EXTH-42. COMBINATION OF MAPK PATHWAY INHIBITORS AND IMMUNE CHECKPOINT BLOCKADE IN BRAF-MUTANT HIGH-GRADE GLIOMA

BACKGROUND

Despite successes, clinical MAPK pathway inhibitors show limited anti-tumor activity in the majority of patients with BRAF-mutant high-grade glioma. Because of the presence of higher fraction of CD8+ tumor-infiltrating T cells in MAPK pathway-altered glioma, we explored the possibility that combined BRAF and MEK inhibition with immune checkpoint blockade enhances anti-tumor response.

METHODS

We engineered mice to carry BRAF V600E expression and CDKN2A deletion in various hemispheric areas. We treated syngeneic tumor-bearing mice with dabrafenib, trametinib, anti-PD-L1 and anti-CTLA-4 antibodies, and analyzed the tumor immune infiltrate by high-dimensional single-cell mass cytometry (CyTOF). RNA sequencing and Gene Set Enrichment Analysis were conducted using patient-derived BRAF-mutant glioma lines upon the inhibitor treatment.

RESULTS

The transcriptome analysis demonstrated that antigen processing and presentation feature is strongly enriched upon dual MAPK pathway inhibition. Consistent with these molecular changes, dabrafenib and trametinib treatment led to dynamic changes in tumor-infiltrating immune cells, including CD8+ and CD4+ T cells. In line with this, combination of MAPK pathway and immune checkpoint inhibitors elicit a significant survival benefit over MAPK pathway inhibition alone in mice with orthotopic BRAF-mutant glioma.

CONCLUSIONS

Clinically relevant molecular targeted therapy by dabrafenib and trametinib and immune checkpoint blockade synergize in pre-clinical models.

Abstract 3072: Clinically relevant MAPK pathway inhibition reverses stem cell fate defects and sensitize BRAF mutant glioma to immune modulatory therapies

Pediatric low-grade gliomas (pLGG) are the most common central nervous system (CNS) tumor in children. Although the majority of pLGG has unregulated MAPK pathway activity, and clinical MAPK pathway inhibitors are available, pLGG treatment remains challenging. Tumor progression, rebound, and therapy resistance limit the success of MAPK pathway inhibitors, especially in high risk subgroups, such as BRAF V600E mutant CDKN2A deleted gliomas.To define mechanisms for progression and overcome resistance, we generated BRAF V600E mutant, CDKN2A deleted murine glioma models of different grades and drug sensitivities. Here, we use genetically engineered mice to target expression of BRAF V600E and deletion of CDKN2A to various germinal and non-germinal areas, assessing neuro-anatomical aspects of oncogene sensitivity. Using neural stem cell (NSC)- and astrocyte-, and oligodendrocyte progenitor cell (OPC)-selective driver line, we investigate the likely origin for pLGG and therapy resistant glioma stem cells, developing models that faithfully recapitulate human tumor development. Our numerous immunocompetent, orthotopic BRAF V600E mutant glioma model derived from endogenous tumors exhibit a range of sensitivities to MAPK inhibitor therapies, allowing us to define mechanisms for therapy resistance. In syngeneic, orthotopic BRAF V600E mutant CDKN2A deleted gliomas, we define MAPK pathway inhibitor-induced-changes in the tumor microenvironment, including the immune infiltrate, and investigate immune modulatory therapy as novel treatment strategy for children with BRAF mutant gliomas. Our numerous models for BRAF mutant glioma will facilitate research on tumor development, progression and the tumor-immune interactions. Collectively, our results describe genetic alteration- and microenvironment-based mechanisms for BRAF mutant tumor initiation and progression, and suggest therapeutic combination therapies for glioma.

Citation Format: Claudia K. Petritsch, Stefan Grossauer, Wei Wang, Anne Marie Barrette, Jongwhi Park. Clinically relevant MAPK pathway inhibition reverses stem cell fate defects and sensitize BRAF mutant glioma to immune modulatory therapies [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 3072.

