The drugs are not the main treatment for Parkinson's, but one is also sometimes used to treat restless legs syndrome.
A study by Italian researchers found that roughly one-fourth of Parkinson's patients taking pergolide or cabergoline, sold as Permax, Dostinex and other brands, had moderate to severe heart valve problems. Another study, by German doctors, found that users of either drug were five to seven times more likely to have leaky heart valves than those on other types of Parkinson's medications. Both studies were reported in Thursday's New England Journal of Medicine.
"This is an extraordinarily high risk," said Dr. Bryan Roth, a pharmacology professor at the University of North Carolina at Chapel Hill.
"It's a bad side effect. As far as I know, there are no medications that can reverse it," and valve replacement surgery is the only solution, he said.
Roth had no role in the studies but directs a drug screening program for the National Institute of Mental Health. He also published a paper several years ago warning that these drugs appeared to trigger the same heart-related mechanism that the fen-phen diet combination did. The diet pills, sold as Pondimin and Redux, were pulled from the market in 1997 after they were linked to valve problems.
One of the Parkinson's drugs — pergolide, sold as Permax and other brands — also is used to treat restless legs syndrome. Cabergoline, sold as Dostinex, Cabaser and other names, is mostly used in Europe.
About half a million people had taken Permax during its first 14 years on the market when its developer, Eli Lilly and Co., added valve damage to the potential side effects listed on the package insert in 2003. But the company said the risk was extremely low — five in 100,000 users.
Roth believed there were more cases, a theory he said the new studies confirmed.
"This is an example of, if you don't look for it, you don't see it," said Dr. C. Warren Olanow, chairman of neurology at Mount Sinai School of Medicine in New York, who had no role in the work. The findings will lead more doctors to prescribe other Parkinson's treatments, he said.
About 1.5 million Americans and 6 million people worldwide have Parkinson's disease, which results in tremors, loss of muscle control and sometimes death.
It is caused by a lack of the brain chemical, dopamine. The main treatment is levodopa, which spurs the body to make more dopamine. Pergolide and cabergoline often are given in addition to that drug or in place of it, especially if symptoms worsen over time.
In one study, Dr. Renzo Zanettini and others at the Instituti Clinici di Perfezionamento in Milan obtained echocardiogram images of the hearts of 155 patients taking various Parkinson's medications and a comparison group of 90 healthy people.
Moderate to severe valve problems were seen in 23 percent of those on pergolide and nearly 29 percent of those on cabergoline but none of those on other Parkinson's drugs and less than 6 percent of the comparison group. The study was paid for by the Milan clinic and two Parkinson's foundations.
In the other study, Dr. Rene Schade and colleagues in Berlin and in Montreal used records from more than 11,400 Parkinson's patients in the United Kingdom. The rate of newly diagnosed leaky valves was increased among pergolide and cabergoline users but not the others, they found. The Canadian government and a drug company provided partial support for the study. Many researchers in both studies have consulted for Parkinson drug makers.
Pergolide sales have dropped in recent years but still amounted to more than $10 million (euro7.56 million) last year in the United States, according to IMS Health, a health care information firm.
The rights to Permax in the U.S. now belong to Valeant Pharmaceuticals of Aliso Viejo, Calif. A company statement said Permax is safe and effective, but Valeant is no longer promoting the product. All such drugs should be used "with caution," the statement says.
Cabergoline is approved in the U.S. for treating a hormone problem, excessive prolactin in the blood, but not Parkinson's.
Roth has been urging companies developing new drugs to test for the mechanism involved in the Parkinson and fen-phen pills, saying those that that have it should not be sold.