Molecular pathways: CDK4 inhibitors for cancer therapy.

Unrestrained growth is the hallmark of cancer, and disrupted cell-cycle regulation is, therefore, common. CDK4 is the key regulator of the G1-S transition. In complex with cyclin D, CDK4 phosphorylates retinoblastoma protein (Rb) and drives cell-cycle progression, a process inhibited by p16. The p16-CDK4-cyclin D-Rb is aberrant in the majority of cancers and is, thus, a logical target for anticancer therapy. Previous attempts to block CDK4 with nonselective cyclin-dependent kinase (CDK) inhibitors led to toxicity and little efficacy. However, the recent development of selective CDK4 inhibitors launched the first successful efforts to target the pathway for cancer therapy. Three oral selective CDK4 inhibitors have entered clinical trials: palbociclib (PD0332991), LEE011, and LY2835219. CDK4 inhibitors have in vitro activity against a broad range of cancers and in patients have shown antitumor activity in breast cancer, lymphoma, sarcoma, and other tumors. Major efforts are under way to develop biomarkers of response, understand potential mechanisms of resistance, and develop rational combinations of CDK4 inhibitors with chemotherapy and other targeted drugs.