IDO1 Inhibition Synergizes with Radiation and PD-1 Blockade to Durably Increase Survival Against Advanced Glioblastoma

Pharmacological targeting of the tumor–immune symbiosis in glioblastoma

Glioblastoma (GBM) is the most common and highly lethal form of primary brain tumor in adults. The median survival of GBM patients is approximately 14-16 months despite multimodal therapies. Emerging evidence has substantiated the critical role of symbiotic interactions between GBM cells and noncancerous immune cells (e.g., myeloid cells and T cells) in regulating tumor progression and therapy resistance. Approaches to target the tumor-immune symbiosis have emerged as a promising therapeutic strategy for GBM. Here, we review the recent developments for pharmacological targeting of the GBM-immune symbiosis and highlight the role of such strategies to improve the effectiveness of immunotherapies in GBM.

Coming in from the cold: overcoming the hostile immune microenvironment of medulloblastoma

Medulloblastoma is an aggressive brain tumor that occurs predominantly in children. Despite intensive therapy, many patients die of the disease, and novel therapies are desperately needed. Although immunotherapy has shown promise in many cancers, the low mutational burden, limited infiltration of immune effector cells, and immune-suppressive microenvironment of medulloblastoma have led to the assumption that it is unlikely to respond to immunotherapy. However, emerging evidence is challenging this view. Here we review recent preclinical and clinical studies that have identified mechanisms of immune evasion in medulloblastoma, and highlight possible therapeutic interventions that may give new hope to medulloblastoma patients and their families.

The intrinsic and microenvironmental features of diffuse midline glioma; implications for the development of effective immunotherapeutic treatment strategies

Diffuse midline glioma (DMG), including those of the brainstem (diffuse intrinsic pontine glioma), are pediatric tumors of the central nervous system (CNS). Recognized as the most lethal of all childhood cancers, palliative radiotherapy remains the only proven treatment option, however, even for those that respond, survival is only temporarily extended. DMG harbor an immunologically “cold” tumor microenvironment (TME) with few infiltrating immune cells. The mechanisms underpinning the cold TME are not well understood. Low expression levels of immune checkpoint proteins, including PD-1, PD-L1, and CTLA-4, are recurring features of DMG and likely contribute to the lack of response to immune checkpoint inhibitors (ICIs). The unique epigenetic signatures (including stem cell-like methylation patterns), a low tumor mutational burden, and recurring somatic mutations (H3K27M, TP53, ACVR1, MYC, and PIK3CA), possibly play a role in the reduced efficacy of traditional immunotherapies. Therefore, to circumvent the lack of efficacy thus far seen for the use of ICIs, adoptive cell transfer (including chimeric antigen receptor T cells) and the use of oncolytic viruses, are currently being evaluated for the treatment of DMG. It remains an absolute imperative that we improve our understanding of DMG’s intrinsic and TME features if patients are to realize the potential benefits offered by these sophisticated treatments. Herein, we summarize the limitations of immunotherapeutic approaches, highlight the emerging safety and clinical efficacy shown for sophisticated cell-based therapies, as well as the evolving knowledge underpinning the DMG-immune axis, to guide the development of immunotherapies that we hope will improve outcomes.

Meningeal lymphatics regulate radiotherapy efficacy through modulating anti-tumor immunity

As a first-line treatment, radiotherapy (RT) is known to modulate the immune microenvironment of glioma, but it is unknown whether the meningeal lymphatic vessel (MLV)-cervical lymph node (CLN) network regulates the process or influences RT efficacy. Here, we show that the MLV-CLN network contributes to RT efficacy in brain tumors and mediates the RT-modulated anti-tumor immunity that is enhanced by vascular endothelial growth factor C (VEGF-C). Meningeal lymphatic dysfunction impaired tumor-derived dendritic cell (DC) trafficking and CD8⁺ T cell activation after RT, whereas tumors overexpressing VEGF-C with meningeal lymphatic expansion were highly sensitive to RT. Mechanistically, VEGF-C-driven modulation of RT-triggered anti-tumor immunity was attributed to C-C Motif Chemokine Ligand 21 (CCL21)-dependent DC trafficking and CD8⁺ T cell activation. Notably, delivery of VEGF-C mRNA significantly enhanced RT efficacy and anti-tumor immunity in brain tumors. These findings suggest an essential role of the MLV-CLN network in RT-triggered anti-tumor immunity, and highlight the potential of VEGF-C mRNA for brain tumor therapy.

Glioma targeted therapy: insight into future of molecular approaches

Gliomas are the common type of brain tumors originating from glial cells. Epidemiologically, gliomas occur among all ages, more often seen in adults, which males are more susceptible than females. According to the fifth edition of the WHO Classification of Tumors of the Central Nervous System (WHO CNS5), standard of care and prognosis of gliomas can be dramatically different. Generally, circumscribed gliomas are usually benign and recommended to early complete resection, with chemotherapy if necessary. Diffuse gliomas and other high-grade gliomas according to their molecule subtype are slightly intractable, with necessity of chemotherapy. However, for glioblastoma, feasible resection followed by radiotherapy plus temozolomide chemotherapy define the current standard of care. Here, we discuss novel feasible or potential targets for treatment of gliomas, especially IDH-wild type glioblastoma. Classic targets such as the p53 and retinoblastoma (RB) pathway and epidermal growth factor receptor (EGFR) gene alteration have met failure due to complex regulatory network. There is ever-increasing interest in immunotherapy (immune checkpoint molecule, tumor associated macrophage, dendritic cell vaccine, CAR-T), tumor microenvironment, and combination of several efficacious methods. With many targeted therapy options emerging, biomarkers guiding the prescription of a particular targeted therapy are also attractive. More pre-clinical and clinical trials are urgently needed to explore and evaluate the feasibility of targeted therapy with the corresponding biomarkers for effective personalized treatment options.

Expression, prognostic significance and therapeutic implications of PD-L1 in gliomas

The advent of checkpoint immunotherapy, particularly with programmed death-1 (PD-1) and programmed death-ligand 1 (PD-L1) inhibitors, has provided ground-breaking results in several advanced cancers. Substantial efforts are being made to extend these promising therapies to other refractory cancers such as gliomas, especially glioblastoma, which represents the most frequent and malignant glioma and carries an exceptionally grim prognosis. Thus, there is a need for new therapeutic strategies with related biomarkers. Gliomas have a profoundly immunosuppressive tumour micro-environment and evade immunological destruction by several mechanisms, one being the expression of inhibitory immune checkpoint molecules such as PD-L1. PD-L1 is recognised as an important therapeutic target and its expression has been shown to hold prognostic value in different cancers. Several clinical trials have been launched and some already completed, but PD-1/PD-L1 inhibitors have yet to show convincing clinical efficacy in gliomas. Part of the explanation may reside in the vast molecular heterogeneity of gliomas and a complex interplay within the tumour micro-environment. In parallel, critical knowledge about PD-L1 expression is beginning to accumulate including knowledge on expression levels, testing methodology, co-expression with other checkpoint molecules and prognostic and predictive value. This article reviews these aspects and points out areas where biomarker research is needed to develop more successful checkpoint-related therapeutic strategies in gliomas.

