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What therapies are currently in development?

A great deal of research is being conducted on new treatments to (1) improve the symptoms of PD; (2) protect or restore the nerve cells affected by PD-and thereby slow down disease progression; (3) prevent or suppress dyskinesias; and (4) improve non-motor symptoms, such as depression or cognitive impairment.

Some of the most interesting recent clinical trials have examined neurotrophic factors for the treatment of Parkinson's disease. In preclinical (non-human) studies, the neurotrophic factors GDNF and neurturin (i.e., a ligand used to bind to GFRA2 receptors. It is related to Glial cell line-derived neurotrophic factor.) have shown both symptomatic and neuroprotective benefits. This promising preclinical work makes neurotrophic factors one of the most exciting current areas of research.

The challenge for clinical use of neurotrophic factors is delivering them effectively to the brain. Pump infusion of GDNF was used in two small unblinded clinical trials, with positive results, but the results were not replicated in a more recent blinded clinical trial. Currently, a neurturin gene therapy safety trial has been completed and a larger trial is planned to examine its clinical efficacy.

The National Institute for Neurological Disorders and Stroke (NINDS) has conducted several trials to explore the effectiveness of certain compounds that have shown potential to slow the progression of Parkinson's disease in non-human preclinical studies. To date, tests have been done on CoQ10, a substance widely used as a dietary supplement that may have neuroprotective effects, and GPI-1485, an investigational drug that may have a regenerative effect. A second study looking at minocycline, an antibiotic with anti-inflammatory effects, and creatine, a compound that may help maintain healthy mitochondrial function and thus counteract the possible role of oxidative stress in Parkinson's disease, has also been completed. These studies were futility trials-designed to show which compounds should be further pursued and which should not. To date, only the creatine and minocycline trials have been published, with this data concluding that further studies should be done.

Cell replacement therapy is designed to replace the neurons that degenerate in PD. While there are a variety of potential sources of cells for this therapy, most work currently focuses on stem cells. A stem cell has the potential to differentiate into any cell type in the body. The premise of this approach is that stem cells in a dish can be coaxed to become healthy dopamine neurons, which can then be transplanted into the brain to replace the sick and dying neurons in Parkinson's disease.

While U.S. government restrictions on embryonic stem cell research have undoubtedly slowed the pace of this work in recent years, some key insights have emerged. These findings, however, have highlighted the complexity of cell replacement, and have made it clear that significant basic research remains to be done.