SAN DIEGO (AP) - Neurosurgeons have injected genetically modified cells into the brain of an Alzheimer's patient in a pioneering procedure that may hold the promise of halting or reversing brain cell loss caused by the disease.

The 11-hour procedure at the University of California, San Diego, marked the first use of human gene therapy in the treatment of brain disease, researchers said Tuesday.

If the procedure works as hoped, improved brain function might be seen in the patient over the next few weeks, but doctors cautioned it will take years to learn if the therapy would benefit Alzheimer's patients in general.

Scientists took skin cells from a 60-year-old Oregon woman in the early stages of Alzheimer's disease and isolated genes that secrete a protein found in healthy brains called nerve growth factor. On Thursday, two drops of a solution containing those genes were injected into the woman's brain.

She was discharged from the hospital two days later.

``Our hope is that this procedure will be a way of delaying the progress of the disease and improving the quality of life for several years,'' said Dr. Mark Tuszynski, who led the study. ``It's unlikely to be a cure.''

The federal government approved human trials for nerve growth factor two years ago after a team of UCSD researchers showed the protein reversed deterioration in the brains of aging monkeys.

Another patient will undergo the procedure in three months, and researchers are looking for six more candidates for initial studies to determine whether the therapy is safe. Future tests will gauge whether patients maintain their mental abilities.

Four million Americans have Alzheimer's disease, which causes a decline in memory and the ability to care for oneself. Ten percent of people over 65 and nearly half of those over 85 have Alzheimer's disease, according to the Alzheimer's Association.

Bill Thies, vice president of medical and scientific affairs for the Chicago-based group, said he was cautious but optimistic about the new procedure.

``Anytime you start a clinical trial, you don't know whether the benefits outweigh the risks,'' Thies said. ``You always want to be cautious at the beginning.''

He noted that Alzheimer's only afflicts humans, and doctors may not experience the same success they had with monkeys. He also said the complexity of the procedure may hamper wider use.

``We're not going to do neurosurgery on 4 million people,'' Thies said.

The therapy targeted an area about the size of an aspirin tablet deep within the brain of the former Oregon schoolteacher.

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