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

Immunomodulatory agents (IMiDs) and the next-generation Cereblon (CRBN) E3 ligase modulators (CELMoDs), 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 have 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 a single agent in myeloma and in combination treatments to overcome drug resistance in other malignancies. In this study KMS-11 and RPMI-8226 myeloma cell lines are used as models of primary IMID resistance, demonstrating persistent Interferon regulatory factor 4 (IRF4) expression after IMiDs/CELMoDs exposure without loss of cell viability. The combination of Tazemetostat, an FDA-approved EZH2 inhibitor, with IMiDs/CELMoDs significantly reduces IRF4 expression, induces apoptosis, and leads to synergistic cell death in these resistant cell lines. Further investigations reveal that the synergistic effect of EZH2 inhibition appears specific to IMiDs/CELMoDs, is CRBN-dependent and rescued by IRF4 overexpression. Mechanistically, Tazemetostat appears to reduce IKZF1 binding to the IRF4 promoter and super-enhancer, explaining how the combination with IMiDs/CELMoDs 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.