Diaryl hydroxylamines as pan or dual inhibitors of indoleamine 2,3-dioxygenase-1, indoleamine 2,3-dioxygenase-2 and tryptophan dioxygenase

IDO2 Drives Autoantibody Production and Joint Inflammation in a Preclinical Model of Arthritis by Repressing Runx1 Function in B Cells

The immunomodulatory enzyme IDO2 is an essential mediator of autoantibody production and joint inflammation in preclinical models of autoimmune arthritis. Although originally identified as a tryptophan-catabolizing enzyme, we recently discovered a previously unknown nonenzymatic pathway is essential for the proarthritic function of IDO2. We subsequently identified Runx1 (Runt-related transcription factor 1) as a potential component of the nonenzymatic pathway IDO2 uses to drive arthritis. In this study, we find that IDO2 directly binds Runx1 and inhibits its localization to the nucleus, implicating Runx1 as a downstream component of IDO2 function. To directly test whether Runx1 mediates the downstream pathway driving B cell activation in arthritis, we bred B cell conditional Runx1-deficient (CD19cre Runx1flox/flox) mice onto the KRN.g7 arthritis model in the presence or absence of IDO2. Runx1 loss did not affect arthritis in the presence of IDO2; however, deleting Runx1 reversed the antiarthritic effect of IDO2 loss in this model. Further studies demonstrated that the IDO2-Runx1 interaction could be blocked with a therapeutic anti-IDO2 mAb in vitro and that Runx1 was required for IDO2 Ig's therapeutic effect in vivo. Taken together, these data demonstrate that IDO2 mediates autoantibody production and joint inflammation by acting as a repressor of Runx1 function in B cells and implicate therapeutic targeting of IDO2-Runx1 binding as a strategy to inhibit autoimmune arthritis and other autoantibody-mediated diseases.

Blocking Tryptophan Catabolism Reduces Triple-Negative Breast Cancer Invasive Capacity

SIGNIFICANCE

TDO2 is more highly expressed than the nonhomologous TRP-catabolizing enzyme IDO1 in TNBC. We find that TDO2 knockdown can lead to a compensatory increase in IDO1. Therefore, we tested a newly developed TDO2/IDO1 dual inhibitor and found that it decreases TRP catabolism, anchorage-independent survival, and invasive capacity.

Structural Insights into Protein-Inhibitor Interactions in Human Tryptophan Dioxygenase

Human tryptophan dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) are two important targets in cancer immunotherapy. Extensive research has led to a large number of potent IDO inhibitors; in addition, 52 structures of IDO in complex with inhibitors with a wide array of chemical scaffolds have been documented. In contrast, progress in the development of TDO inhibitors has been limited. Only four structures of TDO in complex with competitive inhibitors that compete with the substrate L-tryptophan for binding to the active site have been reported to date. Here we systematically evaluated the structures of TDO in complex with competitive inhibitors with three types of pharmacophores, imidazo-isoindole, indole-tetrazole, and indole-benzotriazole. The comparative assessment of the protein-inhibitor interactions sheds new light into the structure-based design of enzyme-selective inhibitors.

N-Hydroxypiridinedione: A Privileged Heterocycle for Targeting the HBV RNase H

Hepatitis B virus (HBV) remains a global health threat. Ribonuclease H (RNase H), part of the virus polymerase protein, cleaves the pgRNA template during viral genome replication. Inhibition of RNase H activity prevents (+) DNA strand synthesis and results in the accumulation of non-functional genomes, terminating the viral replication cycle. RNase H, though promising, remains an under-explored drug target against HBV. We previously reported the identification of a series of N-hydroxypyridinedione (HPD) imines that effectively inhibit the HBV RNase H. In our effort to further explore the HPD scaffold, we designed, synthesized, and evaluated 18 novel HPD oximes, as well as 4 structurally related minoxidil derivatives and 2 barbituric acid counterparts. The new analogs were docked on the RNase H active site and all proved able to coordinate the two Mg2+ ions in the catalytic site. All of the new HPDs effectively inhibited the viral replication in cell assays exhibiting EC50 values in the low μM range (1.1-7.7 μM) with low cytotoxicity, resulting in selectivity indexes (SI) of up to 92, one of the highest reported to date among HBV RNase H inhibitors. Our findings expand the structure-activity relationships on the HPD scaffold, facilitating the development of even more potent anti-HBV agents.

Indoleamine 2, 3-dioxygenase 1 activation in macrophage exacerbates hepatic ischemia-reperfusion injury by triggering hepatocyte ferroptosis

BACKGROUND

Hepatic Ischemia-reperfusion (I/R) injury, critical challenge in liver surgery and transplantation, exerts a significant impact on the prognosis and survival of patients. Inflammation and cell death play pivotal roles in pathogenesis of hepatic I/R injury. Indoleamine 2, 3-dioxygenase 1 (IDO-1), a key enzyme involved in the kynurenine pathway, has been extensively investigated for its regulatory effects on innate immune responses and cell ferroptosis. However, the precise involvement of IDO-1 in hepatic I/R injury remains unclear.

