CDK4 and CDK6

Class

CDK4/6 inhibitor (small molecule)

Pathway

Blocks retinoblastoma protein (Rb) phosphorylation, halting G1-to-S cell-cycle progression

CDK4 and CDK6 (cyclin-dependent kinases 4 and 6) are enzymes that, in partnership with D-type cyclin proteins, govern whether a cell advances from its resting growth phase into the phase where it copies its DNA and prepares to divide. Active CDK4/6 adds phosphate groups to the retinoblastoma protein (Rb), releasing transcription factors that switch on genes needed for DNA replication. Many cancers, particularly those driven by hormone-receptor signaling, exploit this pathway by increasing cyclin D levels or losing natural brakes on CDK4/6, so cells divide without normal restraint. Small-molecule inhibitors of CDK4/6 block the phosphorylation step, keeping Rb in its growth-suppressing state and holding susceptible cells in the resting phase rather than allowing them to divide. Because CDK4/6 activity is also needed for turnover of some healthy tissues, particularly bone marrow progenitor cells and gut lining, inhibition predictably affects these tissues as well, producing manageable, reversible on-target effects. This mechanism is broadly important in cancers where cell division depends heavily on this checkpoint, and more generally illustrates how targeting cell-cycle regulators, rather than a specific cancer driver mutation, can slow proliferation across tumor types that share this dependency.