Systemic Bevacizumab for Recurrent Respiratory Papillomatosis: A Single Institution's Experience

OBJECTIVES

Medical therapies to limit disease recurrence are critically needed for recurrent respiratory papillomatosis (RRP). Systemic bevacizumab is emerging as an exciting adjuvant therapy toward this end, but uptake has been poor due to the lack of experience and awareness of best prescribing practices. The objective of this study was to describe a single tertiary care academic medical center's experience using systemic bevacizumab for the treatment of RRP.

METHODS

A retrospective review was performed to identify patients with RRP on systemic bevacizumab. Demographic and clinical characteristics, findings on imaging reports, and disease response at all anatomic subsites involved in papilloma were documented.

RESULTS

Of the 17 RRP patients on systemic bevacizumab, 9 (52.9%) were male, and 12 (70.6%) were diagnosed with juvenile-onset RRP. The total lifetime number of surgeries was high, with more than half (n = 9; 52.9%) undergoing more than 50 surgeries. Following induction of systemic bevacizumab, a significant reduction in patients with laryngeal (n = 15; 94.1% vs. n = 7; 41.2%, p < 0.001) and tracheal (n = 11; 64.7% vs. n = 5; 29.4%, p = 0.04) RRP was noted. Surgical frequency was significantly lower following systemic bevacizumab (2.5 vs. 0.5 surgeries per year; p < 0.001). The most common complications were new-onset hypertension (n = 4; 23.5%) and proteinuria (n = 5; 29.4%).

CONCLUSION

Systemic bevacizumab is effective in reducing the number of surgeries needed for RRP while exhibiting a relatively safe complication profile. Papillomas in the larynx and trachea are most responsive to systemic bevacizumab, while pulmonary RRP is most likely to exhibit a partial-to-stable response.

LEVEL OF EVIDENCE

4 Laryngoscope, 2024.

PI3K/mTOR is a therapeutically targetable genetic dependency in diffuse intrinsic pontine glioma

Diffuse midline glioma (DMG), including tumors diagnosed in the brainstem (diffuse intrinsic pontine glioma; DIPG), are uniformly fatal brain tumors that lack effective treatment. Analysis of CRISPR/Cas9 loss-of-function gene deletion screens identified PIK3CA and MTOR as targetable molecular dependencies across patient derived models of DIPG, highlighting the therapeutic potential of the blood-brain barrier-penetrant PI3K/Akt/mTOR inhibitor, paxalisib. At the human-equivalent maximum tolerated dose, mice treated with paxalisib experienced systemic glucose feedback and increased insulin levels commensurate with patients using PI3K inhibitors. To exploit genetic dependence and overcome resistance while maintaining compliance and therapeutic benefit, we combined paxalisib with the antihyperglycemic drug metformin. Metformin restored glucose homeostasis and decreased phosphorylation of the insulin receptor in vivo, a common mechanism of PI3K-inhibitor resistance, extending survival of orthotopic models. DIPG models treated with paxalisib increased calcium-activated PKC signaling. The brain penetrant PKC inhibitor enzastaurin, in combination with paxalisib, synergistically extended the survival of multiple orthotopic patient-derived and immunocompetent syngeneic allograft models; benefits potentiated in combination with metformin and standard-of-care radiotherapy. Therapeutic adaptation was assessed using spatial transcriptomics and ATAC-Seq, identifying changes in myelination and tumor immune microenvironment crosstalk. Collectively, this study has identified what we believe to be a clinically relevant DIPG therapeutic combinational strategy.

DNA methylation landscapes in DIPG reveal methylome variability that can be modified pharmacologically

Background

Diffuse intrinsic pontine glioma (DIPG) is a uniformly lethal brainstem tumor of childhood, driven by histone H3 K27M mutation and resultant epigenetic dysregulation. Epigenomic analyses of DIPG have shown global loss of repressive chromatin marks accompanied by DNA hypomethylation. However, studies providing a static view of the epigenome do not adequately capture the regulatory underpinnings of DIPG cellular heterogeneity and plasticity.

Methods

To address this, we performed whole-genome bisulfite sequencing on a large panel of primary DIPG specimens and applied a novel framework for analysis of DNA methylation variability, permitting the derivation of comprehensive genome-wide DNA methylation potential energy landscapes that capture intrinsic epigenetic variation.

Results

We show that DIPG has a markedly disordered epigenome with increasingly stochastic DNA methylation at genes regulating pluripotency and developmental identity, potentially enabling cells to sample diverse transcriptional programs and differentiation states. The DIPG epigenetic landscape was responsive to treatment with the hypomethylating agent decitabine, which produced genome-wide demethylation and reduced the stochasticity of DNA methylation at active enhancers and bivalent promoters. Decitabine treatment elicited changes in gene expression, including upregulation of immune signaling such as the interferon response, STING, and MHC class I expression, and sensitized cells to the effects of histone deacetylase inhibition.

Conclusions

This study provides a resource for understanding the epigenetic instability that underlies DIPG heterogeneity. It suggests the application of epigenetic therapies to constrain the range of epigenetic states available to DIPG cells, as well as the use of decitabine in priming for immune-based therapies.

Epigenetic upregulation of Schlafen11 renders WNT- and SHH-activated medulloblastomas sensitive to cisplatin

BACKGROUND

Intensive chemotherapeutic regimens with craniospinal irradiation have greatly improved survival in medulloblastoma patients. However, survival markedly differs among molecular subgroups and their biomarkers are unknown. Through unbiased screening, we found Schlafen family member 11 (SLFN11), which is known to improve response to DNA damaging agents in various cancers, to be one of the top prognostic markers in medulloblastomas. Hence, we explored the expression and functions of SLFN11 in medulloblastoma.

METHODS

SLFN11 expression for each subgroup was assessed by immunohistochemistry in 98 medulloblastoma patient samples and by analyzing transcriptomic databases. We genetically or epigenetically modulated SLFN11 expression in medulloblastoma cell lines and determined cytotoxic response to the DNA damaging agents cisplatin and topoisomerase I inhibitor SN-38 in vitro and in vivo.

RESULTS

High SLFN11 expressing cases exhibited significantly longer survival than low expressing cases. SLFN11 was highly expressed in the WNT-activated subgroup and in a proportion of the SHH-activated subgroup. While WNT activation was not a direct cause of the high expression of SLFN11, a specific hypomethylation locus on the SLFN11 promoter was significantly correlated with high SLFN11 expression. Overexpression or deletion of SLFN11 made medulloblastoma cells sensitive and resistant to cisplatin and SN-38, respectively. Pharmacological upregulation of SLFN11 by the brain-penetrant histone deacetylase-inhibitor RG2833 markedly increased sensitivity to cisplatin and SN-38 in SLFN11-negative medulloblastoma cells. Intracranial xenograft studies also showed marked sensitivity to cisplatin by SLFN11-overexpression in medulloblastoma cells.

CONCLUSIONS

High SLFN11 expression is one factor which renders favorable outcomes in WNT-activated and a subset of SHH-activated medulloblastoma possibly through enhancing response to cisplatin.

Generation and multi-dimensional profiling of a childhood cancer cell line atlas defines new therapeutic opportunities

Pediatric solid and central nervous system tumors are the leading cause of cancer-related death among children. Identifying new targeted therapies necessitates the use of pediatric cancer models that faithfully recapitulate the patient's disease. However, the generation and characterization of pediatric cancer models has significantly lagged behind adult cancers, underscoring the urgent need to develop pediatric-focused cell line resources. Herein, we establish a single-site collection of 261 cell lines, including 224 pediatric cell lines representing 18 distinct extracranial and brain childhood tumor types. We subjected 182 cell lines to multi-omics analyses (DNA sequencing, RNA sequencing, DNA methylation), and in parallel performed pharmacological and genetic CRISPR-Cas9 loss-of-function screens to identify pediatric-specific treatment opportunities and biomarkers. Our work provides insight into specific pathway vulnerabilities in molecularly defined pediatric tumor classes and uncovers biomarker-linked therapeutic opportunities of clinical relevance. Cell line data and resources are provided in an open access portal.

Abstract 6720: Using proteasome inhibition to hyperactivate the integrated stress response in aggressive pediatric brain tumors

H3K27-altered diffuse midline glioma (DMG) and atypical teratoid/rhabdoid tumor (AT/RT) are aggressive pediatric brain tumors with dismal outcomes. DMG and AT/RT have increased baseline activation of the integrated stress response (ISR), an evolutionarily conserved system that enables cells to tolerate various forms of stress. This activation is manifested by elevated levels of ATF4 and NRF2 in DMG and AT/RT, respectively. Transient or low-level expression of these transcription factors protect cells from stress, while sustained, high-level activation leads to cell death. Ixazomib is an orally bioavailable proteasome inhibitor that causes endoplasmic reticulum stress, which is a major upstream activator of the ISR. We hypothesized that the high baseline level of ATF4 in DMG and NRF2 in AT/RT would make these tumors susceptible to ISR activators such as ixazomib. After determining the IC50 of ixazomib, DMG and AT/RT cell lines (JHH-DIPG1, JHH-DIPG16A, and CHLA06) were treated with increasing concentrations of the drug. Cleaved caspase 3 immunofluorescence showed a significant increase of apoptosis in all cell lines (JHH-DIPG16A and CHLA06 p&lt;0.0001, JHH-DIPG1 p=0.0008 by ANOVA). Additionally, western blots for cleaved PARP and phospho-Rb expression detected induction of apoptosis and suppression of cell proliferation in JHH-DIPG1 treated with ixazomib. We are currently testing ixazomib in combination with idarubicin, a brain-penetrant anthracycline, and gemcitabine, a brain-penetrant nucleoside analog. Synergy testing in CHLA06 showed an overall zero-interaction-potency (ZIP) synergy score of 23.464 with inhibitory concentrations of ixazomib and idarubicin in the low nanomolar range (scores of 10 or above indicate synergy). Similarly, ixazomib and gemcitabine synergized to suppress CHLA06 growth and induce apoptosis (67% Annexin V+ cells in combination compared to 14 and 24% Annexin V+ with single agent treatment). In the future, we will determine if ixazomib selectively kills DMG and AT/RT while sparing normal cells in iPSC brain organoids. We will also test ixazomib singly and in combination with traditional chemotherapy in orthotopic xenografts of AT/RT. These results suggest that ixazomib, especially in combination with idarubicin, gemcitabine or other ISR activators, has the potential to serve as an effective therapy for aggressive pediatric brain tumors.

Citation Format: Orlandi Valencia Novak, Tyler Findlay, Jeffrey Rubens, Charles Eberhart, Eric H. Raabe. Using proteasome inhibition to hyperactivate the integrated stress response in aggressive pediatric brain tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6720.

A neurodevelopmental epigenetic programme mediated by SMARCD3-DAB1-Reelin signalling is hijacked to promote medulloblastoma metastasis

How abnormal neurodevelopment relates to the tumour aggressiveness of medulloblastoma (MB), the most common type of embryonal tumour, remains elusive. Here we uncover a neurodevelopmental epigenomic programme that is hijacked to induce MB metastatic dissemination. Unsupervised analyses of integrated publicly available datasets with our newly generated data reveal that SMARCD3 (also known as BAF60C) regulates Disabled 1 (DAB1)-mediated Reelin signalling in Purkinje cell migration and MB metastasis by orchestrating cis-regulatory elements at the DAB1 locus. We further identify that a core set of transcription factors, enhancer of zeste homologue 2 (EZH2) and nuclear factor I X (NFIX), coordinates with the cis-regulatory elements at the SMARCD3 locus to form a chromatin hub to control SMARCD3 expression in the developing cerebellum and in metastatic MB. Increased SMARCD3 expression activates Reelin-DAB1-mediated Src kinase signalling, which results in a MB response to Src inhibition. These data deepen our understanding of how neurodevelopmental programming influences disease progression and provide a potential therapeutic option for patients with MB.

