Prostate cancer is one of the most frequently diagnosed cancers and the second leading cause of cancer death in American men. Current treatments for androgen-independent prostate cancer have not shown a definitive increase in survival. The treatment options employed for patients with advanced and metastatic prostate cancer are limited. Stat3 is a critical mediator of oncogenic signaling, which is active in 82% of prostate cancers as well as many other cancers. However, drugs targeting Stat3 have not been used to treat prostate cancer. Our goal is to develop a novel, potent inhibitor of Stat3 as a therapeutic drug for prostate cancer therapy. Developing a novel and promising treatment for prostate cancer will be greatly helpful both in terms of single agent treatment and as part of a combination therapy. We propose a novel strategy to inhibit Stat3, which could be very useful in development of prostate cancer therapy, and have developed a G-quartet oligodeoxynucleotide (GQ-ODN), T40214, as a lead compound that preferentially inhibits the DNA-binding activation of Stat3 among STAT protein members, resulting in the suppression of the anti-apoptotic genes, bcl-x and Mcl-1, in cancer cells. We also have constructed a model of GQ-ODN binding onto Stat3a dimer for rational drug design. The molecular model suggests that the GQ-ODN insert between the two SH2 domains of Stat3 dimer resulting in destabilization of the homodimers. We further show that T40214 and its derivative, T40231, have dramatic in vivo effects on prostate cancer growth in nude mice when the agent is given by intravenous or intraperitoneal injections, dramatically retarding tumor growth and significantly increasing the length of survival time. Also GQ-ODNs greatly increase the number of apoptotic cells in tumors, showing the agents using apoptosis to eliminate tumor cells. The Specific Aims designed to achieve this objective are outlined as below. (1) To develop a potent inhibitor of Stat3 to suppress the growth of prostate cancer cells in cell culture. The strategy to achieve this specific aim contains three parts: (i) rational drug design, which will include structure-based drug design and rational optimization, (ii) oligonucleotide synthesis, and (iii) Stat3 activity assays in vitro. (2) To develop an optimally effective intracellular and intranuclear delivery system for GQ-ODNs. In this aim, we will use our novel drug delivery system to determine the optimum conditions for intracellular delivery of G-quartet inhibitors, e.g. T40214 and its derivatives, in vivo. (3) To determine the effect of GQ-ODNs on Stat3 activity and the growth of prostate tumors in vivo. Using nude mice tumor xenograft assays, we will determine whether GQ-ODNs, T40214 and its derivatives, can block or control the growth of prostate tumors in xenografted mice; whether the anti-tumor effects of GQ-ODNs are on both non-androgen and androgen responsiveness; whether the suppression of tumor growth for GQ-ODNs is mediated by enhanced apoptosis of tumor cells.