Abstract PO-53: Combined EZH2 and BCL2 inhibitors as precision therapy for genetically defined DLBCL subtypes
Authors: Scholze Hanna , Stephenson Regan E. , Reynolds Raymond , Shah Shivem , Puri Rishi , Teater Matthew R. , van Besien Herman , Gibbs-Curtis Destini , Ueno Hideki , Parvin Salma , Letai Anthony G. , Mathew Susan , Singh Ankur , Cesarman Ethel , Melnick Ari , Giulino-Roth Lisa 
Published: December 2020

Abstract
Molecular alterations in the histone methyltransferase EZH2 and the antiapoptotic protein BCL2 frequently co-occur in diffuse large B-cell lymphoma (DLBCL). We hypothesized that EZH2 inhibition and BCL2 inhibition would be synergistic in DLBCL. To test this, we evaluated the EZH2 inhibitor tazemetostat and the BCL2 inhibitor venetoclax in DLBCL cells, 3D lymphoma organoids, and patient-derived xenografts (PDXs). We found that tazemetostat and venetoclax are synergistic in DLBCL cells that harbor both an EZH2 mutation and a BCL2/IGH translocation, as demonstrated by CI values &lt;1 (CI: 0.034, 0.259 and 0.074 in SUDHL-6, WSU-DLCL2, and OCI-Ly1 respectively), but not in wild-type cells. Since cell lines in suspension do not reflect lymph node architecture, we developed a 3D lymphoma organoid culture system that consists of extracellular matrix, lymphoma cells, and stromal cells (Tian et al., Biomaterials 2015). We observed synergy between the two agents in two organoid model systems: 1) OCI-LY1; 2) PDX derived from a DLBCL with BCL2/IGH translocation and EZH2 mutation. To investigate mechanisms of synergy, we evaluated previously published RNA-seq profiles of DLBCL cell lines (n=26) treated with vehicle or tazemetostat to investigate changes in BCL2 family members (Brach et al., Mol Can Ther 2017). Tazemetostat-treated cells showed enhanced expression of proapoptotic BCL2 family members including BCL2L11 (p=0.012), BMF (p&lt;0.001), and BCL2L14 (p=0.002), suggesting that these may be direct or indirect EZH2 target genes that are de-repressed upon EZH2 inhibition. To assess mitochondrial priming to apoptosis as a result of EZH2 inhibition, we performed BH3 profiling of DLBCL PDX organoids treated with vehicle vs. tazemetostat. Tazemetostat-treated cells had increased priming as evidenced by cytochrome c release in response to general apoptotic signaling peptides BIM and PUMA (p&lt;0.0001) and to the BCL2 specific peptide BAD (p&lt;0.0001), suggesting that pretreatment with tazemetostat increases mitochondrial sensitivity to BCL2 inhibition. We next evaluated combination therapy in vivo. In SUDHL-6 xenografts, the combination resulted in attenuation of tumor growth compared to either drug alone (combination vs. venetoclax p&lt;0.0001, combination vs. tazemetostat p=0.0004) and improved overall survival. In DLBCL PDXs, combination therapy resulted in complete resolutions of tumors, which were durable over time and associated with improved overall survival. Strikingly, after 197 days of follow-up there was no detectable disease in any combination-treated animal. In summary, we demonstrate that combined BCL2 and EZH2 inhibition results in synergistic anti-lymphoma effects. We expect this combination to be especially effective as precision therapy for the newly identified cluster 3/EZB DLBCL subtype, which frequently harbors both EZH2 and BCL2 alterations. A clinical trial of this combination is currently in development.
Citation Format: Hanna Scholze, Regan E. Stephenson, Raymond Reynolds, Shivem Shah, Rishi Puri, Matthew R. Teater, Herman van Besien, Destini Gibbs-Curtis, Hideki Ueno, Salma Parvin, Anthony G. Letai, Susan Mathew, Ankur Singh, Ethel Cesarman, Ari Melnick, Lisa Giulino-Roth. Combined EZH2 and BCL2 inhibitors as precision therapy for genetically defined DLBCL subtypes [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-53.