Targeting the IDO1/TDO2–KYN–AhR Pathway for Cancer Immunotherapy – Challenges and Opportunities

MicroRNA-153 Decreases Tryptophan Catabolism and Inhibits Angiogenesis in Bladder Cancer by Targeting Indoleamine 2,3-Dioxygenase 1

Background: Metastasis is the primary cause of cancer deaths, warranting further investigation. This study assessed microRNA-153 (miR-153) expression in bladder cancer tissues and investigated the underlying molecular mechanism of miR-153-mediated regulation of bladder cancer cells.

Methods: Paired tissue specimens from 45 bladder cancer patients were collected for qRT-PCR. The Cancer Genome Atlas (TCGA) dataset was used to identify associations of miR-153 with bladder cancer prognosis. Bladder cancer tissues and immortalized cell lines were used for the following experiments: miR-153 mimics and indoleamine 2,3-dioxygenase 1 (IDO1) siRNA transfection; Western blot, cell viability, colony formation, and Transwell analyses; nude mouse xenograft; and chicken embryo chorioallantoic membrane angiogenesis (CAM) assays. Human umbilical vein endothelial cells (HUVECs) were co-cultured with bladder cancer cells for the tube formation assay. The luciferase reporter assay was used to confirm miR-153-targeting genes.

Results: miR-153 expression was downregulated in bladder cancer tissues and cell lines, and reduced miR-153 expression was associated with advanced tumor stage and poor overall survival of patients. Moreover, miR-153 expression inhibited bladder cancer cell growth by promoting tumor cell apoptosis, migration, invasion, and endothelial mesenchymal transition (EMT) in vitro and tumor xenograft growth in vivo, while miR-153 expression suppressed HUVEC and CAM angiogenesis. At the gene level, miR-153 targeted IDO1 expression and inhibited bladder cancer cell tryptophan metabolism through inhibiting IL6/STAT3/VEGF signaling.

Conclusions: Collectively, our data demonstrate that miR-153 exerts anti-tumor activity in bladder cancer by targeting IDO1 expression. Future studies will investigate miR-153 as a novel therapeutic target for bladder cancer patients.

Cancer immunoediting and resistance to T cell-based immunotherapy

Anticancer immunotherapies involving the use of immune-checkpoint inhibitors or adoptive cellular transfer have emerged as new therapeutic pillars within oncology. These treatments function by overcoming or relieving tumour-induced immunosuppression, thereby enabling immune-mediated tumour clearance. While often more effective and better tolerated than traditional and targeted therapies, many patients have innate or acquired resistance to immunotherapies. Cancer immunoediting is the process whereby the immune system can both constrain and promote tumour development, which proceeds through three phases termed elimination, equilibrium and escape. Throughout these phases, tumour immunogenicity is edited, and immunosuppressive mechanisms that enable disease progression are acquired. The mechanisms of resistance to immunotherapy seem to broadly overlap with those used by cancers as they undergo immunoediting to evade detection by the immune system. In this Review, we discuss how a deeper understanding of the mechanisms underlying the cancer immunoediting process can provide insight into the development of resistance to immunotherapies and the strategies that can be used to overcome such resistance.

Inhibition of Human DNA Polymerases Eta and Kappa by Indole-Derived Molecules Occurs through Distinct Mechanisms

Overexpression of human DNA polymerase kappa (hpol κ) in glioblastoma is associated with shorter survival time and resistance to the alkylating agent temozolomide (TMZ), making it an attractive target for the development of small-molecule inhibitors. We previously reported on the development and characterization of indole barbituric acid-derived (IBA) inhibitors of translesion DNA synthesis polymerases (TLS pols). We have now identified a potent and selective inhibitor of hpol κ based on the indole-aminoguanidine (IAG) chemical scaffold. The most promising IAG analogue, IAG-10, exhibited greater inhibitory action against hpol κ than any other human Y-family member, as well as pols from the A-, B-, and X-families. Inhibition of hpol κ by IAG analogues appears to proceed through a mechanism that is distinct from inhibition of hpol η based on changes in DNA binding affinity and nucleotide insertion kinetics. By way of comparison, both IAG and IBA analogues inhibited binary complex formation by hpol κ and ternary complex formation by hpol η. Decreasing the concentration of enzyme and DNA in the reaction mixture lowered the IC₅₀ value of IAG-10 to submicromolar values, consistent with inhibition of binary complex formation for hpol κ. Chemical footprinting experiments revealed that IAG-10 binds to a cleft between the finger, little finger, and N-clasp domains on hpol κ and that this likely disrupts the interaction between the N-clasp and the TLS pol core. In cell culture, IAG-10 potentiated the antiproliferative activity and DNA damaging effects of TMZ in hpol κ-proficient cells but not in hpol κ-deficient cells, indicative of a target-dependent effect. Mutagenic replication across alkylation damage increased in hpol κ-proficient cells treated with IAG-10, while no change in mutation frequency was observed for hpol κ-deficient cells. In summary, we developed a potent and selective small-molecule inhibitor of hpol κ that takes advantage of structural features unique to this TLS enzyme to potentiate TMZ, a standard-of-care drug used in the treatment of malignant brain tumors. Furthermore, the IAG scaffold represents a new chemical space for the exploration of TLS pol inhibitors, which could prove useful as a strategy for improving patient response to genotoxic drugs.

