Imidazo[2,1-b]thiazole based indoleamine-2,3-dioxygenase 1 (IDO1) inhibitor: Structure based design, synthesis, bio-evaluation and docking studies.

Indoleamine-2,3-dioxygenase 1 (IDO1) is an immunomodulatory enzyme known to catalyse the initial and rate limiting step of kynurenine pathway of l-tryptophan metabolism. IDO1 enzyme over expression plays a crucial role in progression of cancer, malaria, multiple sclerosis and other life-threatening diseases. Several efforts over the last two decades have been invested by the researchers for the discovery of different IDO1 inhibitors and the plasticity of the IDO1 enzyme ligand binding pocket provide ample opportunities to develop new heterocyclic scaffolds targeting this enzyme. In the present work, based on the X-ray crystal structure of human IDO1 coordinated with few ligands, we designed and synthesized new fused heterocyclic compounds and evaluated their potential human IDO1 inhibitory activity (compound 30 and 41 showed IC50 values of 23 and 13 µM, respectively). The identified HITs were observed to be non-toxic to HEK293 cells at 100 µM concentration. The observed activity of the synthesized compounds was correlated with the specific interactions of their structures at the enzyme pocket using docking studies. A detailed analysis of docking results of the synthesized analogues as well as selected known IDO1 inhibitors revealed that most of the inhibitors have some reasonable docking scores in at least two crystal structures and have similar orientation as that of co-crystal ligands.

IDO1 can impair NK cells function against non-small cell lung cancer by downregulation of NKG2D Ligand via ADAM10.

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the rate-limiting step in tryptophan catabolism along the kynurenine (Kyn) pathway and exerts immunosuppressive properties mainly via activation of transcription factor aryl hydrocarbon receptor (AhR) pathway. IDO1 induces NK cells dysfunction via downregulation of the activating receptor NKG2D on NK cells, but whether and how it affects the expression of NKG2D Ligand (NKG2DL) on tumor cells remains unclear. Since a disintegrin and metalloprotease 10 (ADAM10) plays a potential role in the shedding of NKG2DL and the releasing of soluble NKG2DL (sNKG2DL), we investigated how IDO1 modulates the expression of NKG2DL via ADAM10 in non-small cell lung cancer (NSCLC). We found that IDO1 expression was negatively correlated with NKG2DL expression while positively correlated with ADAM10 expression with human lung cancer brain metastasis tissue, NSCLC cells and LLC tumor-bearing mice. IDO1 could regulate ADAM10 expression via IDO1-Kyn-AhR signaling pathway and subsequently regulate NKG2DL expression. IDO1 deficiency led to retarded tumor growth and improved NK cells function in NSCLC mice. IDO1 inhibitors improved NK cells function in vitro and in vivo. The combo of IDO1 inhibitor and NK cells exhibited more therapeutic efficacy than either of the single IDO1 inhibitor or NK cells treatment.

SAR towards indoline and 3-azaindoline classes of IDO1 inhibitors.

A novel series of IDO1 inhibitors have been identified with good IDO1 Hela cell and human whole blood activity. These inhibitors contain an indoline or a 3-azaindoline scaffold. Their structure-activity-relationship studies have been explored. Compounds 37 and 41 stood out as leads due to their good potency in IDO1 Hela assay, good IDO1 unbound hWB IC50s, reasonable unbound clearance, and good MRT in rat and dog PK studies.

Discovery of novel IDO1 inhibitors via structure-based virtual screening and biological assays.

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme-containing enzyme that catalyzes the first and rate-limiting step in catabolism of tryptophan via the kynurenine pathway, which plays a pivotal role in the proliferation and differentiation of T cells. IDO1 has been proven to be an attractive target for many diseases, such as breast cancer, lung cancer, colon cancer, prostate cancer, etc. In this study, docking-based virtual screening and bioassays were conducted to identify novel inhibitors of IDO1. The cellular assay demonstrated that 24 compounds exhibited potent inhibitory activity against IDO1 at micromolar level, including 8 compounds with IC50 values below 10 µM and the most potent one (compound 1) with IC50 of 1.18 ± 0.04 µM. Further lead optimization based on similarity searching strategy led to the discovery of compound 28 as an excellent inhibitor with IC50 of 0.27 ± 0.02 µM. Then, the structure-activity relationship of compounds 1, 2, 8 and 14 analogues is discussed. The interaction modes of two compounds against IDO1 were further explored through a Python Based Metal Center Parameter Builder (MCPB.py) molecular dynamics simulation, binding free energy calculation and electrostatic potential analysis. The novel IDO1 inhibitors of compound 1 and its analogues could be considered as promising scaffold for further development of IDO1 inhibitors.

