Potential of Varicella zoster virus thymidine kinase as a suicide gene in breast cancer cells

To investigate the potential of the thymidine kinase gene from Varicella zoster virus (VZVtk) to act as a suicide gene, VZVtk was transferred via a dicistronic retroviral construct into MCF7, T-47D and MDA-MB-435 human breast cancer cells. The cytotoxicity of antiviral drugs was then evaluated in vitro on the wild-type and transduced cells. Acyclovir and ganciclovir did not show any selective toxicity for the modified cells. In contrast, (E)-5-(2-bromovinyl-2'-deoxyuridine (BVDU) was extremely toxic for the VZVtk expressing cells, with IC50 values of 0.6 mu M, 0.1 mu M and 0.06 mu M for MC57, T-47D and MDA-MB-435 cells, respectively. The selectivity index of BVDU (ie the IC50 value ratio of the wild-type to the VZVtk(+) cells) was 400 for MCF7, 750 for T-47D and 2000 for MDA-MB-435 cells. To test the system in vivo, VZVtk carrying MDA-MB-435 cells were inoculated subcutaneously into nude mice. An intraperitoneal treatment with BVDU administered at the emergence of the tumors, led to a prolonged arrest of the tumor growth and a reduced tumor mass. This effect was BVDU dose-depentent. No bystander effect of BVDU killing could be demonstrated in vitro on mixed populations of VZVtk positive and negative MDA-MB-435 cells. However, an important bystander effect was observed in identical experiments performed on 9L rat gliosarcoma cells infected with the VZVtk-carrying vector. These results demonstrate the efficiency of VZVtk as a suicide]gene when BVDU is used as prodrug. The bystander effect measured in vitro, depends however on the tumoral cell type used.