The oncolytic adenovirus Delta-24-RGD in combination with ONC201 induces a potent antitumor response in pediatric high-grade and diffuse midline glioma models

BACKGROUND

Pediatric high-grade gliomas (pHGGs), including diffuse midline gliomas (DMGs), are aggressive pediatric tumors with one of the poorest prognoses. Delta-24-RGD and ONC201 have shown promising efficacy as single agents for these tumors. However, the combination of both agents has not been evaluated.

METHODS

The production of functional viruses was assessed by immunoblotting and replication assays. The antitumor effect was evaluated in a panel of human and murine pHGG and DMG cell lines. RNAseq, the seahorse stress test, mitochondrial DNA content, and γH2A.X immunofluorescence were used to perform mechanistic studies. Mouse models of both diseases were used to assess the efficacy of the combination in vivo. The tumor immune microenvironment was evaluated using flow cytometry, RNAseq and multiplexed immunofluorescence staining.

RESULTS

The Delta-24-RGD/ONC201 combination did not affect the virus replication capability in human pHGG and DMG models in vitro. Cytotoxicity analysis showed that the combination treatment was either synergistic or additive. Mechanistically, the combination treatment increased nuclear DNA damage and maintained the metabolic perturbation and mitochondrial damage caused by each agent alone. Delta-24-RGD/ONC201 cotreatment extended the overall survival of mice implanted with human and murine pHGG and DMG cells, independent of H3 mutation status and location. Finally, combination treatment in murine DMG models revealed a reshaping of the tumor microenvironment to a proinflammatory phenotype.

CONCLUSIONS

The Delta-24-RGD/ONC201 combination improved the efficacy compared to each agent alone in in vitro and in vivo models by potentiating nuclear DNA damage and in turn improving the antitumor (immune) response to each agent alone.

TIM-3 blockade in diffuse intrinsic pontine glioma models promotes tumor regression and antitumor immune memory

Diffuse intrinsic pontine glioma (DIPG) is an aggressive brain stem tumor and the leading cause of pediatric cancer-related death. To date, these tumors remain incurable, underscoring the need for efficacious therapies. In this study, we demonstrate that the immune checkpoint TIM-3 (HAVCR2) is highly expressed in both tumor cells and microenvironmental cells, mainly microglia and macrophages, in DIPG. We show that inhibition of TIM-3 in syngeneic models of DIPG prolongs survival and produces long-term survivors free of disease that harbor immune memory. This antitumor effect is driven by the direct effect of TIM-3 inhibition in tumor cells, the coordinated action of several immune cell populations, and the secretion of chemokines/cytokines that create a proinflammatory tumor microenvironment favoring a potent antitumor immune response. This work uncovers TIM-3 as a bona fide target in DIPG and supports its clinical translation.

Ataxia-telangiectasia mutated ( Atm ) disruption sensitizes spatially-directed H3.3K27M/TP53 diffuse midline gliomas to radiation therapy

Diffuse midline gliomas (DMGs) are lethal brain tumors characterized by p53-inactivating mutations and oncohistone H3.3K27M mutations that rewire the cellular response to genotoxic stress, which presents therapeutic opportunities. We used RCAS/tv-a retroviruses and Cre recombinase to inactivate p53 and induce K27M in the native H3f3a allele in a lineage- and spatially-directed manner, yielding primary mouse DMGs. Genetic or pharmacologic disruption of the DNA damage response kinase Ataxia-telangiectasia mutated (ATM) enhanced the efficacy of focal brain irradiation, extending mouse survival. This finding suggests that targeting ATM will enhance the efficacy of radiation therapy for p53-mutant DMG but not p53-wildtype DMG. We used spatial in situ transcriptomics and an allelic series of primary murine DMG models with different p53 mutations to identify transactivation-independent p53 activity as a key mediator of such radiosensitivity. These studies deeply profile a genetically faithful and versatile model of a lethal brain tumor to identify resistance mechanisms for a therapeutic strategy currently in clinical trials.

Recombinant polio-rhinovirus immunotherapy for recurrent paediatric high-grade glioma: a phase 1b trial

BACKGROUND

Outcomes of recurrent paediatric high-grade glioma are poor, with a median overall survival of less than 6 months. Viral immunotherapy, such as the polio-rhinovirus chimera lerapolturev, is a novel approach for treatment of recurrent paediatric high-grade glioma and has shown promise in adults with recurrent glioblastoma. The poliovirus receptor CD155 is ubiquitously expressed in malignant paediatric brain tumours and is a treatment target in paediatric high-grade glioma. We aimed to assess the safety of lerapolturev when administered as a single dose intracerebrally by convection enhanced delivery in children and young people with recurrent WHO grade 3 or grade 4 glioma, and to assess overall survival in these patients.

METHODS

This phase 1b trial was done at the Duke University Medical Center (Durham, NC, USA). Patients aged 4-21 years with recurrent high-grade malignant glioma (anaplastic astrocytoma, glioblastoma, anaplastic oligoastrocytoma, anaplastic oligodendroglioma, or anaplastic pleomorphic xanthoastrocytoma) or anaplastic ependymoma, atypical teratoid rhabdoid tumour, or medulloblastoma with infusible disease were eligible for this study. A catheter was tunnelled beneath the scalp for a distance of at least 5 cm to aid in prevention of infection. The next day, lerapolturev at a dose of 5 × 107 median tissue culture infectious dose in 3 mL infusate loaded in a syringe was administered via a pump at a rate of 0·5 mL per h as a one-time dose. The infusion time was approximately 6·5 h to compensate for volume of the tubing. The primary endpoint was the proportion of patients with unacceptable toxic effects during the 14-day period after lerapolturev treatment. The study is registered with ClinicalTrials.gov, NCT03043391.

FINDINGS

Between Dec 5, 2017, and May 12, 2021, 12 patients (11 unique patients) were enrolled in the trial. Eight patients were treated with lerapolturev. The median patient age was 16·5 years (IQR 11·0-18·0), five (63%) of eight patients were male and three (38%) were female, and six (75%) of eight patients were White and two (25%) were Black or African American. The median number of previous chemotherapeutic regimens was 3·50 (IQR 1·25-5·00). Six of eight patients had 26 treatment-related adverse events attributable to lerapolturev. There were no irreversible (ie, persisted longer than 2 weeks) treatment-related grade 4 adverse events or deaths. Treatment-related grade 3 adverse events included headaches in two patients and seizure in one patient. Four patients received low-dose bevacizumab on-study for treatment-related peritumoural inflammation or oedema, diagnosed by both clinical symptoms plus fluid-attenuated inversion recovery MRI. The median overall survival was 4·1 months (95% CI 1·2-10·1). One patient remains alive after 22 months.

INTERPRETATION

Convection enhanced delivery of lerapolturev is safe enough in the treatment of recurrent paediatric high-grade glioma to proceed to the next phase of trial.

FUNDING

Solving Kids Cancer, B+ Foundation, Musella Foundation, and National Institutes of Health.

Novel genetically engineered H3.3G34R model reveals cooperation with ATRX loss in upregulation of Hoxa cluster genes and promotion of neuronal lineage

Background

Pediatric high-grade gliomas (pHGGs) are aggressive pediatric CNS tumors and an important subset are characterized by mutations in H3F3A, the gene that encodes Histone H3.3 (H3.3). Substitution of Glycine at position 34 of H3.3 with either Arginine or Valine (H3.3G34R/V), was recently described and characterized in a large cohort of pHGG samples as occurring in 5-20% of pHGGs. Attempts to study the mechanism of H3.3G34R have proven difficult due to the lack of knowledge regarding the cell-of-origin and the requirement for co-occurring mutations for model development. We sought to develop a biologically relevant animal model of pHGG to probe the downstream effects of the H3.3G34R mutation in the context of vital co-occurring mutations.

Methods

We developed a genetically engineered mouse model (GEMM) that incorporates PDGF-A activation, TP53 loss and the H3.3G34R mutation both in the presence and loss of Alpha thalassemia/mental retardation syndrome X-linked (ATRX), which is commonly mutated in H3.3G34 mutant pHGGs.

Results

We demonstrated that ATRX loss significantly increases tumor latency in the absence of H3.3G34R and inhibits ependymal differentiation in the presence of H3.3G34R. Transcriptomic analysis revealed that ATRX loss in the context of H3.3G34R upregulates Hoxa cluster genes. We also found that the H3.3G34R overexpression leads to enrichment of neuronal markers but only in the context of ATRX loss.

Conclusions

This study proposes a mechanism in which ATRX loss is the major contributor to many key transcriptomic changes in H3.3G34R pHGGs.

Accession number

GSE197988.

