A new clinical trial has shown that stem-cell-derived brain cells can be safely transplanted into people with Parkinson’s disease. The cells survived, produced dopamine, and showed early signs of benefit. This article will explain what the study did, what the results mean, and why this could be an important step toward new treatments for Parkinson’s.
Parkinson’s disease is a progressive neurodegenerative disorder that affects movement and quality of life. It is caused by the gradual loss of dopamine-producing neurons in the brain, particularly in a region called the substantia nigra. Current treatments, such as levodopa and deep-brain stimulation, can reduce symptoms but do not stop disease progression or restore lost neurons.
For decades, researchers have explored whether new cells could be introduced into the brain to replace those lost to Parkinson’s disease. A recently published study in Nature (April 2025) reports the results of a first-in-human trial of a stem-cell-based therapy designed to address this challenge. The trial evaluated safety and early outcomes in a small group of patients and provides important data for the future of regenerative treatments
Study overview
The therapy tested in this trial is called bemdaneprocel. It is produced by converting human embryonic stem cells into dopaminergic neuron progenitors—the type of brain cells that normally produce dopamine. The goal is to replace the lost neurons and restore dopamine activity in the affected brain regions.
Participants and method
- The study enrolled 12 patients with moderate to advanced Parkinson’s disease.
- Each participant underwent surgery to receive bilateral transplants of the dopaminergic neuron progenitors into the putamen, a brain structure involved in motor control.
- Two dose levels were tested: a lower dose of 0.9 million cells per putamen and a higher dose of 2.7 million cells per putamen.
- To reduce the risk of rejection, patients were placed on immunosuppressive medication for one year.
- Participants were followed for 18 months to assess safety, cell survival, and potential clinical benefit.
Key findings
Safety
The primary objective of this Phase I trial was to evaluate safety. The results showed that the procedure and the transplanted cells were generally well tolerated. There were no serious adverse events directly related to the cell product. Importantly, there were no cases of tumor formation, which has been a theoretical concern with stem-cell-based therapies. In addition, no patients developed graft-induced dyskinesia, a complication observed in earlier transplantation studies using fetal tissue.
Evidence of cell survival and function
To assess whether the transplanted neurons survived and produced dopamine, the researchers used 18F-DOPA positron emission tomography (PET) imaging. At 18 months after transplantation, patients showed increased dopaminergic activity in the regions where the cells had been implanted. This indicates that the transplanted cells were not only alive but also functioning in a way that could restore dopamine signaling.
Clinical outcomes
Although the trial was not designed to test efficacy, the researchers collected clinical data using established measures of Parkinson’s symptoms. In the high-dose group, patients showed an average improvement of around 23 points on the MDS-UPDRS Part III OFF score, which assesses motor symptoms when patients are not taking medication. This improvement suggests a potential clinical benefit, although results varied among individuals and the study was not powered to demonstrate effectiveness conclusively.
Interpretation of results
The findings of this study are significant for several reasons. First, they provide evidence that human embryonic stem-cell-derived dopaminergic progenitors can be manufactured at scale, delivered to patients, and survive in the human brain for more than a year. Second, the increase in dopamine activity shown on PET scans suggests that the transplanted cells are performing their intended function. Third, the motor improvements observed in some patients, particularly in the high-dose group, support the potential for meaningful clinical benefit.
At the same time, caution is necessary. The study involved a small number of patients, lacked a control group, and was primarily designed to evaluate safety. Longer-term studies with larger cohorts will be essential to confirm whether the therapy is effective, durable, and safe over many years.
Broader context
The use of cell transplantation in Parkinson’s disease has been investigated for decades. Early studies using fetal tissue showed that transplanted neurons could survive and release dopamine, but results were inconsistent, and complications such as dyskinesia limited the approach. Ethical and practical challenges in sourcing fetal tissue also prevented widespread adoption.
Stem-cell-derived therapies such as bemdaneprocel address many of these limitations. Stem cells can be expanded in the laboratory, providing a consistent and renewable source of cells. They can also be carefully differentiated into specific neuron types, improving the precision of the therapy. Several groups worldwide are now investigating both embryonic stem-cell and induced pluripotent stem-cell approaches. The results of this trial represent one of the most advanced demonstrations of the technology in patients.
Next steps
This Phase I trial establishes a foundation for future research. The logical next step is a Phase II study, designed to evaluate efficacy in a larger and more controlled setting. Such a trial would include a placebo or sham-surgery group to determine whether the observed motor improvements are due to the therapy itself rather than other factors.
Key questions remain:
- What is the optimal cell dose for sustained benefit?
- Will patients require long-term immunosuppression, or can the cells integrate permanently without it?
- How durable are the benefits, and will they continue to improve over time?
- Can the therapy be scaled to meet the needs of large patient populations if proven effective?
Answering these questions will determine whether stem-cell-based treatments become a standard option for Parkinson’s disease in the future.
Conclusion
The Phase I trial of bemdaneprocel provides important new evidence that stem-cell-derived dopaminergic progenitors can be safely transplanted into patients with Parkinson’s disease. The transplanted cells survived, produced dopamine, and were associated with signs of motor improvement in some participants. While further research is needed, these results represent a key step in the development of regenerative therapies for Parkinson’s disease and demonstrate the feasibility of this approach in a clinical setting.
The findings do not yet change clinical practice, but they mark progress toward a long-standing goal: developing treatments that address the underlying loss of neurons in Parkinson’s disease rather than only managing symptoms.
Disclaimer: The information shared here should not be taken as medical advice. The opinions presented here are not intended to treat any health conditions. For your specific medical problem, consult with your healthcare provider.
