EXTH-122. A novel S100A4/TFR bispecific antibody to reprogram the GBM tumor microenvironment and reactivate anti-tumor immunity

Glioblastomas (GBM) are the most common and aggressive primary adult brain tumors, and advances in immunotherapy have failed to significantly improve patient outcomes. The presence of the blood brain barrier (BBB), abundance of immunosuppressive myeloid cells, and relative paucity of cytotoxic lymphocytes are major challenges preventing immunotherapy success in GBM. We previously identified the small calcium-binding protein S100A4 as a major regulator of GBM immune suppression. Targeting S100a4 via genetic depletion in tumor or stromal cells is sufficient to reprogram the tumor immune-landscape and extend survival of GBM-bearing mice. We recently developed an S100A4 blocking monoclonal antibody to target S100A4 signaling in the tumor microenvironment; however, this antibody has limited BBB penetrance. To overcome this challenge, we generated a bispecific S100A4-Transferrin-Receptor (TFR) antibody (BsA) that utilizes TFR-mediated transcytosis to cross the BBB. We assessed efficacy of BsA treatment in the preclinical syngeneic S100ß-vErbB;p53 (SVP) GBM mouse model. Using ELISA, confocal immunofluorescent microscopy, and bulk RNA-sequencing, we report that the BsA treatment accumulates in the CNS at nearly twenty-fold the concentration of the monoclonal antibody and reprograms the tumor immune microenvironment to promote immune cell infiltration and pro-inflammatory activation. BsA monotherapy modestly extends survival in the SVP model in an immune-cell dependent manner, as treatment in SVP-tumor bearing NSG mice abolishes this survival benefit. We further assessed if BsA treatment sensitizes immune cold GBMs to immune checkpoint blockade. Combination with αPD-1 checkpoint-blockade nearly doubled median overall survival in the SVP mouse model, with increased tumor infiltrating reactive CD4 and cytotoxic CD8 T cells. Single cell RNA-sequencing revealed an enhanced interferon-γ response signature in GBM cells and macrophages from combination therapy treated-tumors, with an accompanying upregulation of MHC-I and MHC-II associated genes, respectively. These results indicate that the BsA is a promising agent to reprogram the GBM immune-landscape and reactivate anti-tumor immunity.