Indoleamine 2,3-Dioxygenase 1: A Promising Therapeutic Target in Malignant Tumor

Tumorigenesis is a complex multifactorial and multistep process in which tumors can utilize a diverse repertoire of immunosuppressive mechanisms to evade host immune attacks. The degradation of tryptophan into immunosuppressive kynurenine is considered an important immunosuppressive mechanism in the tumor microenvironment. There are three enzymes, namely, tryptophan 2,3-dioxygenase (TDO), indoleamine 2,3-dioxygenase 1 (IDO1), and indoleamine 2,3-dioxygenase 2 (IDO2), involved in the metabolism of tryptophan. IDO1 has a wider distribution and higher activity in catalyzing tryptophan than the other two; therefore, it has been studied most extensively. IDO1 is a cytosolic monomeric, heme-containing enzyme, which is now considered an authentic immune regulator and represents one of the promising drug targets for tumor immunotherapy. Collectively, this review highlights the regulation of IDO1 gene expression and the ambivalent mechanisms of IDO1 on the antitumoral immune response. Further, new therapeutic targets via the regulation of IDO1 are discussed. A comprehensive analysis of the expression and biological function of IDO1 can help us to understand the therapeutic strategies of the inhibitors targeting IDO1 in malignant tumors.

Advances in Immunotherapies for Gliomas

PURPOSE OF REVIEW

Immunotherapy-based treatment of glioblastoma has been challenging because of the tumor's limited neoantigen profile and weakly immunogenic composition. This article summarizes the current clinical trials underway by evaluating the leading immunotherapy paradigms, the encountered barriers, and the future directions needed to overcome such tumor evasion.

RECENT FINDINGS

A limited number of phase III trials have been completed for checkpoint inhibitor, vaccine, as well as gene therapies, and have been unable to show improvement in survival outcomes. Nevertheless, these trials have also shown these strategies to be safe and promising with further adaptations. Further large-scale studies for chimeric antigen receptors T cell therapies and viral therapies are anticipated. Many current trials are broadening the number of antigens targeted and modulating the microtumor environment to abrogate early mechanisms of resistance. Future GBM treatment will also likely require synergistic effects by combination regimens.

Tumor Cell IDO Enhances Immune Suppression and Decreases Survival Independent of Tryptophan Metabolism in Glioblastoma

PURPOSE

Glioblastoma (GBM) is an incurable primary brain tumor that has not benefited from immunotherapy to date. More than 90% of GBM expresses the tryptophan (Trp) metabolic enzyme, indoleamine 2,3-dioxygenase 1 (IDO). This observation supported the historical hypothesis that IDO suppresses the antitumor immune response solely through a mechanism that requires intratumoral Trp depletion. However, recent findings led us to investigate the alternative hypothesis that IDO suppresses the anti-GBM immune response independent of its association with Trp metabolism.

EXPERIMENTAL DESIGN

IDO-deficient GBM cell lines reconstituted with IDO wild-type or IDO enzyme-null cDNA were created and validated in vitro and in vivo. Microarray analysis was conducted to search for genes that IDO regulates, followed by the analysis of human GBM cell lines, patient GBM and plasma, and The Cancer Genome Atlas (TCGA) database. Ex vivo cell coculture assays, syngeneic and humanized mouse GBM models, were used to test the alternative hypothesis.

RESULTS

Nonenzymic tumor cell IDO activity decreased the survival of experimental animals and increased the expression of complement factor H (CFH) and its isoform, factor H like protein 1 (FHL-1) in human GBM. Tumor cell IDO increased CFH and FHL-1 expression independent of Trp metabolism. Increased intratumoral CFH and FHL-1 levels were associated with poorer survival among patients with glioma. Similar to IDO effects, GBM cell FHL-1 expression increased intratumoral regulatory T cells (Treg) and myeloid-derived suppressor cells while it decreased overall survival in mice with GBM.

CONCLUSIONS

Our study reveals a nonmetabolic IDO-mediated enhancement of CFH expression and provides a new therapeutic target for patients with GBM.

Glial and myeloid heterogeneity in the brain tumour microenvironment

Brain cancers carry bleak prognoses, with therapeutic advances helping only a minority of patients over the past decade. The brain tumour microenvironment (TME) is highly immunosuppressive and differs from that of other malignancies as a result of the glial, neural and immune cell populations that constitute it. Until recently, the study of the brain TME was limited by the lack of methods to de-convolute this complex system at the single-cell level. However, novel technical approaches have begun to reveal the immunosuppressive and tumour-promoting properties of distinct glial and myeloid cell populations in the TME, identifying new therapeutic opportunities. Here, we discuss the immune modulatory functions of microglia, monocyte-derived macrophages and astrocytes in brain metastases and glioma, highlighting their disease-associated heterogeneity and drawing from the insights gained by studying these malignancies and other neurological disorders. Lastly, we consider potential approaches for the therapeutic modulation of the brain TME.

Indoleamine-2,3-Dioxygenase as a Perioperative Marker of the Immune System

Indoleamine-2,3-dioxygenase (IDO) is the “rate-limiting” enzyme in the kynurenine (Kyn) pathway of the tryptophan (Trp) catabolism. By its immune-modulatory effect, IDO initiates changes to the physiologically balanced immune state and plays a key role in the pathogenesis of various diseases, as well as in the perioperative setting during surgery. In autoimmune processes, highly malignant cancers such as glioblastoma or organ transplantation, IDO’s involvement has been studied extensively. However, in severe systemic infections, as present in sepsis, it is not yet completely understood. Hereafter, in this narrative review, we present the current knowledge of IDO’s implication on such complex immune-related processes. Moreover, we address the role of IDO as a predictive biomarker as well as a therapeutic target for immune-mediated diseases. Finally, we discuss IDO in the setting of surgical trauma-induced stress and highlight its promising use as a biomarker in the pre-operative setting for all disciplines involved in the decision-making process and treatment of patients undergoing surgery.

Glioblastoma as an age-related neurological disorder in adults

Background

Advanced age is a major risk factor for the development of many diseases including those affecting the central nervous system. Wild-type isocitrate dehydrogenase glioblastoma (IDH^wt GBM) is the most common primary malignant brain cancer and accounts for ≥90% of all adult GBM diagnoses. Patients with IDH^wt GBM have a median age of diagnosis at 68–70 years of age, and increasing age is associated with an increasingly worse prognosis for patients with this type of GBM.

Methods

The Surveillance, Epidemiology, and End Results, The Cancer Genome Atlas, and the Chinese Glioma Genome Atlas databases were analyzed for mortality indices. Meta-analysis of 80 clinical trials was evaluated for log hazard ratio for aging to tumor survivorship.

