Synergy between the herpes simplex virus tk/ganciclovir prodrug suicide system and the topoisomerase I inhibitor topotecan.

An established principle of antineoplastic chemotherapy is that multidrug regimens are generally superior to single-agent therapy. This prompted us to elucidate whether the topoisomerase inhibitor topotecan (TPT) could enhance the efficacy of the herpes simplex virus thymidine kinase gene/ganciclovir (HSV-tk/GCV) system for the treatment of cancer. We assessed the interaction between these two treatments in murine MC38 and human HT-29 colon carcinoma cell lines that were genetically modified to constitutively express HSV-tk, sensitizing them to GCV. 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 (Adv. Enzyme Regul. 1984; 22:27-55). Subcutaneous tumor models, using the same cell lines in C57BL/6 and athymic nude mice, respectively, demonstrated that the combination of GCV and TPT resulted in statistically significant enhanced survival relative to single-agent treatment. In addition, nude mice bearing HT-29 tumor xenografts were treated with an Ad5 E1b Mr 55,000 attenuated replication-competent adenovirus expressing HSV-tk (Ad.TK(RC)) either alone or in combination with GCV and/or TPT. These experiments demonstrated that Ad.TK(RC) followed by GCV and TPT was more efficacious than any other treatment tested. Our results suggest that for antineoplastic therapy, molecular chemotherapy based on the HSV-tk/GCV system combined with traditional chemotherapy is a logical and practical future direction to pursue. Suicide gene therapy is the approach whereby genetically altering a cell makes it susceptible to an otherwise relatively nontoxic prodrug. By this approach it is possible to achieve relatively high concentrations of the toxic metabolites in the transduced cells while maintaining low systemic levels of the active drug. The most often used metabolic suicide gene transfer system is the HSV-tk/GCV paradigm, which is currently being used in cancer therapy or as a safety modality. The low response rate observed in the early clinical HSV-tk cancer trials may be due to failure in achieving adequate transduction efficiency and/or prodrug concentration within the tumor. The combination of such suicide gene prodrug systems with adjunctive drugs resulting in synergistic cytotoxicity might improve the clinical utility of this approach.