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Long-term tests on monkeys using Oxford BioMedica's gene therapy ProSavin suggest it can treat Parkinson's disease without causing the jerky, involuntary movements associated with current drugs, researchers said on Wednesday.

Parkinson's is caused by lack of the brain chemical dopamine. Standard treatment involves oral drugs that briefly raise dopamine levels -- but levels of the chemical still remain unstable, leading to a movement disorder called dyskinesias.

By contrast, tests on macaque monkeys found the gene therapy safely restored concentrations of dopamine in the brain, corrected motor deficits and prevented dyskinesias -- with no severe adverse side effects.

French researchers observed the animals for up to three and a half years in the study, after first inducing Parkinsonian syndrome by giving a neurotoxin and then treating them with gene therapy injections.

"Gene therapy-mediated dopamine replacement may be able to correct Parkinsonism in patients without the complications of dyskinesias," Bechir Jarraya and colleagues wrote in the journal Science Translational Medicine.

Oxford BioMedica is currently conducting Phase I/II clinical trials with ProSavin and in July announced encouraging initial results.

ProSavin, which is administered directly to the striatum in the brain, delivers three genes required to convert cells that normally do not produce dopamine into cells that do.