Targeting CD73 and downstream adenosine receptor signaling in triple-negative breast cancer

Design, synthesis and structure-activity relationship of malonic acid non-nucleoside derivatives as potent CD73 inhibitors

High levels of extracellular adenosine in tumor microenvironment (TME) has extensive immunosuppressive effect. CD73 catalyzes the conversion of AMP into adenosine and regulates its production. Inhibiting CD73 can reduce the level of adenosine and reverse adenosine-mediated immune suppression. Therefore, CD73 has emerged as a valuable target for cancer immunotherapy. Here, a new series of malonic acid non-nucleoside derivatives were designed, synthesized and evaluated as CD73 inhibitors. Among them, compounds 18 and 19 exhibited significant inhibition activities against hCD73 with IC50 values of 0.28 μM and 0.10 μM, respectively, suggesting the feasibility of replacing the benzotriazole moiety in the lead compound. This study explored the novelty and structural diversity of CD73 inhibitors.

CD73 contributes to the pathogenesis of fusion-negative rhabdomyosarcoma through the purinergic signaling pathway

Rhabdomyosarcoma (RMS) is the most common type of soft tissue sarcoma in children and adolescents. Fusion-negative RMS (FN-RMS) accounts for more than 80% of all RMS cases. The long-term event-free survival rate for patients with high-grade FN-RMS is below 30%, highlighting the need for improved therapeutic strategies. CD73 is a 5' ectonucleotidase that hydrolyzes AMP to adenosine and regulates the purinergic signaling pathway. We found that CD73 is elevated in FN-RMS tumors that express high levels of TWIST2. While high expression of CD73 contributes to the pathogenesis of multiple cancers, its role in FN-RMS has not been investigated. We found that CD73 knockdown decreased FN-RMS cell growth while up-regulating the myogenic differentiation program. Moreover, mutation of the catalytic residues of CD73 rendered the protein enzymatically inactive and abolished its ability to stimulate FN-RMS growth. Overexpression of wildtype CD73, but not the catalytically inactive mutant, in CD73 knockdown FN-RMS cells restored their growth capacity. Likewise, treatment with an adenosine receptor A2A-B agonist partially rescued FN-RMS cell proliferation and bypassed the CD73 knockdown defective growth phenotype. These results demonstrate that the catalytic activity of CD73 contributes to the pathogenic growth of FN-RMS through the activation of the purinergic signaling pathway. Therefore, targeting CD73 and the purinergic signaling pathway represents a potential therapeutic approach for FN-RMS patients.

Immune Checkpoint Blockade Therapy for Breast Cancer: Lessons from Epithelial-Mesenchymal Transition

Immune checkpoint blockade therapies have generated efficacious responses in certain tumor types; however, the responses of breast carcinomas have been largely limited. Moreover, the identity of various parameters that can predict responses to immunotherapies, and at the same time, serve as putative biomarkers that can be therapeutically targeted to enhance the effectiveness of immunotherapies for breast cancers, remains to be comprehensively delineated. Activation of epithelial-mesenchymal plasticity in cancer cells, including those of the breast, increases their tumor-initiating potential and promotes their aggressiveness and resistance to multiple treatment regimens. Moreover, the residence of cancer cells in alternating epithelial or mesenchymal plastic phenotypic states can also influence their immuno-modulatory properties and susceptibilities to immune checkpoint blockade therapies. In this current opinion, we discuss the lessons that can be learnt from epithelial-mesenchymal transition to potentiate the efficacy of immunotherapy for breast cancers. We also discuss strategies to sensitize more-mesenchymal cancer cells to anti-tumor immunity and immune checkpoint blockade therapies, with the hope that these can serve as new translational avenues for the treatment of human breast tumors.

Endosome Traffic Modulates Pro-Inflammatory Signal Transduction in CD4+ T Cells-Implications for the Pathogenesis of Systemic Lupus Erythematosus

Endocytic recycling regulates the cell surface receptor composition of the plasma membrane. The surface expression levels of the T cell receptor (TCR), in concert with signal transducing co-receptors, regulate T cell responses, such as proliferation, differentiation, and cytokine production. Altered TCR expression contributes to pro-inflammatory skewing, which is a hallmark of autoimmune diseases, such as systemic lupus erythematosus (SLE), defined by a reduced function of regulatory T cells (Tregs) and the expansion of CD4⁺ helper T (Th) cells. The ensuing secretion of inflammatory cytokines, such as interferon-γ and interleukin (IL)-4, IL-17, IL-21, and IL-23, trigger autoantibody production and tissue infiltration by cells of the adaptive and innate immune system that induce organ damage. Endocytic recycling influences immunological synapse formation by CD4⁺ T lymphocytes, signal transduction from crosslinked surface receptors through recruitment of adaptor molecules, intracellular traffic of organelles, and the generation of metabolites to support growth, cytokine production, and epigenetic control of DNA replication and gene expression in the cell nucleus. This review will delineate checkpoints of endosome traffic that can be targeted for therapeutic interventions in autoimmune and other disease conditions.

