Abstract Currently advanced prostate cancer is treated by suppressing androgen receptor (AR) signaling. Since AR could be activated via multiple mechanisms, inhibition of one or two of these mechanisms could be initially effective, but drug resistance can rapidly develop due to kick-in of other AR-activating mechanisms, resulting in lethal castration-resistant prostate cancer (CRPC). The therapeutic effects of all of the currently available AR-targeting agents are short-lived. To achieve long lasting therapeutic effect for prostate cancer, it appears we need to get out of the box of AR. Here, we propose to develop chemical inhibitors of oncogenic protein ERG, which is an ETS transcriptional factor. The rationale for inhibiting ERG as a possible treatment for prostate cancer has two folds: i) ERG plays a critical role in prostate cancer initiation and progression, and ii) Half of all prostate cancer patients are TMPRSS2-ERG fusion positive, which result in high level of ERG oncoprotein. ERG is a transcriptional factor that directly regulates EZH2, oncogene c-Myc and tumor suppressor Nkx3.1 and many other targets in prostate cancer. ERG cooperates with PI3K/AKT and AR in tumor initiation. ERG knockdown inhibits tumor growth in xenograft of VCaP cells. To date, accumulated studies in patients and experimental models have indicated that ERG overexpression as a result of TMPRSS2-ERG fusion plays a critical role in prostate cancer initiation and progression. However, ERG protein has no obvious functional sites or enzymatic activity to be inhibited and is considered ‘undruggable’. In contrast to such view, our laboratory has recently discovered a novel chemical inhibitor of ERG by combining virtual screening and reporter assay. We have built a structural model of ERG DBD and identified a putative binding-site for virtual screening study. We discovered a novel compound, which substantially inhibits ERG-dependent reporter activity in HEK293 cells and inhibits expression of ERG-regulated genes, such as EZH2 and c-Myc, in VCaP cells. We further showed that our compound inhibits proliferation of VCaP cells, and disrupts association of ERG with its essential coactivator PARP1 by co-IP assay. Surface Plasmon Resonance (SPR) analysis has confirmed that our compound has direct binding with recombinant ERG protein. As recent studies indicated that advanced prostate cancer with TMPRSS2-ERG fusion have substantial higher tendency to metastasize to lymph nodes, ERG inhibitors could be an effective therapeutics to prevent this progression. Importantly, ERG inhibitors could be effective therapeutics for advanced prostate cancer. Citation Format: Xiaohong Tian. Development of novel small-molecule inhibitors of ETS oncoproteins as anti-prostate cancer agents. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3661. doi:10.1158/1538-7445.AM2015-3661