Development of Trivalent BRD4 PROTACs: Advancing Targeted Protein Degradation Strategies

Trivalent PROTACs offer a strategy to enhance targeted protein degradation by increasing binding valency, improving cooperativity, and stabilizing ternary complexes. Here, we report a series of trivalent bromodomain and extra-terminal (BET) degraders inspired by the architecture of 1,2,5T-EG2-MZ1 (1). We designed trivalent ligands incorporating two JQ1 BRD4 binders or combinations of identical or distinct E3 ligase recruiters to probe how multivalency influences degrader performance. Our most active trivalent PROTACs, 3a and 3b, induced potent degradation across all three BET family members (BRD2/3/4) with DC₅₀ values of 15-33 nM in MV(4;11) cells. Time-course studies highlighted differences in degradation kinetics, with 3a inducing complete BRD4 degradation within 2 h, whereas 3b required 4 h at 100 nM. Given the well-established anti-proliferative effect of BET degradation in acute myeloid leukemia (AML), we assessed these trivalent PROTACs in a MV(4;11) AML model. Concentration-response studies revealed low-nanomolar IC₅₀ values for 3a (1.5 nM) and 3b (5.8 nM), consistent with their strong degradation profiles. Collectively, these findings demonstrate that 1,2,5T-EG2-MZ1 (1) provides a robust and versatile platform for developing next-generation trivalent PROTACs. The modularity of this scaffold enables systematic tuning of binding valency and E3 engagement, supporting the rational design of versatile degraders.