CDK4 and breast cancer: Among all three, CDK 4/6-Is available in clinical practice, abemaciclib is the most potent with reported IC50s of 2 nm and 10 nm for CDK4 and CDK6, respectively.24 In addition, abemaciclib has shown a potential for crossing the blood–brain barrier25 and, in this context, some trials are evaluating abemaciclib in ER-positive BC patients with brain metastases.