Pembrolizumab (Keytruda) is an immunotherapy medicine that uses the body’s own immune system to attack cancer cells. It is approved by the Food and Drug Administration to be given with chemotherapy to treat people with metastatic or triple-negative breast cancer that cannot be removed with surgery or tests positive for the PD-L1 protein. It is also FDA approved to treat early-stage triple-negative breast cancer that has a high risk of coming back (recurrence) and that can be removed with surgery. For early-stage breast cancer, pembrolizumab can only be given before surgery.

Pembrolizumab also is FDA approved for another use – in people with types of cancers that grow in part because of problems that occur when DNA makes copies of itself. These tumors are called microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR). This approval is not specific to breast cancer but in some situations breast cancer may have this mutation.


How pembrolizumab works

Pembrolizumab works by targeting a protein present on the surface of certain immune cells, including the white blood cells known as T-cells. This protein, known as PD-1 (Programmed Death Receptor-1), has an important role in regulating the immune system and telling it not to attack healthy cells.

Your body’s healthy cells have another protein called PD-L1 that binds to PD-1 on the T-cells, instructing them not to launch an immune response. However, some breast cancer cells and surrounding normal cells express PD-L1 as well. When a T-cell encounters a cancer cell with PD-L1 interacting with PD-1, the protein tells the T-cell that the cancer cell is part of the body and it should not attack. The PD-1 on the T-cell binds to the PD-L1 in and around the tumor cells, keeping them safe from an immune response.

Pembrolizumab is a PD-1 inhibitor that blocks the PD-1 and PD-L1 from interacting, enabling the immune system to fight the cancer. That’s why it is often referred to as an immune checkpoint inhibitor.


Who gets pembrolizumab

Pembrolizumab may be used for some people with high-risk early-stage and metastatic triple-negative breast cancer, as well as others with metastatic breast cancer that has a microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) cancer.

Pembrolizumab for metastatic breast cancer

Pembrolizumab is FDA approved to be given along with chemotherapy to treat metastatic, triple-negative breast cancer, as well as advanced triple-negative breast cancer that can’t be removed with surgery. The breast cancer cells must test positive for the PD-L1 protein. If your medical team thinks you may benefit from pembrolizumab, they will use an FDA-approved test on a sample of cancer tissue that was taken during surgery or a biopsy to look for the level of PD-L1 expression.

Pembrolizumab for early-stage breast cancer

Pembrolizumab also can be used to treat early-stage triple-negative breast cancer that has a high risk of coming back (recurrence). It can be used along with chemotherapy medicines before surgery and may be continued by itself after surgery. In early-stage triple-negative disease, the cancer is considered high risk when the tumor is greater than 2 centimeters, or it’s smaller but there is cancer in the lymph nodes.

Pembrolizumab for solid tumors

Usually, the FDA approves medicines to treat certain kinds of cancer identified by where they first started growing in the body. Some medicines have been approved for multiple types of cancer. Pembrolizumab is the first medicine approved to treat any cancer that has certain characteristics, regardless of where in the body it started.

This approval means pembrolizumab can be used in breast cancer as other types of solid tumors that cannot be removed with surgery or have traveled away from the original site (become metastatic), and that test positive for characteristics such as:

  • microsatellite instability-high (MSI-H)
  • mismatch repair deficient (dMMR)
  • high tumor mutational burden (TMB)

Microsatellite instability-high and mismatch repair deficient tumors can be identified with common lab tests performed on tumor samples from your biopsy, such as an immunohistochemistry test or a tumor biomarker (genomic) test. Tumor mutational burden (TMB) can be identified with tumor biomarker tests. Breast cancer tumors don’t often have these features, but in rare cases it may and would make a person able to take pembrolizumab for that reason alone.


How pembrolizumab is given

Pembrolizumab is given through a port or a line placed with a needle to drip medicine directly into a vein, a process called infusion. It is typically given every 3 weeks, and the infusion usually takes about 30 minutes.

For triple-negative breast cancer that is PD-L1 positive, pembrolizumab is given with chemotherapy that will also be given by infusion.

Your care team will advise you on the treatment combination and schedule that makes the most sense for you. In metastatic triple-negative breast cancer, pembrolizumab can be given as long as the cancer responds to it and there are no difficult or dangerous side effects. For early-stage triple-negative breast cancer, treatment can continue for up to a year.


Side effects and things to remember

Because pembrolizumab affects the way your immune system recognizes your cells, it also may cause your immune system to target healthy cells in your body. Like the cancer cells, your healthy cells may not be able to bind to T-cells, causing the T-cells to attack. Although people’s bodies can react to this differently, pembrolizumab can cause side effects such as inflammation of endocrine glands, the colon, or the lungs. For example, some people develop an overactive or underactive thyroid gland, and others may develop colitis, where the inner lining of the intestine becomes inflamed. Skin, kidney, and liver problems have also been known to happen. It’s important to talk through all of this with your care team so you know what side effects to watch out for. You also may need to have regular lab testing to check on the function of your thyroid gland and other organs.

If you already have a immune system disorder such as rheumatoid arthritis, lupus, or inflammatory bowel disease, you may not be eligible for pembrolizumab. Your care team can help you decide what makes sense for your situation.

Other common side effects of pembrolizumab may include:


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Reviewed and updated: August 12, 2022

Reviewed by: Douglas Yee, MD


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