Targeting Cancer with Amino-Acid Auxotroph Salmonella typhimurium A1-R

Abstract

Salmonella, Clostridium and Bifidobacterium have been shown to ­control tumour growth and promote survival in animal models. However, Clostridium and Bifidobacterium are obligate anaerobes which limits their growth to the necrotic region of tumour and thus limits their effectiveness. In contrast, Salmonella is a facultative anaerobe which can grow in the viable as well as necrotic regions of tumours giving it greater potential as an anti-tumour agent. However, previous experiments with Salmonella, including clinical trials, have used over-attenuated mutants, limiting the anti-tumour efficacy. We have developed an effective bacterial cancer therapy strategy using Salmonella typhimurium auxotrophs which grow in viable as well as necrotic areas of tumours. The auxotrophy severely restricts growth of these bacteria in normal tissue. The S. typhimurium A1-R mutant, which is auxotrophic for leu-arg, has high anti-tumour virulence. In vitro, A1-R infects tumour cells and causes nuclear destruction. A1-R was initially used to treat metastatic human prostate and breast tumours that had been orthotopically implanted in nude mice. Forty percent of treated mice were cured completely and survived as long as non-tumour-bearing mice. A1-R administered i.v. to nude mice with primary osteosarcoma and lung metastasis was highly effective, especially against metastasis. A1-R was also targeted to both axillary lymph and popliteal lymph node metastasis of human pancreatic cancer and fibrosarcoma, respectively, as well as lung metastasis of the fibrosarcoma in nude mice. The bacteria were delivered via a lymphatic channel to target the lymph node metastases and systemically via the tail vein to target the lung metastasis. The metastases were cured without the need of chemotherapy or any other treatment. A1-R was administered intratumorally to nude mice with an orthotopically transplanted human pancreatic tumour. The primary pancreatic cancer regressed without additional chemotherapy or any other treatment. A1-R was also effective against pancreatic cancer liver metastasis when administered intrasplenically to nude mice. A1-R was also highly effective against spinal cord glioma in orthotopic nude mouse models, a highly treatment-resistant disease. Tumour vascularity positively correlates with susceptibility to A1-R therapy. Substrains with tumour-specific promoters and mutants which enhance selective tumour targeting have been identified. The approach described here, where bacterial monotherapy effectively treats ­primary and metastatic tumours, is a significant improvement over previous bacterial tumour therapy strategies that require combination with toxic chemotherapy.