CDK 4/6 Inhibitors

CDK 4/6, when bound to cyclin D, hyperphosphorylates the Rb protein to proceed through the cell cycle past the G1 checkpoint and eventually divide. Therefore, by inhibiting this CDK 4/6 complex, cell division is halted as the cell cannot proceed past this checkpoint. As of today, there are three US FDA approved CDK 4/6 inhibitors on the market: ribociclib (Kisqali), palbociclib (Ibrance), and abemaciclib (Verzenio). These inhibitors are effective for individuals with metastatic hormone receptor-positive, HER2-negative breast cancer (2). Both ribociclib and palbociclib are approved to be taken in combination therapy with letrozole (Femara), an aromatase inhibitor (4). Aromatase inhibitors lower the estrogen production at the site of the cancer, which can be helpful in hormone-sensitive breast cancers in postmenopausal women as they respond to estrogen (1).