Recent advances in the discovery of indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors

Qingfei mixture mitigates immunosuppression of tumor microenvironment in non-small cell lung cancer by blocking stat1/Ido1-mediated tryptophan-kynurenine pathway

Programmed death-1 (PD-1) acts as a T cell checkpoint and is important in controlling T cell exhaustion. Blocking the intercommunication across PD-1 and PD-L1 is promising for advanced lung cancer treatment. However, the response rate requires being strengthened. This study aimed to determine whether the combination treatment of Qingfei mixture (QFM) and PD-1 inhibitor could improve the sensitivity of monoclonal antibody by regulating STAT1/IDO1-mediated tryptophan (Trp)-kynurenine (Kyn) pathway. The in vivo imaging system, immunofluorescence, hematoxylin-eosin staining, TUNEL, flow cytometry, HPLC, and ELISA were used to estimate the anti-tumor effects in LLC-luc tumor-bearing C57BL/6 mice treated with QFM, PD-1 inhibitor, 2-NP (enhancer of STAT1 transcription), and FICZ (AhR agonist) alone or in combination. IFN-γ-mediated A549 and LLC cells were treated with QFM-containing serum and fludarabine (FLU, STAT1 inhibitor), and cell viability, apoptosis, and Kyn content were then evaluated using CCK-8 assays, flow cytometry, and HPLC assays, respectively. Additionally, the expressions of STAT1, IDO1, AhR, NFATc1, TRIP12, PD-1, and PD-L1 were measured in vivo and in vitro. We found QFM increased the anti-cancer actions of PD-1 inhibitors by increasing the CD8+IFNγ+ T cells infiltration and decreasing the ratio of Kyn/Trp. Besides, QFM-containing serum suppressed the proliferation and promoted apoptosis in A549 and LLC cells, meanwhile, FLU boosted the effects of QFM-containing serum. Moreover, the suppression of tumor growth in the combination therapy was attenuated in the mice receiving 2-NP or FICZ. The occurrence of the above results was accompanied by a decrease in STAT1, IDO1, AhR, PD-1, and PD-L1 expressions. Collectively, the findings suggested that QFM may increase the influences of PD-1 inhibitors at least partially by blocking the STAT1/IDO1-mediated tryptophan-kynurenine pathway in lung cancer.

Small-molecule modulators of tumor immune microenvironment

In recent years, tumor immunotherapy, aimed at increasing the activity of immune cells and reducing immunosuppressive effects, has attracted wide attention. Among them, immune checkpoint blocking (ICB) is the most commonly explored therapeutic approach. All approved immune checkpoint inhibitors (ICIs) are clinically effective monoclonal antibodies (mAbs). Compared with biological agents, small-molecule drugs have many unique advantages in tumor immunotherapy. Therefore, they also play an important role. Immunosuppressive signals such as PD-L1, IDO1, and TGF-β, etc. overexpressed in tumor cells form the tumor immunosuppressive microenvironment. In addition, the efficacy of multi-pathway combined immunotherapy has also been reported and verified. Here, we mainly reviewed the mechanism of tumor immunotherapy, analyzed the research status of small-molecule modulators, and discussed drug candidates' structure-activity relationship (SAR). It provides more opportunities for further research to design more immune small-molecule modulators with novel structures.

An updated patent review of IDO1 inhibitors for cancer (2018-2022)

INTRODUCTION

Indoleamine 2,3-dioxygenase 1 (IDO1) is highly related to the immune evasion of a wide range of malignancies due to its role in the immune suppression caused by the depletion of tryptophan (Trp) and the accumulation of kynurenine (Kyn). The combination of IDO1 inhibitors with other treatments represents a promising strategy in immunotherapy, although considerable challenges lie ahead.

AREAS COVERED

This review focuses on patent publications searched from Espacenet and Google Scholar, and related to IDO1 inhibitors with potential anti-cancer utilization during the period 2018-2022.

EXPERT OPINION

Despite the clinical trial failure of the first-in-class IDO1 inhibitor epacadostat in combination with pembrolizumab, numerous studies have been carried on to pursue more efficient IDO1-based immune-modulating therapeutic solutions. A large number of IDO1 inhibitors with new structures and design concepts have been produced with the impetus of crystallographic studies, and have shown great research potential. The elaboration on the combination of IDO1 inhibitors with other targeting agents, the more precise selection of patients, the identification of more reliable biomarkers for evaluating the IDO1 treatment, and the investigation of possible toxicity, are critical factors to promote IDO1-based immunotherapies from bench to bedside.