Results

Despite significant advances in the understanding of intratumoral genetic alterations, molecular characteristics of tumor microenvironments, and relationships between tumor molecular characteristics and the use of targeted therapeutics, life expectancy for older adults with GBM has yet to improve.

Conclusions

Based upon the results of our analysis, we propose that age-dependent factors that are yet to be fully elucidated, contribute to IDH^wt GBM patient outcomes.

Anti-PD-1/Anti-PD-L1 Drugs and Radiation Therapy: Combinations and Optimization Strategies

Simple Summary

Although immune checkpoint blockade has yielded unprecedented and durable responses in cancer patients, the efficacy of this treatment remains limited. Radiation therapy can induce immunogenic cell death that contributes to the local efficacy of irradiation. However, radiation-induced systemic responses are scarce. Studies combining radiation with checkpoint inhibitors suggest a synergistic potential of this strategy. In this review, we focused on parameters that can be optimized to enhance the anti-tumor immune response that results from this association, in order to achieve data on dose, fractionation, target volume, lymph nodes sparing, radiation particles, and other immunomodulatory agents. These factors should be considered in future trials for better clinical outcomes. To this end, we discussed the main preclinical and clinical data available to optimize the efficacy of the treatment combination.

Abstract

Immune checkpoint inhibitors have been associated with long-term complete responses leading to improved overall survival in several cancer types. However, these novel immunotherapies are only effective in a small proportion of patients, and therapeutic resistance represents a major limitation in clinical practice. As with chemotherapy, there is substantial evidence that radiation therapy promotes anti-tumor immune responses that can enhance systemic responses to immune checkpoint inhibitors. In this review, we discuss the main preclinical and clinical evidence on strategies that can lead to an enhanced response to PD-1/PD-L1 blockade in combination with radiation therapy. We focused on central issues in optimizing radiation therapy, such as the optimal dose and fractionation for improving the therapeutic ratio, as well as the impact on immune and clinical responses of dose rate, target volume, lymph nodes irradiation, and type of radiation particle. We explored the addition of a third immunomodulatory agent to the combination such as other checkpoint inhibitors, chemotherapy, and treatment targeting the tumor microenvironment components. The strategies described in this review provide a lead for future clinical trials.

Interrogating the immune-modulating roles of radiation therapy for a rational combination with immune-checkpoint inhibitors in treating pancreatic cancer

BACKGROUND

Radiation therapy (RT) has the potential to enhance the efficacy of immunotherapy, such as checkpoint inhibitors, which has dramatically altered the landscape of treatments for many cancers, but not yet for pancreatic ductal adenocarcinoma (PDAC). Our prior studies demonstrated that PD ligand-1 and indoleamine 2,3-dioxygenase 1 (IDO1) were induced on tumor epithelia of PDACs following neoadjuvant therapy including RT, suggesting RT may prime PDAC for PD-1 blockade antibody (αPD-1) or IDO1 inhibitor (IDO1i) treatments. In this study, we investigated the antitumor efficacy of the combination therapies with radiation and PD-1 blockade or IDO1 inhibition or both.

METHODS

We developed and used a mouse syngeneic orthotopic model of PDAC suitable for hypofractionated RT experiments.

RESULTS

The combination therapy of αPD-1 and RT improved survival. The dual combination of RT/IDO1i and triple combination of RT/αPD-1/IDO1i did not improve survival compared with RT/αPD-1, although all of these combinations offer similar local tumor control. RT/αPD-1 appeared to result in the best systemic interferon-γ response compared with other treatment groups and the highest local expression of immune-activation genes, including Cd28 and Icos.

CONCLUSION

Our RT model allows examining the immune-modulatory effects of RT alone and in combination with immune-checkpoint inhibitors in the pancreas/local microenvironment. This study highlights the importance of choosing the appropriate immune-modulatory agents to be combined with RT to tip the balance toward antitumor adaptive immune responses.

Emerging strategies based on nanomaterials for ionizing radiation-optimized drug treatment of cancer

Drug-radiotherapy is a common and effective combinational treatment for cancer. This study aimed to explore the ionizing radiation-optimized drug treatment based on nanomaterials so as to improve the synergistic efficacy of drug-radiotherapy against cancer and limit the adverse effect on healthy organs. In this review, these emerging strategies were divided into four parts. First, the delivery of the drug-loaded nanoparticles was optimized owing to the strengthened passive targeting process, active targeting process, and cell targeting process of nanoparticles after ionizing radiation exposure. Second, nanomaterials were designed to respond to the ionizing radiation, thus leading to the release of the loading drugs controllably. Third, radiation-activated pro-drugs were loaded onto nanoparticles for radiation-triggered drug therapy. In particular, nontoxic nanoparticles with radiosensitization capability and innocuous radio-dynamic contrast agents can be considered as radiation-activated drugs, which were discussed in this review. Fourth, according to the various synergetic mechanisms, radiotherapy could improve the drug response of cancer, obtaining optimized drug-radiotherapy. Finally, relative suggestions were provided to further optimize these aforementioned strategies. Therefore, a novel topic was selected and the emerging strategies in this region were discussed, aiming to stimulate the inspiration for the development of ionizing radiation-optimized drug treatment based on nanomaterials.

Treating ICB-resistant glioma with anti-CD40 and mitotic spindle checkpoint controller BAL101553 (lisavanbulin)

Glioblastoma is a highly malignant brain tumor with no curative treatment options, and immune checkpoint blockade has not yet shown major impact. We hypothesized that drugs targeting mitosis might affect the tumor microenvironment and sensitize cancer cells to immunotherapy. We used 2 glioblastoma mouse models with different immunogenicity profiles, GL261 and SB28, to test the efficacy of antineoplastic and immunotherapy combinations. The spindle assembly checkpoint activator BAL101553 (lisavanbulin), agonistic anti-CD40 antibody, and double immune checkpoint blockade (anti–programmed cell death 1 and anti–cytotoxic T lymphocyte–associated protein 4; anti–PD-1 and anti–CTLA-4) were evaluated individually or in combination for treating orthotopic GL261 and SB28 tumors. Genomic and immunological analyses were used to predict and interpret therapy responsiveness. BAL101553 monotherapy increased survival in immune checkpoint blockade–resistant SB28 glioblastoma tumors and synergized with anti-CD40 antibody, in a T cell–independent manner. In contrast, the more immunogenic and highly mutated GL261 model responded best to anti–PD-1 and anti–CTLA-4 therapy and more modestly to BAL101553 and anti-CD40 combination. Our results show that BAL101553 is a promising therapeutic agent for glioblastoma and could synergize with innate immune stimulation. Overall, these data strongly support immune profiling of glioblastoma patients and preclinical testing of combination therapies with appropriate models for particular patient groups.

