To date, the prognosis of multiple myeloma (MM) in patients harboring cytogenetic abnormalities (CA) involving t (4; 14) and deletion of chromosome 17 remains poor despite recent advances in drug development that include the use of immunomodulatory drugs (IMiDs) such as lenalidomide for MM. To address this issue, we have developed a novel phenylphthalimide derivative, TC11, that is structurally related to IMiDs. It remains unclear how TC11 induces apoptosis of MM cells with high-risk CA. Here, we show that TC11 does not induce degradation of CRBN's substrates, IKZF1/3 and CK1 alpha, and induces apoptosis of CRBN-silenced MM; this effect was independent of the cereblon (CRBN) pathway, which is involved in the mechanism of action of IMiDs used for the treatment of MM. We also revealed that TC11, in contrast to existing IMiDs, induced degradation of MCL1 and activation of caspase-9. Furthermore, inhibition of CDK1 by CGP74514A prevented TC11-induced MCL1 degradation, caspase-9 activation, and the subsequent apoptotic cell death. We showed that ectopic MCL1 expression rescued apoptosis of MM. These observations suggest that TC11 induces apoptotic death caused by degradation of MCL1 during prolonged mitotic arrest. Therefore, our findings suggest that TC11 is a potential drug candidate for high-risk MM. (C) 2019 Published by Elsevier Inc.