The A3 adenosine receptor: history and perspectives

Steric influence of 4'-position substituents and C2-Hexynyl group on A3AR antagonism in truncated 4'-Thioadenosine derivatives

A3 adenosine receptor (A3AR) has attracted significant interest due to its therapeutic potential in inflammation, neurodegenerative disorders, and cancer. This study investigates the structure-activity relationships (SAR) of truncated 4'-thioadenosine derivatives with modifications at the 4'- and C2-positions. Alkyl substituents of varying sizes (methyl, ethyl, isopropyl) were introduced at the 4'-position to evaluate steric effects, while C2-propynyl and hexynyl groups were incorporated to explore binding enhancement. The 4'-ethyl derivatives exhibit potent A3AR antagonistic effects, compared to the 4'-methyl series which exhibits partial agonist activity. C2-hexynyl substitution significantly enhanced binding affinity and antagonistic properties. In contrast, 4'-isopropyl derivatives lacked measurable binding, highlighting steric constraints at 4'-position. Docking studies revealed that none of the compounds formed interactions with Thr94, a residue critical for agonistic activity. This lack of interaction likely explains their observed antagonistic behavior, with the 4'-alkyl and C2-hexynyl substitutions appearing to play a significant role in this effect. This work underscores the role of steric bulk at the 4'-position and hydrophobic modifications at the C2-position in modulating A3AR activity, providing valuable insights for designing selective A3AR antagonists.

Progress in the study of anti-Alzheimer's disease activity of pyrimidine-containing bioactive molecules

Pyrimidines are aromatic, heterocyclic organic compounds characterized by a six-membered ring that contains four carbon atoms and two nitrogen atoms. They have been reported to exhibit a variety of biological activities such as antifungal, antiviral, and anti-Parkinsonian effects. Recently, there has been an increased focus on their potential anti-Alzheimer's properties. Several pyrimidine-based drugs and their analogs are currently undergoing various phases of clinical trials, indicating pyrimidine as a promising chemical structure for drug development. Notably, modifications to the pyrimidine structure significantly influence their activity against Alzheimer's disease. For instance, the introduction of heteroatoms into the pyrimidine ring or alternations in the length of the linkage region have been shown to enhance therapeutic efficacy. This review provides a comprehensive overview of pyrimidine derivatives as potential therapeutics for Alzheimer's disease, with a focus on structure-activity relationship (SAR) studies, design strategies, and binding mechanisms. These insights could pave the way for the development of more effective anti-Alzheimer's medications.

Role of A1 adenosine receptor in cardiovascular diseases: Bridging molecular mechanisms with therapeutic opportunities

Adenosine serves as a critical homeostatic regulator, exerting influence over physiological and pathological conditions in the cardiovascular system. During cellular stress, increased extracellular adenosine levels have been implicated in conferring cardioprotective effects through the activation of adenosine receptors with the A1 adenosine receptor subtype showing the highest expression in the heart. A1 adenosine receptor stimulation inhibits adenylyl cyclase activity via heterotrimeric Gi proteins, leading to the activation of distinct downstream effectors involved in cardiovascular homeostasis. While the comprehensive characterization of the pharmacological functions and intracellular signaling pathways associated with the A1 adenosine receptor subtype is still ongoing, this receptor is widely recognized as a crucial pharmacological target for the treatment of various states of cardiovascular diseases (CVDs). In this review, we focus on elucidating signal transduction of A1 adenosine receptor, particularly Gi protein-dependent and -independent pathways, and their relevance to cardiovascular protective effects as well as pathological consequences during cellular and tissue stresses in the cardiovascular system. Additionally, we provide comprehensive updates and detailed insights into a range of A1 adenosine receptor agonists and antagonists, detailing their development and evaluation through preclinical and clinical studies with a specific focus on their potential for the management of CVDs, especially heart diseases.

Photoswitchable Diazocine Derivative for Adenosine A3 Receptor Activation in Psoriasis

Incorporating photoisomerizable moieties within drugs offers the possibility of rapid and reversible light-dependent switching between active and inactive configurations. Here, we developed a photoswitchable adenosine A3 receptor (A3R) agonist that confers optical control on this G protein-coupled receptor through noninvasive topical skin irradiation in an animal model of psoriasis. This was achieved by covalently bonding an adenosine-5'-methyluronamide moiety to a diazocine photochrome, whose singular photoswitching properties facilitated repeated interconversion between a thermally stable, biologically inactive Z agonist form and a photoinduced, pharmacologically active E configuration. As a result, our photoswitchable agonist allowed the precise modulation of A3R function both in vitro and in vivo, which led to a clear light-controlled pharmacotherapeutic effect on mouse skin lesions. This breakthrough not only demonstrates the potential of diazocine photoswitches for in vivo photopharmacology but also paves the way for the development of new strategies for skin-related diseases that require localized and temporally controlled drug action.

Adenosine relaxes vagina smooth muscle through the cyclic guanosine monophosphate- and cyclic guanosine monophosphate-dependent pathways

BACKGROUND

In males, adenosine (ADO) is known to relax penile smooth muscles, although its role in the vagina is not yet fully elucidated.

AIM

This study investigated the effect of ADO on vagina smooth muscle activity, using a validated female Sprague-Dawley rat model.

METHODS

Contractility studies, using noradrenaline-precontracted vaginal strips, tested the effects of ADORA1/3 antagonists and ADORA2A/2B antagonists and agonists. Increasing doses of ADO were tested after in vivo or in vitro treatment with Nω-nitro-L-arginine-methyl-ester hydrochloride (L-NAME) or with guanylate or adenylate cyclase inhibitors. Immunopositivity for ADORA2A and ADORA2B was assessed, and messenger RNA (mRNA) analysis was performed. Cyclic ADO monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) were quantified both in rat vagina smooth muscle cells (rvSMCs) and in vaginal tissues with increasing doses of ADO.

OUTCOMES

Demonstrating ADO's role in the relaxing/contractile mechanism in distal vagina smooth muscle.

RESULTS

All ADO receptors mRNAs were expressed in vaginal tissue, with a prevalent content of ADORA2B. A high expression of genes regulating ADO catabolism (ADK) and de novo synthesis (NT5E) was found. In vaginal strips, ADO induced relaxation with IC50 = 144.7 μM and a flat pseudo-Hill coefficient value = -0.42, indicating an activity on heterogeneous receptors. Blocking ADORA1/3 shifted ADO response to the left and with a steeper slope. ADORA2A/2B agonists showed a higher potency than ADO in inducing relaxation. Immunolocalization confirmed the presence of ADORA2A/2B in vaginal musculature, in the blood vessels endothelium, and in the epithelium. ADO stimulation of vagina tissues induced a significant increase in cAMP and cGMP contents. Experiments on rvSMCs confirmed that ADO time- and dose-dependently stimulated cAMP production in these cells. However, ADORA2A/2B antagonists, although reducing the ADO-induced relaxation, did not completely block it. A similar inhibition was obtained by blocking adenylate cyclase. Overall, these findings suggest that ADO relaxation involves other pathways, eg, nitric oxide (NO)/cGMP. Accordingly, blocking NO formation through L-NAME substantially blunted ADO responsiveness, as it does the block of cGMP formation through 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one. Simultaneous incubation with cGMP and cAMP blockers completely inhibited ADO responsiveness.

CLINICAL TRANSLATION

The study highlights ADO's role in regulating vaginal smooth muscle activity, suggesting its potential effect on the vagina.

