Abemaciclib: The Newest CDK4/6 Inhibitor for the Treatment of Breast Cancer.

Objective: To review the pharmacology, efficacy, and safety of the cyclin-dependent kinase (CDK) inhibitor, abemaciclib, in the treatment of advanced or metastatic breast cancer (MBC). Data Sources: Relevant information was identified through a MEDLINE/PubMed (January 2000 to June 2018) literature search. The new drug application, prescribing information, and abstracts and posters from scientific meetings were also reviewed. Study Selection/Data Extraction: The literature search was limited to human studies published in the English language. Phase 1, 2, and 3 studies evaluating the pharmacology, efficacy, or safety of abemaciclib for breast cancer were included. Data Synthesis: Abemaciclib is an oral, potent, small molecule inhibitor of CDK4 and CDK6 activity, which blocks retinoblastoma tumor suppressor protein phosphorylation and thereby prevents progression through the cell cycle. Three major clinical trials, MONARCH 1, 2, and 3, established the efficacy and safety of abemaciclib for the treatment of hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative advanced or MBC. In these trials, response rates were promising, ranging from 19.7% to 59%, and median progression-free survival was significantly prolonged over the control arm in 2 of the trials. Common adverse effects included diarrhea, neutropenia, nausea, abdominal pain, infections, and fatigue. Relevance to Patient Care and Clinical Practice: Although no head-to-head studies have been completed between the CDK4/6 inhibitors, abemaciclib may be an attractive option because of its continuous dosing and ability to be used as monotherapy. Conclusions: Abemaciclib is an effective and well-tolerated treatment for HR-positive, HER2-negative advanced or metastatic breast cancer.