목적: Uterine leiomyosarcoma (LMS) is a rare uterine malignancy that arises from the smooth muscle of the uterine wall. Although LMS is an aggressive tumor associated with a high risk of recurrence, current radiotherapy and chemotherapy is limited role in the treatment. Therefore, the introduction of a new therapy for LMS of the uterus is urgently needed. Ulipristal acetate (UPA, selective progesterone receptor modulator) is used for pre-operative treatment of moderate to severe symptoms of uterine fibroids because UPA-treated fibroids shown about inhibition of proliferation and induction of apoptosis and remodeling of the extracellular matrix. However, anti-cancer effect of LMS has not yet been reported. This study was designed to investigate preclinical efficacy of UPA as anti-cancer agent in LMS cells. 방법: Methods We treated UPA in human LMS cells including SK-UT-1 and MESSA to evaluate the effect on cell proliferation using MTT assay. To check the apoptosis according to UPA treatment, we performed ELISA in LMS cells. In addition, in vivo therapy experiments of UPA were done using xenografts using SK-UT-1 and MESSA in nude mice. SK-UT-1 or MESSA cells were injected s.c, into mice (n = 10 per group) and these were randomly assigned to two groups: PBS (control), oral UPA 2mg/kg once daily. All specimens were fixed in 4% paraformaldehyde and paraffin-enclosed for examination. Sections 4 μm thick were stained with TUNEL and immunohistochemistry. 결과: UPA significantly inhibited the cell survival the growth and increased the apoptosis in SK-UT-1 and MESSA cells. In in vivo experiments, UPA significantly decreased the tumor growth in both mice with SK-UT-1 and MESSA cells compared with control (both p<0.001). Moreover, the immunohistochemical analysis using in vivo tumor samples showed that UPA treated group decreased Ki-67 expression and increased apoptosis revealed by TUNEL assay compared with control. 결론: We found that UPA have anti-cancer-effects in LMS cells via the action to cell proliferation and apoptosis through in vivo and in vivo tests. Further studies using patient-derived tumor xenograft model (Avatar mice) and detailed mechanisms of anti- cancer effects for UPA will be needed.