CDK4 and breast carcinoma: In large clinical trials evaluating use of CDK4/6i in female patients with breast cancer as 1L or 2L+, response rates for palbociclib, ribociclib, and abemaciclib were 42.1%, 42.5%, and 49.7%, respectively, when coadministered with an AI, and 25.0%, 32.4%, and 35.2% when coadministered with fulvestrant [11, 28–32].