EXTH-30. Improving intravenous B7-H3 CAR T cell treatment for DMG using MRgFUS

Diffuse midline glioma (DMG) is one of the most lethal pediatric brain tumors with no targeted therapies approved to treat this devastating disease. One treatment being investigated for DMG is chimeric antigen receptor (CAR) T cell therapy targeting the tumor antigen B7-H3. However, the blood-brain barrier (BBB) prevents most systemically delivered CAR T cells from entering the brain. Current clinical trials overcome this using an intracerebroventricular catheter; however, implanting the catheter is both expensive and invasive, limiting accessibility to CAR T cell treatment for DMG. Magnetic resonance imaging-guided focused ultrasound (MRgFUS) is a noninvasive technique that has been proven to safely and temporarily permeabilize the BBB in a targeted area of the brain. Herein, we used a patient-derived xenograft (PDX) model of DMG to evaluate the synergism between intravenous B7-H3 targeted CAR T cell therapy and MRgFUS. Firstly, our results suggest that clinically relevant doses of B7-H3 CAR T cells administered intravenously do not provide a significant survival benefit in a PDX model of DMG. Next, we observed that our MRgFUS protocol results in spatially restricted BBB opening, as shown via MRI with a non-BBB penetrant contrast enhancing agent. We then tested different timings of MRgFUS relative to CAR T cell administration. B7-H3 CAR T cells were transduced to express firefly luciferase (ffLuc) to permit CAR T cell tracking following intravenous injection. Our results show that MRgFUS increased the trafficking of CAR T cells to the brain and that administering CAR T cells 12 hours prior to MRgFUS treatment resulted in the highest CAR T signal in the brain. This body of work supports the continued investigation of combinatorial CAR T therapy and MRgFUS for DMG. Future studies will determine if combination therapy produces a survival advantage over monotherapies and is translatable to a fully immunocompetent model of DMG.