Hydroxyamidine inhibitors of indoleamine-2,3-dioxygenase potently suppress systemic tryptophan catabolism and the growth of IDO-expressing tumors

The role of nonesterified fatty acids in cancer biology: Focus on tryptophan and related metabolism

Plasma nonesterified fatty acids (NEFA) are elevated in cancer, because of decreased albumin levels and of fatty acid oxidation, and increased fatty acid synthesis and lipolysis. Albumin depletion and NEFA elevation maximally release albumin-bound tryptophan (Trp) and increase its flux down the kynurenine pathway, leading to increased production of proinflammatory kynurenine metabolites, which tumors use to undermine T-cell function and achieve immune escape. Activation of the aryl hydrocarbon receptor by kynurenic acid promotes extrahepatic Trp degradation by indoleamine 2,3-dioxygenase and leads to upregulation of poly (ADP-ribose) polymerase, activation of which and also of SIRT1 (silent mating type information regulation 2 homolog 1) could lead to depletion of NAD+ and ATP, resulting in cell death. NEFA also modulate heme synthesis and degradation, changes in which impact homocysteine metabolism and production of reduced glutathione and hydrogen sulphide. The significance of the interactions between heme and homocysteine metabolism in cancer biology has received little attention. Targeting Trp disposition in cancer to prevent the NEFA effects is suggested.

Exploring tryptophan metabolism: The transition from disturbed balance to diagnostic and therapeutic potential in metabolic diseases

The rapidly rising prevalence of metabolic diseases has turned them into an escalating global health concern. By producing or altering metabolic products, the gut microbiota plays a pivotal role in maintaining human health and influencing disease development. These metabolites originate from the host itself or the external environment. In the system of interactions between microbes and the host, tryptophan (Trp) plays a central role in metabolic processes. As the amino acid in the human body that must be obtained through dietary intake, it is crucial for various physiological functions. Trp can be metabolized in the gut into three main products: The gut microbiota regulates the transformation of 5-hydroxytryptamine (5-HT, serotonin), kynurenine (Kyn), and various indole derivatives. It has been revealed that a substantial correlation exists between alterations in Trp metabolism and the initiation and progression of metabolic disorders, including obesity, diabetes, non-alcoholic fatty liver disease, and atherosclerosis, but Trp metabolites have not been comprehensively reviewed in metabolic diseases. As such, this review summarizes and analyzes the latest research, emphasizing the importance of further studying Trp metabolism within the gut microbiota to understand and treat metabolic diseases. This carries potential significance for improving human health and may introduce new therapeutic strategies.

Efferocytosis drives a tryptophan metabolism pathway in macrophages to promote tissue resolution

Macrophage efferocytosis prevents apoptotic cell (AC) accumulation and triggers inflammation-resolution pathways. The mechanisms linking efferocytosis to resolution often involve changes in macrophage metabolism, but many gaps remain in our understanding of these processes. We now report that efferocytosis triggers an indoleamine 2,3-dioxygenase-1 (IDO1)-dependent tryptophan (Trp) metabolism pathway that promotes several key resolution processes, including the induction of pro-resolving proteins, such interleukin-10, and further enhancement of efferocytosis. The process begins with upregulation of Trp transport and metabolism, and it involves subsequent activation of the aryl hydrocarbon receptor (AhR) by the Trp metabolite kynurenine (Kyn). Through these mechanisms, macrophage IDO1 and AhR contribute to a proper resolution response in several different mouse models of efferocytosis-dependent tissue repair, notably during atherosclerosis regression induced by plasma low-density lipoprotein (LDL) lowering. These findings reveal an integrated metabolism programme in macrophages that links efferocytosis to resolution, with possible therapeutic implications for non-resolving chronic inflammatory diseases, notably atherosclerosis.

Nanodelivery Optimization of IDO1 Inhibitors in Tumor Immunotherapy: Challenges and Strategies

Tryptophan (Trp) metabolism plays a vital role in cancer immunity. Indoleamine 2.3-dioxygenase 1 (IDO1), is a crucial enzyme in the metabolic pathway by which Trp is degraded to kynurenine (Kyn). IDO1-mediated Trp metabolites can inhibit tumor immunity and facilitate immune evasion by cancer cells; thus, targeting IDO1 is a potential tumor immunotherapy strategy. Recently, numerous IDO1 inhibitors have been introduced into clinical trials as immunotherapeutic agents for cancer treatment. However, drawbacks such as low oral bioavailability, slow onset of action, and high toxicity are associated with these drugs. With the continuous development of nanotechnology, medicine is gradually entering an era of precision healthcare. Nanodrugs carried by inorganic, lipid, and polymer nanoparticles (NPs) have shown great potential for tumor therapy, providing new ways to overcome tumor diversity and improve therapeutic efficacy. Compared to traditional drugs, nanomedicines offer numerous significant advantages, including a prolonged half-life, low toxicity, targeted delivery, and responsive release. Moreover, based on the physicochemical properties of these nanomaterials (eg, photothermal, ultrasonic response, and chemocatalytic properties), various combination therapeutic strategies have been developed to synergize the effects of IDO1 inhibitors and enhance their anticancer efficacy. This review is an overview of the mechanism by which the Trp-IDO1-Kyn pathway acts in tumor immune escape. The classification of IDO1 inhibitors, their clinical applications, and barriers for translational development are discussed, the use of IDO1 inhibitor-based nanodrug delivery systems as combination therapy strategies is summarized, and the issues faced in their clinical application are elucidated. We expect that this review will provide guidance for the development of IDO1 inhibitor-based nanoparticle nanomedicines that can overcome the limitations of current treatments, improve the efficacy of cancer immunotherapy, and lead to new breakthroughs in the field of cancer immunotherapy.

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.

Targeting amino acid-metabolizing enzymes for cancer immunotherapy

Despite the immune system's role in the detection and eradication of abnormal cells, cancer cells often evade elimination by exploitation of various immune escape mechanisms. Among these mechanisms is the ability of cancer cells to upregulate amino acid-metabolizing enzymes, or to induce these enzymes in tumor-infiltrating immunosuppressive cells. Amino acids are fundamental cellular nutrients required for a variety of physiological processes, and their inadequacy can severely impact immune cell function. Amino acid-derived metabolites can additionally dampen the anti-tumor immune response by means of their immunosuppressive activities, whilst some can also promote tumor growth directly. Based on their evident role in tumor immune escape, the amino acid-metabolizing enzymes glutaminase 1 (GLS1), arginase 1 (ARG1), inducible nitric oxide synthase (iNOS), indoleamine 2,3-dioxygenase 1 (IDO1), tryptophan 2,3-dioxygenase (TDO) and interleukin 4 induced 1 (IL4I1) each serve as a promising target for immunotherapeutic intervention. This review summarizes and discusses the involvement of these enzymes in cancer, their effect on the anti-tumor immune response and the recent progress made in the preclinical and clinical evaluation of inhibitors targeting these enzymes.

Inhibition of kynurenine production by N,O-substituted hydroxylamine derivatives

The inhibition of kynurenine production is considered a promising target for cancer immunotherapy. In this study, an amino acid derivative, compound 1 was discovered using a cell-based assay with our screening library. Compound 1 suppressed kynurenine production without inhibiting indoleamine 2,3-dioxygenase 1 (IDO1) activity. The activity of 1 was derived from the inhibition of IDO1 by a metabolite of 1, O-benzylhydroxylamine (OBHA, 2a). A series of N-substituted 2a derivatives that exhibit potent activity in cell-based assays may represent effective prodrugs. Therefore, we synthesized and evaluated novel N,O-substituted hydroxylamine derivatives. The structure-activity relationships revealed that N,O-substituted hydroxylamine 2c inhibits kynurenine production in a cell-based assay. We conducted an in vivo experiment with 2c, although the effectiveness of O-substituted hydroxylamine derivatives in vivo has not been previously reported. The results indicate that N,O-substituted hydroxylamine derivatives are promising IDO1 inhibitors.

