Some strains of bacteria have unique capabilities: (1) the ability to specifically target tumors, (2) preferential growth in tumor-specific microenvironment, (3) intra-tumoral penetration, (4) native bacterial cytotoxicity. Motility is the key feature of bacterial therapies that enables intratumoral targeting. Bacteria can actively swim away from the vasculature and penetrate deep into tumor tissue. Within tumor, bacteria actively proliferate, resulting in 1000-fold or even higher increases in bacterial numbers in tumor tissue relative to normal organs. Because their genetics can be easily manipulated, bacteria can be engineered to synthesize drugs at sufficient concentrations to induce therapeutic effects. Although this strategy led to significantly greater therapeutic effects, they still have significant limitations; for example, multiple injections of bacteria are required, the tumors tend to recur quickly, and efficacy is unclear in the treatment of metastatic disease.
Our group developed an attenuated strain of S. typhimurium, which was defective in guanosine 5’-diphosphate-3’-diphosphate synthesis (ΔppGpp S. typhimurium) and genetically engineered this strain to express diverse cargo molecules that can provoke anticancer immunogenicity or direct cell killing. In this lecture, I will introduce several strategies of genetic engineering of bacteria for cancer treatment and describe unique mechanisms related to bacteria-mediated cancer therapy.
Different strategies have been used to deliver therapeutic agents such as cytotoxic proteins, cytokines, antigens, and antibodies, or genetic materials such as short hairpin RNA, to tumor tissues using engineered Salmonellae. Salmonella engineered to produce flagellin B (FlaB) of Vibrio vulnificus or cytolysin A (ClyA) of S. typhi showed excellent anticancer effects in diverse mouse tumor models, suggesting that this strategy could be applied to a wide spectrum of malignancies. This approach is based on the cooperative activity of ΔppGpp S. typhimurium and its payload, FlaB or ClyA, combined with the finding that bacterial colonization and proliferation in tumors strongly induced tumor infiltration and subsequent activation of immune cells.
Imaging strategies for bacterial trafficking have been tried using diverse technologies such as optical bioluminescence, fluorescence, PET, MRI and acoustic strategies. Bacteria expressing fluorescent proteins or bioluminescence reporters are most widely used imaging approaches. In particular, bacteria expressing bacterial luciferase (lux) provided highly sensitive and specific image qualities in mouse models. Despite the merit of optical imaging, its low potential in human application limits the use in numerous types of animal models. More translatable techniques such as PET or MRI have been explored to image bacteria. For PET imaging, bacteria were transformed with exogenous reporter gene such as herpes simplex virus type I thymidine kinase (hsv1-tk) and imaged with radioactive nucleosides such as 124I-FIAU or 18F-FHBG. Recently, bacteria-specific radiotracer, 18F-FDS (fluorodeoxy sorbitol), which accumulates in gram negative bacteria through transporter, successfully imaged bacteria without transformation with reporter gene.