Scientists have discovered that Prozac, the antidepressant taken by millions of people around the world, may stimulate the growth of brain tumours by blocking the body's natural ability to kill cancer cells.
An international team of researchers led by John Gordon, professor of immunology at Birmingham University, found evidence to suggest cancer cells can be killed by "positive thinking", which could be blocked when people take Prozac.
The study, to be published in the journal Blood next week, examined the effects of Prozac and other antidepressants on a group of tumour cells growing in a test tube. The researchers found that the drug prevented the cancer cells from committing "suicide", thereby leading to a more vigorous growth of the tumours.
Although an increased risk of cancer has not so far been detected in Prozac patients, the latest findings could lead to a global re-evaluation of the drug's long-term safety.
Prozac, a "happiness pill" that was first approved in the United States in 1987, is widely used for the treatment of depression, obsessive-compulsive disorder and bulimia nervosa. Doctors in Britain issue about three million prescriptions for it each year and worldwide sales reached £1.8bn in 1999.
Professor Gordon, whose study was jointly funded by Birmingham University and the Medical Research Council, emphasised that the results of his study cannot be taken as proof that Prozac stimulates the growth of tumours.
He said: "Although that extrapolation could be valid, there is no direct evidence from large-scale epidemiological studies currently to back it up. However, it's important that we look again and again."
The research work was designed to find new ways of treating lymphomas, a type of blood cancer, by investigating how the brain communicates with the immune system to induce "positive thinking" through a neuro-transmitter in the brain called serotonin.
"Serotonin is a natural chemical that regulates people's moods, keeping them balanced. Too much serotonin affects appetite and sleep and too little affects the mood – often causing depression," Professor Gordon said.
Prozac, along with other members of the class of antidepressants known as selective serotonin re-uptake inhibitors (SSRIs), works by preventing serotonin from being quickly reabsorbed by nerve cells in the brain.
The scientists tested other SSRIs such as Paxil and Celexa and found they, too, had the same effect in stimulating the growth of a type of tumour known as Burkitt's lymphoma.
"An exciting property of serotonin is that it can tell some cells to self-destruct. We have found that serotonin can get inside the lymphoma cells and instruct them to commit suicide, thereby providing the potential for an effective therapy," Professor Gordon said.
The researchers found that Prozac blocked the entry of serotonin into the test-tube tumour cells and therefore stopped them from committing suicide. That raised the question of whether Prozac can do the same in the brains of people taking the drug.
Professor Gordon said it was still premature to suggest that the drug was unsafe. "We must stress the effects shown for the SSRI on cancer cells is indirect and should cause no concern whatsoever to the many millions of people throughout the world who are prescribed this class of antidepressants," he said.
Further work is underway to test Prozac further in this field. In particular, the scientists want to develop drugs that will mimic the cancer-destroying feature of serotonin which is blocked by Prozac.
A spokeswoman for Eli Lilly, the manufacturer of Prozac, said that the research is too new for the company to make a detailed response. "It's not something we can directly comment on because we haven't been involved in it," she said.Reuse content