AI-2 quorum sensing controlled delivery of cytolysin-A by tryptophan auxotrophic low-endotoxic Salmonella and its anticancer effects in CT26 mice with colon cancer

INTRODUCTION

The limitations of conventional cancer therapies necessitate target-oriented, highly invasive, and safe treatment approaches. Hence, the intrinsic anti-tumor activity of Salmonella can offer better options to combat cancers.

OBJECTIVES

This study aims to utilize attenuated Salmonella and deliver cytolytic protein cytolysin A (ClyA) under quorum sensing (QS) signaling for precise localized expression in tumors but not in healthy organs.

METHODS

The therapeutic delivery strain was imposed with tryptophan auxotroph for selective colonization in tumors by trpA and trpE deletion, and lipid-A and O-antigen were altered by pagL and rfaL deletions using lambda red recombination method. The strain was transformed with the designed QS-controlled ClyA expression vector which was validated by western blot. The in vivo passaged therapeutic strain was used for treatment four times at a weekly interval, with a dose of 5 × 106 CFU/mouse for cancer therapy.

RESULTS

The attenuated strain induced minimal endotoxicity-related cytokines TNF-α, IL-1β, and IFN-γ and exhibited adequate colonization despite earlier exposure in mice. The QS-controlled ClyA expression was confirmed by western blot from bacterial cultures grown at different cell densities. The results demonstrated that the in vivo passaged strain preferentially colonized the tumor after vacating the spleen, liver, and lung, leaving no outward histological scars. The anti-cancer effect of the designed construct was evaluated in the murine CT26 colon cancer model. The expression of ClyA increased tumoricidal activity by 67 % compared to vector control.

CONCLUSION

Hence, the anti-tumor effect of the engineered Salmonella strain was improved by ClyA expression via QS activation after achieving the threshold bacterial cell density. Further, immunohistochemical staining of the tumor and other organs corroborated the QS-controlled tumor-specific expression of ClyA. Overall, the results imply that the developed anti-cancer Salmonella has low endotoxicity and QS-controlled expression of ClyA as beneficial safety elements and supports recurrent Salmonella inoculation by O-antigen deficiency.

The Toxoplasma oxygen-sensing protein, TgPhyA, is required for resistance to interferon gamma-mediated nutritional immunity in mice

As Toxoplasma gondii disseminates through its host, the parasite must sense and adapt to its environment and scavenge nutrients. Oxygen (O2) is one such environmental factor and cytoplasmic prolyl 4-hydroxylases (PHDs) are evolutionarily conserved O2 cellular sensing proteins that regulate responses to changes in O2 availability. Toxoplasma expresses 2 PHDs. One of them, TgPHYa hydroxylates SKP1, a subunit of the SCF-E3 ubiquitin ligase complex. In vitro, TgPHYa is important for growth at low O2 levels. However, studies have yet to examine the role that TgPHYa or any other pathogen-encoded PHD plays in virulence and disease. Using a type II ME49 Toxoplasma TgPHYa knockout, we report that TgPHYa is important for Toxoplasma virulence and brain cyst formation in mice. We further find that while TgPHYa mutant parasites can establish an infection in the gut, they are unable to efficiently disseminate to peripheral tissues because the mutant parasites are unable to survive within recruited immune cells. Since this phenotype was abrogated in IFNγ knockout mice, we studied how TgPHYa mediates survival in IFNγ-treated cells. We find that TgPHYa is not required for release of parasite-encoded effectors into host cells that neutralize anti-parasitic processes induced by IFNγ. In contrast, we find that TgPHYa is required for the parasite to scavenge tryptophan, which is an amino acid whose levels are decreased after IFNγ up-regulates the tryptophan-catabolizing enzyme, indoleamine dioxygenase (IDO). We further find, relative to wild-type mice, that IDO knockout mice display increased morbidity when infected with TgPHYa knockout parasites. Together, these data identify the first parasite mechanism for evading IFNγ-induced nutritional immunity and highlight a novel role that oxygen-sensing proteins play in pathogen growth and virulence.

Tryptophan metabolism in digestive system tumors: unraveling the pathways and implications

Tryptophan (Trp) metabolism plays a crucial role in influencing the development of digestive system tumors. Dysregulation of Trp and its metabolites has been identified in various digestive system cancers, including esophageal, gastric, liver, colorectal, and pancreatic cancers. Aberrantly expressed Trp metabolites are associated with diverse clinical features in digestive system tumors. Moreover, the levels of these metabolites can serve as prognostic indicators and predictors of recurrence risk in patients with digestive system tumors. Trp metabolites exert their influence on tumor growth and metastasis through multiple mechanisms, including immune evasion, angiogenesis promotion, and drug resistance enhancement. Suppressing the expression of key enzymes in Trp metabolism can reduce the accumulation of these metabolites, effectively impacting their role in the promotion of tumor progression and metastasis. Strategies targeting Trp metabolism through specific enzyme inhibitors or tailored drugs exhibit considerable promise in enhancing therapeutic outcomes for digestive system tumors. In addition, integrating these approaches with immunotherapy holds the potential to further enhance treatment efficacy.

Targeting cellular metabolism in head and neck cancer precision medicine era: A promising strategy to overcome therapy resistance

Head and neck squamous cell carcinoma (HNSCC) is among the most prevalent cancer worldwide, with the most severe impact on quality of life of patients. Despite the development of multimodal therapeutic approaches, the clinical outcomes of HNSCC are still unsatisfactory, mainly caused by relatively low responsiveness to treatment and severe drug resistance. Metabolic reprogramming is currently considered to play a pivotal role in anticancer therapeutic resistance. This review aimed to define the specific metabolic programs and adaptations in HNSCC therapy resistance. An extensive literature review of HNSCC was conducted via the PubMed including metabolic reprogramming, chemo- or immune-therapy resistance. Glucose metabolism, fatty acid metabolism, and amino acid metabolism are closely related to the malignant biological characteristics of cancer, anti-tumor drug resistance, and adverse clinical results. For HNSCC, pyruvate, lactate and almost all lipid categories are related to the occurrence and maintenance of drug resistance, and targeting amino acid metabolism can prevent tumor development and enhance the response of drug-resistant tumors to anticancer therapy. This review will provide a better understanding of the altered metabolism in therapy resistance of HNSCC and promote the development of new therapeutic strategies against HNSCC, thereby contribute to a more efficacious precision medicine.

Tryptophan metabolism in health and disease

Tryptophan (Trp) metabolism primarily involves the kynurenine, 5-hydroxytryptamine, and indole pathways. A variety of bioactive compounds produced via Trp metabolism can regulate various physiological functions, including inflammation, metabolism, immune responses, and neurological function. Emerging evidence supports an intimate relationship between Trp metabolism disorder and diseases. The levels or ratios of Trp metabolites are significantly associated with many clinical features. Additionally, studies have shown that disease progression can be controlled by modulating Trp metabolism. Indoleamine-2,3-dioxygenase, Trp-2,3-dioxygenase, kynurenine-3-monooxygenase, and Trp hydroxylase are the rate-limiting enzymes that are critical for Trp metabolism. These key regulatory enzymes can be targeted for treating several diseases, including tumors. These findings provide novel insights into the treatment of diseases. In this review, we have summarized the recent research progress on the role of Trp metabolites in health and disease along with their clinical applications.

