Levodopa is currently the most common drug treatment for the brain condition but the pills can leave people susceptible to involuntary movements such as twitches. It is normally prescribed a few years after diagnosis as the symptoms get worse.
In additionally, disease progression and long-term oral treatment with levodopa may lead to the development of motor fluctuations and dyskinesias
This new approach involved taking levadopa producing eye cells taken from a dead donor and implanting the directly into the brains of six Parkinson's patients. The researchers reported no serious side effects.
Eye cells that form retinal pigment epithelial tissue produce levodopa and can be isolated from human eye tissue and implanted in the brain. Research on animals has shown that the cell implants can help treat the symptoms safely.
The retina cells were cultivated and implanted in the brains of six patients with advanced Parkinson's, said researcher Natividad Stover of the University of Alabama.
One year later, the patients scored 48 per cent higher on tests of movement and coordination, and the improvement was sustained after two years, Stover wrote in the report.
"The implants were well tolerated," the report said. "Improvement was also observed in activities of daily living (and) quality of life."
The study is additional proof that the technique could work on humans and paves the way for a larger, more thorough study.
Indeed, Stover added that a randomised, double-blind, placebo-controlled study was now in the pipeline to test the treatment further.
For most patients, the levodopa pills lose their effectiveness over five years or less, and larger and larger doses are needed to keep at bay the involuntary movements and shaking symptomatic of the disease.
Some scientists have viewed implanting foetal stem cells into the brains of Parkinson's patients as a promising avenue to restoring dopamine production. But preliminary human trials were disappointing, and animal experiments have yielded mixed results.
Other treatments showing promise include deep brain stimulation with implanted electrodes, drugs that promote brain cell growth, and gene therapy.
The study was published in the Archives of Neurology journal.