Synthesis of novel tryptanthrin derivatives as dual inhibitors of indoleamine 2,3-dioxygenase 1 and tryptophan 2,3-dioxygenase.

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are promising drug development targets due to their implications in pathologies such as cancer and neurodegenerative diseases. The search for IDO1 inhibitor has been intensely pursued but there is a paucity of potent TDO and IDO1/TDO dual inhibitors. Natural product tryptanthrin has been confirmed to bear IDO1 and/or TDO inhibitory activities. Herein, twelve novel tryptanthrin derivatives were synthesized and evaluated for the IDO1 and TDO inhibitory potency. All of the compounds were found to be IDO1/TDO dual inhibitors, in particular, compound 9a and 9b bore IDO1 inhibitory activity similar to that of INCB024360, and compound 5a and 9b had remarkable TDO inhibitory activity superior to that of the well-known TDO inhibitor LM10. This work enriches the collection of IDO1/TDO dual inhibitors and provides chemical molecules for potential development into drugs.

Design, Synthesis and Biological Evaluation of Phenyl Urea Derivatives as IDO1 Inhibitors.

Indoleamine 2,3-dioxygenase 1 (IDO1) is a heme-containing intracellular enzyme that catalyzes the first and rate-determining step of tryptophan metabolism and is an important immunotherapeutic target for the treatment of cancer. In this study, we designed and synthesized a new series of compounds as potential IDO1 inhibitors. These compounds were then evaluated for inhibitory activity against IDO1 and tryptophan 2,3-dioxygenase (TDO). Among them, the three phenyl urea derivatives i12, i23, i24 as showed potent IDO1 inhibition, with IC50 values of 0.1-0.6 µM and no compound exhibited TDO inhibitory activity. Using molecular docking, we predicted the binding mode of compound i12 within IDO1. Compound i12 was further investigated by determining its in vivo pharmacokinetic profile and anti-tumor efficacy. The pharmacokinetic study revealed that compound i12 had satisfactory properties in mice, with moderate plasma clearance (22.45 mL/min/kg), acceptable half-life (11.2 h) and high oral bioavailability (87.4%). Compound i12 orally administered at 15 mg/kg daily showed tumor growth inhibition (TGI) of 40.5% in a B16F10 subcutaneous xenograft model and 30 mg/kg daily showed TGI of 34.3% in a PAN02 subcutaneous xenograft model. In addition, the body weight of i12-treated mice showed no obvious reduction compared with the control group. Overall, compound i12 is a potent lead compound for developing IDO1 inhibitors and anti-tumor agents.

Design, synthesis and biological evaluation of 2,5-dimethylfuran-3-carboxylic acid derivatives as potential IDO1 inhibitors.

Indoleamine 2,3-dioxygenase 1 (IDO1) plays a vital role in tumor immune escape and has emerged as a promising target for cancer immunotherapy. In this study, a novel series of 2,5-dimethylfuran-3-carboxylic acid derivatives were designed, synthesized and evaluated for inhibitory activities against IDO1, and their structure-activity relationship was investigated. Among these, compound 19a exhibited excellent IDO1 inhibitory activity (HeLa cellular IC50 = 4.0 nM, THP-1 cellular IC50 = 4.6 nM). Further molecular docking studies revealed that the compound 19a formed a coordinate bond with the heme iron through the carboxylic acid moiety. These results indicate that compound 19a is a potential IDO1 inhibitor for further investigation.

Inhibitory effects of flavonoids isolated from Sophora flavescens on indoleamine 2,3-dioxygenase 1 activity.

Indoleamine 2,3-dioxygenase 1 (IDO1), a tryptophan catabolising enzyme, is known as a tumour cell survival factor that causes immune escape in several types of cancer. Flavonoids of Sophora flavescens have a variety of biological benefits for humans; however, cancer immunotherapy effect has not been fully investigated. The flavonoids (1-6) isolated from S. flavescens showed IDO1 inhibitory activities (IC50 4.3-31.4 µM). The representative flavonoids (4-6) of S. flavescens were determined to be non-competitive inhibitors of IDO1 by kinetic analyses. Their binding affinity to IDO1 was confirmed using thermal stability and surface plasmon resonance (SPR) assays. The molecular docking analysis and mutagenesis assay revealed the structural details of the interactions between the flavonoids (1-6) and IDO1. These results suggest that the flavonoids (1-6) of S. flavescens, especially kushenol E (6), as IDO1 inhibitors might be useful in the development of immunotherapeutic agents against cancers.