Discovery of a Series of Potent, Selective, and Orally Bioavailable Nucleoside Inhibitors of CD73 That Demonstrates In Vivo Antitumor Activity

CD73 (ecto-5'-nucleotidase) has emerged as an attractive target for cancer immunotherapy of many cancers. CD73 catalyzes the hydrolysis of adenosine monophosphate (AMP) into highly immunosuppressive adenosine that plays a critical role in tumor progression. Herein, we report our efforts in developing orally bioavailable and highly potent small-molecule CD73 inhibitors from the reported hit molecule 2 to lead molecule 20 and then finally to compound 49. Compound 49 was able to reverse AMP-mediated suppression of CD8⁺ T cells and completely inhibited CD73 activity in serum samples from various cancer patients. In preclinical in vivo studies, orally administered 49 showed a robust dose-dependent pharmacokinetic/pharmacodynamic (PK/PD) relationship that correlated with efficacy. Compound 49 also demonstrated the expected immune-mediated antitumor mechanism of action and was efficacious upon oral administration not only as a single agent but also in combination with either chemotherapeutics or checkpoint inhibitor in the mouse tumor model.

Pan-cancer analysis identifies NT5E as a novel prognostic biomarker on cancer-associated fibroblasts associated with unique tumor microenvironment

Background: Ecto-5'-nucleotidase (NT5E) encodes the cluster of differentiation 73 (CD73), whose overexpression contributes to the formation of immunosuppressive tumor microenvironment and is related to exacerbated prognosis, increased risk of metastasis and resistance to immunotherapy of various tumors. However, the prognostic significance of NT5E in pan-cancer is obscure so far. Methods: We explored the expression level of NT5E in cancers and adjacent tissues and revealed the relationship between the NT5E expression level and clinical outcomes in pan-cancer by utilizing the UCSC Xena database. Then, correlation analyses were performed to evaluate the relationship between NT5E expression and immune infiltration level via EPIC, MCP-counter and CIBERSORT methods, and the enrichment analysis were employed to identify NT5E-interacting molecules and functional pathways. Furthermore, we conducted single-cell analysis to explore the potential role of NT5E on single-cell level based on the CancerSEA database. Meanwhile, gene set enrichment analysis (GSEA) in single-cell level was also conducted in TISCH database and single-cell signature explorer was utilized to evaluate the epithelial-mesenchymal transition (EMT) level in each cell type. Results: The expression level of NT5E was aberrant in almost all cancer types, and was correlated with worse prognosis in several cancers. Notably, NT5E overexpression was related to worse overall survival (OS) in pancreatic adenocarcinoma (PAAD), head and neck squamous cell carcinoma (HNSC), mesothelioma (MESO), stomach adenocarcinoma (STAD), uveal melanoma (UVM) and cervical squamous cell carcinoma and endocervical adenocarcinoma (CESC) (p < 0.01). NT5E-related immune microenvironment analysis revealed that NT5E is associated positively with the degree of infiltration of cancer-associated fibroblasts (CAFs) and endothelial cells in most cancers. Enrichment analysis of cellular component (CC) demonstrated the critical part of NT5E played in cell-substrate junction, cell-substrate adherens junction, focal adhesion and external side of plasma membrane. Finally, single-cell analysis of NT5E illuminated that EMT function of CAFs was elevated in basal cell carcinoma (BCC), skin cutaneous melanoma (SKCM), HNSC and PAAD. Conclusion: NT5E could serve as a potential prognostic biomarker for cancers. The potential mechanism may be related to the upregulated EMT function of CAFs, which provides novel inspiration for immunotherapy by targeting CAFs with high NT5E expression.

A Novel Antagonistic CD73 Antibody for Inhibition of the Immunosuppressive Adenosine Pathway

Despite some impressive clinical results with immune checkpoint inhibitors, the majority of patients with cancer do not respond to these agents, in part due to immunosuppressive mechanisms in the tumor microenvironment. High levels of adenosine in tumors can suppress immune cell function, and strategies to target the pathway involved in its production have emerged. CD73 is a key enzyme involved in adenosine production. This led us to identify a novel humanized antagonistic CD73 antibody, mAb19, with distinct binding properties. mAb19 potently inhibits the enzymatic activity of CD73 in vitro, resulting in an inhibition of adenosine formation and enhanced T-cell activation. We then investigated the therapeutic potential of combining CD73 antagonism with other immune modulatory and chemotherapeutic agents. Combination of mAb19 with a PD-1 inhibitor increased T-cell activation in vitro Interestingly, this effect could be further enhanced with an agonist of the adenosine receptor ADORA3. Adenosine levels were found to be elevated upon doxorubicin treatment in vivo, which could be blocked by CD73 inhibition. Combining CD73 antagonism with doxorubicin resulted in superior responses in vivo Furthermore, a retrospective analysis of rectal cancer patient samples demonstrated an upregulation of the adenosine pathway upon chemoradiation, providing further rationale for combining CD73 inhibition with chemotherapeutic agents.This study demonstrates the ability of a novel CD73 antibody to enhance T-cell function through the potent suppression of adenosine levels. In addition, the data highlight combination opportunities with standard of care therapies as well as with an ADORA3 receptor agonist to treat patients with solid tumors.

Currently Used Laboratory Methodologies for Assays Detecting PD-1, PD-L1, PD-L2 and Soluble PD-L1 in Patients with Metastatic Breast Cancer

Simple Summary

Several methods targeting the programmed death protein-1 (PD-1) axis have been developed and evaluated for the detection of immune checkpoint levels that are strongly involved in immunotherapy for patients with metastatic breast cancer. Variations in different assays used in diverse studies have affected their result interpretation and clinical utility. When applying these assays to the laboratory, a comprehensive understanding of the characteristics of them should be recognized. We reviewed applied laboratory techniques for detecting PD-1, PD-ligand (L)1, PD-L2, and soluble PD-L1, which are important for selecting metastatic cancer patients for immunotherapy. Advances in methodologies according to the epoch are also investigated to gain insight into immunologic techniques and to facilitate appropriate laboratory settings for evaluating the PD-1 axis status, which are useful for estimating outcomes and planning patient-tailored immunotherapy strategies.

