PRMT5 inhibition drives therapeutic vulnerability to combination treatment with BCL-2 inhibition in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell malignancy that comprises up to 6% of non-Hodgkin lymphomas diagnosed annually and is associated with a poor prognosis. The average overall survival of patients with MCL is 5 years, and for most patients who progress on targeted agents, survival remains at a dismal 3 to 8 months. There is a major unmet need to identify new therapeutic approaches that are well tolerated to improve treatment outcomes and quality of life. The protein arginine methyltransferase 5 (PRMT5) enzyme is overexpressed in MCL and promotes growth and survival. Inhibition of PRMT5 drives antitumor activity in MCL cell lines and preclinical murine models. PRMT5 inhibition reduced the activity of prosurvival AKT signaling, which led to the nuclear translocation of FOXO1 and modulation of its transcriptional activity. Chromatin immunoprecipitation and sequencing identified multiple proapoptotic BCL-2 family members as FOXO1-bound genomic loci. We identified BAX as a direct transcriptional target of FOXO1 and demonstrated its critical role in the synergy observed between the selective PRMT5 inhibitor, PRT382, and the BCL-2 inhibitor, venetoclax. Single-agent and combination treatments were performed in 9 MCL lines. Loewe synergy scores showed significant levels of synergy in most MCL lines tested. Preclinical, in vivo evaluation of this strategy in multiple MCL models showed therapeutic synergy with combination venetoclax/PRT382 treatment with an increased survival advantage in 2 patient-derived xenograft models (P ≤ .0001, P ≤ .0001). Our results provide mechanistic rationale for the combination of PRMT5 inhibition and venetoclax to treat patients with MCL.

PRMT5 supports multiple oncogenic pathways in mantle cell lymphoma

Mantle cell lymphoma (MCL) is an incurable B-cell malignancy with an overall poor prognosis, particularly for patients that progress on targeted therapies. Novel, more durable treatment options are needed for patients with MCL. Protein arginine methyltransferase 5 (PRMT5) is overexpressed in MCL and plays an important oncogenic role in this disease via epigenetic and posttranslational modification of cell cycle regulators, DNA repair genes, components of prosurvival pathways, and RNA splicing regulators. The mechanism of targeting PRMT5 in MCL remains incompletely characterized. Here, we report on the antitumor activity of PRMT5 inhibition in MCL using integrated transcriptomics of in vitro and in vivo models of MCL. Treatment with a selective small-molecule inhibitor of PRMT5, PRT-382, led to growth arrest and cell death and provided a therapeutic benefit in xenografts derived from patients with MCL. Transcriptional reprograming upon PRMT5 inhibition led to restored regulatory activity of the cell cycle (p-RB/E2F), apoptotic cell death (p53-dependent/p53-independent), and activation of negative regulators of B-cell receptor-PI3K/AKT signaling (PHLDA3, PTPROt, and PIK3IP1). We propose pharmacologic inhibition of PRMT5 for patients with relapsed/refractory MCL and identify MTAP/CDKN2A deletion and wild-type TP53 as biomarkers that predict a favorable response. Selective targeting of PRMT5 has significant activity in preclinical models of MCL and warrants further investigation in clinical trials.

Abstract 1031: PRMT5 inhibition alters mitochondrial dynamics in mantle cell lymphoma, creating vulnerability to BH3 mimetic compounds

Mantle cell lymphoma (MCL) is an aggressive and incurable blood cancer comprising 5% of all non-Hodgkin lymphomas diagnosed annually. The median age of diagnosis is 68yo, and while many patients initially respond to frontline treatment, relapse is common. There is an unmet need to develop novel therapeutic strategies for the treatment of MCL. Our group has identified protein arginine methyltransferase 5 (PRMT5) as a key driver of MCL pathogenesis. PRMT5 symmetrically dimethylates arginine residues on a number of proteins (P53, E2F1, P65) and histones (H4R3, H3R8, H2AR3), which support tumorigenesis. Selectively inhibiting PRMT5 has shown significant anti-tumor activity in preclinical MCL models, and a Phase 1 clinical trial with PRT543 (Prelude), a novel PRMT5 inhibitor, is underway. While exploring pathways that converge with PRMT5 activity as potential avenues for combination treatment, we identified the intrinsic apoptotic pathway as an attractive target in MCL. BCL2 family proteins either promote or inhibit intrinsic apoptosis at the outer mitochondrial membrane through a dynamic set of binding interactions. Prior work has shown PRMT5 inhibition to drive the expression of multiple pro-death BCL2 family gene products (BAX, BAK, and BBC3/PUMA) in MCL. We hypothesized that combining PRMT5 inhibition with BH3 mimetics, compounds that target pro-survival BCL2 proteins, would induce synergistic cell death in MCL. Selective PRMT5 inhibition with PRT382 inhibits the viability of MCL cell lines with an IC50 below 1uM in eight of nine lines (IC50 44.8nM - 1905.5nM). BH3 mimetics navitoclax (BCL2, BCLXL, and BCLw inhibitor), A852 (BCLXL inhibitor), and AMG176 (MCL1 inhibitor) were found to have IC50s below 1uM in five, two, and four of the nine cell lines respectively. We chose six cell lines to test combination treatment ranging in sensitivity to BH3 mimetics and expression of BCL2 family proteins (CCMCL1, Z-138, UPN1, Granta 519, Mino, and Maver1). Synergistic decreases in viability were tested via MTS assay and analyzed with the Loewe model of synergy. Mino, which showed sensitivity to all three mimetics, exhibited a synergistic reduction in viability with combination PRT382 treatment. Granta 519 and Z-138 exhibited a similar effect with the combination of PRT382 and navitoclax or A852. These observations were confirmed through BH3 profiling, supporting MCL cell line dependence on BCL2, BCLXL, BCLw and MCL1, and increased sensitivity to BCL2 family protein targeting with PRMT5 inhibition. Two patient derived xenograft models were tested ex vivo after treatment with 10mg/kg of the PRMT5 inhibitor PRT382 or vehicle. iBH3 flow-based analysis revealed increased sensitivity of ex vivo PDX cells to pan BCL2, BCLXL, and MCL1 inhibition. These results provide rationale for combining BH3 mimetics and PRMT5 inhibition in clinical trials as a novel treatment strategy for MCL.

Citation Format: Claire Hinterschied, Fiona Brown, Janani Ravikrishnan, JoBeth Helmig-Mason, Kris Vaddi, Peggy Scherle, Jennifer Woyach, Selina Chen-Kiang, Oliver Elemento, Jihye Paik, Robert Baiocchi. PRMT5 inhibition alters mitochondrial dynamics in mantle cell lymphoma, creating vulnerability to BH3 mimetic compounds [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1031.