21179 Background: Selective cyclooxygenase 2 (COX2) inhibitors, including celecoxib, has shown its antitumor effects against various cancers including breast cancer. Gabexate mesilate (GM) can be found to have anti-cancer activities against tumors such as colon cancer. We tried to determine whether synergism can exist between GM and celecoxib against human breast cancer cells. Methods: MCF7/Her18 (estrogen receptor positive) and MDA-MB-436 (estrogen receptor negative) human breast cancer cells were used. Celecoxib and GM were added into the culture media at a dose of 30 and 50 μM for estrogen receptor (ER) (+) cells and 50 and 75 μM for ER (-) cells. Cellular proliferation assays wer performed and trypan blue exclusion method was adopted to count the viable cells after culture. Cell cycle analysis and fluorescein-assorted cell sortings were done to check the cell cycle changes after treatment. Immunoblotting using Akt, phosphorylated Akt (p- Akt), β-catenin, COX2 and VEGF was done to evaluate changes in expression of these proteins. Results: Combination treatment group showed significantly enhanced tumoricidal effect than either mono-treatment group on cellular proliferation study. Combined treatment yielded more G1 phase population and G2-M phase cells. (p<0.05) The level of Akt, p-Akt, VEGF, COX2 expression were decreased more significantly than either mono-treatment group. (p<0.05) Conclusions: We think that there could be synergistic effect between GM and celecoxib on the human breast cancer cells. Either drug has been used for many cases without significant problem, and can be tried for further ex vivo studies to determine synergistic effects between them. Possible antiangiogenentic actions after the combination should be evaluated thoroughly in other cell lines to get more preclinical data. No significant financial relationships to disclose.