CDK4/6 Inhibitors

What are CDK4/6 inhibitors?

Cyclin-dependent kinase 4 and 6 (CDK4 and CDK6) are enzymes important in cell division. CDK4/6 inhibitors are drugs designed to interrupt the growth of cancer cells.

CDK4/6 inhibitors are used in combination with hormone therapy to treat some breast cancers.

CDK4/6 inhibitors are pills.

Drug name

Brand name

Used to treat early or metastatic breast cancer?



Early and metastatic breast cancer



Metastatic breast cancer



Metastatic breast cancer

Learn about CDK4/6 inhibitors and metastatic breast cancer treatment.

Abemaciclib (Verzenio) and early breast cancer treatment

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

Abemaciclib in combination with hormone therapy is FDA-approved for the treatment of early breast cancers that are all of the following [8,133]:

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 [134].


For a summary of research studies on abemaciclib and early breast cancer treatment, visit the Breast Cancer Research Studies section.

How is abemaciclib given?

Abemaciclib is a pill taken twice a day for 2 years. It’s only given in combination with tamoxifen or an aromatase inhibitor.

You begin taking abemaciclib after breast cancer surgery.

Learn more about the hormone therapy drugs tamoxifen and aromatase inhibitors.

Side effects of abemaciclib

CDK4/6 Inhibitor

Side Effects


Possible side effects include diarrhea, low white blood cell counts, anemia (low red blood cell counts), blood clots, nausea, abdominal pain, fatigue and vomiting.

In some cases, it can cause liver problems. Your liver function will be checked before treatment begins and throughout your treatment. This is to help make sure there are no problems, or if there are problems, they’re caught early.

In rare cases, it 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.

Adapted from select sources [133].

Grapefruit and grapefruit juice should be avoided while taking abemaciclib, as they may increase blood levels of abemaciclib [133].

Before you begin taking abemaciclib, talk with your health care provider about possible side effects and how to manage them.

CDK4/6 inhibitors and metastatic breast cancer treatment

The CDK4/6 inhibitors abemaciclib, palbociclib and ribociclib are FDA-approved for the treatment of some metastatic breast cancers.

Learn about CDK4/6 inhibitors and metastatic breast cancer treatment.

Under study

Palbociclib (Ibrance) and ribociclib (Kisqali)

The CDK4/6 inhibitors palbociclib and ribociclib are under study for the treatment of early breast cancer. They are FDA-approved for the treatment of metastatic breast cancer.

Neoadjuvant therapy

Abemaciclib is FDA-approved for the adjuvant treatment (given after surgery) of some early breast cancers. Abemaciclib and other CDK4/6 inhibitors are under study for use in the neoadjuvant treatment (before surgery) setting [135].

Learn more about emerging drug therapies for early breast cancer.

Treatment guidelines

Although the exact treatment for breast cancer varies from person to person, evidence-based guidelines help ensure high-quality care. These guidelines are based on the latest research and agreement among experts.

The National Comprehensive Cancer Network (NCCN) and the American Society of Clinical Oncology (ASCO) are respected organizations that regularly review and update their guidelines.

In addition, the National Cancer Institute (NCI) has treatment overviews.

Talk with your health care team about which treatment guidelines they use.

After you get a recommended treatment plan from your health care team, study your treatment options. Together with your health care team, make thoughtful, informed decisions that are best for you. Each treatment option has risks and benefits to consider along with your own values and lifestyle.

Prescription drug assistance

Prescription drug costs can quickly become a financial burden for you and your family.

Medicare and many insurance companies offer prescription drug plans. One may already be included in your policy, or you may be able to buy an extra plan for prescriptions.

Abemaciclib (Verzenio) is a pill, so it’s covered under your health insurance plan’s prescription drug benefit rather than the plan’s medical benefit. This means there are usually out-of-pocket costs, which can add up over time.

You may qualify for programs that help with drug costs or offer low-cost or free prescriptions.

Many cancer centers have patient financial counselors who can discuss insurance and cost coverage with you.

Learn more about insurance plans and prescription drug assistance programs.

Learn more about other financial assistance programs.

Komen Financial Assistance Program

Susan G. Komen® created the Komen Financial Assistance Program to help those struggling with the costs of breast cancer treatment by providing financial assistance to eligible individuals.

Funding is available for eligible individuals undergoing breast cancer treatment at any stage or living with metastatic breast cancer (stage 4).

To learn more about this program and other helpful resources, call the Komen Breast Care Helpline at 1-877 GO KOMEN (1-877-465-6636) or email

Se habla español.

Susan G. Komen®‘s position on fairness in oral cancer drug coverage

Insurance coverage of oral cancer drugs

Cancer medications given by vein (through an IV) or injection (under the skin or into a muscle) are usually covered under a health insurance plan’s medical benefit. However, cancer medications that are pills (oral cancer drugs) are usually covered under a health insurance plan’s prescription drug benefit.

As a result, people often find themselves facing high out-of-pocket costs when filling prescriptions for oral cancer drugs. Sometimes these costs can be thousands of dollars a month.

The impact of high cost-sharing

High prescription drug costs and the resulting out-of-pocket burden on patients are a barrier to care. They can prevent people from getting the medications prescribed by their health care providers.

No one should be forced to get less appropriate treatment because an insurer gives more coverage for IV and injectable drugs than pills.

Efforts to increase fairness in drug coverage

Komen supports state and federal efforts to require insurers to provide the same or better coverage for oral cancer drugs as they do for IV and injectable cancer drugs. This would help make sure patients have access to affordable, appropriate treatment.

Become a Komen Advocacy Insider

Sign up to be a Komen Advocacy Insider and get informed when action is needed on drug coverage issues at the state or national level.

Clinical trials

Research is ongoing to improve all areas of treatment for breast cancer.

New therapies are being studied in clinical trials. The results of these studies will decide whether these therapies become part of the standard of care.

After discussing the benefits and risks with your health care provider, we encourage you to consider joining a clinical trial.

Susan G. Komen® Breast Care Helpline

If you or a loved one needs information or resources about clinical trials, call the Komen Breast Care Helpline at 1-877 GO KOMEN (1-877- 465- 6636) or email

Se habla español. in collaboration with Komen offers a custom matching service to help find clinical trials that fit your needs.

Learn what Komen is doing to help people find and participate in breast cancer clinical trials, including trials supported by Komen.

Learn more about clinical trials.


Komen Perspectives

Read our perspective on clinical trials.*

Learn More

Susan G. Komen® Support Resources

*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date.

Updated 09/28/23



Educational Resources



1-877 GO KOMEN

Research Fast Facts