PROTAC repurposing uncovers a noncanonical binding surface that mediates chemical degradation of nuclear receptors

Proteolysis-targeting chimeras (PROTACs) containing a target protein ligand linked to an E3 ubiquitin ligase ligand induce target protein degradation through E3 recruitment. Most PROTACs bind a surface cleft of the protein of interest rather than a buried pocket. Using the nuclear receptor PXR, we previously described the inherent difficulties of PROTAC targeting via a deep solvent-inaccessible ligand binding pocket. Here, we discover that the CRBN-dependent MDM2 PROTAC MD-224 is a potent PXR degrader that achieves its activity from binding adjacent to the ligand-binding pocket. Furthermore, because the proximal region is a structural feature common among nuclear receptors, MD-224 also targets additional receptors for proteasomal degradation. Using structure- and activity-guided medicinal chemistry, we ablated MDM2 degradation and generated MD-224 analogs with activities skewed toward different receptors. Thus, we describe (1) PROTAC repurposing as a potential route of degrader discovery and (2) nuclear receptor-targeted degradation through a noncanonical binding site.