Although new, effective treatments – such as targeted antibodies or immunotherapies – have been available for several years, most patients with advanced-stage abdominal tumors such as colon or pancreatic cancer die within a few years of diagnosis. One problem is that the tumor cells become resistant to the drugs over time and are no longer recognized by the immune system. Now, the research group led by Pierre-Alain Clavien and Anurag Gupta has discovered the role serotonin plays in this tumor cell resistance mechanism.
Without serotonin, the immune system recognizes the cancer cells again
Cancer cells use serotonin to boost the production of a molecule that is immunoinhibitory, known as PD-L1. This molecule binds to killer T cells, a specific type of immune cell that recognizes and eliminates tumor cells, and renders them dysfunctional. The cancer cells thus avoid being destroyed by the immune system. In experiments with mice, the researchers were able to show that SSRIs or peripheral serotonin synthesis inhibitors prevent this mechanism. “This class of antidepressants and other serotonin blockers cause immune cells to recognize and efficiently eliminate tumor cells again. This slowed the growth of colon and pancreatic cancers in the mice,” Clavien says.
Hope for novel combination therapies
PD-L1, via which serotonin exerts its effect, is also the target of modern immunotherapies, also called immune checkpoint inhibitors. In a next step, the researchers tested a dual treatment approach in mice: They combined immunotherapy, which increases the activity of killer T cells, with drugs that reduce peripheral serotonin. The results were impressive: Cancer growth was suppressed in the animal models in the long term, and in some mice the tumors disappeared completely.
“Our results provide hope for cancer patients, as the drugs used are already approved for clinical use. Testing such drug combinations on cancer patients in clinical trials can be fast-forwarded due to the known safety and efficacy of the drugs,” says Pierre-Alain Clavien.