Patients given exantide showed improvements in their movement, which persisted after the 12-week follow-up. Those who had injected the placebo showed a decline in their motor scores at both the 48 and 60 week tests.
Whether exenatide affects the underlying disease or just its symptoms is uncertain, the researchers say. Further, longer-term trials are required.
Australian Professor Bryce Vissel, director of the Centre for Neuroscience and Regenerative Medicine at the University of Technology Sydney, has hailed the study as "exceptionally important".
He says the finding provides hope of a new way forward.
"Despite decades of research efforts, there are no cures and the disease continues to worsen over time until the symptomatic treatments are less and less useful. No currently available treatments slow the underlying disease process in humans. There is a urgent need for treatments that slow the underlying disease," said Vissel.
"If this finding holds up to further investigations, this will have been a watershed," he said.