OTS514 HCl (OTS 514; OTS-514), the hydrochloride salt of OTS514, is a novel and highly potent inhibitor of the TOPK (T-LAK cell-originated protein kinase) with potential antineoplastic activity. Its IC50 value of 2.6 nM indicates that it inhibits TOPK. The growth of TOPK-positive cancer cells was significantly suppressed by it. Moreover, it demonstrates a strong growth-inhibitory effect on ovarian cancer cell lines, with IC50 values ranging from 3.0 to 46 nM. OTS514 can cause apoptotic cell death in small cell lung cancer by exhibiting growth inhibitory effects, with an IC50 range of 0.4 ~ 42.6 nM. The activity of a protein involved in the stemness of CSC, forkhead box protein M1 (FOXM1), was also suppressed by OTS514. Additionally, OTS514 treatment exhibited increased cytotoxicity against lung sphere-derived CSC-like SCLC cells and decreased CD90-positive SCLC cells.