Targeting Indoleamine Dioxygenase and Tryptophan Dioxygenase in Cancer Immunotherapy: Clinical Progress and Challenges

Indoleamine 2.3-dioxygenases (IDO1/2) and tryptophan 2.3-dioxygenase (TDO) are the initial and rate-limiting enzymes in tryptophan metabolism, which play an essential role in mediating immunosuppression in tumor microenvironment. Accumulating evidence has indicated that both IDO1 and TDO are highly expressed in many malignant tumors, and their expression is generally associated with reduced tumor-infiltrating immune cells, increased regulatory T-cell infiltration, as well as cancer progression and poor prognosis for malignancies. A large number of IDO1 and TDO inhibitors have been screened or synthesized in the last two decades. Thus far, at least 12 antagonists targeting IDO1 and TDO have advanced to clinical trials. In this account, we conducted a comprehensive review of the development of IDO1 and TDO inhibitors in cancer immunotherapy, particularly their clinical research progress, and presented the current challenges and corresponding solutions.

PAMAM Dendritic Nanoparticle-Incorporated Hydrogel to Enhance the Immunogenic Cell Death and Immune Response of Immunochemotherapy

The efficiency of chemotherapy is frequently affected by its multidrug resistance, immune suppression, and severe side effects. Its combination with immunotherapy to reverse immune suppression and enhance immunogenic cell death (ICD) has emerged as a new strategy to overcome the aforementioned issues. Herein, we construct a pH-responsive PAMAM dendritic nanocarrier-incorporated hydrogel for the co-delivery of immunochemotherapeutic drugs. The stepwise conjugation of moieties and drug load was confirmed by various techniques. In vitro experimental results demonstrated that PAMAM dendritic nanoparticles loaded with a combination of drugs exhibited spherical nanosized particles, facilitated the sustained release of drugs, enhanced cellular uptake, mitigated cell viability, and induced apoptosis. The incorporation of PAB-DOX/IND nanoparticles into thermosensitive hydrogels also revealed the formation of a gel state at a physiological temperature and further a robust sustained release of drugs at the tumor microenvironment. Local injection of this formulation into HeLa cell-grafted mice significantly suppressed tumor growth, induced immunogenic cell death-associated cytokines, reduced cancer cell proliferation, and triggered a CD8⁺ T-cell-mediated immune response without obvious systemic toxicity, which indicates a synergistic ICD effect and reverse of immunosuppression. Hence, the localized delivery of immunochemotherapeutic drugs by a PAMAM dendritic nanoparticle-incorporated hydrogel could provide a promising strategy to enhance antitumor activity in cancer therapy.

Novel 1,2,3-Triazole Erlotinib Derivatives as Potent IDO1 Inhibitors: Design, Drug-Target Interactions Prediction, Synthesis, Biological Evaluation, Molecular Docking and ADME Properties Studies

Indoleamine 2,3-dioxygenase 1 (IDO1) plays a predominant role in cancer immunotherapy which catalyzes the initial and rate limiting steps of the kynurenine pathway as a key enzyme. To explore novel IDO1 inhibitors, five derivatives of erlotinib-linked 1,2,3-triazole compounds were designed by using a structure-based drug design strategy. Drug-target interactions (DTI) were predicted by DeePurpose, an easy-to-use deep learning library that contains more than 50 algorithms. The DTI prediction results suggested that the designed molecules have potential inhibitory activities for IDO1. Chemical syntheses and bioassays showed that the compounds exhibited remarkable inhibitory activities against IDO1, among them, compound e was the most potent with an IC₅₀ value of 0.32 ± 0.07 μM in the Hela cell assay. The docking model and ADME analysis exhibited that the effective interactions of these compounds with heme iron and better drug-likeness ensured the IDO1 inhibitory activities. The studies suggested that compound e was a novel and interesting IDO1 inhibitor for further development.

