Abstract
Multiple myeloma (MM) remains an incurable disease in need of the development of novel therapeutic agents and drug combinations. ABT-199 is a specific Bcl-2 inhibitor in clinical trials for MM; however, its activity as a single agent was limited to myeloma patients with the t(11;14) translocation who acquire resistance due to co-expression of Mcl-1 and Bcl-xL. These limitations preclude its use in a broader patient population. We have recently found that a sphingosine kinase 2-specific inhibitor (ABC294640) induces apoptosis in primary human CD138(+) cells and MM cell lines. ABC294640 is currently in phase I/II clinical trials for myeloma (clinicaltrials.gov: #NCT01410981). Interestingly, ABC294640 down-regulates c-Myc and Mcl-1, but does not have any effects on Bcl-2. We first evaluated the combinatorial anti-myeloma effect of ABC294640 and ABT-199 in vitro in 7 MM cell lines, all of which harbor no t(11;14) translocation. Combination index calculation demonstrated a synergistic anti-myeloma effect of the combination of ABC294640 and ABT-199. This synergistic anti-myeloma effect was maintained even in the presence of bone marrow (BM) stromal cells. The combination of ABC294640 and ABT-199 led to enhanced cleavage of PARP and caspase-3/9 and increased Annexin-V expression, consistent with the induction of apoptosis by the combination treatment. In addition, the combination of ABC294640 and ABT-199 resulted in the down-regulation of the anti-apoptotic proteins Mcl-1, Bcl-2, and Bcl-xL and the cleavage of Bax and Bid. The combination induced both the mitochondrial mediated- and caspase-mediated apoptosis pathways. Finally, the combination of ABC294640 and ABT-199 resulted in augmented anti-myeloma effect in vivo in a mouse xenograft model. These findings demonstrate that the co-administration of ABC294640 and ABT-199 exhibits synergistic anti-myeloma activity in vitro and in vivo, providing justification for a clinical study of this novel combination in patients with relapsed/refractory multiple myeloma.