METHODS

IDO-1 knockout mice were generated to establish a murine model of liver partial warm ischemia and reperfusion, while an in vitro Hypoxia/Reoxygenation (H/R) model was employed to simulate ischemia/reperfusion injury.

RESULTS

The involvement of ferroptosis was observed to be involved in hepatic I/R injury, and effective mitigation of liver injury was achieved through the inhibition of ferroptosis. In the context of hepatic I/R injury, up-regulation of IDO-1 was found in macrophages exhibiting prominent M1 polarization and impaired efferocytosis. Deficiency or inhibition of IDO-1 alleviated hepatocytes ferroptosis and M1 polarization induced by hepatic I/R injury, while also enhancing M2 polarization and promoting efferocytosis in macrophages. Furthermore, depletion of macrophages attenuated ferroptosis in hepatocytes induced by hepatic I/R injury.

CONCLUSION

This study highlights the crucial role of IDO-1 activation in macrophages in triggering ferroptosis in hepatocytes during hepatic ischemia-reperfusion injury. Our findings suggest that targeting IDO-1 could be a promising therapeutic strategy for mitigating hepatic I/R injury associated with liver surgery and transplantation.

Discovery of novel IDO1/TDO2 dual inhibitors: a consensus Virtual screening approach with molecular dynamics simulations, and binding free energy analysis

The pursuit of effective cancer immunotherapy drugs remains challenging, with overexpression of indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase 2 (TDO2) allowing cancer cells to evade immune attacks. While several IDO1 inhibitors have undergone clinical testing, only three dual IDO1/TDO2 inhibitors have reached human trials. Hence, this study focuses on identifying novel IDO1/TDO2 dual inhibitors through consensus structure-based virtual screening (SBVS). ZINC15 natural products library was refined based on molecular descriptors, and the selected compounds were docked to the holo form IDO1 and TDO2 using two different software programs and ranked according to their consensus docking scores. The top-scoring compounds underwent in silico evaluations for pharmacokinetics, toxicity, CYP3A4 affinity, molecular dynamics (MD) simulations, and MM-GBSA binding free energy calculations. Five compounds (ZINC00000079405/10, ZINC00004028612/11, ZINC00013380497/12, ZINC00014613023/13, and ZINC00103579819/14) were identified as potential IDO1/TDO2 dual inhibitors due to their high consensus docking scores, key residue interactions with the enzymes, favorable pharmacokinetics, and avoidance of CYP3A4 binding. MD simulations of the top three hits with IDO1 indicated conformational changes and compactness, while MM-GBSA analysis revealed strong binding free energy for compounds 10 (ΔG: -20.13 kcal/mol) and 11 (ΔG: -16.22 kcal/mol). These virtual hits signify a promising initial step in identifying candidates as supplementary therapeutics to immune checkpoint inhibitors in cancer treatment. Their potential to deliver potent dual inhibition of IDO1/TDO2, along with safety and favorable pharmacokinetics, makes them compelling. Validation through in vitro and in vivo assays should be conducted to confirm their activity, selectivity, and preclinical potential as holo IDO1/TDO2 dual inhibitors.

Developing reliable classification of dual IDO1/TDO inhibitors using data fusion and majority voting

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are promising dual-targeting inhibitors in cancer and neurodegenerative diseases treatment. Data fusion of receptor-based and ligand-based information of dual IDO1/TDO inhibitors were employed for active/inactive classification performance. A reliable decision making procedure was used here to identify active/inactive dual IDO1/TDO inhibitors using majority voting method and pools of individual classifications instead of individual models. All classification models were validated using prediction set, cross-validation and y-scrambling tests. The classification outcomes indicate that the sensitivity, specificity, precision, accuracy, G-mean and F1 score values increases up to ∼90% using data fusion and majority voting method. Compare to individual classification models with a single prediction point, the majority voting method has more reliable results due to the integration of the pool of individual classification models. This classification strategy may lead to more reliable identification of active/inactive dual-targeting inhibitors in cancer immunotherapy.Communicated by Ramaswamy H. Sarma.

The kynurenine pathway presents multi-faceted metabolic vulnerabilities in cancer

The kynurenine pathway (KP) and associated catabolites play key roles in promoting tumor progression and modulating the host anti-tumor immune response. To date, considerable focus has been on the role of indoleamine 2,3-dioxygenase 1 (IDO1) and its catabolite, kynurenine (Kyn). However, increasing evidence has demonstrated that downstream KP enzymes and their associated metabolite products can also elicit tumor-microenvironment immune suppression. These advancements in our understanding of the tumor promotive role of the KP have led to the conception of novel therapeutic strategies to target the KP pathway for anti-cancer effects and reversal of immune escape. This review aims to 1) highlight the known biological functions of key enzymes in the KP, and 2) provide a comprehensive overview of existing and emerging therapies aimed at targeting discrete enzymes in the KP for anti-cancer treatment.

