Abstract B25: BRD4 inhibition improves the efficacy of ABT-199 in T-cell acute lymphoblastic leukemia
Authors: Peirs Sofie , Matthijssens Filip , Pieters Tim , Vandamme Niels , Berx Geert , Poppe Bruce , Vlierberghe Pieter Van 
Published: May 2016

Abstract
Introduction: Despite the improved survival of patients with T-cell acute lymphoblastic leukemia (T-ALL), therapy-resistant or refractory T-ALL remains a major clinical challenge. Recently, we and others reported promising therapeutic activity for ABT-199, a highly specific inhibitor of the anti-apoptotic protein B-cell lymphoma 2 (BCL-2), especially in immature T-ALL. Nevertheless, the occurrence of dose-limiting toxicities and the emergence of therapy resistance provides a rationale for the evaluation of ABT-199 as part of combination therapies. Indeed, previous studies have shown that ABT-199 can synergize with conventional chemotherapeutic agents in human T-ALL.
BRD4 is an important bromodomain and extra-terminal (BET) protein that facilitates the recruitment of the transcriptional apparatus to acetylated chromatin. Small molecule inhibitors that target BET-bromodomain proteins, such as JQ1, can broadly affect oncogenic gene transcription and have recently emerged as powerful anti-cancer agents in a variety of hematological malignancies, including T-ALL.
Here, we evaluated JQ1 and ABT-199 combination therapy and provide in vitro and in vivo evidence that combined BCL-2 and BRD4 inhibition synergistically affects leukemic tumor growth in T-ALL.
Experimental Procedures: In vitro synergism between ABT-199 and JQ1 in a panel of T-ALL cell lines was determined by the combination index (CI) based on cell viability 48h after compound administration. For in vivo evaluation of the ABT-199/JQ1 combination therapy, luciferase-labeled T-ALL cells were administered by tail vein injection in immune compromised NOD scid gamma mice. Upon engraftment of leukemic cells, mice were divided into four groups and treated daily with 50mg ABT-199/kg body weight and/or 50mg JQ1/kg body weight for 13 days. The percentage of leukemic cells in the blood and bone marrow after treatment was analyzed using FACS analysis upon staining with an anti-human CD45 antibody.
Results: In vitro evaluation of ABT-199 and JQ1 combination therapy revealed strong synergism (CI&lt;0.5) in the T-ALL cell lines LOUCY, DND-41, ALL-SIL and TALL-1. In contrast, no or only slight synergistic effects (CI&gt;0.5) were observed in other T-ALL lines, including KOPTK1, JURKAT, PEER and HPB-ALL. Notably, the degree of synergism was not defined by the IC50 values for ABT-199 or JQ1, but was significantly correlated with BCL-2 expression levels. Next, in vivo drug treatment experiments using luciferase-positive LOUCY T-ALL cells confirmed the synergistic activity of the JQ1/ABT-199 combination therapy as evaluated by bioluminescence, percentage of leukemic blasts in blood or bone marrow and spleen size. Interestingly, although ABT-199 strongly reduced the leukemic burden during the first days of treatment, the therapeutic response of single ABT-199 therapy was not durable. In contrast, JQ1/ABT-199 combination therapy resulted in a sustained therapeutic response associated with tumor regression. As expected, combination therapy resulted in enhanced levels of cleaved PARP and increased caspase activity in synergistic T-ALL cell lines, in line with induction of cellular apoptosis upon ABT-199/JQ1 treatment. To better understand the molecular mechanism by which JQ1 enhances the anti-leukemic properties of ABT-199, we defined its effect on gene expression in T-ALL cell lines. Notably, this analysis showed that the balance of pro- and anti-apoptotic BCL2 family factors was affected by JQ1 treatment. For example, JQ1 treatment caused a significant reduction in BCL2 levels, but also induced a concomitant increase in anti-apoptotic MCL1. In addition, pro-apoptotic factors, such as BIM, were also upregulated upon JQ1 treatment.
Conclusions: Here, we show that BET-bromodomain inhibition can sensitize BCL2-positive human T-ALL to ABT-199 treatment by interfering with the balance of pro- and anti-apoptotic BCL2 family members.
Citation Format: Sofie Peirs, Filip Matthijssens, Tim Pieters, Niels Vandamme, Geert Berx, Bruce Poppe, Pieter Van Vlierberghe. BRD4 inhibition improves the efficacy of ABT-199 in T-cell acute lymphoblastic leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr B25.