Stem Cells for Parkinson’s Just Took a Big Step Forward. But We Don't Have a Cure Yet
Neuroepidemiology Series Part 6: Two new clinical trials were published this month. Both look promising in terms of safety measures.
For decades, cell therapy for Parkinson’s disease has lived in an awkward limbo. It was too scientifically plausible to dismiss, but too technically fraught to trust. Early trials using fetal tissue offered flashes of hope, but the results were scattered, the side effects were serious, and the logistics were a nightmare. Eventually, enthusiasm cooled. If you knew a neurologist who followed the field, chances are they’d sigh and call it “promising, but not ready.”
That may have changed in the last few weeks.
Two early-stage clinical trials just published in Nature suggest we’ve finally crossed a threshold. One, led by a Japanese team using induced pluripotent stem (iPS) cells, and another by a U.S.-based group using embryonic stem cells, both delivered the same quiet headline: This is safe now.
No tumors. No rejection. No major complications from the surgery or the cells. That alone is a huge deal. Let’s take a closer look.
The Japanese Trial: Induced Pluripotent Cells, Homegrown Dopamine
The Kyoto University team enrolled seven patients with moderate Parkinson’s and implanted them with dopaminergic progenitors derived from iPS cells. These are essentially stem cells reprogrammed from adult human tissue.
The cells were transplanted directly into the putamen, a key part of the brain’s motor circuit, with some patients receiving higher doses than others. Immunosuppressants were given short-term to prevent rejection. Over the next two years, researchers tracked safety outcomes, motor symptoms, and brain imaging.
Here’s what they found:
No serious adverse events related to the transplant
Modest but real motor improvement: Four of six patients in the efficacy analysis improved in their “OFF” state (without medication), with a ~9.5-point drop on the MDS-UPDRS Part III scale. Five improved in the “ON” (with medication) state as well.
PET scans confirmed dopamine activity in the brain had increased, especially in the high-dose group
No tumor growth or graft-induced dyskinesias, which plagued earlier fetal-tissue efforts
Is that a cure? No. But it’s something medicine rarely delivers: plausible functional benefit with no severe red flags in early trials.
The American Trial: An Off-the-Shelf Embryonic Stem Cell Product
Meanwhile, the U.S. team (based out of Memorial Sloan Kettering and working with BlueRock Therapeutics) tested a cryopreserved, ready-to-use dopaminergic neuron product called bemdaneprocel.
Twelve patients received either a low or high dose of the graft, delivered bilaterally into the putamen. Everyone got a year of immunosuppressants. Follow-up lasted 18 months.
The results were striking:
Again, no serious adverse events from the cells or procedure (just one post-op seizure, one unrelated GI bleed)
No tumors, no dyskinesias, and again, clean imaging
In the high-dose group, the average OFF-med motor score improved by 23 points
“Good ON” time (waking hours with minimal symptoms) also increased substantially
Even in a small, open-label trial, that level of improvement raises eyebrows.
Why This Matters
For years, the story of Parkinson’s cell therapy has been one of caveats. Promising science, disappointing trials. Hype, then retreat. The early fetal tissue transplants of the 1990s showed that replacing lost dopamine neurons could work, but the field got stuck: unpredictable results, graft-induced dyskinesia, ethical headaches over sourcing, and a total lack of scalability.
Now? We have GMP-grade, quality-controlled cell lines. We have patients showing real functional improvement on both clinical scales and imaging. We have two trials in two countries, with two different technologies, and both cleared the biggest hurdle: they didn’t hurt people.
This is the part of the story where everyone wants to jump ahead. But we can’t. Not yet. These studies have their limitations
Both trials were small, as Phase I/II trails tend to be. They were also both were open-label, meaning there was no blinding/placebo group. That means we can’t separate true benefit from placebo, from patient expectation, or from confirmation bias.
Still: these results are much better than nothing. They suggest we’ve moved from “can we even do this safely?” to “can we make this actually work at scale?” That’s a big shift.
The Long View
There’s a temptation in science writing to declare a field reborn every time a good early-phase trial comes out. But that’s not how this works. These are foundational studies. They don’t prove cell therapy works for Parkinson’s, but they show it can be delivered safely, can survive in the brain, and might provide benefit.
In other words, they make legitimacy possible.
The next step is the hard part: larger, blinded, placebo-controlled trials that can really tell us whether these cells do what we want them to do; replace lost dopamine neurons, restore function, and last over time without causing harm.
Even if they don’t fully succeed, these trials have already moved the field forward. They give researchers a map. They tell funders this isn’t just a fantasy. And maybe most importantly, they show patients that progress is happening, even if it’s slower than anyone would like.
So: we’re not there yet. But we’re further than we’ve ever been. These results don’t mean stem cells are the cure for Parkinson’s. But they do suggest that, for the first time in decades, the dream isn’t dead. It’s walking. Slowly, carefully, and with support.
And in a disease defined by slowness, maybe that’s exactly the kind of progress we should expect.
Studies Referenced
Sawamoto, N., Doi, D., Nakanishi, E. et al. Phase I/II trial of iPS-cell-derived dopaminergic cells for Parkinson’s disease. Nature (2025). https://doi.org/10.1038/s41586-025-08700-0
Tabar, V., Sarva, H., Lozano, A.M. et al. Phase I trial of hES cell-derived dopaminergic neurons for Parkinson’s disease. Nature (2025). https://doi.org/10.1038/s41586-025-08845-y
Tempting to pass this read along to friend. He has early stage Parkinson's... just diagnosed this year and early 60's.