CDK4/6

Class

Selective CDK4/6 inhibitor (IC50 2.7 nM CDK4 / 16 nM CDK6), dosed on a continuous schedule

Pathway

Cyclin D-CDK4/6-Rb axis controlling G1/S cell-cycle checkpoint transition

CDK4 and CDK6 are closely related enzymes, cyclin-dependent kinases, that partner with cyclin D proteins to control the transition of a cell from the G1 phase of the cell cycle into S phase, when DNA is replicated. Active cyclin D-CDK4/6 complexes phosphorylate the retinoblastoma protein (Rb), releasing transcription factors that switch on genes needed for DNA synthesis. In many cancers, this checkpoint is dysregulated by amplification of cyclin D, loss of the natural CDK inhibitor p16, or other upstream alterations, allowing cells to divide without the normal restraint. A selective CDK4/6 inhibitor blocks this kinase activity, holding susceptible tumor cells in G1 and preventing further proliferation. Targeting both CDK4 and CDK6 together, rather than either alone, is important because tumor cells can rely on either kinase interchangeably to phosphorylate Rb, so dual coverage closes an escape route that single-kinase inhibition would leave open. This mechanism is most extensively studied in hormone receptor-driven breast cancer, where cyclin D expression is often estrogen-dependent, but the same cell-cycle logic applies broadly across tumor types in which the cyclin D-CDK4/6-Rb axis remains intact and functionally required for proliferation.