Abstract 143: MAPK pathway blockade effects on glioma stem cells and immunotherapy in BRAFV600E mutant gliomas
Authors: Grossauer Stefan , Koeck Katharina , Chouchane Malek , Phillips Joanna J. , Petritsch Claudia K. , Nicolaides Theodore 
Published: August 2018

Abstract
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 &lt;0.05), additional treatment with PD-L1 and CTLA-4 mAbs further increased survival to a median of 150 days (P &lt;0.001). Notably, PD-1 and CTLA-4 blockade, alone, only marginally increased survival to 60.5 days (P &lt;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.