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

Treatment of hormone refractory prostate cancer requires new treatment stra tegies. Genetic prodrug activation therapy (GPAT) may provide a new therape utic avenue. In this study the antitumour efficacy of the gene encoding her pes simplex virus thymidine kinase (HSVtk) activating the prodrug ganciclov ir (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 ce ll lines were stably transfected with HSVtk and were rendered highly sensit ive to GCV Little or no bystander killing effect was observed by tk-transfe cted cells on wild-type cells in vitro. However, a significant in vivo byst ander effect was observed suggesting an immune-mediated response. Indeed, s uch an immune response was capable of slowing the growth of distant wild-ty pe tumours and increased overall animal survival. A T helper I immune respo nse was generated as a result of GCV activation and cell kill, demonstrated by the secretion of IFN gamma by cultured splenocytes in response to tumou r cells. BrDU staining of tk-transfected cells treated with GCV in vitro su ggested apoptotic cell death, but Annexin V staining was less marked for on e of the cell lines. Serial in vivo monitoring by non-invasive magnetic res onance spectroscopy (MRS) of the tk-transfected MATLyLu tumours demonstrate d a decreased ATP/Pi ratio (a measure of cell energy status) during growth and an increase in the ATP/Pi ratio during regression initiated by treatmen t with GCV Further, significant differences were found in the phosphomonest er (PME) to total phosphate (SigmaP) ratios in treated compared with untrea ted tumours, a result rarely seen in animal models, but commonly observed i n patients. This study showed that a Th1-biased immune response generated b y killing prostate tumour cells with tk/GCV can kill distant as well as loc al wild-type tumour cells. These findings suggest that GPAT may have a pote ntial application in patients with both confined and metastatic prostate ca ncer and MRS may provide a method of monitoring response to treatment.