ARQ-621 is 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. In proliferation assays, ARQ-621 showed broad spectrum cytotoxicity against a panel of human cancer cell lines, with cell cycle analysis confirming a G2/M arrest followed by apoptosis. When administered intraperitoneally to nude mice bearing human pancreatic tumors (MIA PaCa-2), complete tumor stasis was achieved with a 6.25 mg/kg dose 3 times weekly for 4 weeks. At the completion of ARQ-621 dosing, the mice showed hematology profiles indistinguishable from untreated controls with no evidence of bone marrow toxicity. A phase I trial has been completed, however research has being discontinued.