Immunotherapy approaches for the treatment of diffuse midline gliomas

Diffuse midline gliomas (DMG) are a highly aggressive and universally fatal subgroup of pediatric tumors responsible for the majority of childhood brain tumor deaths. Median overall survival is less than 12 months with a 90% mortality rate at 2 years from diagnosis. Research into the underlying tumor biology and numerous clinical trials have done little to change the invariably poor prognosis. Continued development of novel, efficacious therapeutic options for DMGs remains a critically important area of active investigation. Given that DMGs are not amenable to surgical resection, have only limited response to radiation, and are refractory to traditional chemotherapy, immunotherapy has emerged as a promising alternative treatment modality. This review summarizes the various immunotherapy-based treatments for DMG as well as their specific limitations. We explore the use of cell-based therapies, oncolytic virotherapy or immunovirotherapy, immune checkpoint inhibition, and immunomodulatory vaccination strategies, and highlight the recent clinical success of anti-GD2 CAR-T therapy in diffuse intrinsic pontine glioma (DIPG) patients. Finally, we address the challenges faced in translating preclinical and early phase clinical trial data into effective standardized treatment for DMG patients.

A novel mouse model of diffuse midline glioma initiated in neonatal oligodendrocyte progenitor cells highlights cell-of-origin dependent effects of H3K27M

Diffuse midline glioma (DMG) is a type of lethal brain tumor that develops mainly in children. The majority of DMG harbor the K27M mutation in histone H3. Oligodendrocyte progenitor cells (OPCs) in the brainstem are candidate cells-of-origin for DMG, yet there is no genetically engineered mouse model of DMG initiated in OPCs. Here, we used the RCAS/Tv-a avian retroviral system to generate DMG in Olig2-expressing progenitors and Nestin-expressing progenitors in the neonatal mouse brainstem. PDGF-A or PDGF-B overexpression, along with p53 deletion, resulted in gliomas in both models. Exogenous overexpression of H3.3K27M had a significant effect on tumor latency and tumor cell proliferation when compared with H3.3WT in Nestin+ cells but not in Olig2+ cells. Further, the fraction of H3.3K27M-positive cells was significantly lower in DMGs initiated in Olig2+ cells relative to Nestin+ cells, both in PDGF-A and PDGF-B-driven models, suggesting that the requirement for H3.3K27M is reduced when tumorigenesis is initiated in Olig2+ cells. RNA-sequencing analysis revealed that the differentially expressed genes in H3.3K27M tumors were non-overlapping between Olig2;PDGF-B, Olig2;PDGF-A, and Nestin;PDGF-A models. GSEA analysis of PDGFA tumors confirmed that the transcriptomal effects of H3.3K27M are cell-of-origin dependent with H3.3K27M promoting epithelial-to-mesenchymal transition (EMT) and angiogenesis when Olig2 marks the cell-of-origin and inhibiting EMT and angiogenesis when Nestin marks the cell-of-origin. We did observe some overlap with H3.3K27M promoting negative enrichment of TNFA_Signaling_Via_NFKB in both models. Our study suggests that the tumorigenic effects of H3.3K27M are cell-of-origin dependent, with H3.3K27M being more oncogenic in Nestin+ cells than Olig2+ cells.

Novel cancer gene discovery using a forward genetic screen in RCAS-PDGFB-driven gliomas

BACKGROUND

Malignant gliomas, the most common malignant brain tumors in adults, represent a heterogeneous group of diseases with poor prognosis. Retroviruses can cause permanent genetic alterations that modify genes close to the viral integration site.

METHODS

Here we describe the use of a high-throughput pipeline coupled to the commonly used tissue-specific retroviral RCAS-TVA mouse tumor model system. Utilizing next-generation sequencing, we show that retroviral integration sites can be reproducibly detected in malignant stem cell lines generated from RCAS-PDGFB-driven glioma biopsies.

RESULTS

A large fraction of common integration sites contained genes that have been dysregulated or misexpressed in glioma. Others overlapped with loci identified in previous glioma-related forward genetic screens, but several novel putative cancer-causing genes were also found. Integrating retroviral tagging and clinical data, Ppfibp1 was highlighted as a frequently tagged novel glioma-causing gene. Retroviral integrations into the locus resulted in Ppfibp1 upregulation, and Ppfibp1-tagged cells generated tumors with shorter latency on orthotopic transplantation. In human gliomas, increased PPFIBP1 expression was significantly linked to poor prognosis and PDGF treatment resistance.

CONCLUSIONS

Altogether, the current study has demonstrated a novel approach to tagging glioma genes via forward genetics, validating previous results, and identifying PPFIBP1 as a putative oncogene in gliomagenesis.

Abstract 2001: Delta-24-RGDOX oncolytic adenovirus improves the survival by modulating the immune system in DMG models

Purpose of Study: Diffuse Midline Gliomas (DMG) are aggressive pediatric brain tumors that arise in the brainstem of children, with a peak of incidence between 5-10 years old. The median survival of DMG patients is only 9 months, being the leading cause of pediatric death caused by a brain tumor. On this project we set to characterize the efficacy of Delta-24-RGDOX, an oncolytic adenovirus based on Delta-24-RGD platform, which has demonstrated safety and a therapeutic benefit in different pediatric tumors, armed with the ligand of OX40. The binding of OX40 to OX40L leads to the co-stimulation of CD4 and CD8 cells, generating effector and memory T cells. Therefore, the aim of this project is to improve the antitumor effect of the virus, providing a greater co-stimulation in the tumor.

Experimental Procedures: Murine and human DMG cell lines were used. Viral protein expression was measured by western blot, viral replication was analyzed using a method based on hexon detection and the oncolytic effect by MTS assay. OX40L expression was measured by flow cytometry and qPCR. For in vivo experiments, cells were injected in the pons of mice using a screw-guided system. The adenovirus was administered once into the tumor using the same procedure. Tumor immune populations were analyzed by flow cytometry.

Results: We first confirmed the oncolytic effect of Delta-24-RGDOX in DMG murine and human cell lines in vitro. The virus was able to infect the cells, produce viral proteins, and to cause cell death in a dose-dependent manner. In addition, we observed effective viral replication in human cell lines but not in mouse cells, as previously described. We then measured OX40L expression in infected cells both at protein and mRNA level, observing that at 50 PFU/cell almost 100% of cells expressed the ligand on their membrane. More importantly, the OX40L was functional as it activated CD8 lymphocytes in vitro. Once we confirmed that the viral administration in vivo was safe, we assess the efficacy of the virus in murine DMG models. We observed a significant survival benefit in mice bearing NP53 tumor treated with Delta-24-RGDOX, which lead to 30% of long-term survivors (P=0.003, median OS PBS 25.5 days vs 35.5 days for treated mice). More importantly, we administered the virus in an already stablished tumor model using XFM cells and also obtained a significant improve of survival (P=0.018, median OS PBS 9 days vs 12.5 treated mice). Finally, we analyzed the immune mechanisms underlying the survival benefit, both in the tumor and in spleen. We observed a significant increase of activated immune cells in the tumor microenvironment 7 days after the viral administration. In addition, splenocytes from virus-treated mice were also significantly more activated.

Conclusions: These data show that Delta-24-RGDOX adenovirus expresses a functional OX40L that can modulate the immune response, leading to a significantly improved survival outcome in DMG models.

Citation Format: Virginia Laspidea, Sara Labiano, Sumit Gupta, Hong Jiang, Iker Ausejo-Mauleon, Daniel de la Nava, Marc García-Moure, Javier Marco-Sanz, Reyes Hernández-Osuna, Oren J Becher, Juan Fueyo, Ana Patiño-García, Candelaria Gomez-Manzano, Marta M Alonso. Delta-24-RGDOX oncolytic adenovirus improves the survival by modulating the immune system in DMG models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2001.

Abstract 912: A novel genetically engineered H3.3G34R model reveals cooperation with ATRX loss in upregulation of PRC2 target genes

Pediatric High Grade Glioma (pHGG) is a collection of molecularly distinct subtypes with different prognoses depending on the genetic drivers. One particularly aggressive subtype is H3.3G34R mutant gliomas, which are currently incurable and in need of improved therapies. Interestingly, H3.3G34R mutant gliomas commonly harbor TP53mutations, ATRX mutations, and alterations in PDGFRA signaling. The mechanism by which H3.3G34R promotes gliomagenesis is still somewhat unclear and additional models are needed to dissect its role in tumorigenesis. We used the RCAS Tv-a system to model H3.3G34R pHGG in mice. Nestin expressing progenitors in the frontal cortex of Nestin-Tva (Ntva); p53 fl/fl; ATRX fl/fl or Ntva; p53 fl/fl mice were infected with H3.3G34R or H3.3WT, PDGFA, and Cre, and monitored for signs and symptoms of tumor formation. RNAseq analysis and immunophenotype were used to characterize the tumors. We observed that H3.3G34R did not significantly impact tumor latency independent of ATRX status. ATRX loss significantly increased tumor latency independent of H3.3G34R (P < 0.01) from 90 days to 143 days. H&E analysis revealed that the majority of tumors in all groups were high grade and that mice with ATRX loss were more likely to develop low grade tumors though this trend did not reach significance (P = 0.075). Expression of Ki67, GFAP and Olig2 was present in all groups, as shown by Immunophenotypic analysis. RNAseq analysis of murine tumor tissue revealed that ATRX loss in the context of G34R led to significant differential expression of 113 genes (78 upregulated and 35 downregulated) including upregulation of PRC2 target genes including HoxA genes. These findings highlight the cooperation between ATRX loss and H3.3G34R in gliomagenesis.

