Co-targeting BCL-2 and PI3K induces BAX-dependent mitochondrial apoptosis in AML cells

Inhibitors targeting BCL-2 apoptotic proteins have significant potential for the treatment of acute myeloid leukemia (AML); however, complete responses are observed in only 20% of patients suggesting targeting BCL-2 alone is insufficient to yield durable responses. Here we assessed the efficacy of co-administration of the PI3K/mTOR inhibitor GDC-0980 or the p110β-sparing PI3K inhibitor taselisib with the selective BCL-2 antagonist venetoclax in AML cells. Tetracycline-inducible downregulation of BCL-2 significantly sensitized MV4-11 and MOLM-13 AML cells to PI3K inhibition. Venetoclax/GDC-0980 co-administration induced rapid and pronounced BAX mitochondrial translocation, cytochrome c release, and apoptosis in various AML cell lines in association with AKT/mTOR inactivation and MCL-1 downregulation; ectopic expression of MCL-1 significantly protected cells from this regimen. Combined treatment was also effective against primary AML blasts from 17 patients, including those bearing various genetic abnormalities. Venetoclax/GDC-0980 markedly induced apoptosis in primitive CD34+/38-/123+ AML cell populations but not in normal hematopoietic progenitor CD34+ cells. The regimen was also active against AML cells displaying intrinsic or acquired venetoclax resistance or tumor microenvironment-associated resistance. Either combinatorial treatment markedly reduced AML growth and prolonged survival in a systemic AML xenograft mouse model and diminished AML growth in two patient-derived xenograft models. Venetoclax/GDC-0980 activity was partially diminished in BAK-/- cells and failed to induce apoptosis in BAX-/- and BAX-/-BAK-/- cells, whereas BIM-/- cells were fully sensitive. Similar results were observed with venetoclax alone in in vitro and in vivo systemic xenograft models. Collectively, these studies demonstrate that venetoclax/GDC-0980 exhibits potent anti-AML activity primarily through BAX and, to a lesser extent, BAK. These findings argue that dual BCL-2 and PI3K inhibition warrants further evaluation in AML.

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