Differential toxicity of ganciclovir for rat neurons and astrocytes in primary culture following adenovirus-mediated transfer of the HSVtk gene

The toxicity of the suicide HSVtk gene approach is known to be targeted to DNA synthesis and, consequently, to dividing cells. This system is therefore useful for the treatment of brain tumors which contain dividing cells surrounded by a quiescent normal tissue. Adenoviruses are efficient vectors for the transfer of the HSVtk gene into the tumor but this can lead to the transduction of quiescent cells. In this study, we focused on the toxicity of the HSVtk/ganciclovir treatment for the two main cell types of the normal brain: astrocytes and neurons. Astrocytes and neurons in primary culture were infected by an adenoviral vector bearing the HSVtk gene (Ad.tk) and cells were exposed to different concentrations of ganciclovir. After 5 days of treatment, an MTT test measured a dramatic decrease in cell viability for treated astrocytes while a small decrease in cell viability was observed for neurons treated in the same experimental conditions. The differential toxicity of the HSVtk/ganciclovir treatment was also observed in cocultures of astrocytes and neurons: an immunocytochemical analysis of the treated cells showed major morphological modifications for astrocytes but not for neurons. Furthermore, our data suggest that a bystander effect is able to kill all the astrocytes while neurons from the same culture remain unaffected.