TP-110 is a new proteasome inhibitor, which shows potent growth inhibition in various tumor cell lines. Treatment with TP-110 for 24 h in vitro induced apoptosis in multiple myeloma cell line RPMI8226. TP-110 reduced the intrinsic inhibitor of apoptosis proteins (IAPs), cIAP-1 and XIAP, that suppress executioner caspases.