Yeast cytosine deaminase improves radiosensitization and bystander effect by 5-fluorocytosine of human colorectal cancer xenografts

The efficacy of cancer gene therapy using bacterial cytosine deaminase (bCD )/5-fluorocytosine (5-FC) enzyme/prodrug strategy is limited by the ineffic iency of cytosine deaminase (CD)-catalyzed conversion of 5-FC into 5-fluoro uracil (5-FU). We have shown previously that yeast CD (yCD) its more effici ent at the conversion of 5-FC than bCD, In the current study, we hypothesiz ed that the increased production of 5-FU by yCD would enhance the efficacy of the CD/5-FC treatment strategy by increasing the bystander effect as wel l as the efficacy of radiotherapy because of the radiosensitizing capacity of 5-FU. To test this hypothesis, we generated stable HT29 human colon canc er cell lines expressing either bCD (HT29/bCD) or yCD (HT29/yCD), The amoun t of 5-FU produced in HT29/yCD tumors after a single injection of 5-FC (100 0 mg/kg, i.p.) was 15-fold higher than that produced in HT29/bCD tumors. In tumor-bearing nude mice, the average minimum relative tumor size (compared with pretreat ment values) of HT29/bCD tumors treated with 5-FC and radiat ion (500 mg/kg i.p. and 3 Gy, 5 days a week for 2 weeks) was 0.55 +/- 0.1, compared with 0.01 +/- 0.01 in HT29/yCD tumors (P = 0.002). Moreover, an in creased cytotoxic and radiosensitizing effect of 5-FC on bystander cells wa s observed in vitro and in vivo when yCD was expressed in HT29 cells instea d of bCD, In mice bearing HT29 tumors containing 10% HT29/yCD cells, the co mbined treatment resulted in a minimum tumor size of 0.20 +/- 0.07 compared with 0.60 +/- 0.1 in 10% HT29/bCD cells (P < 0.001). These results demonst rate that the use of yCD in the CD/5-FC strategy has a high potential to im prove the therapeutic outcome of combined gene therapy and radiotherapy in cancer patients.