Involvement of the kynurenine pathway in breast cancer: updates on clinical research and trials

Breast cancer (BrCa) is the leading cause of cancer incidence and mortality in women worldwide. While BrCa treatment has been shown to be highly successful if detected at an early stage, there are few effective strategies to treat metastatic tumours. Hence, metastasis remains the main cause in most of BrCa deaths, highlighting the need for new approaches in this group of patients. Immunotherapy has been gaining attention as a new treatment for BrCa metastasis and the kynurenine pathway (KP) has been suggested as one of the potential targets. The KP is the major biochemical pathway in tryptophan (TRP) metabolism, catabolising TRP to nicotinamide adenine dinucleotide (NAD+). The KP has been reported to be elevated under inflammatory conditions such as cancers and that its activity suppresses immune surveillance. Dysregulation of the KP has previously been reported implicated in BrCa. This review aims to discuss and provide an update on the current mechanisms involved in KP-mediated immune suppression and cancer growth. Furthermore, we also provide a summary on 58 studies about the involvement of the KP and BrCa and five clinical trials targeting KP enzymes and their outcome.

Preclinical PK investigation of a novel IDO1/TDO dual inhibitor-SHR9146 in mouse plasma and tissues by LC-MS/MS

Purpose

The aim of the present study was to establish a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of SHR9146, a novel IDO1/TDO dual inhibitor, in mouse plasma and tissues, and to apply it to investigate the preclinical plasma pharmacokinetics and tissue distribution of SHR9146 in mice.

Methods

Samples were spiked with deuterated SHR9146-d4 as an internal standard and pretreated by protein-precipitation extraction with methanol. Chromatographic separation was performed on a Venusil ABS C18 column (150 × 4.6 mm, 5 μm) by isocratic elution with 10 mM ammonium acetate buffer containing 0.1% formic acid solution and methanol as mobile phases. MS detection was conducted in positive electrospray ionization with multiple reaction monitoring at m/z 444.1/229.4 for SHR9146 and m/z 448.4/229.2 for the internal standard.

Results

The method showed good linearity in the calibration range from 0.05 to 50.0 μg/mL. Precisions (intra- and inter-run) were in the range from 0.5% to 5.1%, and accuracies (RE) were between -3.0% and 4.4% for all the concentration levels. SHR9146 was stable in all the tested bio-samples with recoveries >90%. Pharmacokinetic parameters were obtained by non-compartmental analysis. SHR9146 has a half-life of 0.713 h when IV-injected, with CL 12 mL/min/kg and Vd 0.666 L/kg. After oral dosing from 20 to 80 mg/kg, Cmax (range from 8.751 to 12.893 μg/mL) and AUC0-t (range from 15.606 to 69.971 μg·h/mL) of SHR9146 showed dose proportionality. Other post-oral pharmacokinetic parameters in plasma were as follows (n=6): Tmax 0.79 ± 0.36 h, t1/2 1.586 ± 0.853 h, CL 19.8 ± 0.9 mL/min/kg, Vd 3.427± 1.617 L/kg, and absolute bioavailability of 54.2% ± 12.6% (range from 40.2% to 64.7%). In addition, SHR9146 was found to be absorbed rapidly and distributed widely and mainly in the stomach, adrenal gland, liver, and lung.

Conclusion

The method was simple, sensitive, accurate, and specific and was successfully applied for the preclinical pharmacokinetic and tissue distribution study of SHR9146 in mice. The results showed that SHR9146 had dose-independent kinetics in mice via oral administration and was absorbed rapidly and distributed widely. The study provides a good basis for further drug development assessment.

Tryptophanase Expressed by Salmonella Halts Breast Cancer Cell Growth In Vitro and Inhibits Production of Immunosuppressive Kynurenine

Tryptophan is an essential amino acid required for tumor cell growth and is also the precursor to kynurenine, an immunosuppressive molecule that plays a role in limiting anticancer immunity. Tryptophanase (TNase) is an enzyme expressed by different bacterial species that converts tryptophan into indole, pyruvate and ammonia, but is absent in the Salmonella strain VNP20009 that has been used as a therapeutic delivery vector. We cloned the Escherichia coli TNase operon tnaCAB into the VNP20009 (VNP20009-tnaCAB), and were able to detect linear production of indole over time, using Kovács reagent. In order to conduct further experiments using the whole bacteria, we added the antibiotic gentamicin to stop bacterial replication. Using a fixed number of bacteria, we found that there was no significant effect of gentamicin on stationary phase VNP20009-tnaCAB upon their ability to convert tryptophan to indole over time. We developed a procedure to extract indole from media while retaining tryptophan, and were able to measure tryptophan spectrophotometrically after exposure to gentamicin-inactivated whole bacterial cells. Using the tryptophan concentration equivalent to that present in DMEM cell culture media, a fixed number of bacteria were able to deplete 93.9% of the tryptophan in the culture media in 4 h. In VNP20009-tnaCAB depleted tissue culture media, MDA-MB-468 triple negative breast cancer cells were unable to divide, while those treated with media exposed only to VNP20009 continued cell division. Re-addition of tryptophan to conditioned culture media restored tumor cell growth. Treatment of tumor cells with molar equivalents of the TNase products indole, pyruvate and ammonia only caused a slight increase in tumor cell growth. Using an ELISA assay, we confirmed that TNase depletion of tryptophan also limits the production of immunosuppressive kynurenine in IFNγ-stimulated MDA-MB-468 cancer cells. Our results demonstrate that Salmonella VNP20009 expressing TNase has improved potential to stop tumor cell growth and reverse immunosuppression.

The role of amino acid metabolism alterations in pancreatic cancer: From mechanism to application

The incidence of pancreatic cancer is increasing in both developed and developing Nations. In recent years, various research evidence suggested that reprogrammed metabolism may play a key role in pancreatic cancer tumorigenesis and development. Therefore, it has great potential as a diagnostic, prognostic and therapeutic target. Amino acid metabolism is deregulated in pancreatic cancer, and changes in amino acid metabolism can affect cancer cell status, systemic metabolism in malignant tumor patients and mistakenly involved in different biological processes including stemness, proliferation and growth, invasion and migration, redox state maintenance, autophagy, apoptosis and even tumor microenvironment interaction. Generally, the above effects are achieved through two pathways, energy metabolism and signal transduction. This review aims to highlight the current research progress on the abnormal alterations of amino acids metabolism in pancreatic cancer, how they affect tumorigenesis and development of pancreatic cancer and the application prospects of them as diagnostic, prognostic and therapeutic targets.

IDO1 Is a Therapeutic Target for Pancreatic Cancer-Associated Depression

Metabolites of tryptophan degradation are known to alter mood. Their effects have only been superficially examined in the context of pancreatic cancer. Herein, we study the role of indoleamine 2,3-dioxygenase 1 (IDO1), an enzyme important in the conversion of tryptophan to kynurenine, in a murine model of pancreatic cancer-associated depression. Behavioral tests (open field, forced swim, tail suspension, and elevated plus maze) and biochemical assays (LC-MS metabolomics) were used to characterize a depressive-phenotype in tumor-bearing mice (relative to non-tumor-bearing mice). In addition, we determine whether pharmacologic blockade of IDO1 affects mood in tumor-bearing mice. Immunocompetent mice bearing orthotopic pancreatic tumors exhibit depressive-like behavior relative to non-tumor-bearing mice. Pancreatic tumors strongly express IDO1. Consequently, serum kynurenine levels in tumor-bearing mice are elevated relative to non-tumor-bearing mice. Tumor-bearing mice treated with epacadostat, an IDO1 inhibitor, exhibited improved mood relative to mice receiving vehicle. There was a 95% reduction in serum kynurenine levels in mice receiving epacadostat relative to mice treated with vehicle. As confirmatory evidence of on-target activity, tumors of mice treated with epacadostat exhibited a compensatory increase in IDO1 protein levels. Escitalopram, an approved antidepressant, was ineffective at improving mood in tumor-bearing mice as measured by behavioral assays and did not affect kynurenine levels. Neither epacadostat, nor escitalopram, affected overall survival relative to vehicle. Mice with pancreatic cancer exhibit depressive-like behavior. Epacadostat was effective as an antidepressant for pancreatic cancer-associated depression in mice. These data offer a rationale to consider IDO1 inhibition as a therapeutic strategy to mitigate depressive symptoms in patients with pancreatic cancer.