Abstract

Approximately 20% of breast cancer (BC) patients suffer from distant metastasis. The incidence and prevalence rates of metastatic BC have increased annually. Immune checkpoint inhibitors are an emerging area of treatment, especially for metastatic patients with poor outcomes. Several antibody drugs have been developed and approved for companion testing of the programmed death protine-1 (PD-1) axis. We reviewed currently used laboratory methodologies for assays determining PD-1 axis to provide a comprehensive understanding of principles, advantages, and drawbacks involved in their implementation. The most commonly used method is immunohistochemistry (92.9%) for PD-L1 expression using tissue samples (96.4%). The commonly used anti-PD-L1 antibody clone were commercially available 22C3 (30.8%), SP142 (19.2%), SP263 (15.4%), and E1L3N (11.5%). Enzyme-linked immunosorbent assay and electrochemiluminescent immunoassay that target soluble PD-ligand (L)1 were developed and popularized in 2019–2021, in contrast to 2016–2018. Easy accessibility and non-invasiveness due to the use of blood samples, quantitative outputs, and relatively rapid turnaround times make them more preferable. Regarding scoring methods, a combination of tumor and immune cells (45.5% in 2016–2018 to 57.1% in 2019–2021) rather than each cell alone became more popular. Information about antibody clones, platforms, scoring methods, and related companion drugs is recommended for reporting PD-L1 expression.

Targeting the purinergic pathway in breast cancer and its therapeutic applications

Breast cancer (BC) is the most frequent cause of death among women, representing a global public health problem. Here, we aimed to discuss the correlation between the purinergic system and BC, recognizing therapeutic targets. For this, we analyzed the interaction of extracellular nucleotides and nucleosides with the purinergic receptors P1 and P2, as well as the influence of ectonucleotidase enzymes (CD39 and CD73) on tumor progression. A comprehensive bibliographic search was carried out. The relevant articles for this review were found in the PubMed, Scielo, Lilacs, and ScienceDirect databases. It was observed that among the P1 receptors, the A1, A2A, and A2B receptors are involved in the proliferation and invasion of BC, while the A3 receptor is related to the inhibition of tumor growth. Among the P2 receptors, the P2X7 has a dual function. When activated for a short time, it promotes metastasis, but when activated for long periods, it is related to BC cell death. P2Y2 and P2Y6 receptors are related to BC proliferation and invasiveness. Also, the high expression of CD39 and CD73 in BC is strongly related to a worse prognosis. The receptors and ectonucleotidases involved with BC become possible therapeutic targets. Several purinergic pathways have been found to be involved in BC cell survival and progression. In this review, in addition to analyzing the pathways involved, we reviewed the therapeutic interventions already studied for BC related to the purinergic system, as well as to other possible therapeutic targets.

Direct and Indirect Regulators of Epithelial-Mesenchymal Transition-Mediated Immunosuppression in Breast Carcinomas

The epithelial-to-mesenchymal transition, which conveys epithelial (E) carcinoma cells to quasi-mesenchymal (qM) states, enables them to metastasize and acquire resistance to certain treatments. Murine tumors composed of qM mammary carcinoma cells assemble an immunosuppressive tumor microenvironment (TME) and develop resistance to anti-CTLA4 immune-checkpoint blockade (ICB) therapy, unlike their E counterparts. Importantly, minority populations of qM cells within a tumor can cross-protect their more E neighbors from immune attack. The underlying mechanisms of immunosuppression and cross-protection have been unclear. We demonstrate that abrogation of qM carcinoma cell-derived factors (CD73, CSF1, or SPP1) prevents the assembly of an immunosuppressive TME and sensitizes otherwise refractory qM tumors partially or completely to anti-CTLA4 ICB. Most strikingly, mixed tumors in which minority populations of carcinoma cells no longer express CD73 are now sensitized to anti-CTLA4 ICB. Finally, loss of CD73 also enhances the efficacy of anti-CTLA4 ICB during the process of metastatic colonization. SIGNIFICANCE: Minority populations of qM carcinoma cells, which likely reside in human breast carcinomas, can cross-protect their E neighbors from immune attack. Understanding the mechanisms by which qM carcinoma cells resist antitumor immune attack can help identify signaling channels that can be interrupted to potentiate the efficacy of checkpoint blockade immunotherapies.This article is highlighted in the In This Issue feature, p. 995.

Orally Bioavailable Small-Molecule CD73 Inhibitor (OP-5244) Reverses Immunosuppression through Blockade of Adenosine Production

The adenosinergic pathway represents an attractive new therapeutic approach in cancer immunotherapy. In this pathway, ecto-5-nucleotidase CD73 has the unique function of regulating production of immunosuppressive adenosine (ADO) through the hydrolysis of AMP. CD73 is overexpressed in many cancers, resulting in elevated levels of ADO that correspond to poor patient prognosis. Therefore, reducing the level of ADO via inhibition of CD73 is a potential strategy for treating cancers. Based on the binding mode of adenosine 5'-(α,β-methylene)diphosphate (AOPCP) with human CD73, we designed a series of novel monophosphonate small-molecule CD73 inhibitors. Among them, OP-5244 (35) proved to be a highly potent and orally bioavailable CD73 inhibitor. In preclinical studies, 35 completely inhibited ADO production in both human cancer cells and CD8⁺ T cells. Furthermore, 35 lowered the ratio of ADO/AMP significantly and reversed immunosuppression in mouse models, indicating its potential as an in vivo tool compound for further development.