BIOL-05. MAPK PATHWAY INHIBITION SENSITIZES TO IMMUNOTHERAPY IN BRAF-MUTANT GLIOMAS

Background

BRAF alterations frequently occur in pediatric low-grade gliomas. Previously, we showed that dabrafenib and trametinib (D+T) that target MAPK pathway can mediate the antitumor effect in a preclinical model of BRAF-mutant glioma (PMC5342782). Here, we further investigate the effect of MAPK pathway inhibitors on cancer cells and tumor-infiltrating immune cells to maximize the therapeutic efficacy in malignant gliomas.

Methods

Drug concentrations in tumor, brain and plasma were assessed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). RNA sequencing and Gene Set Enrichment Analysis were performed using patient-derived BRAF-mutant glioma lines upon D+T treatment. Molecular profiles of drug-resistant clones were assessed for understanding of glioma heterogeneity and exploring new therapeutic targets. Results. BRAF-mutant stem-like glioma cells were particularly resistant to BRAF or MAPK inhibitor, along with aggressive phenotype in mice. LC-MS/MS showed effective D+T drug delivery in tumor regions. The transcriptome analysis demonstrated that D+T upregulate HLA molecules and downregulate immunosuppressive factors in patient-derived BRAF-mutant glioma lines. Consistent with these molecular changes, D+T led to changes in the proportions of tumor-infiltrating immune cells, including CD8+ cytotoxic T lymphocytes and FOXP3+ regulatory T cells. Furthermore, the therapeutic effect of D+T was further enhanced in combination with immune checkpoint inhibition.

Conclusions

The present study highlights the immunomodulatory activity of MAPK pathway inhibitors in BRAF-mutant gliomas.

TMOD-23. A NOVEL BRAF V600E LOW-GRADE GLIOMA MOUSE MODEL HIGHLIGHTS EXOMIC AND TUMOR IMMUNE ALTERATIONS AND DIFFERING THERAPEUTIC RESPONSES IN LOW- AND HIGH-GRADE GLIOMAS

Pediatric low-grade glioma (pLGG), the most common brain cancer in children, is difficult to treat especially at recurrence. The BRAF V600E mutation is the second most common mutation in pLGG, and in a high-risk group for progression is associated with deletion of the tumor suppressor CDKN2A. A better understanding of the factors contributing to progression, in particular the role of the immune infiltrate is needed, but studies have been hindered by the lack of low-grade glioma mouse models. We utilized transgenic mice with a cre-activatable (CA) allele of BRAF V600E to generate endogenous models for low-grade gliomas. We found that BRAF V600E expression cooperates with hemizygous CDKN2A deletion to induce low-grade gliomas, with tumors forming at a greater latency than by homozygous deletion. Cell line derivatives from low-grade lesions continue to grow slower upon orthotopic injection than those we previously derived from high-grade tumors. Murine LGG can progress to higher grade tumors within the mouse lifespan and we observe exomic changes and alterations in the tumor immune infiltrate associated with progression, the details of which will be discussed at the meeting. High-grade cells’ phenotypic changes within in vivo passage are accompanied by exomic changes. The high-grade glioma immune infiltrate is altered by dual MAPK pathway inhibition with dabrafenib and trametinib. Adding dual immune checkpoint inhibition by anti-PD-L1 and anti-CTLA-4 antibodies significantly extends survival of dabrafenib-trametinib dual treatment. Human BRAF V600E mutant tumors reportedly have a higher tumor immune infiltrate than that of BRAF wildtype gliomas, consistent with our murine RESULTS: Here we present a novel model for BRAF V600E mutant gliomas in mice that has a frequent rate of progression, similar to human BRAF V600E mutant gliomas, and an active immune infiltrate in high grade tumors which makes them susceptible to the immunostimulatory effects of dual checkpoint inhibition.