Tryptophan metabolism and disposition in cancer biology and immunotherapy

Tumours utilise tryptophan (Trp) and its metabolites to promote their growth and evade host defences. They recruit Trp through up-regulation of Trp transporters, and up-regulate key enzymes of Trp degradation and down-regulate others. Thus, Trp 2,3-dioxygenase (TDO2), indoleamine 2,3-dioxygenase 1 (IDO1), IDO2, N'-formylkynurenine formamidase (FAMID) and Kyn aminotransferase 1 (KAT1) are all up-regulated in many cancer types, whereas Kyn monooxygenase (KMO), kynureninase (KYNU), 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) and quinolinate phosphoribosyltransferase (QPRT) are up-regulated in a few, but down-regulated in many, cancers. This results in accumulation of the aryl hydrocarbon receptor (AhR) ligand kynurenic acid and in depriving the host of NAD+ by blocking its synthesis from quinolinic acid. The host loses more NAD+ by up-regulation of the NAD+-consuming poly (ADP-ribose) polymerases (PARPs) and the protein acetylaters SIRTs. The nicotinamide arising from PARP and SIRT activation can be recycled in tumours to NAD+ by the up-regulated key enzymes of the salvage pathway. Up-regulation of the Trp transporters SLC1A5 and SLC7A5 is associated mostly with that of TDO2 = FAMID > KAT1 > IDO2 > IDO1. Tumours down-regulate enzymes of serotonin synthesis, thereby removing competition for Trp from the serotonin pathway. Strategies for combating tumoral immune escape could involve inhibition of Trp transport into tumours, inhibition of TDO and IDOs, inhibition of FAMID, inhibition of KAT and KYNU, inhibition of NMPRT and NMNAT, inhibition of the AhR, IL-4I1, PARPs and SIRTs, and by decreasing plasma free Trp availability to tumours by albumin infusion or antilipolytic agents and inhibition of glucocorticoid induction of TDO by glucocorticoid antagonism.

IDO/kynurenine pathway in cancer: possible therapeutic approaches

Cancer is one of the leading causes of death in both men and women worldwide. One of the main changes associated with cancer progression, metastasis, recurrence, and chemoresistance is the change in the tumor immune microenvironment, especially immunosuppression. Cancer immunosuppression appears in multiple forms, such as inhibition of immuno-stimulant cells with downregulation of immuno-stimulant mediators or through stimulation of immuno-suppressive cells with upregulation of immunosuppressive mediators. One of the most immunosuppressive mediators that approved potency in lung cancer progression is indoleamine 2,3-dioxygenase (IDO) and its metabolite kynurenine (Kyn). The current review tries to elucidate the role of IDO/Kyn on cancer proliferation, apoptosis, angiogenesis, oxidative stress, and cancer stemness. Besides, our review investigates the new therapeutic modalities that target IDO/Kyn pathway and thus as drug candidates for targeting lung cancer and drugs that potentiate IDO/Kyn pathway and thus can be cancer-promoting agents.

Subversion of Serotonin Receptor Signaling in Osteoblasts by Kynurenine Drives Acute Myeloid Leukemia

Remodeling of the microenvironment by tumor cells can activate pathways that favor cancer growth. Molecular delineation and targeting of such malignant-cell nonautonomous pathways may help overcome resistance to targeted therapies. Herein we leverage genetic mouse models, patient-derived xenografts, and patient samples to show that acute myeloid leukemia (AML) exploits peripheral serotonin signaling to remodel the endosteal niche to its advantage. AML progression requires the presence of serotonin receptor 1B (HTR1B) in osteoblasts and is driven by AML-secreted kynurenine, which acts as an oncometabolite and HTR1B ligand. AML cells utilize kynurenine to induce a proinflammatory state in osteoblasts that, through the acute-phase protein serum amyloid A (SAA), acts in a positive feedback loop on leukemia cells by increasing expression of IDO1-the rate-limiting enzyme for kynurenine synthesis-thereby enabling AML progression. This leukemia-osteoblast cross-talk, conferred by the kynurenine-HTR1B-SAA-IDO1 axis, could be exploited as a niche-focused therapeutic approach against AML, opening new avenues for cancer treatment.

SIGNIFICANCE

AML remains recalcitrant to treatments due to the emergence of resistant clones. We show a leukemia-cell nonautonomous progression mechanism that involves activation of a kynurenine-HTR1B-SAA-IDO1 axis between AML cells and osteoblasts. Targeting the niche by interrupting this axis can be pharmacologically harnessed to hamper AML progression and overcome therapy resistance. This article is highlighted in the In This Issue feature, p. 873.

Metabolic adaptation of ovarian tumors in patients treated with an IDO1 inhibitor constrains antitumor immune responses

To uncover underlying mechanisms associated with failure of indoleamine 2,3-dioxygenase 1 (IDO1) blockade in clinical trials, we conducted a pilot, window-of-opportunity clinical study in 17 patients with newly diagnosed advanced high-grade serous ovarian cancer before their standard tumor debulking surgery. Patients were treated with the IDO1 inhibitor epacadostat, and immunologic, transcriptomic, and metabolomic characterization of the tumor microenvironment was undertaken in baseline and posttreatment tumor biopsies. IDO1 inhibition resulted in efficient blockade of the kynurenine pathway of tryptophan degradation and was accompanied by a metabolic adaptation that shunted tryptophan catabolism toward the serotonin pathway. This resulted in elevated nicotinamide adenine dinucleotide (NAD⁺), which reduced T cell proliferation and function. Because NAD⁺ metabolites could be ligands for purinergic receptors, we investigated the impact of blocking purinergic receptors in the presence or absence of NAD⁺ on T cell proliferation and function in our mouse model. We demonstrated that A2a and A2b purinergic receptor antagonists, SCH58261 or PSB1115, respectively, rescued NAD⁺-mediated suppression of T cell proliferation and function. Combining IDO1 inhibition and A2a/A2b receptor blockade improved survival and boosted the antitumor immune signature in mice with IDO1 overexpressing ovarian cancer. These findings elucidate the downstream adaptive metabolic consequences of IDO1 blockade in ovarian cancers that may undermine antitumor T cell responses in the tumor microenvironment.

Oxetane Promise Delivered: Discovery of Long-Acting IDO1 Inhibitors Suitable for Q3W Oral or Parenteral Dosing

3,3-Disubstituted oxetanes have been utilized as bioisosteres for gem-dimethyl and cyclobutane functionalities. We report the discovery of a novel class of oxetane indole-amine 2,3-dioxygenase (IDO1) inhibitors suitable for Q3W (once every 3 weeks) oral and parenteral dosing. A diamide class of IDO inhibitors was discovered through an automated ligand identification system (ALIS). Installation of an oxetane and fluorophenyl dramatically improved the potency. Identification of a biaryl moiety as an unconventional amide isostere addressed the metabolic liability of amide hydrolysis. Metabolism identification (Met-ID)-guided target design and the introduction of polarity resulted in the discovery of potent IDO inhibitors with excellent pharmacokinetic (PK) profiles in multiple species. To enable rapid synthesis of the key oxetane intermediate, a novel oxetane ring cyclization was also developed, as well as optimization of a literature route on kg scale. These IDO inhibitors may enable unambiguous proof-of-concept testing for the IDO1 inhibition mechanism for oncology.