Citation Format: Aalaa Abdallah, Herminio J. Cardona, David J. Picketts, Daniel J. Brat, Xiao-Nan Li, Oren J. Becher. A novel genetically engineered H3.3G34R model reveals cooperation with ATRX loss in upregulation of PRC2 target genes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 912.

DIPG-22. Modifying the tumor microenvironment with a TIM-3 monoclonal antibody as a therapeutic strategy for DIPGs

Diffuse Midline Glioma, encompassing Diffuse Intrinsic Pontine Gliomas (DIPGs), are the most aggressive pediatric brain tumors. Their meager survival has not changed despite the combination of radiotherapy with targeted therapies emphasizing the urgent need for effective treatments. TIM-3 (HAVCR2) is a member of the T-cell immunoglobulin and mucin domain protein family. It is expressed on multiple immune cell types, including T cells, NK cells, myeloid populations, and microglia, regulating adaptive and innate immunity. In silico assessment of TIM-3 expression in DIPG datasets showed a robust expression of this gene. Single-cell sequencing analyses of DIPG biopsies uncover TIM-3 expression, especially in microglia. In vivo efficacy studies showed that treatment with AbTIM-3 significantly increased overall survival in two DIPG immunocompetent orthotopic models, led to long-term survivors (50%), and showed immune memory. TIM-3 treatment led to a significant increase in the tumor microenvironment of microglia, granulocytes, NK, and CD8+ cells and higher levels of IFNγ, GrzB and TNFα corresponding with an NK and T-cell activate phenotypes. Interestingly, there was a decrease in the Treg population which causes an increase in the pro-inflammatory CD8/Treg ratio. CD4, CD8 or NK cell depletion leads to a significant but not a total loss of treatment efficacy. CD4+ and CD8+ cells were aumented in treated draining lymph nodes and expressed higher amounts of pro-inflamattory cytokines than control-mice. Population analysis and depletion experiments demonstrated the relevance of NK, CD4, CD8 and myeloid populations in the response to anti-TIM-3 therapy. Interestingly, the depletion of the different immune populations combined or using immunodeficient Rag2 mice, did not completely abrogate the treatment efficacy. These results suggest the concurrence of an additional mechanism of action that together with the immune response leads to a robust anti-DIPG effect. In conclusion, these data demonstrate that TIM-3 is a potential target for the treatment of DIPG.

DIPG-17. CD155 regulates cell growth and immune evasion in diffuse intrinsic pontine glioma

There is an unmet need for more effective treatment strategies for diffuse intrinsic pontine glioma (DIPG), a devastating brain tumour arising in children and young adults. While immunotherapy is emerging as a powerful approach to treatment of other cancers, clinical trials with immune checkpoint inhibitors have failed to show a survival benefit for DIPG patients. In this study, we analysed the expression of immune checkpoint molecules on the surface of human and murine DIPG cells by flow cytometry and identified CD155 and B7-H3 as the most highly expressed checkpoint molecules, with minimal expression of PD-L1, PD-L2, Galectin-9, CEACAM-1, CD86, CD252 and CD137. These findings were confirmed in primary patient samples from pediatric brain tumours, including high-grade gliomas, medulloblastomas and ependymomas. To test whether CD155 inhibition increases susceptibility to CD8+ T cell killing in vitro, we cultured DIPG cells expressing ovalbumin (OVA) with CD8+ T cells from OT-I mice, which express T cell receptors specific for OVA. Addition of CD155 blocking antibodies to these cultures increased expression of T cell activation markers (CD25, CD44 and CD69) as well as T cell-mediated tumour killing, supporting the notion that CD155 can function as an immune checkpoint in DIPG. In addition to its effects on T cells, CD155 also exerted direct effects on tumour cells: treatment with anti-CD155 antibodies led to impaired cell viability, and shRNA-mediated knockdown of CD155 resulted in reduced cell proliferation in vitro. Strikingly, knockdown of CD155 also led to reduced growth of DIPG cells in vivo, and mice transplanted with the CD155-deficient cells had a clear survival benefit compared to mice transplanted with wild type cells. These studies demonstrate that CD155 functions as an immune checkpoint and as a regulator of tumor growth in DIPG, and suggest that targeting CD155 could be a valuable therapeutic strategy for this devastating disease.

A tumor suppressor role for EZH2 in diffuse midline glioma pathogenesis

Pediatric high-grade gliomas, specifically diffuse midline gliomas, account for only 20% of clinical cases but are 100% fatal. A majority of the DMG cases are characterized by the signature K27M mutation in histone H3. The H3K27M mutation opposes the function of enhancer of zeste homolog 2 (EZH2), the methyltransferase enzyme of the polycomb repressor complex 2. However, the role of EZH2 in DMG pathogenesis is unclear. In this study, we demonstrate a tumor suppressor function for EZH2 using Ezh2 loss- and gain-of-function studies in H3WT DMG mouse models. Genetic ablation of Ezh2 increased cell proliferation and tumor grade while expression of an Ezh2 gain-of-function mutation significantly reduced tumor incidence and increased tumor latency. Transcriptomic analysis revealed that Ezh2 deletion upregulates an inflammatory response with upregulation of immunoproteasome genes such as Psmb8, Psmb9, and Psmb10. Ezh2 gain-of-function resulted in enrichment of the oxidative phosphorylation/mitochondrial metabolic pathway namely the isocitrate dehydrogenase Idh1/2/3 genes. Pharmacological inhibition of EZH2 augmented neural progenitor cell proliferation, supporting the tumor suppressive role of EZH2. In vivo 7-day treatment of H3K27M DMG tumor bearing mice with an EZH2 inhibitor, Tazemetostat, did not alter proliferation or significantly impact survival. Together our results suggest that EZH2 has a tumor suppressor function in DMG and warrants caution in clinical translation of EZH2 inhibitors to treat patients with DMG.

Radiosensitizing the Vasculature of Primary Brainstem Gliomas Fails to Improve Tumor Response to Radiation Therapy

PURPOSE

Diffuse intrinsic pontine gliomas (DIPGs) arise in the pons and are the leading cause of death from brain tumors in children. DIPGs are routinely treated with radiation therapy, which temporarily improves neurological symptoms but generally fails to achieve local control. Because numerous clinical trials have not improved survival from DIPG over standard radiation therapy alone, there is a pressing need to evaluate new therapeutic strategies for this devastating disease. Vascular damage caused by radiation therapy can increase the permeability of tumor blood vessels and promote tumor cell death.

METHODS AND MATERIALS

To investigate the impact of endothelial cell death on tumor response to radiation therapy in DIPG, we used dual recombinase (Cre + FlpO) technology to generate primary brainstem gliomas which lack ataxia telangiectasia mutated (Atm) in the vasculature.

RESULTS

Here, we show that Atm-deficient tumor endothelial cells are sensitized to radiation therapy. Furthermore, radiosensitization of the vasculature in primary gliomas triggered an increase in total tumor cell death. Despite the observed increase in cell killing, in mice with autochthonous DIPGs treated with radiation therapy, deletion of Atm specifically in tumor endothelial cells failed to improve survival.

CONCLUSIONS

These results suggest that targeting the tumor cells, rather than endothelial cells, during radiation therapy will be necessary to improve survival among children with DIPG.

Prenatal overexpression of platelet-derived growth factor receptor A results in central nervous system hypomyelination

BACKGROUND

Platelet-derived growth factor (PDGF) signaling, through the ligand PDGF-A and its receptor PDGFRA, is important for the growth and maintenance of oligodendrocyte progenitor cells (OPCs) in the central nervous system (CNS). PDGFRA signaling is downregulated prior to OPC differentiation into mature myelinating oligodendrocytes. By contrast, PDGFRA is often genetically amplified or mutated in many types of gliomas, including diffuse midline glioma (DMG) where OPCs are considered the most likely cell-of-origin. The cellular and molecular changes that occur in OPCs in response to unregulated PDGFRA expression, however, are not known.

METHODS

Here, we created a conditional knock-in (KI) mouse that overexpresses wild type (WT) human PDGFRA (hPDGFRA) in prenatal Olig2-expressing progenitors, and examined in vivo cellular and molecular consequences.

RESULTS

The KI mice exhibited stunted growth, ataxia, and a severe loss of myelination in the brain and spinal cord. When combined with the loss of p53, a tumor suppressor gene whose activity is decreased in DMG, the KI mice failed to develop tumors but still exhibited hypomyelination. RNA-sequencing analysis revealed decreased myelination gene signatures, indicating a defect in oligodendroglial development. Mice overexpressing PDGFRA in prenatal GFAP-expressing progenitors, which give rise to a broader lineage of cells than Olig2-progenitors, also developed myelination defects.

CONCLUSION

Our results suggest that embryonic overexpression of hPDGFRA in Olig2- or GFAP-progenitors is deleterious to OPC development and leads to CNS hypomyelination.

