Clinical trial results have emerged simultaneously indicating that it is safe to implant dopamine stem cells in the brains of Parkinson's disease patients. Parkinson's disease is a degenerative disease where nerve cells that secrete dopamine in the brain's substantia nigra are damaged, causing a loss of motor function and shaking throughout the body. In severe cases, cognitive function may also decline.
The number of people with Parkinson's disease worldwide is estimated to exceed 10 million. While medications and electrical stimulation therapies are used for Parkinson's disease, restoring nerve cells through stem cell transplantation in the brain is an active area of research due to the difficulty of fundamental treatment. This clinical trial raises expectations about whether patients with Parkinson's disease can regain their health.
◇Muscles revived in patients with embryonic stem cells
Researchers from the Vivien Tabar Memorial Sloan Kettering Cancer Center in the U.S. stated, “We confirmed the safety by administering dopamine progenitor cells made from human embryonic stem cells to patients with Parkinson's disease,” in a paper published in the international journal Nature on the 17th (local time). Embryonic stem cells are primitive cells that grow into over 220 types of human cells from a fertilized egg formed by the meeting of sperm and egg.
The secret to embryonic stem cell treatment lay in 'cryopreservation.' The research team differentiated embryonic stem cells into dopamine progenitor cells, then cryopreserved them before implanting them into the brains of 12 patients with Parkinson's disease. They drilled holes in the brain to insert the cells into specific areas. Patients were divided into low-dose (5 patients) and high-dose (7 patients) groups based on the administered cells.
The implanted cells became nerve cells that produce dopamine in the brain. After following up with the patients for 18 months, no significant side effects were found. Since embryonic stem cells have excellent differentiation ability, they can potentially turn into tumors. Fortunately, the research team noted that such issues did not arise.
The stem cell transplantation procedure also reduced symptoms of Parkinson's disease. The researchers evaluated symptoms such as muscle rigidity, tremors, and decreased movement speed in the patients. As a result, both the low-dose and high-dose groups showed improvement in Parkinson's disease symptoms. Particularly, the high-dose group showed a 50% reduction in symptoms. This indicates that the more stem cells that are transplanted, the better the treatment effect.
◇Will induced pluripotent stem cells be a hope for Parkinson's disease patients?
On the same day, researchers, including Ryoosuke Takahashi and Jun Takahashi from the University of Tokyo in Japan, noted, “We created dopamine progenitor cells using induced pluripotent stem (iPS) cells and confirmed their safety and efficacy” in Nature.
Induced pluripotent stem cells are made by inserting specific genes into fully grown cells to revert them to a state similar to embryonic stem cells. Since this does not involve using fertilized eggs, it is free from ethical issues.
The research team implanted dopamine progenitor cells made from induced pluripotent stem cells into the brains of 7 patients (3 in the low-dose group and 4 in the high-dose group). After following up with these 7 patients, they stated that there were no safety issues. The research team remarked, “There were no side effects of excessive cell growth leading to tumors.”
The researchers also examined the treatment effects of Parkinson's disease on 6 patients (2 in the low-dose group and 4 in the high-dose group). They noted, “Dopamine increased in both the low-dose and high-dose groups,” adding that “especially in the high-dose group, a significant amount of dopamine was produced.” However, both research teams mentioned, “Due to the short study period, long-term research is needed.”
Hideyuki Okano, a professor at Keio University School of Medicine in Japan, stated in a commentary published alongside in Nature, “Both clinical trials demonstrated safety and showed potential efficacy,” noting that “the method of treating Parkinson's disease with cells represents an important step toward broader applications.”
However, it was pointed out that a limitation of the clinical trials was that both the researchers and patients were aware of what treatments they were receiving. Clinical trials must involve a large number of patients, randomly assigning them to real drug and placebo groups to be accepted.
References
Nature (2025), DOI:
https://doi.org/10.1038/s41586-025-08845-y
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-08700-0
Nature (2025), DOI: https://doi.org/10.1038/d41586-025-00688-x