Inhibition of Mcl-1 with the pan-Bcl-2 family inhibitor (-)BI97D6 overcomes ABT-737 resistance in acute myeloid leukemia

Overexpression of antiapoptotic Bcl-2 proteins such as Bcl-2, Bcl-xL, and Mcl-1 is widely associated with tumor initiation, progression, and chemoresistance. Furthermore, it has been demonstrated that Mcl-1 upregulation renders several types of cancers resistant to the Bcl-2/Bcl-xL inhibitors ABT-737 and ABT-263. The emerging importance of Mcl-1 in pathogenesis and drug resistance makes it a high-priority therapeutic target. In this study, we showed that inhibition of Mcl-1 with a novel pan-Bcl-2 inhibitor (-)BI97D6 potently induced Apoptosis in acute myeloid leukemia (AML) cells. (-)BI97D6 induced hallmarks of mitochondrial Apoptosis, disrupted Mcl-1/Bim and Bcl-2/Bax interactions, and stimulated cell death via the Bak/Bax-dependent mitochondrial Apoptosis pathway, suggesting on-target mechanisms. As a single agent, this pan-Bcl-2 inhibitor effectively overcame AML cell Apoptosis resistance mediated by Mcl-1 or by interactions with bone marrow mesenchymal stromal cells. (-)BI97D6 was also potent in killing refractory primary AML cells. Importantly, (-)BI97D6 killed AML leukemia stem/progenitor cells while largely sparing normal hematopoietic stem/progenitor cells. These findings demonstrate that pan-Bcl-2 inhibition by an Mcl-1-targeting inhibitor not only overcomes intrinsic drug resistance ensuing from functional redundancy of Bcl-2 proteins, but also abrogates extrinsic resistance caused by the protective tumor microenvironment.