CDK4 and CDK6 are enzymes important in cell division. CDK4/6 inhibitors are drugs designed to interrupt the growth of cancer cells.

Abemaciclib, or Verzenio, is a CDK4/6 inhibitor drug. Abemaciclib is given in combination with hormone therapy, and either tamoxifen or an aromatase inhibitor. Hormone therapy drugs are used to treat hormone receptor-positive breast cancers.

Abemaciclib in combination with hormone therapy is used to treat early breast cancers that are:

• Hormone receptor-positive
• HER2-negative
• Lymph node-positive, and
• At high risk of recurrence

Study findings show hormone therapy plus abemaciclib may reduce the risk of breast cancer recurrence better than hormone therapy alone in women with hormone receptor- positive, HER2-negative breast cancers at high risk of recurrence.

Abemaciclib is a pill taken twice a day for 2 years.

Some possible side effects of abemaciclib include diarrhea, low white blood cell counts, nausea, abdominal pain, fatigue, anemia (which is low red blood cell count), vomiting, hair loss and blood clots.

In some cases, abemaciclib can cause liver problems. Your liver function will be checked before treatment begins and throughout your treatment.

In rare cases, abemaciclib can cause lung inflammation, which can lead to death. Tell your health care provider right away if you have shortness of breath or other breathing problems while taking abemaciclib.

Abemaciclib is also used to treat HER2-positive metastatic breast cancer. For more information, see the Living with Metastatic Breast Cancer section in this tool.