Stem cell clinical trials have expanded rapidly over the past decade, spanning conditions from blood cancers and autoimmune diseases to Parkinson’s, type 1 diabetes, and spinal cord injury. As of late 2024, at least 115 clinical trials using human pluripotent stem cell products had received regulatory approval worldwide, testing 83 distinct products and dosing more than 1,200 patients with no broad safety red flags so far.1Cell Stem Cell. Pluripotent stem-cell-derived therapies in clinical trial: A 2025 update That figure only captures one category of stem cell therapy. Counting all cell types, including mesenchymal and hematopoietic stem cells, the clinical trial landscape is far larger and the results more varied than headlines suggest.
Where Trials Have the Longest Track Record
Hematopoietic stem cell transplantation, often called bone marrow transplant, remains the most mature form of stem cell therapy. It has been used for decades to treat blood cancers, immune deficiencies, and certain genetic blood disorders. A Children’s Oncology Group report on high-risk acute myeloid leukemia found that transplant roughly doubled five-year disease-free survival compared with chemotherapy alone, from about 26% to nearly 50% in one subgroup and from about 17% to 51% in another.2PubMed Central. Hematopoietic Stem Cell Transplantation Outcomes for High-Risk AML: A Report From the Children’s Oncology Group These are established procedures, not experimental ones, and they form the foundation on which newer stem cell therapies are being built.
Another area with accumulating clinical data is autologous hematopoietic stem cell transplant for treatment-resistant multiple sclerosis. In a prospective trial of 35 patients who had failed a median of four disease-modifying therapies, event-free survival at three years was 60%, and nearly half the patients showed sustained improvement in disability scores. There was no treatment-related mortality.3Journal of Neurology, Neurosurgery & Psychiatry. Prospective phase II clinical trial of autologous haematopoietic stem cell transplant for treatment refractory multiple sclerosis The procedure essentially reboots the immune system, and the trial documented a sustained rise in regulatory immune cells that help keep autoimmune attacks in check.
Parkinson’s Disease and the Push Toward Cell Replacement
Some of the most closely watched trials aim to replace lost brain cells, not just modulate the immune system. In Parkinson’s disease, the core problem is the death of dopamine-producing neurons. Three separate early-phase trials, running simultaneously on three continents, are now testing whether transplanting stem cell-derived dopamine-producing cells into the brain can restore some of that lost function.
A multicenter phase 1/2 trial reported on STEM-PD, a product derived from human embryonic stem cells. The trial reached its 12-month safety endpoint and reported interim efficacy data, offering the first structured look at how these off-the-shelf dopaminergic progenitors behave in a clinical setting.4Nature Medicine. Human embryonic stem cell-derived dopaminergic cells for Parkinson’s disease: a phase 1/2 open-label trial A separate trial in China transplanted embryonic stem cell-derived dopaminergic progenitors into 12 patients at two dose levels. No dose-limiting toxicities appeared, and motor scores improved at 12 months, with the high-dose group showing greater gains. Brain imaging confirmed increased dopamine transporter uptake in the transplant region, suggesting the grafted cells survived and were producing dopamine.5Cell. Clinical trial of human embryonic stem cell-derived dopaminergic progenitors for Parkinson’s disease
Meanwhile, a phase 1/2 trial at Kyoto University used induced pluripotent stem (iPS) cells rather than embryonic stem cells. Seven patients received transplants of iPS-cell-derived dopaminergic progenitors. The cells survived, produced dopamine, and did not form tumors.6PubMed Central. Phase I/II trial of iPS-cell-derived dopaminergic cells for Parkinson’s disease These are all small, early trials. None proves that stem cell transplants will become a standard Parkinson’s treatment. But the convergence of three independent groups showing safety and biological signals of graft survival is a meaningful step.