STRENGTHS AND LIMITATIONS

This is the first study on ADO in the vagina, although the results are preliminary and limited to the rat model.

CONCLUSION

These results show that ADO acts as a vaginal relaxing modulator through selective activation of receptors involving not only cAMP but also cGMP.

Stereochemical influence of 4'-methyl substitutions on truncated 4'-thioadenosine derivatives: Impact on A3 adenosine receptor binding and antagonism

Herein, we investigated the stereochemical effects of 4'-methyl substitution on A3 adenosine receptor (A3AR) ligands by synthesizing and evaluating a series of truncated 4'-thioadenosine derivatives featuring 4'-α-methyl, 4'-β-methyl, and 4',4'-dimethyl substitutions. We successfully synthesized these derivatives, using the stereoselective addition of an organometallic reagent, KSAc-mediated sulfur cyclization, and Vorbrüggen condensation. Binding assays demonstrated that the 4'-β-methyl substitution conferred the highest affinity for A3AR, with compound 1 h exhibiting a Ki = 3.5 nM, followed by the 4',4'-dimethyl and 4'-α-methyl substitutions. Notably, despite the absence of the 5'-OH group, compound 1 h unexpectedly displayed partial agonism. Computational docking studies indicated that compound 1 h, the β-methyl derivative, adopted a South conformation and maintained strong interactions within the receptor, including a critical interaction with Thr94, a residue known to be notable for agonistic effects. Conversely, compound 2 h, the α-methyl derivative, also adopted a South conformation but resulted in a flattened structure that hindered interactions with Thr94 and Asn250. The dimethyl derivative 3 h exhibited steric clashes with Thr94, contributing to a reduction in binding affinity. However, the docking results for 3 h indicated a North conformation, suggesting that the change in sugar conformation due to the additional 4'-methyl group altered the angle between the α-methyl group and the sugar plane, enabling binding despite the increased steric bulk. These findings suggest that not only do the substituents and their stereochemistry influence receptor-ligand interactions, but the conformation and the resulting spatial orientation of the substituents also play a crucial role in modulating receptor-ligand interaction. This stereochemical insight offers a valuable framework for the design of new, selective, and potent A3AR ligands, potentially facilitating the development of novel therapeutics for A3AR-related diseases such as glaucoma, inflammation, and cancer.

Adenosine Receptor 3 in Liver Cancer: Expression Variability, Epigenetic Modulation, and Enhanced Histone Deacetylase Inhibitor Effects

Background and Aims

Primary liver cancer, including hepatocellular carcinoma (HCC) and cholangiocarcinoma (CCA), has low response rates to existing treatments, highlighting the urgent need for novel treatment options. Adenosine A3 receptor (ADORA3) signaling has emerged as a potential target. Namodenoson, an ADORA3 agonist, has shown promise in early clinical trials for HCC. However, further data are required to clarify ADORA3 expression patterns in liver cancer, mechanisms of action, and the potential for combination therapies to inform patient selection for future clinical trials.

Methods

Patient-derived tissue microarrays and RNA-sequencing were employed to investigate ADORA3 expression. Cellular responses to ADORA3 stimulation and combination treatments were studied in HCC and CCA cell lines and patient-derived organoids (PDOs). Genome-wide RNA-Seq analysis, mRNA analysis, and DigiWest protein profiling were performed.

Results

Tissue microarray analysis revealed higher ADORA3 expression in nonmalignant samples and a subset of tumors with weak or absent ADORA3 expression. This was supported by RNA sequencing data from The Cancer Genome Atlas and needle biopsy samples. Cell lines and PDOs exhibited antiproliferative effects with the ADORA3 agonist Namodenoson, confirmed by receptor dependency tests with specific antagonists and siRNA experiments. Genome-wide RNA-Seq analysis suggested chromatin remodeling events after ADORA3 stimulation. mRNA expression and DigiWest profiling identified downregulation of histone deacetylases and histone H3 modifications. Combination treatments with different ADORA3 agonists and histone deacetylase inhibitors significantly enhanced antiproliferative effects in almost all selected combinations, supported by investigations in PDOs.

Conclusion

ADORA3 expression varies considerably in HCC or CCA, ranging from high to absent receptor detection. This observation might help to identify patients for clinical studies. Additionally, Namodenoson's epigenetic modulating activity suggests epigenetic drugs as promising candidates for combination treatment.

Small-molecule-based targeted therapy in liver cancer

Liver cancer is one of the most prevalent malignant tumors worldwide. According to the Barcelona Clinic Liver Cancer staging criteria, clinical guidelines provide tutorials to clinical management of liver cancer at their individual stages. However, most patients diagnosed with liver cancer are at advanced stage; therefore, many researchers conduct investigations on targeted therapy, aiming to improve the overall survival of these patients. To date, small-molecule-based targeted therapies are highly recommended (first line: sorafenib and lenvatinib; second line: regorafenib and cabozantinib) by current the clinical guidelines of the American Society of Clinical Oncology, European Society for Medical Oncology, and National Comprehensive Cancer Network. Herein, we summarize the small-molecule-based targeted therapies in liver cancer, including the approved and preclinical therapies as well as the therapies under clinical trials, and introduce their history of discovery, clinical trials, indications, and molecular mechanisms. For drug resistance, the revealed mechanisms of action and the combination therapies are also discussed. In fact, the known small-molecule-based therapies still have limited clinical benefits to liver cancer patients. Therefore, we analyze the current status and give our ideas for the urgent issues and future directions in this field, suggesting clues for novel techniques in liver cancer treatment.

Genetic and functional modulation by agonist MRS5698 and allosteric enhancer LUF6000 at the native A3 adenosine receptor in HL-60 cells

The A3 adenosine receptor (AR) is an important inflammatory and immunological target. However, the underlying mechanisms are not fully understood. Here, we report the gene regulation in HL-60 cells treated acutely with highly selective A3AR agonist MRS5698, positive allosteric modulator (PAM) LUF6000, or both. Both pro- and anti-inflammatory genes, such as IL-1a, IL-1β, and NFκBIZ, are significantly upregulated. During our observations, LUF6000 alone produced a lesser effect, while the MRS5698 + LUF6000 group demonstrated generally greater effects than MRS5698 alone, consistent with allosteric enhancement. The number of genes up- and down-regulated are similar. Pathway analysis highlighted the critical involvement of signaling molecules, including IL-6 and IL-17. Important upstream regulators include IL-1a, IL-1β, TNF-α, NF-κB, etc. PPAR, which modulates eicosanoid metabolism, was highly downregulated by the A3AR agonist. Considering previous pharmacological results and mathematical modeling, LUF6000's small enhancement of genetic upregulation suggested that MRS5698 is a nearly full agonist, which we demonstrated in both cAMP and calcium assays. The smaller effect of LUF6000 on MRS5698 in comparison to its effect on Cl-IB-MECA was shown in both HL-60 cells endogenously expressing the human (h) A3AR and in recombinant hA3AR-expressing CHO cells, consistent with its HL-60 cell genetic regulation patterns. In summary, by using both selective agonists and PAM, we identified genes that are closely relevant to immunity and inflammation to be regulated by A3AR in differentiated HL-60 cells, a cell model of neutrophil function. In addition, we demonstrated the previously uncharacterized allosteric signaling-enhancing effect of LUF6000 in cells endogenously expressing the hA3AR.

