EZH1/2 Dual Inhibitor Valemetostat Tosylate (DS-3201b) Acts Differently from EZH2 Selective Inhibitor on Epigenetic Landscape to Exert Greater Anti-Tumor Effect Against Diffuse Large B-Cell Lymphoma

Enhancer of zeste homologous (EZH) 2 and its close homolog EZH1 are catalytic subunits of polycomb repressive complex (PRC) 2 protein complex, and play redundant and crucial role for the maintenance of transcriptional repression by tri-methylating histone H3 lysine 27 (H3K27). Hyper trimethylation of H3K27 has been associated with malignant lymphoma and myeloma progression, thus several small molecules suppressing PRC2 complex activity has been developed for hematological malignancy therapy. We have developed valemetostat tosylate (DS-3201b, also known as valematostat), a potent dual inhibitor of EZH1/2, and demonstrated its superior anti-proliferative effect against DLBCL cells to tazemetostat (EPZ-6438, E7438) a selective EZH2 inhibitor currently in clinic. In addition, valemetostat synergized with wide variety of 1st and 2nd line drugs used in DLBCL therapy both in vitro and in vivo proposing its potential combination opportunities. However, it is still elusive how valemetostat modulates epigenetic landscape and represses malignant B-cell proliferation more potently than selective EZH2 inhibitors. Therefore, impact on epigenetic landscape between valemetostat and tazemetostat was analyzed by RNA/ChIP-sequencing. Though these two inhibitors significantly reduced cellular global H3K27me3 level, we observed ectopic EZH1/2 accumulation in several tumor suppressor gene loci after tazemetostat treatment resulting in partial reduction in H3K27me3 and de-repression of silenced gene expression. Meanwhile valemetostat treatment evidently triggered gene expression by depleting H3K27me3 and enhancing H3K27Ac mark without inducing ectopic enrichment of EZH1/2, suggesting that valemetostat has a distinct effect on genome wide distribution of EZH1/2 from tazemetostat.

In conclusion, these results suggest that valemetostat has a capacity of averting ectopic relocation of EZH1/2 on tumor suppressor genes mainly induced by EZH2 specific inhibition and thereby exerts greater anti-B cell tumor effect than EZH2 preferential inhibitor. A phase 2 clinical study of valemetostat is now ongoing for patients with Relapse/Refractory B-cell Lymphoma [ClinicalTrials.gov Identifier: NCT04842877]