In a world-first clinical trial, Japanese scientists have successfully implanted lab-grown brain cells into Parkinson’s disease patients — and early results suggest the treatment may actually work.

If the results hold, the implications could be enormous — not just for Parkinson’s, but for other neurodegenerative diseases too. Huntington’s, ALS, even Alzheimer’s could be future targets for similar cell-replacement strategies.

Parkinson’s disease is a progressive brain disorder that affects movement, balance, and coordination. It begins subtly, often with tremors in one hand or a slight stiffness in the limbs. But over time, it chips away at a person’s ability to walk, speak, and live independently. It’s estimated that 10 million people live with Parkinson’s, and is prevalence is rising fast as populations age.

The culprit is the gradual loss of dopamine-producing neurons in a part of the brain called the substantia nigra. Without enough dopamine, the brain’s signals to the muscles falter. There’s currently no cure for Parkinson’s disease, just treatments to slow down progression and deal with some of the effects.

Scientists have had the idea to replace dead cells directly in the brain for decades. In the 1980s and ’90s, some researchers tried transplanting fetal brain tissue into patients, but the results were inconsistent. Some patients improved, others didn’t. Tumors and uncontrolled movements were common side effects. And the ethical concerns surrounding fetal tissue never went away.

This new study from Japan takes a new approach to this idea. Instead of fetal tissue, researchers used induced pluripotent stem (iPS) — adult cells that have been genetically reprogrammed to behave like embryonic stem cells. These versatile cells can be turned into nearly any cell type in the body. And that includes the dopamine-producing neurons that Parkinson’s destroys.

Promising, though not perfect

Seven patients were enrolled in the study. One dropped out before surgery due to a COVID-19 infection, and another received the transplant in two stages and was evaluated only for safety (not for drug efficacy).

None of the patients had any major side effects and four showed improvements in movement when off their usual medications. Their average motor scores improved by 20%, and brain scans confirmed that the implanted cells were producing dopamine. A fifth patient improved while on medication. One patient didn’t improve, but their symptoms didn’t worsen either.

The gains were modest but meaningful. One patient improved by two stages on the Hoehn–Yahr scale, a widely used measure of Parkinson’s severity. Another saw a 50% improvement in motor function during off-medication testing.

“It is a significant achievement that we have been able to confirm improvement in patients’ symptoms,” said Jun Takahashi, director of Kyoto University’s Center for iPS Cell Research and Application, who is leading the research. “We hope to deliver this treatment to patients as soon as possible.”

Dopamine is back, to an extent

Researchers carried out brain scans to see how much dopamine production increased. They used a specialized tracer and found that in the patients that received the highest dose, dopamine increased by an average of 63.5%; in the low-dose group, it rose by just 7%.

Notably, in three patients, dopamine activity increased only in specific parts of the brain — right where the cells were injected. That local boost was still enough to improve symptoms in some cases, but there was no generalized return of dopamine production.

Still, the study authors are confident.

This trial is the most advanced test yet of iPS-cell-derived neurons in human patients — and the results may open the door to regulatory approval. Sumitomo Pharma, the company which helped manufacture the cells, plans to seek approval to manufacture and sell the treatment in Japan by the end of fiscal 2025. If successful, it would become only the second approved iPS-cell-based regenerative medicine product in the country, after a treatment for heart disease.

The team’s goal is to develop a standardized, off-the-shelf cell therapy that can be administered to a wide population. The cells used in this study were derived from a donor whose genetic immune profile matches about 17% of the Japanese population, reducing the risk of rejection.

Japan has made regenerative medicine a priority. It was the first country to approve clinical use of iPS cells, and it continues to lead in funding and infrastructure for stem cell-based treatments. This trial may put Japan at the forefront of a new era in neuroscience.

The study has been published in Nature.