Tumor-targeting amino acid auxotrophic Salmonella typhimurium
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We have developed an effective bacterial cancer therapy strategy by targeting viable tumor tissue using Salmonella typhimurium auxotrophs that we have generated which grow in viable as well as necrotic areas of tumors. However, the auxotrophy severely restricts growth of these bacteria in normal tissue. The S. typhimurium A1-R mutant, which is auxotrophic for leu-arg and has high anti-tumor virulence, was developed in our laboratory. In vitro, A1-R infects tumor cells and causes nuclear destruction. A1-R was initially used to treat metastatic human prostate and breast tumors that had been orthotopically implanted in nude mice. Forty percent of treated mice were cured completely and survived as long as non-tumor-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 tumor. 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. The approach described here, where bacterial monotherapy effectively treats primary and metastatic tumors, is a significant improvement over previous bacterial tumor-therapy strategies that require combination with toxic chemotherapy. Three promoter clones engineered in S. enterica typhimurium were identified to have enhanced expression in bacteria growing in tumors relative to those growing in the spleen. The expression of therapeutics in Salmonella under the regulation of one or more promoters that are activated preferentially in tumors has the potential to improve the efficacy of Salmonella tumor therapy. Exploitation of the tumor-killing capability of Salmonella has great promise for a new paradigm of cancer therapy.