A MORE POTENT BYSTANDER CYTOCIDAL EFFECT ELICITED BY TUMOR-CELLS EXPRESSING THE HERPES-SIMPLEX VIRUS-THYMIDINE KINASE GENE THAN BY FIBROBLAST VIRUS-PRODUCER CELLS IN-VITRO

Retrovirus-mediated herpes simplex virus-thymidine kinase (HSV-tk) gen e therapy is a promising approach in the treatment of brain tumors. Pr evious in vitro and in vivo studies have demonstrated a bystander effe ct in which nonmodified tumor cells in proximity to HSV-tk-modified tu mor cells are killed with the modified cells in the presence of gancic lovir. In the present study the authors assessed the contribution of i nfectious HSV-tk retrovirus made by producer cells to the bystander cy tocidal effect in tissue culture using Walker 256 rat breast carcinosa rcoma cells, which represent an established model for carcinomatous me ningitis. The authors observed ganciclovir-dependent growth inhibition even when only one HSV-tk-positive Walker cell was mixed with 1000 HS V-tk-negative Walker cells and showed that the bystander cytocidal eff ect is not mediated by toxic cell lysis products. Walker cells enginee red to produce HSV-tk retrovirus with titers ranging from 10(3) to 10( 5) colony-forming units/ml exert no greater cytocidal effect than nonv iral producer HSV-tk-positive Walker cells in vitro. Murine fibroblast -producer cells with viral titers ranging from 10(6) to 10(7) colony-f orming units/ml exerted a stronger cytocidal effect than nonviral prod ucer HSV-tk-positive murine fibroblasts. Despite the high viral titers of fibroblast producer cells, HSV-tk-modified Walker cells performed better than fibroblast producer cells in their cytotoxic effect on wil d-type Walker tumor cells, Given that HSV-tk-modified tumor cells can become ganciclovir resistant, we tested gamma-irradiation as a means t o overcome resistance. Lethal gamma-irradiation of the HSV-tk-positive Walker cells did not abolish their bystander effect on Walker HSV-tk- negative cells. One can infer from these results that HSV-tk-modified tumor cells, irradiated or not, may be a better alternative to murine fibroblast producer cells in the treatment of central nervous system n eoplasia.