Suicide gene/prodrug therapy using salmonella-mediated delivery of Escherichia coli purine nucleoside phosphorylase gene and 6-methoxypurine 2'-deoxyriboside in murine mammary carcinoma 4T1 model.

Attenuated salmonella have been reported to be capable of both selectively growing in tumors and expressing exogenous genes for tumor-targeted therapy. As 6-methoxypurine 2'-deoxyriboside (MoPdR) is similar to 6-methylpurine 2'-deoxyriboside in structure, we aimed to evaluate the antitumoral effect of the Escherichia coli purine nucleoside phosphorylase (ePNP) gene, using an attenuated salmonella-mediated delivery system, in combination with MoPdR. A novel mutant serovar Typhimurium (SC36) was used to carry the pEGFP-C1-ePNP vector that contains an enhanced green fluorescent protein and an ePNP gene under the control of the cytomegalovirus promoter. The function of the ePNP expression vector was confirmed in vitro using the enzymic conversion of MoPdR into methoxypurine. We also observed a high bystander effect induced by the ePNP/MoPdR system with a very low proportion (1%) of ePNP-positive cells and 5 microg/mL MoPdR, although the growth of parental cells was affected appreciably by MoPdR. The killing effect and increased apoptosis induced by SC36 carrying the ePNP expression vector (SC/ePNP) were detected by cytotoxicity assay and propidium iodide staining flow cytometry analysis, in combination with MoPdR. SC/ePNP was given orally to mice bearing mammary carcinomas, and its antitumor effect was evaluated. SC/ePNP plus MoPdR significantly inhibited tumor growth by approximately 86.6-88.7% and prolonged the survival of tumor-hosting mice. Our data support the view that MoPdR combined with the ePNP gene could be used in gene-directed enzyme prodrug therapy. Attenuated salmonella could be a promising strategy to improve ePNP/MoPdR bystander killing due to its preferential accumulation and anticancer activity in tumors.