Clinical Development of the CDK4/6 Inhibitors Ribociclib and Abemaciclib in Breast Cancer

Clinical and preclinical data support a significant role for inhibitors of the cyclin-dependent kinases (CDKs) 4 and 6 in the treatment of patients with breast cancer. Recently, based on data showing improvement in progression-free survival, the use of palbociclib (Ibrance; Pfizer, Inc.) in combination with endocrine agents was approved to treat patients with hormone receptor-positive advanced disease. Importantly, 2 other CDK4/6 inhibitors, abemaciclib (LY2835219; Lilly) and ribociclib (LEE011; Novartis), are in the late stage of clinical development. In this review, we will focus on clinical data on these 2 new drugs, highlighting their differences compared to palbociclib in terms of single-agent activity, central nervous system penetration, and common adverse events. In addition, we will present the ongoing clinical trials and discuss future directions in the field.