Chemo-immunotherapy by nanoliposomal epacadostat and docetaxel combination to IDO1 inhibition and tumor microenvironment suppression.

The over-activation of tryptophan (Trp) metabolism to kynurenine (Kyn) catalyzed by Indoleamine 2,3-dioxygenase-1 (IDO1) enzyme, is one of the main metabolic pathways involved in tumor microenvironment (TME) immune escape and cancer treatment failure. The most efficient of IDO1 inhibitors is Epacadostat (EPA). Since monotherapy with single-agent IDO1 inhibitor regimen has led to an insufficient anti-tumor activity, we examined the efficacy of simultaneous treatment by Liposomal epacadostat (Lip-EPA) as a potent IDO inhibitor, in combination with docetaxel (DTX) as a complement immunogenic cell death (ICD) agent against B16F10 model. First, the in vitro combination index (CI) of epacadostat (EPA) and DTX was investigated by using the unified theory. Then, the in vivo efficacy of the combination therapy was assessed. Results indicated the synergestic cytotoxic effect of the combination on B16F10 compared to normal fibroblast cells (NIH). The immune profiling demonstrated a significant increase in the percentage of infiltrated T lymphocytes and IFN-γ release, a significant decrease in the percentage of regulatory T cells (Treg) population and the subsequent low levels of IL-10 generation in mice treated with Lip-EPA + DTX. Further, a significant tumor growth delay (TGD = 69.15 %) and an increased life span (ILS > 47.83 %) was observed with the combination strategy. Histopathology analysis revealed a remarkable increase in the Trp concentration following combination treatment, while Kyn levels significantly decreased. Results showed that the nano-liposomal form of IDO1 inhibitor in combination with chemotherapy could significantly improve the imunity response and dominate the tumor immuno-suppressive micro-environment, which merits further investigations.

Role of indoleamine 2, 3-dioxygenase 1 in immunosuppression of breast cancer.

Breast cancer (BC) contributes greatly to global cancer incidence and is the main cause of cancer-related deaths among women globally. It is a complex disease characterized by numerous subtypes with distinct clinical manifestations. Immune checkpoint inhibitors (ICIs) are not effective in all patients and have been associated with tumor resistance and immunosuppression. Because amino acid (AA)-catabolizing enzymes have been shown to regulate immunosuppressive effects, this review investigated the immunosuppressive roles of indoleamine 2,3-dioxygenase 1 (IDO1), a tryptophan (Trp)-catabolizing enzyme, which is overexpressed in various metastatic tumors. It promotes immunomodulatory effects by depleting Trp in the regional microenvironment. This leads to a reduction in the number of immunogenic immune cells, such as effector T and natural killer (NK) cells, and an increase in tolerogenic immune cells, such as regulatory T (Treg) cells. The BC tumor microenvironment (TME) establishes a supportive niche where cancer cells can interact with immune cells and neighboring endothelial cells and is thus a feasible target for cancer therapy. In many immunological contexts, IDO1 regulates immune control by causing regional metabolic changes in the TME and tissue environment, which may further affect the maturation of systemic immunological tolerance. In the development of effective treatment targets and approaches, it is essential to understand the immunomodulatory effects exerted by AA-catabolizing enzymes, such as IDO1, on the components of the TME.

IDO1 inhibitors are synergistic with CXCL10 agonists in inhibiting colon cancer growth.

