EZH2 inhibition overcomes immunomodulatory drug resistance in multiple myeloma via a cereblon-dependent pathway

Immunomodulatory agents (IMiD) and the next-generation cereblon (CRBN) E3 ligase modulators (CELMoD), targeting the IKZF1/IKZF3-IRF4-MYC axis, are effective therapies for multiple myeloma (MM) across all stages of disease. Resistance to treatment can be acquired following exposure, but a subset of patients has primary resistance, with both states necessitating the development of alternative treatment strategies. Enhancer of zeste homolog 2 (EZH2) has been shown to have increased expression at myeloma relapse and higher expression is associated with a shorter progression-free survival from diagnosis. EZH2 inhibitors have been studied as single agents in myeloma and in combination treatments to overcome drug resistance in other malignancies. In this study KMS-11 and RPMI-8226 myeloma cell lines were used as models of primary IMiD resistance, demonstrating persistent interferon regulatory factor 4 (IRF4) expression after IMiD/CELMoD exposure without loss of cell viability. The combination of tazemetostat, a Food and Drug Administration-approved EZH2 inhibitor, with IMiD/CELMoD significantly reduced IRF4 expression, induced apoptosis, and led to synergistic cell death in these resistant cell lines. Further investigations revealed that the synergistic effect of EZH2 inhibition appeared specific to IMiD/ CELMoD, was CRBN-dependent and was rescued by IRF4 overexpression. Mechanistically, tazemetostat appeared to reduce IKZF1 binding to the IRF4 promoter and super-enhancer, explaining how the combination with IMiD/CELMoD which also have this effect may reach the threshold required to suppress IRF4 expression and ultimately inhibit MM cell growth in resistant cell lines. Our findings highlight a potential strategy for treating MM patients with IMiD resistance.