But the investigators cautioned that the results were somewhat ambiguous, as no disease-modifying effect was seen with a higher rasagiline dose of 2 mg per day.
Olanow suggested that 2 mg per day may have been so effective at reducing Parkinson's symptoms that the slowed disease progression was masked.
"It's an answer that is uncertain, however, it's tantalizing," said Dr. Karl Kieburtz, a professor of neurology and community and preventive medicine at the University of Rochester Medical Center, a consultant on the ADAGIO study and a medical expert for the Michael J. Fox Foundation for Parkinson's Research. "We don't know the mechanism for sure ... but does it have evidence of slowing the disease, and the answer, at least in [the 1 mg group] is yes."
But some experts were not inclined to accept the study's results.
"I would be reluctant to accept the conclusion that this provides compelling evidence of slowed Parkinson's disease progression," said Dr. J. Eric Ahlskog, a neurologist at the Mayo Clinic in Rochester, Minn.
Dr. William Weiner, chairman of neurology at the University of Maryland and one of the trial investigators, said he disagreed with the NEJM report's emphasis on the possible disease-modifying effect.
"I would not have agreed to stressing the neuroprotection angle so strongly," he said. "[The outcome] may be statistically significant but it is surely not clinically meaningful."
But while medical data on the potential neuroprotective effects of some Parkinson's drugs is sparse, doctors have been aware that they may have such an effect for several years and prescribe them as such to patients.
Evan Henry, 54, from Newport Beach, Calif., said he was prescribed medications for symptom management and to slow disease progression when he was diagnosed with Parkinson's disease in 2003.
"It was both right from the get go," Henry said. "[I and my doctor] thought it was better to try it and see what happens than to squander an opportunity to help stave something off."
The study did not allow researchers to differentiate whether rasagiline was having a neuroprotective effect or whether it only treated symptoms while the brain compensated for any cognitive losses by dialing up the activity in other areas.
Laboratory studies on tissue cultures and animal models showed rasagiline can protect brain cells from dying, but Olanow noted that the drug may not have the same protective effect in Parkinson's patients.
Further research on other Parkinson's drugs to see if they might slow disease progression similar to rasagiline may help bolster the results of the ADAGIO study.
But any advance that hints at slowing the ravaging effects of Parkinson's disease can be appealing to patients.
"It's unfortunate that it's not black and white at the end of the study," said Henry, who did not participate in the ADAGIO trial and has never taken rasagiline. "The trajectory of Parkinson's advance for me is very slow ... But if something like rasagiline comes on the market and has scientific data behind it to show that it works, I'd be a fool not to try and take advantage of this kind of advance."
Despite skepticism, some neurologists said the ADAGIO results could be practice-changing.