Evaluation of the Bystander Effect in Experimental Brain Tumors Bearing Herpes Simplex Virus–Thymidine Kinase Gene by Serial Magnetic Resonance Imaging

Antitumor effects of herpes simplex virus-thymidine kinase (HSV-tk) gene transfer followed by ganciclovir (GCV) administration were studied by serial magnetic resonance imaging (MRI) with reference to the bystander effect. Mixed populations of 9L-gliosarcoma cells transduced with the HSV-tk gene (TK cells) and wild-type 9L cells were implanted into the brain of syngeneic Fisher rats at various ratios (total cell number, 10⁵ cells; percentage of TK cells, 100%, 25%, 10%, or 0%). Rats were treated with GCV (30 mg/kg per day) or saline for 14 days and tumor masses were visually monitored using MRI. All of the saline-treated rats (regardless of TK cell percentage) and GCV-treated rats inoculated with 0% TK cells died between day 19 and day 31 (mean survival, 22.9 days) due to progressive tumor growth. The GCV-treated rats inoculated with more than 10% of TK cells lived significantly longer than the saline-treated rats (p < 0.01). The mean survivals of GCV-treated groups were 50.7, 70.0, and longer than 100 days for 10%, 25%, and 100% TK cells, respectively. MRI study revealed that reduction in tumor size and disappearance of tumor were observed in the GCV-treated rats inoculated with 10% or 25% TK cells. Complete regression of the tumor was, however, observed only in the rats implanted with 100% TK cells. The present results show that the bystander effect is clearly observed in vivo in a TK percentage-dependent manner, and a population of more than 25% of TK-positive cells is required for complete tumor elimination.

Overview summary

The herpes simplex virus-thymidine kinase (HSV-tk) and prodrug ganciclovir (GCV) system have been proved to have an effective antitumor activity in animal experiments. Complete tumor regression does not require that all tumor cells express HSV-tk. Tumor cells in close proximity to HSV-tk-expressing tumor cells become sensitive to GCV through a phenomenon designated as the bystander effect. In this study, the authors show the process of regression of established brain tumors through the bystander effect in the HSV-tk-GCV system with sequential magnetic resonance imaging in the rat.