The PNP/6-methylpurine 2-deoxyriboside (6MePdR) system is an efficient gene-directed enzyme prodrug therapy system with significant antitumor activities.In this system, Escherichia coli purine nucleoside phosphorylase (ePNP) activates nontoxic 6MePdR into potent antitumor drug 6-methylpurine (6MeP).The Salmonellatyphimurium PNP (sPNP) gene has a 96-％ sequence homology in comparison with ePNP and also has the ability to convert 6MePdR to 6MeP.In this study, we used tumor targeting S.typhimurium VNP20009 expressing endogenous PNP gene constitutively to activate 6MePdR and a combination treatment of bacteria and prodrug in B 16F 10 melanoma model.The conversion of 6MePdR to 6MeP by S.Typhimurium was analyzed by HPLC and the enzyme activity of sPNP was confirmed by in vitro (tetrazolium based colorimetric assay) MTT cytotoxicity assay.After systemic administration of VNP20009 to mice, the bacteria largely accumulated and specifically delivered endogenous sPNP in the tumor.In comparison with VNP20009 or 6MePdR treatment alone, combined administration of VNP20009 followed by 6MePdR treatment significantly delayed the growth of B16F10 tumor and increased the CD8+ T-cell infiltration.In summary, our results demonstrated that the combination therapy of S.Typhimurium and prodrug 6MePdR is a promising strategy for cancer therapy.