Abstract 2909: ST101, a clinical CEBPβ antagonist peptide, promotes an immune-active tumor microenvironment by multiple cellular mechanisms

Abstract CCAAT/Enhancer Binding Protein β (C/EBPβ) is a transcription factor that is an established driver of cellular transformation in several cancer types with poor prognosis, including glioblastoma (GBM), triple negative breast cancer (TNBC) and non-small cell lung cancer (NSCLC). These cancers’ growth is often supported by an immunosuppressive (IS) tumor microenvironment (TME), characterized by high expression of TAMs, MDSCs, Tregs, and in the case of GBM, IS microglia. In addition to its cell-autonomous role of promoting survival and proliferation in cancer cells, C/EBPβ is also critical for IS M2 macrophage polarization, while its role in other TME populations is less understood. ST101 is a peptide antagonist of C/EBPβ that is being evaluated in a Phase 2 clinical study in patients with recurrent and newly diagnosed glioblastoma (NCT04478279). Here we explored the impact of ST101 on IS TME cell populations and on activation of cytotoxic T-cells in macrophage co-culture systems. ST101 increased the M1:M2 ratio up to 40-fold in hPBMC-derived macrophages cultured under conditions that typically drive M2 polarization. Further, ST101 promoted M2 repolarization to the M1 cell type, demonstrating that C/EBPβ is required for maintenance of the M2 program. When added to co-cultures of T cells with M2 macrophages, ST101 induced a 3-4-fold increase in intracellular IFN-γ staining indicating enhanced T cell activation. Notably, in M1 cultures ST101 further enhanced expression of the immune activity marker CD80 and T cell activation. Similar to its effect on M2 repolarization, ST101 induced activation of iPs-derived IS human microglia cultures toward an M1-like program. ST101 also impacted IS cell populations in vivo, as demonstrated in an orthotopic TNBC model in which ST101 exposure was associated with a reduction of the TAM fraction, increased intratumoral M1/M2 ratios, reduction of Tregs, increase of CD8:Treg fraction, and enhanced activity in combination with anti-PD-1. Similar observations were observed in clinical samples, where following ST101 exposure a significant decrease in expression of factors involved in M2 polarization, including CD209, SIGLEC5, and IL-24, and T cell suppression, including FoxP3 and inhibitory KIR proteins, were observed. Gene expression analysis suggests that ST101 induced a decrease in the intratumoral Treg:TIL ratio, indicating a shift towards a more immune-active TME. Overall, these results provide novel evidence for the contribution of C/EBPβ to the expression of multiple IS cancer-associated cell types and support the use of ST101 to antagonize C/EBPβ and promote an immune-active TME. Further, these data provide rational for evaluating the clinical impact of ST101 in combinations with immune checkpoint inhibitors. Citation Format: Claudio Scuoppo, Julia Diehl, Ricardo Ramirez, Mark Koester, Erin Gallagher, Siok Leong, Jerel Gonzales, Karen Mendelson, Zach Mattes, Lila Ghamsari, Gene Merutka, Abi Vainstein-Haras, Barry J. Kappel, Jim A. Rotolo. ST101, a clinical CEBPβ antagonist peptide, promotes an immune-active tumor microenvironment by multiple cellular mechanisms [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2909.