Abstract B77: The BET bromodomain inhibitor OTX015 (MK-8628) in mantle cell lymphoma (MCL): in vivo activity and identification of novel combinations to overcome adaptive resistance
Authors: Gaudio Eugenio , Bernasconi Elena , Tarantelli Chiara , Cascione Luciano , Kwee Ivo , Rinaldi Andrea , Ponzoni Maurilio , Riveiro Maria E. , Zucca Emanuele , Bertoni Francesco 
Published: May 2016

Abstract
OTX015 (MK-8628) has shown anti-lymphoma activity in both the preclinical and clinical settings (Boi et al, CCR 2015; Stathis et al, EORT-NCI-AACR 2014). Now, we report in vivo and in vitro data on OTX015 in MCL, including combination regimens.
Methods. NOD-Scid (NOD.CB17-Prkdcscid/NCrHsd) mice were subcutaneously inoculated with 15 x106 MCL REC1 cells. Tumor size was measured twice weekly using digital calipers. Tumor volumes were calculated using the equation V = [length x width2]/2, in which the width and the length are the shortest and the longest diameters of each tumor, respectively. Gene expression profiling (GEP) was performed with Illumina HumanHT12 Expression BeadChips. Combinations were evaluated using the Chou-Talalay combination index (CI): &lt;0.9, synergism; 0.9-1.1 additive effect; &gt;1.2, no benefit.
Results. Since we had previously reported in vitro activity of OTX015 as single agent in MCL cell lines (Boi et al, CCR 2015), we first confirmed its antitumor activity in an in vivo model. Mice were treated with OTX015 (50 mg/kg once daily; QDx7/w x5w) started 3 days after the graft. OTX015-treated mice did not show any weight loss. OTX015-treated REC1 xenografts presented a 2-fold decrease in volume compared with the control group (p&lt; 0.05). To better understand the activity of OTX015 in MCL and to identify rational combinations that may overcome adaptive resistance mechanisms, GEP was performed on three sensitive (Rec1, Maver1, Jeko1) and one resistant MCL cell lines (Granta519, IC50 &gt;10000 nM) exposed to DMSO or OTX015 (500 nM) for 2, 4, 8 or 12 hr. Despite the difference in the IC50 values between the three OTX015 sensitive cell lines and Granta-519, GEP changes were similar, and support the previously reported synergisms of OTX015 with ibrutinib and lenalidomide (Bernasconi et al, ASH 2014). Downregulated transcripts were enriched for MYC targets, genes involved in interferon response, in NFKB signaling, in MYD88 signaling, in B cell activation, genes more expressed in MCL than in CLL or DLBCL. TNFRSF17, TNFRSF13B, TLR10, CD72, VPREB3, PLEK, PTPN6 were among the most down-regulated transcripts, while the most upregulated ones comprised genes coding different histones of the clusters 1-2, PPP1R13B, CDKN2D (p19), IRF7, FGFR3, HES6. The upregulation of few transcripts, including HES and FGFR3, more pronounced in the resistant cell line, were suggestive of adaptive resistance mechanisms. We thus evaluated the combination of OTX015 with inhibitors of FGFR (PD173074) or MEK (pimasertib), which might counteract FGFR3 upregulation. Both combinations increased the antiproliferative effect of OTX015 in the sensitive cell line Jeko1 (CI values of 0.83 and 0.37, respectively). The combination with pimasertib was able to increase the sensitivity to OTX015 also in the resistant cell line (CI = 0.18), although it was not able to completely reverse its primary resistance.
Conclusions. OTX015 showed in vivo activity as a single agent in MCL. GEP identified new combinations with MEK and FGFR inhibitors with the potential to overcome adaptive resistance.
Citation Format: Eugenio Gaudio, Elena Bernasconi, Chiara Tarantelli, Luciano Cascione, Ivo Kwee, Andrea Rinaldi, Maurilio Ponzoni, Maria E. Riveiro, Emanuele Zucca, Francesco Bertoni. The BET bromodomain inhibitor OTX015 (MK-8628) in mantle cell lymphoma (MCL): in vivo activity and identification of novel combinations to overcome adaptive resistance. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr B77.