Abstract #3867: Degradation of the anti-apoptotic protein Mcl-1 correlates with mitotic cell death in response to the novel KSP inhibitor ARRY-520.

Mitotic kinesin inhibitors are promising cancer drug candidates. Inhibition of the mitotic kinesin KSP/Eg5 prevents the formation of a bipolar spindle and causes a prolonged arrest of the cell cycle in M phase of both normal and cancer cells. Cells respond to mitotic arrest in several ways. Firstly, apoptotic cell death can be triggered while the cell is arrested in mitosis. Alternatively, the mitotic checkpoint can fail (mitotic slippage) and the cell can enter an interphase state. In some cases, the cell can survive mitotic slippage, while in others this leads to cell death. We have studied the fate of four multiple myeloma cell lines (RPMI8226, JJN3, U266 and H929) in response to the novel KSP inhibitor ARRY-520. Upon continuous treatment with ARRY-520, the majority (3/4) of cell lines undergo a sustained mitotic arrest, with the resulting onset of apoptosis (as measured by caspase 3/7 activation and PARP cleavage) occurring within 24-30h while the cell remains in M-phase. The remaining cell line (H929) is temporally refractory to ARRY-520 treatment, initiating mitotic slippage and subsequently showing a peak in apoptotic markers after 72-80h of treatment, while the majority of cells are in interphase. Biochemical analysis of these lines suggests that the loss of the key anti-apoptotic protein MCL-1 correlates with apoptosis in mitotic cells. MCL-1 is a typically short-lived member of the BCL-2 family of anti-apoptotic proteins, and is a key survival protein in hematological malignancies and normal hematopoietic cells. Of interest in H929 cells, MCL-1 remains stable during the mitotic arrest and cells subsequently slip and enter interphase, consistent with the failure of these cells to trigger cell death in mitosis. These data suggest that the loss of survival signaling in mitosis is consistent with the onset of apoptosis during mitosis, and persistent anti-apoptotic signals during mitosis enables the cell to survive until mitotic slippage and entry into interphase. These results indicate that variation in sensitivity to KSP inhibition is governed by a delicate balance between anti-apoptotic proteins and spindle checkpoint activity. Cells that rely on short-lived anti-apoptotic proteins for survival during mitotic arrest are more likely to undergo a rapid onset of apoptosis in response to novel mitotic inhibitors such as KSP inhibitors. These data would suggest that hematological malignancies, which frequently rely on MCL-1, are good candidates for treatment with KSP inhibitors. ARRY-520 is currently in clinical trials in both AML and multiple myeloma.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3867.