IMMU-01. THE ONCOLYTIC VIRUS DELTA-24-RGD IN COMBINATION WITH AN AGONISTIC CD40 MAB INDUCES A DURABLE AND SYNERGISTIC ANTI-TUMOR IMMUNE EFFECT IN DIPG PRECLINICAL MODELS

With a 2-year survival less than 20%, Diffuse Intrinsic Pontine Glioma (DIPG) is the principal cause of pediatric death. Despite recent advances in the current treatments, the outcome for children with DIPGs remains dismal. Since the approval of T-VEC for melanoma by the FDA, oncolytic adenoviruses have emerged as a promising therapeutic strategy for brain tumors. Thus, our group launched the first world clinical trial phase I with the oncolytic adenovirus Delta-24-RGD (DNX-2401 in the clinic) for newly diagnosed DIPG (NCT03178032), which has shown safety and feasibility. Despite DNX-2401 increases the recruitment of T cells into the tumor, they usually become inactive due to the suppressive tumor microenvironment evidencing the urgent need to improve this strategy focusing on the generation of effective long-term immune responses. Therefore, we decided to combine the Delta-24-RGD with the targeting of the costimulatory molecule CD40 in two unique orthotopic immunocompetent mouse models of DIPG. The activation of the CD40 receptor, which is expressed by antigen presenting cells (APC) such as microglia, macrophages and dendritic cells, is known to increase antigen presentation and enable T-cell priming and activation. Here, we have observed that in addition to Delta-24-RGD anti-tumor effects, the stimulation of CD40 on the tumor APCs results in a remodeling of the tumor immune compartment with a more efficient T-cell tumor infiltration. Of importance, there is an increase of the survival of mice treated with the combination as compared to single treatments or non-treated mice. In addition, the combination therapy induced a complete regression of tumors in 25% of treated mice indicating the development of long-term anti-tumor immunity. We believe that these results provide a translational breakthrough in the treatment of these lethal tumors and open the door for a future innovative clinical trial.

IMMU-08. MICROENVIRONMENT MODULATION BY TIM-3 BLOCKADE IMPROVES THE OUTCOME OF PRECLINICAL DIPG MODELS

Diffuse Midline Gliomas (DMGs), encompassing Diffuse Intrinsic Pontine Gliomas (DIPGs), are the most aggressive pediatric brain tumors. Their meagre survival has not changed despite the combination of radiotherapy with targeted therapies emphasizing the urgent need for effective treatments. Recent research suggested that the DIPG tumor microenvironment is neither highly immunosuppressive nor inflammatory. These analyses showed the lack of infiltrating lymphocytes and the abundance of CD11b+ cells. TIM-3 (HAVCR2) is a member of the T-cell immunoglobulin and mucin domain protein family which is expressed on multiple immune cell types including T cells, T_regs, NK cells, monocytes, dendritic cells and microglia, where it potently regulates not only adaptive immunity but also innate immunity. Therefore, the central hypothesis of this study is that TIM-3 inhibitors could stimulate a cytotoxic immune effect and challenge several components in the tumor microenvironment including microglia, thereby providing a potential effective treatment for DMGs. In silico assessment of TIM-3 expression in a DIPG datasets showed a robust expression of this gene. Moreover, single-cell sequencing analyses of DIPG biopsies uncover its expression on tumor cells, especially in the OPCs compartment. In vivo efficacy studies showed that treatment with anti-TIM-3 antibody significantly increase the overall survival in two DIPG immunocompetent orthotopic animal models (doubling the median), lead to long-term survivors (50%) and showed immune memory. Analyses of CD45⁺ populations in the tumor microenvironment showed a significant increase in B, NK and CD8⁺ cells corresponding with a T-cell activate phenotype in treated-mice. The potential therapeutic involvement of NK cells was certified using nude mice and functional studies. Involvement of microglia in currently being analysed. In summary, these data underscore TIM-3 as a potential target DIPGs and uncover the potential involvement of NKs and other immune mechanisms in the efficacy of anti-TIM-3 therapy.

IMMU-09. MODULATING THE MYELOID POPULATION IN DIPG MODELS WITH ONCOLYTIC VIRUS AND COMPLEMENT INHIBITORS SHOWS THERAPEUTIC EFFICACY

Diffuse intrinsic pontine glioma (DIPG) is the leading cause of brain tumor-related death in children. It is characterised for having a non-inflammatory microenvironment and be immunologically inert. Therefore, strategies aiming to break the microenvironment status-quo in this disease could provide therapeutic benefit. The complement system promotes tumor progression due to the continuous production of anaphylatoxins leading to the infiltration of myeloid cells, which express high levels of complement receptors (C3aR and C5aR1). We have in silico data showing the high expression of C5aR1 in DIPGs. Thus, we wanted to assess first whether complement C5aR1 could constitute an actionable target, and second whether combining C5aR1 inhibitors with oncolytic virus could result in a superior antitumor immune response than either agent alone in DIPG. In this study, we used two different peptide inhibitors of C5aR1, PMX53 and PMX205 combined with the virus Delta-24-ACT (an oncolytic virus armed with 4-1BBL). We performed in vivo studies to evaluate the efficacy of this combination in immunocompetent DIPG models. Our data showed that the combination Delta-24-ACT/PMX53 significantly extended the median survival of the animals when compared with either agent alone, and led to long-term survivors that generated immune memory. The combination treatment modulated the tumor microenvironment promoting an increase in lymphocytes, mainly CD8+ cells presenting an active phenotype, and a reduction in C5aR1 expression in the myeloid compartment. We are currently evaluating in vivo whether PMX205, which has an improved ability to cross the blood brain barrier, leads to better therapeutic response. In summary, the combination of Delta-24-ACT with a C5aR1 inhibitor showed the capacity to shake the DIPG tumor microenvironment and unleashed an antitumor immune response. These data underscore the possibilities to combine oncolytic virus with targets of the tumor microenvironment to improve their therapeutic benefit in DIPGs.

Pharmacologic inhibition of lysine-specific demethylase 1 as a therapeutic and immune-sensitization strategy in pediatric high-grade glioma

BACKGROUND

Diffuse midline gliomas (DMG), including brainstem diffuse intrinsic pontine glioma (DIPG), are incurable pediatric high-grade gliomas (pHGG). Mutations in the H3 histone tail (H3.1/3.3-K27M) are a feature of DIPG, rendering them therapeutically sensitive to small-molecule inhibition of chromatin modifiers. Pharmacological inhibition of lysine-specific demethylase 1 (LSD1) is clinically relevant but has not been carefully investigated in pHGG or DIPG.

METHODS

Patient-derived DIPG cell lines, orthotopic mouse models, and pHGG datasets were used to evaluate effects of LSD1 inhibitors on cytotoxicity and immune gene expression. Immune cell cytotoxicity was assessed in DIPG cells pretreated with LSD1 inhibitors, and informatics platforms were used to determine immune infiltration of pHGG.

RESULTS

Selective cytotoxicity and an immunogenic gene signature were established in DIPG cell lines using clinically relevant LSD1 inhibitors. Pediatric HGG patient sequencing data demonstrated survival benefit of this LSD1-dependent gene signature. Pretreatment of DIPG with these inhibitors increased lysis by natural killer (NK) cells. Catalytic LSD1 inhibitors induced tumor regression and augmented NK cell infusion in vivo to reduce tumor burden. CIBERSORT analysis of patient data confirmed NK infiltration is beneficial to patient survival, while CD8 T cells are negatively prognostic. Catalytic LSD1 inhibitors are nonperturbing to NK cells, while scaffolding LSD1 inhibitors are toxic to NK cells and do not induce the gene signature in DIPG cells.

CONCLUSIONS

LSD1 inhibition using catalytic inhibitors is selectively cytotoxic and promotes an immune gene signature that increases NK cell killing in vitro and in vivo, representing a therapeutic opportunity for pHGG.

KEY POINTS

1. LSD1 inhibition using several clinically relevant compounds is selectively cytotoxic in DIPG and shows in vivo efficacy as a single agent.2. An LSD1-controlled gene signature predicts survival in pHGG patients and is seen in neural tissue from LSD1 inhibitor-treated mice.3. LSD1 inhibition enhances NK cell cytotoxicity against DIPG in vivo and in vitro with correlative genetic biomarkers.

Tumor immune landscape of paediatric high-grade gliomas

Over the last decade, remarkable progress has been made towards elucidating the origin and genomic landscape of childhood high-grade brain tumors. It has become evident that pediatric high-grade gliomas (pHGGs) differ from adult HGGs with respect to multiple defining aspects including: DNA copy number, gene expression profiles, tumor locations within the central nervous system, and genetic alterations such as somatic histone mutations. Despite these advances, clinical trials for children with glioma have historically been based on ineffective adult regimens that fail to take into consideration the fundamental biological differences between the two. Additionally, although our knowledge of the intrinsic cellular mechanisms driving tumor progression has considerably expanded, little is known concerning the dynamic tumor immune microenvironment (TIME) in pHGGs. In this review, we explore the genetic and epigenetic landscape of pHGGs and how this drives the creation of specific tumor sub-groups with meaningful survival outcomes. Further, we provide a comprehensive analysis of the pHGG TIME and discuss emerging therapeutic efforts aimed at exploiting the immune functions of these tumors.

