CTIM-24. REPEATED PERIPHERAL INFUSIONS OF ANTI-EGFRVIII CAR T CELLS IN COMBINATION WITH PEMBROLIZUMAB REMODELS THE TUMOR MICROENVIRONMENT IN DE NOVO GLIOBLASTOMA

Abstract Treatment efficacy with chimeric antigen receptor (CAR) T cell therapy in glioblastoma (GBM) is undermined by an immunosuppressive tumor microenvironment (TME). We previously showed that CAR T cell therapy targeting epidermal growth factor receptor variant III (EGFRvIII) produces anti-tumor activity against recurrent GBM and causes upregulation of programmed death-ligand 1 (PD-L1) in the TME. Here, we conducted a phase I trial to study the safety and tolerability of CART-EGFRvIII cells administered concomitantly with the PD-1 inhibitor pembrolizumab in patients with newly diagnosed, EGFRvIII+ GBM (n = 7). Treatment was well tolerated without incidence of dose-limiting toxicity. However, no signal of efficacy was detected with a median progression-free survival of 5.2 months (90% CI, 2.9 – 6.0 months) and overall survival of 11.8 months (90 % CI, 9.2 – 14.2 months). We aimed to elucidate reasons for limited efficacy through correlative analyses. Using BBZ qPCR, we found circulating CAR T cells in 5 out of 7 patients at the time of repeat resection, but only in one patient in the tumor. However, shared TCRs were found between the infusion product and the relapsed tumors, which could indicate an infiltration but lack of persistence of the CART. We further compared the tumor microenvironment of the tumors harvested before and after CAR+aPD1 administration using single cell RNAseq, and observed comparable proportions of the major immune cell subsets. However, the myeloid and T cells infiltrating the tumors significantly evolved, with more exhausted, regulatory and IFN-stimulated T cells at the relapse. At that time, the amount of IFN-stimulated T cells positively correlated with time from relapse to death. Together, these findings suggest that the combination of CAR T cells and PD-1 inhibition in GBM is safe and biologically active but, given the lack of efficacy, also indicate a need to consider alternative immunotherapeutic strategies. ClinicalTrials.gov registration: NCT03726515.