Optimization of Tumor Targeting Bacteria
Several studies have demonstrated the feasibility of engineered bacteria as anticancer agents, further improvements are needed to reduce the virulence of pathogens used to target tumors and to reduce their nonspecific colonization in vivo. The combination of bacteria and chemotherapeutic drugs can make cancer cells more sensitive to treatment and reduce the dose of both components, which in turn reduces toxicity and side effects.
Although tumor-targeting activity is high, attenuating virulence is critical for pathogenic strains of the genera Clostridium, Listeria, and Salmonella, which cause infection. This can be done by deleting virulence genes or creating nutrition-deficient mutations that prevent the strain from dividing itself and causing infection. For example, lipopolysaccharide (LPS), one of the most potent endotoxins produced by Gram-negative bacteria, causes systemic inflammation. In Salmonella typhimurium, deletion of the myristoyl transferase gene msbB, which encodes lipid A biosynthesis, results in altered lipid A in the LPS produced, thereby reducing the amount of TNF-α(tumor necrosis factor-α) produced in mice after bacterial administration. LPS modification significantly reduced bacterial virulence, with a 10,000-fold increase in LD50(median lethal dose) in mice. In addition, the commonly used Salmonella typhimurium VNP20009 is a purine nutrient-deficient mutant carrying the msbB deletion. This bacterium has been extensively studied in various mouse models and has been evaluated for safety in clinical trials.
On the other hand, improving the tumor-targeting specificity of chassis bacteria is essential to improve their performance as therapeutic delivery systems. This is especially important for facultative anaerobic bacteria, including Salmonella, Listeria, and Escherichia, as these microorganisms can proliferate in normal oxygenated tissues and cause tissue damage. One such strategy is to enhance the tumor binding capacity of bacteria by expressing tumor binding modules on the surface of bacteria. These binding modules (synthetic binding proteins or native bacterial adhesins) typically target surface markers that are overexpressed in cancer cells, such as αvβ3 integrin, heparan sulfate, and tumor-associated antigens. The surface presentation of these tumor-binding modules increases bacterial colonization and the enrichment of engineered bacteria in tumors.
Another strategy is to utilize tumor-specific expression systems to ensure the safety of bacteria-mediated therapeutic delivery. In these systems, payload expression is modulated by inducible promoters that respond to TME signals, including hypoxia, acidosis, or radiotherapy delivered directly to the tumor site. In other studies, researchers are also combining materials science and synthetic biology to develop bacteria-mediated photothermal therapy for cancer treatment. In some of these systems, gold nanoparticles (AUNPs) are coated on bacterial surfaces by biomineralization. The bacteria have been genetically engineered to express heat-responsive anti-tumor molecules. After the bacteria are administered and colonized in the tumor site, the tumor cells are killed by increasing the near-infrared (NIR) laser. Upon NIR irradiation, AuNP absorbs transdermal photons and converts them into local heat, which activates the expression of tumor suppressor molecules in bacteria and, together with the generated heat, induces cell death. The application of light-induced biomineralization of Escherichia coli cells with AuNP has been shown to express TNF-α in a mouse breast cancer model and ClyA in a mouse colon cancer model, resulting in significant tumor regression.
Reference
- Shen, H., Aggarwal, N., et al., Matthew Wook Chang. Engineered microbial systems for advanced drug delivery, Adv. Drug Deliv. Rev., 2022, 187, 114364.
