The E3 ubiquitin ligase MDM2 (Murine Double Minute 2) is the most important negative regulator of p53 tumor suppressor primarily by targeting it for ubiquitination and proteasomal degradation. Amplification or overexpression of the MDM2 gene occurs in many human cancers contributing to tumor development, progression and metastasis. Targeting MDM2 to restore p53 function has become an attractive therapeutic strategy for cancers harboring wild-type TP53. Currently, numerous drugs inhibiting the MDM2-p53 interaction, such as RG7112 (Hoffmann-La Roche), MI-773 (Sanofi) and DS-3032b (Daiichi Sankyo), have entered different stages of clinical trials. However, even in p53 wild-type tumors, MDM2 inhibitors can exert limited efficacy as monotherapy in some models, probably due to inadequate p53 induction and side effects. Therefore novel and improved strategies to effectively target the MDM2-p53 pathway are needed. Recently, we have designed a series of MDM2 degraders by conjugating a small molecule MDM2 inhibitor to phthalimide. The phthalimide moiety interacts with its target protein Cereblon (CRBN) and recruits the CUL4-DDB1-CRBN (also known as CRL4CRBN) E3 ubiquitin ligase complex to promote ubiquitination and proteasome degradation of MDM2. Our initial evaluation focused on acute leukemias that are mostly p53 wild-type and express high levels of MDM2. The MDM2 degraders exert significantly improved growth inhibitory activity compared to the inhibitor in human acute leukemia cell lines with IC50s < 5 nM. We performed Western Blots for MDM2 and p53 protein levels and qRT-PCR analysis for mRNA levels of p53 target genes to demonstrate MDM2 protein degradation and activation of p53 downstream pathways. Flow cytometry analysis confirmed the induction of apoptosis by MDM2 degraders in leukemia cells. In agreement with our design, the action of our MDM2 degraders depends on binding to Cereblon, as demonstrated by competition with free phthalimide, and on proteasome function, as demonstrated by proteasome inhibition. siRNA knockdown of p53 significantly abolished the activity of the degraders demonstrating the dependence upon p53 activation. Our studies demonstrate that the MDM2 degraders robustly activate wild-type p53 by inducing rapid degradation of MDM2 and activation of p53 pathways leading to strong apoptosis in leukemia cell lines in a CRBN-binding, proteasome and p53 dependent manner. More importantly, the MDM2 degraders showed great efficacy in inducing complete tumor regression or strong tumor growth inhibition in human leukemia xenograft models and significantly improved survival in disseminated human leukemia models in mice at well-tolerated dose-schedules. Our data provide strong preclinical rationale to further develop MDM2 degraders as a new class of therapy for the treatment of human acute leukemia and potentially other types of human cancer.

Citation Format: Jiuling Yang, Yangbing Li, Angelo Aguilar, Donna McEachern, Sally Przybranowski, Ester Fernandez-Salas, Jianfeng Lu. Targeted MDM2 degradation as a novel and efficacious cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4870.