Indoleamine 2,3-Dioxygenase 1 (IDO1) Promotes Cardiac Hypertrophy via a PI3K-AKT-mTOR-Dependent Mechanism

Indoleamine 2,3-dioxygenase 1 (IDO1) is an enzyme for tryptophan metabolism, involved in immune cell differentiation/maturation and cancer biology. IDO1 is also expressed in cardiomyocytes, but its roles in the cardiovascular system are not fully understood. Here, we reported the functions of IDO1 during cardiac hypertrophy. Quantitative real-time PCR and Western blot experiments demonstrated the upregulation of IDO1 mRNA and protein levels in human and hypertrophic mouse hearts, as well as in angiotensin II (Ang II)-induced hypertrophic rat cardiomyocytes. IDO1 activity and metabolite product kynurenine were upregulated in rodent hypertrophic hearts and cardiomyocytes. Inhibition of IDO1 activity with PF-06840003 reduced Ang II-induced cardiac hypertrophy and rescued cardiac function in mice. siRNA-mediated knockdown of Ido1 repressed Ang II-induced growth in cardiomyocyte size and overexpression of hypertrophy-associated genes atrial natriuretic peptide (Anp or Nppa), brain natriuretic peptide (Bnp or Nppb), β-myosin heavy chain (β-Mhc or Myh7). By contrast, adenovirus-mediated rat Ido1 overexpression in cardiomyocytes promoted hypertrophic growth induced by Ang II. Mechanism analysis showed that IDO1 overexpression was associated with PI3K-AKT-mTOR signaling to activate the ribosomal protein S6 kinase 1 (S6K1), which promoted protein synthesis in Ang II-induced hypertrophy of rat cardiomyocytes. Finally, we provided evidence that inhibition of PI3K with pictilisib, AKT with perifosine, or mTOR with rapamycin, blocked the effects of IDO1 on protein synthesis and cardiomyocyte hypertrophy in Ang II-treated cells. Collectively, our findings identify that IDO1 promotes cardiomyocyte hypertrophy partially via PI3K-AKT-mTOR-S6K1 signaling.

Sex-Specific Differences in Glioblastoma

Sex differences have been well identified in many brain tumors. Even though glioblastoma (GBM) is the most common primary malignant brain tumor in adults and has the worst outcome, well-established differences between men and women are limited to incidence and outcome. Little is known about sex differences in GBM at the disease phenotype and genetical/molecular level. This review focuses on a deep understanding of the pathophysiology of GBM, including hormones, metabolic pathways, the immune system, and molecular changes, along with differences between men and women and how these dimorphisms affect disease outcome. The information analyzed in this review shows a greater incidence and worse outcome in male patients with GBM compared with female patients. We highlight the protective role of estrogen and the upregulation of androgen receptors and testosterone having detrimental effects on GBM. Moreover, hormones and the immune system work in synergy to directly affect the GBM microenvironment. Genetic and molecular differences have also recently been identified. Specific genes and molecular pathways, either upregulated or downregulated depending on sex, could potentially directly dictate GBM outcome differences. It appears that sexual dimorphism in GBM affects patient outcome and requires an individualized approach to management considering the sex of the patient, especially in relation to differences at the molecular level.

Radiation in Combination With Targeted Agents and Immunotherapies for Pediatric Central Nervous System Tumors - Progress, Opportunities, and Challenges

Pediatric brain tumors are the most common solid tumors in children and represent a heterogenous group of diagnoses. While some are treatable with current standard of care, relapsed/refractory disease is common and some high-risk diagnoses remain incurable. A growing number of therapy options are under development for treatment of CNS tumors, including targeted therapies that disrupt key tumor promoting processes and immunotherapies that promote anti-tumor immune function. While these therapies hold promise, it is likely that single agent treatments will not be sufficient for most high-risk patients and combination strategies will be necessary. Given the central role for radiotherapy for many pediatric CNS tumors, we review current strategies that combine radiation with targeted therapies or immunotherapies. To promote the ongoing development of rational combination treatments, we highlight 1) mechanistic connections between molecular drivers of tumorigenesis and radiation response, 2) ways in which molecular alterations in tumor cells shape the immune microenvironment, and 3) how radiotherapy affects the host immune system. In addition to discussing strategies to maximize efficacy, we review principles that inform safety of combination therapies.

Resolution of tissue signatures of therapy response in patients with recurrent GBM treated with neoadjuvant anti-PD1

The response of patients with recurrent glioblastoma multiforme to neoadjuvant immune checkpoint blockade has been challenging to interpret due to the inter-patient and intra-tumor heterogeneity. We report on a comparative analysis of tumor tissues collected from patients with recurrent glioblastoma and high-risk melanoma, both treated with neoadjuvant checkpoint blockade. We develop a framework that uses multiplex spatial protein profiling, machine learning-based image analysis, and data-driven computational models to investigate the pathophysiological and molecular factors within the tumor microenvironment that influence treatment response. Using melanoma to guide the interpretation of glioblastoma analyses, we interrogate the protein expression in microscopic compartments of tumors, and determine the correlates of cytotoxic CD8+ T cells, tumor growth, treatment response, and immune cell-cell interaction. This work reveals similarities shared between glioblastoma and melanoma, immunosuppressive factors that are unique to the glioblastoma microenvironment, and potential co-targets for enhancing the efficacy of neoadjuvant immune checkpoint blockade.

Current Development of Glioblastoma Therapeutic Agents

Glioblastoma multiforme (GBM) is the most common and aggressive malignant primary brain tumor in humans. Over the past several decades, despite improvements in neurosurgical techniques, development of powerful chemotherapeutic agents, advances in radiotherapy, and comprehensive genomic profiling and molecular characterization, treatment of GBM has achieved very limited success in increasing overall survival. Thus, identifying and understanding the key molecules and barriers responsible for the malignant phenotypes and treatment resistance of GBM will yield new potential therapeutic targets. We review the most recent development of receptor tyrosine kinase targeted therapy for GBM and discuss the current status of several novel strategies with the emphasis on blood-brain barrier penetration as a major obstacle for small-molecule drugs to achieve their therapeutic goals. Likewise, a major opportunity for the treatment of GBM lies in the use of biomarkers for the discovery and development of new receptor tyrosine kinase targeted therapy.

Radiotherapy and the immune system: More than just immune suppression

Radiotherapy (RT) is still one of the standard cancer therapies, with up to two third of all cancer patients with solid tumors being irradiated in the course of their disease. The aim of using ionizing radiation in fractionated treatment schedules was always to achieve local tumor control by inducing DNA damage which can be repaired by surrounding normal tissue but leads to cell death in tumor cells. Meanwhile, it is known that RT also has immunological effects reshaping the tumor microenvironment. Nevertheless, RT alone often fails to elicit potent antitumor immune responses as these effects can be immunostimulatory as well as immunosuppressive. Here, we discuss how immunotherapies can be exploited in combined therapies to boost RT-induced antitumor immune responses or to counteract preexisting and RT-mediated immunosuppression to improve local and systemic tumor control. Furthermore, we highlight some parameters of radioimmunotherapies (RITs) which are under investigation for potential optimizations and how RIT approaches are tested in first phases II and III trials. Finally, we discuss how RT might affect normal and cancer stem cells.

