UCLA Study Reveals Common Antidepressants May Boost Immune System’s Ability to Combat Cancer

A widely prescribed class of antidepressants known as selective serotonin reuptake inhibitors (SSRIs) may hold remarkable potential beyond their traditional use in treating depression. New findings from a comprehensive study conducted at UCLA reveal that SSRIs can significantly enhance the immune system’s ability to combat cancer by boosting T cell activity and suppressing tumor growth in multiple cancer types. This insight opens a promising new avenue in cancer treatment by repurposing existing drugs already approved for human use.

Traditionally, SSRIs are recognized for their role in increasing serotonin levels in the brain by inhibiting the serotonin transporter (SERT), thus alleviating symptoms of depression and anxiety. However, the new research uncovers a surprising dimension of serotonin’s function—far from merely regulating mood, serotonin also acts as a critical signaling molecule modulating immune responses. The UCLA team’s rigorous experiments demonstrate how SSRIs amplify the efficacy of killer T cells, specialized immune cells tasked with identifying and eliminating cancerous cells, through enhanced serotonin signaling.

Dr. Lili Yang, the senior author and a leading immunologist at UCLA’s Broad Stem Cell Research Center, explains that this mechanism pivots on serotonin’s interaction with T cells within the suppressive environment of tumors. When T cells infiltrate tumors, their activity tends to diminish due to multiple immunosuppressive factors. The team found that elevating serotonin signaling via SSRIs counteracts this suppression, reinvigorating T cells and empowering them to maintain their cytotoxic functions against cancer cells more aggressively.

The connection between serotonin and immune function emerged from earlier observations that tumor-infiltrating immune cells exhibited elevated expression of serotonin-regulating molecules. Initially, attention focused on monoamine oxidase A (MAO-A), an enzyme that metabolizes serotonin as well as other neurotransmitters. Previous findings by the same group established that MAO-A in T cells reduces their tumor-fighting capacity. While MAO inhibitors improved T cell function in preclinical models, their clinical utility is limited due to potentially severe side effects and dietary restrictions.

Redirecting focus to SERT and SSRIs offered a safer pharmacological target. SSRIs, including well-known drugs such as Prozac and Celexa, provide a more specific modulation of serotonin signaling with a substantially better safety profile. The study tested SSRIs across a range of murine and human tumor models, including melanoma, breast, prostate, colon, and bladder cancers. Remarkably, treated tumors exhibited an average reduction in size exceeding 50%, demonstrating broad applicability and robust anticancer effects.

Beyond monotherapy, the researchers evaluated the synergy between SSRIs and established cancer immunotherapies, specifically immune checkpoint blockade (ICB) therapy using anti-PD-1 antibodies. ICB treatments aim to lift the brakes imposed on T cells by tumor-induced checkpoint molecules but only yield durable responses in a minority of patients. Co-administration of SSRIs with anti-PD-1 antibodies yielded pronounced tumor shrinkage and, in some cases, complete remission in preclinical models. This combination may amplify T cell activation by concurrently restoring serotonin-mediated signaling pathways and removing immunosuppressive constraints.

The implications of repurposing SSRIs for cancer treatment are profound, especially considering the widespread use of these drugs. According to U.S. Centers for Disease Control and Prevention data, roughly 13% of American adults use antidepressants, with SSRIs dominating prescriptions. This prevalence presents a unique opportunity to conduct observational studies comparing cancer outcomes in patients on SSRIs versus those not receiving antidepressants. Such data could accelerate clinical trials and potentially validate SSRIs as adjuncts to immunotherapy regimens.

This approach offers a strategic advantage when viewed through the lens of drug development economics. Innovating new cancer therapies typically entails an investment surpassing $1.5 billion and a long timeline for clinical validation. By contrast, repurposing FDA-approved SSRIs carries a substantially lower estimated cost—around $300 million—making this a financially and logistically attractive route with potentially transformative impact on patient care.

From a mechanistic standpoint, the study elucidates that SSRIs improve T cell function by inhibiting SERT, thereby increasing serotonin availability in the tumor microenvironment. Serotonin acts as a signaling molecule not only in neuronal tissues but also extensively modulates immune cell behavior, metabolism, and homeostasis. The augmentation of serotonin signaling reactivates exhausted T cells, reestablishing their capacity to proliferate and produce cytotoxic molecules essential for tumor cell eradication.

The latest findings have been submitted for patent protection by UCLA’s Technology Development Group, reflecting the translational potential of this therapeutic strategy. Co-inventors Dr. Yang and Bo Li underscore the translational path from molecular immunology insights to new clinical paradigms. Alongside Dr. Yang’s leadership roles at the Jonsson Comprehensive Cancer Center and microbiology departments, the interdisciplinary effort reflects an intersection of immunology, pharmacology, and oncology.

Future research directions will include real-world data analyses to determine whether patients currently prescribed SSRIs exhibit improved cancer survival or response rates, particularly when combined with immune checkpoint inhibitors. The research team is actively designing controlled clinical trials to rigorously test these hypotheses. The hope is that these initiatives will leverage the established safety profiles of SSRIs to bring forth novel immunotherapy adjuncts that increase the fraction of patients benefitting from immune-based cancer treatments.

This groundbreaking research exemplifies the power of reexamining well-known drugs within new biological contexts to unlock unexpected therapeutic potentials. With only a fraction of cancer patients responding to existing immunotherapies, the integration of SSRIs may represent a critical enhancement to the armamentarium of cancer immunotherapy, ultimately leading to more effective and accessible treatments.

Web References:
UCLA Stem Cell Research Center – Lili Yang
CDC Antidepressant Use Data
Published Study in Cell

References:
Yang L. et al., “Selective Serotonin Reuptake Inhibitors Enhance Anti-tumor Immunity via T Cell Serotonin Signaling,” Cell, 2024.