Discovery of IDO1 inhibitors containing a decahydroquinoline, decahydro-1,6-naphthyridine, or octahydro-1H-pyrrolo[3,2-c]pyridine scaffold

A series of IDO1 inhibitors containing a decahydroquinoline, decahydro-1,6-naphthyridine, or octahydro-1H-pyrrolo[3,2-c]pyridine scaffold were identified with good cellular and human whole blood activity against IDO1. These inhibitors contain multiple chiral centers and all diastereomers were separated. The absolute stereochemistry of each isomers were not determined. Compounds 15 and 27 stood out as leads due to their good cellular as well as human whole blood IDO1 inhibition activity, low unbound clearance, and reasonable mean residence time in rat cassette PK studies.

Folate receptor-mediated delivery of 1-MDT-loaded mesoporous silica magnetic nanoparticles to target breast cancer cells

Aims: The efficiency of mesoporous silica magnetic nanoparticles (MSMNP) as a targeted drug-delivery system was investigated. Methods: The superparamagnetic iron oxide nanoparticles (NP) were synthesized, coated with mesoporous silica and conjugated with polyethylene glycol and methotrexate. Next, 1-methyl-D-tryptophan was loaded into the prepared nanosystems (NS). They were characterized using transmission electron microscopy, scanning electron microscopy, dynamic light scattering, vibrating sample magnetometer, x-ray powder diffraction, Fourier transform-infrared spectroscopy and the Brunauer-Emmett-Teller method and their biological impacts on breast cancer cells were evaluated. Results: The prepared NSs displayed suitable properties and showed enhanced internalization by folate-receptor-expressing cells, exerting efficient cytotoxicity, which was further enhanced by the near-infrared radiation irradiation. Conclusion: On the basis of our findings, the engineered NS is a promising multifunctional nanomedicine/theranostic for solid tumors.

Structural Basis of Selective Human Indoleamine-2,3-dioxygenase 1 (hIDO1) Inhibition

hIDO1 is a heme-dioxygenase overexpressed in the tumor microenvironment and is implicated in the survival of cancer cells. Metabolism of tryptophan to N-formyl-kynurenine by hIDO1 leads to immune suppression to result in cancer cell immune escape. In this article, we discuss the discovery of selective hIDO1 inhibitors for therapeutic intervention that have been promoted to clinical trials and for which crystallographic structural information is available for the respective inhibitor-enzyme complex. The structural insights are based on the complex crystal structures and the relative biological data profiles. The structural basis of selective hIDO1 inhibition, as discussed herein, opens new avenues to the discovery of novel inhibitors with improved activity profiles, selectivity, and distinct structure frameworks.

Kynurenines as a Novel Target for the Treatment of Malignancies

Malignancies are unquestionably a significant public health problem. Their effective treatment is still a big challenge for modern medicine. Tumors have developed a wide range of mechanisms to evade an immune and therapeutic response. As a result, there is an unmet clinical need for research on solutions aimed at overcoming this problem. An accumulation of tryptophan metabolites belonging to the kynurenine pathway can enhance neoplastic progression because it causes the suppression of immune system response against cancer cells. They are also involved in the development of the mechanisms responsible for the resistance to antitumor therapy. Kynurenine belongs to the most potent immunosuppressive metabolites of this pathway and has a significant impact on the development of malignancies. This fact prompted researchers to assess whether targeting the enzymes responsible for its synthesis could be an effective therapeutic strategy for various cancers. To date, numerous studies, both preclinical and clinical, have been conducted on this topic, especially regarding the inhibition of indoleamine 2,3-dioxygenase activity and their results can be considered noteworthy. This review gathers and systematizes the knowledge about the role of the kynurenine pathway in neoplastic progression and the findings regarding the usefulness of modulating its activity in anticancer therapy.

SAR towards indoline and 3-azaindoline classes of IDO1 inhibitors

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

A Series of 2-((1-Phenyl-1H-imidazol-5-yl)methyl)-1H-indoles as Indoleamine 2,3-Dioxygenase 1 (IDO1) Inhibitors

Indoleamine 2,3-dioxygenase 1 (IDO1) is a promising therapeutic target in cancer immunotherapy and neurological disease. Thus, searching for highly active inhibitors for use in human cancers is now a focus of widespread research and development efforts. In this study, we report the structure-based design of 2-(5-imidazolyl)indole derivatives, a series of novel IDO1 inhibitors which have been designed and synthesized based on our previous study using N1-substituted 5-indoleimidazoles. Among these, we have identified one with a strong IDO1 inhibitory activity (IC₅₀ =0.16 μM, EC₅₀ =0.3 μM). Structural-activity relationship (SAR) and computational docking simulations suggest that a hydroxyl group favorably interacts with a proximal Ser167 residue in Pocket A, improving IDO1 inhibitory potency. The brain penetrance of potent compounds was estimated by calculation of the Blood Brain Barrier (BBB) Score and Brain Exposure Efficiency (BEE) Score. Many compounds had favorable scores and the two most promising compounds were advanced to a pharmacokinetic study which demonstrated that both compounds were brain penetrant. We have thus discovered a flexible scaffold for brain penetrant IDO1 inhibitors, exemplified by several potent, brain penetrant, agents. With this promising scaffold, we provide herein a basis for further development of brain penetrant IDO1 inhibitors.

