Cytosine deaminase/5-fluorocytosine-based vaccination against liver tumors:: Evidence of distant bystander effect

Background: The cytosine deaminase gene of Escherichia coli converts the nontoxic compound 5-fluorocytosine into 5-fluorouracil (5-FU), thereby acting as a suicide gene when introduced into cancer cells, killing the cells when they are exposed to 5-fluorocytosine. We analyzed the efficacy of using cytosine deaminase-bearing cancer cells as an autologous tumor vaccine in a rat model that mimics liver metastasis from colon carcinoma. Methods: We introduced a plasmid vector containing the E. coli cytosine deaminase gene into a BDM rat colon carcinoma cell line. Intrahepatic injection of the modified cells in syngeneic animals generates a single experimental liver "suicide tumor," We then analyzed the effect of 5-fluorocytosine treatment in terms of regression of cytosine deaminase-expressing cells in vivo as well as protection against mild-type cancer cells. Results: Treatment with 5-fluorocytosine induced regression of cytosine deaminase-expressing (CD+) tumors, with seven of II treated animals being tumor free at the end of 30 days and a statistically significant difference in tumor volumes bet between treated and control animals (two-sided P < .0001), Intrahepatic injection of CD+ cells followed by 5-fluorocytosine treatment rendered the treated animals resistant to challenge with wild-type tumor cells, with no (zero of seven) treated animals developing wild-type tumors in contrast to all (four of four) control animals. Moreover, in animals with established wild-type liver tumors, injection of CD+ tumor cells followed by 5-fluorocytosine treatment produced a statistically significant increase in survival time (two-sided P < .0001). In vivo immunodepletion and immunohistologic analysis of experimental tumors indicate that natural killer cells are the major immune component involved in this antitumor effect. Conclusions and Implications: Taken together, these results suggest the potential use of suicide gene-modified tumor cells as therapeutic vaccines against liver metastasis from colon carcinoma.