ANAEROBIC-BACTERIA AS A GENE DELIVERY SYSTEM THAT IS CONTROLLED BY THE TUMOR MICROENVIRONMENT

A fundamental obstacle in gene therapy for cancer treatment is the spe cific delivery of an anticancer gene product of a solid tumor. Althoug h several strategies exist to control gene expression once a vector is directly introduced into a tumor, as yet no systemic delivery system exists that specifically targets solid tumors. Nonpathogenic, obligate anaerobic bacteria of the genus Clostridium have been used experiment ally as anticancer agents because of their selective growth in the hyp oxic regions of solid tumors after systemic application. In this repor t we further describe a novel approach to cancer gene therapy in which genetically engineered clostridia are used as tumor-specific vectors for the delivery of antitumor genes. We have introduced into a strain of C. beijerinckii the gene for an E. coli nitroreductase known to act ivate the nontoxic prodrug CB 1954 to a toxic anticancer drug. Nitrore ductase produced by these clostridia enhanced the killing of tumor cel ls in vitro by CB 1954, by a factor of 22. To demonstrate the specific ity of this approach far tumor targeting we intravenously injected the inactive spore form of C. beijerinckii, which upon transition to a re productive state will express the E. coli nitroreductase gene. Nitrore ductase activity was detectable in 10 of 10 tumors during the first 5 days after intravenous injection of inactive clostridial spores, indic ating a rapid transition from spore to reproductive state. Tumors harb oring clostridial spores which did not possess the E. coli nitroreduct ase gene were devoid of nitroreductase activity. Most importantly, E. coli nitroreductase protein was not found in a large survey of normal mouse tissues following intravenous injection of nitroreductase contai ning clostridia, strongly suggesting that obligate anaerobic bacteria such as clostridia can be utilized as highly specific gene delivery ve ctors for cancer therapy.