Platelet-derived growth factor beta is a potent inflammatory driver in paediatric high-grade glioma

Paediatric high-grade gliomas (HGGs) account for the most brain tumour-related deaths in children and have a median survival of 12-15 months. One promising avenue of research is the development of novel therapies targeting the properties of non-neoplastic cell-types within the tumour such as tumour associated macrophages (TAMs). TAMs are immunosuppressive and promote tumour malignancy in adult HGG; however, in paediatric medulloblastoma, TAMs exhibit anti-tumour properties. Much is known about TAMs in adult HGG, yet little is known about them in the paediatric setting. This raises the question of whether paediatric HGGs possess a distinct constituency of TAMs because of their unique genetic landscapes. Using human paediatric HGG tissue samples and murine models of paediatric HGG, we demonstrate diffuse midline gliomas possess a greater inflammatory gene expression profile compared to hemispheric paediatric HGGs. We also show despite possessing sparse T-cell infiltration, human paediatric HGGs possess high infiltration of IBA1+ TAMs. CD31, PDGFRβ, and PDGFB all strongly correlate with IBA1+ TAM infiltration. To investigate the TAM population, we used the RCAS/tv-a system to recapitulate paediatric HGG in newborn immunocompetent mice. Tumours are induced in Nestin-positive brain cells by PDGFA or PDGFB overexpression with Cdkn2a or Tp53 co-mutations. Tumours driven by PDGFB have a significantly lower median survival compared to PDGFA-driven tumours and have increased TAM infiltration. NanoString and quantitative PCR analysis indicates PDGFB-driven tumours have a highly inflammatory microenvironment characterized by high chemokine expression. In vitro bone marrow-derived monocyte and microglial cultures demonstrate bone marrow-derived monocytes are most responsible for the production of inflammatory signals in the tumour microenvironment in response to PDGFB stimulation. Lastly, using knockout mice deficient for individual chemokines, we demonstrate the feasibility of reducing TAM infiltration and prolonging survival in both PDGFA and PDGFB-driven tumours. We identify CCL3 as a potential key chemokine in these processes in both humans and mice. Together, these studies provide evidence for the potent inflammatory effects PDGFB has in paediatric HGGs.

Therapeutic targeting of transcriptional elongation in diffuse intrinsic pontine glioma

BACKGROUND

Diffuse intrinsic pontine glioma (DIPG) is associated with transcriptional dysregulation driven by H3K27 mutation. The super elongation complex (SEC) is required for transcriptional elongation through release of RNA polymerase II (Pol II). Inhibition of transcription elongation by SEC disruption can be an effective therapeutic strategy of H3K27M-mutant DIPG. Here, we tested the effect of pharmacological disruption of the SEC in H3K27M-mutant DIPG to advance understanding of the molecular mechanism and as a new therapeutic strategy for DIPG.

METHODS

Short hairpin RNAs (shRNAs) were used to suppress the expression of AF4/FMR2 4 (AFF4), a central SEC component, in H3K27M-mutant DIPG cells. A peptidomimetic lead compound KL-1 was used to disrupt a functional component of SEC. Cell viability assay, colony formation assay, and apoptosis assay were utilized to analyze the effects of KL-1 treatment. RNA- and ChIP-sequencing were used to determine the effects of KL-1 on gene expression and chromatin occupancy. We treated mice bearing H3K27M-mutant DIPG patient-derived xenografts (PDXs) with KL-1. Intracranial tumor growth was monitored by bioluminescence image and therapeutic response was evaluated by animal survival.

RESULTS

Depletion of AFF4 significantly reduced the cell growth of H3K27M-mutant DIPG. KL-1 increased genome-wide Pol II occupancy and suppressed transcription involving multiple cellular processes that promote cell proliferation and differentiation of DIPG. KL-1 treatment suppressed DIPG cell growth, increased apoptosis, and prolonged animal survival with H3K27M-mutant DIPG PDXs.

CONCLUSIONS

SEC disruption by KL-1 increased therapeutic benefit in vitro and in vivo, supporting a potential therapeutic activity of KL-1 in H3K27M-mutant DIPG.

Tumor genotype dictates radiosensitization after Atm deletion in primary brainstem glioma models

Diffuse intrinsic pontine glioma (DIPG) kills more children than any other type of brain tumor. Despite clinical trials testing many chemotherapeutic agents, palliative radiotherapy remains the standard treatment. Here, we utilized Cre/loxP technology to show that deleting Ataxia telangiectasia mutated (Atm) in primary mouse models of DIPG can enhance tumor radiosensitivity. Genetic deletion of Atm improved survival of mice with p53-deficient but not p53 wild-type gliomas after radiotherapy. Similar to patients with DIPG, mice with p53 wild-type tumors had improved survival after radiotherapy independent of Atm deletion. Primary p53 wild-type tumor cell lines induced proapoptotic genes after radiation and repressed the NRF2 target, NAD(P)H quinone dehydrogenase 1 (Nqo1). Tumors lacking p53 and Ink4a/Arf expressed the highest level of Nqo1 and were most resistant to radiation, but deletion of Atm enhanced the radiation response. These results suggest that tumor genotype may determine whether inhibition of ATM during radiotherapy will be an effective clinical approach to treat DIPGs.

DIPG-03. THERAPEUTIC TARGETING OF TRANSCRIPTIONAL ELONGATION IN DIFFUSE INTRINSIC PONTINE GLIOMA

Diffuse intrinsic pontine glioma (DIPG) is highly aggressive brain stem tumor and needed to develop novel therapeutic agents for the treatment. The super elongation complex (SEC) is essential for transcription elongation through release of RNA polymerase II (Pol II). We found that AFF4, a scaffold protein of the SEC, is required for the growth of H3K27M-mutant DIPG cells. In addition, the small molecule SEC inhibitor, KL-1, increased promoter-proximal pausing of Pol II, and reduced transcription elongation, resulting in down-regulate cell cycle, transcription and DNA repair genes. KL-1 treatment decreased cell growth and increased apoptosis in H3K27M-mutant DIPG cells, and prolonged animal survival in our human H3K27M-mutant DIPG xenograft model. Our results demonstrate that the SEC disruption by KL-1 is a novel therapeutic strategy for H3K27M-mutant DIPG.

Magnetic Resonance Imaging-Guided Focused Ultrasound-Based Delivery of Radiolabeled Copper Nanoclusters to Diffuse Intrinsic Pontine Glioma

Diffuse intrinsic pontine glioma (DIPG) is an invasive pediatric brainstem malignancy exclusively in children without effective treatment due to the often-intact blood-brain tumor barrier (BBTB), an impediment to the delivery of therapeutics. Herein, we used focused ultrasound (FUS) to transiently open BBTB and delivered radiolabeled nanoclusters (⁶⁴Cu-CuNCs) to tumors for positron emission tomography (PET) imaging and quantification in a mouse DIPG model. First, we optimized FUS acoustic pressure to open the blood-brain barrier (BBB) for effective delivery of ⁶⁴Cu-CuNCs to pons in wildtype mice. Then the optimized FUS pressure was used to deliver radiolabeled agents in DIPG mouse. Magnetic resonance imaging (MRI)-guided FUS-induced BBTB opening was demonstrated using a low molecular weight, short-lived ⁶⁸Ga-DOTA-ECL1i radiotracer and PET/CT before and after treatment. We then compared the delivery efficiency of ⁶⁴Cu-CuNCs to DIPG tumor with and without FUS treatment and demonstrated the FUS-enhanced delivery and time-dependent diffusion of ⁶⁴Cu-CuNCs within the tumor.

IMMU-14. ONCOLYTIC ADENOVIRUS DELTA-24-RGD ENGINEERED TO EXPRESS 4-1BBL AS A THERAPEUTIC APPROACH FOR DIPG

Despite the advances in our understanding of pediatric diffuse midline gliomas (DMG) they remain the leading cause of pediatric death caused by cancer. Our group has demonstrated that the administration of Delta-24-RGD in DIPG models is safe and therapeutically efficacious. An on-going clinical trial with this virus has proven to be safe for these patients. To further improve the anti-tumoral response of the virus, we armed Delta-24-RGD with 4-1BBL (Delta-24-ACT). 41BB is a costimulatory receptor that promotes the survival and expansion of activated T cells, and the generation and maintenance of memory CD8+ T cells. Here, we showed that in vitro Delta-24-ACT can infect and express 4-1BBL in murine and human DIPG cell lines. Importantly, 4-1BBL expression in DIPG cell lines was able to activate lymphocytes and increase their IFN-gamma production. In addition, Delta-24-ACT triggered immunogenic cell death in DIPG cell lines, as shown by the release of DAMPs such as ATP, HMGB1, Hsp90a and calreticulin translocation. Delta-24-ACT administration in orthotopic DIPG models was well tolerated and safe. We confirmed the expression of the ligand within the tumor. Moreover, using flow cytometry and multispectral immunohistochemistry we observed profound changes in the tumor microenvironment with an increase in the T-cell populations and a remodeling of the myeloid component before and after treatment. Functional studies showed no differences in lymphocytes isolated from mice treated splenocytes but uncovered TILS that showed a significantly increased in the production of IFN-gamma. Delta-24-ACT treatment of mice bearing orthotopic DIPG murine tumors resulted in a significant increase in the median survival and led to free of disease long-term survivors. In summary, Delta-24-ACT is a virus that builds in our clinical experience with Delta-24-RGD in DIPG patients going a step further to boost the antitumor effect of viral therapy while maintaining a safe profile.