Discovery and biological evaluation of a new type of dual inhibitors of indoleamine 2,3-dioxygenase 1 and tryptophan 2,3-dioxygenase from ethnomedicinal plant Dactylicapnos scandens

The root of Dactylicapnos scandens (D.Don.) Hutch (Papaveraceae), one of the most famous ethno-medicinal plants from the Bai communities in P. R. China, is used to treat various inflammations and tumours. Bioassay-guided phytochemical research on D. scandens followed by semi-synthesis led to a series of undescribed tetrahydroisoquinoline alkaloids with dual inhibitory activities against indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO). The previously undescribed dark-green alkaloid dactycapnine A exhibited the best dual inhibitor effects among the identified compounds. Structure-activity relationship analysis revealed the importance of the base skeleton with a hyperconjugation system. The performed semi-synthesis further yielded bioactive dimeric and trimeric compounds with hyperconjugated systems. Performed STD NMR experiments disclosed direct interactions between dactycapnine A and IDO1/TDO. Inhibition kinetics indicated dactycapnine A as a mixed-type dual inhibitor. These findings provided a possible explanation for the anticancer properties of the ethno-medicinal plant species D. scandens.

Dual inhibition of IDO1/TDO2 enhances anti-tumor immunity in platinum-resistant non-small cell lung cancer

BACKGROUND

The impact of non-small cell lung cancer (NSCLC) metabolism on the immune microenvironment is not well understood within platinum resistance. We have identified crucial metabolic differences between cisplatin-resistant (CR) and cisplatin-sensitive (CS) NSCLC cells with elevated indoleamine 2,3-dioxygenase-1 (IDO1) activity in CR, recognized by increased kynurenine (KYN) production.

METHODS

Co-culture, syngeneic, and humanize mice models were utilized. C57BL/6 mice were inoculated with either Lewis lung carcinoma mouse cells (LLC) or their platinum-resistant counterpart (LLC-CR) cells. Humanized mice were inoculated with either A (human CS cells) or ALC (human CR cells). Mice were treated with either IDO1 inhibitor or TDO2 (tryptophan 2,3-dioxygenase-2) inhibitor at 200 mg/kg P.O. once a day for 15 days; or with a new-in-class, IDO1/TDO2 dual inhibitor AT-0174 at 170 mg/kg P.O. once a day for 15 days with and without anti-PD1 antibody (10 mg/kg, every 3 days). Immune profiles and KYN and tryptophan (TRP) production were evaluated.

RESULTS

CR tumors exhibited a more highly immunosuppressive environment that debilitated robust anti-tumor immune responses. IDO1-mediated KYN production from CR cells suppressed NKG2D on immune effector natural killer (NK) and CD8⁺ T cells and enhanced immunosuppressive populations of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs). Importantly, while selective IDO1 inhibition attenuated CR tumor growth, it concomitantly upregulated the TDO2 enzyme. To overcome the compensatory induction of TDO2 activity, we employed the IDO1/TDO2 dual inhibitor, AT-0174. Dual inhibition of IDO1/TDO2 in CR mice suppressed tumor growth to a greater degree than IDO1 inhibition alone. Significant enhancement in NKG2D frequency on NK and CD8⁺ T cells and a reduction in Tregs and MDSCs were observed following AT-1074 treatment. PD-L1 (programmed death-ligand-1) expression was increased in CR cells; therefore, we assessed dual inhibition + PD1 (programmed cell death protein-1) blocking and report profound anti-tumor growth and improved immunity in CR tumors which in turn extended overall survival in mice.

CONCLUSION

Our study reports the presence of platinum-resistant lung tumors that utilize both IDO1/TDO2 enzymes for survival, and to escape immune surveillance as a consequence of KYN metabolites. We also report early in vivo data in support of the potential therapeutic efficacy of the dual IDO1/TDO2 inhibitor AT-0174 as a part of immuno-therapeutic treatment that disrupts tumor metabolism and enhances anti-tumor immunity.

Indoleamine 2,3-dioxygenase (IDO) inhibitors and cancer immunotherapy

Strategies for unlocking immunosuppression in the tumor microenvironment have been investigated to overcome resistance to first-generation immune checkpoint blockade with anti- programmed cell death protein 1 (PD-1)/ programmed death-ligand 1 (PD-L1) and anti-cytotoxic T-lymphocyte associated protein 4 (CTLA-4) agents. Indoleamine 2,3-dioxygenase (IDO) 1, an enzyme catabolizing tryptophan to kynurenine, creates an immunosuppressive environment in preclinical studies. Early phase clinical trials investigating inhibition of IDO1, especially together with checkpoint blockade, provided promising results. Unfortunately, the phase 3 trial of the IDO1 inhibitor epacadostat combined with the PD-1 inhibitor pembrolizumab did not show clinical benefit when compared with pembrolizumab monotherapy in patients with advanced malignant melanoma, which dampened enthusiasm for IDO inhibitors. Even so, several molecules, such as the aryl hydrocarbon receptor and tryptophan 2,3-dioxygenase, were reported as additional potential targets for the modulation of the tryptophan pathway, which might enhance clinical effectiveness. Furthermore, the combination of IDO pathway blockade with agents inhibiting other signals, such as those generated by PIK3CA mutations that may accompany IDO1 upregulation, may be a novel way to enhance activity. Importantly, IDO1 expression level varies by tumor type and among patients with the same tumor type, suggesting that patient selection based on expression levels of IDO1 may be warranted in clinical trials.