Type 1 Diabetes and Insulin Independence
Perhaps the most dramatic recent result from a stem cell trial involves type 1 diabetes. In a trial of a product called zimislecel, stem cell-derived islet cells were infused into patients whose own insulin-producing cells had been destroyed by autoimmune attack. All 12 participants in the main study groups showed engraftment and islet function, confirmed by detectable C-peptide, a marker of insulin production. Ten of the 12 patients, roughly 83%, achieved insulin independence and were no longer using exogenous insulin at one year.7PubMed. Stem Cell-Derived, Fully Differentiated Islets for Type 1 Diabetes All participants were free of severe low-blood-sugar episodes and spent more than 70% of their time in the target glucose range.
The study is small and short-term, and the participants required immunosuppression to prevent rejection of the transplanted cells. But for people with type 1 diabetes who live with constant glucose monitoring and insulin dosing, replacing the missing cells rather than supplementing the missing hormone is a fundamentally different approach. Autologous stem cell strategies, which would use a patient’s own reprogrammed cells and potentially avoid immunosuppression, are also being explored.8PubMed Central. First-ever stem cell therapy restores insulin independence in type 1 diabetes: A medical milestone
Knee Osteoarthritis Trials Have Progressed Through Phase III
Orthopedic applications are among the most commercially hyped areas for stem cells, but rigorous trial data is actually accumulating. Early proof-of-concept work showed that injecting a high dose of adipose-derived mesenchymal stem cells (MSCs) into arthritic knees reduced pain, improved function scores, and shrank cartilage defects visible on imaging and arthroscopy, with histology showing thick, hyaline-like cartilage regeneration.9STEM CELLS. Intra‐Articular Injection of Mesenchymal Stem Cells for the Treatment of Osteoarthritis of the Knee: A Proof‐of‐Concept Clinical Trial A phase IIb randomized, placebo-controlled trial followed, showing a 55% mean reduction in the standard knee-pain score at six months in the MSC group compared with no significant improvement in controls. Cartilage defects held stable in the MSC group while they grew significantly in the control group.10STEM CELLS Translational Medicine. Intra-Articular Injection of Autologous Adipose Tissue-Derived Mesenchymal Stem Cells for the Treatment of Knee Osteoarthritis: A Phase IIb, Randomized, Placebo-Controlled Clinical Trial
A phase III, randomized, double-blind, placebo-controlled trial then tested the same approach in a larger group. The MSC group showed significantly better pain and function scores at six months, and a higher proportion of patients crossed the threshold for a clinically meaningful improvement. No serious treatment-related adverse events occurred. However, imaging at six months did not show a significant difference in cartilage defect size between the treatment and control groups, a disconnect from the earlier trials that tempers expectations about structural repair.11The American Journal of Sports Medicine. Clinical Efficacy and Safety of the Intra-articular Injection of Autologous Adipose-Derived Mesenchymal Stem Cells for Knee Osteoarthritis: A Phase III, Randomized, Double-Blind, Placebo-Controlled Trial
Spinal Cord Injury
Cell therapies for spinal cord injury face perhaps the steepest biological challenge in regenerative medicine. A review in The Lancet Neurology concluded that while clinical trials have established the feasibility and long-term safety of transplanting cells into the injured spinal cord, no trial had yet provided reproducible evidence of clinical efficacy.12The Lancet Neurology. Cell-based therapies for spinal cord injury — past, present, and future That landscape is beginning to shift. A phase 1 trial of iPS-cell-derived neural progenitor cells, transplanted into patients with subacute spinal cord injuries, reported a median motor score improvement of 13 points over 52 weeks. Two of four participants improved from the most severe classification (complete loss of motor and sensory function below the injury) to partial recovery of voluntary movement.13Nature Medicine. An iPSC-derived neural progenitor cell therapy for subacute spinal cord injury: a phase 1 trial with long-term follow-up Those gains exceeded the spontaneous recovery seen in a registry-based comparison group, though the trial was too small to draw firm conclusions.
