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Immunotherapy is the use of medicines that help a person’s own immune system find and destroy cancer cells. This is often a useful form of treatment for Merkel cell carcinoma (MCC), especially if it has spread to other parts of the body.

Immune checkpoint inhibitors

An important part of the immune system is its ability to keep itself from attacking normal cells in the body. To do this, it uses “checkpoint” proteins on immune cells, which act like switches that need to be turned on (or off) to start an immune response.

Cancer cells sometimes use these checkpoints to keep the immune system from attacking them. But drugs that target checkpoint proteins, called checkpoint inhibitors, can help the immune system find and attack cancer cells.

PD-1 and PD-L1 inhibitors

PD-1 is a checkpoint protein on immune cells called T cells. It normally helps keep the T cells from attacking other cells in the body. When it attaches to PD-L1, a protein on some normal (and cancer) cells, it basically tells the T cell to leave the other cell alone. Some cancer cells have large amounts of PD-L1, which helps keep the immune system from attacking them. 

Drugs that block either PD-1 or PD-L1 can stop this binding and boost the body's immune response against cancer cells. Examples of checkpoint inhibitors that can be used to treat MCC include:

• Avelumab (Bavencio), which targets PD-L1
• Pembrolizumab (Keytruda), retifanlimab (Zynyz), and nivolumab (Opdivo), which block PD-1

These drugs are given as an infusion into a vein (IV), usually every 2 to 6 weeks, depending on the drug.

These drugs? can often shrink or slow the growth of advanced MCC tumors. Doctors are now studying the use of these drugs for some earlier-stage MCCs as well. For example, they are testing to see if these drugs might be useful after, or even before, surgery in some people.

Other immune checkpoint inhibitors are being studied for use against MCC as well.

Possible side effects of immunotherapy for Merkel cell cancer (MCC)

Side effects of these types of drugs can include: 

• Fatigue
• Cough
• Nausea
• Rash or itchy skin
• Joint pain
• Loss of appetite
• Diarrhea
• Constipation

Other, more serious side effects occur less often: 

Infusion reactions: Some people might have an infusion reaction while getting one of these drugs. This is like an allergic reaction. It can include fever, chills, flushing of the face, rash, itchy skin, wheezing, and trouble breathing. It’s important to tell your doctor or nurse right away if you have any of these symptoms while getting these drugs.

Autoimmune reactions: These drugs work by removing one of the safeguards that helps keep the body’s immune system in check. Sometimes this can lead to the immune system attacking other parts of the body, which can cause serious or even life-threatening problems in the lungs, intestines, liver, hormone-making glands, kidneys, or other organs.

It’s very important to report any changes or new side effects to your health care team right away. If serious side effects do occur, you might need to stop treatment.