Professor Tom Foltynie, of University College London's Institute of Neurology, said: "This is the strongest evidence we have so far a drug could do more than provide symptom relief for Parkinson's disease".
Parkinson's disease, which affects 127,000 people in the United Kingdom, causes progressive damage to the brain over time and cells that produce dopamine hormone - a chemical that helps control body movement - are lost.
'This is a very promising finding, as the drug holds potential to affect the course of the disease itself, and not merely the symptoms.
Currently, existing treatments relieve most of the symptoms for some years, but the disease continues to worsen.
This study also adds weight to the theory that diabetes and Parkinson's (or Alzheimer's) work in a similar way; just as neurons can become unresponsive to insulin, cells in the pancreas do in type 2 diabetes.
The team used either a once-weekly injection of Exenatide for 48 weeks, or a placebo, in addition to their regular medications.
But the same receptors exist in the brain, and scientists believe activating them can boost dopamine function and stop inflammation. Symptoms typically don't become apparent until over 70% of the brain's dopamine-producing cells have been affected.
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"While we are optimistic about the results of our trial, there is more investigation to be done, and it will be a number of years before a new treatment could be approved and ready for use".
Those who had injected the placebo showed a decline in their motor scores at both the 48 and 60-week tests. This was a statistically significant difference.
Researchers found the benefits were still continuing when participants were checked 12 weeks after they stopped taking the medication.
The participants did not report noticeable improvements in their symptoms during the trial period beyond what their standard medication already did for them.
The research is published in The Lancet today (FRI) and was funded by the Michael J. Fox Foundation for Parkinson's Research (MJFF). "We also hope to learn why exenatide appears to work better for some patients than for others", concluded the study's first author, Dr Dilan Athauda (UCL Institute of Neurology).
"Using approved therapies for one condition to treat another, or drug repurposing, offers new avenues to speed Parkinson's therapeutic development", commented Dr Brian Fiske, senior vice president of research programmes at MJFF.
Next, researchers will try to confirm whether or not the drug can affect the course of the disease and whether it improves quality of life for patients in the long-term.
Parkinson's disease affects 1 in 500 people and around 127,000 people in the United Kingdom live with the condition. The condition results in muscle stiffness, slowness of movement, tremors, sleep disturbance, chronic fatigue and an impaired quality of life.