RETROVIRUS-MEDIATED HERPES-SIMPLEX VIRUS THYMIDINE KINASE GENE TRANSDUCTION RENDERS HUMAN THYROID-CARCINOMA CELL-LINES SENSITIVE TO GANCICLOVIR AND RADIATION IN-VITRO AND IN-VIVO

In an attempt to develop gene therapy for thyroid carcinomas, the pres ent studies were undertaken to evaluate in vitro and in vivo therapeut ic efficacy and toxicity of herpes simplex virus thymidine kinase (HSV -tk) gene and ganciclovir (GCV) treatment, a widely used prodrug/suici de gene therapy, in human thyroid carcinoma cell lines, FRO and WRO ce lls, using a means of retrovirus-mediated gene transduction. In vitro experiments demonstrated dose-and time-dependent cell killing by trans duction of the HSV-tk gene followed by GCV treatment. The IC50 (the co ncentration required to elicit 50% growth inhibition) shifted from 250 to 0.5 mg/liter in FRO cells, and from 3,000 to 0.09 mg/liter in WRO cells with therapeutic indexes of 500 and 33,000, respectively. Treatm ent with 30 mg/liter GCV for 4 days led to complete cell death in HSV- tk tumor cells. Nontransduced cells mixed with transduced cells were a lso effectively killed by GCV (bystander effect). Low concentrations o f GCV, which alone showed little cytotoxicity, enhanced radiation-indu ced cytotoxicity (radiosensitization). In vivo sc FRO-tk tumor models in nude mice also showed dose-and time-dependent tumor regression. The IC50 was less than 2 mg/kg, and treatment with 100 mg/kg GCV for 2 we eks completely eradicated all tumors. The bystander effect and radiose nsitization were also obtained in vivo. These results suggest that the HSV-tk/ GCV approach to human thyroid carcinoma cells appears to be v ery efficacious, with a wide therapeutic range, and exerts a bystander effect and radiosensitization both in vitro and in vivo. Thus, HSV-tk /GCV system, alone or in combination with radiotherapy, may be a promi sing suicide gene therapy for thyroid carcinomas.