1435 Therapeutic mRNA cancer vaccine for GBM reaches complete response in a preclinical model

<h3>Background</h3> Glioblastoma (GBM) is a fatal cancer that has limited response to chemotherapy and radiotherapy. Immunotherapy for GBM has been challenging due to low frequency of neoantigens and immunosuppressive tumor microenvironment. Here we report the efficacy of a novel EGFRvIII mRNA vaccine formulated in next generation lipid nanoparticles (LNP). EGFRvIII is a cancer-driver mutation detected in approximately 30% of GBM patients, it appears as an in-frame epidermal growth factor receptor (EGFR) deletion generating a constitutively active oncogenic protein therefore considered a neoantigen driver mutation. <h3>Methods</h3> A GBM implantable cell line was generated from C3H/HeJ mice by <i>in vivo</i> electroporation to introduce genetic alterations of EGFRvIII-OE, CDKN2A-KO and PTEN-KO into periventricular cells of early postnatal mice. Resulting tumors were propagated in vitro to create cell lines. Following orthotopic implantation in 6-week-old C3H/He mice resulted in syngeneic tumors that recapitulate histologic features of GBM and caused mortality of mice by day 30. Using this GBM model, mice were intramuscularly vaccinated with either formulation buffer (fb) or EGFRv-III mRNA-LNP on day 7or 10 post tumor implantation. Four more immunizations were done before MR imaging. Mice were monitored until day 150. Brains were fixed for histological evaluation and immuno-histochemistry to determine lymphocytes, macrophages and tumor cells. T cells from spleen, and lymph nodes were analyzed to determine KLRG1, IL-7R, PD1, Ly108, CD44 and CD62L populations by flow. Anti-EGFRvIII abs were determined by ELISA. In situ spatial profiling with 10x Xenium was done in tissues for a panel of immunological markers. <h3>Results</h3> Vaccinated mice cleared tumors with no detection by histology or MRI and stayed tumor free until day 150. In contrast, control mice were diseased by day 36. Moreover, surviving vaccinated mice re-challenged on day 42 with the tumor cells, controlled the tumor with no evidence of tumor growth until day 150. The vaccination regimen induced specific EGFRvIII T cells, high numbers of Ly108+ T cells and high amounts of anti EGFRvIII antibodies. T cells are detected in brain tissue of vaccinated mice. LNP mechanism of action involved in the tumor control will be discussed. <h3>Conclusions</h3> An mRNA vaccine encoding a driver mutation in a new LNP induce an immune response that clears tumors in 100% of mice with a high-grade glioma. The vaccine induced strong T cell and antibody responses involved in controlling tumor progression. These new LNP will help progress the development of cancer vaccines for solid tumors and difficult to treat cancers.