Two separate clinical trials published this week in Nature have confirmed that experimental stem cell therapies for Parkinson’s disease are safe and well-tolerated in humans. While further research is needed to prove how well the treatments work, these early trials represent a major step forward in the search for long-lasting treatments for this debilitating condition.
Parkinson’s disease is a progressive disorder caused by the gradual loss of brain cells that produce dopamine, a key chemical that helps control movement.
Over time, patients experience tremors, stiffness, balance issues, and slowness of movement. Current medications such as L-dopa can help relieve symptoms in early stages but often become less effective and can lead to unpleasant side effects like involuntary movements (dyskinesia).
One promising idea is cell therapy—replacing the lost dopamine-producing neurons in the brain using lab-grown cells. Scientists have been exploring this for years, but safety concerns, particularly the risk of uncontrolled cell growth or tumors, have slowed progress.
In the first trial, led by Ryosuke Takahashi and Jun Takahashi in Japan, researchers transplanted lab-grown dopamine-producing cells made from induced pluripotent stem cells (iPSCs) into the brains of seven Parkinson’s patients aged 50 to 69.
These stem cells were originally taken from adult cells and then “reprogrammed” into an embryonic-like state, meaning they can develop into any type of cell in the body.
The transplanted cells survived, integrated into the brain, and produced dopamine for at least 24 months without any signs of tumor growth or serious side effects—an important safety milestone.
Some participants also showed slight improvements in motor symptoms, especially those who continued through the full trial with or without standard medication. However, the results were mixed and depended on which clinical scales were used to measure improvement.
In the second trial, researchers led by Viviane Tabar in the United States tested a different cell product, bemdaneprocel, made from human embryonic stem cells.
These cells were also developed into dopamine-producing neuron precursors and transplanted into both sides of the putamen, a brain region involved in movement control. Twelve patients with an average age of 67 received either a low or high dose of the treatment.
Like the first study, this trial also reported no serious side effects or tumor development over an 18-month follow-up period. Notably, there were no cases of dyskinesia, a complication seen in past trials using fetal brain tissue. Some patients showed signs of improved motor function, but again, the improvements varied.
Both studies were open-label, meaning that participants and researchers knew what treatment was being given, and each trial involved a small number of patients. Still, the fact that two independent groups in different countries used different types of stem cells and arrived at similar safety conclusions is encouraging.
According to Dr. Hideyuki Okano, writing in a Nature commentary, these results “hint at possible efficacy” and represent an “important step” toward broader use of stem cell therapy for Parkinson’s disease.
The next steps include conducting larger, placebo-controlled trials to better understand how effective these therapies are and whether repeated treatments or combination approaches could offer even better outcomes.
For now, though, these trials provide a hopeful glimpse into a future where replacing lost brain cells could ease the burden of Parkinson’s disease for millions of people around the world.
If you care about Parkinson’s disease, please read studies that Vitamin B may slow down cognitive decline, and Mediterranean diet could help lower risk of Parkinson’s.
For more information about brain health, please see recent studies that blueberry supplements may prevent cognitive decline, and results showing Plant-based diets could protect cognitive health from air pollution.
The research findings can be found in Nature.
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