Induced degradation of protein kinases by bifunctional small molecules: a next-generation strategy.

Introduction: Protein kinases are a major target for small-molecule drug development. However, relatively few compounds are free of off-target toxicity and reach the clinic. Because the 500-plus kinases share conserved ATP-binding clefts, the site targeted by competitive inhibitors, generation of specific therapeutics remains a nearly intractable challenge. Areas covered: Inducing degradation, instead of inhibition by occupancy-driven drugs, is an emerging strategy that offers the long-sought specificity, as well as mechanistic benefits. Currently approved inhibitors require steady-state binding and leave proteins intact for interactions in multi-protein complexes. After a general background about induced protein degradation, perspectives on protein kinases are provided. Expert opinion: Induced degradation by state-of-the-art compounds (proteolysis-targeting chimeras, PROTACs) has been shown for protein kinases, albeit in early pre-clinical stages. Further work is required to expand the number of enzymes that could be exploited to direct proteins for degradation by ubiquitylation. In addition, despite the simple modularity of the chimeras, generation of hits will require empirical approaches due to the role of protein-protein interactions and distribution of tagging sites. However, given the advantages of degradation, drug discovery efforts targeting protein kinases should increasingly shift toward generation and screening of inducers of degradation and away from occupancy-based inhibitors of old.