Abstract 143: MAPK pathway blockade effects on glioma stem cells and immunotherapy in BRAFV600E mutant gliomas

BRAFV600E mutations are frequent in pediatric gliomas. We previously showed antitumor activity when using BRAF and/or MEK inhibitors against BRAFV600E glioma syngeneic grafts (Grossauer et al., Oncotarget 2016;7:775839). While early clinical trial reports suggest an encouraging response rate to BRAFV600E inhibitors alone (Burger et al., Oncol Rep 2017;38:3291) and in combination with MEK inhibition (Brown et al., CNS Oncol 2017:6:291), resistance has been reported in BRAFV600E mutant melanoma. Moreover, we reported that resistance to BRAFV600E inhibitor monotherapy occurs in mouse models and is driven by stem-like cells (Lerner et al., Cancer Res 2015;75:5355). Rapid reactivation of MEK pathway and prosurvival signaling via EGFR have been implicated as mechanisms for acquired resistance (Yao et al., Oncotarget 2017;8:583). It is just unclear why stem-like cells survive treatment better than non-stem like tumor cells and how they respond to additional therapies, such as MEK inhibition but also immune checkpoint inhibition. Immunocompetent, syngeneic FVB/N mice were intracranial-engrafted with luciferase-modified BRAFV600E mutant CDKN2A-deficient murine glioma cell line 2341luc and treated with BRAFV600E and/or MEK inhibitors, either alone or in combination, and either with or without PD-L1 (Clone 10F.9G2) and CTLA-4 (Clone 9D9) mAbs. While the combination of BRAF and MEK inhibitors significantly increased the median survival from 46 to 70 days (P <0.05), additional treatment with PD-L1 and CTLA-4 mAbs further increased survival to a median of 150 days (P <0.001). Notably, PD-1 and CTLA-4 blockade, alone, only marginally increased survival to 60.5 days (P <0.05). To understand the mechanism underlying the impact on survival, we examined the tumor microenvironment using CyToF as well as T cell and CD133+ cancer stem cell composition using flow cytometry in treated tumors. Our preliminary findings showed that the survival benefit of quadruple treatment associates with an increased frequency of CD8+ cells and a decrease in T regulatory cells and cancer stem cells. In summary, we report that in BRAFV600E mutant gliomas when MAPK pathway and immune checkpoint blockades are combined, an immune profile indicative of productive tumor immunity is observed. CyToF analyses of treated and untreated tumors and RNA sequencing analyses of stem and non-stem like tumor cells from untreated and BRAFV600E inhibitor treated mice are on the way. These data provide rationale for further understanding this therapeutic approach, as well as the potential for clinical consideration in the near future.

Citation Format: Stefan Grossauer, Katharina Koeck, Malek Chouchane, Joanna J. Phillips, Claudia K. Petritsch, Theodore Nicolaides. MAPK pathway blockade effects on glioma stem cells and immunotherapy in BRAFV600E mutant gliomas [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 143.

Concurrent MEK targeted therapy prevents MAPK pathway reactivation during BRAFV600E targeted inhibition in a novel syngeneic murine glioma model

Inhibitors of BRAF^V600E kinase are currently under investigations in preclinical and clinical studies involving BRAF^V600E glioma. Studies demonstrated clinical response to such individualized therapy in the majority of patients whereas in some patients tumors continue to grow despite treatment. To study resistance mechanisms, which include feedback activation of mitogen-activated protein kinase (MAPK) signaling in melanoma, we developed a luciferase-modified cell line (2341^luc) from a Braf^V600E mutant and Cdkn2a- deficient murine high-grade glioma, and analyzed its molecular responses to BRAF^V600E- and MAPK kinase (MEK)-targeted inhibition. Immunocompetent, syngeneic FVB/N mice with intracranial grafts of 2341^luc were tested for effects of BRAF^V600E and MEK inhibitor treatments, with bioluminescence imaging up to 14-days after start of treatment and survival analysis as primary indicators of inhibitor activity. Intracranial injected tumor cells consistently generated high-grade glioma-like tumors in syngeneic mice. Intraperitoneal daily delivery of BRAF^V600E inhibitor dabrafenib only transiently suppressed MAPK signaling, and rather increased Akt signaling and failed to extend survival for mice with intracranial 2341^luc tumor. MEK inhibitor trametinib delivered by oral gavage daily suppressed MAPK pathway more effectively and had a more durable anti-growth effect than dabrafenib as well as a significant survival benefit. Compared with either agent alone, combined BRAF^V600E and MEK inhibitor treatment was more effective in reducing tumor growth and extending animal subject survival, as corresponding to sustained MAPK pathway inhibition. Results derived from the 2341^luc engraftment model application have clinical implications for the management of BRAF^V600E glioma.

