Abstract 1721: BRD4 antagonist-based therapy exerts lethal activity against FLT3 mutation expressing AML cells resistant to FLT3 tyrosine kinase inhibitors
Authors: Rodriguez Melissa , Fiskus Warren , Sharma Sunil , Qi Jun , Schaub Leasha J. , Shah Bhavin , Devaraj Santhana G.T. , Liu Ka , Iyer Swaminathan , Bradner James E. , Bhalla Kapil N. 
Published: May 2015

Abstract
The BET (bromodomain and extra terminal) protein antagonists, e.g., JQ1, disrupt the binding of the bromodomains of BRD4 to acetylated lysines, displacing the BET proteins, along with the associated transcript initiation and elongation factors, from the chromatin. JQ1 treatment also induces hexamethylene bisacetamide (HMBA)-inducible protein 1 (HEXIM1), which binds and sequesters pTEFb, thereby inhibiting its kinase activity for RNAP II. This inhibits the pause release of RNAP II, thereby attenuating the elongation and expression of the mRNA of oncogenes, e.g., c-MYC and BCL2, and their target genes, leading to growth inhibition and apoptosis of AML cells. Although the second generation FLT3 tyrosine kinase inhibitors (TKI), such as AC220 induce remissions in the aggressive sub-type of AML expressing FLT3 mutations, resistance-causing gatekeeper (F691I/L), activation loop (AL) (D835V/Y/F) or compound FLT3-ITD/F691I mutations are known to impair the in vitro and in vivo activity of the FLT3-TKIs. We have recently determined that JQ1, but not its inactive enantiomer R-JQ1 potently induced apoptosis of not only Ba/F3/FLT3-ITD but also Ba/F3/FLT3-ITD expressing the highly FLT3 TKI-resistant mutations F691L and D835V (IC50 values for JQ1 were 697, 1588 and 909 nM, in the three cell lines, respectively). JQ1 dose-dependently induced apoptosis of human cultured AML MOLM13 and MV4-11 cells, as well as of primary AML cells, all expressing FLT3-ITD. Concomitantly, JQ1 treatment attenuated c-MYC, BCL2 and CDK6, while inducing p21, p27, BIM and cleaved PARP levels. Following engraftment of NOD/SCID mice with MOLM13 xenografts, treatment with JQ1 (50 mg/kg, daily x 5 days per week x 3 weeks), versus treatment with vehicle control, significantly improved survival of the mice (p &lt; 0.01), without causing any toxicity. Co-treatment with JQ1 and AC220 or the pan-histone deacetylase (HDAC) inhibitor (HDI) panobinostat (PS) synergistically induced apoptosis of cultured (MV4-11 and MOLM13) and primary AML cells expressing FLT3-ITD. We also determined the effects of JQ1 against MOLM13/TKIR cells that were generated under the continuous selection pressure of FLT3-TKI, and exhibited &gt; 50-fold resistance to AC220. As compared to the parental MOLM13, the TKI-resistant cells express higher levels of BRD4, c-MYC and class I HDACs. Importantly, as compared to the parental MOLM13, the MOLM13/TKIR cells were significantly more sensitive to JQ1-induced apoptosis (p &lt; 0.001). Additionally, co-treatment with JQ1 and PS, but not AC220, synergistically induced apoptosis of MOLM13/TKIR cells. This was associated with marked attenuation of c-MYC levels. Collectively, these findings support further in vivo testing of BRD4 antagonist-based combinations with AC220 or PS against FLT3 TKI-sensitive, and of BRD4 antagonist with PS against FLT3 TKI-refractory AML expressing FLT3-ITD.
Citation Format: Melissa Rodriguez, Warren Fiskus, Sunil Sharma, Jun Qi, Leasha J. Schaub, Bhavin Shah, Santhana G.T. Devaraj, Ka Liu, Swaminathan Iyer, James E. Bradner, Kapil N. Bhalla. BRD4 antagonist-based therapy exerts lethal activity against FLT3 mutation expressing AML cells resistant to FLT3 tyrosine kinase inhibitors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1721. doi:10.1158/1538-7445.AM2014-1721