Synthesis and anti-renal fibrosis activity of conformationally locked truncated 2-hexynyl-N(6)-substituted-(N)-methanocarba-nucleosides as A3 adenosine receptor antagonists and partial agonists

Synthesis and biological evaluation of sugar-modified truncated carbanucleosides as A2A and A3 adenosine receptor ligands to explore conformational effect to the receptors

This study investigated the impact of conformation on the binding affinity of carbanucleosides to A2A and A3 adenosine receptors (ARs). A series of nucleosides, including saturated, unsaturated, North (N)-methano, and South (S)-methanocarbanucleosides was prepared, and their binding affinities to A2AAR and A3AR were assessed. Biological evaluations revealed that all synthesized (S)-methanocarbanucleosides had negligible binding to both receptors, and most (N)-methanocarbanucleosides exhibited high binding affinities. Molecular docking analysis showed that the (N)-methanocarbanucleoside 6a exhibited favorable interactions and minimal steric clashes in both A2AAR and A3AR. Conversely, the (S)-methanocarbanucleoside 7a appears to encounter significant steric clashes, which impeded its binding to A2AAR. Furthermore, when adopting the South conformation 7a was unable to bind to A3AR. Expanding upon the (N)-methanocarba moiety, various C8-aromatic groups were introduced to convert A2AAR agonists into antagonists and these modified compounds also exhibited strong binding affinity. These results suggest that the North conformation is favored by both A2AAR and A3AR, and that (N)-methanocarbanucleosides can serve as versatile structural moieties for dual targeting of A2AAR and A3AR. These findings offer promising avenues for the development of dual ligands for therapeutic applications in obesity and immunotherapy.

Structure-Activity Relationship of Truncated 2,8-Disubstituted-Adenosine Derivatives as Dual A2A/A3 Adenosine Receptor Antagonists and Their Cancer Immunotherapeutic Activity

Based on hA2AAR structures, a hydrophobic C8-heteroaromatic ring in 5'-truncated adenosine analogues occupies the subpocket tightly, converting hA2AAR agonists into antagonists while maintaining affinity toward hA3AR. The final compounds of 2,8-disubstituted-N6-substituted 4'-thionucleosides, or 4'-oxo, were synthesized from d-mannose and d-erythrono-1,4-lactone, respectively, using a Pd-catalyst-controlled regioselective cross-coupling reaction. All tested compounds completely antagonized hA2AAR, including 5d with the highest affinity (Ki,A2A = 7.7 ± 0.5 nM). The hA2AAR-5d X-ray structure revealed that C8-heteroaromatic rings prevented receptor activation-associated conformational changes. However, the C8-substituted compounds still antagonized hA3AR. Structural SAR features and docking studies supported different binding modes at A2AAR and A3AR, elucidating pharmacophores for receptor activation and selectivity. Favorable pharmacokinetics were demonstrated, in which 5d displayed high oral absorption, moderate half-life, and bioavailability. Also, 5d significantly improved the antitumor effect of anti-PD-L1 in vivo. Overall, this study suggests that the novel dual A2AAR/A3AR nucleoside antagonists would be promising drug candidates for immune-oncology.

Structure–Activity Relationships of Truncated 1′-Homologated Carbaadenosine Derivatives as New PPARγ/δ Ligands: A Study on Sugar Puckering Affecting Binding to PPARs

Peroxisome proliferator-activated receptors (PPARs) are associated with the regulation of metabolic homeostasis. Based on a previous report that 1'-homologated 4'-thionucleoside acts as a dual PPARγ/δ modulator, carbocyclic nucleosides 2-5 with various sugar conformations were synthesized to determine whether sugar puckering affects binding to PPARs. (S)-conformer 2 was synthesized using Charette asymmetric cyclopropanation, whereas (N)-conformer 3 was synthesized using stereoselective Simmons-Smith cyclopropanation. All synthesized nucleosides did not exhibit binding affinity to PPARα but exhibited significant binding affinities to PPARγ/δ. The binding affinity of final nucleosides to PPARγ did not differ significantly based on their conformation, but their affinity to PPARδ depended greatly on their conformation, correlated with adiponectin production. (N)-conformer 3h was discovered to be the most potent PPARδ antagonist with good adiponectin production, which exhibited the most effective activity in inhibiting the mRNA levels of LPS-induced IL-1β expression in RAW 264.7 macrophages, implicating its anti-inflammatory activity.

