Approach 1 of 6
Replace or repair what's lost
Can we put back lost cells or help the brain regrow?
49 steps since 1968. 17 moved us forward. Furthest along. Japan gave a time-limited approval in 2026, based on a trial of 7 people. A final-stage trial, exPDite-2, is due to report in Mar 2027. It replaces lost cells but does not stop the disease. As of Oct 2026
Facts on this page checked 18–22 Oct 2026
Each dot is one step in this approach: a discovery, a new tool or a trial, by the year it started.
- 17 moved us forward
- 7 waiting for results
- 13 still open
- 2 overtaken by better ideas
- 10 dead ends
Where it stands
- How far it has got
- Step 5 of 5 · Approved somewhere
- Has it slowed Parkinson's?
- Not yet shown
- Tried and did not work
- 10 trials
- Next result
- Mar 2027ASPIRO · Phase 2
The idea in brief
Parkinson's destroys the brain cells that make dopamine, a chemical that helps control movement. Most approaches try to slow that loss. This one asks a different question. Can we put back what was lost, or repair the cells that are left?
There are three routes. In cell replacement, new dopamine cells grown in a lab are placed in the brain by surgery. This is called a graft. Growth factors are proteins meant to keep damaged cells alive. In gene therapy, a harmless virus carries a gene into brain cells so they make a helpful protein.
Cell replacement is furthest along. Early grafts used tissue from donated fetuses, and they did not work well enough. Today's grafts use stem cells grown in a lab. In 2026, Japan gave a time-limited approval to one of them, Amchepry (Amchepry approval). It was based on a small trial of 7 people with no comparison group (Kyoto iPS trial). The maker must now prove that it helps.
How we got here
- Rat grafts · 1968
- GDNF discovered · 1993
- Skin cells to stem cells · 2006
- Dopamine cell recipe · 2011
- Graft movements explained · 2010
What has been tried
Each row is one trial in people. Its dot shows how it turned out, using the same key as the chart above.
Results already in
23 trials
Grouped by the line of research they belong to. Open a group to see its trials.
New cells10 finished trials4 did not work3 mixed or early2 passed a stage1 replaced
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First fetal grafts · grafts of dopamine-making cells from donated fetal tissue
Replaced by a newer approach
In 1987 doctors in Sweden grafted fetal cells into one patient's brain. The cells survived and made dopamine. Later controlled trials did not show a clear gain.
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Freed fetal grafts · grafts of dopamine-making cells from donated fetal tissue
Did not work
In 40 people, grafts did not beat sham surgery on the main measure. People aged 60 or under did better on one score. 15% had unwanted movements.
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Olanow fetal grafts · grafts of dopamine-making cells from donated fetal tissue
Did not work
In 34 people, grafts did not clearly beat a placebo operation at 24 months. 56% of grafted people had unwanted movements that continued off medication.
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Spheramine · retinal cells that make levodopa, placed in the brain
Did not work
In 71 people, the implants did no better than a fake operation. Movement scores changed by about the same amount in both groups.
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First own-cell graft · nerve cells made from a patient's own skin cells
Mixed or early results
One man had a graft made from his own skin cells, without immune-suppressing drugs. Scans suggested the cells survived. One person cannot show benefit.
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Kyoto iPS trial · dopamine cell precursors made from donor stem cells (iPS cells)
Passed this stage
In 7 people, with no comparison group, no serious side effects were seen at 24 months. Dopamine-making rose 44.7% on scans. All took drugs to stop rejection.
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Bemdaneprocel Phase 1 · dopamine nerve cells grown from embryonic stem cells
Passed this stage
In 12 people, with no comparison group, the cells were safe and made dopamine. High-dose motor scores improved 23 points. All took immune-suppressing drugs for a year.
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TRANSEURO · grafts of dopamine-making cells from donated fetal tissue
Did not work
Only 11 people could be grafted because fetal tissue was scarce. After 3 years there was no overall gain compared with 16 people who had no graft.
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STEM-PD · dopamine cell precursors made from embryonic stem cells
Mixed or early results
Eight people took part, with no comparison group. One died of a fungal infection while on immune-suppressing drugs. No serious events were blamed on the cells.
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NouvNeu001 · dopamine cell precursors made from donor stem cells with chemicals
Mixed or early results
Several open-label trials in China and the US. No peer-reviewed results exist. Early claims of benefit come only from company releases.
Growth factors6 finished trials3 did not work2 mixed or early1 replaced
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GM1 ganglioside · a fat-like molecule found in nerve cell membranes
Mixed or early results
A 77-person trial from one centre hinted at slower progression. It was small and has not been repeated. Part of the benefit may have been on symptoms.
