybenzoyl)amine 25 (Ki = 0.027 nM) is one of the most potent and selective hA3 antagonists reported so far. By using an in silico receptor-driven approach the obtained binding data were rationalized and the molecular bases of the observed hA3 AR affinities were critically described.

Exploring the 7-oxo-thiazolo[5,4-d]pyrimidine core for the design of new human adenosine A3 receptor antagonists. Synthesis, molecular modeling studies and pharmacological evaluation

A new series of 5-methyl-thiazolo[5,4-d]pyrimidine-7-ones bearing different substituents at position 2 (aryl, heteroaryl and arylamino groups) was synthesized and evaluated in radioligand binding assays to determine their affinities at the human (h) A1, A2A, and A3 adenosine receptors (ARs). Efficacy at the hA(2B) and antagonism of selected ligands at the hA3 were also assessed through cAMP experiments. Some of the new derivatives exhibited good to high hA3AR affinity and selectivity versus all the other AR subtypes. Compound 2-(4-chlorophenyl)-5-methyl-thiazolo[5,4-d]pyrimidine-7-one 4 was found to be the most potent and selective ligand of the series (K(I) hA3 = 18 nM). Molecular docking studies of the reported derivatives were carried out to depict their hypothetical binding mode in our hA3 receptor model.

History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A1 , A2A , A2B , and A3 adenosine receptors (ARs). At the present time, the role of A2A ARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A2A AR signal transduction pathways, together with an analysis of the structure-activity relationships of A2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A1 /A2A ARs and monoamine oxidase-B (MAO-B), both closely implicated in the pathogenesis of PD.