Common Antidepressants Show Promise in Boosting Immune Response Against Cancer, UCLA Study Finds

Los Angeles, May 23, 2025 — In a groundbreaking discovery, researchers at UCLA have found that selective serotonin reuptake inhibitors (SSRIs)—a class of widely prescribed antidepressants—may significantly enhance the immune system’s ability to fight cancer. The study, published in the journal Cell, suggests that these medications, commonly used to treat depression, could be repurposed to improve cancer treatment outcomes.

SSRIs, which include well-known drugs such as Prozac and Celexa, work by increasing serotonin levels in the brain. While serotonin is best known as the “happiness hormone,” it also plays a vital role in other bodily functions, including immune activity. The UCLA team, led by Dr. Lili Yang, discovered that SSRIs help “reinvigorate” killer T cells—immune cells that attack cancer—by increasing their access to serotonin signals. This effect was observed in both mouse and human tumor models representing melanoma, breast, prostate, colon, and bladder cancers.

“Our research shows that SSRIs don’t just make our brains happier; they also make our T cells happier, even while they’re fighting tumors,” said Dr. Yang, a professor at UCLA’s Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research. “These drugs have been widely and safely used to treat depression for decades, so repurposing them for cancer would be a lot easier than developing an entirely new therapy.”

In the study, SSRI treatment reduced average tumor size by more than 50% and significantly improved the effectiveness of T cells in killing cancer cells. The research team also tested SSRIs in combination with immune checkpoint blockade (ICB) therapy—an existing cancer treatment that helps unleash the immune system against tumors. The combination therapy led to even greater tumor reduction, and in some cases, complete remission in mouse models.

“Immune checkpoint blockades are effective in fewer than 25% of patients,” said James Elsten-Brown, a graduate student and co-author of the study. “If a safe, widely available drug like an SSRI could make these therapies more effective, it would be hugely impactful.”

The next step for the researchers is to investigate whether cancer patients who are already taking SSRIs experience better outcomes, particularly those receiving ICB therapies. With about 20% of cancer patients currently on antidepressants—most commonly SSRIs—the team sees a unique opportunity to explore this potential benefit in real-world settings.

Dr. Yang emphasized that repurposing existing FDA-approved drugs could dramatically reduce the time and cost required to bring new cancer treatments to patients. “Studies estimate the bench-to-bedside pipeline for new cancer therapies costs an average of $1.5 billion. When you compare this to the estimated $300 million cost to repurpose FDA-approved drugs, it’s clear why this approach has so much potential,” she said.

The findings have led to a patent application for this new therapeutic strategy, and the team is planning clinical trials to further evaluate the impact of SSRIs on cancer treatment outcomes.

Disclaimer:
This article is based on preliminary research findings from UCLA and is intended for informational purposes only. The use of SSRIs as a cancer treatment has not been approved by regulatory authorities, and further clinical studies are needed to confirm their safety and effectiveness in this context. Patients should not alter their medication or cancer treatment regimen without consulting their healthcare provider.