Abstract 807: Mechanism of action of synergistic activity of EZH2 inhibition and IMiDs in preclinical multiple myeloma models

The EZH2 inhibitor tazemetostat (EPZ-6438) is currently being evaluated in phase 2 clinical trials for the treatment of non-Hodgkin lymphoma (NHL). EZH2 inhibitors have shown anti-proliferative effects in multiple preclinical models of NHL, mesothelioma, and molecularly defined solid tumors. Objective clinical responses have been reported in patients with B-cell lymphomas in phase 1 and in preliminary analysis of phase 2 studies of tazemetostat. Mounting evidence suggests that EZH2 regulates the maturation of both normal B-cells and B-cell lymphomas and may be an important mediator of cell fate in other B-cell malignancies. Consistent with this hypothesis, recent preclinical studies suggest that EZH2 may be a therapeutic target for treatment of multiple myeloma (MM), a disease arising from late stage B-cell lymphocyte plasmablasts. Inhibition of EZH2 has shown potent anti-proliferative effects in both in vitro and in vivo preclinical models of MM. Here, we show that synergistic anti-proliferative activity was observed when EZH2 inhibitors were combined with small molecule immune system modulators in both in vitro and in vivo multiple myeloma models and describe the mechanism of action of this enhanced effect. In particular, combination with IMiDs led to dramatic synergy across multiple MM cell lines. Like EZH2, Ikaros/Aiolos, the two important targets of IMiD response, are repressors of transcription. This led us to investigate the effects of the combination of tazemetostat and IMiDs on gene expression. Previous reports have indicated that both EZH2 and Ikaros/Aiolos play important roles in the regulation of interferon signaling via the IRF4 axis. We found that both agents singly elicited changes in interferon target genes, a phenomenon that could be enhanced when the two drugs were added in combination. Combination treatment also led to an increase in apoptosis as would be expected following downregulation of the IRF4 axis which is critical for MM cell survival. In this study, we provide evidence that EZH2 and Ikaros/Aiolos share a set of target genes, including IRF4, suggesting that these overlapping pathways may, in part, mediate the synergy between EZH2 inhibitors and IMiDs on cell viability. Based upon these in vitro findings, we conducted a xenograft study using the RPMI-8226 myeloma model and observed a notable enhancement in tumor growth inhibition with the combination EZH2 inhibitor and pomalidomide compared to the effect of either single agent. These data provide evidence of synergy between pomalidomide and EZH2 inhibitors in both in vitro and in vivo preclinical models, and provide a rationale for clinical development of this combination strategy for patients with multiple myeloma. Citation Format: Allison E. Drew, Vinny Motwani, Lindsey Eichinger, Jesse Smith, Alejandra Raimondi. Mechanism of action of synergistic activity of EZH2 inhibition and IMiDs in preclinical multiple myeloma models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 807.