Indoleamine 2,3-dioxygenase 1 (IDO1) is an immune checkpoint that degrades L-tryptophan to kynurenine (Kyn) and enhance immunosuppression, which can be an attractive target for treating colon cancer. IDO1 inhibitors have limited efficacy when used as monotherapies, and their combination approach has been shown to provide synergistic benefits. Many studies have shown that targeting chemokines can promote the efficacy of immune checkpoint inhibitors. Therefore, this study explored the use of IDO1 inhibitors with multiple chemokines to develop a new combination regimen for IDO1 inhibitors. We found that IDO1 inhibitors reduce the secretion of C-X-C motif ligand 10(CXCL10) in cancer cells, and CXCL10 supplementation significantly improved the anticancer effect of IDO1 inhibitors. The combination of the IDO1 inhibitor with CXCL10 or its agonist axitinib had a synergistic inhibitory effect on the growth of colon cancer cells and transplanted CT26 tumors. This synergistic effect may be achieved by inhibiting cancer cell proliferation, promoting cancer cell apoptosis, promoting CD8+T cell differentiation and decreasing Tregs. Two downstream pathways of IDO1 affect CXCL10 secretion. One being the Kyn-aryl hydrocarbon receptor (AHR) pathway, the other is the general control nonderepressible 2(GCN2). Our study provides a new reference for combination regimens of IDO1 inhibitors.

Efficacy and Mechanism of a Biomimetic Nanosystem Carrying Doxorubicin and an IDO Inhibitor for Treatment of Advanced Triple-Negative Breast Cancer.

Introduction: Chemotherapy is still the treatment of choice for advanced triple-negative breast cancer. Chemotherapy combined with immunotherapy is being tried in patients with triple-negative breast cancer. As a kind of "cold tumor", triple-negative breast cancer has a bottleneck in immunotherapy. Indoleamine 2, 3-dioxygenase-1 inhibitors can reverse the immunosuppressive state and enhance the immune response. Methods: In this study, mesoporous silica nanoparticles were coated with the chemotherapeutic drug doxorubicin and indoleamine 2, 3-dioxygenase 1 inhibitor 1-Methyl-DL-tryptophan (1-MT), and then encapsulate the surfaces of a triple-negative breast cancer cell membrane to construct the tumor dual-targeted delivery system CDIMSN for chemotherapy and immunotherapy, and to investigate the immunogenic death effect of CDIMSN. Results and discussion: The CDIMSN could target the tumor microenvironment. Doxorubicin induced tumor immunogenic death, while 1-MT reversed immunosuppression. In vivo findings showed that the tumor size in the CDIMSN group was 2.66-fold and 1.56-fold smaller than that in DOX and DIMSN groups, respectively. CDIMSN group was better than naked DIMSN in stimulating CD8+T cells, CD4+T cells and promoting Dendritic Cells(DC) maturation. In addition, blood analysis, biochemical analysis and Hematoxylin staining analysis of mice showed that the bionic nanoparticles had good biological safety.

Design, synthesis, biological evaluation of urea substituted 1,2,5-oxadiazole-3-carboximidamides as novel indoleamine 2,3-dioxygenase-1 (IDO1) inhibitors.

Indoleamine 2,3-dioxygenase-1 (IDO1) has been considered as an attractive target for oncology immunotherapy due to its immunosuppressive effects on the tumor microenvironment. The most advanced IDO1 inhibitor epacadostat in combination with anti-PD-1 antibody failed to show desirable objective response. Epacadostat is now reevaluated in phase III clinical trials, but its pharmacokinetic (PK) properties are unsatisfactory. To further unravel the antitumor efficacy of IDO1 inhibitors, we designed a series of epacadostat analogues by introducing various urea-containing side chains. In particular, the most active compound 3 showed superior inhibitory potency against recombinant hIDO1 and hIDO1 in HeLa cells induced by interferon γ (IFNγ) relative to epacadostat (3, biochemical hIDO1 IC50 = 67.4 nM, HeLa hIDO1 IC50 = 17.6 nM; epacadostat, biochemical hIDO1 IC50 = 75.9 nM, HeLa hIDO1 IC50 = 20.6 nM). Moreover, compound 3 exhibited improved physicochemical properties and rat PK profile with better oral exposure and bioavailability compared with epacadostat. Importantly, this compound exhibited comparable antitumor efficacy with epacadostat in LLC syngeneic xenograft models. Hence, compound 3 represents a promising lead compound for discovery of more effective IDO1 inhibitors.

A Review on the Synthesis and Anticancer Potentials of Imidazothiazole Derivatives.