Role of IDO and TDO in Cancers and Related Diseases and the Therapeutic Implications

Kynurenine (Kyn) pathway is a significant metabolic pathway of tryptophan (Trp). The metabolites of the Kyn pathway are closely correlated with numerous diseases. Two main enzymes, indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO or TDO2), regulate the first and rate-limiting step of the Kyn pathway. These enzymes are directly or indirectly involved in various diseases, including inflammatory diseases, cancer, diabetes, and mental disorders. Presently, an increasing number of potential mechanisms have been revealed. In the present review, we depict the structure of IDO and TDO and explicate their functions in various diseases to facilitate a better understanding of them and to indicate new therapeutic plans to target them. Moreover, we summarize the inhibitors of IDO/TDO that are currently under development and their efficacy in the treatment of cancer and other diseases.

AhR Activation in Pharmaceutical Development: Applying Liver Gene Expression Biomarker Thresholds to Identify Doses Associated with Tumorigenic Risks in Rats

Aryl hydrocarbon receptor (AhR) activation is associated with carcinogenicity of non-genotoxic AhR-activating carcinogens such as 2, 3, 7, 8-tetrachlorodibenzodioxin (TCDD), and is often observed with drug candidate molecules in development and raises safety concerns. As downstream effectors of AhR signaling, the expression and activity of Cyp1a1 and Cyp1a2 genes are commonly monitored as evidence of AhR activation to inform carcinogenic risk of compounds in question. However, many marketed drugs and phytochemicals are reported to induce these Cyps modestly and are not associated with dioxin-like toxicity or carcinogenicity. We hypothesized that a threshold of AhR activation needs to be surpassed in a sustained manner in order for the dioxin-like toxicity to manifest, and a simple liver gene expression signature based on Cyp1a1 and Cyp1a2 from a short-term rat study could be used to assess AhR activation strength and differentiate tumorigenic dose levels from non-tumorigenic ones. To test this hypothesis, short term studies were conducted in Wistar Han rats with two AhR-activating carcinogens (TCDD and PCB126) at minimally carcinogenic and non-carcinogenic dose levels, and three AhR-activating non-carcinogens (omeprazole, mexiletine, and canagliflozin) at the top doses used in their reported 2-year rat carcinogenicity studies. A threshold of AhR activation was identified in rat liver that separated a meaningful "tumorigenic-strength AhR signal" from a statistically-significant AhR activation signal that was not associated with dioxin-like carcinogenicity. These studies also confirmed the importance of the sustainability of AhR activation for carcinogenic potential. A sustained activation of AhR above the threshold could thus be used in early pharmaceutical development to identify dose levels of drug candidates expected to exhibit dioxin-like carcinogenic potential.

