Cell death associated with genetic prodrug activation therapy of colorectal cancer

Genetic prodrug activation therapy (GPAT) is a form of cancer gene therapy that has potential use against tumours such as colorectal malignancy. The characterization of such therapies using laboratory models provides a basis for clinical trials. In this study the gene encoding Herpes Simplex Virus thymidine kinase (HSVtk) was delivered to colorectal tumour cells using an Adenoviral (Ad) vector in vitro. In this way the cells were made susceptible to killing with the prodrug ganciclovir to various degrees depending on cell infectability with Ad. Bystander killing effect appeared minimal both in vitro and when transduced cells were injected in vivo. Mechanisms of cell death, measured in vitro using anti-BrDU (DNA-break labelling) and propidium iodide staining variously showed a combination of apoptosis in the G1 cell cycle phase and late apoptotic or necrotic sub-G1 DNA fragmentation, depending on the tumour cell line. These findings suggest that gene therapy of colorectal cancer by GPAT gives rise to therapeutic forms of direct cell death, but requires improvements in transduction, and possibly immune augmentation.