Anti-tumoral effect of the mitochondrial target domain of Noxa delivered by an engineered Salmonella typhimurium

Salmonella-Mediated Cancer Therapy: Roles and Potential

The use of bacteria for cancer therapy, which was proposed many years ago, was not recognized as a potential therapeutic strategy until recently. Technological advances and updated knowledge have enabled the genetic engineering of bacteria for their safe and effective application in the treatment of cancer. The efficacy of radiotherapy depends mainly on tissue oxygen levels, and low oxygen concentrations in necrotic and hypoxic regions are a common cause of treatment failure. In addition, the distribution of a drug is important for the therapeutic effect of chemotherapy, and the poor vasculature in tumors impairs drug delivery, limiting the efficacy of a drug, especially in necrotic and hypoxic regions. Bacteria-mediated cancer therapy (BMCT) relies on facultative anaerobes that can survive in well or poorly oxygenated regions, and it therefore improves the therapeutic efficacy drug distribution throughout the tumor mass. Since the mid-1990s, the number of published bacterial therapy papers has increased rapidly, with a doubling time of 2.5 years in which the use of Salmonella increased significantly. BMCT is being reevaluated to overcome some of the drawbacks of conventional therapies. This review focuses on Salmonella-mediated cancer therapy as the most widely used type of BMCT.2.

Combined prokaryotic-eukaryotic delivery and expression of therapeutic factors through a primed autocatalytic positive-feedback loop

Progress in bacterial therapy for cancer and infectious diseases is hampered by the absence of safe and efficient vectors. Sustained delivery and high gene expression levels are critical for the therapeutic efficacy. Here we developed a Salmonella typhimrium strain to maintain and safely deliver a plasmid vector to target tissues. This vector is designed to allow dual transcription of therapeutic factors, such as cytotoxic proteins, short hairpin RNAs or combinations, in the nucleus or cytoplasm of eukaryotic cells, with this expression sustained by an autocatalytic positive-feedback loop. Mechanisms to prime the system and maintain the plasmid in the bacterium are also provided. Synergistic effects of attenuated Salmonella and our inter-kingdom system allow the precise expression of Diphtheria toxin A chain (DTA) gene in tumor microenvironment and eradicate large established tumors in immunocompetent animals. In the experiments reported here, 26% of mice (n=5/19) with aggressive tumors were cured and the others all survived until the end of the experiment. We also demonstrated that ST4 packaged with shRNA-encoding plasmids has sustained knockdown effects in nude mice bearing human MDA-MB-231 xenografts. Three weeks after injection of 5×10(6) ST4/pIKT-shPlk, PLK1 transcript levels in tumors were 62.5±18.6% lower than the vector control group (P=0.015). The presence of PLK1 5' RACE-PCR cleavage products confirmed a sustained RNAi-mediated mechanism of action. This innovative technology provides an effective and versatile vehicle for efficient inter-kingdom gene delivery that can be applied to cancer therapy and other purposes.

Salmonella typhimurium Suppresses Tumor Growth via the Pro-Inflammatory Cytokine Interleukin-1β

Although strains of attenuated Salmonella typhimurium and wild-type Escherichia coli show similar tumor-targeting capacities, only S. typhimurium significantly suppresses tumor growth in mice. The aim of the present study was to examine bacteria-mediated immune responses by conducting comparative analyses of the cytokine profiles and immune cell populations within tumor tissues colonized by E. coli or attenuated Salmonellae. CT26 tumor-bearing mice were treated with two different bacterial strains: S. typhimurium defective in ppGpp synthesis (ΔppGpp Salmonellae) or wild-type E. coli MG1655. Cytokine profiles and immune cell populations in tumor tissue colonized by these two bacterial strains were examined at two time points based on the pattern of tumor growth after ΔppGpp Salmonellae treatment: 1) when tumor growth was suppressed ('suppression stage') and 2) when they began to re-grow ('re-growing stage'). The levels of IL-1β and TNF-α were markedly increased in tumors colonized by ΔppGpp Salmonellae. This increase was associated with tumor regression; the levels of both IL-1β and TNF-α returned to normal level when the tumors started to re-grow. To identify the immune cells primarily responsible for Salmonellae-mediated tumor suppression, we examined the major cell types that produce IL-1β and TNF-α. We found that macrophages and dendritic cells were the main producers of TNF-α and IL-1β. Inhibiting IL-1β production in Salmonellae-treated mice restored tumor growth, whereas tumor growth was suppressed for longer by local administration of recombinant IL-1β or TNF-α in conjunction with Salmonella therapy. These findings suggested that IL-1β and TNF-α play important roles in Salmonella-mediated cancer therapy. A better understanding of host immune responses in Salmonella therapy may increase the success of a given drug, particularly when various strategies are combined with bacteriotherapy.

