"This is a very promising finding, as the drug holds potential to affect the course of the disease itself, and not merely the symptoms," said senior study author Tom Foltynie, from University College London's Institute of Neurology.
"With existing treatments, we can relieve most of the symptoms [of Parkinson's] for some years, but the disease continues to worsen," he said in a university news release. "This is the strongest evidence we have so far that a drug could do more than provide symptom relief for Parkinson's disease."
Parkinson's is the second most common neurodegenerative disease worldwide, the researchers noted. The condition results in muscle stiffness, slowed movement, tremors, sleep disturbance and chronic fatigue.
In the study, 60 people with Parkinson's received either a weekly injection of exenatide or an inactive placebo for 48 weeks, along with their regular medications.
At the end of that period, those who took the diabetes drug scored four points higher on a 132-point scale of agility, speech and tremors than those who took the placebo. The difference was statistically significant, the study authors said.
The findings were published Aug. 3 in the The Lancet.
According to Brian Fiske, senior vice president of research programs at The Michael J. Fox Foundation for Parkinson's Research, "Using approved therapies for one condition to treat another, or drug repurposing, offers new avenues to speed Parkinson's therapeutic development." The foundation funded the study.
"The results from the exenatide studies justify continued testing, but clinicians and patients are urged not to add exenatide to their regimens until more is known about their safety and impact on Parkinson's," Fiske said.
Another Parkinson's expert agreed that more research is in order.
"While these are exciting findings, the observed benefit was small and only in one outcome-measure," said Dr. Martin Niethammer, a neurologist at Northwell Health's Neuroscience Institute, in Manhasset, N.Y.
"This might relate to the study being relatively small and of short duration, rather than lack of efficacy [effectiveness] of exenatide, and more study is certainly needed," he noted.
"This trial does provide an excellent rationale for larger and longer trials, and it remains to be seen if exenatide, and drugs like it, truly have a disease-modifying effect or merely improve the symptoms of Parkinson's disease," Niethammer said.
"There's absolutely no doubt the most important unmet need in Parkinson's is a drug to slow down disease progression, it's unarguable," Prof Tom Foltynie, one of the researchers, told the BBC.
In Parkinson's, the brain is progressively damaged and the cells that produce the hormone dopamine are lost.
It leads to a tremor, difficulty moving and eventually memory problems.
Therapies help manage symptoms by boosting dopamine levels, but the death of the brain continues and the disease gets worse.
No drug stops that happening.
In the trial, half of patients were given the diabetes drug exenatide and the rest were given a placebo (dummy treatment). All the patients stayed on their usual medication.
As expected, those on just their usual medication declined over 48 weeks of treatment. But those given exenatide were stable.
And three months after the experimental treatment stopped, those who had been taking exenatide were still better off.
Prof Foltynie told the BBC News website: "This is the first clinical trial in actual patients with Parkinson's where there has been anything like this size of effect.
"It gives us confidence exenatide is not just masking symptoms, it's doing something to the underlying disease.
"We have to be excited and encouraged, but also cautious as we need to replicate these findings."
They also need to trial the drug for much longer periods of time.
An effective drug would need to hold back the disease for years in order to make a significant difference to patients.
Parkinson's progresses slowly and the difference in this 60-week trial was definitely there, but was "trivial" in terms of the impact on day-to-day life, say the researchers.
The drug helps control blood sugar levels in diabetes by acting on a hormone sensor called GLP-1.
Those sensors are found in brain cells too. It is thought the drug makes those cells work more efficiently or helps them to survive.
It is why the drug is being tested in other neurodegenerative diseases including Alzheimer's.
David Dexter, the deputy director of research at Parkinson's UK, said: "The findings offer hope that drugs like exenatide can slow the course of Parkinson's - something no current treatment can do.
"Because Parkinson's can progress quite gradually, this study was probably too small and short to tell us whether exenatide can halt the progression of the condition, but it's certainly encouraging and warrants further investigation."
Dr Brian Fiske, from The Michael J Fox Foundation for Parkinson's Research, said: "The results from the exenatide studies justify continued testing, but clinicians and patients are urged not to add exenatide to their regimens until more is known about their safety and impact on Parkinson's."