Discovery of the First Selective IDO2 Inhibitor As Novel Immunotherapeutic Avenues for Rheumatoid Arthritis

Indoleamine 2,3-dioxygenase 2 (IDO2), a closely related homologue of well-studied immunomodulatory enzyme IDO1, has been identified as a pathogenic mediator of inflammatory autoimmunity in preclinical models. Therapeutic targeting IDO2 in autoimmune diseases has been challenging due to the lack of small-molecule IDO2 inhibitors. Here, based on our previously developed IDO1/IDO2 dual inhibitor, guided by the homology model of the IDO2 structure, we discovered compound 22, the most potent inhibitor targeting IDO2 with good in vitro inhibitory activity (IDO2 IC₅₀ = 112 nM). Notably, treatment with 22 alleviated disease severity and reduced inflammatory cytokines in both the collagen-induced arthritis (CIA) mice model and adjuvant arthritis (AA) rat model. Our study offered for the first time a selective small-molecule IDO2 inhibitor 22 with IC₅₀ at the nanomolar level, which may be used not only as a candidate compound for the treatment of autoimmune diseases but also as a tool compound for further IDO2-related mechanistic study.

Inhibitory effect of ascorbate on tryptophan 2,3-dioxygenase

Tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) catalyze the same reaction, oxidative cleavage of L-tryptophan (L-Trp) to N-formyl-kynurenine. In both enzymes, the ferric (FeIII) form is inactive, and ascorbate (Asc) is frequently used as a reductant in in vitro assays to activate the enzymes by reducing the heme iron. Recently, it has been reported that Asc activates IDO2 by acting as a reductant, however, it is also a competitive inhibitor of the enzyme. Here, the effect of Asc on human TDO (hTDO) is investigated. Similar to its interaction with IDO2, Asc acts as both a reductant and a competitive inhibitor of hTDO in the absence of catalase, and its inhibitory effect was enhanced by the addition of H2O2. Interestingly, however, no inhibitory effect of Asc was observed in the presence of catalase. TDO is known to be activated by H2O2 and a ferryl-oxo (FeIV=O) intermediate (Compound II) is generated during the activation process. The observation that Asc acts as a competitive inhibitor of hTDO only in the absence of catalase can be explained by assuming that the target of Asc is Compound II. Asc seems to compete with L-Trp in an unusual manner.

O-((Ferrocenyl)(3-fluorophenyl)methyl)hydroxylamine

Based on the diaryl hydroxylamine scaffold, which exhibits the potential to inhibit all three enzymes of the first step of the kynurenine pathway, the main tryptophan degradation pathway in mammals, which is often activated in cancer, we report herein the synthesis of a ferrocenyl analogue as an attempt to improve the scaffold’s pan-inhibitory potency through the isosteric replacement of a phenyl group with the ferrocenyl moiety. The synthetic methodology followed gives access to O-((ferrocenyl)(aryl)methyl)hydroxylamines, a class of compounds not yet reported in the literature.

The Immunomodulatory Enzyme IDO2 Mediates Autoimmune Arthritis through a Nonenzymatic Mechanism

IDO2 is one of two closely related tryptophan catabolizing enzymes induced under inflammatory conditions. In contrast to the immunoregulatory role defined for IDO1 in cancer models, IDO2 has a proinflammatory function in models of autoimmunity and contact hypersensitivity. In humans, two common single-nucleotide polymorphisms have been identified that severely impair IDO2 enzymatic function, such that <25% of individuals express IDO2 with full catalytic potential. This, together with IDO2's relatively weak enzymatic activity, suggests that IDO2 may have a role outside of its function in tryptophan catabolism. To determine whether the enzymatic activity of IDO2 is required for its proinflammatory function, we used newly generated catalytically inactive IDO2 knock-in mice together with established models of contact hypersensitivity and autoimmune arthritis. Contact hypersensitivity was attenuated in catalytically inactive IDO2 knock-in mice. In contrast, induction of autoimmune arthritis was unaffected by the absence of IDO2 enzymatic activity. In pursuing this nonenzymatic IDO2 function, we identified GAPDH, Runx1, RANbp10, and Mgea5 as IDO2-binding proteins that do not interact with IDO1, implicating them as potential mediators of IDO2-specific function. Taken together, our findings identify a novel function for IDO2, independent of its tryptophan catabolizing activity, and suggest that this nonenzymatic function could involve multiple signaling pathways. These data show that the enzymatic activity of IDO2 is required only for some inflammatory immune responses and provide, to our knowledge, the first evidence of a nonenzymatic role for IDO2 in mediating autoimmune disease.

