Synergistic anticancer effects of ganciclovir/thymidine kinase and 5-fluorocytosine/cytosine deaminase gene therapies.

Abstract

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. 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. 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.