Gene therapy is a hot topic in Parkinson’s disease. Researchers are now testing this novel approach in clinical trials of people with mid- to late-stage Parkinson’s. These therapies are intended to treat movement symptoms (tremor, slowness, stiffness), lessen levodopa needs and, in turn, limit possible medication complications (such as dyskinesia — involuntary, uncontrolled movement).
Gene therapy aims to restore or repair the function of the brain’s dopamine cells, which are damaged or die in Parkinson’s, by giving them the machinery (proteins, or enzymes) they need to make dopamine. One enzyme, AADC, is responsible for converting levodopa to dopamine. As disease progresses, less AADC is available so that despite levodopa, people may experience increasing “off” time (when symptoms return because medication isn’t working well).
Voyager Therapeutics is testing a therapy that delivers AADC, through a one-time surgical procedure, to a brain area populated with dopamine cells. With support from MJFF, Phase I trials showed the gene therapy was safe and well-tolerated for up to three years in 15 people who had Parkinson’s for an average of 10 years. Although not designed to show efficacy, the trial found some evidence to suggest that participants’ movement symptoms improved as well. Based on this data, Voyager now is recruiting for a larger, multicenter Phase II placebo-controlled trial to further assess the therapy’s safety and tolerability.
Axovant Sciences is testing another form of gene therapy that infuses the genetic instructions for making dopamine into a different area of the brain through a one-time surgical procedure. The Phase I trial aims to evaluate safety and tolerability of the therapy in 30 people with Parkinson’s; the first part of the study will test different doses across participant groups and the second part will test continued therapy versus placebo.
These types of gene therapy could expand options for treating the symptoms of Parkinson’s, especially as disease progresses. But, like all available therapies, they do not slow or stop disease progression. Early results are promising, but researchers must evaluate longer-term safety and efficacy as well as the optimal amounts of drug and delivery locations in the brain.