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.

Fluorine-18-Labeled PET Radiotracers for Imaging Tryptophan Uptake and Metabolism: a Systematic Review

Due to its metabolism via the serotonin and kynurenine pathways, tryptophan plays a key role in multiple disease processes including cancer. Imaging tryptophan uptake and metabolism in vivo can be achieved with tryptophan derivative positron emission tomography (PET) radiotracers. While human studies with such tracers have been confined to C-11-labeled compounds, preclinical development of F-18-labeled tryptophan-based radiotracers has surged in recent years. We performed a systematic review of studies reporting on such F-18-labeled tryptophan tracers to summarize and compare their biological characteristics and their potential for tumor imaging, with a particular focus on key enzymes of the kynurenine pathway (indoleamine 2,3-dioxygenase [IDO] and tryptophan 2,3-dioxygenase [TDO]), which play an important role in tumoral immune resistance. From a PubMed search, English language articles including data on the preparation and radiochemical and/or biological characteristics of F-18-labeled tryptophan derivative radiotracers were reviewed. A total of 19 original papers included data on 15 unique radiotracers, the majority of which were synthesized with an adequate radiochemical yield. Automated synthesis was reported for 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan, the most extensively evaluated tracer thus far. Biodistribution studies showed high uptake in the pancreas, while the L-type amino acid transporter was the dominant transport mechanism for most of the reviewed tracers. Tracers tested for tumor uptake showed accumulation in tumor cell lines in vitro and in xenografts in vivo, often with favorable tumor-to-background uptake ratios in comparison with clinically used F-18-labeled radiotracers. Five tracers showed promise for imaging IDO activity, including 1-(2-[¹⁸F]fluoroethyl)-L-tryptophan and a F-18-labeled analog of alpha-[¹¹C]methyl-L-tryptophan tested clinically in previous studies. Two radiotracers were metabolized by TDO but showed defluorination in vivo. In summary, most F-18-labeled tryptophan derivative PET tracers share common transport mechanisms and biodistribution characteristics. Several reported tracers could be candidates for further testing and validation toward PET imaging applications in a variety of human diseases.

Current Challenges for IDO2 as Target in Cancer Immunotherapy

Immune checkpoint inhibitors have revolutionized the clinical approach of untreatable tumors and brought a breath of fresh air in cancer immunotherapy. However, the therapeutic effects of these drugs only cover a minority of patients and alternative immunotherapeutic targets are required. Metabolism of l-tryptophan (Trp) via the kynurenine pathway represents an important immune checkpoint mechanism that controls adaptive immunity and dampens exaggerated inflammation. Indoleamine 2,3-dioxygenase 1 (IDO1), the enzyme catalyzing the first, rate–limiting step of the pathway, is expressed in several human tumors and IDO1 catalytic inhibitors have reached phase III clinical trials, unfortunately with disappointing results. Although much less studied, the IDO1 paralog IDO2 may represent a valid alternative as drug target in cancer immunotherapy. Accumulating evidence indicates that IDO2 is much less effective than IDO1 in metabolizing Trp and its functions are rather the consequence of interaction with other, still undefined proteins that may vary in distinct inflammatory and neoplastic contexts. As a matter of fact, the expression of IDO2 gene variants is protective in PDAC but increases the risk of developing tumor in NSCLC patients. Therefore, the definition of the IDO2 interactome and function in distinct neoplasia may open innovative avenues of therapeutic interventions.

Tryptophan metabolism in brain tumors - IDO and beyond

Metabolism of the essential amino acid tryptophan is a key metabolic pathway that restricts antitumor immunity and is a drug development target for cancer immunotherapy. Tryptophan metabolism is active in brain tumors including gliomas and promotes a malignant phenotype and contributes to the immunosuppressive tumor microenvironment. In recent years, improved understanding of the regulation and downstream function of tryptophan metabolism has been significantly expanded beyond the initial in vitro observation that the enzyme indoleamine-2,3-dioxygenase 1 (IDO1) promotes the depletion of intracellular tryptophan. Here, we revisit the specific roles of tryptophan metabolites in regulating brain functioning and neuronal integrity as well as in the context of brain tumors. This review summarizes recent developments in identifying key regulators, as well as the cellular and molecular effects of tryptophan metabolism with a particular focus on potential therapeutic targets in glioma.