Synthesis, antitumor and antibacterial activities of cordycepin derivatives

Twelve novel cordycepin derivatives were designed and synthesized with modification at positions of 2', 5'-hydroxyl and N⁶ amino groups of cordycepin. The results showed that the inhibitory activities of 3, 4b, 6c and 6d on A549 were comparable to the positive control gefitinib, and the inhibitory activity of 6a on A549 was better than that of gefitinib. Also, the inhibitory activities of twelve cordycepin derivatives against E. coli 1924, S. aureus 4220 and S. mutans 3289 were studied. Among them, 4b showed certain inhibitory on S. mutans 3289, while 6b showed certain inhibition on S. aureus 4220.

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.

Catalyst-controlled regioselective Sonogashira coupling of 9-substituted-6-chloro-2,8-diiodopurines

We have established a catalyst-dependent regioselective Sonogashira coupling methodology where both regioisomeric products can be obtained independently with remarkably high selectivity.

Novel cyanothiouracil and cyanothiocytosine derivatives as concentration-dependent selective inhibitors of U87MG glioblastomas: Adenosine receptor binding and potent PDE4 inhibition

Thiouracil and thiocytosine are important heterocyclic pharmacophores having pharmacological diversity. Antitumor and antiviral activity is commonly associated with thiouracil and thiocytosine derivatives, which are well known fragments for adenosine receptor affinity with many associated pharmacological properties. In this respect, 33 novel compounds have been synthesized in two groups: 24 thiouracil derivatives (4a-x) and 9 thiocytosine derivatives (5a-i). Antitumor activity of all the compounds was determined in the U87 MG glioblastoma cell line. Compound 5e showed an anti-proliferative IC₅₀ of 1.56 μM, which is slightly higher activity than cisplatin (1.67 μM). The 11 most active compounds showed no signficant binding to adenosine A₁, A_2A or A_2B receptors at 1 μM. Brain tumors express high amounts of phosphodiesterases. Compounds were tested for PDE4 inhibition, and 5e and 5f showed the best potency (5e: 3.42 μM; 5f: 0.97 μM). Remakably, those compounds were also the most active against U87MG. However, the compounds lacked a cytotoxic effect on the HEK293 healthy cell line, which encourages further investigation.

Breakthrough in GPCR Crystallography and Its Impact on Computer-Aided Drug Design

Recent crystallographic structures of G protein-coupled receptors (GPCRs) have greatly advanced our understanding of the recognition of their diverse agonist and antagonist ligands. We illustrate here how this applies to A_2A adenosine receptors (ARs) and to P2Y₁ and P2Y₁₂ receptors (P2YRs) for ADP. These X-ray structures have impacted the medicinal chemistry aimed at discovering new ligands for these two receptor families, including receptors that have not yet been crystallized but are closely related to the known structures. In this Chapter, we discuss recent structure-based drug design projects that led to the discovery of: (a) novel A₃AR agonists based on a highly rigidified (N)-methanocarba scaffold for the treatment of chronic neuropathic pain and other conditions, (b) fluorescent probes of the ARs and P2Y₁₄R, as chemical tools for structural probing of these GPCRs and for improving assay capabilities, and (c) new more drug-like antagonists of the inflammation-related P2Y₁₄R. We also describe the computationally enabled molecular recognition of positive (for A₃AR) and negative (P2Y₁R) allosteric modulators that in some cases are shown to be consistent with structure-activity relationship (SAR) data. Thus, computational modeling has become an essential tool for the design of purine receptor ligands.

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.

Regioselective C–H and N–H functionalization of purine derivatives and analogues: a synthetic and mechanistic perspective

This perspective provides a systematic and concise overview of the recent development in C–H/N–H bond functionalization in purine derivatives and analogues.

Control of α/β Anomer Formation by a 2',5' Bridge: Toward Nucleoside Derivatives Locked in the South Conformation

We describe a novel stereoselective synthesis of nucleoside derivatives with the ribose ring locked in the South conformation by a bridge between C2' and C5'. Despite the intrinsic constraints of the bicyclic structure, we demonstrate that their synthesis can be achieved by ring closing metathesis of readily accessible precursors. The obtained ribose derivatives are, however, very poor substrates for further installation of the nucleobases, and even simple nucleophiles, such as azido or cyano anions, react with unexpected stereo- or regioselectivity under standard glycosylation conditions. Here we explain this behavior by employing density functional theory (DFT) computations and devise an alternative approach resulting in isomers with the desired orientation of the nucleobase.

