The CDK inhibitor CR8 acts as a molecular glue degrader that depletes cyclin K

Molecular glue compounds induce protein-protein interactions that, in the context of a ubiquitin ligase, lead to protein degradation.(1) Unlike traditional enzyme inhibitors, such molecular glue degraders act sub-stoichiometrically to catalyse rapid depletion of previously inaccessible targets.(2) They are clinically effective and highly sought-after, but have thus far only been discovered serendipitously. Through systematic mining of databases for correlations between the cytotoxicity of 4,518 clinical and pre-clinical small molecules and E3 ligase expression levels across hundreds of human cancer cell lines,(3–5) we identified CR8, a cyclin-dependent kinase (CDK) inhibitor,(6) as a compound that acts as a molecular glue degrader. A solvent-exposed pyridyl moiety of CR8, in its CDK-bound form, induces CDK12-cyclin K complex formation with DDB1, the CUL4 adaptor protein, bypassing the requirement for a substrate receptor and presenting cyclin K (cycK) for ubiquitination and degradation. Our studies demonstrate that chemical alteration of surface-exposed moieties can confer gain-of-function glue properties to an inhibitor, and we propose this as a broader strategy to turn target binders into molecular glues.