Role and mechanisms of mast cells in brain disorders

Mast cells serve as crucial effector cells within the innate immune system and are predominantly localized in the skin, airways, gastrointestinal tract, urinary and reproductive tracts, as well as in the brain. Under physiological conditions, brain-resident mast cells secrete a diverse array of neuro-regulatory mediators to actively participate in neuroprotection. Meanwhile, as the primary source of molecules causing brain inflammation, mast cells also function as the "first responders" in brain injury. They interact with neuroglial cells and neurons to facilitate the release of numerous inflammatory mediators, proteases, and reactive oxygen species. This process initiates and amplifies immune-inflammatory responses in the brain, thereby contributing to the regulation of neuroinflammation and blood-brain barrier permeability. This article provides a comprehensive overview of the potential mechanisms through which mast cells in the brain may modulate neuroprotection and their pathological implications in various neurological disorders. It is our contention that the inhibition of mast cell activation in brain disorders could represent a novel avenue for therapeutic breakthroughs.

Hit-to-Lead Optimization of Heterocyclic Carbonyloxycarboximidamides as Selective Antagonists at Human Adenosine A3 Receptor

Antagonism of the human adenosine A3 receptor (hA3R) has potential therapeutic application. Alchemical relative binding free energy calculations of K18 and K32 suggested that the combination of a 3-(2,6-dichlorophenyl)-isoxazolyl group with 2-pyridinyl at the ends of a carbonyloxycarboximidamide group should improve hA3R affinity. Of the 25 new analogues synthesized, 37 and 74 showed improved hA3R affinity compared to K18 (and K32). This was further improved through the addition of a bromine group to the 2-pyridinyl at the 5-position, generating compound 39. Alchemical relative binding free energy calculations, mutagenesis studies and MD simulations supported the compounds' binding pattern while suggesting that the bromine of 39 inserts deep into the hA3R orthosteric pocket, so highlighting the importance of rigidification of the carbonyloxycarboximidamide moiety. MD simulations highlighted the importance of rigidification of the carbonyloxycarboximidamide, while suggesting that the bromine of 39 inserts deep into the hA3R orthosteric pocket, which was supported through mutagenesis studies 39 also selectively antagonized endogenously expressed hA3R in nonsmall cell lung carcinoma cells, while pharmacokinetic studies indicated low toxicity enabling in vivo evaluation. We therefore suggest that 39 has potential for further development as a high-affinity hA3R antagonist.

Current Understanding of the Role of Adenosine Receptors in Cancer

Cancer, a complex array of diseases, involves the unbridled proliferation and dissemination of aberrant cells in the body, forming tumors that can infiltrate neighboring tissues and metastasize to distant sites. With over 200 types, each cancer has unique attributes, risks, and treatment avenues. Therapeutic options encompass surgery, chemotherapy, radiation therapy, hormone therapy, immunotherapy, targeted therapy, or a blend of these methods. Yet, these treatments face challenges like late-stage diagnoses, tumor diversity, severe side effects, drug resistance, targeted drug delivery hurdles, and cost barriers. Despite these hurdles, advancements in cancer research, encompassing biology, genetics, and treatment, have enhanced early detection methods, treatment options, and survival rates. Adenosine receptors (ARs), including A1, A2A, A2B, and A3 subtypes, exhibit diverse roles in cancer progression, sometimes promoting or inhibiting tumor growth depending on the receptor subtype, cancer type, and tumor microenvironment. Research on AR ligands has revealed promising anticancer effects in lab studies and animal models, hinting at their potential as cancer therapeutics. Understanding the intricate signaling pathways and interactions of adenosine receptors in cancer is pivotal for crafting targeted therapies that optimize benefits while mitigating drawbacks. This review delves into each adenosine receptor subtype's distinct roles and signaling pathways in cancer, shedding light on their potential as targets for improving cancer treatment outcomes.

Translational Experimental Basis of Indirect Adenosine Receptor Agonist Stimulation for Bone Regeneration: A Review

Bone regeneration remains a significant clinical challenge, often necessitating surgical approaches when healing bone defects and fracture nonunions. Within this context, the modulation of adenosine signaling pathways has emerged as a promising therapeutic option, encouraging osteoblast activation and tempering osteoclast differentiation. A literature review of the PubMed database with relevant keywords was conducted. The search criteria involved in vitro or in vivo models, with clear methodological descriptions. Only studies that included the use of indirect adenosine agonists, looking at the effects of bone regeneration, were considered relevant according to the eligibility criteria. A total of 29 articles were identified which met the inclusion and exclusion criteria, and they were reviewed to highlight the preclinical translation of adenosine agonists. While preclinical studies demonstrate the therapeutic potential of adenosine signaling in bone regeneration, its clinical application remains unrealized, underscoring the need for further clinical trials. To date, only large, preclinical animal models using indirect adenosine agonists have been successful in stimulating bone regeneration. The adenosine receptors (A1, A2A, A2B, and A3) stimulate various pathways, inducing different cellular responses. Specifically, indirect adenosine agonists act to increase the extracellular concentration of adenosine, subsequently agonizing the respective adenosine receptors. The agonism of each receptor is dependent on its expression on the cell surface, the extracellular concentration of adenosine, and its affinity for adenosine. This comprehensive review analyzed the multitude of indirect agonists currently being studied preclinically for bone regeneration, discussing the mechanisms of each agonist, their cellular responses in vitro, and their effects on bone formation in vivo.

Manual acupuncture ameliorates inflammatory pain by upregulating adenosine A3 receptor in complete Freund's adjuvant-induced arthritis rats

BACKGROUND

Adenosine A3 receptor (A3R) exerts analgesic, anti-inflammatory, and anti-nociceptive effects. In this study, we determined the analgesic mechanism of manual acupuncture (MA) in rats with complete Freund's adjuvant (CFA)-induced arthritis and explored whether MA ameliorates inflammation in these rats by upregulating A3R.

METHODS

Sixty Sprague Dawley (SD) rats were randomly divided into the following groups: Control, CFA, CFA + MA, CFA + sham MA, CFA + MA + DMSO, CFA + MA + IB-MECA, and CFA + MA + Reversine groups. The arthritis rat model was induced by injecting CFA into the left ankle joints. Thereafter, the rats were subjected to MA (ST36 acupoint) for 3 days. The clinical indicators paw withdrawal latency (PWL), paw withdrawal threshold (PWT), and open field test (OFT) were used to determine the analgesic effect of MA. In addition, to explore the effect of A3R on inflammation after subjecting arthritis rats to MA, IB-MECA (A3R agonist) and Reversine (A3R antagonist) were injected into ST36 before MA.

RESULTS

MA ameliorated the pathological symptoms of CFA-induced arthritis, including the pain indicators PWL and PWT, number of rearing, total ambulatory distance, and activity trajectory. Furthermore, after MA, the mRNA and protein expression of A3R was upregulated in CFA-induced arthritis rats. In contrast, the protein levels of TNF-α, IL-1β, Rap1, and p-p65 were downregulated after MA. Interestingly, the A3R agonist and antagonist further downregulated and upregulated inflammatory cytokine expression, respectively, after MA. Furthermore, the A3R antagonist increased the degree of ankle swelling after MA.

CONCLUSION

MA can alleviate inflammatory pain by inhibiting the NF-κB signaling pathway via upregulating A3R expression of the superficial fascia of the ST36 acupoint site in CFA-induced arthritis rats.