Synthesis, conformational study and antiviral activity of l-like neplanocin derivatives

The l-like enantiomer of 9-(trans-2', trans-3'-dihydroxycyclopent-4'-enyl)-3-deazaadenine (DHCDA) (1), its 3-deaza-3-bromo derivative (3), and the conformational restricted methanocarba (MC) nucleoside analogues (2 and 4) were synthesized. X-ray crystal structures showed the L isomer MC analogue 4 adopts a similar North-like locked conformation as conventional D-MC nucleosides, while the DHCDA analogue 3 preferred south-like conformer. Compounds 1 and 4 showed potent antiviral activity against norovirus, while compound 2 and 3 were less potent or inactive. The conformational behavior of "sugar" puckering (north/south) and nucleobase orientation (syn /anti) may contribute to the antiviral activity differences. For compound 3, antiviral activity was also found against Ebola virus.

Exploring the Role of N6-Substituents in Potent Dual Acting 5'-C-Ethyltetrazolyladenosine Derivatives: Synthesis, Binding, Functional Assays, and Antinociceptive Effects in Mice ∇

Structural determinants of affinity of N⁶-substituted-5'-C-(ethyltetrazol-2-yl)adenosine and 2-chloroadenosine derivatives at adenosine receptor (AR) subtypes were studied with binding and molecular modeling. Small N⁶-cycloalkyl and 3-halobenzyl groups furnished potent dual acting A₁AR agonists and A₃AR antagonists. 4 was the most potent dual acting human (h) A₁AR agonist (K_i = 0.45 nM) and A₃AR antagonist (K_i = 0.31 nM) and highly selective versus A_2A; 11 and 26 were most potent at both h and rat (r) A₃AR. All N⁶-substituted-5'-C-(ethyltetrazol-2-yl)adenosine derivatives proved to be antagonists at hA₃AR but agonists at the rA₃AR. Analgesia of 11, 22, and 26 was evaluated in the mouse formalin test (A₃AR antagonist blocked and A₃AR agonist strongly potentiated). N⁶-Methyl-5'-C-(ethyltetrazol-2-yl)adenosine (22) was most potent, inhibiting both phases, as observed combining A₁AR and A₃AR agonists. This study demonstrated for the first time the advantages of a single molecule activating two AR pathways both leading to benefit in this acute pain model.

N6-Substituted 5'-N-Methylcarbamoyl-4'-selenoadenosines as Potent and Selective A3 Adenosine Receptor Agonists with Unusual Sugar Puckering and Nucleobase Orientation

Potent and selective A₃ adenosine receptor (AR) agonists were identified by the replacement of 4'-oxo- or 4'-thionucleosides with bioisosteric selenium. Unlike previous agonists, 4'-seleno analogues preferred a glycosidic syn conformation and South sugar puckering, as shown in the X-ray crystal structure of 5'-N-methylcarbamoyl derivative 3p. Among the compounds tested, N⁶-3-iodobenzyl analogue 3d was found to be the most potent A₃AR full agonist (K_i = 0.57 nM), which was ≥800- and 1900-fold selective for A₁AR and A_2AAR, respectively. In the N⁶-cycloalkyl series, 2-Cl analogues generally exhibited better hA₃AR affinity than 2-H analogues, whereas 2-H > 2-Cl in the N⁶-3-halobenzyl series. N⁷ isomers 3t and 3u were much weaker in binding than corresponding N⁹ isomers, but compound 3t lacked A₃AR activation, appearing to be a weak antagonist. 2-Cl-N⁶-3-iodobenzyl analogue 3p inhibited chemoattractant-induced migration of microglia/monocytes without inducing cell death at ≤50 μM. This suggests the potential for the development of 4'-selenonucleoside A₃AR agonists as novel antistroke agents.

Nickel-catalyzed C–H activation of purine bases with alkyl halides

C–H alkylations of purine nucleosides were achieved by means of user-friendly nickel catalysis with ample substrate scope and high levels of chemo, site and regio control, which among others enabled the direct fluorescent labeling of purines in terms of late stage diversification.

The A3 adenosine receptor: history and perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein-coupled adenosine receptors (ARs): A(1), A(2)A, A(2)B, and A(3). The A(3) adenosine receptor (A(3)AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A(3)AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A(3) agonists or antagonists would be the best choice. Today, the choice has been made and A(3)AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A(3)AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Palladium-catalysed ortho-acylation of 6-anilinopurines/purine nucleosides via C–H activation

Purinyl N1-directed ortho-acylation of 6-anilinopurines was achieved in the presence of [Pd]-catalyst using aldehydes/α-oxocarboxylic acids as the acylating sources.