Abstract 4782: STACT: A novel tumor-targeting, systemically-administered delivery platform capable of targeting intractable pathways and precise immunomodulation of the tumor microenvironment
Authors: Rae Chris S. , Besprozvannaya Marina , Faulhaber John , Makarova Anastasia M. , King Beverly , Glickman Laura Hix , Thanos Christopher D. , Skoble Justin 
Published: August 2019

Abstract
We have engineered a tumor-targeted, microbial immunotherapy platform called STACT (Salmonella Typhimurium (Attenuated) and Checkpoint Therapy). The platform strain has been engineered to increase tumor-specific enrichment, reduce immunosuppressive inflammation and enhance tolerability. Upon phagocytosis by tumor-resident immune cells, the microbe delivers plasmid DNA, which can either encode inhibitory microRNAs to knockdown immune targets of interest, or encode immuno-modulatory cDNA expression cassettes, alone or in specific combinations.
The STACT platform strain is over 10,000-fold attenuated for virulence due to disruptions in the msbB and purI genes, that result in the detoxification of the surface LPS and purine/adenosine auxotrophy, respectively. The STACT platform strain required addition of exogenous adenosine or purines at concentrations found in the tumor micro-environment, but not in healthy tissue for replication in vitro. The STACT platform strain was unable to replicate significantly in infected macrophage cell lines, but was able to colonize tumors in mice and deliver functional plasmids. This strain was further engineered by precise genome deletions of the fljB and fliC genes, encoding the bacterial flagellar subunits that are strong TLR5 agonists and induce inflammasome-mediated pyroptosis in macrophages. Deletion of the flagellin genes prevented bacterial cell motility but did not affect tumor-specific colonization after IV administration, and tumor enrichment was observed at levels over 100,000 times greater than in spleens. A plasmid maintenance system was engineered by deletion of the chromosomal asd gene and complementation of the asd gene on a copy-number optimized plasmid to ensure plasmid maintenance in vivo. The asd system allowed for significantly improved plasmid retention in vivo without antibiotic selection. Furthermore, the vector incorporates immunostimulatory CpG motifs into the strain to help promote a TLR9-mediated adaptive immune response to tumor antigens. A library of inhibitory RNAi’s against a set of immuno-modulatory targets, including TREX1, PD-L1, and TGF-beta were screened for optimal knockdown of gene expression by qPCR and western blot. Pairwise combinatorial knockdown of specific targets was also observed in human cells. IV administration of STACT encoding TGF-beta RNAi demonstrated significant tumor growth inhibition in a subcutaneous tumor model.
STACT is a highly attenuated microbial immunotherapy platform engineered to deliver immunomodulatory molecules to phagocytic cells in the tumor microenvironment after systemic administration. This platform can be engineered to knock down combinations of immune checkpoints or express immunostimulatory genes in a single therapeutic modality.
Citation Format: Chris S. Rae, Marina Besprozvannaya, John Faulhaber, Anastasia M. Makarova, Beverly King, Laura Hix Glickman, Christopher D. Thanos, Justin Skoble. STACT: A novel tumor-targeting, systemically-administered delivery platform capable of targeting intractable pathways and precise immunomodulation of the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4782.