Targeting CSF1R Alone or in Combination with PD1 in Experimental Glioma

Glioblastoma is an aggressive primary tumor of the central nervous system. Targeting the immunosuppressive glioblastoma-associated microenvironment is an interesting therapeutic approach. Tumor-associated macrophages represent an abundant population of tumor-infiltrating host cells with tumor-promoting features. The colony stimulating factor-1/ colony stimulating factor-1 receptor (CSF-1/CSF1R) axis plays an important role for macrophage differentiation and survival. We thus aimed at investigating the antiglioma activity of CSF1R inhibition alone or in combination with blockade of programmed death (PD) 1. We investigated combination treatments of anti-CSF1R alone or in combination with anti-PD1 antibodies in an orthotopic syngeneic glioma mouse model, evaluated post-treatment effects and assessed treatment-induced cytotoxicity in a coculture model of patient-derived microtumors (PDM) and autologous tumor-infiltrating lymphocytes (TILs) ex vivo. Anti-CSF1R monotherapy increased the latency until the onset of neurological symptoms. Combinations of anti-CSF1R and anti-PD1 antibodies led to longterm survivors in vivo. Furthermore, we observed treatment-induced cytotoxicity of combined anti-CSF1R and anti-PD1 treatment in the PDM/TILs cocultures ex vivo. Our results identify CSF1R as a promising therapeutic target for glioblastoma, potentially in combination with PD1 inhibition.

Immunostimulant hydrogel for the inhibition of malignant glioma relapse post-resection

Immunotherapies have revolutionized intervention strategies for many primary cancers, but have not improved the outcomes of glioblastoma multiforme (GBM), which remains one of the most lethal malignant cerebral tumours. Here we present an injectable hydrogel system that stimulates tumoricidal immunity after GBM surgical resection, which mitigates its relapse. The hydrogel comprises a tumour-homing immune nanoregulator, which induces immunogenic cell death and suppression of indoleamine 2,3-dioxygenase-1, and chemotactic CXC chemokine ligand 10, for a sustained T-cell infiltration. When delivered in the resected tumour cavity, the hydrogel system mimics a 'hot' tumour-immunity niche for attacking residual tumour cells and significantly suppresses postoperative GBM recurrence. Our work provides an alternative strategy for conferring effective tumoricidal immunity in GBM patients, which may have a broad impact in the immunotherapy of 'cold' tumours after surgical intervention.

Parallel discovery of selective and dual inhibitors of tryptophan dioxygenases IDO1 and TDO2 with a newly-modified enzymatic assay

The expression of tryptophan catabolising enzyme indoleamine 2,3-dioxygenase 1 (IDO1) or tryptophan 2,3-dioxygenase 2 (TDO2) in cancers is associated with suppressed immunity and poor patient prognosis. Results from human clinical trials of IDO1 inhibitors have been disappointing. There is now a strong interest in the development of TDO2-selective or dual IDO1/TDO2 inhibitors that may surpass IDO1 inhibitors by providing broader efficacy and blocking constitutively-expressed hepatic TDO2. To expedite the discovery of novel TDO2-specific and dual inhibitors, an assay that enabled the efficient and accurate measurement of the inhibitory activity of compounds against both IDO1 and TDO2 enzymes, concurrently in the same experiment was established to screen 5,682 compounds that included the National Cancer Institute Diversity set 5, for inhibition of IDO1 and TDO2 activity. This screen identified 82 compounds that inhibited either IDO1, TDO2 or both enzymes > 50% at 20 µM. Thirty Pan Assay Interference compounds were removed from the list and the IC₅₀ of the remaining 52 compounds against IDO1 and TDO2 was subsequently determined using the newly-developed concurrent assay. Ten compounds were confirmed as dual IDO1/TDO2 inhibitors having IC₅₀ values under 50 µM against both enzymes and within 2-fold of each other. Six compounds with IC₅₀ values between 1.39 and 8.41 µM were identified as potential TDO2-selective leads. The use of this concurrent protocol is anticipated to expedite the discovery of novel leads for dual and selective inhibitors against IDO1 and or TDO2 and speed the evaluation of novel analogues that will ensue.

An Immune-Related Gene Pairs Signature Predicts Prognosis and Immune Heterogeneity in Glioblastoma

Purpose

Glioblastoma is one of the most aggressive nervous system neoplasms. Immunotherapy represents a hot spot and has not been included in standard treatments of glioblastoma. So in this study, we aim to filtrate an immune-related gene pairs (IRGPs) signature for predicting survival and immune heterogeneity.

Methods

We used gene expression profiles and clinical information of glioblastoma patients in the TCGA and CGGA datasets, dividing into discovery and validation cohorts. IRGPs significantly correlative with prognosis were selected to conduct an IRGPs signature. Low and high risk groups were separated by this IRGPs signature. Univariate and multivariate cox analysis were adopted to check whether risk can be a independent prognostic factor. Immune heterogeneity between different risk groups was analyzed via immune infiltration and gene set enrichment analysis (GSEA). Some different expressed genes between groups were selected to determine their relationship with immune cells and immune checkpoints.

Results

We found an IRGPs signature consisting of 5 IRGPs. Different risk based on IRGPs signature is a independent prognostic factor both in the discovery and validation cohorts. High risk group has some immune positive cells and more immune repressive cells than low risk group by means of immune infiltration. We discovered some pathways are more active in the high risk group, leading to immune suppression, drug resistance and tumor evasion. In two specific signaling, some genes are over expressed in high risk group and positive related to immune repressive cells and immune checkpoints, which indicate aggression and immunotherapy resistance.

Conclusion

We identified a robust IRGPs signature to predict prognosis and immune heterogeneity in glioblastoma patients. Some potential targets and pathways need to be further researched to make different patients benefit from personalized immunotherapy.

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.

Challenging Hurdles of Current Targeting in Glioblastoma: A Focus on Immunotherapeutic Strategies

Glioblastoma is the most frequent primary neoplasm of the central nervous system and still suffers from very poor therapeutic impact. No clear improvements over current standard of care have been made in the last decade. For other cancers, but also for brain metastasis, which harbors a very distinct biology from glioblastoma, immunotherapy has already proven its efficacy. Efforts have been pursued to allow glioblastoma patients to benefit from these new approaches, but the road is still long for broad application. Here, we aim to review key glioblastoma immune related characteristics, current immunotherapeutic strategies being explored, their potential caveats, and future directions.

