Transduction of thymidine phosphorylase cDNA facilitates efficacy of cytosine deaminase/5-FC gene therapy for malignant brain tumor

The in vivo gene delivery of E. coli cytosine deaminase (cd) cDNA and syste mic 5-fluorocytosine (5-FC) administration have been studied extensively be cause of their clinical relevance to cancer gene therapy. This approach has the potent advantage of a stronger bystander effect compared to the previo us thymidine kinase suicide gene system of the herpes simplex virus. Howeve r, 5-fluorouracil (5-FU), an active metabolite in cd with 5-FC therapy, is not always effective for every type of tumor since the enzymes responsible for further drug metabolism vary significantly in each tissue. In this stud y, we aimed to increase the sensitivity of 5-FU by transduction of thymidin e phosphorylase (dThdPase) cDNA into brain tumor cells. After retroviral tr ansfer of the cDNA, we obtained 9L murine gliosarcoma cells showing stable expression of the target enzyme (9L-dThdPase). The growth of the cells was identical to wild type (9L-WT) or control-vector transfected (9L-Neo) cells in vitro. Sensitivity to 5-FU was increased in 9L-dThdPase cells. After th e adenoviral delivery of cytosine deaminase gene into these cells, 9L-dThdP ase cells also demonstrated an increased sensitivity to 5-FC. Moreover, we showed that transduction of dThdPase cDNA prolongs the survival of animals bearing intracerebral tumors after experimental in vivo cytosine deaminase gene therapy. These results suggest that transduction of thymidine phosphor ylase may, be a beneficial approach to increasing the efficacy of cd/5-FC s uicide gene therapy in certain types of tumor.