Small-Molecule Immuno-Oncology Therapy: Advances, Challenges and New Directions

Oncology immunotherapy has gained significant advances in recent years and benefits cancer patients with superior efficacy and superior clinical responses. Currently over ten immune checkpoint antibodies targeting CTLA-4 and PD-1/PD-L1 have received regulatory approval worldwide and over thousands are under active clinical trials. However, compared to the rapid advance of Monoclonal Antibody (mAb), studies on immunotherapeutic small molecules have far lagged behind. Small molecule immunotherapy not only can target immunosuppressive mechanisms similar to mAbs, but also can stimulate intracellular pathways downstream of checkpoint proteins in innate or adaptive immune cells that mAbs are unable to access. Therefore, small molecule immunotherapy can provide an alternative treatment modality either alone or complementary to or synergistic with extracellular checkpoint mAbs to address low clinical response and drug resistance. Fortunately, remarkable progress has achieved recently in the pursuit of small molecule immunotherapy. This review intends to provide a timely highlight on those clinically investigated small molecules targeting PD-1/PD-L1, IDO1, and STING. The most advanced IDO1 inhibitor epacadostat have been aggressively progressed into multiple clinical testings. Small molecule PD-1/PD-L1 inhibitors and STING activators are still in a premature state and their decisive application needs to wait for the ongoing clinical outcomes. Since no small molecule immunotherapy has been approved yet, the future research should continue to focus on discovery of novel small molecules with distinct chemo-types and higher potency, identification of biomarkers to precisely stratify patients, as well as validation of many other immune-therapeutic targets, such as LAG3, KIRs, TIM-3, VISTA, B7-H3, and TIGIT.

Carnosic acid enhances the anti-lung cancer effect of cisplatin by inhibiting myeloid-derived suppressor cells

Cisplatin and other platinum-based drugs are used frequently for treatment of lung cancer. However, their clinical performance are usually limited by drug resistance or toxic effects. Carnosic acid, a polyphenolic diterpene isolated from Rosemary (Rosemarinus officinalis), has been reported to have several pharmacological and biological activities. In the present study, the combination effect of cisplatin plus carnosic acid on mouse LLC (Lewis lung cancer) xenografts and possible underlying mechanism of action were examined. LLC-bearing mice were treated with intraperitoneal injection with cisplatin, oral gavage with carnosic acid, or combination with cisplatin and carnosic acid, respectively. Combination of carnosic acid and cisplatin yielded significantly better anti-growth and pro-apoptotic effects on LLC xenografts than drugs alone. Mechanistic study showed that carnosic acid treatment boosted the function of CD8⁺ T cells as evidenced by higher IFN-γ secretion and higher expression of FasL, perforin as well as granzyme B. In the meantime, the proportion of MDSC (myeloid-derived suppressor cells) in tumor tissues were reduced by carnosic acid treatment and the mRNA levels of iNOS2, Arg-1, and MMP9, which are the functional markers for MDSC, were reduced. In conclusion, our study proved that the functional suppression of MDSC by carnosic acid promoted the lethality of CD8⁺ T cells, which contributed to the enhancement of anti-lung cancer effect of cisplatin.

Tryptophan (Trp) modulates gut homeostasis via aryl hydrocarbon receptor (AhR)

The intestinal homeostasis is an orchestrated dynamic equilibrium state composed of the coexistence and interactions among the nutrients, microbial flora, and immune system. The intestinal balance disorder can trigger a series of diseases, such as inflammatory bowel disease (IBD). Many of tryptophan (Trp) metabolites, such as kynurenine and indole, generated under a series of endogenous enzymes or microbial metabolism, have been reported enable to bind and activate the aryl hydrocarbon receptor (AhR), this series of process is termed the Trp-AhR pathway. The activated Trp-AhR pathway can induce the expression of downstream cytokines such as interleukin-22 (IL-22) and interleukin-17 (IL-17), thereby regulating the intestinal homeostasis. This review highlights the advance of Trp-AhR pathway in the regulation of intestinal homeostasis and provides some insights for the clinical strategies that expect to effectively prevent and treat gut diseases via intervening the Trp-AhR pathway.

Reimagining IDO Pathway Inhibition in Cancer Immunotherapy via Downstream Focus on the Tryptophan-Kynurenine-Aryl Hydrocarbon Axis

Significant progress has been made in cancer immunotherapy with checkpoint inhibitors targeting programmed cell death protein 1 (PD-1)-programmed death-ligand 1 signaling pathways. Tumors from patients showing sustained treatment response predominately demonstrate a T cell-inflamed tumor microenvironment prior to, or early on, treatment. Not all tumors with this phenotype respond, however, and one mediator of immunosuppression in T cell-inflamed tumors is the tryptophan-kynurenine-aryl hydrocarbon receptor (Trp-Kyn-AhR) pathway. Multiple mechanisms of immunosuppression may be mediated by this pathway including depletion of tryptophan, direct immunosuppression of Kyn, and activity of Kyn-bound AhR. Indoleamine 2,3-dioxygenase 1 (IDO1), a principle enzyme in Trp catabolism, is the target of small-molecule inhibitors in clinical development in combination with PD-1 checkpoint inhibitors. Despite promising results in early-phase clinical trials in a range of tumor types, a phase III study of the IDO1-selective inhibitor epacadostat in combination with pembrolizumab showed no difference between the epacadostat-treated group versus placebo in patients with metastatic melanoma. This has led to a diminution of interest in IDO1 inhibitors; however, other approaches to inhibit this pathway continue to be considered. Novel Trp-Kyn-AhR pathway inhibitors, such as Kyn-degrading enzymes, direct AhR antagonists, and tryptophan mimetics are advancing in early-stage or preclinical development. Despite uncertainty surrounding IDO1 inhibition, ample preclinical evidence supports continued development of Trp-Kyn-AhR pathway inhibitors to augment immune-checkpoint and other cancer therapies.

