Suicide gene therapy of chemically induced mammary tumor in rat: efficacy and distant bystander effect.

We examined the efficacy of suicide gene therapy for nitrosomethylurea-induced mammary tumors in rats. Individual tumors were directly injected with a retrovirus-producing cell line that releases retroviral vectors that transduce the herpes simplex virus type 1 thymidine kinase (HSV1-TK) gene. HSV1-TK specifically converts the nucleoside analogue ganciclovir (GCV) into a toxic metabolite. Compared to control rats receiving saline, we observed a significant tumor regression of the injected tumors following GCV administration, accompanied by a stromal inflammation and an extensive lymphocyte infiltration invading the tumor epithelium. It is noteworthy that the neighboring uninjected tumors also regressed, demonstrating the occurrence of a distant bystander effect. This is the first demonstration that HSV1-TK/GCV can efficiently treat multiple solid tumors directly generated from an epithelial tissue.