Ganciclovir mediated regression of rat brain tumors expressing the herpes simplex virus thymidine kinase imaged by magnetic resonance.

Using a rat C6 brain tumor model, we studied the antitumor effects of Herpes simplex virus type 1 thymidine kinase (HSV-tk) gene transfer followed by ganciclovir treatment. C6 glioma cells were transfected in vitro with the HSV-tk gene, and tested for their sensitivity to ganciclovir. Although there was no surviving cell at a 30 microM ganciclovir concentration, unmodified C6 cells were not affected by the drug. For in vivo experiments, intracerebral tumors were induced in rats by stereotactic injection of 10(4) HSV-tk-modified C6 cells. Ten days later, the animals were treated with intraperitoneal injections of ganciclovir for 21 days. The tumors evolution was evaluated by high resolution magnetic resonance imaging. In 33% of the rats, the signal intensity of the tumors became heterogeneous, with development of highly hyperintense areas, and a complete tumor regression was subsequently noted. Histological examination of successfully treated tumors revealed progressive necrosis with formation of cysts. The survival time of the HSV-tk/ganciclovir treated animals was consistently increased, all rats surviving more than 30 days and 33% of them being still alive after 80 days.