Detection of Cerebral Vasospasm Following Aneurysmal Subarachnoid Hemorrhage Using Motor Evoked Potentials

BACKGROUND

Early detection of vasospasm (VS) following aneurysmal subarachnoid hemorrhage (aSAH) is vital to trigger therapy and to prevent infarction and subsequent permanent neurological deficit. Although motor evoked potentials (MEPs) are a well-established method for intraoperative detection of cerebral VS and cerebral ischemia during aneurysm surgery, there are no studies investigating the diagnostic value of MEPs for detecting delayed VS following aSAH in an intensive care unit.

OBJECTIVE

A prospective study was conceived to assess the diagnostic accuracy of MEPs in comparison with digital subtraction angiography.

METHODS

MEP threshold changes were determined in patients both with and without angiographic VS following high-grade aSAHs. Sensitivity, specificity, and the positive and negative predictive values of significant MEP threshold increases, which indicate angiographic VS, were calculated.

RESULTS

In all patients experiencing VS of the arteries supplying cerebral motor areas, a minimal MEP threshold increase of 50 mA (mean 66.25 mA) was observed, whereas a maximum MEP threshold increase of 30 mA was observed in patients without VS. Therefore, an increase from a baseline of ≥50 mA was considered significant and resulted in a sensitivity of 0.83, a specificity of 0.92, a positive predictive value of 0.83, and a negative predictive value of 0.92.

CONCLUSION

VS following aSAH can be detected accurately by using MEPs. MEPs are a feasible bedside tool for online VS detection in an intensive care unit and, therefore, may complement existing diagnostic tools.

Targeting a Plk1-Controlled Polarity Checkpoint in Therapy-Resistant Glioblastoma-Propagating Cells

The treatment of glioblastoma (GBM) remains challenging in part due to the presence of stem-like tumor-propagating cells that are resistant to standard therapies consisting of radiation and temozolomide. Among the novel and targeted agents under evaluation for the treatment of GBM are BRAF/MAPK inhibitors, but their effects on tumor-propagating cells are unclear. Here, we characterized the behaviors of CD133(+) tumor-propagating cells isolated from primary GBM cell lines. We show that CD133(+) cells exhibited decreased sensitivity to the antiproliferative effects of BRAF/MAPK inhibition compared to CD133(-) cells. Furthermore, CD133(+) cells exhibited an extended G2-M phase and increased polarized asymmetric cell divisions. At the molecular level, we observed that polo-like kinase (PLK) 1 activity was elevated in CD133(+) cells, prompting our investigation of BRAF/PLK1 combination treatment effects in an orthotopic GBM xenograft model. Combined inhibition of BRAF and PLK1 resulted in significantly greater antiproliferative and proapoptotic effects beyond those achieved by monotherapy (P < 0.05). We propose that PLK1 activity controls a polarity checkpoint and compensates for BRAF/MAPK inhibition in CD133(+) cells, suggesting the need for concurrent PLK1 inhibition to improve antitumor activity against a therapy-resistant cell compartment.

Behavioral disorders and cognitive impairment associated with cerebellar lesions

In the last decade evidence has accumulated that suggests that the cerebellum is involved not only in motor but also in behavioral and cognitive functions. A myriad of anatomical, clinical and imaging studies support that assumption. The lengthened survival of patients with cerebellar tumors has also brought an increased awareness of neurocognitive deficits to the neurooncological community. Although evidence from neurosurgical case series exists that clearly demonstrates that patients afflicted from posterior fossa tumors are at high risk for long-term cognitive or adaptive deficits, there is still a lack of systematic translational review on this issue. Accordingly a systematic review was conducted to summarize the impact of cerebellar lesions on behavior and cognition. The findings and clinical implications are discussed in the light of the recent advances in neuroimaging techniques.

