ACTM-838, a novel systemically delivered bacterial immunotherapy that enriches in solid tumors and delivers IL-15/IL-15Rα and STING payloads to engage innate and adaptive immunity in the TME and enable a durable anti-tumor immune response

Abstract

STACT is a modular, genetically engineered live attenuated S. Typhimurium bacterial platform that enables tissue-specific localization and cell-targeted delivery of large, multiplexed payloads via systemic administration. It has been engineered to minimize systemic toxicity and to enrich in the tumor microenvironment (TME) via metabolic dependency and showed a decreased systemic inflammatory cytokine profile compared to its parent strain VNP20009. ACTM-838 utilizes the STACT platform to deliver IL-15/IL15Rα and a constitutively active STING to tumor-resident phagocytic antigen-presenting cells. Upon intravenous (IV) dosing to tumor-bearing mice, ACTM-838 distributed and enriched in the TME, exhibited specific uptake in tumor-resident phagocytic cells and led to expression of human IL-15/IL15Rα and murine IFNα in the tumor. ACTM-838 induced comprehensive TME changes to an immune permissive anti-tumor phenotype with a decrease in exhausted T-cells and Tregs and an increase in cytolytic T-cells and MHCII-high proliferating myeloid cells. ACTM-838-treated tumors exhibited upregulated anti-tumor innate and adaptive immunity expression profiles, T-, NK- and B-cell infiltration and downregulated cell cycle, DNA damage and TGFβ responses. Single-cell RNAseq and flow cytometry data confirmed activation and infiltration of both innate and adaptive immune cells. ACTM-838 showed durable anti-tumor efficacy in multiple murine tumor models and synergized with anti-PD1 therapy in combination.