Abstract 736: ACTM-838 safely and comprehensively re-activates the immunosuppressive TME by exploiting myeloid biology intrinsic in many cancers
Authors: Lu William , Cron Kyle , Janes Julie , Berryman Leigh , Singh Gurvinder , Thanos Christopher , Udyavar Akshata 
Published: March 2024

Abstract
ACTM-838 is a genetically modified bacterial vehicle encoding a DNA plasmid with two engineered payloads, IL-15plex and constitutively active eSTING. The bacterial vehicle is a highly attenuated S. typhimurium, engineered to facilitate safe IV administration. ACTM-838 is designed to specifically enrich in the TME via auxotrophic dependency on extracellular adenosine and purine metabolites. ACTM-838 is then selectively internalized by tumor-resident myeloid cells achieving tumor-specific payload delivery.
Previously, we showed ACTM-838 exhibited significant and durable anti-tumor efficacy across syngeneic tumor mouse models (EMT6 breast, MC38 colon) and MMTV-PyMT GEMM.
To elucidate TME changes induced by ACTM-838, EMT6 tumors were analyzed post-treatment by scRNA-seq, which demonstrated that ACTM-838 induced significant transcriptional changes in several cell populations in the TME. Cytolytic pathways were upregulated in T and NK cells, while type I IFN and PDL1 were upregulated in myeloid populations consistent with previous bulk RNAseq analyses.
To understand the tolerability and safety of ACTM-838, a GLP toxicology study was performed on naïve cynomolgus monkeys (NHP). No ACTM-838-related clinical or veterinary observations were reported at any dose level. Acute dose-dependent increases in serum cytokines were observed at 4 hours which reverted to baseline by 24 hours. ACTM-838 was cleared from blood by 24 hours post-dose with no significant shedding in urine or feces. ACTM-838 was only detected in the spleen in 2 and 1 animals at 22- and 49-days post treatment, respectively. Pre-existing ADA was observed in some animals with subsequent increases in ADA upon dosing in most animals. However, no correlation of serum ADA and blood PK levels suggested that the presence of ADA does not impact drug stability.
The ACTM-838 mechanism of action requires high levels of adenosine or purine metabolites to enrich and colonize the TME as well as phagocytic internalization by tumor-intrinsic myeloid cells to achieve tumor-specific payload delivery. Myeloid cells comprise roughly half of all cells in the TME in human tumors. Bioinformatics analyses of the public patient datasets identified key tumor indications enriched in both adenosine and myeloid pathways. To further understand the spatial heterogeneity in the TME across tumors, we performed multiplex immunofluorescence on patient tumor microarrays to validate tumor types with a high prevalence of myeloid populations coupled with an active adenosine pathway, which will help guide future clinical study design.
In summary, we demonstrate that ACTM-838 modulates the TME and is stable and safe in animal models. Additionally, our studies recommend the prioritization of certain tumor indications for treatment with ACTM-838 in the clinic.
Citation Format: William Lu, Kyle Cron, Julie Janes, Leigh Berryman, Gurvinder Singh, Christopher Thanos, Akshata Udyavar. ACTM-838 safely and comprehensively re-activates the immunosuppressive TME by exploiting myeloid biology intrinsic in many cancers [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 736.