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 still 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. It can be taken by postmenopausal women, women who have not completed menopause yet, and men, although recommendations for these groups can vary.

Abemaciclib in early-stage breast cancer

Abemaciclib may be given to people with high-risk, early-stage breast cancer that is hormone-receptor positive and 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. However, some doctors may still recommend the medicine if the score is below 20 and the cancer has high-risk features.

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


Pre- and perimenopausal women, as well as men, usually need to take a luteinizing hormone-releasing hormone (LHRH) agonist, such as goserelin (Zoladex), along with abemaciclib and an aromatase inhibitor. This suppresses hormone production in the body.

“When deciding on the combination of abemaciclib 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:

  • If you are a postmenopausal woman or a man, it is given with an aromatase inhibitor as the first treatment for metastatic breast cancer.
  • If you are a pre- or postmenopausal woman or a man, 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.

Pre- and perimenopausal women, as well as men, often need to take a luteinizing hormone-releasing hormone (LHRH) agonist to suppress hormone production in the body.

Abemaciclib was FDA approved for these uses on the basis of several clinical trials:

  • monarchE: Compared abemaciclib plus tamoxifen or an aromatase inhibitor, versus hormonal therapy alone, in people with lymph node-positive breast cancer at high risk of recurrence
  • MONARCH3: Compared abemaciclib plus an aromatase inhibitor (anastrozole or letrozole) to an AI alone in postmenopausal women with metastatic breast cancer
  • MONARCH2: Compared abemaciclib plus fulvestrant to fulvestrant alone in women with cancer that had grown during past hormonal therapy
  • MONARCH1: Assessed abemaciclib alone in people with metastatic breast cancer that continued to grow on previous therapies

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.

Aromatase inhibitors and tamoxifen are also pills taken by mouth. Fulvestrant is given as a shot to the butt: two shots every two weeks in the first month, and then two shots every four weeks.



Side effects & 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 so that you can begin preventive treatment.

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.

If you’re a man, abemaciclib may affect your fertility (ability to have a child). Discuss this with your doctor if this is a concern for you.

Talk to your doctor about any side effects you experience and how to manage them. If you have a serious side effect, the dose may need to be lowered, or you may need to take a break from the medication. You can also visit our Side effects page to learn more.


Related resources


Reviewed and updated: December 23, 2022

Reviewed by: Sameer Gupta MD, MPH


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