Suppression of murine mammary carcinoma growth and metastasis by HSVtk/GCV gene therapy using in vivo electroporation.

The effectiveness of electroporation as a means of gene transfection, both in vitro and in vivo, was tested using the herpes simplex virus 1 thymidine kinase (HSVtk) gene in combination with ganciclovir (GCV) administration as therapy against murine mammary cancer. Approximately 80% of BJMC3879 metastatic mammary carcinoma cells, derived from MMTV-infected BALB/c mice, died as a result of HSVtk/GCV treatment 72 hours after the transfection; decreased DNA synthesis was also seen. Mammary tumors induced by inoculation of syngeneic mice with BJMC3879 cells were subsequently treated by direct injection of vector containing HSVtk (pHSVtk) alone, empty vector or saline alone twice a week. After each injection, the tumors were subjected to in vivo electroporation. Mice treated with pHSVtk or saline were intraperitoneally injected with GCV at 40 mg/kg five times a week. Significantly reduced tumor volumes were observed for the pHSVtk+GCV group in experimental week 2 and thereafter throughout the 2-month study. DNA synthesis was significantly decreased as well in the pHSVtk+GCV group compared with all other groups. Furthermore, metastasis to lymph nodes and lungs was significantly suppressed by HSVtk/GCV treatment. Expression of HSVtk in the tumors was confirmed by RT-PCR. Macrophage accumulations were frequently observed in the peripheries of necrotic regions in HSVtk/GCV-treated tumors, where levels of apoptosis were significantly higher than those observed in other groups. We therefore conclude that in vivo electroporation can result in efficient gene transfer and that the HSVtk/GCV prodrug system strongly suppresses tumor growth and metastases in this model.