The ups, downs and new trends of IDO1 inhibitors

Cancer immunotherapy has become an emerging driving force in the development of innovative strategies to fight against cancer. Despite the significant clinical benefits that many cancer patients have gained, the generally average response rate of ~ 20% is far behind the expectation for immune checkpoint inhibitors (ICIs). Combination of ICIs with indoleamine 2,3-dioxygenase-1 (IDO1) inhibitors is considered as an alternative solution and has proved effective in tremendous preclinical studies. However, the failure of phase III ECHO-301/KEYNOTE-252 trial seriously dampened the enthusiasm on the rationality of IDO1-targeting strategy. Fortunately, in spite of the ups and downs in the developmental journey of IDO1 inhibitors, multiple new approaches have been proposed to bridge the gap between lab to the clinic. Here, we review the recent advances in the development of small molecule inhibitors targeting IDO1 especially the new trend of IDO1 inhibitors after ECHO-301 clinical trials, including dual or pan-inhibitors targeting IDO1 and TDO or IDO2, apo-IDO1 inhibitors, IDO1 PROTACs, as well as other IDO1 inhibitors.

Development of Indoleamine 2,3-Dioxygenase 1 Inhibitors for Cancer Therapy and Beyond: A Recent Perspective

Indoleamine 2,3-dioxygenase 1 (IDO1) has received increasing attention due to its immunosuppressive function in connection with various diseases, including cancer. A recent increase in the understanding of IDO1 has significantly contributed to the discovery of numerous novel inhibitors, but the latest clinical outcomes raised questions and have indicated a future direction of IDO1 inhibition for therapeutic approaches. Herein, we present a comprehensive review of IDO1, discussing the latest advances in understanding the IDO1 structure and mechanism, an overview of recent IDO1 inhibitor discoveries and potential therapeutic applications to provide helpful information for medicinal chemists investigating IDO1 inhibitors.

Indoleamine and tryptophan 2,3-dioxygenases as important future therapeutic targets

Conversion of tryptophan to N-formylkynurenine is the first and rate-limiting step of the tryptophan metabolic pathway (i.e., the kynurenine pathway). This conversion is catalyzed by three enzyme isoforms: indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), and tryptophan 2,3-dioxygenase (TDO). As this pathway generates numerous metabolites that are involved in various pathological conditions, IDOs and TDO represent important targets for therapeutic intervention. This pathway has especially drawn attention due to its importance in tumor resistance. Over the last decade, a large number of IDO and TDO inhibitors have been developed, many of which have entered clinical trials. Here, detailed structural comparisons of these three enzymes (with emphasis on their active sites), their involvement in cellular signaling, and their role(s) in pathological conditions are discussed. Furthermore, the most important recent inhibitors described in papers and patents and involved in clinical trials are reviewed, with a focus on both selective and multiple inhibitors. A short overview of the biochemical and cellular assays used for inhibitory potency evaluation is also presented. This review summarizes recent advances on IDO and TDO as potential drug targets, and provides the key features and perspectives for further research and development of potent inhibitors of the kynurenine pathway.

Evaluation and comparison of the commonly used bioassays of human indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO)

Inhibitors of indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are potential drugs for the treatment of tumor and neurological diseases. A variety of bioassays have been developed to evaluate IDO1/TDO (IDO1 and/or TDO) inhibitors, with uncertainty regarding how the differences in the assay methods or protocols may influence the assay outcomes. The enzymatic assays of IDO1/TDO are usually performed with NFK assay and Kyn adduct assay while the cellular assays of IDO1 are carried out with Hela assay and HEK293 assay. The present study focused on the comparison of the most common bioassays of IDO1/TDO. In addition, the effects of major factors of bioassays such as reaction time and culture medium on the assay outcomes were evaluated. The study will provide reference for the researchers to select IDO1/TDO inhibitors with bioassays, and promote the development of IDO1/TDO inhibitors.

Tumor resistance mechanisms and their consequences on γδ T cell activation

Human γδ T lymphocytes are predominated by two major subsets, defined by the variable domain of the δ chain. Both, Vδ1 and Vδ2 T cells infiltrate in tumors and have been implicated in cancer immunosurveillance. Since the localization and distribution of tumor-infiltrating γδ T cell subsets and their impact on survival of cancer patients are not completely defined, this review summarizes the current knowledge about this issue. Different intrinsic tumor resistance mechanisms and immunosuppressive molecules of immune cells in the tumor microenvironment have been reported to negatively influence functional properties of γδ T cell subsets. Here, we focus on selected tumor resistance mechanisms including overexpression of cyclooxygenase (COX)-2 and indolamine-2,3-dioxygenase (IDO)-1/2, regulation by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/TRAIL-R4 pathway and the release of galectins. These inhibitory mechanisms play important roles in the cross-talk of γδ T cell subsets and tumor cells, thereby influencing cytotoxicity or proliferation of γδ T cells and limiting a successful γδ T cell-based immunotherapy. Possible future directions of a combined therapy of adoptively transferred γδ T cells together with γδ-targeting bispecific T cell engagers and COX-2 or IDO-1/2 inhibitors or targeting sialoglycan-Siglec pathways will be discussed and considered as attractive therapeutic options to overcome the immunosuppressive tumor microenvironment.