Recent progress and novel approaches to treating atypical teratoid rhabdoid tumor

Atypical teratoid rhabdoid tumors (AT/RT) are malignant central nervous system (CNS) tumors that occur mostly in young children and have historically carried a very poor prognosis. While recent clinical trial results show that this tumor is curable, outcomes are still poor compared to other central nervous system embryonal tumors. We here review prior AT/RT clinical trials and highlight promising pre-clinical results that may inform novel clinical approaches to this aggressive cancer.

Abstract 5227: Combination therapy activating the integrated stress response synergistically suppresses proliferation and induces apoptosis in diffuse intrinsic pontine glioma

Diffuse intrinsic pontine glioma (DIPG) is a lethal pediatric brain tumor. Radiation, the standard of care, extends life for 6-18 months but has never cured a patient. Therefore, the discovery of novel treatments is imperative. DIPG tumors have elevated baseline activation of the integrated stress response (ISR), an evolutionarily conserved system that allows cells to tolerate various forms of stress. Increased expression of activating transcription factor 4 (ATF4) indicates activation of the ISR. Low levels of ATF4 protect cells from stress, while sustained high-levels of ATF4 result in cell death. Because DIPG has a high baseline level of ATF4, we hypothesized that the ISR activators Sal003 and ONC201 would synergize and kill DIPG cells. After determining the IC25 of Sal003 and ONC201, we treated three patient-derived cell lines: JHH-DIPG1, SF-7761, and JHH-DIPG16A with low micromolar doses. To measure proliferation, we performed immunofluorescence staining for bromodeoxyuridine (BrdU) incorporation. The combination treatment significantly reduced BrdU incorporation (JHH-DIPG1 p=0.0026, SF-7761 p=0.0002, JHH-DIPG16A p&lt;0.0001 by ANOVA and Dunnett’s multiple comparisons test compared to DMSO control). In all three cell lines, the combination also significantly reduced proliferation compared to monotherapy. We next measured apoptosis by staining for cleaved caspase-3 (CC3) and performing western blots for cleaved PARP. The combination of Sal003 and ONC201 significantly increased apoptosis as measured by CC3 immunofluorescence in comparison to DMSO (JHH-DIPG1 p&lt;0.0001, SF-7761 p&lt;0.0001, JHH-DIPG16A p&lt;0.0001 by ANOVA and Dunnett’s multiple comparisons test compared to DMSO control). In all cell lines, combination therapy significantly increased CC3 positivity compared to single treatment. Western blots for cleaved PARP expression detected induction of apoptosis in all three cell lines treated with the combination over DMSO and monotherapy treated cells. In JHH-DIPG1 and SF-7761, the combination increased ATF4 expression. Since Sal003 is not yet available for clinical testing in humans, we will next investigate treatment with ONC201 and the well-tolerated ATF4 inducer fenretinide. The combination of ONC201 with another ISR activating agent has the potential to serve as a therapy for DIPG.

Citation Format: Orlandi V. Novak, Antje Arnold, Charles Eberhart, Eric H. Raabe. Combination therapy activating the integrated stress response synergistically suppresses proliferation and induces apoptosis in diffuse intrinsic pontine glioma [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 5227.

Dual mTORC1/2 inhibition compromises cell defenses against exogenous stress potentiating Obatoclax-induced cytotoxicity in atypical teratoid/rhabdoid tumors

Atypical teratoid/rhabdoid tumors (AT/RT) are the most common malignant brain tumors of infancy and have a dismal 4-year event-free survival (EFS) of 37%. We have previously shown that mTOR activation contributes to AT/RT’s aggressive growth and poor survival. Targeting the mTOR pathway with the dual mTORC1/2 inhibitor TAK-228 slows tumor growth and extends survival in mice bearing orthotopic xenografts. However, responses are primarily cytostatic with limited durability. The aim of this study is to understand the impact of mTOR inhibitors on AT/RT signaling pathways and design a rational combination therapy to drive a more durable response to this promising therapy. We performed RNASeq, gene expression studies, and protein analyses to identify pathways disrupted by TAK-228. We find that TAK-228 decreases the expression of the transcription factor NRF2 and compromises AT/RT cellular defenses against oxidative stress and apoptosis. The BH3 mimetic, Obatoclax, is a potent inducer of oxidative stress and apoptosis in AT/RT. These complementary mechanisms of action drive extensive synergies between TAK-228 and Obatoclax slowing AT/RT cell growth and inducing apoptosis and cell death. Combination therapy activates the integrative stress response as determined by increased expression of phosphorylated EIF2α, ATF4, and CHOP, and disrupts the protective NOXA.MCL-1.BIM axis, forcing stressed cells to undergo apoptosis. Combination therapy is well tolerated in mice bearing orthotopic xenografts of AT/RT, slows tumor growth, and extends median overall survival. This novel combination therapy could be added to standard upfront therapies or used as a salvage therapy for relapsed disease to improve outcomes in AT/RT.

Unbiased Proteomic and Phosphoproteomic Analysis Identifies Response Signatures and Novel Susceptibilities After Combined MEK and mTOR Inhibition in BRAFV600E Mutant Glioma

The mitogen-activated protein kinase pathway is one of the most frequently altered pathways in cancer. It is involved in the control of cell proliferation, invasion, and metabolism, and can cause resistance to therapy. A number of aggressive malignancies, including melanoma, colon cancer, and glioma, are driven by a constitutively activating missense mutation (V600E) in the v-Raf murine sarcoma viral oncogene homolog B (BRAF) component of the pathway. Mitogen-activated protein kinase kinase (MEK) inhibition is initially effective in targeting these cancers, but reflexive activation of mammalian target of rapamycin (mTOR) signaling contributes to frequent therapy resistance. We have previously demonstrated that combination treatment with the MEK inhibitor trametinib and the dual mammalian target of rapamycin complex 1/2 inhibitor TAK228 improves survival and decreases vascularization in a BRAFV600E mutant glioma model. To elucidate the mechanism of action of this combination therapy and understand the ensuing tumor response, we performed comprehensive unbiased proteomic and phosphoproteomic characterization of BRAFV600E mutant glioma xenografts after short-course treatment with trametinib and TAK228. We identified 13,313 proteins and 30,928 localized phosphosites, of which 12,526 proteins and 17,444 phosphosites were quantified across all samples (data available via ProteomeXchange; identifier PXD022329). We identified distinct response signatures for each monotherapy and combination therapy and validated that combination treatment inhibited activation of the mitogen-activated protein kinase and mTOR pathways. Combination therapy also increased apoptotic signaling, suppressed angiogenesis signaling, and broadly suppressed the activity of the cyclin-dependent kinases. In response to combination therapy, both epidermal growth factor receptor and class 1 histone deacetylase proteins were activated. This study reports a detailed (phospho)proteomic analysis of the response of BRAFV600E mutant glioma to combined MEK and mTOR pathway inhibition and identifies new targets for the development of rational combination therapies for BRAF-driven tumors.

OTME-9. Comprehensive Metabolic Profiling Of high MYC Medulloblastoma Reveals Key Differences Between In Vitro And In Vivo Glucose And Glutamine Usage

Reprograming of cellular metabolism is a hallmark of cancer. The metabolic alterations in cancer cells is not only defined by series of genetic mutations, but also reflecting the crosstalk between cancer cells and other factors in the microenvironment. Altering metabolism allows cancer cells to overcome unfavorable conditions, to proliferate and invade. Medulloblastoma is the most common malignant brain tumor of children. Genomic amplification of MYC is a hallmark of a subset of poor-prognosis medulloblastoma. However, the metabolism of high MYC amplified medulloblastoma subgroup remains underexplored. We performed comprehensive metabolic studies of human MYC-amplified medulloblastoma by comparing the metabolic profiles of tumor cells in different environments – in vitro, in flank xenografts and in orthotopic xenografts. Principal component analysis showed that the metabolic profiles of brain and flank high-MYC medulloblastoma tumors clustered closely together and separated away from normal brain and the high-MYC medulloblastoma cells in culture. Compared to normal brain, MYC-amplified medulloblastoma orthotopic xenograft tumors showed upregulation of nucleotide, hexosamine biosynthetic pathway (HBP), TCA cycle, and amino acid and glutathione pathways. There was significantly higher glucose up taking and usage in orthotopic xenograft tumor compared to flank xenograft and cells in culture. The data demonstrated that glucose was the main carbon source for the glutamate, glutamine and glutathione synthesis through the TCA cycle. The glutaminase ii pathway was the main pathway utilizing glutamine in MYC-amplified medulloblastoma in vivo. Glutathione was found as the most abundant upregulated metabolite. Glutamine derived glutathione was mainly synthesized through glutamine transaminase K (GTK) enzyme in vivo. In conclusion, we demonstrated that high MYC medulloblastoma adapt to different environments by altering its metabolic pathways despite carrying the same genetic mutations. Glutamine antagonists may have therapeutic applications in human patients.

Abstract 2321: Comprehensive metabolic profiling of high MYC medulloblastoma revealed key differences between in vitro and in vivo in glucose and glutamine usage

Medulloblastoma is the most common malignant brain tumor of children. Genomic amplification of MYC is a hallmark of a subset of poor-prognosis medulloblastoma. In other cancers, high-level expression of MYC increase glutamine transport and fuels the TCA cycle, supporting cell growth and proliferation. However, the metabolism of high MYC amplified medulloblastoma subgroup remains underexplored. We performed comprehensive metabolic studies of xenografts of human MYC-amplified medulloblastoma tumors. We found that glucose was the main carbon source for TCA cycle. We also found that glutaminase ii pathway was the main pathway utilizing glutamine. In orthotopic xenografts, glutathione was the most abundant upregulated metabolite in tumor compared to normal brain. Glutamine derived glutathione was synthesized through glutamine transaminase K (GTK) enzyme in vivo, and it was significantly inhibited by glutamine analog 6-diazo-5-oxo-l-norleucine (DON). We engineered a DON prodrug with higher lipophilicity (JHU395) and found that JHU395 suppressed medulloblastoma growth in vitro and induced apoptosis. A single dose of JHU395 induced apoptosis in orthotopic D425 MED tumors but not in normal cerebellum or cortex. Twice weekly 15mg/kg dosing of JHU395 significantly extended the survival of the mice with D425 MED orthotopic xenografts (p&lt;0.001 by log-rank test comparing treated vs vehicle control). Glutamine antagonists exploit MYC-amplified medulloblastoma's reliance on glutamine metabolism and may have therapeutic applications in human patients.

Citation Format: Khoa Dang Pham, Bradley Poore, Allison Hanaford, Micah J. Maxwell, Heather Sweeney, Akhila Parthasarathy, Jesse Alt, Rana Rais, Barbara S. Slusher, Charles G. Eberhart, Eric H. Raabe. Comprehensive metabolic profiling of high MYC medulloblastoma revealed key differences between in vitro and in vivo in glucose and glutamine usage [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2321.

