Development of versatile solid-phase methods for syntheses of PROTACs with diverse E3 ligands.

Developing highly active proteolysis-targeting chimeras (PROTACs) requires investigating a variety of ubiquitin ligase (E3 ligase) ligands and linker structures as well as their lengths. In this study, we developed a solid-phase synthesis method that affords PROTAC design diversity. We expanded the E3 ligand range to include Von Hippel-Lindau (VHL) and inhibitor of apoptosis protein (IAP) ligands because only the cereblon (CRBN) ligand thalidomide and its derivatives have been investigated for solid-phase synthesis of PROTACs. Moreover, we examined the suitability of a polyethylene glycol (PEG) rather than an alkyl linker used in our previous study for synthesizing PROTACs. Facile and rapid solid-phase synthesis methods using the above E3 ligands for developing PROTACs targeting bromodomain-containing protein 4 (BRD4) were accomplished. Western blotting analysis revealed that minor differences in the E3 ligand and linker type significantly affected the activity of the synthesized PROTACs. Our solid-phase PROTAC synthesis methods enable rapid synthesis of multiple PROTACs with various combinations of ligands for the protein-of-interest and E3 ligands and linkers that connect these ligands.