Challenges and Emerging Strategies in the Development of BET Protein Inhibitors

Bromodomain and extra-terminal domain (BET) proteins are epigenetic readers that recognize acetylated lysine residues and regulate transcriptional programs involved in inflammation, cell survival, and tumor progression. BRD4 has attracted particular interest because of its role in super-enhancer regulation, P-TEFb recruitment, and c-MYC-driven oncogenic transcription. Early pan-BET inhibitors established BET bromodomains as druggable targets, but their clinical translation has been limited by broad transcriptional suppression, dose-limiting toxicities, narrow therapeutic windows, limited monotherapy efficacy, and drug resistance. To overcome these limitations, BET-targeted drug discovery has shifted toward domain- or protein-selective inhibitors, multi-target BET inhibitors, and targeted protein degradation. Selective BET inhibitors aim to improve therapeutic selectivity by exploiting context-dependent BET functions, whereas multi-target inhibitors combine BET blockade with modulation of kinase signaling, histone deacetylation, or DNA damage repair pathways. BET degraders induce the degradation of BET proteins through the ubiquitin-proteasome system, thereby suppressing both bromodomain-dependent chromatin binding and non-bromodomain scaffold functions. Overall, next-generation BET-targeted therapy will require rational indication selection, biomarker-guided patient stratification, optimized combination or multi-target strategies, and improved drug-like properties of BET degraders to achieve clinically meaningful efficacy with acceptable safety.