Potent, cell-permeable, selective inhibitor of mitotic kinesin Eg5, a protein required for establishing and maintaining a bipolar spindle. Inhibits basal ATPase activity (IC50 = 1 mM) and microtubule-activated ATPase activity of Eg5 (IC50 = 140 nM). Induces mitotic arrest in HeLa cells with an IC50 of 700 nM. Displays antitumor activity.