Bystander effect caused by cytosine deaminase gene and 5-fluorocytosine invitro is substantially mediated by generated 5-fluorouracil

Because it appears impossible to transfer toxic genes to all the cells of a cancer, the bystander effect is critical to induce effective antitumor eff ects. In the present study, possible in vitro mechanisms of the bystander e ffect by the cytosine deaminase (CD) gene and 5-fluorocytosine (5-FC) were investigated. CD-transduced cancer cells exhibited much higher sensitivity to 5-FC compared to parental cells. CD-transduced cells caused killing of n eighboring parental cells in the presence of 5-FC, irrespective of direct c ell- to-cell contact. Media conditioned by CD-transduced cells and 5-FC con tained considerable amounts of 5-fluorouracil (5-FU) and exhibited profound cytotoxicity on parental cells. Furthermore, this killing ability of condi tioned media correlated well with 5-FU levels converted from 5-FC by CD-tra nsduced cells. CD was shown not to be secreted into media from cells. These results indicate that diffusible 5-FU plays the substantially causative ro le in the in vitro bystander effect caused by the CD/5-FC system.