EZH2 PROTACs are unable to abolish EZH2 activator function but inhibit certain prostate cancer by rapid senescence induction

Enhancer of Zeste Homolog 2 (EZH2) is the enzymatic subunit of the Polycomb Repressive Complex 2 (PRC2) that catalyzes H3K27 methylation for epigenetic silencing. EZH2 up-regulation or mutations contribute to the progression of various cancers, including prostate cancer (PCa). Enzymatic EZH2 inhibitors (EZH2i) exert limited efficacy in PCa, partially due to PRC2-independent roles of EZH2 in activating androgen receptor (AR) expression. Several proteolysis-targeting chimera (PROTAC) degraders of EZH2 have been developed, which, surprisingly, continued to have limited efficacy in PCa cells. Here, we designed and developed a series of EZH2 PROTACs using EZH2i EPZ-6438 as a ligand and identified PROTAC-6272 as a potent lead compound. We demonstrated that 6272 effectively degraded EZH2 and other PRC2 subunits across diverse PCa cell lines. Interestingly, EZH2 PROTACs were consistently unable to decrease AR and p300 due to their inability to engage and target EZH2 outside of the PRC2 complex. Regardless, EZH2 PROTAC-6272 exhibited greater anti-proliferative activities than EPZ-6438 in some PCa cell lines, wherein it rapidly induced cellular senescence and p21 expression independently of H3K27 methylation. In summary, we report that EZH2i-based PROTACs could benefit certain PCa but are unable to target the EZH2 activator function, shedding significant light on further drug development.