Improved efficacy against malignant brain tumors with EGFRwt/EGFRvIII targeting immunotoxin and checkpoint inhibitor combinations

Background: D2C7-IT is a novel immunotoxin (IT) targeting wild-type epidermal growth factor receptor (EGFRwt) and mutant EGFR variant III (EGFRvIII) proteins in glioblastoma. In addition to inherent tumoricidal activity, immunotoxins induce secondary immune responses through the activation of T cells. However, glioblastoma-induced immune suppression is a major obstacle to an effective and durable immunotoxin-mediated antitumor response. We hypothesized that D2C7-IT-induced immune response could be effectively augmented in combination with alpha CTLA-4/alpha PD-1/alpha PD-L1 therapies in murine models of glioma. Methods: To study this, we overexpressed the D2C7-IT antigen, murine EGFRvIII (dmEGFRvIII), in established glioma lines, CT-2A and SMA560. The reactivity and therapeutic efficacy of D2C7-IT against CT-2A-dmEGFRvIII and SMA560-dmEGFRvIII cells was determined by flow cytometry and in vitro cytotoxicity assays, respectively. Antitumor efficacy of D2C7-IT was examined in immunocompetent, intracranial murine glioma models and the role of T cells was assessed by CD4+ and CD8+ T cell depletion. In vivo efficacy of D2C7-IT/alpha CTLA-4/alpha PD-1 monotherapy or D2C7-IT+alpha CTLA-4/alpha PD-1 combination therapy was evaluated in subcutaneous unilateral and bilateral CT-2A-dmEGFRvIll glioma-bearing immunocompetent mice. Further, antitumor efficacy of D2C7-IT+alpha CTLA-4/alpha PD-1/alpha PD-L1/alpha Tim-3/alpha Lag-3/alpha CD73 combination therapy was evaluated in intracranial CT-2A-dmEGFRvIII and SMA560-dmEGFRvIII glioma-bearing mice. Pairwise differences in survival curves were assessed using the generalized Wilcoxon test. Results: D2C7-IT effectively killed CT-2A-dmEGFRvIII (IC50 = 0.47 ng/mL) and SMA560-dmEGFRvIII (IC50 = 1.05 ng/mL) cells in vitro. Treatment of intracranial CT-2A-dmEGFRvIII and SMA560-dmEGFRvIII tumors with D2C7-IT prolonged survival (P = 0.0188 and P = 0.0057, respectively), which was significantly reduced by the depletion of CD4+ and CD8+ T cells. To augment antitumor immune responses, we combined D2C7-IT with alpha CTLA-4/alpha PD-1 in an in vivo subcutaneous CT-2A-dmEGFRvIII model. Tumor-bearing mice exhibited complete tumor regressions (4/10 in D2C7-IT+alpha CTLA-4 and 5/10 in D2C7-IT+alpha PD-1 treatment groups), and combination therapy-induced systemic antitumor response was effective against both dmEGFRvlll-positive and dmEGFRvIII-negative CT-2A tumors. In a subcutaneous bilateral CT-2A-dmEGFRvIII model, D2C7-IT+alpha CTLA-4/alpha PD-1 combination therapies showed dramatic regression of the treated tumors and measurable regression of untreated tumors. Notably, in CT-2A-dmEGFRvIII and SMA560-dmEGFRvIII intracranial glioma models, D2C7-IT+alpha PD-1/alpha PD-L1 combinations improved survival, and in selected cases generated cures and protection against tumor re-challenge. Conclusions: These data support the development of D2C7-IT and immune checkpoint blockade combinations for patients with malignant glioma.