Role of kynurenine in promoting the generation of exhausted CD8+ T cells in colorectal cancer

Although blocking programmed cell death protein 1 (PD-1) has emerged as a standard treatment for metastatic colorectal cancer (CRC), a vast majority of CRC patients still respond poorly to anti-PD-1 immunotherapy. In this study, we showed that the levels of indoleamine 2,3-dioxygenase 1 (IDO1) and its catabolite kynurenine (Kyn) were higher in late stages (stages III and IV) than in early stages (stages I and II) of CRC patients. We found that Kyn could induce the expression of immune checkpoints and exhaustion markers in CD8⁺ tumor-infiltrating T cells. Knockdown of IDO1 expression using small hairpin RNAs (shRNAs) in the MC38 and CT26 colorectal cell lines led downregulation of Kyn expression and activation of CD8⁺ T cells in MC38- or CT26-bearing mice. Subsequent mechanistic study revealed significantly reduced thymocyte selection-associated HMG box (TOX) mRNA levels in CD8⁺ tumor-infiltrating T cells isolated from IDO1 knockdown MC38-Scr- and CT26-bearing mice. Kyn-induced CD8⁺ T cell exhaustion was reversed by knockdown of TOX expression. Finally, the application of the well-known IDO1 inhibitors 1MT or NLG919 substantially improved the therapeutic effect of CRC in vivo and restored CD8⁺ tumor-infiltrating T cells anti-tumor activity. This improvement was further enhanced by an anti-PD-1 combined therapy. In conclusion, our study revealed a novel mechanism underlying the metabolic factors found in tumor microenvironment which could induce CD8⁺ T cells exhaustion. Our findings provided a new strategy of restoring the antitumor activity of CD8⁺ T cells through combined targeting of the IDO1/Kyn and PD-1/PD-L1 pathways in patients with CRC.

NTRK2 Fusion driven pediatric glioblastoma: Identification of oncogenic Drivers via integrative Genome and transcriptome profiling

This is the first report of a NACC2-NTRK2 fusion in a histological glioblastoma. Oncogenomic analysis revealed this actionable fusion oncogene in a pediatric cerebellar glioblastoma, which would not have been identified through routine diagnostics, demonstrating the value of clinical genome profiling in cancer care.

Combination immunotherapy strategies for glioblastoma

INTRODUCTION

Despite recent advances in treatment for a number of cancers with immune checkpoint blockade (ICB), immunotherapy has had limited efficacy in glioblastoma (GBM). The recent multi-centered CheckMate 143 trial in first time recurrent GBM and the Checkmate 498 trial in newly diagnosed unmethylated GBM showed that antibodies against programmed cell death protein 1 (PD-1) failed to improve overall survival in patients with GBM. Recent preclinical and clinical studies have explored combining ICB with several other therapies including additional ICB against alternative checkpoint molecules, activation of costimulatory checkpoint molecules such as 4-1BB, radiation-induced tumor cell lysis and immunogenic recruitment, local chemotherapy, neoadjuvant ICB therapy, and myeloid cell reactivation.

METHODS

We have reviewed the literature on ICB seminal to the progression of several preclinical studies and clinical trials in order to provide a compendium of the current state of combination immunotherapy for GBM. For ongoing clinical trials without associated publications, we searched clinicaltrials.gov for ongoing studies using the keywords, "GBM" and "glioblastoma", as well as names of checkpoint molecules.

RESULTS

Recent trends from clinical trials demonstrate that despite a variety of different combination strategies involving ICB, GBM remains largely elusive to current immunotherapies. There is a discordance of survival outcomes between GBM pre-clinical models and clinical trials, likely due to the heterogeneity of GBM in patients as well as other adaptive immune mechanisms not otherwise represented in murine models. However, in clinical studies, neoadjuvant ICB in GBM was found to diversify the T cell receptor (TCR) repertoire and increase chemokine mRNA transcripts when comparing pre- and post- surgical time points. Moreover, an increase in peripheral and tumor-infiltrating lymphocyte (TIL) clonotypes were also observed when comparing adjuvant and neoadjuvant cohorts.

DISCUSSION

Despite the lack of clinical survival benefit, immune modulation was observed in multiple different combination strategies for GBM in both preclinical and clinical studies, indicating that ICB combination therapy results in a significant immunological impact on the tumor microenvironment.

Pathogenetic Features and Current Management of Glioblastoma

Glioblastoma (GBM) is the most common form of primary malignant brain tumor with a devastatingly poor prognosis. The disease does not discriminate, affecting adults and children of both sexes, and has an average overall survival of 12-15 months, despite advances in diagnosis and rigorous treatment with chemotherapy, radiation therapy, and surgical resection. In addition, most survivors will eventually experience tumor recurrence that only imparts survival of a few months. GBM is highly heterogenous, invasive, vascularized, and almost always inaccessible for treatment. Based on all these outstanding obstacles, there have been tremendous efforts to develop alternative treatment options that allow for more efficient targeting of the tumor including small molecule drugs and immunotherapies. A number of other strategies in development include therapies based on nanoparticles, light, extracellular vesicles, and micro-RNA, and vessel co-option. Advances in these potential approaches shed a promising outlook on the future of GBM treatment. In this review, we briefly discuss the current understanding of adult GBM's pathogenetic features that promote treatment resistance. We also outline novel and promising targeted agents currently under development for GBM patients during the last few years with their current clinical status.

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.

Inhibition of 2-hydroxyglutarate elicits metabolic reprogramming and mutant IDH1 glioma immunity in mice

Mutant isocitrate dehydrogenase 1 (IDH1-R132H; mIDH1) is a hallmark of adult gliomas. Lower grade mIDH1 gliomas are classified into 2 molecular subgroups: 1p/19q codeletion/TERT-promoter mutations or inactivating mutations in α-thalassemia/mental retardation syndrome X-linked (ATRX) and TP53. This work focuses on glioma subtypes harboring mIDH1, TP53, and ATRX inactivation. IDH1-R132H is a gain-of-function mutation that converts α-ketoglutarate into 2-hydroxyglutarate (D-2HG). The role of D-2HG within the tumor microenvironment of mIDH1/mATRX/mTP53 gliomas remains unexplored. Inhibition of D-2HG, when used as monotherapy or in combination with radiation and temozolomide (IR/TMZ), led to increased median survival (MS) of mIDH1 glioma-bearing mice. Also, D-2HG inhibition elicited anti-mIDH1 glioma immunological memory. In response to D-2HG inhibition, PD-L1 expression levels on mIDH1-glioma cells increased to similar levels as observed in WT-IDH gliomas. Thus, we combined D-2HG inhibition/IR/TMZ with anti-PDL1 immune checkpoint blockade and observed complete tumor regression in 60% of mIDH1 glioma-bearing mice. This combination strategy reduced T cell exhaustion and favored the generation of memory CD8+ T cells. Our findings demonstrate that metabolic reprogramming elicits anti-mIDH1 glioma immunity, leading to increased MS and immunological memory. Our preclinical data support the testing of IDH-R132H inhibitors in combination with IR/TMZ and anti-PDL1 as targeted therapy for mIDH1/mATRX/mTP53 glioma patients.