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CERE-120 · a gene therapy that makes the brain produce neurturin, a growth factor
Did not work
Two trials, in 58 and 51 people, did no better than sham surgery. Autopsies of two people suggested too little of the target area was reached.
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PDGF-BB · a growth-factor protein given into the brain's fluid spaces
Mixed or early results
A safety test in 12 people for 12 days. It was well tolerated and symptom scores did not change. It was too small to show any slowing.
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GDNF infusions · a growth-factor protein pumped into the brain
Did not work
Two controlled trials, in 34 and 35 people, did not improve symptoms. One showed more dopamine activity on scans, but patients were not better.
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CDNF · a growth-factor protein infused into the brain
Replaced by a newer approach
Safe enough in 17 people, but symptoms did not improve significantly. The company has moved to a copy of the protein given by injection under the skin.
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Fosgonimeton · an injected drug meant to boost a growth-factor pathway
Did not work
Stopped early at 28 of a planned 75 people, who had dementia linked to Parkinson's. It missed its main target. Too small to close the idea.
Gene therapy7 finished trials3 did not work2 mixed or early2 passed a stage
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AAV-AADC · a gene therapy that helps the brain turn levodopa into dopamine
Passed this stage
Two small trials, in 10 and 15 people, had no comparison group. Dopamine-making rose and symptoms eased. It treats symptoms and does not slow the disease.
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VY-AADC · a gene therapy that helps the brain make dopamine
Did not work
A trial with sham surgery was put on hold in 2020 after MRI changes. The maker ended the Parkinson's work in 2021. No result on benefit was found.
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Focused ultrasound · sound waves that briefly open the barrier between blood and brain
Mixed or early results
Small safety studies showed the blood-brain barrier can be opened safely in a chosen spot. None was designed to show benefit, and none had a placebo group.
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AAV2-GAD · a gene therapy that calms an overactive movement area of the brain
Passed this stage
In 45 people, motor scores fell 8.1 points with gene therapy and 4.7 with sham surgery. It treats symptoms. A later 14-person study is a company report.
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ProSavin · a gene therapy giving brain cells three dopamine-making genes
Did not work
Safe in 15 people in 2014, with no comparison group. Its successor stopped in 2022 when the sponsor collapsed, with 6 people treated.
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CAP-003 · a gene therapy for GBA gene Parkinson's, given by vein
Did not work
Withdrawn before anyone was treated, after a child died in a trial of another drug with the same virus shell. It was not a test of the gene itself.
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VGN-R08b · a gene therapy giving a working GBA1 gene, injected into the brain's fluid spaces
Mixed or early results
Planned for 17 people with a GBA1 gene change. The estimated start date has passed with no report that anyone was treated. Safety comes first.
Still to come
7 trials · dates are expected, not promised
The next result is due in Mar 2027, from ASPIRO and exPDite-2. The largest trial still to report, REGENERATE-PD, is expected in Aug 2028.
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ASPIRO · dopamine cell precursors made from each person's own skin cells
Result due Mar 2027
It will tell us whether grafts stay safe at one year. The first 8 people had no comparison group, so gains cannot be told from a placebo effect.
Who: Adults aged 40 to 75 with Parkinson's. Enrolment is by invitation. · Last checked 19 Oct 2026
Details and sources: ASPIRO Check if it is recruiting: ASPIRO
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exPDite-2 · bemdaneprocel, dopamine nerve cells grown from embryonic stem cells
Result due Mar 2027
The first sham-controlled test of these cells, in about 102 people. It will tell us whether they raise good on-time more than a fake operation at 78 weeks.
Who: Adults with Parkinson's. The trial is recruiting. · Last checked 18 Oct 2026
Details and sources: exPDite-2 Check if it is recruiting: exPDite-2
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REPLACE (Kenai) · ready-made dopamine cell precursors from donor stem cells
Result due Sep 2027
It will tell us whether this frozen donor-cell product is safe over 15 months. It has no control group, and no results exist yet.
Who: People aged 45 to 75 with Parkinson's that has no known gene cause. Enrolment is complete. · Last checked 19 Oct 2026
Details and sources: REPLACE (Kenai) Check if it is recruiting: REPLACE (Kenai)
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Mass General own-cell trial · dopamine cell precursors made from each person's own skin cells
Result due Dec 2027
It will tell us whether repeating the 2020 one-person graft is safe in up to 8 people over 2 years. It has no control group.
Who: People aged 45 to 80, at least 5 years from first symptoms, with at least 3 hours of daily off time. · Last checked 19 Oct 2026
Details and sources: Mass General own-cell trial Check if it is recruiting: Mass General own-cell trial
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UC San Diego trial · dopamine cell precursors made from donor stem cells, the same cells as Kyoto
Result due Dec 2027
It will tell us whether Kyoto's donor cells are safe in the US and keep a stable size on MRI. It has no control group.
