Replacing the Neurons Parkinson’s Takes Away
Parkinson’s disease gradually damages the brain cells that produce dopamine, a chemical that plays an important role in movement. As these neurons are lost, symptoms such as tremor, stiffness, and slowed movement can become more difficult to manage.
A Phase I clinical trial explored whether researchers could replace some of these lost cells using dopamine-producing neuron progenitor cells grown from human embryonic stem cells. The treatment, called bemdaneprocel, was designed to develop into functioning dopamine-producing neurons after being transplanted into the brain.
How Did the Study Work?
The open-label trial included 12 people with Parkinson’s disease.
Participants were divided into two groups:
- 5 received a lower dose of approximately 0.9 million cells
- 7 received a higher dose of approximately 2.7 million cells
The cells were transplanted into both sides of the putamen, an area of the brain involved in movement and dopamine signaling.
Participants also received immune-suppressing medication for one year to help protect the transplanted cells while they became established.
Did the Cells Survive?
One of the most important questions was whether the transplanted cells would survive after being placed in the brain.
At 18 months, PET imaging showed increased dopamine-related activity in the areas where the cells had been transplanted. This provided evidence that the grafted cells survived and were producing dopamine-related signals.
The increases appeared greater in the higher-dose group, suggesting that the number of transplanted cells may influence the biological response.
Encouraging Changes in Movement
Although the main purpose of this Phase I trial was to evaluate safety, researchers also tracked changes in Parkinson’s symptoms.
Participants in the higher-dose group showed an average improvement of about 23 points on a motor-function assessment performed while patients were off their usual Parkinson’s medication. The amount of time participants spent in a good “ON” state without troublesome involuntary movements also increased, particularly in the higher-dose group.
These findings are encouraging, but because the study was small and did not include a placebo group, researchers cannot yet say how much of the improvement was directly caused by the transplanted cells.
What About Safety?
The study met its main safety and tolerability goals and researchers reported no adverse events directly related to the cell product during the first year. They also observed no graft-induced dyskinesias, a potential concern with dopamine-producing cell transplantation.
No tumors or abnormal tissue growth associated with the transplanted cells were reported during the study period.
Some participants did experience medical complications related to the surgical procedure or immune-suppressing medication, which is an important consideration with this type of treatment.
Why This Approach Is Different
Most Parkinson’s treatments are designed to increase dopamine activity or manage symptoms. This approach is different because it aims to replace the neurons that have been lost.
Instead of repeatedly supplying medication to compensate for declining dopamine production, researchers hope transplanted cells may eventually become a more lasting source of dopamine inside the brain. That makes this study a particularly interesting example of true cell-replacement therapy.
A Promising Step Forward
This was only a Phase I study with 12 participants, so it was not designed to prove that the treatment works. Still, the combination of encouraging safety findings, evidence that the transplanted cells survived, and improvements in motor symptoms provides a strong reason for larger controlled trials.
For now, this research offers an exciting glimpse at how stem cell-derived neurons may one day help replace some of the brain cells lost in Parkinson’s disease, potentially opening a new path for regenerative neurological care.
Source
Tabar V, Sarva H, Lozano AM, et al. Phase I trial of hES cell-derived dopaminergic neurons for Parkinson’s disease. Nature. 2025;641:978–983. doi: 10.1038/s41586-025-08845-y. Available from: https://www.nature.com/articles/s41586-025-08845-y
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