Computational study on new natural compound inhibitors of indoleamine 2,3-dioxygenase 1

Indoleamine 2,3-Dioxygenase (IDO), is a speed limiting enzyme that catalyzes the decomposition and metabolism of Tryptophan along Tryptophan-IDO-Kynurenine pathway [1]. Tryptophan is a necessary amino acid for activating cell growth and metabolism. Additionally, the insufficiency of Tryptophan can lead to immune system dysfunction. Raising the level of Indoleamine 2,3-Dioxygenase protein can promote stagnation and apoptosis of effector T cells [2].

In contrast, the decline in the number of effect T cells naturally protects cancer cells from attack. Therefore, Indoleamine 2,3-Dioxygenase is a potential target for tumour immunotherapy, such as melanoma, ovarian cancer, lung cancer, leukaemia, and so on, especially in solid tumours [3]. In the study, we have done sets of virtual screening aided by computer techniques in order to find potentially effective inhibitors of Indoleamine 2,3-Dioxygenase. Firstly, screening based on structure was carried out by Libdock. Then, ADME (adsorption, distribution, metabolism, excretion) and toxicity prediction were also analyzed. Molecular docking and 3D-QSAR pharmacophore generation were used to study the mechanism of these compounds and Indoleamine 2,3-Dioxygenase’s binding. A molecular dynamic analysis was carried out to assess if these potential compound’s binding is stable enough. According to the results of the analysis above, two potential compounds (ZINC000012495022 and ZINC000003791817) from the ZINC database were discovered to interact with Indoleamine 2,3-Dioxygenase with appropriate energy and proved to be none toxic. The study offered valuable information of Indoleamine 2,3-Dioxygenase inhibitor-based drug discovery in cancer therapy by increasing the activity of T cells and releasing immunity suppression [4, 5].

Trial watch: IDO inhibitors in cancer therapy

Indoleamine 2,3-dioxygenase 1 (IDO1) catalyzes the first, rate-limiting step of the so-called “kynurenine pathway”, which converts the essential amino acid L-tryptophan (Trp) into the immunosuppressive metabolite L-kynurenine (Kyn). While expressed constitutively by some tissues, IDO1 can also be induced in specific subsets of antigen-presenting cells that ultimately favor the establishment of immune tolerance to tumor antigens. At least in part, the immunomodulatory functions of IDO1 can be explained by depletion of Trp and accumulation of Kyn and its derivatives. In animal tumor models, genetic or pharmacological IDO1 inhibition can cause the (re)activation of anticancer immune responses. Similarly, neoplasms expressing high levels of IDO1 may elude anticancer immunosurveillance. Therefore, IDO1 inhibitors represent promising therapeutic candidates for cancer therapy, and some of them have already entered clinical evaluation. Here, we summarize preclinical and clinical studies testing IDO1-targeting interventions for oncologic indications.

DNA-Encoded Library Technology-Based Discovery, Lead Optimization, and Prodrug Strategy toward Structurally Unique Indoleamine 2,3-Dioxygenase-1 (IDO1) Inhibitors

We report the discovery of a novel indoleamine 2,3-dioxygenase-1 (IDO1) inhibitor class through the affinity selection of a previously unreported indole-based DNA-encoded library (DEL). The DEL exemplar, spiro-chromane 1, had moderate IDO1 potency but high in vivo clearance. Series optimization quickly afforded a potent, low in vivo clearance lead 11. Although amorphous 11 was highly bio-available, crystalline 11 was poorly soluble and suffered disappointingly low bio-availability because of solubility-limited absorption. A prodrug approach was deployed and proved effective in discovering the highly bio-available phosphonooxymethyl 31, which rapidly converted to 11 in vivo. Obtaining crystalline 31 proved problematic, however; thus salt screening was performed in an attempt to circumvent this obstacle and successfully delivered greatly soluble and bio-available crystalline tris-salt 32. IDO1 inhibitor 32 is characterized by a low calculated human dose, best-in-class potential, and an unusual inhibition mode by binding the IDO1 heme-free (apo) form.

The therapeutic potential of targeting tryptophan catabolism in cancer

Based on its effects on both tumour cell intrinsic malignant properties as well as anti-tumour immune responses, tryptophan catabolism has emerged as an important metabolic regulator of cancer progression. Three enzymes, indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) and tryptophan-2,3-dioxygenase (TDO2), catalyse the first step of the degradation of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). The notion of inhibiting IDO1 using small-molecule inhibitors elicited high hopes of a positive impact in the field of immuno-oncology, by restoring anti-tumour immune responses and synergising with other immunotherapies such as immune checkpoint inhibition. However, clinical trials with IDO1 inhibitors have yielded disappointing results, hence raising many questions. This review will discuss strategies to target Trp-degrading enzymes and possible down-stream consequences of their inhibition. We aim to provide comprehensive background information on Trp catabolic enzymes as targets in immuno-oncology and their current state of development. Details of the clinical trials with IDO1 inhibitors, including patient stratification, possible effects of the inhibitors themselves, effects of pre-treatments and the therapies the inhibitors were combined with, are discussed and mechanisms proposed that might have compensated for IDO1 inhibition. Finally, alternative approaches are suggested to circumvent these problems.