Extracellular NAD+ enhances PARP-dependent DNA repair capacity independently of CD73 activity

Changes in nicotinamide adenine dinucleotide (NAD⁺) levels that compromise mitochondrial function trigger release of DNA damaging reactive oxygen species. NAD⁺ levels also affect DNA repair capacity as NAD⁺ is a substrate for PARP-enzymes (mono/poly-ADP-ribosylation) and sirtuins (deacetylation). The ecto-5′-nucleotidase CD73, an ectoenzyme highly expressed in cancer, is suggested to regulate intracellular NAD⁺ levels by processing NAD⁺ and its bio-precursor, nicotinamide mononucleotide (NMN), from tumor microenvironments, thereby enhancing tumor DNA repair capacity and chemotherapy resistance. We therefore investigated whether expression of CD73 impacts intracellular NAD⁺ content and NAD⁺-dependent DNA repair capacity. Reduced intracellular NAD⁺ levels suppressed recruitment of the DNA repair protein XRCC1 to sites of genomic DNA damage and impacted the amount of accumulated DNA damage. Further, decreased NAD⁺ reduced the capacity to repair DNA damage induced by DNA alkylating agents. Overall, reversal of these outcomes through NAD⁺ or NMN supplementation was independent of CD73. In opposition to its proposed role in extracellular NAD⁺ bioprocessing, we found that recombinant human CD73 only poorly processes NMN but not NAD⁺. A positive correlation between CD73 expression and intracellular NAD⁺ content could not be made as CD73 knockout human cells were efficient in generating intracellular NAD⁺ when supplemented with NAD⁺ or NMN.

Clinical significance of CD73 in triple-negative breast cancer: multiplex analysis of a phase III clinical trial

Background

CD73 is an ecto-enzyme that promotes tumor immune escape through the production of immunosuppressive extracellular adenosine in the tumor microenvironment. Several CD73 inhibitors and adenosine receptor antagonists are being evaluated in phase I clinical trials.

Patients and methods

Full-face sections from formalin-fixed paraffin-embedded primary breast tumors from 122 samples of triple-negative breast cancer (TNBC) from the BIG 02-98 adjuvant phase III clinical trial were included in our analysis. Using multiplex immunofluorescence and image analysis, we assessed CD73 protein expression on tumor cells, tumor-infiltrating leukocytes and stromal cells. We investigated the associations between CD73 protein expression with disease-free survival (DFS), overall survival (OS) and the extent of tumor immune infiltration.

Results

Our results demonstrated that high levels of CD73 expression on epithelial tumor cells were significantly associated with reduced DFS, OS and negatively correlated with tumor immune infiltration (Spearman's R= -0.50, P < 0.0001). Patients with high levels of CD73 and low levels of tumor-infiltrating leukocytes had the worse clinical outcome.

Conclusions

Taken together, our study provides further support that CD73 expression is associated with a poor prognosis and reduced anti-tumor immunity in human TNBC and that targeting CD73 could be a promising strategy to reprogram the tumor microenvironment in this BC subtype.

Inhibition of the adenosinergic pathway: the indispensable part of oncological therapy in the future

In recent years, immunotherapy has produced many unexpected breakthroughs in oncological therapy; however, it still has many deficiencies. For example, the number of patients who are unresponsive to anti-programmed death-ligand 1 (PD-L1), anti-cytotoxic T-like antigen-4 (CTLA4), and anti-programmed death-1 (PD1) therapies cannot be ignored, and the search for an undiscovered immunosuppressive pathway is imminent. Five decades ago, researchers found that activation of the adenosinergic pathway was negatively correlated with prognosis in many cancers. This review describes the entire process of the adenosinergic pathway in the tumor microenvironment and the mechanism of immunosuppression, which promotes tumor metastasis and drug resistance. Additionally, the review explores factors that regulate this pathway, including signaling factors secreted by the tumor microenvironment and certain anti-tumor drugs. Additionally, the combination of adenosinergic pathway inhibitors with chemotherapy, checkpoint blockade therapy, and immune cell-based therapy is summarized. Finally, certain issues regarding treatment via inhibition of this pathway and the use of targeted nanoparticles to reduce adverse reactions in patients are put forward in this review. Graphical Abstract The inhibitors of adenosinergic pathway loaded nanoparticles enter tumor tissue through EPR effect, and inhibit adenosinergic pathway to enhance or restore the effect of immune checkpoint blockade therapy, chemotherapies and immune cell-based therapy. Note: EPR means enhanced penetration and retention, × means blockade.