Targeting Tryptophan Catabolism in Cancer Immunotherapy Era: Challenges and Perspectives

Metabolism of tryptophan (Trp), an essential amino acid, represent a major metabolic pathway that both promotes tumor cell intrinsic malignant properties as well as restricts antitumour immunity, thus emerging as a drug development target for cancer immunotherapy. Three cytosolic enzymes, namely indoleamine 2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan 2,3-dioxygenase (TDO2), catalyzes the first-rate limiting step of the degradation of Trp to kynurenine (Kyn) and modulates immunity toward immunosuppression mainly through the aryl hydrocarbon receptor (AhR) activation in numerous types of cancer. By restoring antitumor immune responses and synergizing with other immunotherapies, the encouraging preclinical data of IDO1 inhibitors has dramatically failed to translate into clinical success when combined with immune checkpoints inhibitors, reigniting the debate of combinatorial approach. In this review, we i) provide comprehensive evidences on immunomodulatory role of the Trp catabolism metabolites that highlight this pathway as relevant target in immuno-oncology, ii)ii) discuss underwhelming results from clinical trials investigating efficacy of IDO1 inhibitors and underlying mechanisms that might have contributed to this failure, and finally, iii) discuss the current state-of-art surrounding alternative approaches of innovative antitumor immunotherapies that target molecules of Trp catabolism as well as challenges and perspectives in the era of immunotherapy.

Clinical development of metabolic inhibitors for oncology

Metabolic inhibitors have been used in oncology for decades, dating back to antimetabolites developed in the 1940s. In the past 25 years, there has been increased recognition of metabolic derangements in tumor cells leading to a resurgence of interest in targeting metabolism. More recently there has been recognition that drugs targeting tumor metabolism also affect the often acidic, hypoxic, immunosuppressive tumor microenvironment (TME) and non-tumor cell populations within it, including immune cells. Here we review small-molecule metabolic inhibitors currently in clinical development for oncology applications. For each agent, we evaluate the preclinical studies demonstrating antitumor and TME effects and review ongoing clinical trials. The goal of this Review is to provide an overview of the landscape of metabolic inhibitors in clinical development for oncology.

Immune System in Action

Tumor exists as a complex network of structures with an ability to evolve and evade the host immune surveillance mechanism. The immune milieu which includes macrophages, dendritic cells, natural killer cells, neutrophils, mast cells, B cells, and T cells is found in the core, the invasive margin, or the adjacent stromal or lymphoid component of the tumor. The immune infiltrate is heterogeneous and varies within a patient and between patients of the same tumor histology. The location, density, functionality, and the crosstalk between the immune cells in the tumor microenvironment influence the nature of immune response, prognosis, and treatment outcomes in cancer patients. Therefore, an understanding of the characteristics of the immune cells and their role in tumor immune surveillance is of paramount importance to identify immune targets and to develop novel immune therapeutics in the war against cancer. In this chapter, we provide an overview of the individual components of the human immune system and the translational relevance of predictive biomarkers.

Recent advances in clinical trials targeting the kynurenine pathway

The kynurenine pathway (KP) is the major catabolic pathway for the essential amino acid tryptophan leading to the production of nicotinamide adenine dinucleotide. In inflammatory conditions, the activation of the KP leads to the production of several bioactive metabolites including kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, kynurenic acid and quinolinic acid. These metabolites can have redox and immune suppressive activity, be neurotoxic or neuroprotective. While the activity of the pathway is tightly regulated under normal physiological condition, it can be upregulated by immunological activation and inflammation. The dysregulation of the KP has been implicated in wide range of neurological diseases and psychiatric disorders. In this review, we discuss the mechanisms involved in KP-mediated neurotoxicity and immune suppression, and its role in diseases of our expertise including cancer, chronic pain and multiple sclerosis. We also provide updates on the clinical trials evaluating the efficacy of KP inhibitors and/or analogues in each respective disease.

Deep learning model enables the discovery of a novel immunotherapeutic agent regulating the kynurenine pathway

Kynurenine (Kyn) is a key inducer of an immunosuppressive tumor microenvironment (TME). Although indoleamine 2,3-dioxygenase (IDO)-selective inhibitors have been developed to suppress the Kyn pathway, the results were not satisfactory due to the presence of various opposing mechanisms. Here, we employed an orally administered novel Kyn pathway regulator to overcome the limitation of anti-tumor immune response. We identified a novel core structure that inhibited both IDO and TDO. An orally available lead compound, STB-C017 (designated hereafter as STB), effectively inhibited the enzymatic and cellular activity of IDO and TDO in vitro. Moreover, it potently suppressed Kyn levels in both the plasma and tumor in vivo. STB monotherapy increased the infiltration of CD8⁺ T cells into TME. In addition, STB reprogrammed the TME with widespread changes in immune-mediated gene signatures. Notably, STB-based combination immunotherapy elicited the most potent anti-tumor efficacy through concurrent treatment with immune checkpoint inhibitors, leading to complete tumor regression and long-term overall survival. Our study demonstrated that a novel Kyn pathway regulator derived using deep learning technology can activate T cell immunity and potentiate immune checkpoint blockade by overcoming an immunosuppressive TME.

IDO1/TDO dual inhibitor RY103 targets Kyn-AhR pathway and exhibits preclinical efficacy on pancreatic cancer

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzing the conversion of tryptophan (Trp) to kynurenine (Kyn) in kynurenine pathway (KP) is involved in the immunosuppression in pancreatic cancer (PC), but the value of IDO1 as an independent prognostic marker for PC is uncertain. Moreover, the correlation between tryptophan 2,3-dioxygenase (TDO), an isozyme of IDO1, and PC is largely unknown. Using TCGA database, the correlation between IDO1 and/or TDO expression and PC patients' survival was analyzed. The expressions of IDO1 and TDO in PC cells and PC mice were examined. The effects of IDO1, TDO or dual inhibition on IDO1 and TDO effector pathway (Aryl hydrocarbon receptor, AhR) and on migration and invasion of PC cells were investigated. The block effect of IDO1/TDO dual inhibitor RY103 on KP was evaluated. The preclinical efficacy of RY103 and its immunomodulatory effect on KPIC orthotopic PC mice and Pan02 tumor-bearing mice were explored. Results showed that IDO1/TDO co-expression is an independent prognostic marker for PC. RY103 can significantly block KP and target Kyn-AhR pathway to blunt the migration and invasion of PC cells, exhibit preclinical efficacy and ameliorate IDO1/TDO-mediated immunosuppression in PC mice.

Peptide vaccination directed against IDO1-expressing immune cells elicits CD8+ and CD4+ T-cell-mediated antitumor immunity and enhanced anti-PD1 responses

Background

The tryptophan-catabolizing enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which subverts T-cell immunity at multiple levels, is itself subject to inherent T-cell reactivity. This intriguing deviation from central tolerance has been interpreted as counterbalancing IDO1-mediated immunosuppression. Based on this hypothesis, clinical studies employing an IDO1 peptide-based vaccine approach for cancer treatment have been initiated, but there remains a pressing need to further investigate the immunological ramifications of stimulating the anti-IDO1 T-cell response in this manner.

Methods

CT26 colon carcinoma tumors were evaluated for expression of IDO1 protein by western blot analysis, immunofluorescence microscopy and flow cytometry. Mouse IDO1-derived peptides, predicted to bind either major histocompatibility complex (MHC) class I or II of the H2^d BALB/c strain, were emulsified in 50% Montanide for prophylactic or therapeutic vaccine treatment of CT26 tumor-bearing mice initiated either 7 days prior to or following tumor cell injection, respectively. In some therapeutic treatment experiments, administration of programmed cell death protein 1-binding antibody (anti-PD1 antibody) or epacadostat was concurrently initiated. Tumor size was determined by caliper measurements and comparative tumor growth suppression was assessed by longitudinal analyses of tumor growth data. For adoptive transfer, T cells from complete responder animals were isolated using paramagnetic beads and fluorescence-activated cell sorting.