Abstract 324: Unbiased proteomic and phosphoproteomic analysis identifies response signatures and novel susceptibilities after combined MEK and mTOR inhibition in BRAFV600E mutant glioma

The mitogen-activated protein kinase (MAPK) pathway is one of the most frequently altered pathways in cancer. It is involved in the control of cell proliferation, invasion, metabolism, and can cause resistance to therapy. A number of aggressive malignancies including melanoma, colon cancer, and glioma, are driven by a constitutively activating missense mutation (V600E) in the BRAF component of the pathway. MEK inhibition is initially effective in targeting these cancers, but reflexive activation of mTOR signaling contributes to frequent therapy resistance. We have previously demonstrated that combination treatment with the MEK inhibitor trametinib and the dual mTORC1/2 inhibitor TAK228 improves survival and decreases vascularization in a BRAFV600E mutant glioma model. To elucidate the mechanism of action of this combination therapy and understand the ensuing tumor response, we performed comprehensive unbiased proteomic and phosphoproteomic characterization of BRAFV600E mutant glioma xenografts after short-course treatment with trametinib and TAK228. We identified 13,313 proteins and 30,928 localized phosphosites, of which 12,526 proteins and 17,444 phosphosites were quantified across all samples (data available via ProteomeXchange; identifier PXD022329). We identified distinct response signatures for each monotherapy and combination therapy and validated that combination treatment inhibited activation of the MAPK and mTOR pathways. Combination therapy also increased apoptotic signaling, suppressed angiogenesis signaling, and broadly suppressed the activity of the cyclin-dependent kinases. In addition, combination therapy had a profound impact on cancer cell metabolic pathways, increasing the expression of proteins (and their activating phosphorylations) involved in glycolysis, the tricarboxylic acid (TCA) cycle, nucleotide biosynthesis, and DNA replication. In response to combination therapy, both receptor tyrosine kinase and histone deacetylase proteins were activated. This study reports a detailed (phospho)proteomic analysis of the response of BRAFV600E mutant glioma to combined MEK and mTOR pathway inhibition and identifies new targets for the development of rational combination therapies for aggressive BRAF-driven tumors.

Citation Format: Micah J. Maxwell, Antje Arnold, Heather Sweeney, Ljun Chen, Tung-Shing M. Lih, Michael Schnaubelt, Charles G. Eberhart, Jeffrey A. Rubens, Hui Zhang, David J. Clark, Eric H. Raabe. Unbiased proteomic and phosphoproteomic analysis identifies response signatures and novel susceptibilities after combined MEK and mTOR inhibition in BRAFV600E mutant glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 324.

ATRT-05. REPURPOSED ANTI-MALARIAL QUINACRINE ACTIVATES P53 AND INHIBITS ATRT TUMORIGENICITY

Atypical teratoid rhabdoid tumors (ATRTs) are fatal pediatric brain tumors that warrant improved therapies urgently. ATRTs are characterized by loss of INI1, a subunit of the SWI/SNF chromatin-remodeling complex. ATRTs grow aggressively despite majority of primary tumors expressing p53, suggesting inactivation of this tumor suppressor pathway. Reactivation of p53 could be a potential therapeutic strategy for inhibiting ATRT growth. Our laboratory specializes in researching mechanisms contributing to ATRT pathogenesis and potential therapies. In line with this, we investigated an anti-malarial drug called quinacrine that has been safely used in children for decades and can induce p53 in renal cell carcinoma. We used 5 patient-derived ATRT cell lines (BT-37, BT-12, CHLA-06, CHLA-266, CHLA-05) for our studies. We show that ATRT cell lines treated with quinacrine for 6 hours show increased expression of p53, suggesting its activation. Treatment of ATRT cell lines with increasing doses of quinacrine for 24 hours showed dose-dependent decrease in cell growth and proliferation (assessed by MTS assay and BrdU incorporation, P<0.05) and increase in apoptotic cell death (CC-3 and cleaved PARP expression). Nude mice harboring flank tumors of ATRT cell lines and treated with quinacrine for 3 weeks showed significant reduction in tumor growth compared to control animals (P<0.05). Since quinacrine is a substrate for the drug-efflux proteins P-gp/BCRP, we used quinacrine in combination with elacridar (Pgp/BCRP inhibitor) in our intracranial xenograft experiments to increase quinacrine’s retention in the brain. Mice harboring intracranial xenografts of ATRT cells showed increased survival when treated with quinacrine and elacridar (median survival 46 days) compared to control animals (median survival 25 days). These results suggest that quinacrine inhibits ATRT growth, partly by activating p53. Our studies are the first to show quinacrine’s effect on ATRTs and our current experiments include further investigation of quinacrine’s mechanism.

Synergistic activity of mTORC1/2 kinase and MEK inhibitors suppresses pediatric low-grade glioma tumorigenicity and vascularity

BACKGROUND

Pediatric low-grade glioma (pLGG) is the most common childhood brain tumor. Many patients with unresectable or recurrent/refractory tumors have significant lifelong disability. The majority of pLGG have mutations increasing the activity of the Ras/mitogen-activated protein kinase (MAPK) pathway. Activation of mammalian target of rapamycin (mTOR) is also a hallmark of pLGG. We therefore hypothesized that the dual target of rapamycin complexes 1 and 2 (TORC1/2) kinase inhibitor TAK228 would synergize with the mitogen-activated extracellular signal-regulated kinase (MEK) inhibitor trametinib in pLGG.

METHODS

We tested TAK228 and trametinib in patient-derived pLGG cell lines harboring drivers of pLGG including BRAFV600E and neurofibromatosis type 1 loss. We measured cell proliferation, pathway inhibition, cell death, and senescence. Synergy was analyzed via MTS assay using the Chou-Talalay method. In vivo, we tested for overall survival and pathway inhibition and performed immunohistochemistry for proliferation and vascularization. We performed a scratch assay and measured angiogenesis protein activation in human umbilical vein endothelial cells (HUVECs).

RESULTS

TAK228 synergized with trametinib in pLGG at clinically relevant doses in all tested cell lines, suppressing proliferation, inducing apoptosis, and causing senescence in a cell line-dependent manner. Combination treatment increased median survival by 70% and reduced tumor volume compared with monotreatment and control cohorts. Vascularization of tumors decreased as measured by CD31 and CD34. Combination treatment blocked activation of focal adhesion kinase (FAK) and sarcoma proto-oncogene non-receptor tyrosine kinase (SRC) in HUVEC cells and reduced HUVEC migration compared with each drug alone.

CONCLUSIONS

The combination of TAK228 and trametinib synergized to suppress the growth of pLGG. These agents synergized to reduce tumor vascularity and endothelial cell growth and migration by blocking activation of FAK and SRC.

BCOR Internal Tandem Duplication Expression in Neural Stem Cells Promotes Growth, Invasion, and Expression of PRC2 Targets

Central nervous system tumor with BCL6-corepressor internal tandem duplication (CNS-BCOR ITD) is a malignant entity characterized by recurrent alterations in exon 15 encoding the essential binding domain for the polycomb repressive complex (PRC). In contrast to deletion or truncating mutations seen in other tumors, BCOR expression is upregulated in CNS-BCOR ITD, and a distinct oncogenic mechanism has been suggested. However, the effects of this change on the biology of neuroepithelial cells is poorly understood. In this study, we introduced either wildtype BCOR or BCOR-ITD into human and murine neural stem cells and analyzed them with quantitative RT-PCR and RNA-sequencing, as well as growth, clonogenicity, and invasion assays. In human cells, BCOR-ITD promoted derepression of PRC2-target genes compared to wildtype BCOR. A similar effect was found in clinical specimens from previous studies. However, no growth advantage was seen in the human neural stem cells expressing BCOR-ITD, and long-term models could not be established. In the murine cells, both wildtype BCOR and BCOR-ITD overexpression affected cellular differentiation and histone methylation, but only BCOR-ITD increased cellular growth, invasion, and migration. BCOR-ITD overexpression drives transcriptional changes, possibly due to altered PRC function, and contributes to the oncogenic transformation of neural precursors.

FSMP-18. COMPREHENSIVE METABOLIC PROFILING OF HIGH MYC MEDULLOBLASTOMA REVEALS KEY DIFFERENCES BETWEEN IN VITRO AND IN VIVO GLUCOSE AND GLUTAMINE USAGE

Reprograming of cellular metabolism is a hallmark of cancer. Altered metabolism can overcome unfavorable conditions, allowing cancer cells to proliferate and invade in different tumor microenvironments. Medulloblastoma is the most common malignant brain tumor of children. Genomic amplification of MYC is a hallmark of a subset of poor-prognosis medulloblastoma. However, the metabolism of high MYC amplified medulloblastoma subgroup remains underexplored. We performed comprehensive metabolic studies of human MYC-amplified medulloblastoma by comparing the metabolic profiles of tumor cells in different environments – in vitro, in flank xenografts and in orthotopic xenografts. Principal component analysis showed that the metabolic profiles of brain and flank high-MYC medulloblastoma tumors clustered closely together and separated away from normal brain and the high-MYC medulloblastoma cells in culture. Compared to normal brain, MYC-amplified medulloblastoma orthotopic brain tumor xenografts showed upregulation of nucleotide, amino acid and glutathione pathways. Glucose was the main carbon source for the nucleotide synthesis and the TCA cycle in vivo. The glutaminase ii pathway was the main pathway utilizing glutamine in MYC-amplified medulloblastoma. In brain and flank xenografts, glutathione was the most abundant upregulated metabolite. Glutamine derived glutathione was synthesized through glutamine transaminase K (GTK) enzyme in vivo. The glutamine analog 6-diazo-5-oxo-l-norleucine (DON) significantly inhibited glutathione, amino acid, and nucleotide synthesis. In conclusion, we found that MYC-amplified medulloblastoma relied on glutamine metabolism in synthesizing glutathione in vivo. Glutamine antagonists may have therapeutic applications in human patients.

Novel Glutamine Antagonist JHU395 Suppresses MYC-Driven Medulloblastoma Growth and Induces Apoptosis

Medulloblastoma is the most common malignant pediatric brain tumor. Amplification of c-MYC is a hallmark of a subset of poor-prognosis medulloblastoma. MYC upregulates glutamine metabolism across many types of cancer. We modified the naturally occurring glutamine antagonist 6-diazo-5-oxo-l-norleucine (DON) by adding 2 promoeities to increase its lipophilicity and brain penetration creating the prodrug isopropyl 6-diazo-5-oxo-2-(((phenyl (pivaloyloxy) methoxy) - carbonyl) amino) hexanoate, termed JHU395. This prodrug was shown to have a 10-fold improved CSF-to-plasma ratio and brain-to-plasma ratio relative to DON. We hypothesized that JHU395 would have superior cell penetration compared with DON and would effectively and more potently kill MYC-expressing medulloblastoma. JHU395 treatment caused decreased growth and increased apoptosis in multiple human high-MYC medulloblastoma cell lines at lower concentrations than DON. Parenteral administration of JHU395 in Nu/Nu mice led to the accumulation of micromolar concentrations of DON in brain. Treatment of mice bearing orthotopic xenografts of human MYC-amplified medulloblastoma with JHU395 increased median survival from 26 to 45 days compared with vehicle control mice (p < 0.001 by log-rank test). These data provide preclinical justification for the ongoing development and testing of brain-targeted DON prodrugs for use in medulloblastoma.