Multiple steps determine CD73 shedding from RPE: lipid raft localization, ARA1 interaction, and MMP-9 up-regulation

Physiologically, retinal pigment epithelium (RPE) expresses high levels of CD73 in their membrane, converting AMP to immune suppressive adenosine, mediates an anti-inflammatory effect. However, after being exposed to inflammatory factors, RPE rapidly becomes CD73-negative cells, which render RPE's immune suppressive function and accelerate local inflammation. Here, we investigated the mechanism leading to the loss of membrane CD73 in RPE. We found the controversy that when membrane CD73 was significantly diminished in inflammatory RPE, Cd73 mRNA levels were not changed at all. It was further verified that, matrix metalloproteinase-9 (MMP-9) mediated the shedding of CD73 from the cell membrane of inflammatory RPE by catalyzing its K547/F548 site. However, MMP-9 could not catalyze uncomplexed CD73, the interaction of CD73 with adenosine receptor A1 subtype (ARA1) is necessary for being catalyzed by MMP-9. After being treated by LPS and TNF-α, the formation of CD73/ARA1 complex in RPE was verified by co-immunoprecipitation and FRET-based assays. It was also revealed that CD73 need to be localized in lipid rafts to be capable of interacting with ARA1, since CD73/ARA1 interaction and CD73 shedding were completely blocked by the addition of lipid raft synthesis inhibitor. As a conclusion, multiple steps are involved in CD73 shedding in RPE, including up-regulation of MMP-9 activity, localization of CD73 in lipid rafts, and the formation of CD73/ARA1 complex. Lipid rafts committed CD73 with high mobility, shuttled CD73 to ARA1 to form a complex, which was capable of being recognized and catalyzed by MMP-9.

Controlling the Immune Suppressor: Transcription Factors and MicroRNAs Regulating CD73/NT5E

The NT5E (CD73) molecule represents an ecto-5′-nucleotidase expressed on the cell surface of various cell types. Hydrolyzing extracellular adenosine monophosphate into adenosine and inorganic phosphate, NT5E performs numerous homeostatic functions in healthy organs and tissues. Importantly, NT5E can act as inhibitory immune checkpoint molecule, since free adenosine generated by NT5E inhibits cellular immune responses, thereby promoting immune escape of tumor cells. MicroRNAs (miRNAs) are small non-coding RNA molecules regulating gene expression on posttranscriptional level through binding to mRNAs, resulting in translational repression or degradation of the targeted mRNA molecule. In tumor cells, miRNA expression patterns are often altered which in turn might affect NT5E surface expression and eventually influence the efficacy of antitumor immune responses. This review describes the diverse roles of NT5E, summarizes current knowledge about transcription factors controlling NT5E expression, and highlights the significance of miRNAs involved in the posttranscriptional regulation of NT5E expression.

Adenosine A3 receptor elicits chemoresistance mediated by multiple resistance-associated protein-1 in human glioblastoma stem-like cells

MRP1 transporter correlates positively with glioma malignancy and the Multiple Drug Resistance (MDR) phenotype in Glioblastoma Multiforme (GBM). Evidence shows that the MRP1 transporter is controlled by the adenosine signalling axis. The aim of this study was to identify the role of adenosine on the MDR phenotype in Glioblastoma Stem-like Cells (GSCs), the cell population responsible for the tumorigenic and chemoresistance capabilities of this tumour. We found that GSCs have increased intrinsic capacity to generate extracellular adenosine, thus controlling MRP1 transporter expression and activity via activation of the adenosine A3 receptor (A3AR). We showed PI3K/Akt and MEK/ERK1/2 signaling pathways downstream A3AR to control MRP1 in GSCs. In vitro pharmacological blockade of A3AR had a chemosensitizing effect, enhancing the actions of antitumour drugs and decreasing cell viability and proliferation of GSCs. In addition, we produced an in vivo xenograft model by subcutaneous inoculation of human GSCs in NOD/SCID-IL2Rg null mice. Pharmacological blockade of A3AR generated a chemosensitizing effect, enhancing the effectiveness of the MRP1 transporter substrate, vincristine, reducing tumour size and the levels of CD44 and Nestin stem cell markers as well as the Ki-67 proliferation indicator. In conclusion, we demonstrated the chemosensitizing effect of A3AR blockade on GSCs.

A Robust Multiplex Mass Spectrometric Assay for Screening Small-Molecule Inhibitors of CD73 with Diverse Inhibition Modalities

CD73/Ecto-5'-nucleotidase is a membrane-tethered ecto-enzyme that works in tandem with CD39 to convert extracellular adenosine triphosphate (ATP) into adenosine. CD73 is highly expressed on various types of cancer cells and on infiltrating suppressive immune cells, leading to an elevated concentration of adenosine in the tumor microenvironment, which elicits a strong immunosuppressive effect. In preclinical studies, targeting CD73 with anti-CD73 antibody results in favorable antitumor effects. Despite initial studies using antibodies, inhibition of CD73 catalytic activity using small-molecule inhibitors may be more effective in lowering extracellular adenosine due to better tumor penetration and distribution. To screen small-molecule libraries, we explored multiple approaches, including colorimetric and fluorescent biochemical assays, and due to some limitations with these assays, we developed a mass spectrometry (MS)-based assay. Only the MS-based assay offers the sensitivity and dynamic range required for screening small-molecule libraries at a substrate concentration close to the K_m value of substrate and for evaluating the mode of binding of screening hits. To achieve a throughput suitable for high-throughput screening (HTS), we developed a RapidFire-tandem mass spectrometry (RF-MS/MS)-based multiplex assay. This assay allowed a large diverse compound library to be screened at a speed of 1536 reactions per 40-50 min.

