Effect of Herpes Simplex Virus Thymidine Kinase Expression Levels on Ganciclovir-Mediated Cytotoxicity and the “Bystander Effect”

Transfer of the herpes simplex virus type-1 thymidine kinase (HSV-tk) gene into tumor cells followed by ganciclovir (GCV) administration, will provide selective tumor cell killing. We studied the effect of herpes simplex virus thymidine kinase (HSV-tk) expression level on the HSV-tk/GCV-mediated “bystander effect.” Clones of HSV-tk-transduced rat glioma cells (9L) were isolated that stably expressed with different levels of HSV-tk. All clones studied had similar sensitivity to ganciclovir with IC50 values ranging from 0.45 to 1.3 μM. Within certain enzyme level thresholds, in vitro evaluation of the bystander effect has shown that clones with higher level of HSV-tk expression exhibited a better bystander effect. Interestingly, the bystander effect was observed between different cell types. Both the transduction efficiency and bystander effect are essential factors for the success of the antitumor effect by the HSV-tk/prodrug GCV suicide gene system. Many studies have described the herpes simplex virus type-1 thymidine kinase and prodrug ganciclovir (GCV) system for antitumor effect Complete tumor regression does not require that all tumor cells express herpes simplex virus thymidine kinase (HSV-tk). Tumor cells in close proximity to HSV-tk-expressing tumor cells become GCV-sensitive through a phenomenon described as the “bystander effect.” The bystander effect can function between different cell types. Combination of the direct cytotoxicity of target cells via the HSV-tk/prodrug GCV system and the bystander effect are essential in achieving antitumor effects in vitro and in vivo. In this study, we demonstrate the effect of HSV-tk expression level on the HSV-tk/GCV-mediated bystander effect Within a certain level of TK enzyme level threshold, the higher levels of HSV-tk expression correlated with better bystander effect in mediating cell killing.