The Role of CDK4/6 Inhibitors in Breast Cancer.

OPINION STATEMENT: Oral inhibitors of CDK4/6 have been shown to increase response rates and prolong disease control when combined with endocrine therapy in hormone-responsive (HR+) HER2-negative advanced breast cancer. Palbociclib, ribociclib and abemaciclib are all approved in combination with non-steroidal aromatase inhibitors in first-line therapy for post-menopausal women, with a 40-45% improvement in progression-free survival seen with the addition of any of these CDK4/6 inhibitors. Additional approved indications, including first- and second-line combination therapy for pre-menopausal women, combination with fulvestrant and use as monotherapy, vary with each agent and are reviewed fully in the subsequent texts. These agents also differ in their toxicity profiles and monitoring requirements, and prescribers should be aware of the individual requirements for each agent. Current clinical trials are investigating the expanded use of these agents in other breast cancer subtypes, such as HER2-positive and triple-negative breast cancer, as well as in the adjuvant and neoadjuvant treatments of early breast cancer. Resistance to CDK4/6 inhibition can occur through multiple mechanisms. Rational combinations with other therapies, such as PI3K inhibitors, HER2-directed therapies and immunotherapy, are being explored.