Some anaerobes and facultative anaerobes have been used in
tumor-specific gene therapy by reason of their selective growth in
tumors. In this work, we aimed to evaluate the anticancer efficacy of attenuated Salmonella typhimurium as a carrier to deliver the Escherichia coli
purine nucleoside phosphorylase (
ePNP) gene for GDEPT (gene-directed
enzyme-
prodrug therapy). A live attenuated
purine-auxotrophic strain of S. typhimurium (SC36) was used to carry the pEGFP-C1-ePNP vector that contains a
green fluorescent protein (GFP) and an
ePNP gene under the control of the human cytomegalovirus (CMV) promoter. The function of the
ePNP expression vector was confirmed in vitro using the enzymic conversion of
6-methylpurine 2'-deoxyriboside (MePdR) into
6-methylpurine. We also observed a high bystander effect induced by the
ePNP/MePdR system with a very low proportion (1%) of
ePNP-positive cells. The killing effect and increased apoptosis induced by SC/
ePNP (SC36 carrying the
ePNP expression vector)
infection were detected by cytotoxicity assay and PI staining flow cytometry analysis, in combination with MePdR administration. Furthermore, SC/
ePNP was administered orally into mice bearing
melanomas or pulmonary
tumors, and its anti-
tumor effect was evaluated. When the
tumor was huge (500 mm(3)) at the beginning of MePdR administration, SC/
ePNP plus MepdR significantly inhibited
tumor growth by about 59-80% and prolonged survival of mice. Complete
tumor regression and long-term cure were achieved by MePdR administration, even when the
tumor was large (100 mm(3)) at the beginning of MePdR treatment. Our data support a hopeful view that
tumor-targeting SC36 could improve antitumor efficacy of the
ePNP/MePdR system due to its preferential accumulation and anticancer activity in
tumors.