Gene-Edited Stem Cells for Sickle Cell Disease
One of the fastest-moving areas combines stem cell therapy with gene editing. In sickle cell disease, patients’ own blood-forming stem cells are removed, edited using CRISPR to reactivate fetal hemoglobin production, and then transplanted back after chemotherapy clears the bone marrow. Early results published in the New England Journal of Medicine showed that edited cells engrafted, fetal hemoglobin levels rose substantially, and patients became transfusion-independent, with vaso-occlusive crises eliminated in the patient with sickle cell disease.14PubMed. CRISPR-Cas9 Gene Editing for Sickle Cell Disease and β-Thalassemia
Subsequent and larger datasets have confirmed the pattern. A study of three patients receiving a different CRISPR-edited product showed stable fetal hemoglobin induction (19% to 27% of total hemoglobin) with fetal hemoglobin broadly distributed across red cells, and reduced sickle cell symptoms during follow-up.15PubMed Central. CRISPR-Cas9 Editing of the HBG1 and HBG2 Promoters to Treat Sickle Cell Disease A more recent trial using a next-generation CRISPR-Cas12a approach treated 28 patients. By six months, average total hemoglobin had risen from about 9.8 to 13.8 grams per deciliter, and fetal hemoglobin jumped from roughly 2.5% to over 48% of total hemoglobin.16PubMed. CRISPR-Cas12a Gene Editing of HBG1 and HBG2 Promoters to Treat Sickle Cell Disease Gene-edited stem cell therapy for sickle cell disease has already received regulatory approval in some markets, making it among the first CRISPR-based therapies to reach patients outside of trials.
Where Trials Have Struggled
Not every application has panned out as hoped. Heart failure trials illustrate a persistent challenge. A systematic review and meta-analysis of stem cell therapy for heart failure found that mesenchymal stem cells failed to produce a significant improvement in heart pumping function, likely because of very low cellular retention: preclinical work showed that after injection into heart tissue, almost no cells remained after 24 hours.17PubMed Central. Cardiac Repair and Clinical Outcomes of Stem Cell Therapy in Heart Failure: A Systematic Review and Meta-Analysis The cells may exert short-lived anti-inflammatory and anti-fibrotic effects through the molecules they secrete, but they do not stick around long enough to rebuild tissue in a meaningful way. Researchers have called for better delivery methods, such as injectable scaffolds or hydrogels, that could keep transplanted cells alive at the target site for longer.18Laboratory Investigation. Stem cell therapies for myocardial infarction in clinical trials: bioengineering and biomaterial aspects
Acute respiratory distress syndrome is another area where safety has been confirmed but efficacy remains elusive. Phase 1 and phase 2 trials of MSCs for ARDS found no serious infusion-related toxicity, even at high doses.19The Lancet Respiratory Medicine. Mesenchymal stem cells for acute respiratory distress syndrome: a phase 1 safety trial But a phase 2 trial showed no mortality benefit, with 28-day death rates of 30% in the MSC group versus 15% in the placebo group, a difference that was not statistically significant but pointed in the wrong direction.20The Lancet Respiratory Medicine. Treatment with mesenchymal stem cells for acute respiratory distress syndrome Some trials have noted decreases in inflammatory markers and hints of benefit in subgroups, but the overall picture for ARDS mortality is unresolved.21PubMed Central. Mesenchymal stem cell therapies for ARDS: translational promise and challenges
Graft-versus-host disease, a common and sometimes deadly complication of bone marrow transplant, has been another area of mixed signals. An influential open-label study reported that 30 of 55 patients with severe acute graft-versus-host disease had a complete response to MSC infusions, and complete responders had markedly higher two-year survival than partial or nonresponders.22The Lancet. Mesenchymal stem cells for treatment of severe acute graft-versus-host disease But a large phase III trial of an industrially produced MSC product failed to meet its primary endpoint of significantly increasing complete response rates compared with placebo.23Cytotherapy. The mesenchymal stromal cells dilemma—does a negative phase III trial of random donor mesenchymal stromal cells in steroid-resistant graft-versus-host disease represent a death knell or a bump in the road? Whether the discrepancy comes from differences in cell preparation, patient selection, or the inherent variability of MSC products remains debated.