Potent and tumor specific: arming bacteria with therapeutic proteins

Bacteria are perfect vessels for targeted cancer therapy. Conventional chemotherapy is limited by passive diffusion, and systemic administration causes severe side effects. Bacteria can overcome these obstacles by delivering therapeutic proteins specifically to tumors. Bacteria have been modified to produce proteins that directly kill cells, induce apoptosis via signaling pathways, and stimulate the immune system. These three modes of bacterial treatment have all been shown to reduce tumor growth in animal models. Bacteria have also been designed to convert nontoxic prodrugs to active therapeutic compounds. The ease of genetic manipulation enables creation of arrays of bacteria that release many new protein drugs. This versatility will allow targeting of multiple cancer pathways and will establish a platform for individualized cancer medicine.

L-Asparaginase delivered by Salmonella typhimurium suppresses solid tumors

Bacteria can be engineered to deliver anticancer proteins to tumors via a controlled expression system that maximizes the concentration of the therapeutic agent in the tumor. L-asparaginase (L-ASNase), which primarily converts asparagine to aspartate, is an anticancer protein used to treat acute lymphoblastic leukemia. In this study, Salmonellae were engineered to express L-ASNase selectively within tumor tissues using the inducible araBAD promoter system of Escherichia coli. Antitumor efficacy of the engineered bacteria was demonstrated in vivo in solid malignancies. This result demonstrates the merit of bacteria as cancer drug delivery vehicles to administer cancer-starving proteins such as L-ASNase to be effective selectively within the microenvironment of cancer tissue.

Engineered bacteria as therapeutic agents

Although bacteria are generally regarded as the causative agents of infectious diseases, most bacteria inhabiting the human body are non-pathogenic and some of them can be turned, after proper engineering, into 'smart' living therapeutics of defined properties for the treatment of different illnesses. This review focuses on recent developments to engineer bacteria for the treatment of diverse human pathologies, including inflammatory bowel diseases, autoimmune disorders, cancer, metabolic diseases and obesity, as well as to combat bacterial and viral infections. We discuss significant advances provided by synthetic biology to fully reprogram bacteria as human therapeutics, including novel measures for strict biocontainment.

Impediments to enhancement of CPT-11 anticancer activity by E. coli directed beta-glucuronidase therapy

CPT-11 is a camptothecin analog used for the clinical treatment of colorectal adenocarcinoma. CPT-11 is converted into the therapeutic anti-cancer agent SN-38 by liver enzymes and can be further metabolized to a non-toxic glucuronide SN-38G, resulting in low SN-38 but high SN-38G concentrations in the circulation. We previously demonstrated that adenoviral expression of membrane-anchored beta-glucuronidase could promote conversion of SN-38G to SN-38 in tumors and increase the anticancer activity of CPT-11. Here, we identified impediments to effective tumor therapy with E. coli that were engineered to constitutively express highly active E. coli beta-glucuronidase intracellularly to enhance the anticancer activity of CPT-11. The engineered bacteria, E. coli (lux/βG), could hydrolyze SN-38G to SN-38, increased the sensitivity of cultured tumor cells to SN-38G by about 100 fold and selectively accumulated in tumors. However, E. coli (lux/βG) did not more effectively increase CPT-11 anticancer activity in human tumor xenografts as compared to non-engineered E. coli. SN-38G conversion to SN-38 by E. coli (lux/βG) appeared to be limited by slow uptake into bacteria as well as by segregation of E. coli in necrotic regions of tumors that may be relatively inaccessible to systemically-administered drug molecules. Studies using a fluorescent glucuronide probe showed that significantly greater glucuronide hydrolysis could be achieved in mice pretreated with E. coli (lux/βG) by direct intratumoral injection of the glucuronide probe or by intratumoral lysis of bacteria to release intracellular beta-glucuronidase. Our study suggests that the distribution of beta-glucuronidase, and possibly other therapeutic proteins, in the tumor microenvironment might be an important barrier for effective bacterial-based tumor therapy. Expression of secreted therapeutic proteins or induction of therapeutic protein release from bacteria might therefore be a promising strategy to enhance anti-tumor activity.

Improved cytotoxic effects of Salmonella-producing cytosine deaminase in tumour cells

In order to increase the cytotoxic activity of a Salmonella strain carrying a salicylate-inducible expression system that controls cytosine deaminase production, we have modified both, the vector and the producer bacterium. First, the translation rates of the expression module containing the Escherichia coli codA gene cloned under the control of the Pm promoter have been improved by using the T7 phage gene 10 ribosome binding site sequence and replacing the original GUG start codon by AUG. Second, to increase the time span in which cytosine deaminase may be produced by the bacteria in the presence of 5-fluorocytosine, a 5-fluorouracyl resistant Salmonella strain has been constructed by deleting its upp gene sequence. This new Salmonella strain shows increased cytosine deaminase activity and, after infecting tumour cell cultures, increased cytotoxic and bystander effects under standard induction conditions. In addition, we have generated a purD mutation in the producer strain to control its intracellular proliferation by the presence of adenine and avoid the intrinsic Salmonella cell death induction. This strategy allows the analysis and comparison of the cytotoxic effects of cytosine deaminase produced by different Salmonella strains in tumour cell cultures.