780 PIGMENT EPITHELIUM-DERIVED FACTOR PROLONGS SURVIVAL AND ENHANCES THE IN VIVO ANTI-TUMOR ACTIVITIES OF LOW-DOSE CHEMOTHERAPY IN CASTRATION-REFRACTORY PROSTATE CANCER

Abstract

INTRODUCTION AND OBJECTIVES: The development of metronomic/low dosing of conventional chemotherapeutic drugs have recently showed great promise in the treatment of castration-refractory prostate cancer (CPRC). Pigment Epithelium-Derived Factor (PEDF) is a natural angio-inhibitor which has been shown to be down-regulated in prostate cancer. We have previously demonstrated that the overexpression of PEDF in human CRPC PC3 cells decreased tumor growth in vivo. In the present study, we further validated PEDF antitumor properties in the highly metastatic CRPC LNCaP-derivative CL1 cells. We also hypothesized that PEDF could enhance the cytotoxicity effects of low dose docetaxel (Doc) and cyclophosphamide (CTX) chemotherapies in vivo. METHODS: Human CRPC PC3 and CL1 cell lines were genetically modified to stably co-express the fluorescent DsRed Express protein with PEDF. Resulting cell lines were characterized in vitro for PEDF expression by western blot and, for proliferation by growth curves and clone formation in matrigel. PEDF anti-tumor effects were assessed on established s.c. xenografts in mice treated with Doc (5mg/kg ip every 4 days, 1mg/kg ip daily for 10 days, 0.5mg/kg every other day), CTX (10-20mg/kg in the drinking water) or placebo. Survival studies were performed by injecting CL1 cells, pre-mixed with 50nM hrPEDF or carrier control, into the left lobe of the dorsal prostate of anesthetized mice. RESULTS: We showed that PEDF inhibits the proliferation and induces the differentiation of all CPRC cells tested in vitro. Similarly, PEDF expression decreases by 85% and 70% the development of s.c. PC3 and CL1 tumors in immuno-compromised mice, respectively. In the survival study, we found that all control animals die within 61 days after surgery while 50% of PEDF-pretreated animals were still alive. Most importantly, we demonstrated that PEDF enhances the cytotoxicity effects of low dose chemotherapy on established tumors (best Doc dose: 1mg/kg for PC3 and 5mg/kg for CL1; best CTX dose: 10mg/kg for both PC3 and CL1). CONCLUSIONS: These data reinforce the significance of PEDF as a potential new target for the treatment of advanced prostate cancer. It also emphasizes PEDF as a promising new agent to enhance the anti-tumor efficacy of low dose Doc and CTX chemotherapies.