Combined suicide gene therapy for human colon cancer cells using adenovirus-mediated transfer of Escherichia coli cytosine deaminase gene and Escherichia coli uracil phosphoribosyltransferase gene with 5-fluorocytosine

The virus-directed enzyme/prodrug system using the Escherichia coli cytosin e deaminase (CD) gene and 5-fluorocytosine (5-FC) suffers from a sensitivit y limitation in many tumor cells. The E. coli uracil phosphoribosyltransfer ase (UPRT), which is a pyrimidine salvage enzyme, directly converts 5-fluor ouracil (5-FU) to 5-fluorouridine monophosphate at the first step of its ac tivating pathway. To improve the antitumoral effect of the CD/5-FC system, we investigated a combined suicide gene transduction therapy for human colo n cancer cells using two separate adenovirus vectors expressing the E. coli CD and E. coli UPRT genes and systemic 5-FC administration (the CD, UPRT/5 -FC system). The present study demonstrates that the CD, UPRT/5-FC system g enerates a co-operative effect of CD and UPRT, resulting in dramatic increa ses in both RNA- and DNA-directed active forms, including 5-fluorouridine t riphosphate incorporated into RNA, 5-fluorodeoxyuridine monophosphate, and the thymidylate synthase inhibition rate, compared with the CD/5-FC system. Furthermore a significant increase in the 5-FC sensitivity of colon cancer cells was demonstrated in the CD, UPRT/5-FC system compared with the CD/5- FC system in vitro and in vivo. These results suggest that the CD, UPRT/5-F C system is a powerful approach in gene therapy for colorectal cancer.