Methylene blue and ascorbate interfere with the accurate determination of the kinetic properties of IDO2

Indoleamine 2,3-dioxygenases (IDOs) catalyze the oxidative cleavage of L-tryptophan (Trp) to N-formylkynurenine. Two IDOs, IDO1 and IDO2, are present in vertebrates. IDO1 is a high-affinity Trp-degrading enzyme involved in several physiological processes. By comparison, IDO2 generally has been reported to have low affinity (high K_m -value) for Trp, and the enzyme's in vivo function remains unclear. Using IDOs from different species, we show that compared with ferrous-oxy (Fe²⁺ -O₂ ) IDO1, Fe²⁺ -O₂ IDO2 is substantially more stable and engages in multiple turnovers of the reaction in the absence of a reductant. Without reductant, Fe²⁺ -O₂ IDO2 showed K_m -values in the range of 80-356 μM, that is, values substantially lower than reported previously and close to the physiological concentrations of Trp. Methylene blue and ascorbate (Asc), used commonly as the reducing system for IDO activity determination, significantly affected the enzymatic activity of IDO2: In combination, the two reductants increased the apparent K_m - and k_cat -values 8- to 117-fold and 2-fold, respectively. Asc alone both activated and inhibited IDO2 by acting as a source of electrons and as a weak competitive inhibitor, respectively. In addition, ferric (Fe³⁺ ) IDO1 and IDO2 exhibited weak dioxygenase activity, similar to tryptophan 2,3-dioxygenase. Our results shed new light in the enzymatic activity of IDO2, and they support the view that this isoform of IDO also participates in the metabolism of Trp in vivo.

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.

Jejucarbazoles A–C, carbazole glycosides with indoleamine 2,3-dioxygenase 1 inhibitory activity from Streptomyces sp. KCB15JA151

A bioassay-guided investigation led to the isolation of three new carbazole glycosides, jejucarbazoles A–C (1–3), from Streptomyces sp. KCB15JA151. Their planar structures were elucidated by detailed NMR and MS spectroscopic analysis with a literature study. Their relative and absolute configurations were established by ROESY correlations, coupling constants, LC-MS analysis of thiocarbamoyl-thiazolidine carboxylate derivatives, and ECD calculation. Compounds 1–3 showed indoleamine 2,3-dioxygenase 1 (IDO1) inhibitory activity with IC₅₀ values of 18.38, 9.17, and 8.81 μM. The molecular docking analysis suggested that all compounds act as heme-displacing inhibitors against IDO1 enzyme.

This study presents the isolation and structure elucidation of jejucarbazoles A–C, isolated from Streptomyces sp. KCB15JA15 and their inhibitory effect and molecular docking analysis against the IDO1 enzyme.

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

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

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.

Discovery of Potent and Orally Available Bicyclo[1.1.1]pentane-Derived Indoleamine-2,3-dioxygenase 1 (IDO1) Inhibitors

Indoleamine-2,3-dioxygenase 1 (IDO1) inhibition and its combination with immune checkpoint inhibitors like pembrolizumab have drawn considerable attention from both academia and the pharmaceutical industry. Here, we describe the discovery of a novel class of highly potent IDO1 heme-displacing inhibitors featuring a unique bicyclo[1.1.1]pentane motif. Compound 1, evolving from an ALIS (automated ligand identification system) hit, exhibited excellent potency but lacked the desired pharmacokinetic profile due to extensive amide hydrolysis of the benzamide moiety. Replacing the central phenyl ring in 1 with a bicyclo[1.1.1]pentane bioisostere effectively circumvented the amide hydrolysis issue, resulting in the discovery of compound 2 with a favorable overall profile such as excellent potency, selectivity, pharmacokinetics, and a low predicted human dose.