IMMU-39. TIM-3 APTAMER IN COMBINATION WITH RADIOTHERAPY RESULTS IN ENHANCED SURVIVAL IN DIPG MODELS

Pediatric diffuse midline gliomas-H3-K27M-mutant are aggressive brain tumours that arise during childhood. Despite new advances in genomic knowledge and the significant number of clinical trial testing new targeted therapies, patient outcome is still insufficient. Cancer immunotherapy is opening new therapeutic options representing a hope for this orphan disease. Aptamers are single-stranded nucleic acid ligands design to achieve a remarkable affinity and specificity to their targets, comparable to antibodies. TIM-3, is a potential immune checkpoint target, typically involved in T-cell exhaustion. Recent studies showed that TIM-3 is also expressed in tumour and glial cells and it plays an important role in brain tumour responses mediated by myeloid cells. In this work, we examined the anti-tumour effect of an aptamer against TIM-3 alone or in combination with radiotherapy. Of importance, we tested TIM-3 aptamer in a murine glioma and DIPG model, where we not observed any toxicity. TIM-3 administration increased overall survival but was unable to control the disease. Of importance, TIM-3 combination with radiotherapy improved the overall survival of treated mice when compared with single treatments leading to 50% of long-term survivors. TIM-3 aptamer administration increase T-infiltration in the tumour site compared to non-treated or library control. Mechanistic studies performed on day 16 showed an increase in CD8 effector cells, a decrease in T-regulators Foxp3+ cells and an increase in IFN-gamma expression suggesting the triggering of an antitumor-immune response. Rechallenge experiments demonstrated immune memory in the long-term responders that led to reject tumour re-implantation, confirming that TIM-3 aptamer treatment in combination with RT elicits specific antitumor immunity in mouse glioma models. These results suggest that immuno-therapies approaches in combination with radiotherapy would be worth exploring in the treatment of deadly DMG-H3K27-Mutant tumours.

Convection-Enhanced Delivery of Enhancer of Zeste Homolog-2 (EZH2) Inhibitor for the Treatment of Diffuse Intrinsic Pontine Glioma

BACKGROUND

Diffuse intrinsic pontine glioma (DIPG) is a fatal childhood brain tumor and the majority of patients die within 2 yr after initial diagnosis. Factors that contribute to the dismal prognosis of these patients include the infiltrative nature and anatomic location in an eloquent area of the brain, which precludes total surgical resection, and the presence of the blood-brain barrier (BBB), which reduces the distribution of systemically administered agents. Convection-enhanced delivery (CED) is a direct infusion technique to deliver therapeutic agents into a target site in the brain and able to deliver a high concentration drug to the infusion site without systemic toxicities.

OBJECTIVE

To assess the efficacy of enhancer of zeste homolog-2 (EZH2) inhibitor by CED against human DIPG xenograft models.

METHODS

The concentration of EZH2 inhibitor (EPZ-6438) in the brainstem tumor was evaluated by liquid chromatography-mass spectrometry (LC/MS). We treated mice-bearing human DIPG xenografts with EPZ-6438 using systemic (intraperitoneal) or CED administration. Intracranial tumor growth was monitored by bioluminescence image, and the therapeutic response was evaluated by animal survival.

RESULTS

LC/MS analysis showed that the concentration of EPZ-6438 in the brainstem tumor was 3.74% of serum concentration after systemic administration. CED of EPZ-6438 suppressed tumor growth and significantly extended animal survival when compared to systemic administration of EPZ-6438 (P = .0475).

CONCLUSION

Our results indicate that CED of an EZH2 inhibitor is a promising strategy to bypass the BBB and to increase the efficacy of an EZH2 inhibitor for the treatment of DIPG.

Abstract IA11: Overcoming immune evasion in pediatric brain tumors

Many immunotherapies act by enhancing T-cell killing of tumor cells. Cytotoxic T cells recognize antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. Our studies suggest that medulloblastomas and high-grade gliomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. Treatment with tumor necrosis factor or lymphotoxin beta receptor agonist rescues expression of Erap1, Tap1, and MHC-I on p53 mutant tumor cells. In vivo, TNF treatment prolongs survival and markedly augments the efficacy of the immune checkpoint inhibitor anti-PD-1. These studies identify p53 as a key regulator of immune evasion in vivo and suggest that TNF could be used to enhance sensitivity of p53-mutant tumors to immunotherapy.

Citation Format: Alexandra Garancher, Hiromichi Suzuki, Svasti Haricharan, Meher B. Masihi, Jessica M. Rusert, Paula S. Norris, Florent Carrette, Megan M. Romero, Sorana A. Morrissy, Patryk Skowron, Florence M.G. Cavalli, Hamza Farooq, Vijay Ramaswamy, Alaide Morcavallo, Jacob J. Henderson, James M. Olson, Yoon-Jae Cho, Xiao-Nan Li, Louis Chesler, Marco A. Marra, Oren J. Becher, Linda M. Bradley, Carl F. Ware, Michael D. Taylor, Robert J. Wechsler-Reya. Overcoming immune evasion in pediatric brain tumors [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr IA11.

Tumor necrosis factor overcomes immune evasion in p53-mutant medulloblastoma

Many immunotherapies act by enhancing the ability of cytotoxic T cells to kill tumor cells. Killing depends on T cell recognition of antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. In this study, we showed that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. In vitro, tumor necrosis factor (TNF) or lymphotoxin-β receptor agonist can rescue expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, low doses of TNF prolong survival and synergize with immune checkpoint inhibitors to promote tumor rejection. These studies identified p53 as a key regulator of immune evasion and suggest that TNF could be used to enhance sensitivity of tumors to immunotherapy.

ABC Transporter Inhibition Plus Dexamethasone Enhances the Efficacy of Convection Enhanced Delivery in H3.3K27M Mutant Diffuse Intrinsic Pontine Glioma

BACKGROUND

An impermeable blood-brain barrier and drug efflux via ATP-binding cassette (ABC) transporters such as p-glycoprotein may contribute to underwhelming efficacy of peripherally delivered agents to treat diffuse intrinsic pontine glioma (DIPG).

OBJECTIVE

To explore the pharmacological augmentation of convection-enhanced delivery (CED) infusate for DIPG.

METHODS

The efficacy of CED dasatinib, a tyrosine kinase inhibitor, in a transgenic H3.3K27M mutant murine model was assessed. mRNA expression of ABCB1 (p-glycoprotein) was analyzed in 14 tumor types in 274 children. In Vitro viability studies of dasatinib, the p-glycoprotein inhibitor, tariquidar, and dexamethasone were performed in 2 H3.3K27M mutant cell lines. Magnetic resonance imaging (MRI) was used to evaluate CED infusate (gadolinium/dasatinib) distribution in animals pretreated with tariquidar and dexamethasone. Histological assessment of apoptosis was performed.

RESULTS

Continuous delivery CED dasatinib improved median overall survival (OS) of animals harboring DIPG in comparison to vehicle (39.5 and 28.5 d, respectively; P = .0139). Mean ABCB1 expression was highest in K27M gliomas. In Vitro, the addition of tariquidar and dexamethasone further enhanced the efficacy of dasatinib (P < .001). In Vivo, MRI demonstrated no difference in infusion dispersion between animals pretreated with dexamethasone plus tariquidar prior to CED dasatinib compared to the CED dasatinib. However, tumor apoptosis was the highest in the pretreatment group (P < .001). Correspondingly, median OS was longer in the pretreatment group (49 d) than the dasatinib alone group (39 d) and no treatment controls (31.5 d, P = .0305).

CONCLUSION

ABC transporter inhibition plus dexamethasone enhances the efficacy of CED dasatinib, resulting in enhanced tumor cellular apoptosis and improved survival in H3.3K27M mutant DIPG.

Epigenetic reprogramming and chromatin accessibility in pediatric diffuse intrinsic pontine gliomas: a neural developmental disease

Diffuse intrinsic pontine glioma (DIPG) is a rare but deadly pediatric brainstem tumor. To date, there is no effective therapy for DIPG. Transcriptomic analyses have revealed DIPGs have a distinct profile from other pediatric high-grade gliomas occurring in the cerebral hemispheres. These unique genomic characteristics coupled with the younger median age group suggest that DIPG has a developmental origin. The most frequent mutation in DIPG is a lysine to methionine (K27M) mutation that occurs on H3F3A and HIST1H3B/C, genes encoding histone variants. The K27M mutation disrupts methylation by polycomb repressive complex 2 on histone H3 at lysine 27, leading to global hypomethylation. Histone 3 lysine 27 trimethylation is an important developmental regulator controlling gene expression. This review discusses the developmental and epigenetic mechanisms driving disease progression in DIPG, as well as the profound therapeutic implications of epigenetic programming.

