(726f) Testing the Intratumoral Targeting Efficacy of Trg Mutant Salmonella Typhimurium In Vivo

Motile Salmonella typhimurium have been known to preferentially colonize tumors over normal tissue and actively penetrate into regions beyond the reach of passively diffusing drugs. We have previously shown in tumor cylindroids that S. typhimurium lacking ribose chemoreceptors specifically localized in quiescent regions, the hosts of viable, therapeutically-resistent cancer cells, exhibiting great potential as precursors for bacterial anti-cancer therapy. We hypothesized that this was because they could not sense the ribose gradient present in quiescent tumor regions that normally drives wild-type bacteria further into distant necrotic regions. To evaluate the intratumoral targeting efficacy of ribose receptor deficient S. typhimurium in vivo, we deleted the ribose receptor gene trg in the attenuated strain VNP20009, and intravenously administered the trg mutant bacteria and VNP20009 into 4T1 murine breast cancer model. Tumor samples were harvested and embedded in paraffin two days after infection. Immunohistochemistry were performed on the paraffin sections to identify S. typhimurium and functional vascular. The RGB images of these sections were then analyzed to quantify the distances between bacteria and their closest surrounding vascular. We found that both the trg mutant S. typhimurium and VNP20009 were concentrated in necrotic tissue, surrounded by infiltrated neutrophils, with the former penetrating longer distances from vasculature and covering larger fractions of tissue. One mechanism to explain this colonization pattern is that prolonged bacteria presence and proliferation inside viable tissue could cause cell death and transform it into necrotic tissue. Our in vivo findings are clearly further evidences of trg mutant S. typhimurium's ability to target viable tumor tissue.