IMPS-38: CETUXIMAB THERAPY REQUIRES IMMUNE STIMULATION FOR EFFICACY IN PEDIATRIC GLIOBLASTOMA

Pediatric glioblastoma (GBM) patients have <10% 5-year survival rate, and numerous experimental therapeutic approaches have failed to significantly improve survival. Effective therapies will therefore require a better understanding of the tumor and its environment. Myeloid cells and t-cells are increased in GBM compared to normal brain tissue, and have been shown to be immunosuppressive. This immunosuppressive tumor microenvironment will negatively impact immunotherapeutic approaches, such as EGFR-targeted antibody treatments, notably EGFR-targeted cetuximab and nimotuzumab. This class of antibodies has shown modest activity in pediatric GBM and we propose that adjuvant immune stimulatory treatment, to reverse inherent immunosuppression, will be required for improved efficacy. Here we investigate the effect of cetuximab in GBM cell line/immune effector cell co-cultures with and without immune stimulating adjuvants, IL-2 and GM-CSF. Different GBM cell lines were co-cultured with HLA-matched allogenic peripheral blood mononuclear cells and treated with cetuximab, GM-CSF, IL-2, or combinations of these. Cetuximab alone had no effect on cytotoxicity. However, addition of immune stimulants to cetuximab resulted in cytotoxicity in GBM cells as measured by accumulation of cleaved caspase 3/7. Combination of cetuximab with both GM-CSF and IL-2 resulted in the greatest cytotoxicity. Additionally, and ex-vivo pediatric GBM organotypic slice culture model was used to better evaluate immunotherapeutic strategies. Organotypic slice cultures contain patient, therefore autologous, infiltrating myeloid cells and t-cells, providing a clinically relevant model of GBM. The slices were treated for 10 days and followed by flow cytometric measurement of proliferation and cell death. Cetuximab alone reduced growth of tumor cells and also reduced cell death. Combination therapy of GM-CSF, IL-2, and cetuximab, however, increased immune cell proliferation, and reduced tumor cell proliferation and increased tumor cell cytotoxicity. We suggest that cetuximab, along with any other therapeutic antibody approach, requires combination immune stimulation in the treatment of GBM.