KIF11 and plasma cell myeloma: In a phase II study, the single use of ARRY-520 in multiple myeloma generated a 16% response rate.32 Some of the reasons why KIF11 inhibitors did not achieve a successful clinical response as single agents are that nonresponsive tumors showed low KIF11 expression, had a low mitotic index, bore treatment-resistant mutations in KIF11, and exhibited KIF15 activity.31