To enhance the efficacy of suicide gene therapy for prostate cancer under androgen deprivation, we designed a promoter system that consists of the prostate-specific membrane antigen (PSMA) promoter / enhancer (PEPM) and Cre-loxP DNA recombination system. We constructed two kinds of plasmids. One plasmid contains a Cre recombinase (Cre) under the control of PEPM and the other expresses CMV-lox-luciferase / herpes simplex virus thymidine kinase (TK). In PSMA-positive LNCaP cells, the promoter activity of the PEPM-Cre plus CMV-lox-luciferase demonstrated 800-fold greater activity compared with that of the PSMA promoter alone. However, no enhancement of the promoter activity was observed in the PSMA-negative cells. Furthermore, in contrast to prostate specific antigen promoter / enhancer (PP), the promoter activity of PEPM did not decrease when the LNCaP cells were cultured in charcoal-stripped fetal bovine serum (CFBS). In an in vitro gene therapy model with LNCaP cells, the cell growth inhibition in the presence of ganciclovir (GCV) was more evident in the cells transfected with the PEPM-Cre plus CMV-lox-TK than in the cells with the PP-TK, and the difference in efficacy between the two plasmids was more remarkable when the cells were maintained in CFBS medium. The therapeutic effect of PEPM-Cre plus CMV-lox-TK was also observed in xenografted LNCaP cells on nude mice when the plasmids were directly injected into tumors and GCV was administered intraperitoneally. These findings indicate that the combination of the PSMA promoter / enhancer and the Cre-loxP system can enhance the PSMA promoter activity even under androgen ablation conditions and can exert its anti-tumor effect both in vitro and in vivo.