Results

This study identifies mouse MHC class I-directed and II-directed, IDO1-derived peptides capable of eliciting antitumor responses, despite finding IDO1 expressed exclusively in tumor-infiltrating immune cells. Treatment of established tumors with anti-PD1 antibody and class I-directed but not class II-directed IDO1 peptide vaccines produced an enhanced antitumor response. Likewise, class I-directed and II-directed IDO1 peptides elicited an enhanced combinatorial response, suggesting distinct mechanisms of action. Consistent with this interpretation, adoptive transfer of isolated CD8⁺ T cells from class I and CD4⁺ T cells from class II peptide-vaccinated responder mice delayed tumor growth. The class II-directed response was completely IDO1-dependent while the class I-directed response included an IDO1-independent component consistent with antigen spread.

Conclusions

The in vivo antitumor effects demonstrated with IDO1-based vaccines via targeting of the tumor microenvironment highlight the utility of mouse models for further exploration and refinement of this novel vaccine-based approach to IDO1-directed cancer therapy and its potential to improve patient response rates to anti-PD1 therapy.

Prognostic value of the PrPC-ILK-IDO1 axis in the mesenchymal colorectal cancer subtype

The CMS4 mesenchymal subtype of colorectal cancer (CRC) is associated with poor prognosis and resistance to treatment. The cellular prion protein PrP^C is overexpressed in CMS4 tumors and controls the expression of a panel of CMS4-specific genes in CRC cell lines. Here, we sought to investigate PrP^C downstream pathways that may underlie its role in CMS4 CRC. By combining gene set enrichment analyses and gain and loss of function approaches in CRC cell lines, we identify the integrin-linked kinase ILK as a proximal effector of PrP^C that mediates its control on the CMS4 phenotype. We further leveraged three independent large CRC cohorts to assess correlations in gene expression pattern with patient outcomes and found that ILK is overexpressed in CMS4 mesenchymal tumors and confers a poor prognosis, especially when combined with high expression of the PrP^C encoding gene PRNP. Of note, we discovered that the PrP^C-ILK signaling axis controls the expression and activity of the tryptophan metabolizing enzyme indoleamine 2,3 dioxygenase IDO1, a key player in immune tolerance. In addition, we monitored alterations in the levels of tryptophan and its metabolites of the kynurenine pathway in the plasma of metastatic CRC patients (n = 325) and we highlight their prognostic value in combination with plasma PrP^C levels. Thus, the PrP^C-ILK-IDO1 axis plays a key role in the mesenchymal subtype of CRC. PrP^C and IDO1-targeted strategies may represent new avenues for patient stratification and treatment in CRC.

Review of 10 years of research on breast cancer patients: Focus on indoleamine 2,3-dioxygenase

Therapeutic manipulation of the immune system in cancer has been an extensive area of research in the field of oncoimmunology. Immunosuppression regulates antitumour immune responses. An immunosuppressive enzyme, indoleamine 2,3-dioxygenase (IDO) mediates tumour immune escape in various malignancies including breast cancer. IDO upregulation in breast cancer cells may lead to the recruitment of regulatory T (T-regs) cells into the tumour microenvironment, thus inhibiting local immune responses and promoting metastasis. Immunosuppression induced by myeloid derived suppressor cells activated in an IDO-dependent manner may enhance the possibility of immune evasion in breast cancer. IDO overexpression has independent prognostic significance in a subtype of breast cancer of emerging interest, basal-like breast carcinoma. IDO inhibitors as adjuvant therapeutic agents may have clinical implications in breast cancer. This review proposes future prospects of IDO not only as a therapeutic target but also as a valuable prognostic marker for breast cancer.

Reliable chromatographic assay for measuring of indoleamine 2,3-dioxygenase 1 (IDO1) activity in human cancer cells

The kynurenine pathway is the major tryptophan degradation routes generating bioactive compounds important in physiology and diseases. Depending on cell type it is initiated enzymatically by tryptophan-2,3-dioxygenase (TDO) or indoleamine-2,3-dioxygenase 1 and 2 (IDO1 and IDO2) to yield N-formylkynurenine as the precursor of further metabolites. Herein, we describe an accurate high-pressure liquid chromatography coupled with a diode array detector (HPLC-DAD) method to serve for IDO1 activity determination in human cancer cells cultured in vitro. Enzymatic activity was expressed as the rate of ʟ-kynurenine generation by 1 mg of proteins obtained from cancer cells. Our approach shows the limit of detection and limit of quantification at 12.9 and 43.0 nM Kyn, respectively. Applicability of this method was demonstrated in different cells (ovarian and breast cancer)exposed to various conditions and has successfully passed the validation process. This approach presents a useful model to study the role of kynurenine pathway in cancer biology.

Tryptophanemia is controlled by a tryptophan-sensing mechanism ubiquitinating tryptophan 2,3-dioxygenase

Maintaining stable tryptophan levels is required to control neuronal and immune activity. We report that tryptophan homeostasis is largely controlled by the stability of tryptophan 2,3-dioxygenase (TDO), the hepatic enzyme responsible for tryptophan catabolism. High tryptophan levels stabilize the active tetrameric conformation of TDO through binding noncatalytic exosites, resulting in rapid catabolism of tryptophan. In low tryptophan, the lack of tryptophan binding in the exosites destabilizes the tetramer into inactive monomers and dimers and unmasks a four-amino acid degron that triggers TDO polyubiquitination by SKP1-CUL1-F-box complexes, resulting in proteasome-mediated degradation of TDO and rapid interruption of tryptophan catabolism. The nonmetabolizable analog alpha-methyl-tryptophan stabilizes tetrameric TDO and thereby stably reduces tryptophanemia. Our results uncover a mechanism allowing a rapid adaptation of tryptophan catabolism to ensure quick degradation of excess tryptophan while preventing further catabolism below physiological levels. This ensures a tight control of tryptophanemia as required for both neurological and immune homeostasis.

Protective Effects of Necrostatin-1 in Acute Pancreatitis: Partial Involvement of Receptor Interacting Protein Kinase 1

Acute pancreatitis (AP) is a severe and potentially fatal disease caused predominantly by alcohol excess and gallstones, which lacks a specific therapy. The role of Receptor-Interacting Protein Kinase 1 (RIPK1), a key component of programmed necrosis (Necroptosis), is unclear in AP. We assessed the effects of RIPK1 inhibitor Necrostatin-1 (Nec-1) and RIPK1 modification (RIPK1K45A: kinase dead) in bile acid (TLCS-AP), alcoholic (FAEE-AP) and caerulein hyperstimulation (CER-AP) mouse models. Involvement of collateral Nec-1 target indoleamine 2,3-dioxygenase (IDO) was probed with the inhibitor Epacadostat (EPA). Effects of Nec-1 and RIPK1K45A were also compared on pancreatic acinar cell (PAC) fate in vitro and underlying mechanisms explored. Nec-1 markedly ameliorated histological and biochemical changes in all models. However, these were only partially reduced or unchanged in RIPK1K45A mice. Inhibition of IDO with EPA was protective in TLCS-AP. Both Nec-1 and RIPK1K45A modification inhibited TLCS- and FAEE-induced PAC necrosis in vitro. Nec-1 did not affect TLCS-induced Ca2+ entry in PACs, however, it inhibited an associated ROS elevation. The results demonstrate protective actions of Nec-1 in multiple models. However, RIPK1-dependent necroptosis only partially contributed to beneficial effects, and actions on targets such as IDO are likely to be important.

