OTSSP167 is an inhibitor of maternal embryonic leucine zipper kinase (MELK) with IC50 value of 0.41nM . MELK is a member of the AMPK serine/threonine kinase family and is involved in the mammalian embryonic development. It plays roles in cancer cell growth and formation or maintenance of cancer stem cells. OTSSP167 is a small-molecule inhibitor and can inhibit MELK’s activity effectively and selectively. OTSSP167 inhibits cell proliferation of a variety of cancer cell lines including A549, T47D, DU4475 and 22Rv1. The IC50 values are 6.7nM, 4.3nM, 2.3nM and 6nM, respectively. In mice bearing MDA-MB-231 xenograft, administration of OTSSP167 inhibits 70% tumor growth at dose of 20 mg/kg. Besides that, OTSSP167 is also found to have effects on PSMA1 and DBNL which are the novel substrates of MELK. It inhibits the phosphorylation of PSMA1 and DBNL causes the subsequent suppression of mammosphere formation .