Targeting of NT5E by miR-30b and miR-340 attenuates proliferation, invasion and migration of gallbladder carcinoma

MicroRNAs (miRNAs) have been closely associated with the proliferation, invasion and migration of various cancers, including gallbladder carcinoma (GBC). Previous studies have revealed dysregulation of miR-30b and miR-340 in many types of cancer. However, the role of miR-30b and miR-340 in the development and progression of GBC remains unclear. Moreover, epithelial-to-mesenchymal transition (EMT) has been gradually viewed as a significant contributor to tumor metastasis. In this study, the cell line GBC-SD was used and we explored that EMT promoted GBC cells invasion and migration and inhibited the expression level of miR-30b and miR-340 compared with the control. We showed that overexpression of miR-30b and miR-340 suppressed GBC cells proliferation, invasion and migration, as well as the expression of EMT-associated genes. In addition, we identified ecto-5'-nucleotidase (NT5E) as a common target of miR-30b and miR-340 using bioinformatics analysis and a luciferase assay. Further experiments found that exogenous expression of NT5E in GBC cells could partially reverse the inhibitory effect of miR-30b and miR-340 on cell proliferation, invasion and migration. Our findings suggest that NT5E-targeting miRNAs (miR-30b and miR-340) function as tumor suppressors and may represent promising therapeutic targets for GBC.

Switching off CD73: a way to boost the activity of conventional and targeted antineoplastic therapies

Over the past few years, several preclinical studies have highlighted the value of CD73 (ecto-5'-nucleotidase) as a potential therapeutic target for cancer therapy. Indeed, the pharmacological blockade of CD73, via monoclonal antibodies or small molecules, has promise in counteracting cancer development, growth and spread. Synergistic combinations of anti-CD73 drugs with conventional cancer treatments (i.e., chemotherapy, radiation therapy, immunotherapy, targeted therapy) have increased therapeutic potential. In this review, we discuss the potential synergistic effects of CD73 blockers and conventional antineoplastic therapies in the treatment of cancer.

Increased efficacy of a dendritic cell–based therapeutic cancer vaccine with adenosine receptor antagonist and CD73 inhibitor

Dendritic cells are important in initiating immune responses; therefore, a range of dendritic cell–based approaches have been established to induce immune response against cancer cells. However, the presence of immunosuppressive mediators such as adenosine in the tumor microenvironment reduces the efficacy of dendritic cell–based cancer immunotherapy. In this study, we investigated whether blockade of the A2A adenosine receptor with a selective antagonist and a CD73 inhibitor may increase the efficacy of a dendritic cell–based cancer vaccine. According to the findings, this therapeutic combination reduced tumor growth, prolonged survival of tumor-bearing mice, and enhanced specific antitumor immune responses. Thus, we suggest that targeting cancer-derived adenosine improves the outcomes of dendritic cell–based cancer immunotherapy.

The ectonucleotidases CD39 and CD73: Novel checkpoint inhibitor targets

Cancers are able to grow by subverting immune suppressive pathways, to prevent the malignant cells as being recognized as dangerous or foreign. This mechanism prevents the cancer from being eliminated by the immune system and allows disease to progress from a very early stage to a lethal state. Immunotherapies are newly developing interventions that modify the patient's immune system to fight cancer, by either directly stimulating rejection-type processes or blocking suppressive pathways. Extracellular adenosine generated by the ectonucleotidases CD39 and CD73 is a newly recognized "immune checkpoint mediator" that interferes with anti-tumor immune responses. In this review, we focus on CD39 and CD73 ectoenzymes and encompass aspects of the biochemistry of these molecules as well as detailing the distribution and function on immune cells. Effects of CD39 and CD73 inhibition in preclinical and clinical studies are discussed. Finally, we provide insights into potential clinical application of adenosinergic and other purinergic-targeting therapies and forecast how these might develop in combination with other anti-cancer modalities.

The ectonucleotidases CD39 and CD73: Novel checkpoint inhibitor targets

Cancers are able to grow by subverting immune suppressive pathways, to prevent the malignant cells as being recognized as dangerous or foreign. This mechanism prevents the cancer from being eliminated by the immune system and allows disease to progress from a very early stage to a lethal state. Immunotherapies are newly developing interventions that modify the patient's immune system to fight cancer, by either directly stimulating rejection-type processes or blocking suppressive pathways. Extracellular adenosine generated by the ectonucleotidases CD39 and CD73 is a newly recognized "immune checkpoint mediator" that interferes with anti-tumor immune responses. In this review, we focus on CD39 and CD73 ectoenzymes and encompass aspects of the biochemistry of these molecules as well as detailing the distribution and function on immune cells. Effects of CD39 and CD73 inhibition in preclinical and clinical studies are discussed. Finally, we provide insights into potential clinical application of adenosinergic and other purinergic-targeting therapies and forecast how these might develop in combination with other anti-cancer modalities.

The ectonucleotidases CD39 and CD73: Novel checkpoint inhibitor targets

Cancers are able to grow by subverting immune suppressive pathways, to prevent the malignant cells as being recognized as dangerous or foreign. This mechanism prevents the cancer from being eliminated by the immune system and allows disease to progress from a very early stage to a lethal state. Immunotherapies are newly developing interventions that modify the patient's immune system to fight cancer, by either directly stimulating rejection-type processes or blocking suppressive pathways. Extracellular adenosine generated by the ectonucleotidases CD39 and CD73 is a newly recognized "immune checkpoint mediator" that interferes with anti-tumor immune responses. In this review, we focus on CD39 and CD73 ectoenzymes and encompass aspects of the biochemistry of these molecules as well as detailing the distribution and function on immune cells. Effects of CD39 and CD73 inhibition in preclinical and clinical studies are discussed. Finally, we provide insights into potential clinical application of adenosinergic and other purinergic-targeting therapies and forecast how these might develop in combination with other anti-cancer modalities.

