Patients with Parkinson’s disease often do well for years by taking levodopa-a drug their brain cells turn into dopamine, the neurotransmitter whose decline causes debilitating spasmodic movements. Eventually, though, the drug loses effectiveness, as the patient’s brain stops converting it, and symptoms worsen. But gene therapy-in which therapeutic genes supplement or replace missing or damaged ones-may keep the drug working much longer, offering hope that the lives of many Parkinson’s patients can be dramatically improved.

Biologists at the University of California, San Francisco, have injected viruses carrying an enzyme-producing gene, called AADC, into monkeys’ brains. The enzyme transforms levodopa into dopamine and could keep the drug working in late-stage Parkinson’s patients. Krys Bankiewicz, the UCSF gene therapist who is leading the study, says levodopa still controls the symptoms of test monkeys suffering from a severe, chemically induced form of Parkinson’s three and a half years after gene transfer. “There are no adverse effects that we can detect, and it’s efficacious,” he says. Indeed, Bankiewicz is cautiously optimistic that a very small human trial, which would also require injections into the brain, could begin as early as the end of the year.

While this approach could help patients with advanced Parkinson’s, the long-term goal is to slow or halt the disease’s early progression. A different gene, called GDNF, could help by protecting and perhaps regenerating dopamine-producing neurons. While much work is still needed-for one thing, the researchers must find a way to effectively turn the gene on and off-Jeffrey H. Kordower, director of the Research Center for Brain Repair at Rush Presbyterian Medical Center in Chicago, hopes that clinical trials could take place within three years.

As a means of tackling Parkinson’s, gene therapy is gaining momentum. In February, a consortium of scientists at seven research centers-organized by University of Rochester neuroscientist Howard Federoff and including Bankiewicz and Kordower-received $8.8 million in federal funding to pursue gene therapy for Parkinson’s. Richard Mulligan, a gene transfer expert at Harvard Medical School, says questions remain-including which gene is best-but that some form of gene therapy could very well prove effective as a Parkinson’s treatment. “It’s really an excellent disease target for gene therapy,” he says, because the therapeutic genes need to be delivered to a specific location in the brain, which is difficult for other therapeutics to reach. If the consortium is successful, it will be a victory for both the field of gene therapy and millions of Parkinson’s patients.