5065 Background: We identified Purα as a crucial component of a transcriptional RC that binds to specific sequences in the 5’-UTR of the AR gene (ARS). Purα knockdown in androgen-dependent (AD) LNCaP cells and forced expression in an androgen- independent-derivative (AI) established the loss of Purα is determinant of AR overexpression and AI growth (Can Res.2008). We have now analyzed: 1. Purα expression and binding to the ARS in hormone naïve (HN) and HR-PC tissues. 2. The effect of histone deacetylase inhibitors (HDACI) on Purα and AR expression and resistance of AI LNCaP cells to hormones. Methods: Tissue Microarray immunohistochemistry (IH), real time PC (QPCR), Oncomine analysis and ChIP assays were used for analysis of Purα and AR expression in tissues. MTT, Western, and EMSA measured the effect on growth and targets of HDACI’s ± casodex treatment of AD and AI LNCaP cells. Results: AR was predominantly nuclear in all tumors with significantly lower (p=0.011) IH staining intensity in HN (mean ±SD=15.4±16.4) than HR (24.4±17.8). The percentage of Purα positive nuclei was significantly lower (P=0.005) for HR (mean ±SD=34.1±37.9; median =12.0) than HN tumors (mean ±SD=80.5±37.2; median=100). AR mRNA levels by QPCR were significantly (p= 0.0317) lower in HN versus HR tumors (mean ±SD=4,172.1±5,992.0 and 52,341.0±102,570.0, respectively) while Purα-mRNA levels were significantly (p=0.0317) higher (5,282.3±6,498.4 and 1.764.5±1.055.6) respectively. An Oncomine database showed similar Purα-mRNA pattern. Purα bound to the ARS-containing DNA sequence by Q-ChIP was 4-fold lower in 3 HR (one sided p = 0.029) than 4 HN tumors. Treatment of AI cells with LBH589 or SAHA restored Purα binding to the RC, reduced AR mRNA levels and growth. The HDACI-bicalutamide combinations were highly synergistic at low concentrations. Conclusion: Loss of Purα binding from a novel transcriptional RC of AR is linked to AR mRNA and protein overexpression in HRPC and LNCaP cells with AI growth. HDACI’s that restore Purα and decrease AR mRNA levels reverse the resistance to antiandrogens. These combinations may translate into a new strategy for hormonal manipulation of AI PC. No significant financial relationships to disclose.