Targeted degradation of P-TEFb via MDM2 substrate substitution collapses oncogenic transcriptional programs in MDM2-amplified cancers

Abstract MDM2 is an oncogenic E3 ubiquitin ligase best known for targeting the tumor suppressor p53 and is frequently amplified in human cancers, including dedifferentiated liposarcoma (DDLPS). We show that supraphysiological MDM2 expression can be exploited to reprogram its ligase activity toward selective degradation of oncogenic transcriptional machinery. Using a substrate substitution strategy, we developed an MDM2-recruiting degrader dCDK9-010 that targets positive transcription elongation factor b, inducing its proteasomal degradation while simultaneously stabilizing p53. This dual action disrupts RNA polymerase II abundance and elongation, preferentially impairing enhancer-driven transcriptional programs that sustain tumor growth. In DDLPS models, this approach produces potent antitumor activity with favorable pharmacokinetic and safety profiles, triggers apoptosis, and enhances macrophage-mediated tumor cell clearance. These findings establish substrate substitution-based reprogramming of MDM2 as a generalizable targeted protein degradation strategy and identify MDM2 amplification as a predictive biomarker for therapeutic response across cancer types, with particular relevance to liposarcoma.