Adenosine and Its Receptors in the Pathogenesis and Treatment of Inflammatory Skin Diseases

Inflammatory skin diseases highlight inflammation as a central driver of skin pathologies, involving a multiplicity of mediators and cell types, including immune and non-immune cells. Adenosine, a ubiquitous endogenous immune modulator, generated from adenosine triphosphate (ATP), acts via four G protein-coupled receptors (A1, A2A, A2B, and A3). Given the widespread expression of those receptors and their regulatory effects on multiple immune signaling pathways, targeting adenosine receptors emerges as a compelling strategy for anti-inflammatory intervention. Animal models of psoriasis, contact hypersensitivity (CHS), and other dermatitis have elucidated the involvement of adenosine receptors in the pathogenesis of these conditions. Targeting adenosine receptors is effective in attenuating inflammation and remodeling the epidermal structure, potentially showing synergistic effects with fewer adverse effects when combined with conventional therapies. What is noteworthy are the promising outcomes observed with A2A agonists in animal models and ongoing clinical trials investigating A3 agonists, underscoring a potential therapeutic approach for the management of inflammatory skin disorders.

Adenosine A3 Receptor: From Molecular Signaling to Therapeutic Strategies for Heart Diseases

Cardiovascular diseases (CVDs), particularly heart failure, are major contributors to early mortality globally. Heart failure poses a significant public health problem, with persistently poor long-term outcomes and an overall unsatisfactory prognosis for patients. Conventionally, treatments for heart failure have focused on lowering blood pressure; however, the development of more potent therapies targeting hemodynamic parameters presents challenges, including tolerability and safety risks, which could potentially restrict their clinical effectiveness. Adenosine has emerged as a key mediator in CVDs, acting as a retaliatory metabolite produced during cellular stress via ATP metabolism, and works as a signaling molecule regulating various physiological processes. Adenosine functions by interacting with different adenosine receptor (AR) subtypes expressed in cardiac cells, including A1AR, A2AAR, A2BAR, and A3AR. In addition to A1AR, A3AR has a multifaceted role in the cardiovascular system, since its activation contributes to reducing the damage to the heart in various pathological states, particularly ischemic heart disease, heart failure, and hypertension, although its role is not as well documented compared to other AR subtypes. Research on A3AR signaling has focused on identifying the intricate molecular mechanisms involved in CVDs through various pathways, including Gi or Gq protein-dependent signaling, ATP-sensitive potassium channels, MAPKs, and G protein-independent signaling. Several A3AR-specific agonists, such as piclidenoson and namodenoson, exert cardioprotective impacts during ischemia in the diverse animal models of heart disease. Thus, modulating A3ARs serves as a potential therapeutic approach, fueling considerable interest in developing compounds that target A3ARs as potential treatments for heart diseases.

Deep learning-assisted mass spectrometry imaging for preliminary screening and pre-classification of psychoactive substances

Currently, it is of great urgency to develop a rapid pre-classification and screening method for suspected drugs as the constantly springing up of new psychoactive substances. In most researches, psychoactive substances classification approaches depended on the similar chemical structures and pharmacological action with known drugs. Such approaches could not face the complicated circumstance of emerging new psychoactive substances. Herein, mass spectrometry imaging and convolutional neural networks (CNN) were used for preliminary screening and pre-classification of suspected psychoactive substances. Mass spectrometry imaging was performed simultaneously on two brain slices as one was from blank group and another one was from psychoactive substance-induced group. Then, fused neurotransmitter variation mass spectrometry images (Nv-MSIs) reflecting the difference of neurotransmitters between two slices were achieved through two homemade programs. A CNN model was developed to classify the Nv-MSIs. Compared with traditional classification methods, CNN achieved better estimation accuracy and required minimal data preprocessing. Also, the specific region on Nv-MSIs and weight of each neurotransmitter that affected the classification most could be unraveled by CNN. Finally, the method was successfully applied to assist the identification of a new psychoactive substance seized recently. This sample was identified as cannabinoids, which greatly promoted the screening process.

Targeting the adenosine signaling pathway in macrophages for cancer immunotherapy

One of the ways in which macrophages support tumorigenic growth is by producing adenosine, which acts to dampen antitumor immune responses and is generated by both tumor and immune cells in the tumor microenvironment (TME). Two cell surface expressed molecules, CD73 and CD39, boost catalytic adenosine triphosphate, leading to further increased adenosine synthesis, under hypoxic circumstances in the TME. There are four receptors (A1, A2A, A2B, and A3) expressed on macrophages that allow adenosine to perform its immunomodulatory effect. Researchers have shown that adenosine signaling is a key factor in tumor progression and an attractive therapeutic target for treating cancer. Several antagonistic adenosine-targeting biological therapies that decrease the suppressive action of tumor-associated macrophages have been produced and explored to transform this result from basic research into a therapeutic advantage. Here, we'll review the newest findings from studies of pharmacological compounds that target adenosine receptors, and their potential therapeutic value based on blocking the suppressive action of macrophages in tumors.

Cryo-EM structures of adenosine receptor A3AR bound to selective agonists

The adenosine A3 receptor (A3AR), a key member of the G protein-coupled receptor family, is a promising therapeutic target for inflammatory and cancerous conditions. The selective A3AR agonists, CF101 and CF102, are clinically significant, yet their recognition mechanisms remained elusive. Here we report the cryogenic electron microscopy structures of the full-length human A3AR bound to CF101 and CF102 with heterotrimeric Gi protein in complex at 3.3-3.2 Å resolution. These agonists reside in the orthosteric pocket, forming conserved interactions via their adenine moieties, while their 3-iodobenzyl groups exhibit distinct orientations. Functional assays reveal the critical role of extracellular loop 3 in A3AR's ligand selectivity and receptor activation. Key mutations, including His3.37, Ser5.42, and Ser6.52, in a unique sub-pocket of A3AR, significantly impact receptor activation. Comparative analysis with the inactive A2AAR structure highlights a conserved receptor activation mechanism. Our findings provide comprehensive insights into the molecular recognition and signaling of A3AR, paving the way for designing subtype-selective adenosine receptor ligands.

Adenosine receptors in breast cancer

Adenosine receptors are important in the normal physiological function of cells and the pathogenesis of various cancer cells, including breast cancer cells. The activity of adenosine receptors in cancer cells is related to cell proliferation, angiogenesis, metastasis, immune system evasion, and interference with apoptosis. Considering the different roles of adenosine receptors in cancer cells, we intend to investigate the function of adenosine receptors and their biological pathways in breast cancer to improve understanding of therapeutically relevant signaling pathways.

Covalently Binding Adenosine A3 Receptor Agonist ICBM Irreversibly Reduces Voltage-Gated Ca2+ Currents in Dorsal Root Ganglion Neurons

Interest has been focused in recent years on the analgesic effects exerted by adenosine and its receptors, A1, A2A, A2B, and A3 adenosine receptor (AR) subtypes, in different in vivo models of chronic pain. In particular, it was demonstrated that selective A3AR agonists reduced pro-nociceptive N-type Ca2+ channels in dorsal root ganglion (DRG) neurons isolated from rats and, by this mechanism, inhibit post inflammatory visceral hypersensitivity. In the present study, we investigate the effect of a previously reported irreversibly binding A3AR agonist, ICBM, on Ca2+ currents (ICa) in rat DRG neurons. Present data demonstrate that ICBM, an isothiocyanate derivative designed for covalent binding to the receptor, concentration-dependently inhibits ICa. This effect is irreversible, since it persists after drug removal, differently from the prototypical A3AR agonist, Cl-IB-MECA. ICBM pre-exposure inhibits the effect of a subsequent Cl-IB-MECA application. Thus, covalent A3AR agonists such as ICBM may represent an innovative, beneficial, and longer-lasting strategy to achieve efficacious chronic pain control versus commonly used, reversible, A3AR agonists. However, the possible limitations of this drug and other covalent drugs may be, for example, a characteristic adverse effect profile, suggesting that more pre-clinical studies are needed.

