LAUSR

Background and Rationale: Glioblastoma multiforme (GBM) is the most common type of malignant glioma and has a poor prognosis, which calls for the exploration of alternative therapeutic approaches. Checkpoint blockade immunotherapies have produced significant clinical responses among a spectrum of malignancies. There are ongoing clinical trials for treatment of GBM with checkpoints inhibitors; however, it is anticipated that responses will be varied. With a preclinical GBM mouse model, we systematically evaluated the antitumor efficacy of antibodies targeting PD-1 and CTLA-4 administered as single agent monotherapy or combination in an EGFR genetically driven immunocompetent mouse model of GBM. In addition, we performed comprehensive immunophenotyping in EGFR-driven GBM. Methods: Transgenic EGFR, CDKN2A null, PTEN floxed mice with a floxed luciferase reporter were stereotactically injected intracranially with a TGFa- iCre lentivirus. Cohorts of mice were imaged with bioluminescence to detect growing tumors and were treated with murine monoclonal antibodies against PD-1, CTLA-4 and combination every 3 days for 3 doses beginning post tumor detection. Treated mice and controls were followed for overall survival and analysis of tumor infiltrating immune cells. Another cohort of mice was sacrificed 3 days post completion of treatment and immunophenotyping was performed by flow cytometry. Results: We observe that only mice treated with combination blockade of PD-1 and CTLA-4 had improved survival compared to untreated controls. Single agent monotherapies were ineffective as measured in no improvement in survival compared to controls. A small subset of mice treated with combination blockade of PD-1 and CTLA-4 displayed long term survival up to 100 days post tumor detection. Bioluminescence imaging revealed cytostatic effects in mice treated with combination blockade of PD-1 and CTLA-4 and in a smaller subset, sustained regression. Monotherapy treatment displayed continuing tumor growth similar to untreated controls. Characterization of tumor infiltrating immune cells displayed evidence that only combination treatment of PD-1 and CTLA-4 correlates with an increase in the number of CD8 T cell infiltrate and a decrease in the number of granulocytic MDSC (G-MDSC) infiltrate while monotherapy does not affect the ratio of CD8 to G-MDSC. Conclusions: Immune checkpoint blockade of both PD-1 and CTLA-4 provides survival benefit whereas monotherapies are ineffective in our genetically engineered mouse model of GBM. Studies aimed at revealing the mechanisms of immune-mediated anti-tumor activity from combination blockade are ongoing. Analyses of tumor infiltrating immune cells suggest that the ratio of CD8 to G-MDSCs correlate with efficacy of combination blockade of PD-1 and CTLA-4. Depletion experiments are ongoing to address the role of CD8 and MDSCs in mediating anti-tumor immunity upon single and combination blockade of PD-1 and CTLA-4 in EGFR-driven GBM. Citation Format: Alan T. Yeo, Alain Charest. Understanding mechanisms of checkpoint blockade in EGFR-driven glioblastoma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr A34.