Targeting A2 adenosine receptors in cancer

Tumor cells use various ways to evade anti-tumor immune responses. Adenosine, a potent immunosuppressive metabolite, is often found elevated in the extracellular tumor microenvironment. Therefore, targeting adenosine-generating enzymes (CD39 and CD73) or adenosine receptors has emerged as a novel means to stimulate anti-tumor immunity. In particular, the A2 (A2a and A2b) adenosine receptors exhibit similar immunosuppressive and pro-angiogenic functions, yet have distinct biological roles in cancer. In this review, we describe the common and distinct biological consequences of A2a and A2b adenosine receptor signaling in cancer. We discuss recent pre-clinical studies and summarize the different mechanisms-of-action of adenosine-targeting drugs. We also review the rationale for combining inhibitors of the adenosine pathway with other anticancer therapies such immune checkpoint inhibitors, tumor vaccines, chemotherapy and adoptive T cell therapy.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

CD73s protection of epithelial integrity: Thinking beyond the barrier

The prevailing view of CD73 in cancer is that it is overexpressed in tumors and promotes cancer progression by dampening local T cell-mediated immune responses. We recently found that CD73 is down-regulated in poorly-differentiated and advanced stage endometrial carcinoma compared to normal endometrium and well-differentiated, early stage tumors. We revealed that CD73-generated adenosine induces a physiological response to protect epithelial integrity in well-differentiated, early stage endometrial carcinoma. The ability of CD73-generated adenosine to protect the barrier is not so different from its ability to induce immunosuppression and other physiological responses in cancerous tissues. In this commentary we examine the complexity of CD73 in cancer and suggest that a "one size fits all" approach to the role of CD73/adenosine in cancer is no longer warranted. Given that tumors often hijack normal cellular responses, we also provide consideration on how CD73s known role to protect barrier function may have implications in promoting tumor progression.

Purinergic regulation of the immune system

Cellular stress or apoptosis triggers the release of ATP, ADP and other nucleotides into the extracellular space. Extracellular nucleotides function as autocrine and paracrine signalling molecules by activating cell-surface P2 purinergic receptors that elicit pro-inflammatory immune responses. Over time, extracellular nucleotides are metabolized to adenosine, leading to reduced P2 signalling and increased signalling through anti-inflammatory adenosine (P1 purinergic) receptors. Here, we review how local purinergic signalling changes over time during tissue responses to injury or disease, and we discuss the potential of targeting purinergic signalling pathways for the immunotherapeutic treatment of ischaemia, organ transplantation, autoimmunity or cancer.

Inhibition of CD73 AMP hydrolysis by a therapeutic antibody with a dual, non-competitive mechanism of action

CD73 (ecto-5′-nucleotidase) has recently been established as a promising immuno-oncology target. Given its role in activating purinergic signaling pathways to elicit immune suppression, antagonizing CD73 (i.e., releasing the brake) offers a complimentary pathway to inducing anti-tumor immune responses. Here, we describe the mechanistic activity of a new clinical therapeutic, MEDI9447, a human monoclonal antibody that non-competitively inhibits CD73 activity. Epitope mapping, structural, and mechanistic studies revealed that MEDI9447 antagonizes CD73 through dual mechanisms of inter-CD73 dimer crosslinking and/or steric blocking that prevent CD73 from adopting a catalytically active conformation. To our knowledge, this is the first report of an antibody that inhibits an enzyme's function through 2 distinct modes of action. These results provide a finely mapped epitope that can be targeted for selective, potent, and non-competitive inhibition of CD73, as well as establish a strategy for inhibiting enzymes that function in both membrane-bound and soluble states.

CD73-adenosine: a next-generation target in immuno-oncology

Cancer immunotherapy has entered in a new era with the development of first-generation immune checkpoint inhibitors targeting the PD1/PD-L1 and CTLA-4 pathways. In this context, considerable research effort is being deployed to find the next generation of cancer immunotherapeutics. The CD73-adenosine axis constitutes one of the most promising pathways in immuno-oncology. We and others have demonstrated the immunosuppressive role of CD73-adenosine in cancer and established proof-of-concept that the targeted blockade of CD73 or adenosine receptors could effectively promote anti-tumor immunity and enhance the activity of first-generation immune checkpoint blockers. With Phase I clinical trials now underway evaluating anti-CD73 or anti-A2A therapies in cancer patients, we here discuss the fundamental, preclinical and clinical findings related to the role of the CD73-adenosinergic pathway in tumor immunity.

CD73-adenosine reduces immune responses and survival in ovarian cancer patients

Multiple non-redundant immunosuppressive pathways are active within the microenvironment of cancers to avoid tumor eradication by the immune system. Our results demonstrate that the CD73-adenosine pathway is a major immunosuppressive mechanism co-opted by ovarian tumors to escape antitumor immunity. In ovarian cancer patients, high CD73 expression correlates with poor outcome and impaired CD8(+) T cell immunosurveillance.

Triple negative breast cancer chemosensitivity and chemoresistance: current advances in biomarkers indentification

INTRODUCTION

Triple negative breast cancer (TNBC) is a heterogeneous clinicopathological entity constituting approximately 15 - 20% of all breast cancer (BC) patients. It shows high recurrence rate and poor prognosis. At this juncture, because of the lack of specific targeted therapies available and the development in patients of resistance to some therapeutic agents, clinical and translational settings have gained importance over the past decades.

