Cell-based therapy for Parkinson’s disease: past, present and future.
Background: Cell Replacement for Parkinson’s Disease
Parkinson’s disease (PD) is a chronic neurodegenerative condition that affects dopaminergic neurons in the midbrain. It typically appears around age 50, with mainly motor symptoms: tremor (shaking), slowness of movement, rigidity, and postural instability. Drugs like L-Dopa and MAO-B inhibitors can relieve symptoms. But the ultimate treatment goal is replacing the degenerating dopaminergic neurons with new, healthy ones.
Early Clinical Trials: Fetal Tissue Transplants
Cell replacement therapy for PD dates back to the mid-1980s, when adrenal medullary tissue was transplanted into patients’ striatum [1-3]. The results showed only moderate improvement. Around the same time, researchers in Sweden transplanted fetal ventral mesencephalic tissue from aborted fetuses [4, 5]. These early studies saw meaningful, lasting improvement across several clinical measures. Postmortem examination of these patients’ brains later confirmed that the graft survived and re-innervated the striatum [6].
In 1993, President Clinton lifted the federal funding ban on using fetal tissue for research and therapy. This let the United States begin its own clinical trials using fetal ventral mesencephalic tissue [7, 8]. Unfortunately, patients in these trials showed no significant improvement. Worse, they developed new abnormal, involuntary movements (called graft-induced dyskinesia) as a result of the surgery — a problem also seen in other trials.
Lessons from the Fetal Tissue Trials
A closer look at these transplantation studies revealed three key lessons:
- Younger patients with newly developed disease improved more than older patients with severe PD.
- Some patients kept improving 3-4 years after surgery, even though they saw no benefit in the first year. This shows that clinical improvement can take time to appear, and that patients respond differently to dopaminergic neuron transplants.
- How the fetal tissue was prepared, and how patients were selected for transplantation, varied a lot between the centers running these trials — highlighting the need to standardize tissue preparation, patient selection, and implant site.
The biggest problem with fetal ventral mesencephalic tissue grafts is standardization. It has been hard to standardize the number and quality of fetal dopaminergic cells in graft preparations, and purity varies from batch to batch. On top of that, many ethical — and sometimes legal — concerns limit how widely fetal tissue can be used in the clinic.
Stem Cells as an Alternative Source
Is there an alternative source free of these problems? The answer is yes, but not yet in practice. Human embryonic stem cells (hESCs) were isolated in 1998, and human induced pluripotent stem cells (iPSCs) were introduced in 2007. Since then, stem cell-derived dopaminergic neurons have become the leading candidate for replacing degenerating neurons in PD. hESCs have been the main source of these neurons so far [9-11]. As iPSCs gain popularity, though, future transplantation efforts are likely to shift toward iPSC-derived dopaminergic neurons.
Studies using stem cell-derived dopaminergic neurons in animal models of PD have reported promising results over the years. Still, clinical trials in humans remain a distant goal. Several questions need answers first: how long the transplanted cells remain stable, whether functional recovery is sustained, whether the cells can re-innervate the host striatum, how to generate GMP-grade cells, and — especially — the long-term safety risk of tumor formation. Answering these questions is critical for moving stem cell-derived dopaminergic neurons into the clinic. Even so, the field of regenerative medicine is advancing quickly toward that goal. It’s also worth noting that other therapeutic approaches — like gene therapy and growth factor infusions — are being developed alongside cell transplantation to restore dopaminergic function in PD patients.
Looking Ahead
Stem cell-derived dopaminergic neurons hold real promise for the future of cell-based PD therapy. But there’s a lot to learn from the early clinical trials that used fetal ventral mesencephalic tissue. Fetal dopamine neurons will likely remain the benchmark for future stem cell-derived neuron trials, since we know these transplants survived, re-innervated the striatum, and provided meaningful symptom relief in some patients for more than a decade after surgery. For PD patients considering cell-based therapy today, deciding whether to wait for stem cell-derived neuron trials or proceed with currently available fetal tissue grafts is a difficult choice — one that should weigh the strengths and weaknesses of each approach described above.
References:
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