EXTH-46. IL-15 armored glypican-3 chimeric antigen receptor T cells exhibit antitumor in vitro and in murine models of glioblastoma

INTRODUCTION Glioblastoma is the most common primary brain tumor in adults and is lethal in most patients. Recently, chimeric antigen receptor T cell (CAR-T) therapy has emerged as a potential treatment option for patients with multiple malignancies, including glioblastoma. CAR-T cells directed against glypican-3 (GPC3) have shown promise in the treatment of hepatocellular carcinoma. METHODS We determined the expression level of GPC3 in seven glioma cell lines with flow cytometry. We then cocultured each cell line with varying effector to target ratios of CAR-T cells and IL.15-armored CAR-T cells (1:1, 5:1, 10:1) and performed bioluminescence- and impedance-based assays of tumor cell viability. Next, we implanted the brains of SCID mice with 5e4 tumor cells and, following confirmation of engraftment, treated the mice with intratumoral CAR-T cells, non-transduced T cells, or vehicle control. Mice were monitored for the development of neurological symptoms and survival. RESULTS GPC3 expression varied ranging from 2.1% to 68.0% in glioma cell lines. Coculture experiments demonstrated dose- and time-dependent cytotoxicity of CAR-T cells against 4 of 6 glioma cell lines including GSC 7-2 (53.0% GPC3 expression, p=0.0092), GSC 5-22 (64.4% GPC3 expression, p<0.0001), U373 (3.05% GPC3 expression, p<0.0001), and LN229 (48.8% GPC3 expression, p<0.0001). Furthermore, cytotoxicity increased with greater surface GPC3 expression. There was no cytotoxicity observed in cell line (GSC-20) with very low GPC3 positivity (2.05%) (p=0.92) and in one line (GSC-28) with high GPC3 positivity (68.0%), but known to highly express the immunosuppressive mediator TGFb (p=0.09). The treatment of orthotopic tumor-bearing mice with GPC3 CAR-T cells resulted in significantly improved survival compared to control mice (median survival 146.5 vs 81 days, p=0.0480), with three CAR-T treated mice surviving long term beyond 170 days. CONCLUSIONS GPC3 CAR-T cells show both in vitro and in vivo cytotoxicity against glioma cells. Further investigation in human subjects is planned.