Cell-Based Identification of New IDO1 Modulator Chemotypes

The immunoregulatory enzyme indoleamine-2,3-dioxygenase (IDO1) strengthens cancer immune escape, and inhibition of IDO1 by means of new chemotypes and mechanisms of action is considered a promising opportunity for IDO1 inhibitor discovery. IDO1 is a cofactor-binding, redox-sensitive protein, which calls for monitoring of IDO1 activity in its native cellular environment. We developed a new, robust fluorescence-based assay amenable to high throughput, which detects kynurenine in cells. Screening of a ca. 150 000-member compound library discovered unprecedented, potent IDO1 modulators with different mechanisms of action, including direct IDO1 inhibitors, regulators of IDO1 expression, and inhibitors of heme synthesis. Three IDO1-modulator chemotypes were identified that bind to apo-IDO1 and compete with the heme cofactor. Our new cell-based technology opens up novel opportunities for medicinal chemistry programs in immuno-oncology.

IDO1-Targeted Therapy Does Not Control Disease Development in the Eµ-TCL1 Mouse Model of Chronic Lymphocytic Leukemia

Simple Summary

The tryptophan-catabolizing enzyme IDO1 and its metabolite kynurenine were shown to be enhanced in patients with chronic lymphocytic leukemia (CLL), and their involvement in T cell suppression and immune escape was suggested. As we have observed increased IDO1 expression and kynurenine serum levels in the Eµ-TCL1 mouse model of CLL, we evaluated the therapeutic potential of targeting IDO1 in preclinical treatment studies with two IDO1 inhibitors in mice developing CLL. As both studies revealed only minor effects of IDO1 inhibition on leukemia development and the immune compartment at early time points of treatment which disappeared over time, our data suggest that even though IDO1 might be involved in immunosuppressive mechanisms in CLL, its targeting is not sufficient for preventing immune escape. Thus, compensatory mechanisms beyond IDO1 seem to be of relevance to prevent clinically relevant benefits with IDO1-targeting drugs.

Abstract

Indoleamine-2,3-dioxygenase 1 (IDO1), a tryptophan (Trp)-catabolizing enzyme producing metabolites such as kynurenine (Kyn), is expressed by myeloid-derived suppressor cells (MDSCs) and associated with cancer immune escape. IDO1-expressing monocytic MDSCs were shown to accumulate in patients with chronic lymphocytic leukemia (CLL) and to suppress T cell activity and induce suppressive regulatory T cells (Tregs) in vitro. In the Eµ-TCL1 mouse model of CLL, we observed a strong upregulation of IDO1 in monocytic and granulocytic MDSCs, and a significantly increased Kyn to Trp serum ratio. To explore the potential of IDO1 as a therapeutic target for CLL, we treated mice after adoptive transfer of Eµ-TCL1 leukemia cells with the IDO1 modulator 1-methyl-D-tryptophan (1-MT) which resulted in a minor reduction in leukemia development which disappeared over time. 1-MT treatment further led to a partial rescue of the immune cell changes that are induced with CLL development. Similarly, treatment of leukemic mice with the clinically investigated IDO1 inhibitor epacadostat reduced the frequency of Tregs and initially delayed CLL development slightly, an effect that was, however, lost at later time points. In sum, despite the observed upregulation of IDO1 in CLL, its inhibition is not sufficient to control leukemia development in the Eµ-TCL1 adoptive transfer model.

Indoleamine 2,3-Dioxygenase 1 Inhibitor-Loaded Nanosheets Enhance CAR-T Cell Function in Esophageal Squamous Cell Carcinoma

In chimeric antigen receptor (CAR)-T cell therapy, the role and mechanism of indoleamine 2, 3 dioxygenase 1 (IDO1) in enhancing antitumor immunity require further study. IDO1 is one of the most important immunosuppressive proteins in esophageal squamous cell carcinoma (ESCC). However, the IDO1 inhibitor, epacadostat, has failed in phase III clinical trials; its limited capacity to inhibit IDO1 expression at tumor sites was regarded as a key reason for clinical failure. In this study, we innovatively loaded the IDO1 inhibitor into hyaluronic acid-modified nanomaterial graphene oxide (HA-GO) and explored its potential efficacy in combination with CAR-T cell therapy. We found that inhibition of the antitumor effect of CAR-T cells in ESCC was dependent on the IDO1 metabolite kynurenine. Kynurenine could suppress CAR-T cell cytokine secretion and cytotoxic activity. Inhibiting IDO1 activity significantly enhanced the antitumor effect of CAR-T cells in vitro and in vivo. Our findings suggested that IDO1 inhibitor-loaded nanosheets could enhance the antitumor effect of CAR-T cells compared with free IDO1 inhibitor. Nanosheet-loading therefore provides a promising approach for improving CAR-T cell therapeutic efficacy in solid tumors.

Discovery of Imidazopyridines as Potent Inhibitors of Indoleamine 2,3-Dioxygenase 1 for Cancer Immunotherapy

Indoleamine 2,3-dioxygenase 1 (IDO1) has been identified as a target for small-molecule immunotherapy for the treatment of a variety of cancers including renal cell carcinoma and metastatic melanoma. This work focuses on the identification of IDO1 inhibitors containing replacements or isosteres for the amide found in BMS-986205, an amide-containing, IDO1-selective inhibitor currently in phase III clinical trials. Detailed subsequently are efforts to identify a structurally differentiated IDO1 inhibitor via the pursuit of a variety of heterocyclic isosteres, leading to the discovery of highly potent, imidazopyridine-containing IDO1 inhibitors.

Polypeptide nanoformulation-induced immunogenic cell death and remission of immunosuppression for enhanced chemoimmunotherapy

Many conventional chemotherapeutics play an immune-modulating effect by inducing immunogenic cell death (ICD) in tumor cells. However, they hardly arouse strong antitumor immune response because the immunosuppressive lymphocytes are present in the tumor microenvironment. These immunosuppressive lymphocytes include regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). We used a low dose of doxorubicin (DOX) to induce ICD in combination with immune regulator 1-methyl-DL-tryptophan (1MT) to suppress indoleamine 2,3-dioxygenase and overcome Treg- and MDSC-associated immune suppression. By co-encapsulation of DOX and 1MT into a reduction-responsive polypeptide nanogel, the drugs were simultaneously released in the tumor cells and synergistically performed antitumor efficacy. After treatment, recruitment of Tregs and MDSCs was inhibited, and the frequency of tumor-infiltrating CD8⁺ T cells was remarkably enhanced. These results demonstrated that the chemoimmunotherapy strategy effectively suppressed tumor growth without causing evident adverse effects, indicating its great potential in clinical cancer therapy.

Tryptophan: A Rheostat of Cancer Immune Escape Mediated by Immunosuppressive Enzymes IDO1 and TDO

Blockade of the immunosuppressive tryptophan catabolism mediated by indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) holds enormous promise for sensitising cancer patients to immune checkpoint blockade. Yet, only IDO1 inhibitors had entered clinical trials so far, and those agents have generated disappointing clinical results. Improved understanding of molecular mechanisms involved in the immune-regulatory function of the tryptophan catabolism is likely to optimise therapeutic strategies to block this pathway. The immunosuppressive role of tryptophan metabolite kynurenine is becoming increasingly clear, but it remains a mystery if tryptophan exerts functions beyond serving as a precursor for kynurenine. Here we hypothesise that tryptophan acts as a rheostat of kynurenine-mediated immunosuppression by competing with kynurenine for entry into immune T-cells through the amino acid transporter called System L. This hypothesis stems from the observations that elevated tryptophan levels in TDO-knockout mice relieve immunosuppression instigated by IDO1, and that the vacancy of System L transporter modulates kynurenine entry into CD4+ T-cells. This hypothesis has two potential therapeutic implications. Firstly, potent TDO inhibitors are expected to indirectly inhibit IDO1 hence development of TDO-selective inhibitors appears advantageous compared to IDO1-selective and dual IDO1/TDO inhibitors. Secondly, oral supplementation with System L substrates such as leucine represents a novel potential therapeutic modality to restrain the immunosuppressive kynurenine and restore anti-tumour immunity.

