THYMIDINE KINASE-MEDIATED KILLING OF RAT-BRAIN TUMORS

Gene therapy has many potential applications in central nervous system (CNS) disorders, including the selective killing of tumor cells in th e brain. A rat brain tumor model was used to test the herpes simplex v irus (HSV)-thymidine kinase (TK) gene for its ability to selectively k ill C6 and 9L tumor cells in the brain following systemic administrati on of the nucleoside analog ganciclovir. The HSV-TK gene was introduce d in vitro into tumor cells (C6-TK and 9L-TK), then these modified tum or cells were evaluated for their sensitivity to cell killing by ganci clovir. In a dose-response assay, both C6-TK and 9L-TK cells were 100 times more sensitive to killing by ganciclovir (median lethal dose: C6 -TK, 0.1 mug ganciclovir/ml; C6, 5.0 mug ganciclovir/ml) than unmodifi ed wild-type tumor cells or cultured fibroblasts. In vivo studies conf irmed the ability of intraperitoneal ganciclovir administration to kil l established brain tumors in rats as quantified by both stereological assessment of brain tumor volumes and studies of animal survival over 90 days. Rats with brain tumors established by intracerebral injectio n of wild-type or HSV-TK modified tumor cells or by a combination of w ild-type and HSV-TK-modified cells were studied with and without ganci clovir treatments. Stereological methods determined that ganciclovir t reatment eliminated tumors composed of HSV-TK-modified cells while con trol tumors grew as expected (p < 0.001). In survival studies, all 10 rats with 9L-TK tumors treated with ganciclovir survived 90 days while all untreated rats died within 25 days. Curiously, tumors composed of combinations of 9L and 9L-TK cells could be eliminated by ganciclovir treatments even when only one-half of the tumor cells carried the HSV -TK gene. While not completely understood, this additional tumor cell killing appears to be both tumor selective and local in nature. lt is concluded that HSV-TK gene therapy with ganciclovir treatment does sel ectively kill tumor cells in the brain and has many potential applicat ions in CNS disorders, including the treatment of cancer.