Bridging the translational gap: adenosine as a modulator of neuropathic pain in preclinical models and humans

OBJECTIVES

This review aims to analyse the published data on preclinical and human experimental and clinical adenosine modulation for pain management. We summarise the translatability of the adenosine pathway for further drug development and aim to reveal subgroups of pain patients that could benefit from targeting the pathway.

CONTENT

Chronic pain patients suffer from inadequate treatment options and drug development is generally impaired by the low translatability of preclinical pain models. Therefore, validating the predictability of drug targets is of high importance. Modulation of the endogenous neurotransmitter adenosine gained significant traction in the early 2000s but the drug development efforts were later abandoned. With the emergence of new drug modalities, there is a renewed interest in adenosine modulation in pain management. In both preclinical, human experimental and clinical research, enhancing adenosine signalling through the adenosine receptors, has shown therapeutic promise. A special focus has been on the A1 and A3 receptors both of which have shown great promise and predictive validity in neuropathic pain conditions.

SUMMARY

Adenosine modulation shows predictive validity across preclinical, human experimental and clinical investigations. The most compelling evidence is in the field of neuropathic pain, where adenosine has been found to alleviate hyperexcitability and has the potential to be disease-modifying.

OUTLOOK

Adenosine modulation show therapeutic potential in neuropathic pain if selective and safe drugs can be developed. New drug modalities such as RNA therapeutics and cell therapies may provide new options.

[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.

Namodenoson Inhibits the Growth of Pancreatic Carcinoma via Deregulation of the Wnt/β-catenin, NF-κB, and RAS Signaling Pathways

Namodenoson, an A3 adenosine receptor (A3AR) agonist, is currently being used in a phase III trial in advanced liver cancer. We examined the anti-growth effect of namodenoson on pancreatic carcinoma cells and investigated the molecular mechanism involved. BxPC-3 pancreatic carcinoma cells were cultured with namodenoson (5-20 nM for 24 h at 37 °C), and the Presto Blue assay was used to monitor cell growth. Western blot analyses were performed on BxPC-3 cells (20 nM namodenoson for 24 h at 37 °C) to evaluate the expression levels of cell growth regulatory proteins. In vivo studies involved the subcutaneous inoculation of BxPC-3 cells into nude mice, randomizing the mice into namodenoson (10 μg/kg twice daily for 35 days) vs. control, and monitoring tumor size twice weekly. Treatment with namodenoson was associated with the significant dose-dependent inhibition of BxPC-3 cell growth, which was mitigated by the A3AR antagonist MRS1523. Western blot analyses showed that namodenoson treatment modulated the expression of NF-κB, as well as proteins in the Wnt/β-catenin and the RAS signaling pathways, leading to the upregulation of apoptotic proteins (Bad, Bax). In vivo studies also showed the significant inhibition of pancreatic carcinoma tumor growth with namodenoson. In conclusion, our findings support the continued development of namodenoson as a treatment for pancreatic cancer.

Assay-Dependent Inverse Agonism at the A3 Adenosine Receptor: When Neutral Is Not Neutral

The A3 adenosine receptor (A3AR) is implicated in a variety of (patho)physiological conditions. While most research has focused on agonists and antagonists, inverse agonism at A3AR has been scarcely studied. Therefore, this study aimed at exploring inverse agonism, using two previously engineered cell lines (hA3ARLgBiT-SmBiTβarr2 and hA3ARLgBiT-SmBiTminiGαi), both employing the NanoBiT technology. The previously established inverse agonist PSB-10 showed a decrease in basal signal in the β-arrestin 2 (βarr2) but not the miniGαi recruitment assay, indicative of inverse agonism in the former assay. Control experiments confirmed the specificity and reversibility of this observation. Evaluation of a set of presumed neutral antagonists (MRS7907, MRS7799, XAC, and MRS1220) revealed that all displayed concentration-dependent signal decreases when tested in the A3AR-βarr2 recruitment assay, yielding EC50 and Emax values for inverse agonism. Conversely, in the miniGαi recruitment assay, no signal decreases were observed. To assess whether this observation was caused by the inability of the ligands to induce inverse agonism in the G protein pathway, or rather by a limitation inherent to the employed A3AR-miniGαi recruitment assay, a GloSensor cAMP assay was performed. The outcome of the latter also suggests inverse agonism by the presumed neutral antagonists in this latter assay. These findings emphasize the importance of prior characterization of ligands in the relevant test system. Moreover, it showed the suitability of the NanoBiT βarr2 recruitment and the GloSensor cAMP assays to capture inverse agonism at the A3AR, as opposed to the NanoBiT miniGαi recruitment assay.

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

Combination immunotherapy of PEG-modified Preladenant thermosensitive liposomes and PD-1 inhibitor effectively enhances the anti-tumor immune response and therapeutic effects

Immunotherapy is a promising cancer treatment strategy. In contrast, PD-1/PD-L1 inhibitors are associated with low response rates and are only useful in a small group of cancer patients. A combination of treatments may be effective for overcoming this clinical issue. Preladenant is an adenosine receptor inhibitor that can block the adenosine pathway and improve the tumor microenvironment (TME), thereby enhancing the immunotherapeutic effect of PD-1 inhibitors. However, its poor water solubility and low targeting limit its clinical applications. We designed a PEG-modified thermosensitive liposome (pTSL) loaded with adenosine small molecule inhibitor Preladenant (P-pTSL) to overcome these problems and enhance the effect of PD-1 inhibitor on breast cancer immunotherapy. The prepared P-pTSL was round and uniformly distributed with a particle size of (138.9 ± 1.22) nm, PDI: 0.134 ± 0.031 and Zeta potential (-10.1 ± 1.63) mV; Preladenant was released slowly at 37 °C but released fast at 42 °C from P-pTSL, which was 76.52 ± 0.44%. P-pTSL has good long-term and serum stability and excellent tumor-targeting ability in mice. Moreover, the combination with PD-1 inhibitor significantly enhanced the anti-tumor effect, and the improvement of related factors in serum and lymph was more obvious under the condition of 42 °C thermotherapy in vitro.

C2-linked alkynyl poly-ethylene glycol(PEG) adenosine conjugates as water-soluble adenosine receptor agonists

A series of 12 novel polyethylene-glycol(PEG)-alkynyl C2-adenosine(ADN) conjugates were synthesized using a robust Sonogashira coupling protocol and characterized by NMR spectroscopy and mass spectrometry analysis. The ADN-PEG conjugates showed null to moderate toxicity in murine macrophages and 12c was active against Mycobacterium aurum growth (MIC = 62.5 mg/L). The conjugates were not active against Mycobacterium bovis BCG. Conjugates 10b and 11b exhibited high water solubility with solubility values of 1.22 and 1.18 mg/ml, respectively, in phosphate buffer solutions at pH 6.8. Further, 10b and 11b induced a significant increase in cAMP accumulation in RAW264.7 cells comparable with that induced by adenosine. Analogues 10c, 11c and 12c were docked to the A₁ , A_2A , A_2B and A₃ adenosine receptors (ARs) using crystal-structures and homology models. ADN-PEG-conjugates bearing chains with up to five ethyleneoxy units could be well accommodated within the binding sites of A₁ , A_2A and A₃ ARs. Docking studies showed that compound 10b and 11b were the best A_2A receptor binders of the series, whereas 12c was the best binder for A₁ AR. In summary, introduction of hydrophilic PEG substituents at the C2 of adenine ring significantly improved water solubility and did not affect AR binding properties of the ADN-PEG conjugates.