AREAS COVERED

The development of novel, safe and effective alternatives for the treatment of TNBC are in high demand. Therefore, this review aims to summarize the state of the art of TNBC, its current therapies and potential therapeutic targets. In particular, focus is put on recent advances regarding the identification of emerging biomarkers as prognostic and/or predictive markers, including surrogate markers for molecular tumor subtyping and identifying potential responders to new therapies.

EXPERT OPINION

Effective development of informative markers could constitute an important armamentarium tool for identifying appropriate therapies to challenge the aggressiveness of TNBC.

CD73 is associated with poor prognosis in high-grade serous ovarian cancer

The cell surface nucleotidase CD73 is an immunosuppressive enzyme involved in tumor progression and metastasis. Although preclinical studies suggest that CD73 can be targeted for cancer treatment, the clinical impact of CD73 in ovarian cancer remains unclear. In this study, we investigated the prognostic value of CD73 in high-grade serous (HGS) ovarian cancer using gene and protein expression analyses. Our results demonstrate that high levels of CD73 are significantly associated with shorter disease-free survival and overall survival in patients with HGS ovarian cancer. Furthermore, high levels of CD73 expression in ovarian tumor cells abolished the good prognosis associated with intraepithelial CD8(+) cells. Notably, CD73 gene expression was highest in the C1/stromal molecular subtype of HGS ovarian cancer and positively correlated with an epithelial-to-mesenchymal transition gene signature. Moreover, in vitro studies revealed that CD73 and extracellular adenosine enhance ovarian tumor cell growth as well as expression of antiapoptotic BCL-2 family members. Finally, in vivo coinjection of ID8 mouse ovarian tumor cells with mouse embryonic fibroblasts showed that CD73 expression in fibroblasts promotes tumor immune escape and thereby tumor growth. In conclusion, our study highlights a role for CD73 as a prognostic marker of patient survival and also as a candidate therapeutic target in HGS ovarian cancers.

CD73 Expression Is an Independent Prognostic Factor in Prostate Cancer

PURPOSE

CD73 is an adenosine-generating ecto-enzyme that suppresses antitumor immunity in mouse models of cancer, including prostate cancer. Although high levels of CD73 are associated with poor prognosis in various types of cancer, the clinical impact of CD73 in prostate cancer remains unclear.

EXPERIMENTAL DESIGN

We evaluated the prognostic value of CD73 protein expression and CD8(+) cell density in 285 cases of prostate cancer on tissue microarray (TMA). Normal adjacent and tumor tissues were evaluated in duplicates.

RESULTS

Univariate and multivariate analyses revealed that high levels of CD73 in normal adjacent prostate epithelium were significantly associated with shorter biochemical recurrence (BCR)-free survival. Notably, CD73 expression in normal epithelium conferred a negative prognostic value to prostate-infiltrating CD8(+) cells. Surprisingly, high levels of CD73 in the tumor stroma were associated with longer BCR-free survival in univariate analysis. In vitro studies revealed that adenosine signaling inhibited NF-κB activity in human prostate cancer cells via A2B adenosine receptors. Consistent with these results, CD73 expression in the prostate tumor stroma negatively correlated with p65 expression in the nuclei of prostate tumor cells.

CONCLUSIONS

Our study revealed that CD73 is an independent prognostic factor in prostate cancer. Our data support a model in which CD73 expression in the prostate epithelium suppresses immunosurveillance by CD8(+) T cells, whereas CD73 expression in the tumor stroma reduces NF-κB signaling in tumor cells via A2B adenosine receptor signaling. CD73 expression, including in normal adjacent prostate epithelium, can thus effectively discriminate between aggressive and indolent forms of prostate cancer.

Adenosine inhibits tumor cell invasion via receptor-independent mechanisms

Extracellular adenosine mediates diverse anti-inflammatory, angiogenic, and other signaling effects via binding to adenosine receptors, and it also regulates cell proliferation and death via activation of the intrinsic signaling pathways. Given the emerging role of adenosine and other purines in tumor growth and metastasis, this study evaluated the effects of adenosine on the invasion of metastatic prostate and breast cancer cells. Treatment with low micromolar concentrations of adenosine, but not other nucleosides or adenosine receptor agonists, inhibited subsequent cell invasion and migration through Matrigel- and laminin-coated inserts. These inhibitory effects occurred via intrinsic receptor-independent mechanisms, despite the abundant expression of A2B adenosine receptors (ADORA2B). Extracellular nucleotides and adenosine were shown to be rapidly metabolized on tumor cell surfaces via sequential ecto-5'-nucleotidase (CD73/NT5E) and adenosine deaminase reactions with subsequent cellular uptake of nucleoside metabolites and their intracellular interconversion into ADP/ATP. This was accompanied by concurrent inhibition of AMP-activated protein kinase and other signaling pathways. No differences in the proliferation rates, cytoskeleton assembly, expression of major adhesion molecules [integrin-1β (ITGB1), CD44, focal adhesion kinase], and secretion of matrix metalloproteinases were detected between the control and treated cells, thus excluding the contribution of these components of invasion cascade to the inhibitory effects of adenosine. These data provide a novel insight into the ability of adenosine to dampen immune responses and prevent tumor invasion via two different, adenosine receptor-dependent and -independent mechanisms.

IMPLICATIONS

This study suggests that the combined targeting of adenosine receptors and modulation of intracellular purine levels can affect tumor growth and metastasis phenotypes.