TORC1/2 kinase inhibition depletes glutathione and synergizes with carboplatin to suppress the growth of MYC-driven medulloblastoma

Medulloblastoma is the most common malignant pediatric brain tumor. Tumors having high levels of c-MYC have the worst clinical prognosis, with only a minority of patients surviving. To address this unmet clinical need, we generated a human neural stem cell model of medulloblastoma that recapitulated the most aggressive subtype phenotypically and by mRNA expression profiling. An in silico analysis of these cells identified mTOR inhibitors as potential therapeutic agents. We hypothesized that the orally bioavailable TORC1/2 kinase inhibitor TAK228 would have activity against MYC-driven medulloblastoma. TAK228 inhibited mTORC1/2, decreased cell growth and caused apoptosis in high-MYC medulloblastoma cell lines. Comprehensive metabolic profiling of medulloblastoma orthotopic xenografts showed upregulation of glutathione compared to matched normal brain. TAK228 suppressed glutathione production. Because glutathione is required to detoxify platinum-containing chemotherapy, we hypothesized that TAK228 would cooperate with carboplatin in medulloblastoma. TAK228 synergized with carboplatin to inhibit cell growth and induce apoptosis and extended survival in orthotopic xenografts of high-MYC medulloblastoma. Brain-penetrant TORC1/2 inhibitors and carboplatin may be an effective combination therapy for high-risk medulloblastoma.

Treatment of Hepatoblastoma With Drug-eluting Bead Transarterial Chemoembolization in a 13-Month-Old Infant: A Case Report and Review of the Literature

BACKGROUND

Prior reports have shown the utility of conventional lipiodol-based transarterial chemoembolization in hepatoblastoma. The authors describe the first reported case of hepatoblastoma treated with drug-eluting bead transarterial chemoembolization (DEB-TACE).

OBSERVATIONS

An 11-month-old infant presented with hepatoblastoma measuring 14.3 cm. A trial of cisplatin chemotherapy followed by sequential DEB-TACE to the tumor's feeding vasculature reduced the mass to 5.3 cm. The patient tolerated both sessions of DEB-TACE without any major complication. Having demonstrated adequate disease control, the patient then underwent successful liver transplantation.

CONCLUSION

This report provides promising evidence for the treatment of hepatoblastoma with DEB-TACE.

Delta-24-RGD, an Oncolytic Adenovirus, Increases Survival and Promotes Proinflammatory Immune Landscape Remodeling in Models of AT/RT and CNS-PNET

PURPOSE

Atypical teratoid/rhabdoid tumors (AT/RT) and central nervous system primitive neuroectodermal tumors (CNS-PNET) are pediatric brain tumors with poor survival and life-long negative side effects. Here, the aim was to characterize the efficacy and safety of the oncolytic adenovirus, Delta-24-RGD, which selectively replicates in and kills tumor cells.

EXPERIMENTAL DESIGN

Delta-24-RGD determinants for infection and replication were evaluated in patient expression datasets. Viral replication and cytotoxicity were assessed in vitro in a battery of CNS-PNET and AT/RT cell lines. In vivo, efficacy was determined in different orthotopic mouse models, including early and established tumor models, a disseminated AT/RT lesion model, and immunocompetent humanized mouse models (hCD34⁺-NSG-SGM3).

RESULTS

Delta-24-RGD infected and replicated efficiently in all the cell lines tested. In addition, the virus induced dose-dependent cytotoxicity [IC₅₀ value below 1 plaque-forming unit (PFU)/cell] and the release of immunogenic markers. In vivo, a single intratumoral Delta-24-RGD injection (10⁷ or 10⁸ PFU) significantly increased survival and led to long-term survival in AT/RT and PNET models. Delta-24-RGD hindered the dissemination of AT/RTs and increased survival, leading to 70% of long-term survivors. Of relevance, viral administration to established tumor masses (30 days after engraftment) showed therapeutic benefit. In humanized immunocompetent models, Delta-24-RGD significantly extended the survival of mice bearing AT/RTs or PNETs (ranging from 11 to 27 days) and did not display any toxicity associated with inflammation. Immunophenotyping of Delta-24-RGD-treated tumors revealed increased CD8⁺ T-cell infiltration.

CONCLUSIONS

Delta-24-RGD is a feasible therapeutic option for AT/RTs and CNS-PNETs. This work constitutes the basis for potential translation to the clinical setting.

MEK Inhibition Suppresses Growth of Atypical Teratoid/Rhabdoid Tumors

Atypical teratoid/rhabdoid (AT/RT) tumors are the most common malignant brain tumor of infancy and have a poor prognosis. We have previously identified very high expression of LIN28A and/or LIN28B in AT/RT tumors and showed that AT/RT have corresponding increased expression of the mitogen-activated protein (MAP) kinase pathway. Binimetinib is a novel inhibitor of mitogen-activated protein kinase (MAP2K1 or MEK), and is currently in pediatric phase II clinical trials for low-grade glioma. We hypothesized that binimetinib would inhibit growth of AT/RT cells by suppressing the MAP kinase pathway. Binimetinib inhibited AT/RT growth at nanomolar concentrations. Binimetinib decreased cell proliferation and induced apoptosis in AT/RT cells and significantly reduced AT/RT tumor growth in flank xenografts. Our data suggest that MAP kinase pathway inhibition could offer a potential avenue for treating these highly aggressive tumors.

Therapeutic strategies for diffuse midline glioma from high-throughput combination drug screening

Diffuse midline gliomas (DMGs) are universally lethal malignancies occurring chiefly during childhood and involving midline structures of the central nervous system, including thalamus, pons, and spinal cord. These molecularly related cancers are characterized by high prevalence of the histone H3K27M mutation. In search of effective therapeutic options, we examined multiple DMG cultures in sequential quantitative high-throughput screens (HTS) of 2706 approved and investigational drugs. This effort generated 19,936 single-agent dose responses that inspired a series of HTS-enabled drug combination assessments encompassing 9195 drug-drug examinations. Top combinations were validated across patient-derived cell cultures representing the major DMG genotypes. In vivo testing in patient-derived xenograft models validated the combination of the multi-histone deacetylase (HDAC) inhibitor panobinostat and the proteasome inhibitor marizomib as a promising therapeutic approach. Transcriptional and metabolomic surveys revealed substantial alterations to key metabolic processes and the cellular unfolded protein response after treatment with panobinostat and marizomib. Mitigation of drug-induced cytotoxicity and basal mitochondrial respiration with exogenous application of nicotinamide mononucleotide (NMN) or exacerbation of these phenotypes when blocking nicotinamide adenine dinucleotide (NAD⁺) production via nicotinamide phosphoribosyltransferase (NAMPT) inhibition demonstrated that metabolic catastrophe drives the combination-induced cytotoxicity. This study provides a comprehensive single-agent and combinatorial drug screen for DMG and identifies concomitant HDAC and proteasome inhibition as a promising therapeutic strategy that underscores underrecognized metabolic vulnerabilities in DMG.

Abstract 4943: The stem cell factor LIN28B regulates proliferation and apoptosis in diffuse intrinsic pontine glioma

Diffuse Intrinsic Pontine Glioma (DIPG) is a lethal pediatric brain tumor. With radiation therapy as the primary treatment, the median survival after diagnosis is 6 months. Identifying molecular markers that regulate tumor growth is urgently needed to develop targeted therapies. LIN28B is a stem cell factor expressed during normal neural development and re-expressed in cancer cells. In many cancers, LIN28B acts as an RNA-binding protein and downregulates the tumor-suppressing let-7 microRNAs. We had previously shown that LIN28A, a homolog of LIN28B, regulates invasion and tumorigenicity in adult high-grade gliomas. LIN28B is known to be highly expressed in DIPG. Thus, we hypothesized that LIN28B would be important in DIPG by promoting tumor growth. We observed that LIN28B was expressed in patient-derived DIPG cell lines (JHHDIPG1, JHHDIPG16A, SUDIPGXIII, HSJD-007, SF7761). Knockdown of LIN28B using short hairpin RNAs decreased DIPG cell proliferation (BrdU incorporation) between 50% to 90% (p&lt;0.01). The knockdown of LIN28B also increased DIPG cell apoptosis (cleaved caspase-3 and cleaved PARP expression) by 2- to 3-fold (p&lt;0.01). To determine the molecular mechanism of LIN28B-mediated phenotypes in DIPG, we studied canonical downstream effectors of LIN28B. We found that when LIN28B was suppressed in DIPG cells, there was a decreased expression of the DNA-binding oncoprotein called high mobility group AT-hook 2. These findings suggest that LIN28B could be a potential therapeutic target for this devastating pediatric brain tumor. Future studies will focus on validating the role of LIN28B in DIPG growth in vivo using orthotopic zebrafish and mouse models.

Citation Format: Huizi Guo, Harpreet Kaur, Charles G. Eberhart, Eric H. Raabe. The stem cell factor LIN28B regulates proliferation and apoptosis in diffuse intrinsic pontine glioma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4943.

Abstract 3449: A novel rapid zebrafish model for validation of potential therapies for fatal pediatric brain tumors

Diffuse intrinsic pontine glioma (DIPG) and atypical teratoid/rhabdoid tumors (AT/RT) are deadly pediatric brain tumors. There is an urgent need of better therapeutics for improving survival and quality of life for these patients. Additionally, we need improved animal models to accelerate identification of novel therapeutics and improve through-put. In line with our long-term interests of validating therapies and developing model systems for pediatric brain tumors, we tested a repurposed drug quinacrine as a potential therapy using a new and faster model system in zebrafish. Quinacrine is a safe and widely used treatment for pediatric malaria and parasitic infections. We hypothesized that quinacrine will increase tumor cell death and decrease tumorigenicity of DIPG and ATRT. We used quinacrine in six patient-derived cell lines: three AT/RT (BT37, CHLA-05, CHLA-266) and three DIPG (JHHDIPG1, SUDIPGXIII, SF7761). In all tumor cell lines, quinacrine caused a dose-dependent reduction in proliferation (BrdU) and increase in apoptosis (cleaved caspase-3 and cleaved PARP) compared to vehicle-treated cells (P&lt;0.01). Quinacrine had no effect on growth of normal hindbrain neural stem and progenitor cells. Using quinacrine fluorescence as a surrogate, we achieved micromolar concentration of quinacrine in the mouse and zebrafish brain without overt toxicity. Treatment of mice bearing ATRT flank tumors with quinacrine resulted in decreased tumor volume compared to vehicle-treated mice (P&lt;0.05). To validate quinacrine orthotopically in DIPG, we developed a novel model in zebrafish wherein we injected fluorescent DIPG cells into the developing zebrafish blastula. Two days post injection, DIPG cells had homed to the zebrafish brain. Treatment of xenografted zebrafish with quinacrine for 72 hours decreased DIPG growth by 40% as measured by fluorescence, suggesting that minor groove binding drugs like quinacrine are a viable treatment strategy for these tumors. Using this new system, we were able to validate a potential therapeutic in 7 days versus 7 months in mice. Future studies are aimed at investigating the mechanism of quinacrine in these tumors and optimizing our zebrafish model for high-throughput screening of potential drugs.

