CDK4 and CDK6 are enzymes that help cells grow and divide. CDK4/6 inhibitors are drugs designed to interrupt this cell growth.

Abemaciclib, or Verzenio; and ribociclib, or Kisqali; are CDK4/6 inhibitors given in combination with hormone therapy to treat some hormone receptor-positive, HER2-negative breast cancers.

While the Food and Drug Administration’s approval is similar for each drug, there are some differences.

Abemaciclib can be given when breast cancers are:

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

Ribociclib can be given when breast cancers are:

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

The main difference is that abemaciclib can only be used to treat lymph node-positive breast cancer that has 4 or more positive nodes, or 1 to 3 positive nodes and also have other high-risk features. Ribociclib can be used to treat lymph node-positive breast cancers and can also treat lymph node-negative breast cancers that have other high-risk features.

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

Both drugs are taken as pills, but their schedules differ. Abemaciclib is taken twice a day for about 2 years, while ribociclib is taken once daily for about 3 weeks, followed by 1 week off, for about 3 years. Ribociclib is only given in combination with an aromatase inhibitor.

These treatments begin right after breast cancer surgery. If chemotherapy is a part of your treatment plan, you’ll begin taking one of these drugs after you’ve completed chemotherapy and after you’ve completed radiation therapy.

Before you begin treatment with either of these therapies, talk with your health care team about possible side effects and how to manage them.

Common side effects of abemaciclib include diarrhea, low blood cell counts, blood clots, nausea, vomiting, abdominal pain and fatigue.

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

In rare cases, abemaciclib can also 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.

Ribociclib has similar side effects to abemaciclib including diarrhea, nausea, vomiting and fatigue. Side effects also include hair loss, headache, back pain and constipation.

Like abemaciclib, ribociclib can also cause liver problems and lung inflammation. It may also cause changes on an EKG, or heart test, so your heart is monitored before and throughout your treatment.

Remember though, these treatments help, so once you start on a CDK4/6 inhibitor and throughout the time you’re taking it, let your health care team know how you’re feeling as they may be able to treat many of the side effects you’re experiencing.

For more information, visit the CDK4/6 Inhibitors page within the Treatment section of komen.org.

Abemaciclib and ribociclib are also used to treat HER2-positive metastatic breast cancer. For more information, see the Living with Metastatic Breast Cancer chapter of this tool.