Background: A bacterial enzyme, Escherichia coli cytosine deaminase, which converts the prodrug 5-fluorocytosine into the toxic drug 5-fluorouracil, and a viral enzyme, herpes simplex virus thymidine kinase, which converts ganciclovir from an inactive prodrug to a cytotoxic agent by phosphorylation, are being actively investigated for use in gene therapy for cancer. The purpose of this study was to determine whether combining these prodrug-activating gene therapies might result in enhanced anticancer effects.
Methods: Rat 9L gliosarcoma cells were transfected with plasmids containing the E. coli cytosine deaminase gene (9L/CD cells), with plasmids containing the herpes simplex virus thymidine kinase gene (9L/TK cells), or with both expression plasmids (9L/CD-TK cells). The drug sensitivities of the cell lines were evaluated; in addition, the sensitivities of 9L and 9L/CD-TK cells mixed in varied proportions were measured. The effects of prodrug treatment on 9L/CD-TK tumor growth (i.e., size and volume) in nude mice were monitored. The isobologram method of Loewe and the multiple drug-effect analysis method of Chou-Talalay were used to measure the interaction between the two prodrug-activating gene therapies. To elucidate the mechanism of interaction, the phosphorylation of ganciclovir in 9L/CD-TK cells after varying prodrug treatments was studied.
Results and conclusions: The presence of transfected cytosine deaminase and thymidine kinase genes in 9L gliosarcoma cells reduced cell survival, both in vitro and in vivo, following treatment with the relevant prodrugs; the effects of the two components appeared to be synergistic and related mechanistically to the enhancement of ganciclovir phosphorylation by thymidine kinase following 5-fluorouracil treatment.