GBP1 Facilitates Indoleamine 2,3-Dioxygenase Extracellular Secretion to Promote the Malignant Progression of Lung Cancer

IDO1-mediated immune escape can lead to the malignant progression of tumors. However, the precise mechanism of IDO1 remains unclear. This study showed that IDO1 can bind to GBP1 and increase the extracellular secretion of IDO1 with the assistance of GBP1, thereby promoting the malignant proliferation and metastasis of lung cancer. In vitro study showed that the high expression levels of IDO1 and GBP1 in lung cancer cells promoted cell invasion and migration. In vivo study revealed that knock-down of IDO1 and GBP1 inhibited tumor growth and metastasis. In addition, Astragaloside IV reduces the extracellular secretion of IDO1 by blocking the interaction of IDO1 and GBP1, thereby reducing T cell exhaustion and inhibiting tumor progression. These results suggest that blocking the extracellular secretion of IDO1 may prevent T cell exhaustion and thereby enhance the effect of PD-1 inhibitors on cancer treatment.

Inhibition of Colony-Stimulating Factor-1 Receptor Enhances the Efficacy of Radiotherapy and Reduces Immune Suppression in Glioblastoma

AIM

To use inhibition of colony-stimulating factor-1 receptor (CSF-1R) to target tumor-associated macrophages (TAMs) and improve the efficacy of radiotherapy in glioblastoma (GBM).

MATERIALS AND METHODS

The CSF-1R inhibitor BLZ-945 was used to examine the impact of CSF-1R inhibition on M2 polarization in vitro. Using an orthotopic, immunocompetent GBM model, mice were treated with vehicle, RT, BLZ-945, or RT plus BLZ-945.

RESULTS

BLZ-945 reduced M2 polarization in vitro. BLZ-945 alone did not improve median overall survival (mOS=29 days) compared to control mice (mOS=27 days). RT improved survival (mOS=45 days; p=0.02), while RT plus BLZ-945 led to the longest survival (mOS=not reached; p=0.005). Resected tumors had a relatively large population of M2 TAMs in GBM at baseline, which was increased in response to RT. BLZ-945 reduced RT-induced M2 infiltration.

CONCLUSION

Inhibition of CSF-1R improved response to RT in the treatment of GBM and may represent a promising strategy to improve RT-induced antitumor immune responses.

Regulation of tumor immune suppression and cancer cell survival by CXCL1/2 elevation in glioblastoma multiforme

The invasiveness and high immune suppression of glioblastoma multiforme (GBM) produce poor survival of afflicted patients. Unfortunately, in the past decades, no therapeutic approach has remarkably improved the survival time of patients with GBM. Our analysis of the TCGA database and brain tumor tissue arrays indicated that CXCL1 and CXCL2 overexpression is closely associated with GBM's aggressiveness. Our results showed that elevation of CXCL1 or CXCL2 facilitated myeloid cell migration and simultaneously disrupted CD8⁺ T cell accumulation at tumor sites, causing accelerated tumor progression. Yet, blockade of CXCL1/2 significantly prevented myeloid-derived suppressor cell migration and thereby increased CD8⁺ T cell accumulation in vitro and in vivo. CXCL1/2 also promoted the paracrine factor S100A9 and further activated Erk1/2 and p70S60k, whereas blocking CXCL1/2 down-regulated these prosurvival factors. The combination of targeting CXCL1/2 and standard temozolomide chemotherapy improved upon the antitumor efficacy of chemotherapy alone, extending the overall survival time in GBM.

Immune metabolism in PD-1 blockade-based cancer immunotherapy

Energy metabolism plays an important role in proliferating cells. Recent reports indicate that metabolic regulation or metabolic products can control immune cell differentiation, fate and reactions. Cancer immunotherapy based on blockade of programmed cell death protein 1 (PD-1) has been used worldwide, but a significant fraction of patients remain unresponsive. Therefore, clarifying the mechanisms and overcoming the unresponsiveness are urgent issues. Because cancer immunity consists of interactions between the cancer and host immune cells, there has recently been a focus on the metabolic interactions and/or competition between the tumor and the immune system to address these issues. Cancer cells render their microenvironment immunosuppressive, driving T-cell dysfunction or exhaustion, which is advantageous for cancer cell survival. However, accumulating mechanistic evidence of T-cell and cancer cell metabolism has gradually revealed that controlling the metabolic pathways of either type of cell can overcome T-cell dysfunction and reprogram the metabolic balance in the tumor microenvironment. Here, we summarize the role of immune metabolism in T-cell-based immune surveillance and cancer immune escape. This new concept has boosted the development of combination therapy and predictive biomarkers in cancer immunotherapy with immune checkpoint inhibitors.

Immune system in cancer radiotherapy: Resistance mechanisms and therapy perspectives

Radiotherapy is a common modality for more than half of cancer patients. Classically, radiation is known as a strategy to kill cancer cells via direct interaction with DNA or generation of free radicals. Nowadays, we know that modulation of immune system has a key role in the outcome of radiotherapy. Selecting an appropriate dose per fraction is important for stimulation of anti-tumor immunity. Unfortunately, cancer cells and other cells within tumor microenvironment (TME) promote some mechanisms implicated in the attenuation of anti-tumor immunity via exhaustion of CD8 + T lymphocytes and natural killer (NK) cells. Immunotherapy with immune checkpoint inhibitors (ICIs) has shown to be an interesting adjuvant for induction of more effective anti-tumor immunity. Clinical trial studies are ongoing for uncovering more knowledge about the efficacy of ICI combination with radiotherapy. Some newer pre-clinical studies show more effective therapeutic window for targeting PD-1 and some other targets in combination with hypofractionated radiotherapy. In this review, we explain cellular and molecular consequences in the TME following radiotherapy and promising immune targets to enhance anti-tumor immunity.

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.

What is New in Neuro-oncology?

Neuro-oncology is a rapidly developing field. A continuous evolution in the understanding of the molecular underpinnings of central nervous system tumors has helped reconfigure the classification of brain tumors. More importantly, it has laid the path forward for the development and investigation of new therapeutics. The authors discuss the classification of brain tumors and novel therapies in brain tumors as well as promising treatments underway.

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.

PD-1 Inhibitors: Do they have a Future in the Treatment of Glioblastoma?

Glioblastoma (WHO grade IV glioma) is the most common malignant primary brain tumor in adults. Survival has remained largely static for decades, despite significant efforts to develop new effective therapies. Immunotherapy and especially immune checkpoint inhibitors and programmed cell death (PD)-1/PD-L1 inhibitors have transformed the landscape of cancer treatment and improved patient survival in a number of different cancer types. With the exception of few select cases (e.g., patients with Lynch syndrome) the neuro-oncology community is still awaiting evidence that PD-1 blockade can lead to meaningful clinical benefit in glioblastoma. This lack of progress in the field is likely to be due to multiple reasons, including inherent challenges in brain tumor drug development, the blood-brain barrier, the unique immune environment in the brain, the impact of corticosteroids, as well as inter- and intratumoral heterogeneity. Here we critically review the clinical literature, address the unique aspects of glioma immunobiology and potential immunobiological barriers to progress, and contextualize new approaches to increase the efficacy of PD-1/PD-L1 inhibitors in glioblastoma that may identify gaps and testable relevant hypotheses for future basic and clinical research and to provide a novel perspective to further stimulate preclinical and clinical research to ultimately help patients with glioma, including glioblastoma, which is arguably one of the greatest areas of unmet need in cancer. Moving forward, we need to build on our existing knowledge by conducting further fundamental glioma immunobiology research in parallel with innovative and methodologically sound clinical trials.