Design, synthesis and biological evaluation of indole-2-carboxylic acid derivatives as IDO1/TDO dual inhibitors

Indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) are involved in the key steps of tryptophan metabolism and are potential new targets for tumor immunotherapy. In this work, a variety of indole-2-carboxylic acid derivatives were synthesized, and their inhibitory activities against both enzymes along with structure-activity relationships were investigated. As a result, a number of 6-acetamido-indole-2-carboxylic acid derivatives were found to be potent dual inhibitors with IC₅₀ values at low micromolar levels. Among them, compound 9o-1 was the most potent inhibitor with an IC₅₀ value of 1.17 μM for IDO1, and 1.55 μM for TDO, respectively. In addition, a para-benzoquinone derivative 9p-O, resulted from the oxidation of compound 9p, was also identified and it showed strong inhibition against the two enzymes with IC₅₀ values at the double digit nanomolar level. Using molecular docking and molecular dynamic simulations, we predicted the binding modes of this class of compounds within IDO1 and TDO binding pocket. The results provide insights for further structural optimization of this series of IDO1/TDO dual inhibitors.

Discovery and Characterisation of Dual Inhibitors of Tryptophan 2,3-Dioxygenase (TDO2) and Indoleamine 2,3-Dioxygenase 1 (IDO1) Using Virtual Screening

Cancers express tryptophan catabolising enzymes indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO2) to produce immunosuppressive tryptophan metabolites that undermine patients’ immune systems, leading to poor disease outcomes. Both enzymes are validated targets for cancer immunotherapy but there is a paucity of potent TDO2 and dual IDO1/TDO2 inhibitors. To identify novel dual IDO1/TDO2 scaffolds, 3D shape similarity and pharmacophore in silico screening was conducted using TDO2 as a model for both systems. The obtained hits were tested in cancer cell lines expressing mainly IDO1 (SKOV3—ovarian), predominantly TDO2 (A172—brain), and both IDO1 and TDO2 (BT549—breast). Three virtual screening hits were confirmed as inhibitors (TD12, TD18 and TD34). Dose response experiments showed that TD34 is the most potent inhibitor capable of blocking both IDO1 and TDO2 activity, with the IC₅₀ value for BT549 at 3.42 µM. This work identified new scaffolds able to inhibit both IDO1 and TDO2, thus enriching the collection of dual IDO1/TDO2 inhibitors and providing chemical matter for potential development into future anticancer drugs.

Indoleamine 2,3-dioxygenase 2 depletion suppresses tumor growth in a mouse model of Lewis lung carcinoma

Tryptophan metabolism is important to induce immune tolerance in tumors. To date, 3 types of tryptophan-metabolizing enzymes have been identified: indoleamine 2,3-dioxygenase 1 and 2 (IDO1 and IDO2) and tryptophan 2,3-dioxygenase 2. Numerous studies have focused on IDO1 as its expression is enhanced in various cancers. Recently, IDO2 has been identified as a tryptophan-metabolizing enzyme that is involved in several immune functions and expressed in cancers such as pancreatic cancer. However, the biological role of IDO2 in the induction of immune tolerance in tumors has not yet been reported. In the present study, we examined the effects of Ido2 depletion on tumor growth in a mouse model of Lewis lung carcinoma by using Ido2-knockout mice. Ido2-knockout mice had reduced tumor volumes compared to WT mice. Furthermore, Ido2 depletion altered the tumor microenvironment, such as tryptophan accumulation and kynurenine reduction, leading to enhancement of immune cell invasion. Finally, enzyme-linked immunospot assay revealed that Ido2 depletion enhanced γ-interferon secretion in the tumor. In conclusion, Ido2 is an important immune regulator in the tumor microenvironment. Our data indicate that IDO2 is a potential target for cancer treatment and drug development.

Synthesis and in vivo antitumor evaluation of an orally active potent phosphonamidate derivative targeting IDO1/IDO2/TDO

Targeting Trp-Kyn pathways has been identified as an attractive approach for the cancer immunotherapies. In this study, a novel phosphonamidate containing compound was designed, synthesized and evaluated for its inhibitory activity against key dioxygenases in Trp-Kyn pathway, including IDO1, IDO2 and TDO. This compound showed potent IDO1 inhibitory activity with an IC₅₀ value of 94 nM in an enzymatic assay and 12.6 nM in HeLa cells. In addition, this compound showed promising IDO2 inhibition and TDO inhibition with IC₅₀ values of 310 nM and 2.6 μM, respectively, in enzyme assay. Based on the promising enzyme inhibitory activity toward IDO/TDO, compound F04 was evaluated of its antitumor effects in two tumor models. Further evaluation of mechanism demonstrated compound F04 with the remarkable capacity of reducing kynurenine level in plasma/TME and restoring anti-tumor immune response. F04 could be further developed as a potential immunotherapeutic agent combined with immune checkpoint inhibitors or chemotherapeutic drugs for cancer treatment.