A novel GRK2 inhibitor alleviates experimental arthritis through restraining Th17 cell differentiation

T helper type 17 (Th17) cell which is induced by interleukine-6 (IL-6)-signal transducers and activators of transcription 3 (STAT3) signaling is a central pro-inflammatory T cell subtype in rheumatoid arthritis (RA) and could be significantly reduced by paeoniflorin-6'-O-benzene sulfonate (CP-25) treatment with unclear mechanisms. This study was aimed to found out the mechanism of CP-25 in hampering Th17 cells differentiation in arthritic animals thus explore more therapeutic targets for RA. In mice with collagen-induced arthritis (CIA), both circulating and splenic Th17 subsets were expanded with increased STAT3 phosphorylation and decreased Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1)-β-arrestin2 (arrb2)-STAT3 interaction in CD4⁺ helper T (Th) cells. Either CP-25 or paroxetine (PAR), an established G protein coupled receptor kinase 2 (GRK2) inhibitor treatment effectively relieved the joints inflammation of CIA mice with substantially reduced Th17 cell population through inhibiting STAT3 and restoring the SHP1-arrb2-STAT3 complex. Knockout of arrb2 exacerbated the clinical manifestations of collagen antibody-induced arthritis with upregulated Th17 cells. In vitro studies revealed that depletion of arrb2 or inhibition of SHP1 promoted Th17 cell differentiation. Moreover, stimulation of adenosine A₃ receptor (A₃AR) simultaneously promoted Th17 cell differentiation via accelerating abbr2-A₃AR binding, which could be prevented through inhibiting GRK2 phosphorylation by CP-25 or PAR, or genetically reducing GRK2. This work has demonstrated that CP-25 or PAR treatment recovers the SHP1-arrb2-STAT3 complex which prevents STAT3 activation in Th cells through reducing arrb2 recruitment to A₃AR by inhibiting GRK2 phosphorylation, leading to the reduction in Th17 cell differentiation and arthritis attenuation.

Endless Journey of Adenosine Signaling in Cardioprotective Mechanism of Conditioning Techniques: Clinical Evidence

Myocardial ischemic injury is a primary cause of death among various cardiovascular disorders. The condition occurs due to an interrupted supply of blood and vital nutrients (necessary for normal cellular activities and viability) to the myocardium, eventually leading to damage. Restoration of blood supply to ischemic tissue is noted to cause even more lethal reperfusion injury. Various strategies, including some conditioning techniques, like preconditioning and postconditioning, have been developed to check the detrimental effects of reperfusion injury. Many endogenous substances have been proposed to act as initiators, mediators, and end effectors of these conditioning techniques. Substances, like adenosine, bradykinin, acetylcholine, angiotensin, norepinephrine, opioids, etc., have been reported to mediate cardioprotective activity. Among these agents, adenosine has been widely studied and suggested to have the most pronounced cardioprotective effects. The current review article highlights the role of adenosine signaling in the cardioprotective mechanism of conditioning techniques. The article also provides an insight into various clinical studies that substantiate the applicability of adenosine as a cardioprotective agent in myocardial reperfusion injury.

Structure-Activity Studies of 1H-Imidazo[4,5-c]quinolin-4-amine Derivatives as A3 Adenosine Receptor Positive Allosteric Modulators

We previously reported 1H-imidazo[4,5-c]quinolin-4-amines as A3 adenosine receptor (A3AR) positive allosteric modulators (PAMs). A3AR agonists, but not PAMs, are in clinical trials for inflammatory diseases and liver conditions. We synthesized new analogues to distinguish 2-cyclopropyl antagonist 17 (orthosteric interaction demonstrated by binding and predicted computationally) from PAMs (derivatives with large 2-alkyl/cycloalkyl/bicycloalkyl groups). We predicted PAM binding at a hydrophobic site on the A3AR cytosolic interface. Although having low Caco-2 permeability and high plasma protein binding, hydrophobic 2-cyclohept-4-enyl-N-3,4-dichlorophenyl, MRS7788 18, and 2-heptan-4-yl-N-4-iodophenyl, MRS8054 39, derivatives were orally bioavailable in rat. 2-Heptan-4-yl-N-3,4-dichlorophenyl 14 and 2-cyclononyl-N-3,4-dichlorophenyl 20 derivatives and 39 greatly enhanced Cl-IB-MECA-stimulated [35S]GTPγS binding Emax, with only 12b trending toward decreasing the agonist EC50. A feasible route for radio-iodination at the p-position of a 4-phenylamino substituent suggests a potential radioligand for allosteric site binding. Herein, we advanced an allosteric approach to developing A3AR-activating drugs that are potentially event- and site-specific in action.

Update on the recent development of allosteric modulators for adenosine receptors and their therapeutic applications

Adenosine receptors (ARs) have been identified as promising therapeutic targets for countless pathological conditions, spanning from inflammatory diseases to central nervous system disorders, from cancer to metabolic diseases, from cardiovascular pathologies to respiratory diseases, and beyond. This extraordinary therapeutic potential is mainly due to the plurality of pathophysiological actions of adenosine and the ubiquitous expression of its receptors. This is, however, a double-edged sword that makes the clinical development of effective ligands with tolerable side effects difficult. Evidence of this is the low number of AR agonists or antagonists that have reached the market. An alternative approach is to target allosteric sites via allosteric modulators, compounds endowed with several advantages over orthosteric ligands. In addition to the typical advantages of allosteric modulators, those acting on ARs could benefit from the fact that adenosine levels are elevated in pathological tissues, thus potentially having negligible effects on normal tissues where adenosine levels are maintained low. Several A1 and various A3AR allosteric modulators have been identified so far, and some of them have been validated in different preclinical settings, achieving promising results. Less fruitful, instead, has been the discovery of A2A and A2BAR allosteric modulators, although the results obtained up to now are encouraging. Collectively, data in the literature suggests that allosteric modulators of ARs could represent valuable pharmacological tools, potentially able to overcome the limitations of orthosteric ligands.

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.

Mechanisms underlying paclitaxel-induced neuropathic pain: Channels, inflammation and immune regulations

Paclitaxel is a chemotherapeutic agent widely used for many types of malignancies. However, when paclitaxel is used to treat tumors, patients commonly experience severe neuropathic pain that is difficult to manage. The mechanism underlying paclitaxel-induced neuropathic pain remains unclear. Evidence demonstrates correlations between mechanisms of paclitaxel-mediated pain and associated actions of ion channels, neuroinflammation, mitochondrial damage, and other factors. This review provides a comprehensive analysis of paclitaxel-induced neuropathic pain mechanisms and suggestions for effective interventions.

Adenosine as a Key Mediator of Neuronal Survival in Cerebral Ischemic Injury

Several experimental studies have linked adenosine's neuroprotective role in cerebral ischemia. During ischemia, adenosine is formed due to intracellular ATP breakdown into ADP, further when phosphate is released from ADP, the adenosine monophosphate is formed. It acts via A1, A2, and A3 receptors found on neurons, blood vessels, glial cells, platelets, and leukocytes. It is related to various effector systems such as adenyl cyclase and membrane ion channels via G-proteins. Pharmacological manipulation of adenosine receptors by agonists (CCPA, ADAC, IB-MECA) increases ischemic brain damage in various in vivo and in vitro models of cerebral ischemia whereas, agonist can also be neuroprotective. Mainly, receptor antagonists (CGS15943, MRS1706) indicated neuroprotection. Later, various studies also revealed that the downregulation or upregulation of specific adenosine receptors is necessary during the recovery of cerebral ischemia by activating several downstream signaling pathways. In the current review, we elaborate on the dual roles of adenosine and its receptor subtypes A1, A2, and A3 and their involvement in the pathobiology of cerebral ischemic injury. Adenosine-based therapies have the potential to improve the outcomes of cerebral injury patients, thereby providing them with a more optimistic future.