Citation Format: Harpreet Kaur, Huizi Guo, Kevin Emmerich, David White, Peter Green, Alara Michelle Hector, Sepehr Akhtarkhavari, Anukriti Bhargava, Allison Martin, Smit Shah, Charles G. Eberhart, Jeffrey Mumm, Eric H. Raabe. A novel rapid zebrafish model for validation of potential therapies for fatal pediatric brain tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3449.

Abstract 510: Novel glutamine antagonist JHU-395 suppresses MYC-driven medulloblastoma growth and induces apoptosis

Medulloblastoma is the most common malignant pediatric brain tumor. Genomic amplification of MYC is a hallmark of a subset of poor-prognosis medulloblastoma. MYC upregulates glutamine metabolism, and medulloblastoma tumors with evidence of high glutamate have a poor prognosis. We modified the naturally occurring glutamine analog 6-diazo-5-oxo-l-norleucine (DON) to increase its lipophilicity and its stability in plasma, creating a prodrug termed JHU-395. We hypothesized that this prodrug would have enhanced cell penetration compared to DON and would kill MYC-expressing medulloblastoma. JHU-395 treatment caused decreased growth and increased apoptosis in human MYC-expressing medulloblastoma cell lines at lower concentrations than DON. In D283MED for example IC50 for DON is 20 uM while JHU 395 IC50 is 1 uM (p value &lt; 0.05). Similar results were obtained with other high-MYC medulloblastoma cell lines, including D425MED and D341MED. JHU-395 induced apoptosis at low micromolar concentrations as measured by cleaved PARP Western blot and cleaved caspase 3 (CC3) immunofluorescence (60% CC3+ cells compared to 30% CC3+ in control p&lt;0.05). Parenteral administration of JHU-395 in Nu/Nu mice led to the accumulation of micromolar concentrations of DON in brain as measured by mass spectrometry. These data suggest that JHU-395 may have activity against high MYC medulloblastoma in vivo, and testing this prodrug against MYC-driven medulloblastoma orthotopic xenografts is currently underway.

Citation Format: Khoa Pham, Micah Maxwell, Heather Sweeney, Jesse Alt, Rana Rais, Barbara S. Slusher, Charles G. Eberhart, Eric H. Raabe. Novel glutamine antagonist JHU-395 suppresses MYC-driven medulloblastoma growth and induces apoptosis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 510.

Abstract A38: Validation of potential therapies for treatment of fatal pediatric brain tumors DIPG and AT/RT using a novel rapid intracranial model in zebrafish

Diffuse intrinsic pontine glioma (DIPG) and atypical teratoid/rhabdoid tumors (AT/RT) are deadly pediatric brain tumors. Developing new targets and therapeutics are urgently needed. We have previously shown that multiple primary brain tumors and cell lines express increased amounts of the epigenetic modifier and DNA binding oncoprotein high-mobility group AT-hook 2 (HMGA2). Targeting HMGA2 using short hairpins decreased AT/RT and glioma growth in xenografted mice. We hypothesized that pharmacologic inhibition of HMGA proteins using DNA minor-groove binding drugs such as quinacrine would decrease tumor growth due to displacement of HMGA proteins from DNA. Quinacrine is a safe and widely used treatment for pediatric malaria and parasitic infections. We used quinacrine in six patient-derived cell lines: three AT/RT (BT37, CHLA-05, CHLA-266) and three DIPG (JHHDIPG1, SUDIPGXIII, SF7761). Using quinacrine fluorescence as a surrogate, we can achieve micromolar concentration of quinacrine in the mouse and zebrafish brain after oral administration without overt toxicity. In both tumor cell lines, quinacrine causes a dose-dependent reduction in proliferation (BrdU) and increase in apoptosis (cleaved caspase-3 and cleaved PARP) compared to vehicle-treated cells (P&lt;0.01). Quinacrine had no effect on growth of normal hindbrain neural stem and progenitor cells. Treatment of mice bearing ATRT flank tumors with quinacrine resulted in decreased tumor volume compared to vehicle-treated mice (P&lt;0.05). Current mouse intracranial DIPG models can take 6-9 months due to long latency. To validate quinacrine orthotopically in DIPG, we developed a novel model in zebrafish wherein we injected fluorescent DIPG cells into the developing zebrafish blastula. Five days post injection, DIPG cells had homed to the zebrafish brain. Treatment of xenografted zebrafish with quinacrine for 48 hours decreased DIPG growth by 40% as measured by fluorescence, suggesting that minor groove binding drugs such as quinacrine are a viable treatment strategy for these tumors. Future studies are aimed at investigating the mechanism of quinacrine in these tumors.

Citation Format: Harpreet Kaur, Huizi Guo, David White, Peter Green, Alara M. Hector, Sepehr Akhtarkhavari, Anukriti Bhargava, Smit Shah, Charles G. Eberhart, Jeffrey Mumm, Eric H. Raabe. Validation of potential therapies for treatment of fatal pediatric brain tumors DIPG and AT/RT using a novel rapid intracranial model in zebrafish [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 A38.

Inhibition of mTORC1 in pediatric low-grade glioma depletes glutathione and therapeutically synergizes with carboplatin

BACKGROUND

Pediatric low-grade glioma (pLGG) often initially responds to front-line therapies such as carboplatin, but more than 50% of treated tumors eventually progress and require additional therapy. With the discovery that pLGG often contains mammalian target of rapamycin (mTOR) activation, new treatment modalities and combinations are now possible for patients. The purpose of this study was to determine if carboplatin is synergistic with the mTOR complex 1 inhibitor everolimus in pLGG.

METHODS

We treated 4 pLGG cell lines and 1 patient-derived xenograft line representing various pLGG genotypes, including neurofibromatosis type 1 loss, proto-oncogene B-Raf (BRAF)-KIAA1549 fusion, and BRAFV600E mutation, with carboplatin and/or everolimus and performed assays for growth, cell proliferation, and cell death. Immunohistochemistry as well as in vivo and in vitro metabolomics studies were also performed.

RESULTS

Carboplatin synergized with everolimus in all of our 4 pLGG cell lines (combination index <1 at Fa 0.5). Combination therapy was superior at inhibiting tumor growth in vivo. Combination treatment increased levels of apoptosis as well as gamma-H2AX phosphorylation compared with either agent alone. Everolimus treatment suppressed the conversion of glutamine and glutamate into glutathione both in vitro and in vivo. Exogenous glutathione reversed the effects of carboplatin and everolimus.

CONCLUSIONS

The combination of carboplatin and everolimus was effective at inducing cell death and slowing tumor growth in pLGG models. Everolimus decreased the amount of available glutathione inside the cell, preventing the detoxification of carboplatin and inducing increased DNA damage and apoptosis.

Combined Therapy of AXL and HDAC Inhibition Reverses Mesenchymal Transition in Diffuse Intrinsic Pontine Glioma

PURPOSE

Diffuse intrinsic pontine glioma (DIPG) is an incurable type of pediatric brain cancer, which in the majority of cases is driven by mutations in genes encoding histone 3 (H3K27M). We here determined the preclinical therapeutic potential of combined AXL and HDAC inhibition in these tumors to reverse their mesenchymal, therapy-resistant, phenotype.

EXPERIMENTAL DESIGN

We used public databases and patient-derived DIPG cells to identify putative drivers of the mesenchymal transition in these tumors. Patient-derived neurospheres, xenografts, and allografts were used to determine the therapeutic potential of combined AXL/HDAC inhibition for the treatment of DIPG.

RESULTS

We identified AXL as a therapeutic target and regulator of the mesenchymal transition in DIPG. Combined AXL and HDAC inhibition had a synergistic and selective antitumor effect on H3K27M DIPG cells. Treatment of DIPG cells with the AXL inhibitor BGB324 and the HDAC inhibitor panobinostat resulted in a decreased expression of mesenchymal and stem cell genes. Moreover, this combination treatment decreased expression of DNA damage repair genes in DIPG cells, strongly sensitizing them to radiation. Pharmacokinetic studies showed that BGB324, like panobinostat, crosses the blood-brain barrier. Consequently, treatment of patient-derived DIPG xenograft and murine DIPG allograft-bearing mice with BGB324 and panobinostat resulted in a synergistic antitumor effect and prolonged survival.

CONCLUSIONS

Combined inhibition of AXL and HDACs in DIPG cells results in a synergistic antitumor effect by reversing their mesenchymal, stem cell-like, therapy-resistant phenotype. As such, this treatment combination may serve as part of a future multimodal therapeutic strategy for DIPG.

PD-L1 Expression in Pediatric Low-Grade Gliomas Is Independent of BRAF V600E Mutational Status

To evaluate a potential relationship between BRAF V600E mutation and PD-L1 expression, we examined the expression of PD-L1 in pediatric high- and low-grade glioma cell lines as well as a cohort of pediatric low-grade glioma patient samples. Half of the tumors in our patient cohort were V600-wildtype and half were V600E mutant. All tumors expressed PD-L1. In most tumors, PD-L1 expression was low (<5%), but in some cases over 50% of cells were positive. Extent of PD-L1 expression and immune cell infiltration was independent of BRAF V600E mutational status. All cell lines evaluated, including a BRAF V600E mutant xenograft, expressed PD-L1. Transient transfection of cell lines with a plasmid expressing mutant BRAF V600E had minimal effect on PD-L1 expression. These findings suggest that the PD-1 pathway is active in subsets of pediatric low-grade glioma as a mechanism of immune evasion independent of BRAF V600E mutational status. Low-grade gliomas that are unresectable and refractory to traditional therapy are associated with significant morbidity and continue to pose a treatment challenge. PD-1 pathway inhibitors may offer an alternative treatment approach. Clinical trials will be critical in determining whether PD-L1 expression indicates likely therapeutic benefit with immune checkpoint inhibitors.

Orally bioavailable glutamine antagonist prodrug JHU-083 penetrates mouse brain and suppresses the growth of MYC-driven medulloblastoma

A subset of poor-prognosis medulloblastoma has genomic amplification of MYC. MYC regulates glutamine metabolism in multiple cellular contexts. We modified the glutamine analog 6-diazo-5-oxo-l-norleucine (DON) to mask its carboxylate and amine functionalities, creating a prodrug termed JHU-083 with increased oral bioavailability. We hypothesized that this prodrug would kill MYC-expressing medulloblastoma. JHU-083 treatment caused decreased growth and increased apoptosis in human MYC-expressing medulloblastoma cell lines. We generated a mouse MYC-driven medulloblastoma model by transforming C57BL/6 mouse cerebellar stem and progenitor cells. When implanted into the brains of C57BL/6 mice, these cells formed large cell/anaplastic tumors that resembled aggressive medulloblastoma. A cell line derived from this model was sensitive to JHU-083 in vitro. Oral administration of JHU-038 led to the accumulation of micromolar concentrations of DON in the mouse brain. JHU-083 treatment significantly increased the survival of immune-competent animals bearing orthotopic tumors formed by the mouse cerebellar stem cell model as well as immune-deficient animals bearing orthotopic tumors formed by a human MYC-amplified medulloblastoma cell line. These data provide pre-clinical justification for the ongoing development and testing of orally bioavailable DON prodrugs for use in medulloblastoma patients.

