IMMU-18. TARGETING GD2 IN THALAMIC DIFFUSE MIDLINE GLIOMA WITH TRANSIENT MRNA CAR T CELLS

Chimeric antigen receptor (CAR) T cells targeting the disialoganglioside GD2 have shown promise as a therapeutic for diffuse midline glioma (DMG). However, thalamic tumors have been excluded from clinical trials due to prior studies showing neurotoxicity and fatality when using virally transduced CAR T cells. We hypothesized repeated dosing of transient GD2-directed mRNA CAR T cells could be employed for safe and effective treatment of thalamic DMG. GD2-directed CAR T cells were created using mRNA and first tested against the murine thalamic DMG xenograft 7316-6349 via locoregional delivery with an indwelling catheter for repeated dosing. The previously reported fatal neurotoxicity observed in mice using lentiviral CAR T cells could be recapitulated with aggressive dosing. Four doses of 5x10(6) mRNA CAR T cells delivered intratumorally twice a week resulted in median overall survival of 9 days for GD2-treated mice compared to >30 days for CD19-treated controls (p<0.01). This toxicity could be avoided by decreasing the dose and timing of infusions to 2x10(6) mRNA CAR T cells delivered once weekly. Bioluminescent imaging showed regression of tumor in GD2-treated mice compared to CD19-treated controls (radiance fold change -3x10(6) versus +20x10(6) p/sec/cm2/sr, p<0.01). To assess survival before the onset of graft-versus-host disease, we utilized the aggressive hypermutant thalamic DMG model 7316-3058, treating with 2x10(6) CAR T cells intratumorally once weekly. Overall survival was significantly prolonged for 7316-3058 mice treated with GD2-directed mRNA CAR T cells compared to CD19-directed controls (median OS 48 days versus 33 days, p=0.03). Notably, non-tumor bearing mice treated with GD2-directed CAR T cells developed fatal neurotoxicity within 14 days, suggesting a very narrow therapeutic window in the brain. These data highlight the utility of titratable mRNA-based CAR T cell therapy for CNS tumors and establish GD2-directed mRNA CAR T cells as a safe and effective method for treating thalamic DMG.