WASHINGTON - The first dozen Parkinson's patients to have holes drilled in their skulls for a novel gene therapy attempt weren't harmed - and hints at some improvement have researchers embarking on a larger study to see if the treatment really may work. Doctors reported initial results of the closely watched experiment at a neurology meeting Monday, but cautioned that it's far too soon to raise hopes.

At issue: Using a nerve growth factor to try to rescue dying brain cells.

Some 1.5 million Americans have Parkinson's, a disease that gradually destroys brain cells that produce dopamine, a chemical crucial for the cellular signaling that controls muscle movement. Too little dopamine causes increasingly severe tremors and periodically stiff or frozen limbs.

Standard treatments can control tremors for a while but can't stop the disease's inevitable march. So scientists are hunting ways to protect remaining dopamine-producing neurons, and rescue dying ones.

Previous attempts with growth factors haven't panned out. The new approach uses gene therapy - injecting a virus that carries a gene that in turn produces the growth factor neurturin - to try to get the protective protein right where it's needed.

None of the first 12 patients to undergo the experiment - at the University of California, San Francisco and Chicago's Rush University Hospital - suffered serious side effects, UCSF neurosurgeon Dr. Philip Starr reported Monday.

A year after treatment, three patients showed no difference on a standard rating scale of movement. But the other nine showed a 38 percent improvement, Starr told a meeting of the American Association of Neurological Surgeons.

That doesn't mean the therapy worked, Starr cautioned. It could have been coincidence; some previous attempts found similar hints of effectiveness, only to fail when put to more rigorous testing.

But the results were encouraging enough that researchers are enrolling more Parkinson's sufferers - 56 of them - for the next stage of testing. A third of those patients will undergo sham surgery, getting the holes drilled in their skulls but no gene-carrying virus, to try to tease out whether the therapy really works.

California-based Ceregene Inc. sponsored the research.