Ispinesib is a specific inhibitor of kinesin spindle protein (KSP), with a K_{i app} of 1.7 nM. IC50 & Target: Ki app: 1.7 nM (KSP)^[1] In Vitro: Ispinesib is a potent, highly specific inhibitor of KSP, with a K_{i app} of 1.7 nM^[1]. Ispinesib (150 nM) inhibits BT-474 and MDA-MB-468 cell lines, with GI₅₀s of 45 and 19 nM, respectively^[2]. Ispinesib (SB715992, 15 and 30 nM) suppresses the proliferation of PC-3 prostate cancer cell by 48.65% and 52.16%, and induces apoptosis of prostate cancer cell by 1094.88% and 1516.70%, respectively. Ispinesib up regulates genes responsible for apoptosis and cell cycle arrest, and down regulates genes responsible for cell proliferation and survival. The anti-proliferation and pro-apoptotic activities of Ispinesib can be enhanced by genistein^[3]. In Vivo: Ispinesib (SCID, 8 mg/kg; nude, 10 mg/kg, q4d × 3) reduces tumor volume in mice bearing tumor xenografts of ER-positive (MCF7), HER2-positive (KPL4, HCC1954, and BT-474), and triple-negative (MDA-MB-468) breast cancer cells via i.p. one dose every 4 days repeated three times^[2].