Intraoperative somatosensory evoked potential recovery following opening of the fourth ventricle during posterior fossa decompression in Chiari malformation: case report

The most appropriate surgical technique for posterior fossa decompression in Chiari malformation (CM) remains a matter of debate. Intraoperative electrophysiological studies during posterior fossa decompression of Type I CM (CM-I) aim to shed light on the entity's pathomechanism as well as on the ideal extent of decompression. The existing reports on this issue state that significant improvement in conduction occurs after craniotomy in all cases, but additional durotomy contributes a further improvement in only a minority of cases. This implies that craniotomy alone might suffice for clinical improvement without the need of duraplasty or even subarachnoid manipulation at the level of the craniocervical junction. In contrast to published data, the authors describe the case of a 32-year-old woman who underwent surgery for CM associated with extensive cervicothoracic syringomyelia and whose intraoperative somatosensory evoked potentials (SSEPs) did not notably improve after craniotomy or following durotomy; rather, they only improved after opening of the fourth ventricle and restoration of CSF flow through the foramen of Magendie. Postoperatively, the patient recovered completely from her preoperative neurological deficits. To the authors' knowledge, this is the first report of significant SSEP recovery after opening the fourth ventricle in the decompression of a CM-I. The electrophysiological and operative techniques are described in detail and the findings are discussed in the light of available literature. The authors conclude that there might be a subset of CM-I patients who require subarachnoid dissection at the level of the craniocervical junction to benefit clinically. Prospective studies with detailed electrophysiological analyses seem warranted to answer the question regarding the best surgical approach in CM-I decompression.

Repetitive long-term hyperbaric oxygen treatment (HBOT) administered after experimental traumatic brain injury in rats induces significant remyelination and a recovery of sensorimotor function

Cells in the central nervous system rely almost exclusively on aerobic metabolism. Oxygen deprivation, such as injury-associated ischemia, results in detrimental apoptotic and necrotic cell loss. There is evidence that repetitive hyperbaric oxygen therapy (HBOT) improves outcomes in traumatic brain-injured patients. However, there are no experimental studies investigating the mechanism of repetitive long-term HBOT treatment-associated protective effects. We have therefore analysed the effect of long-term repetitive HBOT treatment on brain trauma-associated cerebral modulations using the lateral fluid percussion model for rats. Trauma-associated neurological impairment regressed significantly in the group of HBO-treated animals within three weeks post trauma. Evaluation of somatosensory-evoked potentials indicated a possible remyelination of neurons in the injured hemisphere following HBOT. This presumption was confirmed by a pronounced increase in myelin basic protein isoforms, PLP expression as well as an increase in myelin following three weeks of repetitive HBO treatment. Our results indicate that protective long-term HBOT effects following brain injury is mediated by a pronounced remyelination in the ipsilateral injured cortex as substantiated by the associated recovery of sensorimotor function.

Intraoperative neurophysiology of the motor system in children: a tailored approach

INTRODUCTION

Intraoperative neurophysiology has moved giant steps forward over the past 15 years thanks to the advent of techniques aimed to reliably assess the functional integrity of motor areas and pathways.

INTRAOPERATIVE NEUROPHYSIOLOGICAL TECHNIQUES

Motor evoked potentials recorded from the muscles and/or the spinal cord (D-wave) after transcranial electrical stimulation allow to preserve the integrity of descending pathways, especially the corticospinal tract (CT), during brain and spinal cord surgery. Mapping techniques allow to identify the motor cortex through direct cortical stimulation and to localize the CT at subcortical levels during brain and brainstem surgery. These techniques are extensively used in adult neurosurgery and, in their principles, can be applied to children. However, especially in younger children, the motor system is still under development, making both mapping and monitoring techniques more challenging. In this paper, we review intraoperative neurophysiological techniques commonly used in adult neurosurgery and discuss their application to pediatric neurosurgery, in the light of preliminary experience from our and other centers. The principles of development and maturation of the motor system, and especially of the CT, are reviewed focusing on clinical studies with transcranial magnetical stimulation.