Targeting breast cancer with CDK inhibitors.

Dysregulation of the cell cycle is a classic hallmark of cancer growth and metastatic potential. Re-establishing cell cycle control through CDK inhibition has emerged as an attractive option in the development of targeted cancer therapy. Three oral agents selectively targeting CDK4/6 have been developed: palbociclib, abemaciclib, and LEE011. Preclinical models show optimal activity in hormone receptor positive breast cancer, which may display biologic features suggesting particular dependence on the CDK4/cyclin D1/Rb interaction. Palbociclib has been studied in a randomized phase 2 clinical trial in metastatic hormone receptor positive breast cancer in which the combination of palbociclib and endocrine therapy significantly prolonged progression-free survival over endocrine therapy alone. The toxicity profile of palbociclib and the other CDK 4/6 inhibitors in early phase I and II trials has been predominantly hematologic, characterized by limited neutropenia, as well as variable gastrointestinal toxicity. Multiple phase II and III studies are ongoing with all three agents, and are designed to explore the role of CDK 4/6 inhibition in metastatic hormone receptor positive breast cancer. The next wave of studies will examine further clinical and scientific topics, including the role of CDK 4/6 inhibition in the neo/adjuvant setting, the combination of CDK 4/6 inhibitors with other targeted therapies, and the activity of CDK 4/6 inhibitors in the HER2 positive subset of breast cancer, as well as in other cancer subtypes. Should ongoing study confirm benefits and tolerability of CDK 4/6 inhibition, combination therapy with endocrine agents may become a new standard of care for hormone receptor positive breast cancer.