Modulating PCAF/GCN5 Immune Cell Function through a PROTAC Approach

Bivalent Ligands for Protein Degradation in Drug Discovery

Targeting the "undruggable" proteome remains one of the big challenges in drug discovery. Recent innovations in the field of targeted protein degradation and manipulation of the ubiquitin-proteasome system open up new therapeutic approaches for disorders that cannot be targeted with conventional inhibitor paradigms. Proteolysis targeting chimeras (PROTACs) are bivalent ligands in which a compound that binds to the protein target of interest is connected to a second molecule that binds an E3 ligase via a linker. The E3 protein is usually either Cereblon or Von Hippel-Lindau. Several examples of selective PROTAC molecules with potent effect in cells and in vivo models have been reported. The degradation of specific proteins via these bivalent molecules is already allowing for the study of biochemical pathways and cell biology with more specificity than was possible with inhibitor compounds. In this review, we provide a comprehensive overview of recent developments in the field of small molecule mediated protein degradation, including transcription factors, kinases and nuclear receptors. We discuss the potential benefits of protein degradation over inhibition as well as the challenges that need to be overcome.

Iterative Design and Optimization of Initially Inactive Proteolysis Targeting Chimeras (PROTACs) Identify VZ185 as a Potent, Fast, and Selective von Hippel-Lindau (VHL) Based Dual Degrader Probe of BRD9 and BRD7

Developing PROTACs to redirect the ubiquitination activity of E3 ligases and potently degrade a target protein within cells can be a lengthy and unpredictable process, and it remains unclear whether any combination of E3 and target might be productive for degradation. We describe a probe-quality degrader for a ligase–target pair deemed unsuitable: the von Hippel–Lindau (VHL) and BRD9, a bromodomain-containing subunit of the SWI/SNF chromatin remodeling complex BAF. VHL-based degraders could be optimized from suboptimal compounds in two rounds by systematically varying conjugation patterns and linkers and monitoring cellular degradation activities, kinetic profiles, and ubiquitination, as well as ternary complex formation thermodynamics. The emerged structure–activity relationships guided the discovery of VZ185, a potent, fast, and selective degrader of BRD9 and of its close homolog BRD7. Our findings qualify a new chemical tool for BRD7/9 knockdown and provide a roadmap for PROTAC development against seemingly incompatible target–ligase combinations.