Small molecule MMRi62 targets MDM4 for degradation and induces leukemic cell apoptosis regardless of p53 status
MDM2 and MDM4 are crucial negative regulators of the tumor suppressor p53. They interact through their RING domains to form a heterodimeric polyubiquitin E3 ligase complex, which is essential for the degradation of p53. MDM4 also pairs with MDM2 isoforms lacking p53-binding domains, creating E3 ligases that regulate the stability of both p53 and MDM4. We are investigating small-molecule inhibitors that target the RING domain of the MDM2-MDM4 heterodimer (MMRi) to block its oncogenic activity. In this study, we report the identification and characterization of MMRi62 as an MDM4 degrader and apoptosis inducer in leukemia cells. Biochemically, MMRi62 bound to preformed RING domain heterodimers, shifting the substrate preference toward MDM4 ubiquitination and promoting MDM2-mediated MDM4 degradation in cells. This MDM4-degrading action of MMRi62 was linked to strong apoptosis induction in leukemia cells. Notably, MMRi62 triggered apoptosis in p53-mutant, multidrug-resistant leukemia cells and patient samples, in addition to p53 wild-type cells. In contrast, MMRi67, which disrupts RING heterodimers and inhibits the enzymatic activity of the MDM2-MDM4 E3 complex, failed to degrade MDM4 or induce apoptosis in these cells. In conclusion, this study identifies MMRi62 as a novel MDM2-MDM4-targeting agent and highlights the potential of small molecules that promote MDM4 degradation as a promising strategy for inducing apoptosis in leukemia cells with non-functional p53.