Abemaciclib (Verzenio) is a targeted therapy approved by the Food and Drug Administration to treat hormone receptor-positive breast cancer. It is a cyclin-dependent kinase 4/6 (CDK 4/6) inhibitor, which means it targets two specific kinases, or proteins, that help tumor cells reproduce. Those kinases are CDK 4 and CDK 6.

CDK 4/6 inhibitors are relatively new treatments. The first, palbociclib (Ibrance), was approved by the FDA in 2015. Another, ribociclib (Kisqali), was approved in early 2017. Later that year, abemaciclib became the third medicine of this type to be approved for hormone receptor-positive, HER2-negative metastatic breast cancer. In 2021, the FDA expanded the approval of abemaciclib for use in high-risk, early-stage breast cancer. It is the only CDK 4/6 inhibitor approved for use in early-stage breast cancer.

While these three medicines mostly work in the same way, abemaciclib differs slightly from the other two in its dosing. Abemaciclib is taken continuously, whereas palbociclib and ribociclib are given in cycles with breaks.


How abemaciclib works

Cyclin-dependent kinases 4 and 6 are two proteins that help some breast cancers to grow. Medicines like abemaciclib are called CDK 4/6 inhibitors because they block these proteins from signaling cancer cells to multiply, which helps slow the growth or spread of the cancer.


Who gets abemaciclib

Abemaciclib can be used in both high-risk early-stage breast cancer and in metastatic breast cancer.

Abemaciclib in early-stage breast cancer

Abemaciclib may be given to people with high-risk early-stage breast cancer that is hormone-receptor positive, HER2-negative. Determining whether a cancer is at high risk for recurrence depends on different features of the cancer. This includes the number of lymph nodes that have cancer, the size and grade of cancer, and results of a Ki-67 index test on the tumor.

Ki-67 is a protein found in cells that affects the way cells divide and grow. Early-stage breast cancers with a higher Ki-67 index are more likely to return after treatment is completed. These cancers are considered “high risk.” The FDA approved the use of a test to measure this protein in tumors, called the Ki-67 IHC MIB-1 pharmDx test, to determine whether abemaciclib is indicated for use. The FDA guideline says abemaciclib may be an option for people whose Ki-67 index is 20 or higher.

When used, abemaciclib is given with tamoxifen or an aromatase inhibitor (anastrozole or exemestane) after the primary treatment (often surgery).


The use of abemaciclib in the early-stage setting depends on reviewing a number of clinical features to determine if a particular patient is high risk for recurrence, and balancing the potential benefit with possible side effects.

Tiffany Avery, MD, MPH


"When deciding on the combination with tamoxifen or aromatase inhibitor in a premenopausal patient, the side effect profile of the two should be considered," Dr. Avery says. "A premenopausal patient could be treated with agents to induce menopause in combination with aromatase inhibitor or with tamoxifen alone. Data for other studies suggest that outcomes may be better with induction of menopause in higher risk patients."

Abemaciclib in metastatic breast cancer

Abemaciclib is FDA approved for three uses in people with hormone receptor-positive, HER2-negative metastatic breast cancer:

  • It is given with an aromatase inhibitor as the first treatment for metastatic breast cancer if you are a postmenopausal woman.
  • It is given alone if you have already been treated with hormonal therapy and chemotherapy for metastatic breast cancer.
  • It is given with fulvestrant (Faslodex) if you have taken hormonal therapy for early-stage or metastatic breast cancer, but have not had chemotherapy since being diagnosed with metastatic breast cancer.

How abemaciclib is given

Abemaciclib is a pill, usually taken twice a day. The recommended dose depends on whether you are taking it alone, with an aromatase inhibitor, with tamoxifen, or with fulvestrant.

Unlike other CDK 4/6 inhibitors, which are taken in cycles with regular breaks, abemaciclib is taken every day through treatment. People with early-stage disease can take abemaciclib for two years. They may stop taking it sooner if the cancer comes back or the side effects are too severe. When given for metastatic breast cancer, abemaciclib may be taken on an ongoing basis for as long as it controls the cancer and side effects are manageable.


Side effects and things to remember

The most common side effect for abemaciclib is diarrhea. In a clinical trial, more than 80 percent of participants had diarrhea, but most cases were controlled with anti-diarrheal medicines and lowering the dose, if necessary. Let your doctor know right away if you notice loose stools.

Abemaciclib also may cause neutropenia, a low white blood cell count that makes infection more likely. Your doctor will watch your blood counts closely, especially during the first two months on the treatment. Other side effects reported in clinical trials of this medication included:

Tell your doctor about any medicines, supplements, or remedies you are taking before starting abemaciclib. Avoid eating grapefruit and any food or drinks that have grapefruit in them while taking this medicine.

Tell your doctor if you are pregnant, breastfeeding, or planning to do either. Abemaciclib may be dangerous to a fetus so it is recommended that you not take this medicine while pregnant and not get pregnant until at least 3 weeks after you have stopped treatment. It is not known if abemaciclib is transferred through breast milk. For this reason, do not breastfeed while taking abemaciclib or for 3 weeks after you stop.

Talk to your doctor about any side effects you experience and how to manage them. You can also visit our Side Effects page to learn more.


Living Beyond Breast Cancer is a national nonprofit organization that seeks to create a world that understands there is more than one way to have breast cancer. To fulfill its mission of providing trusted information and a community of support to those impacted by the disease, Living Beyond Breast Cancer offers on-demand emotional, practical, and evidence-based content. For over 30 years, the organization has remained committed to creating a culture of acceptance — where sharing the diversity of the lived experience of breast cancer fosters self-advocacy and hope. For more information, learn more about our programs and services.