Abstract B028: Bacterial cytotoxin therapy limits tumor growth for pancreatic ductal adenocarcinoma
Authors: Decker Amanda R. , Harimoto Tetsuhiro , Sastra Steve A. , Danino Tal , Olive Kenneth 
Published: November 2022

Treating pancreatic ductal adenocarcinoma (PDAC) with systemic chemotherapeutic drugs has remained a challenge, due in part to the hypovascularized and poorly perfused nature of PDAC tumors, impeding the accumulation of systemically delivered drugs. Several clinical trials aimed at improving drug delivery in PDAC, through targeting of ECM components (HALO-301) or stromal angiogenic signaling (IPI-926-03) have unfortunately not been effective. However, the features that have interfered with systemic therapy in PDAC are potential advantages for the use of bacterial therapies, as bacteria can actively migrate through tissues, thrive in hypoxic microenvironments, and benefit from local immune suppression. Recent developments in the field of synthetic biology have made it possible to engineer complex logic circuits into bacteria, enabling the production of anticancer therapies directly within the tumor parenchyma. Furthermore, live bacteria, once colonized within the tumor niche, are capable of providing a stable source of anticancer compounds directly, rather than relying on repeated systemic doses. We have therefore worked to develop novel bacterial strains and demonstrate preclinical efficacy of a novel strain of therapeutic bacteria for targeting PDAC. We began by testing a range of bacteria-produced toxins and identified the pore-forming protein theta toxin as having the greatest effect in both 2D cell culture and PDAC explant (tissue slice) models. We then engineered a non-toxic probiotic bacteria, E. coli Nissle 1917, to produce either theta toxin or GFP following induction with acyl-homoserine lactone (AHL). To assess preclinical efficacy, we performed intratumoral injections of live GFP- and theta-expressing bacteria into the “KPC” genetically engineered mouse model (Kras LSL.G12D/+; Tp53 LSL.R172H/+; PdxCre tg/+). While GFP-producing bacteria did not induce a change in tumor growth kinetics, treatment with theta toxin-producing bacteria demonstrated prolonged stabilization of tumor growth, increasing the doubling time from 13.7 days (GFP) to 32.5 days (theta) without additional therapy. Indeed, one theta-treated KPC animal lived 113 days following a single bacterial injection, compared to a median of ~12 days for vehicle- or gemcitabine-treated historical controls. Histological analyses demonstrated that diffuse populations of bacteria co-localized with regions of tumor necrosis and cell death, but that bacterial presence and evidence of increased cell death was not observed in healthy tissues, such as the lung, liver, intestine, and diaphragm. Strikingly, while there was minimal spread of bacteria to non-tumor tissues, we observed translocation of the bacteria to regions of liver metastases and distant papillomas following injection of the primary pancreatic tumor, suggesting a mechanism for targeting both known and unknown metastases following local administration. Together these studies demonstrate potent preclinical activity of cytotoxic bacterial therapy as a novel strategy to circumvent the challenges of systemic treatment of PDAC.

Citation Format: Amanda R. Decker, Tetsuhiro Harimoto, Steve A. Sastra, Tal Danino, Kenneth Olive. Bacterial cytotoxin therapy limits tumor growth for pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr B028.