Abemaciclib, a potent cyclin-dependent kinase 4 and 6 inhibitor, for treatment of ER-positive metastatic breast cancer.

CDK 4/6 inhibitors have given patients with estrogen receptor (ER)-positive/HER2-negative (ER+/HER2ࢤ) advanced metastatic breast cancer important new therapeutic options. Abemaciclib is different to the other two licensed and approved CDK 4/6 inhibitors, palbociclib and ribociclib, both in dosing schedule (continuous vs intermittent) and toxicity profile (less neutropenia, more diarrhea), yet the magnitude of clinical benefit seen in first- and second-line studies is very similar. One of the key issues for clinicians is when to use these therapies. Ultimately, the biggest impact of abemaciclib could be in the adjuvant setting if the current MONARCH-E trial in high-risk node-positive patients is positive. The emerging biomarker work in the early breast cancer setting (i.e., neoMONARCH) may determine which tumors are most sensitive to abemaciclib.