A small-molecule inhibitor of the kinesin-related motor protein Eg5 with potential antineoplastic activity. Eg5 kinesin-related motor protein inhibitor ARQ 621 selectively inhibits the activity of Eg5, which may result in mitotic disruption, apoptosis and cell death. The ATP-dependent Eg5 kinesin-related motor protein (also known as KIF11 or kinesin spindle protein-5) is a plus-end directed kinesin motor protein involved in the regulation of spindle dynamics, including assembly and maintenance, during mitosis.