Abstract 637: Glucocorticoids act synergistically with FLT3 inhibitors to enhance cell death in FLT3 mutant acute myeloid leukemia
Authors: Gebru Melat Tsegaye , Wang Hong-Gang , Xavier Jennifer 
Published: September 2020

Abstract
FLT3 internal tandem duplication (FLT3-ITD) is a frequent mutation associated with a poor prognosis of less than 30% five-year survival rate in acute myeloid leukemia (AML). Quizartinib, a selective FLT3 inhibitor, is currently showing promising results in phase III clinical trials for AML patients with mutant FLT3. Despite an impressive initial response to quizartinib, most patients relapse with drug resistance. The mechanism by which resistance arises and the initial response to drug treatment that promotes cell survival is poorly understood. Drug-tolerant “persister” cells (DTPs) are a transiently maintained subpopulation of drug-sensitive cells that survive cytotoxic drug exposure despite lacking resistance-conferring mutations. Here, we explore the impact of DTPs in FLT3-ITD AML cells treated with cytotoxic doses of quizartinib. We demonstrate that treatment with quizartinib induces significant changes in gene expression pattern that promote survival of the DTP population of cells. These changes are transient and result in a window of therapeutically exploitable vulnerabilities. Using an unbiased genome-wide transcriptome analysis, we report that inflammatory pathways to be the most significantly upregulated pathways in DTPs relative to drug naive cells. Independently, a drug-screening using a panel of FDA approved drugs reveals that DTPs are significantly more sensitive to anti-inflammatory drugs, particularly glucocorticoids. Consistent with this observation, a combination of glucocorticoids and quizartinib as well as other FLT3 inhibitors results in a synergistic cell death of FLT3 mutant but not wildtype cells. We show that this synergistic cell death is mediated through the glucocorticoid receptor which, when activated, upregulates the expression of the pro-apoptotic protein BIM. Glucocorticoid-induced increase in BIM together with quizartinib-induced decrease in the expression of the anti-apoptotic protein MCL-1 synergizes and results in augmentation of apoptosis in combination treated cells. CRISPR/Cas9-mediated knockout of the glucocorticoid receptor as well as BIM can completely rescue the observed drug combination synergy. Furthermore, inhibition of MCL-1 using a BH3-mimetic inhibitor, MIK665, displays a significant synergy with quizartinib. The synergy between quizartinib and glucocorticoids has been further validated using AML patient cells ex-vivo as well as AML mouse models in-vivo. Although glucocorticoids are the mainstay of treatment for lymphoblastic leukemia, their use in AML has been shown to be negligible. Our data demonstrates that treatment with FLT3 inhibitors sensitizes AML cells to glucocorticoids and thereby provides a basis for combining glucocorticoids with FLT3 inhibitors in AML patients with FLT3 mutation. Critically, since glucocorticoids have decades of clinical use and experience, the proposed combination therapy could rapidly be explored in the clinic to improve patient outcome.
Citation Format: Melat Tsegaye Gebru, Hong-Gang Wang, Jennifer Xavier. Glucocorticoids act synergistically with FLT3 inhibitors to enhance cell death in FLT3 mutant acute myeloid leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 637.