Genetic prodrug activation therapy (GPAT) in two rat prostate models generates an immune bystander effect and can be monitored by magnetic resonance techniques

Abstract

Treatment of hormone refractory prostate cancer requires new treatment strategies. Genetic prodrug activation therapy (GPAT) may provide a new therapeutic avenue. In this study the antitumour efficacy of the gene encoding herpes simplex virus thymidine kinase (HSVtk) activating the prodrug ganciclovir (GCV) was compared in two models of ectopic (subcutaneous) rat prostate cancer. Both models, which differ in their characteristics, were previously shown to be weakly immunogenic but susceptible to immunotherapy. Tumour cell lines were stably transfected with HSVtk and were rendered highly sensitive to GCV. Little or no bystander killing effect was observed by tk-transfected cells on wild-type cells in vitro. However, a significant in vivo bystander effect was observed suggesting an immune-mediated response. Indeed, such an immune response was capable of slowing the growth of distant wild-type tumours and increased overall animal survival. A T helper 1 immune response was generated as a result of GCV activation and cell kill, demonstrated by the secretion of IFN␥ by cultured splenocytes in response to tumour cells.

Quantification and characterization of the bystander effect in prostate cancer cells following adenovirus-mediated FasL expression

“…However, it is important to note that these observations may be cell-line dependent because other reports indicate that p53 and HSVtk/GCV utilize non-Fas-mediated apoptotic death pathways as well. 17,18 In summary, the data presented in this paper demonstrate that AdGFPFasL TET infection induces bystander killing in certain PCa cell lines through an Fas-mediated mechanism. Currently, we are evaluating the use of FasL as gene therapy in animal models.…”

Immunotherapy of murine prostate cancer using whole tumor cells killed ex vivo by herpes simplex viral thymidine kinase/ganciclovir suicide gene therapy

“…Sensitivity of HSVtK-transfected and nontransfected PAIII and MLL cells to ganciclovir and evidence of an in vitro bystander effect has been previously published by our group. 10 MLL was found to be more sensitive to ganciclovir, but there was incomplete cell kill even at maximum doses of ganciclovir. The predominant mode of cell death as a result of exposure to ganciclovir was by apoptosis, also shown previously by our group.…”

“…Clones were grown in neomycin (G418) (150 mg/ml for MLL and 500 mg/ml for PAIII) and then tested for their sensitivity to killing by ganciclovir (Cymevene, Roche) at a range of concentrations compared to wild-type cells, previously shown. 10 HSVtK transfectants of MLL and PAIII were referred to as MtK and PtK, respectively.…”

“…Cells were transfected as previously described. 10 Viral supernatant harvested from these cells was used to transfect the rat prostate tumor lines in the presence of 4 mg/ml polybrene. Clones were grown in neomycin (G418) (150 mg/ml for MLL and 500 mg/ml for PAIII) and then tested for their sensitivity to killing by ganciclovir (Cymevene, Roche) at a range of concentrations compared to wild-type cells, previously shown.…”

“…6 The antitumor effects of both autologous and allogeneic cell vaccines may be further enhanced by modifying the cells to express cytokines such as GMCSF, 7 costimulatory molecules, 8 heat shock proteins 9 or suicide genes, which induce marked local, systemic and memory immune response. 10,11 Killed tumor cells have been shown to be the source of TAA for processing and presentation by dendritic cells (DCs). 12,13 Their immunostimulatory potential may depend on the mode (apoptosis versus necrosis) of cell death but may be limited by expression of immunosuppressive factors in the tumor microenvironment.…”