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 cautioned 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.'

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.

Kaplitt, who has a financial interest in Neurologix Inc., which paid for the research, noted that the 12 patients in the study still have Parkinson's symptoms. The amount of medication they were already taking for their symptoms didn't change significantly in the year after the surgery.

Current medicines can control symptoms, but can't stop the disease from getting worse over time, and they can produce troublesome side effects like uncontrollable movement.

Some patients gain relief from a surgical treatment called deep brain stimulation, in which electrodes are placed in the brain and connected to a programmable stimulator.

Kaplitt's procedure was aimed at achieving the same goal as that surgery, calming overactive circuitry in the brain. It gets overactive because it loses the normal supply of a chemical called GABA. The gene therapy was designed to make the brain produce more GABA.

For the gene therapy surgery, a tube about the width of a hair was threaded through a hole about the size of a quarter at the top of the skull. The tube delivered a dose of a virus engineered to ferry copies of a gene into cells of a brain region called the subthalamic nucleus. The gene copies enable the cells to pump out more GABA.

The Lancet paper reports that over a year, patients showed no side effects from the procedure. What's more, they showed improvements in an overall assessment of symptoms like tremors, stiffness and walking problems.