A new study from UCLA has found that a common type of antidepressant drug, called SSRIs, may help the immune system fight cancer.
SSRIs, or selective serotonin reuptake inhibitors, are already widely used to treat depression and are known to be safe.
Now, researchers say these drugs may also boost the body’s ability to fight cancer, offering a potential new use for medications like Prozac and Celexa.
The study, published in the journal Cell, found that SSRIs significantly improved how well T cells – the body’s cancer-fighting immune cells – work against tumors. The drugs also helped shrink tumors in both mice and human cancer models, including melanoma, breast, prostate, colon, and bladder cancers.
“This could be a game-changer,” said Dr. Lili Yang, senior author of the study. “SSRIs are already approved and widely used. If we can show they help in cancer too, we might have a fast and safe way to improve treatment.”
The research team began exploring serotonin’s role in cancer after noticing that immune cells in tumors had higher levels of molecules that control serotonin. Serotonin is often called the “happiness hormone” and is best known for its role in the brain, but it also affects the immune system.
In earlier work, Yang’s team discovered that when T cells try to fight cancer, they produce an enzyme called MAO-A, which breaks down serotonin and makes it harder for the T cells to work.
They found that blocking MAO-A helped T cells perform better. However, because drugs that block MAO-A can have serious side effects, the researchers turned their attention to another target: a protein called SERT.
SERT’s only job is to transport serotonin, and drugs that block it — SSRIs — are safer and more commonly used. When the team tested SSRIs in cancer models, they saw tumor sizes shrink by more than 50%, and T cells became more active and effective at killing cancer cells.
The researchers also tested whether SSRIs could work together with existing cancer treatments. They combined an SSRI with a type of immunotherapy called anti-PD-1 therapy. The results were impressive: tumors shrank significantly, and some mice even went into complete remission.
“This combination could help make current treatments more effective,” said graduate student James Elsten-Brown, a co-author of the study.
Next, the team plans to study whether cancer patients who already take SSRIs have better outcomes, especially those receiving immunotherapy. About 20% of cancer patients are prescribed antidepressants, making this an ideal group to study.
Dr. Yang said that using drugs already approved by the FDA could dramatically speed up the process of getting new treatments to patients. “Developing new cancer drugs can cost $1.5 billion,” she said. “Repurposing existing drugs could cost far less—about $300 million—while still offering major benefits.”
UCLA has filed a patent for the new therapy approach, with Yang and Dr. Bo Li listed as inventors.
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The study is published in Cell.
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