Stem Cell Treatments Show Early Promise in Parkinson’s Disease

No serious side effects and positive patient impact seen in two small studies

Kimberly Goad,

AARP

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Published April 16, 2025

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Researchers are one step closer to using stem cells to help people with Parkinson’s disease. After decades of effort to manipulate these specialized cells to become dopamine-producing cells in the brain, two studies, published today in the journal Nature, show early, promising results.

In both studies, researchers enlisted a small number patients with Parkinson’s to determine the safety of stem cell therapy derived from human embryonic stem cells. They also got a hint at the treatment’s impact on patient symptoms.

The study from Kyoto University Hospital in Japan folllowed patients for 24 months after injection. The other study, led by a team at Memorial Sloan Kettering Cancer Center with collaborators from Weill Cornell, UC Irvine, and University of Toronto, followed patients for 18 months after the injections. In both, the cells had taken hold in the brain with no serious side effects. Levels of dopamine, the key chemical messenger that diminishes in Parkinson’s disease, rose in the brain. Some patients appeared to have significant improvement in their Parkinson’s-related symptoms.

Although both studies were too small to draw broad conclusions — the Sloan Kettering study included 12 patients average age 67, and the Kyoto study had six patients, ages 50 to 69 — the findings help pave the way for much larger studies that can evaluate how well the therapy actually works for people. According to the National Institute of Neurological Disorders and Stroke, as many as 1 million people in the United States have Parkinson’s, a brain disorder that causes unintended or uncontrollable movements and can impact cognition, mood and sleep.

“Cell therapy offers the possibility of replacing neurons that are lost in neurodegenerative diseases, such as dopamine in Parkinson’s disease,” says Brian Fiske, chief scientist at the Michael J. Fox Foundation for Parkinson’s Research, which provided early funding for both studies. “For people living with Parkinson’s, the goal of these therapies is to restore motor function, one of the challenging lived aspects of the disease. While these are early results in small numbers of people, these two studies provide encouraging data on safety and potential benefits of cell replacement approaches for Parkinson’s disease.”

The challenges of Parkinson’s disease

Parkinson’s is a progressive disease caused by the damage or death of nerve cells, or neurons, in a part of the brain that controls movement. As these nerve cells die off or become impaired, they no longer produce the chemical dopamine. In most people, dopamine works in concert with other chemical messengers to help coordinate the millions of nerve and muscle cells involved in simply moving. Without enough dopamine, that collaboration is disrupted. The result can include trembling in the hands, arms and legs; slowed movement; stiff muscles; balance and coordination problems.

Although current treatments help alleviate symptoms in the early stages of the disease, they lose their effectiveness over time. But what if the brain’s damaged or lost nerve cells could be replaced with healthy ones? That’s the idea behind stem cell therapy.

What is stem cell therapy for Parkinson’s?

“The grand idea is to potentially develop a once-in-a-lifetime treatment for Parkinson’s patients where you place in their brains … nerve cells to replace the nerve cells that they lose as a consequence of Parkinson’s disease,” says Viviane Tabar, M.D., neurosurgeon, stem cell biologist and chair of the Department of Neurosurgery at Memorial Sloan Kettering. “This idea has been kicking around for a long time now.”

Initial stem cell therapies using fetal tissue showed promise but sparked ethical concerns and caused complications, including graft-induced dyskinesia, which is characterized by involuntary movements like jerking, twisting and head bobbing. The difference now, says Tabar, a co-author of the Sloan Kettering study in Nature, is that it’s possible to make those nerve cells in the petri dish using what’s known as pluripotent stem cells, which are stem cells that have been reprogrammed to revert to an embryonic state. “We directed [the stem cells] to make the exact dopamine neuron that dies in Parkinson’s,” she says. “Then we inject them in the brains of patients.”

The researchers are finally able to make those cells in very large numbers. “And we can actually freeze them, which is not easy with nerve cells,” says Lorenz Studer, M.D., director of the Center for Stem Cell Biology at Memorial Sloan Kettering and a study co-author. “That means it’s a scalable technique. We can make many vials.”

Along with monitoring the treatment’s safety, Tabar, Studer and their team analyzed the therapy’s effect using standard tests, plus the patients kept diaries detailing their experiences. At the end of 18 months, participants reported spending more than 2.5 hours a day of their waking hours feeling better. And some appeared to have significant improvement in their Parkinson’s-related symptoms— most notably those related to movement—with and without taking medications for the disorder.

Green light for larger study

“The results [of both studies] are encouraging because they show that the use of allogeneic (non-self) transplants for the treatment of Parkinson’s is likely to be safe,” physiology professor Hideyuki Okano, M.D., of Japan’s Keiko University, wrote in an editorial in the same journal. To confirm their effectiveness, however, more research is needed, something the researchers in both studies make clear. According to the U.S. researchers, the Food and Drug Administration has already approved a much larger trial to determine the treatment’s efficacy.

“For the Parkinson’s patient who reaches a roadblock after several years of treatment, this (potentially) opens the door for an extended period of life where symptoms are much better controlled,” Tabar says.

“For a long time, there has been some sense that it’s science fiction,” Studer adds. “But we’re getting that much closer to making it real science.”

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Kimberly Goad is a New York-based journalist who has covered health for some of the nation’s top consumer publications. Her work has appeared in Women’s Health, Prevention, Health and Reader's Digest.