Reducing the immune system's activity might be a way to slow Parkinson's progression.
The authors of the study, from Washington University School of Medicine in St. Louis, MO, believe that their results highlight the role that the immune system plays in Parkinson's.
They suggest that immunosuppressant treatments could help to prevent or slow development of the condition.
Approximately 1 million people in the United States are affected by Parkinson's, a neurological disorder that has characteristic symptoms of tremor, stiffness, slow movements, and difficulty walking.
Scientists are not entirely sure what causes Parkinson's, but studies have previously suggested a link between the condition and the immune system.
Patients 'less likely to develop Parkinson's'
The team of scientists looked at data from millions of medical records. Using an algorithm they had designed, they attempted to predict which groups of people would go on to develop Parkinson's.
They found that people with ulcerative colitis and some other autoimmune disorders were at lower risk of developing Parkinson's than the general population.
Because the immune system disorders linked with a reduced risk are associated with a variety of causes and symptoms, it was difficult for the authors to detect a pattern that might indicate a common mechanism of action between the conditions and Parkinson's risk.
Was the link driven by drug or disease?
Eventually, the researchers found a common denominator: immunosuppressant drugs were used to treat many of these conditions.
To investigate further, the team analyzed Medicare prescription data from 48,295 Parkinson's patients and 52,324 people who had not been diagnosed with Parkinson's.
They found that people prescribed immunosuppressants were less likely to develop Parkinson's than people who took no immunosuppressants.
The results showed that people who took corticosteroids were 20 percent less likely to develop Parkinson's, and people taking another type of immunosuppressant, called inosine monophosphate dehydrogenase (IMDH), were approximately one-third less likely to develop Parkinson's.
To test the link between immunosuppressants and reduced risk of Parkinson's, the researchers ran an analysis of specific autoimmune conditions individually, but the results remained the same. This indicates that the reduced risk was linked with the immunosuppressant therapies rather than the autoimmune disorders.
"One group of drugs, in particular, looks really promising, and warrants further investigation to determine whether it can slow disease progression," says senior author Brad Racette.
Their results were published recently in the journal Annals of Clinical and Translational Neurology.
Immunosuppressants come with risks
Although Racette and colleagues' findings suggest that immunosuppressants have a protective effect against Parkinson's, these drugs have a range of unpleasant side effects. Also, they can increase the risk of cancer and some infectious diseases.
So, while immunosuppressants provide more benefits than harm to people with conditions affecting the immune system, doctors would probably not want to risk prescribing these drugs to otherwise healthy people as a preventive measure against Parkinson's.
"What we really need is a drug for people who are newly diagnosed, to prevent the disease from worsening," Racette says. "It's a reasonable assumption that if a drug reduces the risk of getting Parkinson's, it also will slow disease progression, and we're exploring that now."
Because of the relatively high number of side effects linked with corticosteroids, Racette's team is conducting a proof-of-concept study to establish whether IMDH inhibitors could fulfill this role.
"It's too early to be thinking about clinical trials to see whether it modifies the disease," says Racette, "but the potential is intriguing."