Unbiased Metabolic Profiling Predicts Sensitivity of High MYC-Expressing Atypical Teratoid/Rhabdoid Tumors to Glutamine Inhibition with 6-Diazo-5-Oxo-L-Norleucine

PURPOSE

Atypical teratoid/rhabdoid tumors (AT/RT) are aggressive infantile brain tumors with poor survival. Recent advancements have highlighted significant molecular heterogeneity in AT/RT with an aggressive subgroup featuring overexpression of the MYC proto-oncogene. We perform the first comprehensive metabolic profiling of patient-derived AT/RT cell lines to identify therapeutic susceptibilities in high MYC-expressing AT/RT.

EXPERIMENTAL DESIGN

Metabolites were extracted from AT/RT cell lines and separated in ultra-high performance liquid chromatography mass spectrometry. Glutamine metabolic inhibition with 6-diazo-5-oxo-L-norleucine (DON) was tested with growth and cell death assays and survival studies in orthotopic mouse models of AT/RT. Metabolic flux analysis was completed to identify combination therapies to act synergistically to improve survival in high MYC AT/RT.

RESULTS

Unbiased metabolic profiling of AT/RT cell models identified a unique dependence of high MYC AT/RT on glutamine for survival. The glutamine analogue, DON, selectively targeted high MYC cell lines, slowing cell growth, inducing apoptosis, and extending survival in orthotopic mouse models of AT/RT. Metabolic flux experiments with isotopically labeled glutamine revealed DON inhibition of glutathione (GSH) synthesis. DON combined with carboplatin further slowed cell growth, induced apoptosis, and extended survival in orthotopic mouse models of high MYC AT/RT.

CONCLUSIONS

Unbiased metabolic profiling of AT/RT identified susceptibility of high MYC AT/RT to glutamine metabolic inhibition with DON therapy. DON inhibited glutamine-dependent synthesis of GSH and synergized with carboplatin to extend survival in high MYC AT/RT. These findings can rapidly translate into new clinical trials to improve survival in high MYC AT/RT.

Abstract 5267: Unbiased metabolic profiling of atypical teratoid/rhabdoid tumors predicts sensitivity to the glutamine metabolic inhibitor 6-diazo-5-oxo-L-norleucine

Atypical teratoid rhabdoid tumors (AT/RT) are deadly tumors of infancy in need of new, targeted therapies. Molecular analyses have revealed 3 epigenetically distinct subgroups of AT/RT. High MYC-expressing tumors are a particularly aggressive subgroup, with a 5-year median survival of 18.5%. MYC is difficult to directly target given large, flat, featureless protein-protein binding sites. We performed the first unbiased metabolic profile of AT/RT cell models by high-performance liquid chromatography-mass spectrometry. This study revealed that high MYC-expressing AT/RT have a unique metabolic profile (Partial Least Squares-Discriminant Analysis). Pathway analysis highlights a dependence on glutamine metabolism in high-MYC expressing AT/RT. High MYC-expressing cell lines grow poorly in glutamine-free media compared to low-MYC cell lines (MTS assay, glutamine-free media vs normal media). Due to this dependence on glutamine metabolism, we hypothesized that high-MYC expressing AT/RT would be especially sensitive to glutamine metabolic inhibitors. We show that the glutamine analogue, 6-diazo-5-oxo-L-norleucine (DON) slows high-MYC expressing AT/RT cell growth, induces high rates of apoptosis, and improves survival in orthotopic mouse models of high-MYC expressing AT/RT (p =0.0027 by log-rank test). In contrast, low-MYC expressing models are relatively insensitive to DON. In uniformly labeled glutamine metabolic analyses of AT/RT cells, DON treatment led to depletion of glutathione levels by preventing the production of glutamate. DON treatment synergized with carboplatin to kill AT/RT cells in vitro and extend the life of mice bearing AT/RT orthotopic xenografts (p&lt; 0.001 by log rank test). We aim to translate these findings into a new clinical trial to improve survival in this deadly disease.

Citation Format: Sabrina Z. Wang, Brad Poore, Jesse Alt, Antoinette Price, Sariah Allen, Brent Orr, Rana Rais, Barbara Slusher, Charles Eberhart, Eric H. Raabe, Jeffrey H. Rubens. Unbiased metabolic profiling of atypical teratoid/rhabdoid tumors predicts sensitivity to the glutamine metabolic inhibitor 6-diazo-5-oxo-L-norleucine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5267.

Abstract 5277: Glutamine antagonists synergize with L-asparaginase in MYCN-driven rhabdomyosarcoma

Rhabdomyosarcoma is the most common soft tissue sarcoma in children. Chromosomal translocations producing fusion transcription factors (PAX-FOXO1) are associated with poor prognosis. A subset of fusion-positive rhabdomyosarcomas have downstream activation of the transcription factor MYCN, a known oncogenic driver. Attempts to inhibit directly either PAX-FOXO1 or MYCN have been unsuccessful. MYCN is known to drive tumor cell reliance on glutamine metabolism. 6-diazo-5-oxo-L-norleucine (DON) is a well-characterized glutamine analog that irreversibly inhibits glutamine-utilizing enzymes. DON was well tolerated in a phase I pediatric clinical trial, but it has never been systematically tested in rhabdomyosarcoma. We show that high MYCN confers sensitivity to DON therapy in vitro. We have also developed a targeted glutamine antagonist compound, JHU083, which is similarly effective against MYCN-driven rhabdomyosarcoma cell lines. Furthermore, DON administered by intraperitoneal injection twice weekly significantly reduces flank tumor volume in a murine xenograft MYCN-driven rhabdomyosarcoma model (mean tumor volume 757 mm3 vs. 2167 mm3 in control animals, p-value 0.0000197 by t-test). In stable isotope resolved metabolomics experiments, DON primarily prevents asparagine synthesis, depleting intracellular asparagine levels by at least 50% in MYCN-driven cell lines (p-value = 0.0003). Finally, DON combined with L-asparaginase synergistically inhibits growth of MYCN-driven rhabdomyosarcoma cell lines (CI &lt;&lt; 1 by the Chou-Talalay method, indicating strong synergy; p-value = 0.0001). We conclude that DON depletes cellular pools of asparagine, and combination therapy with DON and L-asparaginase synergistically inhibits the growth of MYCN-driven rhabdomyosarcoma. These studies provide the preclinical justification for potential clinical trials for the use DON or DON prodrugs in combination with L-asparaginase as new therapeutic options for patients with MYCN-driven rhabdomyosarcoma.

Citation Format: Micah J. Maxwell, Brad A. Poore, Allison Hanaford, Jesse Alt, Rana Rais, Barbara S. Slusher, Charles G. Eberhart, Eric H. Raabe. Glutamine antagonists synergize with L-asparaginase in MYCN-driven rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5277.

Abstract 2888: Targeting the fatal pediatric brain tumors AT/RT and DIPG with the DNA binding agent quinacrine

Atypical teratoid/rhabdoid tumors (AT/RT) and diffuse intrinsic pontine glioma (DIPG) are incurable pediatric brain tumors. Developing new targets and novel therapeutics are urgently needed. We have previously shown that multiple primary brain tumors and cell lines express increased amounts of the epigenetic modifier high mobility group AT-hook 2 (HMGA2). HMGA2 is a DNA-binding oncoprotein that regulates transcription during normal embryogenesis and in cancer stem cells. Targeting HMGA2 using short hairpins significantly decreased AT/RT and glioma growth and increased survival of xenografted mice. We hypothesized that pharmacological inhibition of HMGA proteins using DNA minor-groove binding drugs like quinacrine will decrease AT/RT and DIPG growth due to displacement of HMGA proteins from the DNA. We used quinacrine in ten patient-derived cell lines: five AT/RT (BT37, CHLA-05, CHLA-06, BT-12, CHLA-266) and five DIPG (JHHDIPG1, SUDIPGXIII, JHHDIPG16A, SF7761, HSJD-007). Quinacrine has been used in millions of humans to treat malaria and parasitic infections, has a well-known safety profile and can penetrate the brain. Using quinacrine fluorescence as a surrogate, we can achieve therapeutically efficacious micromolar concentration of quinacrine in the mouse and zebrafish brain after oral administration without overt toxicity. In both tumor cell lines, quinacrine causes a dose-dependent reduction in growth (MTS) and proliferation (BrdU) compared to vehicle-treated cells (P&lt;0.01). Treatment of both tumor lines with quinacrine significantly increased apoptosis (cleaved caspase-3 and cleaved PARP) compared to control cells (P&lt;0.01). Quinacrine had no effect on growth of normal hindbrain cells. Our results suggest that minor groove binding drugs like quinacrine are a viable potential treatment strategy for these lethal tumors. Ongoing studies include validating the in vivo efficacy of quinacrine using zebrafish and mouse models of AT/RT and DIPG. Future studies are aimed at investigating the mechanism of quinacrine in these devastating tumors.

Citation Format: Harpreet Kaur, Huizi Guo, Peter Green, Sepehr Akhtarkhavari, Smit Shah, Charles G. Eberhart, Eric H. Raabe. Targeting the fatal pediatric brain tumors AT/RT and DIPG with the DNA binding agent quinacrine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2888.

Inhibition of enhancer of zest homologue 2 is a potential therapeutic target for high-MYC medulloblastoma

MYC amplification is common in Group 3 medulloblastoma and is associated with poor survival. Group 3 and Group 4 medulloblastomas are also known to have elevated levels of histone H3-lysine 27-tri-methylation (H3K27me3), at least in part due to high expression of the H3K27 methyltransferase enhancer of zest homologue 2 (EZH2), which can be regulated by MYC. We therefore examined whether MYC expression is associated with elevated EZH2 and H3K27me3 in medulloblastoma, and if high-MYC medulloblastomas are particularly sensitive to pharmacological EZH2 blockade. Western blot analysis of low (DAOY, UW228, CB SV40) and high (DAOY-MYC, UW228-MYC, CB-MYC, D425) MYC cell lines showed that higher levels of EZH2 and H3K27me3 were associated with elevated MYC. In fixed medulloblastoma samples examined using immunohistochemistry, most MYC positive tumors also had high H3K27me3, but many MYC negative ones did as well, and the correlation was not statistically significant. All high MYC lines tested were sensitive to the EZH2 inhibitor EPZ6438. Many low MYC lines also grew more slowly in the presence of EPZ6438, although DAOY-MYC cells responded more strongly than parent DAOY cultures with lower MYC levels. We find that higher MYC levels are associated with increased EZH2, and pharmacological blockade of EZH2 is a potential therapeutic strategy for aggressive medulloblastoma with elevated MYC.