Early Detection of Pancreatic Cancer in Type 2 Diabetes Mellitus Patients Based on 1H NMR Metabolomics

The association of pancreatic cancer with type 2 diabetes mellitus was investigated by ¹H NMR metabolomic analysis of blood plasma. Concentration data of 58 metabolites enabled discrimination of pancreatic cancer (PC) patients from healthy controls (HC) and long-term type 2 diabetes mellitus (T2DM) patients. A panel of eight metabolites was proposed and successfully tested for group discrimination. Furthermore, a prediction model for the identification of at-risk individuals for future development of pancreatic cancer was built and tested on recent-onset diabetes mellitus (RODM) patients. Six of 59 RODM samples were assessed as PC with an accuracy of more than 80%. The health condition of these individuals was re-examined, and in four cases, a correlation to the prediction was found. The current health condition can be retrospectively attributed to misdiagnosed pancreatogenic diabetes or to early-stage pancreatic cancer.

Increased kynurenine concentration attenuates serotonergic neurotoxicity induced by 3,4-methylenedioxymethamphetamine (MDMA) in rats through activation of aryl hydrocarbon receptor

3,4-Methylenedioxymethamphetamine (MDMA) is an amphetamine derivative that has been shown to produce serotonergic damage in the brains of primates, including humans, and of rats. Tryptophan, the precursor of serotonin, is primarily degraded through the kynurenine (KYN) pathway, producing among others KYN, the main metabolite of this route. KYN has been reported as an endogenous agonist of the aryl hydrocarbon receptor (AhR), a transcription factor involved in several neurological functions. This study aims to determine the effect of MDMA on the KYN pathway and on AhR activity and to establish their role in the long-term serotonergic neurotoxicity induced by the drug in rats. Our results show that MDMA induces the activation of the KYN pathway, mediated by hepatic tryptophan 2,3-dioxygenase (TDO). MDMA also activated AhR as evidenced by increased AhR nuclear translocation and CYP1B1 mRNA expression. Autoradiographic quantification of serotonin transporters showed that both the TDO inhibitor 680C91 and the AhR antagonist CH-223191 potentiated the neurotoxicity induced by MDMA, while administration of exogenous l-kynurenine or of the AhR positive modulator 3,3'-diindolylmethane (DIM) partially prevented the serotonergic damage induced by the drug. The results demonstrate for the first time that MDMA increases KYN levels and AhR activity, and these changes appear to play a role in limiting the neurotoxicity induced by the drug. This work provides a better understanding of the physiological mechanisms that attenuate the brain damage induced by MDMA and identify modulation of the KYN pathway and of AhR as potential therapeutic strategies to limit the negative effects of MDMA.

What is the prospect of indoleamine 2,3-dioxygenase 1 inhibition in cancer? Extrapolation from the past

Indoleamine 2,3-dioxygenase 1 (IDO1), a monomeric heme-containing enzyme, catalyzes the first and rate-limiting step in the kynurenine pathway of tryptophan metabolism, which plays an important role in immunity and neuronal function. Its implication in different pathophysiologic processes including cancer and neurodegenerative diseases has inspired the development of IDO1 inhibitors in the past decades. However, the negative results of the phase III clinical trial of the would-be first-in-class IDO1 inhibitor (epacadostat) in combination with an anti-PD1 antibody (pembrolizumab) in patients with advanced malignant melanoma call for a better understanding of the role of IDO1 inhibition. In this review, the current status of the clinical development of IDO1 inhibitors will be introduced and the key pre-clinical and clinical data of epacadostat will be summarized. Moreover, based on the cautionary notes obtained from the clinical readout of epacadostat, strategies for the identification of reliable predictive biomarkers and pharmacodynamic markers as well as for the selection of the tumor types to be treated with IDO1inhibitors will be discussed.

Targeting Indoleamine 2,3-Dioxygenase in Cancer Models Using the Novel Small Molecule Inhibitor NTRC 3883-0

Indoleamine 2,3-dioxygenase (IDO1) is a key regulator of immune suppression by catalyzing the oxidation of L-tryptophan. IDO1 expression has been related to poor prognosis in several cancers and to resistance to checkpoint immunotherapies. We describe the characterization of a novel small molecule IDO1 inhibitor, NTRC 3883-0, in a panel of biochemical and cell-based assays, and various cancer models. NTRC 3883-0 released the inhibitory effect of IDO1 on CD8-positive T cell proliferation in co-cultures of IDO1-overexpressing cells with healthy donor lymphocytes, demonstrating its immune modulatory activity. In a syngeneic mouse model using IDO1-overexpressing B16F10 melanoma cells, NTRC 3883-0 effectively counteracted the IDO1-induced modulation of L-tryptophan and L-kynurenine levels, demonstrating its in vivo target modulation. Finally, we studied the expression and activity of IDO1 in primary cell cultures established from the malignant ascites of ovarian cancer patients. In these cultures, IDO1 expression was induced upon stimulation with IFNγ, and its activity could be inhibited by NTRC 3883-0. Based on these results, we propose the use of ascites cell-based functional assays for future patient stratification. Our results are discussed in light of the recent discontinuation of clinical trials of more advanced IDO1 inhibitors and the reconsideration of IDO1 as a valid drug target.

A theranostic probe of indoleamine 2,3-dioxygenase 1 (IDO1) for small molecule cancer immunotherapy

Discovering novel small molecules for cancer immunotherapy represents a promising but challenging strategy in future cancer treatment. Herein, we designed the first theranostic fluorescent probes to efficiently detect and inhibit the enzymatic activity of 2,3-dioxygenase 1 (IDO1). Probe 6b is a highly active IDO1 inhibitor (IC₅₀ = 12 nM, Cellular IC₅₀ = 10 nM), which can sensitively and specifically detect endogenous IDO1 in living cells. Furthermore, as a theranostic probe, 6b showed excellent in vivo antitumor efficacy in the CT26 xenograft mouse model as well. Therefore, it can be applied as a valuable chemical tool for better understanding the immunotherapy mechanism of IDO1 and improving the therapeutic efficiency.

Tryptophan 2,3-Dioxygenase Expression Identified in Murine Decidual Stromal Cells Is Not Essential for Feto-Maternal Tolerance

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) catalyze the rate-limiting step of tryptophan catabolism along the kynurenine pathway, which has important immuno suppressive properties, particularly in tumor cells and dendritic cells. The prominent expression of IDO1 in the placenta also suggested a role in preventing immune rejection of fetal tissues, and pharmacological inhibition of IDO1 induced abortion of allogeneic fetuses in mice. However, this was later challenged by the lack of rejection of allogeneic fetuses in IDO1-KO mice, suggesting that other mechanisms may compensate for IDO1 deficiency. Here we investigated whether TDO could contribute to feto-maternal tolerance and compensate for IDO1 deficiency in IDO1-KO mice. Expression of TDO mRNA was previously detected in placental tissues. We developed a new chimeric rabbit anti-TDO antibody to confirm TDO expression at the protein level and identify the positive cell type by immunohistochemistry in murine placenta. We observed massive TDO expression in decidual stromal cells, starting at day E3.5, peaking at day E6.5 then declining rapidly while remaining detectable until gestation end. IDO1 was also induced in decidual stromal cells, but only at a later stage of gestation when TDO expression declined. To determine whether TDO contributed to feto-maternal tolerance, we mated TDO-KO and double IDO1-TDO-KO females with allogeneic males. However, we did not observe reduced fertility. These results suggest that, despite its expression in decidual stromal cells, TDO is not a dominant mechanism of feto-maternal tolerance able to compensate for the absence of IDO1. Redundant additional mechanisms of immunosuppression likely take over in these KO mice. The massive expression of TDO during decidualization might suggest a role of TDO in angiogenesis or vessel tonicity, as previously described for IDO1.