INK4 Tumor Suppressor Proteins Mediate Resistance to CDK4/6 Kinase Inhibitors

Cyclin-dependent kinases 4 and 6 (CDK4/6) represent a major therapeutic vulner-ability for breast cancer. The kinases are clinically targeted via ATP competitive inhibitors (CDK4/6i); however, drug resistance commonly emerges over time. To understand CDK4/6i resistance, we surveyed over 1,300 breast cancers and identifi ed several genetic alterations (e.g., FAT1, PTEN, or ARID1A loss) converging on upregulation of CDK6. Mechanistically, we demonstrate CDK6 causes resistance by inducing and binding CDK inhibitor INK4 proteins (e.g., p18(INK4C)). In vitro binding and kinase assays together with physical modeling reveal that the p18 (INK4C)-cyclin D-CDK6 complex occludes CDK4/6i binding while only weakly suppressing ATP binding. Suppression of INK4 expres-sion or its binding to CDK6 restores CDK4/6i sensitivity. To overcome this constraint, we developed bifunctional degraders conjugating palbociclib with E3 ligands. Two resulting lead compounds potently degraded CDK4/6, leading to substantial antitumor effects in vivo , demonstrating the promising thera-peutic potential for retargeting CDK4/6 despite CDK4/6i resistance. SIGNIFICANCE: CDK4/6 kinase activation represents a common mechanism by which oncogenic sign-aling induces proliferation and is potentially targetable by ATP competitive inhibitors. We identify a CDK6-INK4 complex that is resilient to current-generation inhibitors and develop a new strategy for more effective inhibition of CDK4/6 kinases.