Combined targeting of PI3K and MEK effector pathways via CED for DIPG therapy

Background

Midline gliomas like diffuse intrinsic pontine glioma (DIPG) carry poor prognosis and lack effective treatment options. Studies have implicated amplifications in the phosphatidylinositol 3-kinase (PI3K) signaling pathway in tumorigenesis; compensatory activation of parallel pathways (eg, mitogen-activated protein kinase [MEK]) may underlie the resistance to PI3K inhibition observed in the clinic.

Methods

Three patient-derived cell lines (SU-DIPG-IV, SU-DIPG-XIII, and SF8628) and a mouse-derived brainstem glioma cell line were treated with PI3K (ZSTK474) and MEK (trametinib) inhibitors, alone or in combination. Synergy was analyzed using Chou-Talalay combination index (CI). These agents were also used alone or in combination in a subcutaneous SU-DIPG-XIII tumor model and in an intracranial genetic mouse model of DIPG, given via convection-enhanced delivery (CED).

Results

We found that these agents abrogate cell proliferation in a dose-dependent manner. Combination treatments were found to be synergistic (CI < 1) across cell lines tested. They also showed significant tumor suppression when given systemically against a subcutaneous DIPG model (alone or in combination) or when given via direct intracranial injection (CED) in a intracranial DIPG mouse model (combination only, median survival 47 vs 35 days post-induction, P = .038). No significant short- or long-term neurotoxicity of ZSTK474 and trametinib delivered via CED was observed.

Conclusions

Our data indicate that ZSTK474 and trametinib combinatorial treatment inhibits malignant growth of DIPG cells in vitro and in vivo, prolonging survival. These results suggest a promising new combinatorial approach using CED for DIPG therapy, which warrants further investigation.

TNF superfamily cytokines overcome immune evasion in medulloblastoma

Many immunotherapies act by enhancing T cell killing of tumor cells. CD8+ cytotoxic T cells recognize antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. Here we show that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. Treatment with tumor necrosis factor (TNF) or lymphotoxin beta receptor agonist (LTβRag) rescues expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, TNF treatment prolongs survival and markedly augments the efficacy of the immune checkpoint inhibitor anti-PD-1. These studies identify p53 as a key regulator of immune evasion in vivo, and suggest that TNF could be used to enhance sensitivity of p53-mutant tumors to immunotherapy.

Delta-24-RGD combined with radiotherapy exerts a potent antitumor effect in diffuse intrinsic pontine glioma and pediatric high grade glioma models

Pediatric high grade gliomas (pHGG), including diffuse intrinsic pontine gliomas (DIPGs), are aggressive tumors with a dismal outcome. Radiotherapy (RT) is part of the standard of care of these tumors; however, radiotherapy only leads to a transient clinical improvement. Delta-24-RGD is a genetically engineered tumor-selective adenovirus that has shown safety and clinical efficacy in adults with recurrent gliomas. In this work, we evaluated the feasibility, safety and therapeutic efficacy of Delta-24-RGD in combination with radiotherapy in pHGGs and DIPGs models. Our results showed that the combination of Delta-24-RGD with radiotherapy was feasible and resulted in a synergistic anti-glioma effect in vitro and in vivo in pHGG and DIPG models. Interestingly, Delta-24-RGD treatment led to the downregulation of relevant DNA damage repair proteins, further sensitizing tumors cells to the effect of radiotherapy. Additionally, Delta-24-RGD/radiotherapy treatment significantly increased the trafficking of immune cells (CD3, CD4+ and CD8+) to the tumor niche compared with single treatments. In summary, administration of the Delta-24-RGD/radiotherapy combination to pHGG and DIPG models is safe and significantly increases the overall survival of mice bearing these tumors. Our data offer a rationale for the combination Delta-24-RGD/radiotherapy as a therapeutic option for children with these tumors. SIGNIFICANCE: Delta-24-RGD/radiotherapy administration is safe and significantly increases the survival of treated mice. These positive data underscore the urge to translate this approach to the clinical treatment of children with pHGG and DIPGs.

CDK4/6 and PDGFRA Signaling as Therapeutic Targets in Diffuse Intrinsic Pontine Glioma

Diffuse intrinsic pontine gliomas (DIPGs) are incurable childhood brain tumors, whereby the standard of care is focal radiation, a treatment that provides temporary relief for most patients. Surprisingly, decades of clinical trials have failed to identify additional therapies that can prolong survival in this disease. In this conference manuscript, we discuss how genetic engineered mouse modeling techniques with the use of a retroviral gene delivery system can help dissect the complex pathophysiology of this disease. With this approach, autochthonous murine DIPG models can be readily induced to (1) help interrogate the function of novel genetic alterations in tumorigenesis, (2) identify candidate cells of origin for this disease, (3) address how region-specific differences in the central nervous system influence the process of gliomagenesis, and (4) evaluate novel therapeutics in an immunocompetent model.

Vamorolone, a dissociative steroidal compound, reduces pro-inflammatory cytokine expression in glioma cells and increases activity and survival in a murine model of cortical tumor

Corticosteroids, such as dexamethasone, are routinely used as palliative care in neuro-oncology for their anti-inflammatory benefits, however many patients experience dose limiting side effects caused by glucocorticoid response element (GRE)-mediated transcription. The purpose of this study was to use a murine model to investigate a new steroid alternative, vamorolone, which promises to reduce side effects through dissociating GRE-mediated transcription and NF-κB -mediated anti-inflammatory actions. To compare vamorolone to dexamethasone in reducing pro-inflammatory signals in vitro, murine glioma cells were treated with dexamethasone, vamorolone or vehicle control. Changes in mRNA expression were assessed using the nanostring inflammatory platform. Furthermore, drug efficacy, post-treatment behavioral activity and side effects were assessed by treating two cohorts of brain tumor bearing mice with dexamethasone, vamorolone, or vehicle control. Our investigation showed that treatment with vamorolone resulted in a reduction of pro-inflammatory signals in tumor cells in vitro similar to treatment with dexamethasone. Treatment with vamorolone resulted in a better safety profile in comparison to dexamethasone treatment. Vamorolone- treated mice showed similar or better activity and survival when compared to dexamethasone-treated mice. Our data indicate vamorolone is a potential steroid-sparing alternative for treating patients with brain tumors.

Pediatric high-grade glioma: biologically and clinically in need of new thinking

High-grade gliomas in children are different from those that arise in adults. Recent collaborative molecular analyses of these rare cancers have revealed previously unappreciated connections among chromatin regulation, developmental signaling, and tumorigenesis. As we begin to unravel the unique developmental origins and distinct biological drivers of this heterogeneous group of tumors, clinical trials need to keep pace. It is important to avoid therapeutic strategies developed purely using data obtained from studies on adult glioblastoma. This approach has resulted in repetitive trials and ineffective treatments being applied to these children, with limited improvement in clinical outcome. The authors of this perspective, comprising biology and clinical expertise in the disease, recently convened to discuss the most effective ways to translate the emerging molecular insights into patient benefit. This article reviews our current understanding of pediatric high-grade glioma and suggests approaches for innovative clinical management.

A phase I study of perifosine with temsirolimus for recurrent pediatric solid tumors

BACKGROUND

The PI3K/AKT/mTOR pathway is aberrantly activated in many pediatric solid tumors including gliomas and medulloblastomas. Preclinical data in a pediatric glioma model demonstrated that the combination of perifosine (AKT inhibitor) and temsirolimus (mTOR inhibitor) is more potent at inhibiting the axis than either agent alone. We conducted this study to assess pharmacokinetics and identify the maximum tolerated dose for the combination.

PROCEDURE

We performed a standard 3+3 phase I, open-label, dose-escalation study in patients with recurrent/refractory pediatric solid tumors. Four dose levels of perifosine (25-75 mg/m² /day) and temsirolimus (25-75 mg/m² IV weekly) were investigated.

RESULTS

Twenty-three patients (median age 8.5 years) with brain tumors (diffuse intrinsic pontine glioma [DIPG] n = 8, high-grade glioma n = 6, medulloblastoma n = 2, ependymoma n = 1), neuroblastoma (n = 4), or rhabdomyosarcoma (n = 2) were treated. The combination was generally well tolerated and no dose-limiting toxicity was encountered. The most common grade 3 or 4 toxicities (at least possibly related) were thrombocytopenia (38.1%), neutropenia (23.8%), lymphopenia (23.8%), and hypercholesterolemia (19.0%). Pharmacokinetic findings for temsirolimus were similar to those observed in the temsirolimus single-agent phase II pediatric study and pharmacokinetic findings for perifosine were similar to those in adults. Stable disease was seen in 9 of 11 subjects with DIPG or high-grade glioma; no partial or complete responses were achieved.

CONCLUSIONS

The combination of these AKT and mTOR inhibitors was safe and feasible in patients with recurrent/refractory pediatric solid tumors.

