Cereblon modulators: Low molecular weight inducers of protein degradation

Targeted protein degradation has become an exciting new paradigm in drug discovery with the potential to target new protein families for therapeutic intervention. In 2010, Hiroshi Handa and colleagues discovered that the drug thalidomide binds to the protein cereblon, a component of the CRL4^CRBN E3 ubiquitin ligase. In contrast to the heterobifunctional small molecule degraders reported in the literature, thalidomide is of very low molecular weight (∼258Da) with molecular properties (solubility, metabolic stability, permeability etc) that readily support pharmaceutical dosing. It was subsequently shown that thalidomide and the analogues lenalidomide and pomalidomide are able to degrade the transcription factors Ikaros and Aiolos. CK1α and GSPT1 were subsequently identified as substrates for specific ligands, indicating that this molecular class could be tuned for selective protein degradation. Structural studies showed that the thalidomide analogues bind to a shallow hydrophobic pocket on the surface of cereblon, and scaffold a protein-protein interaction with target proteins. Target proteins do not need any affinity for the cereblon modulators, and as such undruggable, or even unligandable, proteins can be targeted for degradation. A similar mechanism of action was subsequently identified for the clinical molecule indisulam, indicating that low molecular weight degraders are not unique to cereblon. The groundbreaking work on cereblon represents a case study for the discovery and characterization of low molecular weight protein degraders for other ligases.