Abstract 951: Direct small-molecule BAX activation in acute myeloid leukemia

Resistance to apoptosis is a hallmark of human cancer. The BCL-2 protein family includes both pro- and anti-apoptotic proteins and has a central role in regulating mitochondrial apoptosis. Cancer cells most frequently ensure their survival and resistance to treatments by overexpression of anti-apoptotic BCL-2 proteins to maintain BAX and other pro-apoptotic members suppressed. Pro-apoptotic BAX is the cardinal executioner BCL-2-family member that, upon conformational activation and oligomerization at the mitochondrial outer membrane (MOM), causes permeabilization of the MOM and release of mitochondrial factors, e.g., cytochrome c, that activate the caspase cascade of apoptosis. Interestingly, the vast majority of cancer cells contain functional BAX in an inactive conformation or suppressed by anti-apoptotic proteins; mutations or alterations in BAX that may cause its inactivation to occur at a low frequency. Therefore, we hypothesized that induction of apoptosis by direct activation of BAX offers the possibility of a new anticancer strategy. Using a structure-based drug design approach we developed BTSA1, a pharmacologically optimized BAX activator that binds with high affinity and specificity to the N-terminal activation site (trigger site) and induces conformational changes to BAX, resulting in BAX mitochondrial translocation, oligomerization and mitochondrial dysfunction (Reyna et al., Cancer Cell 2017). BTSA1 induces activation of cytosolic BAX, leading to prompt and robust mitochondrial apoptosis in acute myeloid leukemia (AML) cell lines. The efficacy of BTSA1 is regulated by the availability of anti-apoptotic BCL-2 proteins to inhibit activated BAX and therefore, higher levels of cytosolic BAX monomer correlated with higher efficacy of BAX-mediated mitochondrial dysfunction. BTSA1-induced BAX activation promotes effectively apoptosis in primary AML samples and has significant antitumor activity in human AML xenograft models, increasing host survival. BTSA1 is orally bioavailable with excellent pharmacokinetics at therapeutically effective doses; it does not show any detectable toxicity in the hematopoietic system or other tissues. Importantly, BTSA1 is also effective in inducing apoptosis in leukemic stem cell-enriched fractions (CD34+CD38−) while sparing healthy counterparts. Consistent with their sensitivity to direct BAX activation, AML blasts and highly purified stem and progenitor cell populations from AML patients when compared to healthy counterparts displayed higher expression of BAX. Overall, our data provide proof of concept for BAX as a druggable target and demonstrate the therapeutic potential of direct BAX activation by BTSA1 as an effective treatment strategy in AML. Citation Format: Denis Reyna, Thomas Garner, Andrea Lopez, Felix Kopp, Gaurav Choudhary, Ashwin Sridharan, Swathi-Rao Narayanagari, Kelly Mitchell, Baoxia Dong, Boris Bartholdy, Loren Walensky, Amit Verma, Ulrich Steidl, Evripidis Gavathiotis. Direct small-molecule BAX activation in acute myeloid leukemia [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 951.