EXTH-32. TARGETED STAT3 INHIBITION IMPAIRS MDSC MOBILIZATION AND RECRUITMENT IN GLIOBLASTOMA

Abstract

Abstract BACKGROUND Glioblastoma tumors (GBM) are comprised upwards of 30 percent by glioma-associated myeloid cells (GAMs). A large fraction of GAMs are myeloid-derived suppressor cells (MDSC) thought to traffic in from peripheral circulation. MDSCs foster a hospitable environment for cancer cells by regulating immune suppression and driving resistance to immunotherapy. While signal transducer and activator of transcription 3 (STAT3) is considered to be a specific marker for MDSCs and is a known player in their phenotypic polarization in cancer, we sought to closely examine its specific role in MDSC trafficking and tumor recruitment. HYPOTHESIS Targeted inhibition of STAT3 blocks MDSC from the bone marrow, mitigating their accumulation within GBM tumors. METHODS Human and murine GBM samples were analyzed for immune-specific STAT3 phosphorylation (P-STAT3). Immune cell characterization and quantification in peripheral and GBM tissue from syngeneic murine models treated with a novel STAT3 inhibitor (LLL12B) was done. Therapeutic responses to LLL12B alone and in combination with immune checkpoint blockade were evaluated. RESULTS We found P-STAT3 to be predominantly expressed by tumor-associated immune cells in both human and murine GBM, particularly MDSCs. Circulating numbers of MDSCs dramatically increase in response to tumor burden. P-STAT3 is elevated in these cells, indicating that its activation occurs prior to tumor infiltration. LLL12B blockade of STAT3 significantly reduced the number of circulating and tumor-infiltrating MDSCs. As a single agent LLL12B reduced tumor volume and extended the survival of murine GBM models. When combined with anti-PD-1 checkpoint blockade, we observed durable survival in a significant fraction of treated mice. CONCLUSIONS These findings advocate a critically important role for STAT3 in regulating MDSC mobilization and trafficking to GBM tumors. It additionally reveals a new therapeutic mechanism of action for LLL12B that may be relevant for other direct inhibitors of STAT3, influencing their clinical applicability to treat GBM.