BASIC INVESTIGATIONS OF GENE THERAPY AGAINST CANCER BY USE OF BACTERIAL CYTOSINE DEAMINASE AND 5-FLUOROCYTOSINE

The gene for bacterial cytosine deaminase (CD) which metabolites the innocuous prodrug 5-fluorocytosine (5-FC) to the highly toxic drug 5-fluorouracil (5-FU) was transduced via retrovirus into several cell lines : hepatocllular carcinoma(HCC), squamous cell carcinoma (SCC) and myoblastoma. Transduction of the CD gene did not affect the growth rate of these cells. However, the cells genetically modifibd to express CD were sensitive to 5-FC in a dose-dependent manner, and the modified HCC, SCC and myoblastoma cells exhibited approximately 120-, 130- and 460-fold higher sensitivity to 5 -FC, respectively, compared with each of the corresponding parental cells. Cells expressing CD were killed with 5-FC at concentrations achievable in sera of patients receiving clinical doses of 5-FC. Conversely, there were no significant differences in the susceptibility to 5 -FU between the modified cells and their parental cells. The cells expressing CD were able to induce the killing of their neighboring parental cells in the presence of 5-FC not only when they were in contact with each other but also when they were not. The killing ability of cells expressing CD was closely correlated with amounts of 5-FU generated by the cells. Most importantly, this killing effect was also observed in vivo. Significant antitumor effects were induced in mice bearing HCC cells composed of the modified and unmodified cells by an intraperitoneal injection of 5-FC. When genetically modified HCC cells com- prised only 10% of a tumormass, significant suppression of HCC development was achieved by administration of subtoxic doses of 5-FC. These results demonstrated the feasibility of gene therapy using the bacterial CD gene and 5-FC for the treatment of cancers