EXTH-136. Combination XPO1 inhibition and cellular immunotherapy for the treatment of pediatric high-grade gliomas

Abstract Pediatric high-grade glioma (PHGG) is the most common cause of pediatric cancer death with current treatments only providing short term survival benefits in the majority of cases. Two promising approaches for PHGG treatment are the use of an inhibitor of the nuclear transporter exportin 1 (XPO1) and cellular immunotherapy using CAR-T cells targeting B7-H3. Both are separately being investigated in clinical trials for effectiveness against PHGG and there is preliminary preclinical and clinical data in adult lymphoma that the combination of XPO1 and sequential CAR-T therapy works synergistically to improve CAR-T efficacy. My hypothesis is that combining the XPO1 inhibitor selinexor along with B7-H3 CAR-T therapy will improve treatment of PHGG. To test this, I performed co-culture experiments with hemispheric PHGG neurosphere cell lines and an engineered B7-H3 + CXCR2 CAR-T (BC2 CAR). These experiments demonstrated that BC2 CAR is highly effective against hemispheric PHGG in vitro. To understand the effect of concurrent verses sequential XPO1 inhibition, I performed co-culture experiments in which the PHGG cells and BC2 CARs were co-cultured in selinexor-containing media (concurrent) verses PHGG cells pretreated with selinexor and then co-cultured with BC2 CARs (sequential). High doses of concurrent selinexor does lead to increased CAR-T PD1 expression and decreased CAR-T proliferation along with reduced tumor clearance in co-culture experiments as compared to BC2 CARs not treated with concurrent selinexor, due potentially to the adverse effect of XPO1 inhibition directly on CAR-T cells. However, sequential selinexor improves tumor clearance, improves CAR-T proliferation as compared to concurrent selinexor treatment, and has no significant effect on CAR- T exhaustion markers as compared to BC2 CARs not exposed to selinexor. Thus, selinexor may prime PHGG cells to be more sensitive to BC2 CARs. These findings make combination selinexor and BC2 CAR-T therapy a promising approach for improving outcomes in PHGG.