How Mesenchymal Stem Cells Actually Work
MSCs appear in nearly every therapeutic area discussed above, and understanding what they do (and do not do) clarifies why trial results are so inconsistent. MSCs do not typically engraft and become new tissue. Instead, they work largely through the molecules they release: cytokines and other signaling factors that dampen inflammation, regulate immune responses, and promote tissue repair in nearby cells.24PubMed Central. Mesenchymal Stem Cell Immunomodulation: Mechanisms and Therapeutic Potential Recent work has shown that even dead or metabolically inactive MSCs can trigger immunomodulatory responses, with regulatory T cells and certain white blood cells doing much of the downstream work. MSCs also interact with both branches of the immune system, making them attractive candidates for autoimmune conditions and transplant complications.25Stem Cell Research & Therapy. Mesenchymal stromal cell therapies: immunomodulatory properties and clinical progress
This paracrine mechanism means MSCs are more like short-lived drug factories than permanent tissue replacements. That distinction explains why conditions requiring a brief immune reset, like graft-versus-host disease, sometimes respond well, while conditions requiring sustained structural repair, like heart failure, have been harder to treat with MSC injections alone.
Immune Compatibility and Rejection
One question hanging over any therapy using donor-derived cells is whether the recipient’s immune system will attack the graft. In the POSEIDON trial, which compared donor-derived and patient-derived MSCs in people with heart disease, more than 30% of patients already had antibodies to donor tissue antigens before any MSC infusion. Most of these patients showed stable antibody levels afterward, meaning the MSC treatment itself did not trigger new immune sensitization. Only two patients in the donor-cell group developed new antibodies, and neither experienced a serious clinical consequence.26JAMA. Comparison of Allogeneic vs Autologous Bone Marrow–Derived Mesenchymal Stem Cells Delivered by Transendocardial Injection in Patients With Ischemic Cardiomyopathy: The POSEIDON Randomized Trial
The question is more nuanced for pluripotent stem cell products. In the Kyoto Parkinson’s trial, donor cells were derived from iPS cells and transplanted into seven patients. Four recipients were fully mismatched at the tested tissue-typing markers, yet the grafted cells survived in all patients without clinically significant immune reactions. The researchers noted that the transplanted neurons expressed low levels of the surface markers that usually trigger rejection, which may partly explain the tolerance. Still, T cells from mismatched recipients did react to donor immune cells in lab tests, suggesting that some degree of immunosuppression remains prudent.27Cell Stem Cell. Control of immune response in an iPSC-based allogeneic cell therapy clinical trial for Parkinson’s disease
The Problem of Unproven Clinics
Alongside the regulated clinical trial pipeline, a large and largely unregulated direct-to-consumer market sells stem cell treatments for conditions ranging from autism to erectile dysfunction, often without meaningful evidence of safety or efficacy. A comprehensive analysis of reported adverse events found that these unproven interventions can pose serious risks, including infections, tumor formation, and vision loss.28Stem Cells Translational Medicine. Concise Review: A Comprehensive Analysis of Reported Adverse Events in Patients Receiving Unproven Stem Cell-Based Interventions Most lawsuits brought by injured patients have ended in settlement rather than judicial decisions, leaving few legal precedents to deter bad actors. The FDA alone cannot adequately police this market, and public health researchers have called for broader regulatory strategies to protect patients.29PubMed. Adverse events related to unapproved stem cell products and other regenerative interventions: recommendations for more robust regulation of the direct-to-consumer marketplace
If you are considering a stem cell treatment, the clearest way to distinguish a legitimate trial from a questionable clinic is to check whether the treatment is listed on a recognized clinical trial registry and whether the provider is charging you to participate. In a properly run clinical trial, patients are not typically billed for the experimental treatment itself. A clinic charging thousands of dollars for an unproven stem cell injection is a fundamentally different operation from a regulated trial, even if the marketing language sounds similar.
