569 Biological consequences of selective inhibition of the first BET bromodomain

Abstract

Background: Single agent OTX015 has shown activity in several preclinical models of lymphoid tumors, including MCL and MM (AACR 2014), and notably in the ongoing phase I study (AACR 2014). We evaluated the synergy of OTX015 administered in combination with other anticancer compounds in several MCL and MM cell lines. Material and Methods: Two mantle cell lymphoma (REC1, MAVER1) and three multiple myeloma (RPMI8226, U266B1, KMS11) cell lines were exposed to increasing doses of OTX015 alone or in combination with increasing doses of other anticancer drugs. MTT assays were performed after 72 hours exposure. Synergy was assessed by Chou– Talalay combination index (CI) with the Synergy R package: CI < 0.3, strong synergy; 0.3−0.9, synergy; 0.9−1.1, additive effects. Changes in CCND1 protein and mRNA expression after OTX015 exposure were assessed in 4 MCL cell lines (REC1, MAVER1, Granta519, JeKo1). Results: Additive effects were seen in 2/2 MCL cell lines when OTX015 was combined with the demethylating agent 5-AZA (CI = 0.8 and 1), the HDAC-inhibitor vorinostat (CI = 0.8 and 1), and the proteasome inhibitor carfilzomib (CI = 0.8 and 0.9). Whereas, synergy was observed with the BTK-inhibitor ibrutinib in 2/2 MCL cells (CI = 0.7 and 0.4), however in ibrutinib-resistant MAVER1 cells, synergy was only seen with high ibrutinib doses (5−10mM). Synergy was observed in 1/2 MCL cell lines with OTX015 combined with the dual PI3K/mTOR inhibitor BEZ235 (CI = 0.1), the mTOR inhibitor everolimus (CI = 0.5), the glucocorticoid dexamethasone (CI = 0.1), and the immunomodulator pomalidomide (CI = 0.2). CCND1 mRNA and protein levels were not altered in 4/4 MCL cell lines after OTX015 (500 nM) treatment for 4 h and 24 h, suggesting that the antiproliferative activity of OTX015 is not CCND1mediated. Synergy was observed in 3/3 MM cell lines following OTX015 combined with carfilzomib (CI = 0.5, 0.7 and 0.9), or vorinostat (CI = 0.7, 0.6 and 0.7), in 2/3 cell lines with pomalidomide (KMS11, CI = 0.3; RPMI8226, CI = 0.4;) or 5-AZA (KMS11, CI = 0.4; RPMI8226, CI = 0.4) and in 1/3 with dexamethasone (U266B1, CI = 0.4). Conclusions: Preclinical experiments support the exploration of OTX015 combined with other anticancer agents in the clinical setting. Additional studies should be performed to clarify the mechanistic or molecular expression profile associated with sensitivity/resistance to individual combinations.