Background
STACTTM is a modular, genetically engineered live attenuated bacterial platform based on S. Typhimurium (VNP20009). The platform enables tissue-specific localization and cell-targeted delivery of large, multiplexed payloads via systemic administration. STACTTM has been engineered to minimize systemic toxicity and is designed to specifically enrich in the TME via auxotrophic dependency on metabolites of the adenosine pathway, to achieve tumor-specific payload delivery. Payload expression is enabled in the bacterial vehicle or targeted human cells with the ability to encode proteins, peptides, DNA, RNA, and gene editing effectors. ACTM-838 is Actym’s first clinical asset utilizing the STACT™ platform to deliver immune-modulatory payloads, engineered IL-15/IL15Rα and a constitutively active STING, specifically to the tumor-resident phagocytic antigen-presenting cells (APCs).
Methods
Payload delivery, cytokine expression and efficacy were assessed in immune excluded checkpoint blockade refractory EMT6 TNBC tumor-bearing mice treated with STACT chassis or ACTM-838 either as single agents or in combination with anti-PD1 or anti-PDL1 (anti-PD(L)1). To study the ACTM-838-mediated kinetics and nature of TME remodeling, bulk and single-cell RNAseq was performed. Flow cytometry was conducted on the tumors, tumor-draining lymph nodes and blood.
Results
Bulk tumor RNAseq showed ACTM-838 chassis and plasmid payload RNA were detected in the TME, confirming tumor colonization and payload delivery. ACTM-838 treated EMT6 tumors exhibited upregulated pro-inflammatory responses (TLR, antigen presentation, anti-viral pathways), NK cell infiltration and downregulated cell cycle, DNA damage and TGFβ responses. Distinct TCR clonotypes were observed in ACTM-838 treated tumors versus vehicle control, showing infiltration of T cells, including / T cells. Single-cell RNAseq and flow cytometry data confirmed engagement and infiltration of both innate and adaptive immunity.
In the combination studies, anti-PD(L)1 agents exhibited low single-agent efficacy but synergistic activity when combined with ACTM-838 in the EMT6 model. The combination therapy was highly synergistic when anti-PD1 or anti-PDL1 were administered either 3 days before or the same day as ACTM-838, and correlated with lower levels of IL-6, TNFα and IL-1β being induced in combination compared to ACTM-838 alone. Ongoing tumor RNAseq analysis on anti-PD(L)1 combinations will further inform the mechanism of synergy.
Conclusions
ACTM-838 is a novel immunotherapy inducing comprehensive TME reactivation to an immune permissive anti-tumor phenotype, with activation of monocytes, macrophages, DCs and B cells and infiltration and activation of cytolytic T and NK cells. ACTM-838 monotherapy showed anti-tumor efficacy and anti-PD(L)1 combinations showed synergy. IV-delivered ACTM-838 is currently being evaluated in a phase 1 clinical trial.
Ethics Approval
All animals were used according to protocols approved by an Institutional Animal Care and Use Committee and maintained in specific pathogen-free conditions in a AAALAC accredited barrier facility.