Inhibition of PRMT5 Increases Sensitivity to BH3 Mimetics in Aggressive B Cell Malignancies

Introduction: Protein Arginine Methyltransferase 5 (PRMT5) is a type II methyltransferase that catalyzes symmetric dimethylation of arginine. PRMT5 is highly expressed in several types of aggressive B cell malignancies, and increased PRMT5 expression has been associated with poor disease prognosis. Several small molecule inhibitors targeting PRMT5 have been developed and showed promising anti-tumor effect in pre-clinical studies including in mantle cell lymphoma (MCL) and diffuse large B cell lymphoma (DLBCL). JNJ-64619178, a potent and specific PRMT5 inhibitor, exhibited broad antiproliferative activity in cancer cell lines from multiple cancer types and cell lines derived xenograft mouse model (Brehmer et al., 2021). In a phase 1 clinical trial of JNJ-64619178 (NCT03573310), patients with advanced solid tumors and non-Hodgkin lymphoma demonstrated manageable toxicity and preliminary evidence of antitumor activity at selected dose levels (Villar et al., 2020). In addition, in patients with lower-risk myelodysplastic syndromes, JNJ-64619178 had manageable hematologic toxicity at the selected expansion dose, but did not have significant clinical activity (Haque et al., 2021). Inhibition of PRMT5 alone could induce cell cycle arrest instead of apoptosis depending on the types of cancer (Zhu et al., 2019), therefore we hypothesized that the combination of PRMT5i and pro-apoptotic BH3 mimetics therapies could be synergistic in aggressive B cell malignancies.

Methods: We used dynamic BH3 profiling (DBP) to measure the net pro-apoptotic signaling induced by ex vivo drug treatments (as per Montero et al., 2015). Other standard techniques included shRNA mediated gene knockdown, Western blot, cell viability assays including PI/Annexin V and CellTiter-Glo, and reactive oxygen species (ROS) measurement by flow cytometry with dihydroethidium (DHE) staining. The DLBCL cell lines (HBL1, TMD8, RI-1, OCI-Ly1, Karpas 422, SUDHL4, Toledo), double hit lymphoma (DHL) patient-derived xenograft (PDX) cell lines (DW19), mantle cell lymphoma (MCL) cell lines (Mino, Jeko-1), T cell acute leukemia (T-ALL) cell line (Jurkat) and Burkitt lymphoma (BL) cell line (Raji) were used to investigate the in vitro anti-cancer activity of JNJ-64619178 (Janssen Pharmaceuticals) and BH3 mimetics (venetoclax [BCL2i], S63845 [MCL-1i] and A1331852 [BCL-xLi], Selleckchem). SynergyFinder was used to calculate drug synergism.

Results: In all 7 DLBCL cell lines and the DHL-PDX cell line, we found synergistic induction of apoptosis with JNJ-64619178 plus venetoclax (synergy score, 2.82~51.42). The lowest level of synergy was observed in OCI-Ly1 cells, which are intrinsically highly sensitive to venetoclax monotherapy. The combination of JNJ-64619178 with S63845 synergistically induced apoptosis in 5 out of 7 cell lines (synergy score, -1.91~35.64). While the combination of JNJ-64619178 with A1331852 synergistically induced apoptosis in all 7 cell lines (synergy score 1.88~46.73), 4 of them had weak synergy score (1.88~9.4). To confirm the effects we observed with pharmacologic inhibition of PRMT5, we performed shRNA mediated knockdown of PRMT5, which similarly revealed increased sensitivity to venetoclax. In MCL cell lines, combining JNJ-64619178 with venetoclax or A1331852 was more synergistic in inducing cell death than combining with S63845. The T-ALL Jurkat cell line was resistant to JNJ-64619178 and single BH3 mimetic-induced apoptosis. Combining JNJ-64619178 with A1331852 but not venetoclax or S63845 strongly induced apoptosis. The Burkitt cell line Raji was resistant to JNJ-64619178, venetoclax and A1331852, but was sensitive to S63845. The addition of JNJ-64619178 and a single BH3 mimetic in these Raji cells had only a minor synergistic effect. The synergy scores are summarized in Figure 1. Using DBP, JNJ-64619178 was found to increase overall apoptotic priming of DLBCL and MCL cell lines (> 15% increase in cytochrome c loss). By measuring ROS levels, we found increased ROS production (> 2-fold increase in median fluorescent intensity) in response to JNJ-64619178 before the onset of apoptosis, suggesting a possible mechanism by which JNJ-64619178 increases apoptotic priming.

Conclusions: The combination of a PRMT5 inhibitor with BH3 mimetics, especially venetoclax, is worthy of further exploration as a potential therapeutic strategy for DLBCL and MCL.

Davids:Eli Lilly and Company: Consultancy, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Consultancy, Membership on an entity's Board of Directors or advisory committees; AstraZeneca: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Consultancy, Membership on an entity's Board of Directors or advisory committees; Ascentage Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Travel expenses, Research Funding; BeiGene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Research to Practice: Honoraria; Ono Pharmaceuticals: Consultancy; Novartis: Research Funding; Merck: Consultancy; Takeda: Consultancy; TG Therapeutics: Consultancy, Research Funding; Verastem: Consultancy, Research Funding.