*2.3.4 Chimeric antigen receptors T cells (CAR T cells)*

A major recent advancement in adoptive cellular therapies has been the development of chimeric antigen receptors as a means for T cells to bypass MHC restriction, and dependence and have specificity for a cell surface antigen. CAR T cell therapy recently received approval targeting CD19 in B cell leukemia and lymphoma [114]. CAR T cells are genetically modified to express an extracellular single-chain variable fragment that specifically recognizes a tumor cell's surface antigen. The extracellular binding fragment is bound to intracellular signaling domains and/or co-stimulatory domains that allow for T cell activation when the fragment is bound to its cognate antigen. CAR T cells have the advantage of recognizing target antigens independent of HLA and also disregarding tumor cell immunoevasion by MHC expression reduction.

A phase I safety study was conducted using autologous CAR T cells targeting EGFRvIII in 10 recurrent EGFRvIII+ GBM patients [115]. The median OS was 251 days (~8 months; PFS could not be calculated due to the confounding factor of neurosurgical intervention in most of the patients). No specific historical controls were mentioned though the authors stated that GBM patients with significant residual disease after surgery have an average survival that is around ~6 months. The group demonstrated that EGFRvIII specific CAR T cells were found in the brain tumor and exerted antigen-directed activity. They also found that most of the patients had decreased expression of EGFRvIII in tumors resected after CAR T therapy [116].

Another member of the family of EGFR-related receptor tyrosine kinase is HER2. HER2 is commonly overexpressed in high-grade gliomas [117–120]. A phase I dose-escalation study was initiated to assess the safety and antitumor efficacy of autologous HER2-specific CAR T cells in 17 patients with progressive recurrent GBM [121]. This study found that though HER2-specific CAR T cells did not expand, they were detected in peripheral blood for up to 12 months. They found that eight patients had clinical benefit from either partial response or stable disease. The median OS was 11.1 months from the first CAR T cell infusion and 24.5 months from diagnosis with an 18-month OS of 29.4% [122]. As a comparator, this study mentions achieving similar outcomes as another study that used bevacizumab and lomustine where the median OS was 12 months with an 18-month OS of 20% [123].

Similar to the aforementioned peptide and DC vaccines, there are CAR T approaches targeting IL-13Rα2 due to its expression in a majority of adult and pediatric GBM tumors but not in normal brains [124, 125]. One group demonstrated that administration of IL-13Rα2-specific CAR T cells was feasible and showed evidence for transient anti-glioma responses in two out of three patients with recurrent GBM [126, 127]. The same group has initiated an ongoing phase I study utilizing IL-13Rα2-specific CAR T cell administration into the resected tumor cavity and the ventricular system in patients with recurrent or refractory malignant glioma [128]. A case report derived from this phase I study observed regression of all CNS tumors along with concomitant increases in cytokines and immune cells in the cerebrospinal fluid. Subsequent relapse was later found to be due to IL-13Rα2 negative tumors [129].

These studies demonstrate the barriers found in targeting single antigens in a highly heterogenous tumor. Newer approaches for enhanced CAR T therapy efficacy will require targeting multiple antigens, a combinatorial approach with other immunotherapies, or the development of CAR T cell designs that induce significant epitope spreading [20]. Aside from antigen target constructs, current work in CAR T therapy looks toward maximizing and maintaining the activity of the administered CAR T cells to overcome barriers in the solid tumor microenvironment [130]. As mentioned with cancer vaccines, the benefit will likely occur with the combination of CAR T therapy and immune checkpoint blockade. Another strategy is to express chemokine receptors in CAR T cells to improve their tumordirected trafficking (discussed below) or, conversely, express blocking chemokines and receptors expressed by tumor cells to inhibit recruitment of inhibitory immune cells. Another strategy is disrupting the tumor vasculature with anti-VEGFR CAR T therapy. Strategies are also looking into the combination of depleting immuneinhibitory cells to then allow for CAR T therapies to maintain durable responses. Though CAR T therapy remains a promising therapy for GBM, further work is needed to lead to clinical benefit.
