Abstract 3351: Identifying novel therapies from unbiased screens in AML with MECOM re-arrangement
Authors: Birdwell Christine E. , Fiskus Warren C. , Mill Christopher P. , Kadia Tapan M. , Daver Naval , DiNardo Courtney D. , Sasaki Koji , Davis John A. , Das Kaberi , Hou Hanxi , Cerebelli Michele , Bhalla Kapil N. 
Published: March 2024

Abstract
EVI1 is a transcriptional regulator encoded from the MECOM locus (3q26.2). Aberrant EVI1 expression is observed in ~10% of de novo AML, of which about half are due to inv3(q21;q26.2) or t(3;3)(q21;q26.2) chromosomal rearrangements [3q26.2-r]. 3q26.2-r AMLs generally have a stem-like phenotype, are refractory to current therapy options and exhibit poor overall survival. Therefore, there is an unmet need to develop novel targeted therapies with improved efficacy in repressing EVI1 and its targets, thereby improving outcomes in AML with EVI1 overexpression. We conducted an unbiased high-throughput drug screen, utilizing a library of mechanistically annotated drugs (NCATS Mechanism Interrogation Plates [MIPE 5.0]), against seven AML cell lines (3q26.2-r: UCSD-AML1, HNT-34, AML191, AML194; non-3q26.2-r AML: SET-2, MV4-11, OCI-AML3). This exploited the mechanistic redundancy in the library to identify druggable, target-level dependencies in 3q26.2-r AML, exhibiting absolute potency and relative activity as compared to other AML subtypes. BRD4 was identified as an absolute dependency in 3q26.2-r AMLs. An unbiased epigenetic modifier CRISPR screen in UCSD-AML further confirmed BRD4 as a dependency, confirming previous reports of BET inhibition (BETi) efficacy in 3q26.2-r AML. Comparison of 3q26.2-r versus other AML cell lines also identified XIAP, mTOR, PIK3CA and Bcl-xL as druggable vulnerabilities in 3q26.2-r AML. In follow-up experiments, IAP family inhibitors/SMAC mimetics induced significantly more dose-dependent apoptosis in 3q26.2-r versus the other AML cell lines, which was associated with reduction of IAP proteins, p-ERK, MCL1 and Bcl-xL, but increase in levels of cleaved caspase-3 and PARP. BETi (mivebresib) also induced dose-dependent apoptosis, and reduced EVI1, c-Myc, c-Myb, IAP proteins, and CDK4/6, while increasing levels of HEXIM1 and cleaved PARP. Consistent with this, co-treatment of mivebresib with SMAC mimetic birinapant or IAP inhibitor LCL161 was synergistically lethal against 3q26.2-r cells. Co-targeting the other identified druggable vulnerabilities from the drug screen with BETi (mTOR/PI3K with BGT-226 and dactolisib, Bcl-xL with navitoclax and A1155463, as well as CBP/p300 (with GNE781, identified through the CRISPR screen), exerted synergistic lethality in 3q26.2-r AML cells. Finally, in a xenograft model of 3q26.2-r AML in immune-depleted mice, while monotherapy with LCL161 or dactolisib significantly reduced the AML burden with minimal toxicity, co-treatment with mivebresib and LCL161 or dactolisib was superior in reducing AML burden in the xenograft model. These findings demonstrate promising preclinical activity of BETi and IAP protein or mTOR/PI3K antagonists in the cellular models of AML with EVI1 overexpression. This supports the rationale to determine in vivo efficacy of these BETi-based combinations against this therapy-resistant AML sub-type.
Citation Format: Christine E. Birdwell, Warren C. Fiskus, Christopher P. Mill, Tapan M. Kadia, Naval Daver, Courtney D. DiNardo, Koji Sasaki, John A. Davis, Kaberi Das, Hanxi Hou, Michele Cerebelli, Kapil N. Bhalla. Identifying novel therapies from unbiased screens in AML with MECOM re-arrangement [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3351.