Control of the Antitumor Immune Response by Cancer Metabolism

The metabolic reprogramming of tumor cells and immune escape are two major hallmarks of cancer cells. The metabolic changes that occur during tumorigenesis, enabling survival and proliferation, are described for both solid and hematological malignancies. Concurrently, tumor cells have deployed mechanisms to escape immune cell recognition and destruction. Additionally, therapeutic blocking of tumor-mediated immunosuppression has proven to have an unprecedented positive impact in clinical oncology. Increased evidence suggests that cancer metabolism not only plays a crucial role in cancer signaling for sustaining tumorigenesis and survival, but also has wider implications in the regulation of antitumor immune signaling through both the release of signaling molecules and the expression of immune membrane ligands. Here, we review these molecular events to highlight the contribution of cancer cell metabolic reprogramming on the shaping of the antitumor immune response.

Revisiting IDO and its value as a predictive marker for anti-PD-1 resistance

Botticelli et al. proposed the activity of indoleamine-2,3-dioxygenase 1 (IDO) as a potential mechanism and predictive marker for primary resistance against anti-PD-1 treatment in the context of non-small cell lung cancer. However, there are a few points for the authors to address in order to strengthen their claims. First, there are many enzymes that modulate the kynurenine to tryptophan ratio, thereby calling into question their use of the ratio as a proxy for IDO activity. Second, the authors could compare IDO to other proposed markers in the literature, providing a better understanding of its predictive value.

PD-1 immunobiology in systemic lupus erythematosus

Programmed death (PD)-1 receptors and their ligands have been identified in the pathogenesis and development of systemic lupus erythematosus (SLE). Two key pathways, toll-like receptor and type I interferon, are significant to SLE pathogenesis and modulate the expression of PD-1 and the ligands (PD-L1, PD-L2) through activation of NF-κB and/or STAT1. These cell signals are regulated by tyrosine kinase (Tyro, Axl, Mer) receptors (TAMs) that are aberrantly activated in SLE. STAT1 and NF-κB also exhibit crosstalk with the aryl hydrocarbon receptor (AHR). Ligands to AHR are identified in SLE etiology and pathogenesis. These ligands also regulate the activity of the Epstein-Barr virus (EBV), which is an identified factor in SLE and PD-1 immunobiology. AHR is important in the maintenance of immune tolerance and the development of distinct immune subsets, highlighting a potential role of AHR in PD-1 immunobiology. Understanding the functions of AHR ligands as well as AHR crosstalk with STAT1, NF-κB, and EBV may provide insight into disease development, the PD-1 axis and immunotherapies that target PD-1 and its ligand, PD-L1.

MiR-218 produces anti-tumor effects on cervical cancer cells in vitro

Background

As indoleamine-2,3-dioxygenase 1 (IDO1) is critical in tumor immune escape, we determined to study the regulatory mechanism of miR-218 on IDO1 in cervical cancer.

Methods

Real-time PCR (RT-qPCR) was carried out to measure the expression of miR-218. RT-qPCR and Western blot were performed to detect the expression of IDO1 in cervical cancer. Dual-luciferase reporter assay was used to determine the binding of miR-218 on the IDO1 3′UTR. Cell viability, apoptosis, and related factors were determined using cell counting kit-8 (CCK-8), Annexin-V/PI (propidium) assay, enzyme-linked immunosorbnent assay (ELISA), RT-qPCR, and Western blot assays after miR-218 mimics has been transfected to HeLa cervical cancer cells.

Results

MiR-218 was downregulated in cervical cancer. The expression of miR-218 was negatively correlated with IDO1 in cervical cancer tissues and cells. IDO1 is a direct target of miR-218. MiR-218 overexpression was found to inhibit cell viability and promoted apoptosis via activating the expression of Cleaved-Caspase-3 and to inhibit the expression of Survivin, immune factors (TGF-β, VEGF, IL-6, PGE2, COX-2), and JAK2/STAT3 pathway.

