Recent Study Raises Questions About Using Adult Stem Cells for Chronic Heart Disease
A recent clinical study (POSEIDON Randomized Trial) investigated the effects of transplanting bone marrow derived adult stem cells into patients with known heart disease. The results were presented at the 2012 American Heart Association (AHA) meeting in Los Angeles and also published in the article "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 article by Dr. Joshua Hare and colleagues appeared in the online edition of the Journal of the American Medical Association on November 6, 2012.
The primary goal of the study was to compare whether adult stem cells from other donors (allogeneic cells) are just as safe as the stem cells derived from the patients' own bone marrow (autologous cells). Thirty patients with a prior heart attack and reduced cardiac function received either allogeneic or autologous cells. The injected cells were mesenchymal stem cells (MSCs), an adult stem cell type that resides within the bone marrow and primarily gives rise to bone, fat or cartilage tissue. MSCs are quite distinct from hematopoietic stem cells (HSCs) which are also present in the bone marrow but give rise to blood cells. In the POSEIDON study, patients underwent a cardiac catheterization and the MSCs were directly injected into the heart muscle. Various measurements of safety and cardiac function were performed before and up to one year after the cell injection.
The good news is that in terms of safety, there was no significant difference when either autologous or allogeneic MSCs were used. Within the first month after the cell injection, only one patient in each group was hospitalized for what may have been a major treatment related side effect. In the long-run, the number of adverse events was very similar in both groups. The implication of this finding is potentially significant. It suggests that one can use off-the-shelf adult stem cells from a healthy donor to treat a patient with heart disease. This is much more practical than having to isolate the bone marrow from a patient and wait for 4-8 weeks to expand his or her own bone marrow stem cells.
The disappointing news from this study is that one year following the stem cell injection, there was minimal improvement in the cardiac function of the patients. The ejection fraction of the heart is an indicator of how well the heart contracts and the normal range for healthy patients is roughly 55-60%. In the current study, patients who received allogeneic cells started out with an average ejection fraction of 27.9% and the value increased to 29.5% one year after the cell injection. The patients who received autologous cells had a mean ejection fraction of 26.2% prior to the cell transplantation and a mean ejection fraction of 28.5% one year after the stem cell therapy. In both groups, the improvement was minimal and not statistically significant. A different measure of the functional capacity of the heart is the assessment of the peak oxygen consumption. This measurement correlates well with the survival of a patient and is also used to help decide if a patient needs a heart transplant. There was no significant change in the peak oxygen consumption in either of the two groups of patients, one year after the treatment. Some other measures did indicate a minor improvement, such as the reduction of the heart attack scar size in both patient groups but this was apparently not enough to improve the ejection fraction or oxygen consumption.
One of the key issues in interpreting the results is the fact that there was no placebo control group. The enrollment in a research study and the cell injection procedure itself could have contributed to minor non-specific or placebo benefits that were unrelated to the stem cell treatments. One odd finding was that the patient sub-group which showed a statistically significant improvement in ejection fraction was the group which received the least stem cells. If the observed minor benefits were indeed the result of the injected cells turning into cardiac cells, one would expect that more cells would lead to greater functional improvement. The efficacy of the lowest number of cells points to non-specific effects from the cell injection or to an unknown mechanism by which the injected cells activate cardiac repair without necessarily becoming cardiac cells themselves.
The results of this study highlight some key problems with current attempts to use adult stem cells in cardiovascular patients. Many studies have shown that adult stem cells have a very limited differentiation potential and that they do not really turn into beating, functional heart cells. Especially in patients with established, long-standing heart disease, the utility of adult stem cells may be very limited. The damage that the heart of these patients has suffered is probably so severe that they need stem cells which can truly regenerate the heart. Examples of such regenerative stem cells are embryonic stem cells or induced pluripotent stem cells which have a very broad differentiation potential. Cardiac stem cells, which exist in very low numbers within the heart itself, are also able to become functional heart cells. Each of these three cell types is challenging to use in patients, which is why many current studies have resorted to using the more convenient adult bone marrow stem cells.
Human embryonic stem cells can develop into functional heart cells, but there have been numerous ethical and regulatory concerns about using them. Induced pluripotent stem cells (iPSCs) appear to have the capacity to become functional heart cells, similar to what has been observed for human embryonic stem cells. However, iPSCs were only discovered six years ago and we still have a lot to learn more about how they work. Lastly, cardiac stem cells are very promising but isolating them from the heart requires an additional biopsy procedure which can also carry some risks for the patients. Hopefully, the fact that adult bone marrow stem cells showed only minimal benefits in the POSEIDON study will encourage researchers to use these alternate stem cells (even if they are challenging to use) instead of adult bone marrow stem cells for future studies in patients with chronic heart disease.
One factor that makes it difficult to interpret the POSEIDON trial is the lack of a placebo control group. This is a major problem for many stem cell studies, because it is not easy to ethically justify a placebo group for invasive procedures such as a stem cell implantation. The placebo patients would also have to receive a cardiac catheterization and injections into the heart tissue, but instead of stem cells, the injections would just contain a cell-free liquid solution. Scientifically, such a placebo control group is necessary to determine whether the stem cells are effective, but this scientific need has to be weighed against the ethics of a “placebo” heart catheterization. Even if one were to ethically justify a “placebo” heart catheterization, it may not be easy to recruit volunteer patients for the study if they knew that they had a significant chance of receiving "empty" injections into their heart muscle.
There is one ongoing study which is very similar in design to the POSEIDON trial and it does contain a placebo group: The TAC-HFT trial. The results of this trial are not yet available, but they may have a major impact on whether or not bone marrow stem cells have a clinical future. If the TAC-HFT trial shows that the bone marrow stem cell treatment for patients with chronic heart disease has no benefits or only minor benefits when compared to the placebo group, it will become increasingly difficult to justify the use of these cells in heart patients.
In summary, the POSEIDON trial has shown that treating chronic heart disease patients with bone marrow derived stem cells is not yet ready for prime time. Bone marrow cells from strangers may be just as safe as one’s own cells, but if bone marrow stem cells are not very effective for treating chronic heart disease, than it may just be a moot point.
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