CDK4

Class

Selective CDK4 inhibitor (small molecule)

Pathway

Cell-cycle arrest via the Rb pathway

CDK4 (cyclin-dependent kinase 4) is an enzyme that, together with its partner protein cyclin D, drives cells from the resting/growth phase (G1) into DNA synthesis (S phase) of the cell division cycle. It does this by adding phosphate groups to the retinoblastoma protein (Rb), which normally restrains cell division; phosphorylation inactivates Rb and releases transcription factors that switch on the genes needed for DNA replication. Many cancers acquire mutations or amplifications that keep this cyclin D–CDK4–Rb axis abnormally active, allowing tumor cells to divide continuously. Blocking CDK4 with a selective small-molecule inhibitor halts this progression, arresting susceptible cancer cells in G1 and slowing tumor growth. CDK4 is closely related to CDK6, and many earlier drugs in this class inhibit both kinases together. Because CDK6 plays a larger role in normal blood cell production, a CDK4-selective agent is designed to preserve the antitumor effect on the Rb pathway while reducing the bone marrow suppression associated with dual CDK4/6 inhibition. This mechanism is broadly relevant in hormone receptor–driven and other cancers where the Rb pathway remains intact and cyclin D-CDK4 signaling is a key growth driver.