Conclusion

MiR-218 inhibits immune escape of cervical cancer cells by direct downregulating IDO1.

Expression of the IDO1/TDO2-AhR pathway in tumor cells or the tumor microenvironment is associated with Merkel cell polyomavirus status and prognosis in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare, aggressive neuroendocrine skin cancer, with approximately 80% of cases related to Merkel cell polyomavirus (MCPyV). Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2) are the key rate-limiting enzymes of the tryptophan-to-kynurenine metabolic pathway. With aryl hydrocarbon receptor (AhR), an intracellular transcription factor, they play a role in escaping the immunosurveillance process in several cancers. IDO1/TDO2/AhR expression associated with the MCPyV status and prognosis in MCC was investigated. Samples included 24 MCPyV-positive MCCs, 12 MCPyV-negative MCCs with squamous cell carcinoma, and 7 MCPyV-negative pure MCCs. They were stained immunohistochemically with IDO1, TDO2, and AhR antibodies and analyzed. Higher IDO1 expression in MCC tumor cells was found in MCPyV-negative than in MCPyV-positive MCC (P < .001). The tumor microenvironment (TME) in MCPyV-negative MCC expressed higher TDO2 than in MCPyV-positive MCC (P < .001). Kaplan-Meier and log-rank tests showed that MCC with lower IDO1 expression in tumor cells and with lower TDO2 and AhR expressions in TME had better overall survival than otherwise (P = .043, .008, and .035, respectively); lower TDO2 expression in TME was also associated with longer disease-specific survival (P = .016). This suggests that IDO1, TDO2, and AhR express differentially in tumor cells or TME and play different roles in tumorigenesis between MCPyV-positive and MCPyV-negative MCC that may affect the MCC biology. Evaluating IDO1/TDO2/AhR expression is important for selecting the most likely patients with MCC for immunotherapies targeting the IDO1/TDO2-AhR pathway.

Targeting the IDO1 pathway in cancer: from bench to bedside

Indoleamine 2, 3-dioxygenases (IDO1 and IDO2) and tryptophan 2, 3-dioxygenase (TDO) are tryptophan catabolic enzymes that catalyze the conversion of tryptophan into kynurenine. The depletion of tryptophan and the increase in kynurenine exert important immunosuppressive functions by activating T regulatory cells and myeloid-derived suppressor cells, suppressing the functions of effector T and natural killer cells, and promoting neovascularization of solid tumors. Targeting IDO1 represents a therapeutic opportunity in cancer immunotherapy beyond checkpoint blockade or adoptive transfer of chimeric antigen receptor T cells. In this review, we discuss the function of the IDO1 pathway in tumor progression and immune surveillance. We highlight recent preclinical and clinical progress in targeting the IDO1 pathway in cancer therapeutics, including peptide vaccines, expression inhibitors, enzymatic inhibitors, and effector inhibitors.

Reversal of indoleamine 2,3-dioxygenase-mediated cancer immune suppression by systemic kynurenine depletion with a therapeutic enzyme

Increased tryptophan (Trp) catabolism in the tumor microenvironment (TME) can mediate immune suppression by upregulation of interferon (IFN)-γ-inducible indoleamine 2,3-dioxygenase (IDO1) and/or ectopic expression of the predominantly liver-restricted enzyme tryptophan 2,3-dioxygenase (TDO). Whether these effects are due to Trp depletion in the TME or mediated by the accumulation of the IDO1 and/or TDO (hereafter referred to as IDO1/TDO) product kynurenine (Kyn) remains controversial. Here we show that administration of a pharmacologically optimized enzyme (PEGylated kynureninase; hereafter referred to as PEG-KYNase) that degrades Kyn into immunologically inert, nontoxic and readily cleared metabolites inhibits tumor growth. Enzyme treatment was associated with a marked increase in the tumor infiltration and proliferation of polyfunctional CD8⁺ lymphocytes. We show that PEG-KYNase administration had substantial therapeutic effects when combined with approved checkpoint inhibitors or with a cancer vaccine for the treatment of large B16-F10 melanoma, 4T1 breast carcinoma or CT26 colon carcinoma tumors. PEG-KYNase mediated prolonged depletion of Kyn in the TME and reversed the modulatory effects of IDO1/TDO upregulation in the TME.