Who: Adults aged 40 to 75 who have had Parkinson's for at least 5 years and respond well to levodopa. · Last checked 19 Oct 2026
Details and sources: UC San Diego trial Check if it is recruiting: UC San Diego trial
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REGENERATE-PD · AB-1005, a gene therapy that makes the brain produce GDNF
Result due Aug 2028
It will tell us whether this gene therapy gives more good on-time than sham surgery at 18 months. GDNF infusions improved scans but not patients.
Who: People aged 45 to 75 with moderate Parkinson's. Enrolment is closed. · Last checked 18 Oct 2026
Details and sources: REGENERATE-PD Check if it is recruiting: REGENERATE-PD
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Brigham own-cell trial · dopamine nerve cells made from each person's own blood cells
Result due Dec 2028
It will tell us whether grafts are safe at 12 and 18 months. With no control group, it cannot show benefit.
Who: Adults aged 55 to 80 who have had Parkinson's for at least 5 years, respond well to levodopa and have had no brain surgery. Enrolment is by invitation. · Last checked 19 Oct 2026
Details and sources: Brigham own-cell trial Check if it is recruiting: Brigham own-cell trial
In words
Of the 30 trials in people, 10 did not work, 4 passed their first stage, 7 had mixed or early results, 2 were replaced by newer approaches, and 7 have results due. The first is due in Mar 2027; the largest, REGENERATE-PD, in Aug 2028.
What the dead ends taught us
- Better scans did not mean better patients. GDNF, a growth factor, was pumped into the brain. Scans showed more dopamine activity, but people did not get better. This is the warning for a GDNF gene therapy now being tested. (GDNF infusions; REGENERATE-PD)
- A fake operation can make people feel better, so trials need one. In some trials the people given sham surgery improved too. Without that comparison, a gain can be wrongly credited to the treatment. (CERE-120; AAV2-GAD)
- Fetal grafts caused problems and could not be scaled up. Some people developed uncontrolled movements, called dyskinesias, that continued off medication. Fetal tissue was also scarce: only 11 people could be grafted in the last big trial. Scientists turned to stem cells. (Freed fetal grafts; Olanow fetal grafts; TRANSEURO)
- Gene therapy can be stopped by safety checks or by the company behind it. One trial was put on hold after MRI changes. One ended when its sponsor collapsed. One closed after a child died in a trial of a different drug with the same virus shell. (VY-AADC; ProSavin; CAP-003)
Most drugs tested against any brain disease do not work. Each one that fails rules something out and shapes the next trial.
What it would mean for you
If cell replacement works, it would replace some lost cells. It would not stop the disease. Grafts can catch Parkinson's. Years later, some grafted cells in a few people showed signs of the disease when checked after death. Other grafts did not (Lewy bodies in grafts). The early cell trials show that the cells are safe enough to test further. They do not yet show that the cells work.
Cell grafts and gene therapy both need brain surgery. Donor cells usually need drugs that damp down the immune system, so the body does not reject them. These drugs carry real risk. In STEM-PD, one person died of a fungal infection while taking them. Some trials use a person's own cells instead, to avoid this. They are still at an early stage.
The biggest controlled trial of cells, exPDite-2, is expected to finish in Mar 2027. The GDNF gene therapy trial, REGENERATE-PD, is expected in Aug 2028. These dates are expected, not promised. Trials often run late. Even with a good result, approval would take further time.
Should I try this now?
No. Approved access is in Japan only, and it is limited and time-limited. Everywhere else, these cells can be had only inside a trial.
Be very careful with clinics that sell stem-cell treatments. These have not been tested in the trials described here. They may cost a great deal and may be unsafe. Talk to your neurologist or Parkinson's nurse before you pay for any treatment or travel for one.
Can I take part?
Trials of cells mostly look for adults who have had Parkinson's for several years and who still respond well to levodopa. Some ask for people with a particular gene change (GBA). Some enrol by invitation only, and some are already full.
Taking part usually means brain surgery. In some trials, some people get a sham operation instead. Each trial sets its own rules.
Related approaches
Sources, and the same lanes for researchers
This page is built from 51 research landmarks. Each has its own sources; follow "Details and sources" on any trial. Facts were checked 18–22 Oct 2026. The summary was written on 22 Oct 2026.
- Cell replacement (26 landmarks)
- Growth factors (11 landmarks)
- Gene therapy & delivery (14 landmarks)
- See these lanes on the full map
How this project checks its facts · Report an error on this page