What are the side effects of monoclonal antibodies?

The two you are most likely to get are a reaction where the needle goes in and a stretch of feeling like you have the flu. Serious problems happen, but they are much less common.

Monoclonal antibodies are immune proteins made in a lab to recognize one specific target. Your body makes antibodies naturally to tag germs for destruction; these are built to tag something on a cancer cell. Because each one hits a different target, the side effect list is not identical from drug to drug. Your own list depends on which antibody you get, the dose, your cancer, and how healthy you were going in.

The two common ones

Needle site reactions include pain, swelling, soreness, redness, itchiness, and rash where the drug enters.

Flu-like symptoms are the other frequent complaint: chills, fatigue, fever, muscle aches, nausea, vomiting, and diarrhea. They usually cluster around infusion day rather than lasting the whole cycle.

Reactions that happen while the drip is running

Monoclonal antibodies can cause allergic reactions during the infusion itself, ranging from mild to severe. In rare cases a reaction is severe enough to be fatal. This is why nurses stay close during your first dose and why the first infusion is deliberately slow. Trastuzumab, for example, is given over 90 minutes the first time, then over 30 minutes for later doses once you have shown you tolerate it.

Cytokine release syndrome can occur, though with monoclonal antibodies it is often mild. Cytokines are immune signaling chemicals, and a sudden surge causes fever, nausea, headache, rash, a fast heartbeat, low blood pressure, and trouble breathing.

Capillary leak syndrome is rarer and more dangerous. Fluid and protein seep out of the smallest blood vessels into surrounding tissue, dropping blood pressure to dangerous levels. It can progress to shock and multiple organ failure.

The serious ones that build over time

Beyond infusion day, monoclonal antibodies can cause mouth and skin sores that turn into serious infections, high blood pressure, congestive heart failure, heart attacks, and inflammatory lung disease.

Some risks belong to one specific drug rather than the whole class, and those show up on the FDA label. Trastuzumab, used for HER2-positive breast and stomach cancers, carries a boxed warning for cardiomyopathy, a weakening of the heart muscle. That is why your team measures your heart's pumping function before treatment and repeats it during treatment, and why the drug is stopped if the heart declines. Its label also warns about lung toxicity and about harm to a fetus, so effective contraception is required during treatment.

When to call, and how fast

Call 911 or go to an emergency room for trouble breathing, swelling of the lips, tongue, or throat, chest pain, fainting, or a rash spreading fast with dizziness. Those point to a severe allergic reaction, capillary leak, or a cardiac problem.

Call your team the same day for a fever of 100.4 °F (38 °C) or higher, since the CDC uses that threshold for people in cancer treatment while NCI's infection page uses 100.5 °F. Also call for new shortness of breath on mild activity, a dry cough that will not settle, swollen ankles or sudden weight gain of several pounds in a few days, or mouth sores that stop you from eating and drinking.

Nobody can predict in advance which side effects you will get or how bad they will be. What you can do is know the warning signs and speak up early rather than waiting for the next scheduled visit.

Sources

https://www.cancer.gov/about-cancer/treatment/types/immunotherapy/monoclonal-antibodies

https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/103792s5345lbl.pdf

https://www.cdc.gov/cancer-preventing-infections/patients/fever.html

https://www.cancer.gov/about-cancer/treatment/side-effects/infection