Abstract Constitutive activation of Signal Transducers and Activators of Transcription 3 (STAT3) signaling is frequently detected in human cancer, including breast cancer, and has emerged as a viable molecular target for breast cancer treatment. Novel therapeutic approaches are needed for breast cancer if significant improvements in survival are to occur. To date, few STAT3 inhibitors are available for the therapy of breast cancer. Conventional high throughput screening and virtual docking drug discovery approaches identified many hits, but only a few of them may be developed into drugs. To accelerate STAT3 drug development, we used a novel drug discovery approach with multiple ligand simultaneous docking (MLSD) which we have developed and drug repositioning to search against FDA approved drug and experimental drug database in DrugBank. Using this new method, we identified the FDA-approved drug celecoxib (a COX-2 inhibitor) with a novel function to target STAT3 [Predictive STAT3 binding dissociation constant (Kd)= 8.6 μM]. In addition, a novel celecoxib analog (8A) was developed exhibiting stronger STAT3 binding (Predicted Kd= 0.12 μM, 71-fold stronger than celecoxib). Although 8A has stronger STAT3 binding, it has weaker COX-2 binding (521-fold weaker than celecoxib), suggesting its selectivity on STAT3 versus COX-2. Minimal COX-2 inhibition by 8A may reduce cardiovascular and gastrointestinal risks associated with COX-2 inhibitors. The ability of 8A to inhibit STAT3 was tested through a combination of Western Blot, MTT cell viability assay, and Immunofluorescence Standing. 8A inhibited the P-STAT3 stimulated by IL-6 but not the induction of STAT1, STAT2, STAT4, STAT5, and STAT6 phosphorylation stimulated by IFN-γ, IFN-γ, and IL-4. 8A also inhibited the induction of STAT3 nuclear localization by IL-6, which could prevent STAT3 function as a transcription factor. 8A inhibited STAT3 phosphorylation and resulted in the induction apoptosis on breast cancer cells as evidenced by caspase-3 cleavage. Further, 8A has a lower IC50 than celecoxib and was more potent than celecoxib to inhibit STAT3 phosphorylation and induce apoptosis in breast cancer cells, which confirmed the stronger STAT3 binding energy than celecoxib. In summary, our study is the first report using MLSD and drug repositioning to develop drug-like STAT3 inhibitors. This novel method has a potential to significantly speed up the STAT3 drug development for the therapy of breast cancer and possibly other types of human cancer which express constitutive STAT3 signaling. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4772. doi:1538-7445.AM2012-4772