Cancer is one of the severe diseases in which abnormal cells divide and proliferate in an uncontrolled manner without any regulation. Globally cancer is among the leading causes of death; according to a recent report of by the WHO, around 10 million people died in 2018 due to cancer. It has also been reported that by 2040, approximately 30 million new cases will be reported every year. The increase in the incidences of cancer is taking a toll on the health care system worldwide. Considerable scientific literature is available on anticancer agents but newer therapeutic strategies are still required in this field to address novel approaches to drug design and discovery to counter this problem. Imidazothiazole represents a privileged scaffold in medicinal chemistry and provides the medicinal chemist the possibility to modulate the physiochemical properties of the lead compound. In recent times, imidazothiazole scaffold is broadly explored for its anticancer activity, which acts through various mechanisms such as EGFR, B-RAF, DHFR kinase inhibition and tubulin polymerization inhibition and other molecular mechanisms of action. Due to their feasible synthetic accessibility and promising pharmacological profile, it has attracted various medicinal chemists to explore and develop imidazothiazole derivatives as potent and safe anticancer agents. In the present article, we have reviewed various potent imidazothiazole scaffold-based derivatives reported as anticancer agents, their synthetic strategies, Structure Activity Relationship (SAR), mechanism of action, and molecular docking along with their future perspective. This review will be very useful for medicinal chemists for drug design and development of imidazothiazole-based potent antiproliferative agents.

Indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors and PROTAC-based degraders for cancer therapy.

Indoleamine 2,3-dioxygenase 1 (IDO1), a known immunosuppressive enzyme that catalyzes the rate-limiting step in the oxidation of tryptophan (Trp) to kynurenine (Kyn), has received increasing attention as an attractive immunotherapeutic target for cancer therapy. Up to now, eleven small-molecule IDO1 inhibitors have entered clinical trials for the treatment of cancers. In addition, proteolysis targeting chimera (PROTAC) based degraders also provide prospects for cancer therapy. Herein we present a comprehensive overview of the medicinal chemistry strategies and potential therapeutic applications of IDO1 inhibitors in nonclinical trials and IDO1-PROTAC degraders.

Combination of apatinib with apo-IDO1 inhibitor for the treatment of colorectal cancer.

Colorectal cancer (CRC) is the third most common cancer in the world. Recently, many clinical studies have demonstrated the therapeutic potential of immune checkpoint therapy combined with inhibitors of vascular endothelial growth factor receptor 2 (VEGFR2) in colon cancer. Compound B37, identified in our previous experiment, is an apo-form indoleamine-2,3-dioxygenase 1 inhibitor (apo-IDO1 inhibitor), which has been shown to significantly suppress tumor growth combined with an anti-PD1 antibody. We speculated whether this apo-IDO inhibitor (B37) combined with a VEGFR2 inhibitor (apatinib) would further improve its anti-tumor activity. Therefore, a syngeneic mouse colon cancer model (mouse colon cancer cell line CT26) was established to investigate the anti-tumor activity of B37 combined with apatinib. As expected, the combination of B37 and apatinib (VEGFR2 inhibitor) improved the therapeutic effect compared with apo-IDO1 inhibitor and apatinib monotherapy, as shown by the reduced growth of transplanted tumors, weakened proliferation, and increased apoptosis of cancer cells. Specifically, there was a 24.8% reduction in tumor volume using apatinib and 31.3% reduction using B37. The combination-treated group showed remarkable inhibition of tumor growth (52.2%). For tumor weight, there was a 29.2% reduction in the apatinib-treated group and 35.0% reduction in the B37-treated group. The combination-treated group showed a 56.3% reduction. Moreover, the combination therapy reprogrammed the immune microenvironment by increasing infiltration of CD4+ and CD8+ T cells, decreasing the ratio of regulatory T cells, and promoting the killing ability of T cells manifested by elevated expression of IFN-γ and granzyme B in the combination-treated group. Our study indicates that the combination of apo-IDO1 inhibitor and apatinib is a promising strategy for CRC therapy.

Indoleamine 2, 3 Dioxygenase 1 Impairs Chondrogenic Differentiation of Mesenchymal Stem Cells in the Joint of Osteoarthritis Mice Model.

