Survivin, a member of the inhibitor of apoptosis protein (IAP) family, is highly expressed in various kinds of tumors. In the present study, we investigated the cytotoxic mechanism of YM155, a unique small-molecule inhibitor of survivin, in human myelogenous leukemia cells. YM155 potently inhibited the cell growth of HL-60 and U937 cells with the half-maximal inhibitory concentration (IC50) value of 0.3 nM and 0.8 nM, respectively. YM155 significantly suppressed the levels of mRNA expression and protein of survivin in HL-60 and U937 cells. In addition, we also found that YM155 down-regulated the level of Mcl-1, another critical anti-apoptotic protein, in both HL-60 and U937 cells. Treatment of HL-60 and U937 cells with YM155 induced apoptosis concomitant with the activation of caspase-8 and caspase-3. Interestingly, we have found that caspase-8 inhibitor Z-IETD-FMK strongly inhibited YM155-induced apoptosis in HL-60 and U937 cells. When cells were pretreated with Z-IETD-FMK, the activation of caspase-3 was completely abolished, suggesting that caspase-8 may be involved in the activation of caspase-3 during YM155-induced apoptosis. We demonstrated for the first time that YM155 induces caspase-8 dependent apoptosis through downregulation of survivin and Mcl-1 in human leukemia cells. (C) 2013 Elsevier Inc. All rights reserved.