Connections between Metabolism and Epigenetic Modification in MDSCs

Myeloid-derived suppressor cells (MDSCs) are major immunosuppressive cells in the tumor microenvironment (TME). During the differentiation and development of MDSCs from myeloid progenitor cells, their functions are also affected by a series of regulatory factors in the TME, such as metabolic reprogramming, epigenetic modification, and cell signaling pathways. Additionally, there is a crosstalk between these regulatory factors. This review mainly introduces the metabolism (especially glucose metabolism) and significant epigenetic modification of MDSCs in the TME, and briefly introduces the connections between metabolism and epigenetic modification in MDSCs, in order to determine the further impact on the immunosuppressive effect of MDSCs, so as to serve as a more effective target for tumor therapy.

1'H-Indole-3'-Carbonyl-Thiazole-4-Carboxylic Acid Methyl Ester Blocked Human Glioma Cell Invasion via Aryl Hydrocarbon Receptor's Regulation of Cytoskeletal Contraction

Blocking glioma cell invasion has been challenging due to cancer cells that can swiftly switch their migration mode, and agents that can block more than one migration mode are sought after. We found that small molecule 2-(1H-indole-3-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE), an endogenous aryl hydrocarbon receptor (AHR) agonist, can block more than one mode of glioma cell migration, based on cultured cell behavior captured by videos. Data from wound-healing assays and mouse xenograft glioma models corroborated ITE's migration-inhibiting effects while knocking down AHR by siRNA abolished these effects. To identify genes that mediated ITE-AHR's effect, we first collected gene expression changes upon ITE treatment by RNA-seq, then compared them against literature reported migration-related genes in glioma and that were potentially regulated by AHR. MYH9, a component of nonmuscle myosin IIA (NMIIA), was confirmed to be reduced by ITE treatment. When MYH9 was overexpressed in the glioma cells, a good correlation was observed between the expression level and the cell migration ability, determined by wound-healing assay. Correspondingly, overexpression of MYH9 abrogated ITE's migration-inhibiting effects, indicating that ITE-AHR inhibited cell migration via inhibiting MYH9 expression. MYH9 is essential for cell migration in 3D confined space and not a discovered target of AHR; the fact that ITE affects MYH9 via AHR opens a new research and development avenue.

Immunotherapy and Response Assessment in Malignant Glioma: Neuro-oncology Perspective

Glioblastoma (GBM) is the deadliest form of brain cancer and recurs uniformly. Despite aggressive treatment with maximal safe surgical resection, adjuvant radiation with temozolomide chemotherapy, and alternating electrical field therapy, median survival for newly diagnosed GBM remains <2 years. Novel therapies are desperately needed. Immunotherapy, which has led to significant improvement in patient outcomes across many tumor types, is currently being studied in a large number of GBM clinical trials. One of the biggest challenges in immunotherapy trials in GBM has been accurate response assessment using currently available imaging modalities, including magnetic resonance imaging. In this review, we will discuss the rationale for immunotherapy for GBM, immunotherapeutic modalities currently under clinical evaluation in GBM, and the challenges and recent advances in imaging response assessment in GBM immunotherapy.

Immunosuppressive IDO in Cancer: Mechanisms of Action, Animal Models, and Targeting Strategies

Indoleamine 2, 3-dioxygenase 1 (IDO; IDO1; INDO) is a rate-limiting enzyme that metabolizes the essential amino acid, tryptophan, into downstream kynurenines. Canonically, the metabolic depletion of tryptophan and/or the accumulation of kynurenine is the mechanism that defines how immunosuppressive IDO inhibits immune cell effector functions and/or facilitates T cell death. Non-canonically, IDO also suppresses immunity through non-enzymic effects. Since IDO targeting compounds predominantly aim to inhibit metabolic activity as evidenced across the numerous clinical trials currently evaluating safety/efficacy in patients with cancer, in addition to the recent disappointment of IDO enzyme inhibitor therapy during the phase III ECHO-301 trial, the issue of IDO non-enzyme effects have come to the forefront of mechanistic and therapeutic consideration(s). Here, we review enzyme-dependent and -independent IDO-mediated immunosuppression as it primarily relates to glioblastoma (GBM); the most common and aggressive primary brain tumor in adults. Our group's recent discovery that IDO levels increase in the brain parenchyma during advanced age and regardless of whether GBM is present, highlights an immunosuppressive synergy between aging-increased IDO activity in cells of the central nervous system that reside outside of the brain tumor but collaborate with GBM cell IDO activity inside of the tumor. Because of their potential value for the in vivo study of IDO, we also review current transgenic animal modeling systems while highlighting three new constructs recently created by our group. This work converges on the central premise that maximal immunotherapeutic efficacy in subjects with advanced cancer requires both IDO enzyme- and non-enzyme-neutralization, which is not adequately addressed by available IDO-targeting pharmacologic approaches at this time.

Advanced Age Increases Immunosuppression in the Brain and Decreases Immunotherapeutic Efficacy in Subjects with Glioblastoma

PURPOSE

Wild-type isocitrate dehydrogenase-expressing glioblastoma (GBM) is the most common and aggressive primary brain tumor with a median age at diagnosis of ≥65 years. It accounts for approximately 90% of all GBMs and has a median overall survival (OS) of <15 months. Although immune checkpoint blockade (ICB) therapy has achieved remarkable survival benefits in a variety of aggressive malignancies, similar success has yet to be achieved for GBM among phase III clinical trials to date. Our study aimed to understand the relationship between subject age and immunotherapeutic efficacy as it relates to survival from glioma.

EXPERIMENTAL DESIGN

(i) Clinical data: GBM patient datasets from The Cancer Genome Atlas, Northwestern Medicine Enterprise Data Warehouse, and clinical studies evaluating ICB were stratified by age and compared for OS. (ii) Animal models: young, middle-aged, and older adult wild-type and indoleamine 2,3 dioxygenase (IDO)-knockout syngeneic mice were intracranially engrafted with CT-2A or GL261 glioma cell lines and treated with or without CTLA-4/PD-L1 mAbs, or radiation, anti-PD-1 mAb, and/or a pharmacologic IDO enzyme inhibitor.

RESULTS

Advanced age was associated with decreased GBM patient survival regardless of treatment with ICB. The advanced age-associated increase of brain IDO expression was linked to the suppression of immunotherapeutic efficacy and was not reversed by IDO enzyme inhibitor treatment.

CONCLUSIONS

Immunosuppression increases in the brain during advanced age and inhibits antiglioma immunity in older adults. Going forward, it will be important to fully understand the factors and mechanisms in the elderly brain that contribute to the decreased survival of older patients with GBM during treatment with ICB.

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.