Osteoarthritis (OA) is a serious joint inflammation that leads to cartilage degeneration and joint dysfunction. Mesenchymal stem cells (MSCs) are used as a cell-based therapy that showed promising results in promoting cartilage repair. However, recent studies and clinical trials explored unsatisfied outcomes because of slow chondrogenic differentiation and increased calcification without clear reasons. Here, we report that the overexpression of indoleamine 2,3 dioxygenase 1 (IDO1) in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs in the joint of the OA mice model. The effect of MSCs mixed with IDO1 inhibitor on the cartilage regeneration was tested compared to MSCs mixed with IDO1 in the OA animal model. Further, the mechanism exploring the effect of IDO1 on chondrogenic differentiation was investigated. Subsequently, miRNA transcriptome sequencing was performed for MSCs cocultured with IDO1, and then TargetScan was used to verify the target of miR-122-5p in the SF-MSCs. Interestingly, we found that MSCs mixed with IDO1 inhibitor showed a significant performance to promote cartilage regeneration in the OA animal model, while MSCs mixed with IDO1 failed to stimulate cartilage regeneration. Importantly, the overexpression of IDO1 showed significant inhibition to Sox9 and Collagen type II (COL2A1) through activating the expression of ß-catenin, since inhibiting of IDO1 significantly promoted chondrogenic signaling of MSCs (Sox9, COL2A1, Aggrecan). Further, miRNA transcriptome sequencing of SF-MSCs that treated with IDO1 showed significant downregulation of miR-122-5p which perfectly targets Wnt1. The expression of Wnt1 was noticed high when IDO1 was overexpressed. In summary, our results suggest that IDO1 overexpression in the synovial fluid of OA patients impairs chondrogenic differentiation of MSCs and cartilage regeneration through downregulation of miR-122-5p that activates the Wnt1/ß-catenin pathway.

Diverse chemical space of indoleamine-2,3-dioxygenase 1 (Ido1) inhibitors.

Indoleamine-2,3-dioxygenase 1 (IDO1) catalyses the first and rate limiting step of kynurenine pathway accounting for the major contributor of L-Tryptophan degradation. The Kynurenine metabolites are identified as essential cofactors, antagonists, neurotoxins, immunomodulators, antioxidants as well as carcinogens. The catalytic active site of IDO1 enzyme consists of hydrophobic Pocket-A positioned in the distal heme site and remains connected to a second hydrophobic Pocket-B towards the entrance of the active site. IDO1 enzyme also relates directly to the modulation of the innate and adaptive immune system. Various studies proved that the over expression of IDO1 enzyme play a predominant role in the escape of immunity during cancer progression. Recently, there has been considerable interest in evaluating the potential of IDO1 inhibitors to mobilize the body's immune system against solid tumours. In the last two decades, enormous attempts to advance new IDO1 inhibitors are on-going both in pharmaceutical industries and in academia which resulted in the discovery of a diverse range of selective and potent IDO1 inhibitors. The IDO1 inhibitors have therapeutic utility in various diseases and in the near future, it may have utility in the treatment of COVID-19. Despite various reviews on IDO1 inhibitors in last five years, none of the reviews provide a complete overview of diverse chemical space including naturally occurring and synthetic IDO1 inhibitors with detailed structure activity relationship studies. The present work provides a complete overview on the IDO1 inhibitors known in the literature so far along with the Structure-Activity Relationship (SAR) in each class of compounds.

Genomic and Immunologic Correlates of Indoleamine 2,3-Dioxygenase Pathway Expression in Cancer.

Immune checkpoint blockade leads to unprecedented responses in many cancer types. An alternative method of unleashing anti-tumor immune response is to target immunosuppressive metabolic pathways like the indoleamine 2,3-dioxygenase (IDO) pathway. Despite promising results in Phase I/II clinical trials, an IDO-1 inhibitor did not show clinical benefit in a Phase III clinical trial. Since, a treatment can be quite effective in a specific subset without being effective in the whole cancer type, it is important to identify the subsets of cancers that may benefit from IDO-1 inhibitors. In this study, we looked for the genomic and immunologic correlates of IDO pathway expression in cancer using the Cancer Genome Atlas (TCGA) dataset. Strong CD8+ T-cell infiltration, high mutation burden, and expression of exogenous viruses [Epstein-Barr virus (EBV), Human papilloma virus (HPV), and Hepatitis C virus (HCV)] or endogenous retrovirus (ERV3-2) were associated with over-expression of IDO-1 in most cancer types, IDO-2 in many cancer types, and TDO-2 in a few cancer types. High mutation burden in ER+ HER2- breast cancer, and ERV3-2 expression in ER- HER2- and HER2+ breast, colon, and endometrial cancers were associated with over-expression of all three genes. These results may have important implications for guiding development clinical trials of IDO-1 inhibitors.