(CBS/AP) An experimental treatment for Parkinson's disease seemed to improve symptoms — dramatically so for one 59-year-old man — without causing side effects in an early study of a dozen patients.

The gene therapy treatment involved slipping billions of copies of a gene into the brain to calm overactive brain circuitry.

The small study focused on testing the safety of the procedure rather than its effectiveness, and experts caution that it's too soon to draw conclusions about how well it works. But they called the results promising and said the approach merits further studies.

"We still have quite a bit more testing to do," said Dr. Michael Kaplitt of Weill Cornell Medical College in New York, an author of the study. Still, "the initial results are extremely encouraging."

"Our hope is that this will, in the future, be considered the first major milestone in the creation of a whole new field of medicine for brain diseases," Kaplitt told CBS News Correspondent Michelle Miller.

Kaplitt and collaborators report their results in this week's issue of the British medical journal, The Lancet.

They're not alone in trying gene therapy for Parkinson's. In April, another team told a medical meeting that its experiments, which delivered a different kind of gene to a different part of the brain, also appeared safe and gave a preliminary hint of benefit.

More than half a million Americans have Parkinson's. They endure symptoms that include tremors, rigidity in their limbs, slowness of movement and impaired balance and coordination. Eventually they can become severely disabled.

Nathan Klein, a 59-year-old freelance television producer in Port Washington, N.Y., said the disease left him "pretty messed up." It weakened his voice, impaired his walking and made his hand tremble so badly he couldn't hold a glass of wine without spilling it all.

Klein was the first patient to be treated with Kaplitt's gene therapy procedure in 2003, and he said his symptoms gradually subsided afterward. Nowadays, he said, apart from freezing now and then when he wants to walk, the symptoms are basically gone.

"I feel great. I swim, ride my bike, I do everything anybody who's normal can do," Klein told Miller.

"I'm elated," added Klein, who continues to take his regular pills for the disease. "It's unbelievable."

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