Manufacturing, Delivery, and the Scale-Up Problem
Even for therapies that work in small trials, getting stem cells from a research lab to a scalable medical product is a formidable challenge. Producing clinical-grade MSCs requires adherence to good manufacturing practice standards, but the inherent variability of cell cultures makes standardization difficult. MSC preparations differ depending on the donor tissue source, the number of times cells have been divided, and the specific culture conditions used, all of which can affect potency.30PubMed Central. Current good manufacturing practice considerations for mesenchymal stromal cells as therapeutic agents The upstream and downstream processing steps, from expanding cells to harvesting, washing, and concentrating them, each introduce potential quality variation.31PubMed Central. Manufacturing human mesenchymal stem cells at clinical scale: process and regulatory challenges
Delivery remains a separate bottleneck. In bone repair trials, using scaffolds to carry MSCs to the defect site has shown bone healing in all treated patients across a systematic review, with most adverse events being mild and related to the surgery itself rather than the cells.32PubMed Central. Bone Regeneration with Mesenchymal Stem Cells in Scaffolds: Systematic Review of Human Clinical Trials But for soft-tissue and organ applications, most trials still inject cells directly into the target tissue or bloodstream and hope enough of them find and stay in the right place. The negligible retention rates seen in cardiac studies suggest that without better delivery vehicles, many trials are working against unfavorable biology.
Cost and Regulatory Pathways
The financial reality of stem cell therapy is stark. Hematopoietic stem cell transplants already carry average estimated billed charges of roughly $378,000 for autologous procedures and $930,000 for transplants using a donor’s cells, covering the hospital stay and about six months of post-transplant management. Nearly 40% of these procedures in the United States are covered by government payers.33STEM CELLS Translational Medicine. Concise Review: The High Cost of High Tech Medicine: Planning Ahead for Market Access Newer cell therapies that require gene editing, iPS cell reprogramming, or pluripotent stem cell differentiation add layers of cost on top of this baseline.
Regulatory agencies have tried to accelerate the path from lab bench to patient. The FDA’s Regenerative Medicine Advanced Therapy (RMAT) designation, introduced in 2017, provides intensive support and streamlined review for cell-based, tissue-based, and combination therapies that show preliminary evidence of addressing serious conditions.34PubMed. Expediting Drug Development: FDA’s New Regenerative Medicine Advanced Therapy Designation But the middle stage of development, after initial safety trials and before large efficacy trials, remains what researchers have bluntly called the “valley of death.” Most cell therapy products fail at this stage, often because funding runs out before efficacy can be demonstrated, diseases prove harder to treat than preclinical work suggested, or regulatory hurdles prove insurmountable for small companies.35PubMed Central. From bench to FDA to bedside: US regulatory trends for new stem cell therapies The therapies that do survive this gauntlet and reach approval will face difficult pricing and reimbursement negotiations, particularly for one-time treatments that may need to recoup years of development cost in a single administration.
Tracking Cells After They Are Transplanted
One reason stem cell trial results can be hard to interpret is that researchers often cannot see what the transplanted cells are doing once they are inside the body. Imaging-based tracking methods, including MRI, nuclear medicine techniques, and optical imaging, have been developed to follow transplanted cells over time. These tools can reveal whether cells have reached their target, how long they survive, and whether they migrate to unintended locations.36PubMed Central. Stem Cell Tracking Technologies for Neurological Regenerative Medicine Purposes Each method comes with trade-offs in image resolution, how deeply it can see into tissue, and whether the tracking label itself affects cell behavior. The Parkinson’s trials described earlier used PET imaging to confirm that transplanted dopamine-producing cells were surviving and functional, which is exactly the kind of biological readout that distinguishes a well-designed trial from one that relies solely on symptom questionnaires.37Cell. Clinical trial of human embryonic stem cell-derived dopaminergic progenitors for Parkinson’s disease As tracking technology improves, it should make future trials easier to interpret and help explain why some patients respond to cell therapy and others do not.