Abstract B14: Establishment and characterization of in vitro models of alternative lengthening of telomeres (ALT) in pediatric high-grade glioma

Telomeres consist of many kilobases of repeated TTAGGG sequences at the ends of chromosomes, protected by a sequence-specific protein cap. Telomeres progressively shorten with each cell division and ultimately become critically short; due to their extensive proliferation, cancer cells must find a way to counteract this telomere loss. While most cancers utilize telomerase to maintain their telomere length, about 5% of cancers use a telomerase-independent telomere maintenance strategy, termed alternative lengthening of telomeres (ALT). While, overall, ALT is rare in cancer (~5-10% of cases), this telomere maintenance mechanism is enriched in pediatric high-grade glioma (pHGG). Previous work in our laboratory suggests that nearly half of pHGG utilize ALT. To date, therapeutic options are largely ineffective for children with HGG, reflected in the five-year survival rate, which is less than 33%. Our goal is to better harness ALT as a clinical marker in pHGG, specifically by the identification of drugs that target ALT-positive cancers. In order to study ALT in this context, we obtained and characterized a panel of six pHGG cell lines. Two of these six pHGG cell lines displayed features of ALT, including the presence of ALT-associated PML bodies and extrachromosomal telomeric DNA in the form of c-circles. Furthermore, these lines lacked measurable telomerase activity. It is well established that ATRX is commonly mutated in ALT-positive cancers, including pHGG. Interestingly, only one of these two ALT-positive pHGG cell lines displayed total loss of ATRX; the second cell line has an in-frame deletion in the ATRX gene, which may provide insight into the mechanism of how ATRX acts to suppress ALT. In addition, we have generated ATRX knockout cell lines from the four ALT-negative pHGG cells identified in this panel. Despite the strong link between ATRX loss and ALT in clinical samples, only one cell line displayed ALT characteristics after ATRX knockout. Comparison of the ALT-competent cell line to the ALT-resistant cell lines will yield important information about additional genetic or epigenetic events that allow ALT to occur. In conclusion, we have established in vitro models of ALT in pHGG cell lines based on endogenous ALT positivity and induction of ALT-like features based on ATRX modulation. These models will be invaluable resources as we strive to understand the molecular characteristics of ALT and translate these findings to better therapies for children with pHGG.

Citation Format: Jacqueline A. Brosnan-Cashman, Mindy K. Graham, Anthony J. Rizzo, Kaylar Myers, Rebecca Zhang, Ezgi Göger, Reza Zarinshenas, Christine Davis, Ming Yuan, Dinesh Rakheja, Eric H. Raabe, Charles G. Eberhart, Christopher M. Heaphy, Alan K. Meeker. Establishment and characterization of in vitro models of alternative lengthening of telomeres (ALT) in pediatric high-grade glioma [abstract]. In: Proceedings of the AACR Special Conference: Pediatric Cancer Research: From Basic Science to the Clinic; 2017 Dec 3-6; Atlanta, Georgia. Philadelphia (PA): AACR; Cancer Res 2018;78(19 Suppl):Abstract nr B14.

ATRX loss induces multiple hallmarks of the alternative lengthening of telomeres (ALT) phenotype in human glioma cell lines in a cell line-specific manner

Cancers must maintain their telomeres at lengths sufficient for cell survival. In several cancer subtypes, a recombination-like mechanism termed alternative lengthening of telomeres (ALT), is frequently used for telomere length maintenance. Cancers utilizing ALT often have lost functional ATRX, a chromatin remodeling protein, through mutation or deletion, thereby strongly implicating ATRX as an ALT suppressor. Herein, we have generated functional ATRX knockouts in four telomerase-positive, ALT-negative human glioma cell lines: MOG-G-UVW, SF188, U-251 and UW479. After loss of ATRX, two of the four cell lines (U-251 and UW479) show multiple characteristics of ALT-positive cells, including ultrabright telomeric DNA foci, ALT-associated PML bodies, and c-circles. However, telomerase activity and overall telomere length heterogeneity are unaffected after ATRX loss, regardless of cellular context. The two cell lines that showed ALT hallmarks after complete ATRX loss also did so upon ATRX depletion via shRNA-mediated knockdown. These results suggest that other genomic or epigenetic events, in addition to ATRX loss, are necessary for the induction of ALT in human cancer.

MELK Inhibition in Diffuse Intrinsic Pontine Glioma

Purpose: Diffuse intrinsic pontine glioma (DIPG) is a highly aggressive pediatric brain tumor, for which no effective therapeutic options currently exist. We here determined the potential of inhibition of the maternal embryonic leucine zipper kinase (MELK) for the treatment of DIPG.Experimental Design: We evaluated the antitumor efficacy of the small-molecule MELK inhibitor OTSSP167 in vitro in patient-derived DIPG cultures, and identified the mechanism of action of MELK inhibition in DIPG by RNA sequencing of treated cells. In addition, we determined the blood-brain barrier (BBB) penetration of OTSSP167 and evaluated its translational potential by treating mice bearing patient-derived DIPG xenografts.Results: This study shows that MELK is highly expressed in DIPG cells, both in patient samples and in relevant in vitro and in vivo models, and that treatment with OTSSP167 strongly decreases proliferation of patient-derived DIPG cultures. Inhibition of MELK in DIPG cells functions through reducing inhibitory phosphorylation of PPARγ, resulting in an increase in nuclear translocation and consequent transcriptional activity. Brain pharmacokinetic analyses show that OTSSP167 is a strong substrate for both MDR1 and BCRP, limiting its BBB penetration. Nonetheless, treatment of Mdr1a/b;Bcrp1 knockout mice carrying patient-derived DIPG xenografts with OTSSP167 decreased tumor growth, induced remissions, and resulted in improved survival.Conclusions: We show a strong preclinical effect of the kinase inhibitor OTSSP167 in the treatment of DIPG and identify the MELK-PPARγ signaling axis as a putative therapeutic target in this disease. Clin Cancer Res; 24(22); 5645-57. ©2018 AACR.

Abstract 3182: A human neural stem cell glial brain tumor model identifies the relative contribution of different oncogenic elements to malignant transformation

Diffuse intrinsic pontine glioma (DIPG) is an invasive malignancy of the brainstem that accounts for greater than 80% of pediatric brainstem gliomas. In the past 40 years, there have been no significant advances in DIPG treatments and survival, so it remains a leading cause of death from pediatric brain tumors. Nearly 80% of DIPG harbor a point mutation in H3F3A or HIST1H3B, and the presence of this H3.3K27M mutation is inversely correlated with reduced OS, suggesting that epigenetic dysregulation is a key driver to cause the pathogenesis of DIPG. However, it is unclear how the H3.3K27M mutation and the other common alterations in DIPG contribute to tumorigenicity in human neural stem cells. To address the relative contributions of different oncogenic elements to DIPG, we sought to add common DIPG elements in a stepwise fashion to human neural stem cells derived from the developing hindbrain. We chose oncogenic elements that are known to present in DIPG, including the stem cell factor BMI1, mutant (R248W) p53, H3.3K27M, constitutively active AKT (representing the activation of mTOR detected in the vast majority of DIPG) and hTERT. We found that the combination of the stem cell factor BMI1 along with mutant (R248W) p53 and hTERT could immortalize neural stem cells, but was insufficient to form orthotopic xenograft tumors when placed in the pons of immunodeficient mice. Addition of activated AKT led to aggressive tumor formation, with a glial phenotype as evidenced by robust GFAP expression and absence of synaptophysin expression. All mice bearing these four genetic alterations succumbed to their tumors within 100 days of implantation. We found that introduction of H3.3K27M mutation reduces the level of tri-methylation of H3.3KK27 to that seen in patient-derived DIPG cell lines. Furthermore, we found that H3.3KK27 mutation introduction increases expression of LIN28B, a stem cell factor, which is also found in patient-derived DIPGs. Our previous study showing that LIN28A, another family member of LIN28 proteins, regulates invasion and tumorigenicity in adult high-grade gliomas suggests that H3.3K27M mutation might facilitate invasiveness of DIPG through LIN28B and its downstream effectors HMGA2, SNAI1, and SLUG. In summary, we have developed a human hindbrain neural stem cell DIPG model that has both accurate cell of origin and genetic defects including H3.3K27M mutation. Our models allow for assessment of the relative contribution to transformation of each genetic element in a stepwise fashion in the likely cell of origin of this deadly tumor.

Citation Format: Youngran Park, Ping An, Dacheng Ding, Charles G. Eberhart, Eric H. Raabe. A human neural stem cell glial brain tumor model identifies the relative contribution of different oncogenic elements to malignant transformation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3182.

Abstract 3484: In vivo metabolomics reveals a potentially potent combination therapy for MYC-driven medulloblastoma

The MYC oncogene is associated with aggressive forms of the pediatric brain tumor medulloblastoma. MYC promotes oncogenesis in part by altering cellular glucose and glutamine metabolism. We hypothesized that MYC-driven medulloblastoma would be sensitive to the glutamine metabolic inhibitor 6-diazo-5-oxo-l-norleucine (DON). In MYC-driven medulloblastoma cell lines, 10uM DON treatment increases apoptosis by up to 280% (p&lt;0.04) as compared to vehicle control. In human neural stem cells transformed with MYC, but not in untransformed cells, DON treatment caused up to a 127% increase in apoptosis compared to vehicle (p&lt;0.001). Once-weekly DON therapy increased median survival by up to 246% (p&lt;0.004) in three different MYC-driven medulloblastoma orthotopic xenograft models. To elucidate the mechanism of DON, we performed stable isotope resolved metabolomics (SIRM) on two MYC-driven medulloblastoma tumor models. SIRM revealed that tumors from DON treated animals had decreased production of asparagine (p&lt;0.016). Production of aspartate was not decreased (p&gt;0.4), suggesting that DON was inhibiting asparaginase synthetase, the enzyme that transfers the ammonia group from glutamine to aspartate to generate asparagine. We hypothesized that DON efficacy could be enhanced by asparaginase (ASNase), an enzyme that breaks down asparagine into aspartate and ammonia. ASNase is a commonly used therapy in pediatric patients with leukemia and lymphoma. In MYC-driven medulloblastoma cell lines, treatment with low-dose DON or ASNase as single agents did not significantly increase apoptosis by cleaved caspase-3 immunofluoresence or cleaved-PARP western blot. The combination of low-dose DON and ASNase increased apoptosis by up to 577% (p&lt;0.0001). Similarly, in human neural stem cells transformed with MYC, the combination of low-dose DON and ASNase increased apoptosis by up to 523% (p&lt;0.0001). We hypothesized that depletion of asparagine from both intracellular and extracellular pools would induce the uncharged tRNA/endoplasmic reticulum stress response. Western blotting revealed that the combination of DON and ASNase increased expression of the transcription factor ATF4 and the pro-apoptotic protein CHOP, which are critical components of the uncharged tRNA response. ATF4 is a known regulator of endoplasmic reticulum stress induced apoptosis through transcription of pro-apoptotic proteins. These data suggest that DON and ASNase could be a powerful therapeutic combination for treating MYC-driven medulloblastoma and possibly other MYC-driven malignancies.

Citation Format: Allison R. Hanaford, Brad Poore, Jesse Alt, Barbara Slusher, Charles G. Eberhart, Eric H. Raabe. In vivo metabolomics reveals a potentially potent combination therapy for MYC-driven medulloblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3484.