DPCPX induces Bim-dependent apoptosis in nasopharyngeal carcinoma cells

ADORA1 promotes tumor growth and development in multiple cancers. DPCPX (a selective adenosine A1 receptor antagonist), a specific ADORA1 antagonist, has shown antitumor effects in many cancer types. Nevertheless, the function of DPCPX in nasopharyngeal carcinoma (NPC) still remains to be unraveled. In this study, we investigated the functional role of DPCPX on NPC cells. We found that DPCPX promotes NPC cells growth inhibition. DPCPX induced Bim-dependent apoptosis in NPC cells irrespective of p53 status via the FoxO3a pathway following PI3K/AKT inhibition. Furthermore, DPCPX enhanced the antitumor effect of cisplatin, 5-FU and Paclitaxel in NPC. Xenograft experiment revealed that deficiency of Bim in vivo stalls apoptosis and antitumor activity of DPCPX. In conclusion, the PI3K/AKT/FoxO3a/Bim axis plays a critical role in the anticancer effects of DPCPX in NPC.

m6A in the Signal Transduction Network

In response to environmental changes, signaling pathways rewire gene expression programs through transcription factors. Epigenetic modification of the transcribed RNA can be another layer of gene expression regulation. N6-adenosine methylation (m6A) is one of the most common modifications on mRNA. It is a reversible chemical mark catalyzed by the enzymes that deposit and remove methyl groups. m6A recruits effector proteins that determine the fate of mRNAs through changes in splicing, cellular localization, stability, and translation efficiency. Emerging evidence shows that key signal transduction pathways including TGFβ (transforming growth factor-β), ERK (extracellular signal-regulated kinase), and mTORC1 (mechanistic target of rapamycin complex 1) regulate downstream gene expression through m6A processing. Conversely, m6A can modulate the activity of signal transduction networks via m6A modification of signaling pathway genes or by acting as a ligand for receptors. In this review, we discuss the current understanding of the crosstalk between m6A and signaling pathways and its implication for biological systems.

Drugs Targeting the A3 Adenosine Receptor: Human Clinical Study Data

The A3 adenosine receptor (A3AR) is overexpressed in pathological human cells. Piclidenoson and namodenoson are A3AR agonists with high affinity and selectivity to A3AR. Both induce apoptosis of cancer and inflammatory cells via a molecular mechanism entailing deregulation of the Wnt and the NF-κB signaling pathways. Our company conducted phase I studies showing the safety of these 2 molecules. In the phase II studies in psoriasis patients, piclidenoson was safe and demonstrated efficacy manifested in significant improvements in skin lesions. Namodenoson is currently being developed to treat liver cancer, where prolonged overall survival was observed in patients with advanced liver disease and a Child-Pugh B score of 7. A pivotal phase III study in this patient population has been approved by the FDA and the EMA and is currently underway. Namodenoson is also being developed to treat non-alcoholic steatohepatitis (NASH). A Phase IIa study has been successfully concluded and showed that namodenoson has anti-inflammatory, anti-fibrosis, and anti-steatosis effects. A phase IIb study in NASH is currently enrolling patients. In conclusion, A3AR agonists are promising drug candidates in advanced stages of clinical development and demonstrate safety and efficacy in their targeted indications.

Purinergic receptors modulators: An emerging pharmacological tool for disease management

Purinergic signaling is mediated through extracellular nucleotides (adenosine 5'-triphosphate, uridine-5'-triphosphate, adenosine diphosphate, uridine-5'-diphosphate, and adenosine) that serve as signaling molecules. In the early 1990s, purines and pyrimidine receptors were cloned and characterized drawing the attention of scientists toward this aspect of cellular signaling. This signaling pathway is comprised of four subtypes of adenosine receptors (P1), eight subtypes of G-coupled protein receptors (P2YRs), and seven subtypes of ligand-gated ionotropic receptors (P2XRs). In current studies, the pathophysiology and therapeutic potentials of these receptors have been focused on. Various ligands, modulating the functions of purinergic receptors, are in current clinical practices for the treatment of various neurodegenerative disorders and cardiovascular diseases. Moreover, several purinergic receptors ligands are in advanced phases of clinical trials as a remedy for depression, epilepsy, autism, osteoporosis, atherosclerosis, myocardial infarction, diabetes, irritable bowel syndrome, and cancers. In the present study, agonists and antagonists of purinergic receptors have been summarized that may serve as pharmacological tools for drug design and development.

A2A Adenosine Receptor: A Possible Therapeutic Target for Alzheimer's Disease by Regulating NLRP3 Inflammasome Activity?

The A2A adenosine receptor, a member of the P1 purinergic receptor family, plays a crucial role in the pathophysiology of different neurodegenerative illnesses, including Alzheimer’s disease (AD). It regulates both neurons and glial cells, thus modulating synaptic transmission and neuroinflammation. AD is a complex, progressive neurological condition that is the leading cause of dementia in the world’s old population (>65 years of age). Amyloid peptide-β extracellular accumulation and neurofibrillary tangles constitute the principal etiologic tracts, resulting in apoptosis, brain shrinkage, and neuroinflammation. Interestingly, a growing body of evidence suggests a role of NLRP3 inflammasome as a target to treat neurodegenerative diseases. It represents a tripartite multiprotein complex including NLRP3, ASC, and procaspase-1. Its activation requires two steps that lead with IL-1β and IL-18 release through caspase-1 activation. NLRP3 inhibition provides neuroprotection, and in recent years adenosine, through the A2A receptor, has been reported to modulate NLRP3 functions to reduce organ damage. In this review, we describe the role of NLRP3 in AD pathogenesis, both alone and in connection to A2A receptor regulation, in order to highlight a novel approach to address treatment of AD.

Pathophysiological Role and Medicinal Chemistry of A2A Adenosine Receptor Antagonists in Alzheimer's Disease

The A_2A adenosine receptor is a protein belonging to a family of four GPCR adenosine receptors. It is involved in the regulation of several pathophysiological conditions in both the central nervous system and periphery. In the brain, its localization at pre- and postsynaptic level in striatum, cortex, hippocampus and its effects on glutamate release, microglia and astrocyte activation account for a crucial role in neurodegenerative diseases, including Alzheimer’s disease (AD). This ailment is considered the main form of dementia and is expected to exponentially increase in coming years. The pathological tracts of AD include amyloid peptide-β extracellular accumulation and tau hyperphosphorylation, causing neuronal cell death, cognitive deficit, and memory loss. Interestingly, in vitro and in vivo studies have demonstrated that A_2A adenosine receptor antagonists may counteract each of these clinical signs, representing an important new strategy to fight a disease for which unfortunately only symptomatic drugs are available. This review offers a brief overview of the biological effects mediated by A_2A adenosine receptors in AD animal and human studies and reports the state of the art of A_2A adenosine receptor antagonists currently in clinical trials. As an original approach, it focuses on the crucial role of pharmacokinetics and ability to pass the blood–brain barrier in the discovery of new agents for treating CNS disorders. Considering that A_2A receptor antagonist istradefylline is already commercially available for Parkinson’s disease treatment, if the proof of concept of these ligands in AD is confirmed and reinforced, it will be easier to offer a new hope for AD patients.

