EGFRvⅢ-targeted immunotoxin combined with temozolomide and bispecific antibody for the eradication of established glioblastoma

EGFRvⅢ is an established target for immunotherapy of glioblastoma (GBM). Current study aims to explore the efficacy of EGFRvⅢ-targeted immunotoxin combined with temozolomide (TMZ) or T cell-engaged bispecific antibody for the treatment of GBM. We generated three rabbit monoclonal antibodies (R1, R2, and R6) that specifically bound to EGFRvⅢ, but not EGFR, with high affinity. Immunotoxins were made by fusing the scFv of these antibodies with engineered Pseudomonas exotoxin PE24. The in vitro cytotoxicity and specificity of the immunotoxins was rigorously validated by EGFRvⅢ and EGFR-expressed cell lines. The in vivo efficacy of immunotoxin monotherapy and in combination with TMZ or EGFRvⅢ-targeted bispecific antibody was evaluated in orthotopic and subcutaneous xenograft mouse models. EGFRvⅢ immunotoxins potently killed U87, U251 and GL261 cells that were forcefully expressing EGFRvⅢ, with IC50 values bellow 1.2 ng/ml. In a subcutaneous model, multiple intratumoral injections of immunotoxin at a dose of 2 mg/kg resulted in complete tumor regression in 3/5 of mice. In a C57BL/6 orthotopic glioblastoma model transplanted with GL261 cells that expressed a mouse version of EGFRvⅢ, two injections of 10 micrograms of immunotoxin in the lateral ventricles significantly improved the survival, with 2/5 mice being completely cured. Furthermore, in a subcutaneous xenograft model transplanted with EGFRvⅢ-expressed U87 cells, a single intratumoral injection of immuntoxin followed by i.v. injections of TMZ or EGFRvⅢ-targeted bispecific antibody achieved complete regression in mice. Taken together, EGFRvⅢ immunotoxin combined with TMZ or T cell-engaged bispecific antibody offers promise for curative treatment of GBM.