Abstract 4849: Targeting the lethal pediatric atypical teratoid/rhabdoid tumors with the DNA minor-groove binding agent quinacrine

Atypical teratoid/rhabdoid tumors (AT/RT) are rare, incurable, and highly proliferative pediatric brain tumors. Discovering new targets and developing novel therapeutics are urgently needed for this devastating tumor. We have previously shown that AT/RT tumors and cell lines express increased amounts of the epigenetic modifier high mobility group AT-hook 2 (HMGA2). HMGA2 is a DNA-binding oncoprotein that regulates transcription during normal embryogenesis and in cancer stem cells. Targeting HMGA2 using short hairpins significantly decreased AT/RT cell growth and increased survival of xenografted mice in our studies. We hypothesized that pharmacological inhibition of HMGA proteins using DNA minor-groove binding drugs will decrease growth of AT/RT cell lines due to displacement of HMGA proteins from the DNA. We used the minor-groove binding agent quinacrine to test our hypothesis in 3 different AT/RT cell lines (BT37, CHLA-06 and CHLA-04). Quinacrine has been used in millions of humans to treat malaria and other parasitic infections and has a well known safety profile. Quinacrine penetrates the brain, and we can achieve micromolar levels of quinacrine in brain after oral administration. Quinacrine causes a dose-dependent reduction in AT/RT cell growth (MTS assay) and proliferation (BrdU incorporation) compared to vehicle-treated cells (P&lt;0.05). Additionally, treatment of AT/RT cells with quinacrine significantly increased apoptotic cell death (increased cleaved caspase-3 and cleaved PARP expression) in a dose-dependent manner compared to vehicle-treated cells (P&lt;0.05). Our results suggest that minor groove binding drugs like quinacrine are a viable potential treatment strategy for AT/RT. Future studies are aimed at testing the in vivo efficacy and validating the mechanism of action of quinacrine in AT/RT.

Citation Format: Harpreet Kaur, Huizi Guo, Charles G. Eberhart, Eric H. Raabe. Targeting the lethal pediatric atypical teratoid/rhabdoid tumors with the DNA minor-groove binding agent quinacrine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4849.

Abstract 4850: The minor groove binding agent quinacrine inhibits growth and increases apoptotic death in diffuse intrinsic pontine glioma tumor cells

Diffuse intrinsic pontine glioma (DIPG) is an invasive, incurable and aggressive pediatric brain tumor found in the brainstem. Improved understanding of the biology of DIPG tumors is urgently needed to develop novel treatments. Our previous studies have shown that DIPG tumors and cell lines express high levels of the DNA-binding stem cell factor high mobility group AT-hook 2 (HMGA2).. Targeting HMGA2 using lentiviral shRNA decreased DIPG cell invasion, proliferation and increased apoptosis in our studies. We hypothesized that inhibiting HMGA2 using DNA minor groove binding drugs like quinacrine would decrease DIPG proliferation and increase apoptotic cell death. We used three DIPG cell lines (JHHDIPG1, JHHDIPG16A, and SUDIPG13) to test the effect of quinacrine. Quinacrine has traditionally been used as an anti-malarial drug and is known to strongly bind to DNA. We used BrdU incorporation as a measure of proliferation and expression of cleaved caspase-3 (CC-3) as a measure of apoptosis. Treatment of DIPG cells with quinacrine showed a dose dependent increase in cell death (CC-3 expression) from 3uM to 20uM, with the most effective doses being from 3uM to 5uM. In all three DIPG cell lines, treatment with 1uM to 5uM quinacrine showed a significant reduction in cell proliferation (BrdU) from 3uM to 5uM compared to vehicle-treated cells (P&lt;0.0001). Additionally, treatment of DIPG cell lines with 1uM to 5uM quinacrine showed a significant increase in apoptotic cell death (CC-3 expression) compared to vehicle control cells from 3uM to 5uM (P&lt;0.001). Our data suggests that minor groove binding agents like quinacrine could be effective therapies for this lethal brain tumor. Our future studies are aimed at testing the efficacy of quinacrine in inhibiting DIPG tumorigenicity and elucidating the mechanism of action.

Citation Format: Huizi Guo, Harpreet Kaur, Sepehr Akhtarkhavari, Charles G. Eberhart, Eric H. Raabe. The minor groove binding agent quinacrine inhibits growth and increases apoptotic death in diffuse intrinsic pontine glioma tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4850.

Abstract 3521: Glutamine metabolic inhibition synergizes with L-asparaginase in MYCN-amplified neuroblastoma

Neuroblastoma is the most common extracranial solid tumor in children. Though it accounts for about 10% of pediatric cancers, it is disproportionately responsible for 15% of pediatric cancer deaths. MYCN is amplified in 20% of neuroblastomas and correlates with adverse outcome. MYCN is involved in the maintenance of cancer stem cells, as well as in driving tumor cell growth, proliferation and tumorigenesis. Attempts to inhibit MYCN directly have been largely unsuccessful. MYCN is known to drive tumor cell reliance on glutamine for cellular metabolism. 6-Diazo-5-oxo-L-norleucine (DON) is a well-characterized glutamine analogue that inhibits glutamine metabolism by irreversibly inactivating multiple glutamine-utilizing enzymes. DON was well tolerated in a previous phase I clinical trial in pediatric patients, but it has never been systematically tested in neuroblastoma patients. We show that MYCN-amplification confers sensitivity to DON therapy in in vitro models of neuroblastoma, and that DON administered by intraperitoneal injection twice weekly significantly reduces flank tumor volume in orthotopic mouse models of MYCN-amplified neuroblastoma (mean tumor volume 1715 mm3 vs. 207 mm3 in control animals, p-value = 0.00017 by t-test). We have also developed an orally bioavailable DON prodrug, JHU083, and we found that this drug administered orally three times weekly was similarly able to suppress neuroblastoma tumor growth in mice (mean tumor volume 1,115 mm3 vs. 217 mm3 in control animals, p-value = 0.0000088 by t-test). In metabolic flux experiments, tracing glutamine and glucose donation of 13C and 15N via liquid chromatography/mass spectrometry, DON prevents asparagine synthesis, depleting intracellular asparagine levels by 40% compared to control treated cells (p-value = 0.006). DON combined with L-asparaginase synergistically inhibits growth of MYCN-amplified neuroblastoma cell lines (CI = 0.25 by the Chou-Talalay method, indicating strong synergy; p-value = 0.00011). We conclude that DON depletes cellular pools of asparagine, and combination therapy with DON and L-asparaginase synergistically inhibits the growth of MYCN-amplified neuroblastoma. These studies provide the preclinical justification for potential clinical trials for the use of DON or DON prodrugs in combination with L-asparaginase as new therapeutic options for patients with MYCN-amplified neuroblastoma.

Citation Format: Micah J. Maxwell, Brad Poore, Allison Hanaford, Jesse Alt, Rana Rais, Barbara S. Slusher, Charles G. Eberhart, Eric H. Raabe. Glutamine metabolic inhibition synergizes with L-asparaginase in MYCN-amplified neuroblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3521.

Abstract 4629: Synergistic growth inhibitor effect on a patient derived NF1 pilocytic astrocytoma cell line with the dual mTORC1/2 inhibitor TAK228 and MEK inhibitor trametinib

Pediatric low grade glioma (PLGG) is the most common brain tumor of childhood. If the tumor is located in a region of the brain that is not accessible for surgical resection, additional therapies are needed. We and others have identified mTORC and MEK-activation in PLGG. The dual mTORC1/2-inhibitor, TAK228, and the FDA approved MEK-inhibitor, trametinib, are promising candidates for targeted PLGG therapy. We hypothesized that TAK228 and trametinib would show synergistic effects in vitro and in vivo PLGG models. We treated in vitro four different patient-derived PLGG cell lines with TAK228, trametinib, or vehicle control: JHH_NF1_PA1 (NF1 mutation), BT66_SV40/hTERT (BRAFfusion), Res186 and Res259 (both cell lines with mTOR and MAPK activation). In vivo, BT40 (BRAFV600E) tumor cells were investigated with both agents in immunodeficient mice. Cell growth was investigated using MTS-assay compared to vehicle control. Activation of MAPK pathway was detected via Western Blot by phosphorylated ERK compared to total ERK, and β-actin. mTOR pathway was investigated with pAKT, pS6, and p4E-BP1 compared to the total protein amount and β-actin. In all of the cell lines, treatment with TAK228 or trametinib reduces cell growth and proliferation in a dose and time depended manner. We have found a robust synergistic (via Chou-Talalay method) effect for JHH_NF1_PA1, Res186, and Res259 cells in clinically relevant doses of both drugs (5nM, 10nM, 20nM). BT66_SV40/hTERT cells have a significant reduction in cell growth under TAK228 treatment after 4 days by up to 70% (p&lt;0.001; via one-way ANOVA), but not under trametinib treatment. Interestingly for this cell line, the MAPK pathway was inactivated in all tested trametinib doses (≥1nM) and in combination treatment. In all cell lines trametinib treatment leads to a pERK inactivation at low nM levels. TAK228 leads to an inactivation of mTORC1 and mTORC2 in all four tested cells lines. In TAK228 treated cells, there was compensatory activation of pERK, which was reduced when trametinib was added. Apoptosis induction was verified through cleaved PARP via western blot and CC-3 via immunocytochemistry. The combination of TAK228 and trametinib increased apoptosis by up to 127% (p&lt;0.001). After determination the optimal dosing schedule for TAK228 (1mg/kg/every other day), trametinib (3mg/kg/daily) and combination, BT40 transplanted nude mice are investigated for tumor size and survival. Our results show that PLGG-derived cell lines are sensitive to TAK228 and trametinib treatment. The increased MAP kinase activity we identified after TAK228 treatment, suggests a compensatory mechanism that may render these cells especially sensitive to treatment with both TORC1/2 and MEK inhibitors. The ongoing in vivo experimentation will provide a pre-clinical rationale for combination therapy of these agents in PLGG.

Citation Format: Antje Arnold, Ming Yuan, Fausto J. Rodriguez, Charles G. Eberhart, Eric H. Raabe. Synergistic growth inhibitor effect on a patient derived NF1 pilocytic astrocytoma cell line with the dual mTORC1/2 inhibitor TAK228 and MEK inhibitor trametinib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4629.

The TORC1/2 inhibitor TAK228 sensitizes atypical teratoid rhabdoid tumors to cisplatin-induced cytotoxicity

Background

Atypical teratoid/rhabdoid tumors (AT/RTs) are deadly pediatric brain tumors driven by LIN28. Mammalian target of rapamycin (mTOR) is activated in many deadly, drug-resistant cancers and governs important cellular functions such as metabolism and survival. LIN28 regulates mTOR in normal cells. We therefore hypothesized that mTOR is activated downstream of LIN28 in AT/RT, and the brain-penetrating mTOR complex 1 and 2 (mTORC1/2) kinase inhibitor TAK228 would reduce AT/RT tumorigenicity.

Methods

Activation of mTOR in AT/RT was determined by measuring pS6 and pAKT (Ser473) by immunohistochemistry on tissue microarray of 18 primary AT/RT tumors. In vitro growth assays (BrdU and MTS), death assays (CC3, c-PARP by western blot), and survival curves of AT/RT orthotopic xenograft models were used to measure the efficacy of TAK228 alone and in combination with cisplatin.

Results

Lentiviral short hairpin RNA-mediated knockdown of LIN28A led to decreased mTOR activation. Primary human AT/RT had high levels of pS6 and pAKT (Ser473) in 21% and 87% of tumors by immunohistochemistry. TAK228 slowed cell growth, induced apoptosis in vitro, and nearly doubled median survival of orthotopic xenograft models of AT/RT. TAK228 combined with cisplatin synergistically slowed cell growth and enhanced cisplatin-induced apoptosis. Suppression of AKT sensitized cells to cisplatin-induced apoptosis and forced activation of AKT protected cells. Combined treatment with TAK228 and cisplatin significantly extended survival of orthotopic xenograft models of AT/RT compared with each drug alone.

Conclusions

TAK228 has efficacy in AT/RT as a single agent and synergizes with conventional chemotherapies by sensitizing tumors to cisplatin-induced apoptosis. These results suggest TAK228 may be an effective new treatment for AT/RT.