Abstract LB-081: Co-targeting BCL-2 and PI3K potently induces BAXdependent mitochondrial apoptosis in AML
Authors: Grant Steven , Rahmani Mohamed , Nkwocha Jewel , Kmieciak Maciej , Leverson Joel , Sampath Deepak 
Published: August 2018

PURPOSE: The purpose of this study was to determine whether and by what mechanisms the PI3K/mTOR inhibitor GDC-0980 or the p110β-sparing PI3K inhibitor taselisib could enhance the activity of the selective BCL-2 antagonist venetoclax (ABT-199) in acute myelogenous leukemia (AML) cells.

EXPERIMENTAL PROCEDURES: Cells were exposed to venetoclax (10-2000 nM) ± GDC-0980 or taselisib (100-500 nM) for varying intervals (2-24 hr) after which effects on cell viability were monitored. These studies included assessment of apoptosis induction (annexin V+ or 7-AAD by flow cytometry) in parental and genetically modified cultured AML cell lines (e.g., by CRISPR-Cas9 or inducible systems), and primary AML specimens. Parallel studies were performed in AML xenograft and PDX models.

FINDINGS: Tetracycline-inducible BCL-2 down-regulation in MV4-11 or MOLM-13 cells significantly sensitized them to PI3K inhibitors. 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 down-regulation. Of note, ectopic MCL-1 expression significantly protected cells from the regimen. Combined treatment (16 hr) 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. Notably, venetoclax (80 mg/kg)/GDC-0980 (5 mg/kg for 2 weeks, then 10 mg/kg until day 55) or venetoclax/taselisib (2.5 mg/kg) co-administration markedly reduced AML growth and prolonged survival versus single-agent treatment (P = .02 and .005) in a systemic AML xenograft mouse model, and diminished AML growth in two PDX models. CRISPR gene knock-out studies in AML cells revealed that venetoclax/GDC-0980 failed to induce apoptosis in BAX-/- or BAX-/-BAK-/- cells. Venetoclax/GDC-0980 activity was partially diminished in BAK-/- cells, whereas BIM-/- cells were fully sensitive. Similar results were observed with venetoclax alone in in vitro and in vivo systemic xenograft models.

CONCLUSIONS: These studies demonstrate that the venetoclax/GDC-0980 and venetoclax/taselisib regimens exhibit potent anti-AML activity both in vitro and in vivo and that this effect occurs primarily through BAX, to a lesser extent BAK, whereas BIM is dispensable for cell death. Taken together, these findings argue that a strategy involving dual BCL-2 and PI3K inhibition warrants further evaluation in AML.

Citation Format: Steven Grant, Mohamed Rahmani, Jewel Nkwocha, Maciej Kmieciak, Joel Leverson, Deepak Sampath. Co-targeting BCL-2 and PI3K potently induces BAXdependent mitochondrial apoptosis in AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-081.