4741 Background: An herbal combination PC-SPES was available in US for prostate cancer treatment and withdrawn in 2002 due to contamination. In China, the number of Prostate cancer recurrence after surgical castration has increased recently. There is an urgent need to find an effective therapy for these patients. We therefore investigated the effect of the seven Chinese herbal extract in PC- SPES on androgen insensitive prostate cancer (AIPC) cell lines and in nude mice inoculated with the cancer cells. Methods: LC-MS was used to detect any synthetic drug contamination. Flow cytometry was used to measure cell cycle and apoptosis. Western blot was used to measure p16, p-RB, p21, Bax, Bad, bcl-2 and Bcl-X, Caspase and PARP protein expressions. IL-6 was detected by the method of ELISA. Each group of 40 nude mice was inoculated with PC-3 and DU-145 separately. Oral administration of the herbal extract continued for 8 weeks. After sacrifice, various tissues were examined for pathological toxicity and the tumors were studied for molecular targets. Results: The extract, found free of drug contamination, prolonged G1 and G2/M phase in PC-3 and DU-145 respectively, and induced apoptosis in both cell lines. At concentrations of 125, 250, 500 mg/kg/d, the extract inhibited the tumor growth by 27.6%, 44.15%, 53.2% in PC-3 (p<0.05) and 29.8%, 44.2%, 66.2% in DU-145 (p<0.05) when compared to the control group (N=10 each). No toxicity was found in all tissues including liver, heart and kidney. Furthermore, p16 was elevated and the phosphorylated RB protein was decreased in both cell lines and animal tumors. The proapoptic proteins Bad and Bax were increased while the antipropotic proteins Bcl-XL and Bcl-2 were decreased. These alterations resulted in the activation of caspase-3 (in cell lines) and PARP (in mice) and finally led to apoptosis. Lastly, IL-6 was found depleted in both animal tumors. Conclusions: Our results demonstrate the activity and mechanism of the herbal extract in suppressing AIPC both in vitro and in mice. We will further investigate its therapeutic benefit in human. No significant financial relationships to disclose.