Enzyme prodrug gene therapy: synergistic use of the herpes simplex virus-cellular thymidine kinase/ganciclovir system and thymidylate synthase inhibitors for the treatment of colon cancer.

The goal of this study was to improve the therapeutic index of the herpes simplex virus-thymidine kinase/ganciclovir (HSV-tk/GCV) system by the addition of thymidylate synthase (TS) inhibitors. For this, we assessed the potential of GCV to synergistically interact with 5-fluorouracil (5-FU), ZD1694 (Tomudex), and (E)-5-(2-bromovinyl)-2'-deoxyuridine in HSV-tk-expressing murine MC38 STK and human HT-29 STK colon carcinoma cell lines. Synergistic cell killing was observed in a clonogenic assay over most of the cytotoxic dose range by the median-effect principle of Chou and Talalay (T. C. Chou and P. Talalay, Adv. Enzyme Regul., 22: 27-55, 1984). In a s.c. HT-29 STK xenograft tumor model, we demonstrated that the combination of GCV and 5-FU resulted in statistically significant enhanced animal survival over single-agent treatment. Furthermore, we showed that the combination of GCV and ZD1694 in association with the HSV-tk/GCV system was at least as effective as GCV/5-FU in vitro and in vivo. The mechanism for the observed synergy is most likely attributable to the increased GCV phosphorylation in the presence of the tested TS inhibitors. Our data suggest that the HSV-tk/GCV metabolic suicide gene transfer system could serve as an adjuvant of the presently used TS inhibitors, thus potentially improving the efficacy of present cancer gene therapy approaches.