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.

Interaction of A3 adenosine receptor ligands with the human multidrug transporter ABCG2

Various adenosine receptor nucleoside-like ligands were found to modulate ATP hydrolysis by the multidrug transporter ABCG2. Both ribose-containing and rigidified (N)-methanocarba nucleosides (C2-, N6- and 5'-modified), as well as adenines (C2-, N6-, and deaza modified), were included. 57 compounds out of 63 tested either stimulated (50) or inhibited (7) basal ATPase activity. Structure-activity analysis showed a separation of adenosine receptor and ABCG2 activities. The 7-deaza modification had favorable effects in both (N)-methanocarba nucleosides and adenines. Adenine 37c (MRS7608) and (N)-methanocarba 7-deaza-5'-ethyl ester 60 (MRS7343) were found to be potent stimulators of ABCG2 ATPase activity with EC50 values of 13.2 ± 1.7 and 13.2 ± 2.2 nM, respectively. Both had affinity in the micromolar range for A3 adenosine receptor and lacked the 5'-amide agonist-enabling group (37c was reported as a weak A3 antagonist, Ki 6.82 μM). Compound 60 significantly inhibited ABCG2 substrate transport (IC50 0.44 μM). Docking simulations predicted the interaction of 60 with 21 residues in the drug-binding pocket of ABCG2.

2‑Cl‑IB‑MECA regulates the proliferative and drug resistance pathways, and facilitates chemosensitivity in pancreatic and liver cancer cell lines

Specific A₃ adenosine receptor (A₃AR) agonist, 2-chloro-N6-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine (2-Cl-IB-MECA), demonstrates anti-proliferative effects on various types of tumor. In the present study, the cytotoxicity of 2-Cl-IB-MECA was analyzed in a panel of tumor and non-tumor cell lines and its anticancer mechanisms in JoPaca-1 pancreatic and Hep-3B hepatocellular carcinoma cell lines were also investigated. Initially, decreased tumor cell proliferation, cell accumulation in the G₁ phase and inhibition of DNA and RNA synthesis was found. Furthermore, western blot analysis showed decreased protein expression level of β-catenin, patched1 (Ptch1) and glioma-associated oncogene homolog zinc finger protein 1 (Gli1), which are components of the Wnt/β-catenin and Sonic hedgehog/Ptch/Gli transduction pathways. In concordance with these findings, the protein expression levels of cyclin D1 and c-Myc were reduced. Using a luciferase assay, it was revealed for the first time a decrease in β-catenin transcriptional activity, as an early event following 2-Cl-IB-MECA treatment. In addition, the protein expression levels of multidrug resistance-associated protein 1 and P-glycoprotein (P-gp) were reduced and the P-gp xenobiotic efflux function was also reduced. Next, the enhancing effects of 2-Cl-IB-MECA on the cytotoxicity of conventional chemotherapy was investigated. It was found that 2-Cl-IB-MECA enhanced carboplatin and doxorubicin cytotoxic effects in the JoPaca-1 and Hep-3B cell lines, and a greater synergy was found in the highly tumorigenic JoPaca-1 cell line. This provides a novel in vitro rationale for the utiliza- tion of 2-Cl-IB-MECA in combination with chemotherapeutic agents, not only for hepatocellular carcinoma, but also for pancreatic cancer. Other currently used conventional chemo- therapeutics, fluorouracil and gemcitabine, showed synergy only when combined with high doses of 2-Cl-IB-MECA. Notably, experiments with A₃AR-specific antagonist, N-[9-Chloro-2-(2-furanyl)(1,2,4)-triazolo(1,5-c)quinazolin-5-yl] benzene acetamide, revealed that 2-Cl-IB-MECA had antitumor effects via both A₃AR-dependent and -independent pathways. In conclusion, the present study identified novel antitumor mechanisms of 2-Cl-IB-MECA in pancreatic and hepatocellular carcinoma in vitro that further underscores the importance of A₃AR agonists in cancer therapy.

Cancer biology and molecular genetics of A3 adenosine receptor

A3 adenosine receptor (A3AR) is a cell membrane protein, which has been found to be overexpressed in a large number of cancer types. This receptor plays an important role in cancer by interacting with adenosine. Specifically, A3AR has a dual nature in different pathophysiological conditions, as it is expressed according to tissue type and stimulated by an adenosine dose-dependent manner. A3AR activation leads to tumor growth, cell proliferation and survival in some cases, while triggering cytostatic and apoptotic pathways in others. This review aims to describe the most relevant aspects of A3AR activation and its ligands whereas it summarizes A3AR activities in cancer. Progress in the field of A3AR modulators, with a potential therapeutic role in cancer treatment are reported, as well.

Synthesis and evaluation of adenosine derivatives as A1, A2A, A2B and A3 adenosine receptor ligands containing boron clusters as phenyl isosteres and selective A3 agonists

A series of adenosine and 2'-deoxyadenosine pairs modified with a 1,12-dicarba-closo-dodecaborane cluster or alternatively with a phenyl group at the same position was synthesized, and their affinity was determined at A1, A2A, A2B and A3 adenosine receptors (ARs). While AR affinity differences were noted, a general tendency to preferentially bind A3 AR over other ARs was observed for most tested ligands. In particular, 5'-ethylcarbamoyl-N6-(3-phenylpropyl)adenosine (18), N6-(3-phenylpropyl)-2-chloroadenosine (24) and N6-(3-phenylpropyl)adenosine (40) showed nanomolar A3 affinity (Ki 4.5, 6.4 and 7.5 nM, respectively). Among the boron cluster-containing compounds, the highest A3 affinity (Ki 206 nM) was for adenosine derivative 41 modified at C2. In the matched molecular pairs, analogs bearing boron clusters were found to show lower binding affinity for adenosine receptors than the corresponding phenyl analogs. Nevertheless, interestingly, several boron cluster modified adenosine ligands showed significantly higher A3 receptor selectivity than the corresponding phenyl analogs: 7vs. 8, 15vs. 16, 17vs. 18.

Remote but not Distant: a Review on Experimental Models and Clinical Trials in Remote Ischemic Conditioning as Potential Therapy in Ischemic Stroke

Stroke is one of the main causes of neurological disability worldwide and the second cause of death in people over 65 years old, resulting in great economic and social burden. Ischemic stroke accounts for 85% of total cases, and the approved therapies are based on re-establishment of blood flow, and do not directly target brain parenchyma. Thus, novel therapies are urgently needed. In this review, limb remote ischemic conditioning (RIC) is revised and discussed as a potential therapy against ischemic stroke. The review targets both (i) fundamental research based on experimental models and (ii) clinical research based on clinical trials and human interventional studies with healthy volunteers. Moreover, it also presents two approaches concerning RIC mechanisms in stroke: (i) description of the underlying cerebral cellular and molecular mechanisms triggered by limb RIC that promote neuroprotection against stroke induced damage and (ii) the identification of signaling factors involved in inter-organ communication following RIC procedure. Limb to brain remote signaling can occur via circulating biochemical factors, immune cells, and/or stimulation of autonomic nervous system. In this review, these three hypotheses are explored in both humans and experimental models. Finally, the challenges involved in translating experimentally generated scientific knowledge to a clinical setting are also discussed.

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.