Scientists have shown that the first gene-based therapy developed for Parkinson's disease substantially alleviates the symptoms of the condition without the need to leave any devices in the body.

The experimental treatment, carried out recently for the first time in humans by American scientists, involved infusing the brain for 100 minutes with a harmless virus that switched on a gene controlling the production of dopamine, a neurotransmitter chemical.

Parkinson's disease is caused by the loss of dopamine-producing cells in the part of the brain that controls movement. Symptoms include shaking and muscle weakness. One in 500 people suffers from the disease and around 10,000 people are diagnosed in the UK every year, mostly over the age of 50.

The results of the phase one clinical trial of the gene therapy, developed by the American biotech company Neurologix, found all 12 patients showed a clinical improvement of 25% as calculated on the Unified Parkinson's Disease Rating Scale (UPDRS) - a measure devised by doctors to track the long-term progress of Parkinson's patients. Nine of the 12 showed an average improvement of 37%, and five of those improved between 40% and 65%.

Phase one trials are designed to show that a new therapy is safe. A few patients are usually given different doses. Before it can be used widely the treatment must undergo two further stages of tests involving larger groups of patients over several years.

Current treatments for Parkinson's disease include directly replacing the lost dopamine, or inserting electrodes into the brain to stimulate it to produce it (deep brain stimulation). The electrodes stay in permanently, powered by an implanted pacemaker. "The next stage is to try and either empower the cells that are there to produce their own dopamine, or to put in cells that will produce dopamine themselves," said Kieran Breen, director of research at the Parkinson's Disease Society.

Matthew During of Neurologix used patients at the New York-Presbyterian Hospital with advanced Parkinson's disease who were not responding to any current treatments. Each patient received an infusion of the non-pathogenic adeno-- associated virus via a catheter inserted into the subthalamic nucleus, a part of the deep brain known to function abnormally in Parkinson's patients. The virus, which was designed to switch on a gene involved in making dopamine, was only exposed to half of each patient's brain

The results were presented yesterday at the Annual Meeting of the Society of Neuroscience in Atlanta.

The patients reported no side effects a year after the therapy, and all showed improved UPDRS scores on the side of their body that correlated to the treated part of their brain. The untreated side of the brain showed no improvement.

"This gene therapy trial is particularly unique and the clinical data unusually promising because the treatment was confined to just one side of the brain," said Dr During. In the next stage the company plans to infuse the treatment into both sides of the brain.

He added that the research aimed to work out whether it was possible to reset a specific group of overactive cells.

"The interim UPDRS scores are highly promising and, if they are borne out with additional data, would be comparable to results seen with deep brain stimulation. Unlike deep brain stimulation, however, our gene therapy approach is much simpler, can be carried out entirely under local anaesthesia, and avoids leaving any devices in the body," he said.

Dr Breen said the work being done by Neurologix was important to get past the current regime of just giving Parkinson's patients doses of dopamine. "We need to move on to the next stage which is to modify the function of the nerve cells."