WASHINGTON (AP) -- A preliminary trial to test the safety of a drug in people with Parkinson's disease surprised scientists when all five patients showed measurable improvement.

The drug eliminated the periods of immobility that had occurred as much as 20 percent of the time before treatment and reduced or stopped the involuntary movements common to the disease, said Clive N. Svendsen of the University of Wisconsin-Madison. Also, the senses improved for three patients who had lost the ability to taste or smell.

While much more work needs to be done, the findings being reported Monday in the online issue of the journal Nature Medicine encouraged researchers.

"All five patients showed improvement, some more than others. Some symptoms were more affected," said Svendsen, one of the researchers on the trial.

The results show that the drug, GDNF, "is worth studying very carefully" as a possible treatment for Parkinson's disease, he said.

Dr. Michael J. Zigmond of the University of Pittsburgh School of Medicine, who has studied the disease for 30 years but was not part of the research team, was enthusiastic about the report.

"I consider this study to be the most exciting advance in the treatment of Parkinson's disease that has come about in years," he said. "I think the findings hold tremendous promise for going beyond" treating the disease's symptoms to treating the underlying disease itself.

Parkinson's is a progressive disease of the nervous system that affects an estimated 1.2 million people in the United States and Canada. Symptoms include tremors, body rigidity and problems in movement.

Former boxing champion Muhammad Ali, actor Michael J. Fox and former Attorney General Janet Reno have Parkinson's.

While the disease's cause is unknown, most symptoms stem from a lack of the brain chemical dopamine. Most drugs used to treat the disease restore dopamine or mimic its action, but they do not act permanently. Deep-brain stimulation using electricity also is used in therapy.

In the new trial, the five patients had a mini-pump implanted under the skin, with tubing connecting to an area within the brain called the putamen.

Tests showed that despite the fact that there were brain cells there that react to dopamine, they were not using the chemical. The new drug stimulated those cells to react to the dopamine present.

The pump delivered a continuous flow of GDNF - glial cell line-derived neurotrophic factor - to the area. GDNF is vital to the development and maintenance of these cells and the human tests were arranged after the drug showed promise in mice and primates with Parkinson's.

When testing turns to humans, the first trial - phase one - is limited to a few people. The aim is just to make sure there are no dangerous side effects. Only then can larger, double-blind tests be set up to see how well the medication works.

This phase one test has continued nearly two years with no side effects to the patients, Svendsen said. The next stage is being arranged.

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