Abstract 704: Therapeutic potential of Delta24-ACT, a novel immunostimulatory oncolytic adenovirus, for the treatment of pediatric solid tumors: Initial study in pHGG, DIPG and osteosarcoma

Pediatric High Grade Glioma (pHGG), including Diffuse Intrinsic Pontine Glioma (DIPG), and osteosarcoma are amongst the most common and lethal pediatric solid tumors. Despite improvements in surgery, radiotherapy and chemotherapy, the outcome for children affected with these malignancies remains dismal since the current treatments are ineffective and inflict severe side effects. Thus, it is critical to implement novel and radically different therapeutic approaches to improve the survival and quality of life of these children. Delta24 is a modified adenovirus with a tumor dependent replication, and its antitumor effect has been tested in preclinical and clinical models of both high grade glioma and osteosarcoma. The adenovirus destroys the tumor mass as it replicates within the tumor. Additionally, the adenovirus infection triggers the recruitment of lymphocytes to the tumor. However, tumors are often characterized by an immunosuppressive environment that hampers the synergistic effect mediated by the virus and the immune system. Here we describe the generation of a new oncolytic adenovirus Delta24-ACT, based on the Delta24 platform, encoding an immunostimulatory protein. For this project we used relevant immunocompetent pediatric solid models. The toxicity of the virus was assessed by dose-escalation experiments. Delta24-ACT was well tolerated and did not lead to important adverse effects. The antitumor effect of Delta24-ACT was evaluated in immunocompetent mice bearing orthotopic supratentorial glioma, DIPG or local osteosarcoma that spontaneously metastasize to lungs, obtaining significant tumor shrinkage or complete elimination in some of them. Delta24-ACT also promoted an increased T-cell infiltration within the tumor and an enhanced antitumor immune response. Of importance, treatment with Delta24-ACT led to a significant increase in median survival in all the tumor models tested and resulted in long-term survivors free of disease. Furthermore, in our osteosarcoma model local treatment of primary tumors led to the regression of spontaneous lung metastases, which are usually the leading cause of death in most of the patients affected by osteosarcoma. Currently, we are performing re-challenge experiments to evaluate a possible memory effect mediated by the virus and mechanistic studies to elucidate the mechanism of action. Altogether, these results demonstrate the potential therapeutic benefit of Delta24-ACT adenovirus in the treatment of pediatric solid tumors such as pHGG, DIPG and osteosarcoma, representing an important milestone in the fight against cancer.

Citation Format: Marc Garcia Moure, Naiara Martínez-Vélez, Enric Xipell, Marisol González-Huarriz, Ana Patiño, Oren J. Becher, Cande Gómez-Manzano, Juan Fueyo, Marta M. Alonso. Therapeutic potential of Delta24-ACT, a novel immunostimulatory oncolytic adenovirus, for the treatment of pediatric solid tumors: Initial study in pHGG, DIPG and osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 704. doi:10.1158/1538-7445.AM2017-704

A phase I study of single-agent perifosine for recurrent or refractory pediatric CNS and solid tumors

The PI3K/Akt/mTOR signaling pathway is aberrantly activated in various pediatric tumors. We conducted a phase I study of the Akt inhibitor perifosine in patients with recurrent/refractory pediatric CNS and solid tumors. This was a standard 3+3 open-label dose-escalation study to assess pharmacokinetics, describe toxicities, and identify the MTD for single-agent perifosine. Five dose levels were investigated, ranging from 25 to 125 mg/m2/day for 28 days per cycle. Twenty-three patients (median age 10 years, range 4-18 years) with CNS tumors (DIPG [n = 3], high-grade glioma [n = 5], medulloblastoma [n = 2], ependymoma [n = 3]), neuroblastoma (n = 8), Wilms tumor (n = 1), and Ewing sarcoma (n = 1) were treated. Only one DLT occurred (grade 4 hyperuricemia at dose level 4). The most common grade 3 or 4 toxicity at least possibly related to perifosine was neutropenia (8.7%), with the remaining grade 3 or 4 toxicities (fatigue, hyperglycemia, fever, hyperuricemia, and catheter-related infection) occurring in one patient each. Pharmacokinetics was dose-saturable at doses above 50 mg/m2/day with significant inter-patient variability, consistent with findings reported in adult studies. One patient with DIPG (dose level 5) and 4 of 5 patients with high-grade glioma (dose levels 2 and 3) experienced stable disease for two months. Five subjects with neuroblastoma (dose levels 1 through 4) achieved stable disease which was prolonged (≥11 months) in three. No objective responses were noted. In conclusion, the use of perifosine was safe and feasible in patients with recurrent/refractory pediatric CNS and solid tumors. An MTD was not defined by the 5 dose levels investigated. Our RP2D is 50 mg/m2/day.

Histone H3.3K27M Represses p16 to Accelerate Gliomagenesis in a Murine Model of DIPG

Diffuse intrinsic pontine glioma (DIPG) is a highly aggressive pediatric brainstem tumor genetically distinguished from adult GBM by the high prevalence of the K27M mutation in the histone H3 variant H3.3 (H3F3A). This mutation reprograms the H3K27me3 epigenetic landscape of DIPG by inhibiting the H3K27-specific histone methyltransferase EZH2. This globally reduces H3K27me2/3, critical repressive marks responsible for cell fate decisions, and also causes focal gain of H3K27me3 throughout the epigenome. To date, the tumor-driving effects of H3.3K27M remain largely unknown. Here, it is demonstrated that H3.3K27M cooperates with PDGF-B in vivo, enhancing gliomagenesis and reducing survival of p53 wild-type (WT) and knockout murine models of DIPG. H3.3K27M expression drives increased proliferation of tumor-derived murine neurospheres, suggesting that cell-cycle deregulation contributes to increased malignancy in mutant tumors. RNA sequencing on tumor tissue from H3.3K27M-expressing mice indicated global upregulation of PRC2 target genes, and a subset of newly repressed genes enriched in regulators of development and cell proliferation. Strikingly, H3.3K27M induced targeted repression of the p16/ink4a (CDKN2A) locus, a critical regulator of the G₀-G₁ to S-phase transition. Increased levels of H3K27me3 were observed at the p16 promoter; however, pharmacologic reduction of methylation at this promoter did not rescue p16 expression. Although DNA methylation is also present at this promoter, it is not K27M dependent. Intriguingly, inhibition of DNA methylation restores p16 levels and is cytotoxic against murine tumor cells. Importantly, these data reveal that H3.3K27M-mediated p16 repression is an important mechanism underlying the proliferation of H3.3K27M tumor cells, as in vivo cdkn2a knockout eliminates the survival difference between H3.3K27M and H3.3WT tumor-bearing mice.Implications: This study shows that H3.3K27M mutation and PDGF signaling act in concert to accelerate gliomagenesis in a genetic mouse model and identifies repression of p16 tumor suppressor as a target of H3.3K27M, highlighting the G₁-S cell-cycle transition as a promising therapeutic avenue. Mol Cancer Res; 15(9); 1243-54. ©2017 AACR.

A Novel Mouse Model of Diffuse Intrinsic Pontine Glioma Initiated in Pax3-Expressing Cells

Diffuse intrinsic pontine glioma (DIPG) is a rare and incurable brain tumor that arises predominately in children and involves the pons, a structure that along with the midbrain and medulla makes up the brainstem. We have previously developed genetically engineered mouse models of brainstem glioma using the RCAS/Tv-a system by targeting PDGF-B overexpression, p53 loss, and H3.3K27M mutation to Nestin-expressing brainstem progenitor cells of the neonatal mouse. Here we describe a novel mouse model targeting these same genetic alterations to Pax3-expressing cells, which in the neonatal mouse pons consist of a Pax3 +/Nestin +/Sox2 + population lining the fourth ventricle and a Pax3 +/NeuN + parenchymal population. Injection of RCAS-PDGF-B into the brainstem of Pax3-Tv-a mice at postnatal day 3 results in 40% of mice developing asymptomatic low-grade glioma. A mixture of low- and high-grade glioma results from injection of Pax3-Tv-a;p53^fl/fl mice with RCAS-PDGF-B and RCAS-Cre, with or without RCAS-H3.3K27M. These tumors are Ki67 +, Nestin +, Olig2 +, and largely GFAP − and can arise anywhere within the brainstem, including the classic DIPG location of the ventral pons. Expression of the H3.3K27M mutation reduces overall H3K27me3 as compared with tumors without the mutation, similar to what has been previously shown in human and mouse tumors. Thus, we have generated a novel genetically engineered mouse model of DIPG, which faithfully recapitulates the human disease and represents a novel platform with which to study the biology and treatment of this deadly disease.

Diffuse Intrinsic Pontine Glioma: Time for Cautious Optimism

Diffuse intrinsic pontine glioma is a lethal brain cancer that arises in the pons of children. The median survival for children with diffuse intrinsic pontine glioma is less than 1 year from diagnosis, and no improvement in survival has been realized in more than 30 years. Currently, the standard of care for diffuse intrinsic pontine glioma is focal radiation therapy, which provides only temporary relief. Recent genomic analysis of tumors from biopsies and autopsies, have resulted in the discovery of K27M H3.3/H3.1 mutations in 80% and ACVR1 mutations in 25% of diffuse intrinsic pontine gliomas, providing renewed hope for future success in identifying effective therapies. In addition, as stereotactic tumor biopsies at diagnosis at specialized centers have been demonstrated to be safe, biopsies have now been incorporated into several prospective clinical trials. This article summarizes the epidemiology, clinical presentation, diagnosis, prognosis, molecular genetics, current treatment, and future therapeutic directions for diffuse intrinsic pontine glioma.