For patients with chronic degenerative diseases such as heart failure, regenerative medicine holds great promise. It is this compelling need that has created a sense of urgency, leading to a faster pace in the field of cardiovascular stem cell therapy than perhaps for any other medical indication. Every day is a game day, and for patients with severe heart failure it is truly a sudden death situation. Although international teams of cardiologists have been called to the playing field of translational stem cell therapy for heart disease, it has been difficult to gauge how close we are to the desired goal of cardiac regeneration.
From the perspective in the stands, it appears we may be both ahead and behind. Clinical trials have surged forward and enrolled hundreds of patients in a myriad of studies around the world. At the same time, the scientific underpinnings for the clinical studies have come into question, and additional in vitro and in vivo animal studies are struggling to catch up. During the past several months, the early returns of the first round of controlled clinical studies have been released, providing an opportunity to determine where we are and what we might have learned about the complexity and the challenges of cardiovascular stem cell therapy. Although our early "shot on goal" may have fallen short, we may now be in a better position to reformulate our next moves down the field.
CONTROLLED CLINICAL TRIALS: LESSONS LEARNED
Results from a handful of randomized, double-blind, placebo-controlled trials have thus far shown a range of efficacy from minimal, to transient, to no improvement in global cardiac function. Yet, a number of important biological insights are beginning to emerge from the clinical studies. Perhaps one of the most important recurring themes is the inherent challenges of achieving true cardiac muscle regeneration in the injured heart with any therapeutic approach thus far. In this regard, randomized clinical studies attempting to repopulate injured myocardium with autologous skeletal myoblasts,1 have recently been terminated owing to a lack of a sufficient therapeutic effect.
In the setting of bone marrow stem cell therapy following acute myocardial infarction, earlier studies in small-animal model systems failed to predict clinical efficacy, and this problem has been compounded by the lack of reproducibility of the original findings of robust transdifferentiation of any nonmuscle cell type into cardiac muscle cell lineages.2 Larger scale, double-blind, randomized clinical studies have now documented the existence of an endogenous pathway for recovery from global cardiac dysfunction following an acute myocardial infarction with current medical regimens alone.3
As such, the finding of anecdotal improvement in heart function in a few patients receiving bone marrow precursor cell-based therapy may not necessarily reflect a true regenerative response.4 This underscores the continued need for carefully controlled, double-blind, placebo-controlled studies.
In the handful of randomized controlled studies to date, evidence in support of clinical efficacy appears to be ambiguous, with a relatively modest improvement of 3%,5 or negligible/no improvement in cardiac function following bone marrow stem cell therapy after acute myocardial infarction.6,7 In the only long-term study to date, a small initial therapeutic benefit was seen in cardiac function associated with bone marrow stem cell therapy after myocardial infarction, but this difference was not evident at the 18-month point (see Trials of the Heart).8 The lack of a long-term effect suggests an acceleration of a natural repair process from the cell-based therapy, but the treated and untreated groups can reach the same plateau level of recovery. At the same time, clinical studies employing stem-cell growth factors that significantly augment circulating hematopoietic stem cells following myocardial infarction, produced negative results in two separate large, randomized, double-blind, placebo-controlled trials.3
Obviously serious challenges remain. There is a compelling need to improve the next generation of clinical studies toward regenerative therapeutic strategies for degenerative heart diseases.
THE PHYSIOLOGICAL BASIS
One of the initial precepts for stem cell therapy for heart disease was based on a vision that the transplantation of diverse muscle or nonmuscle stem cell populations might lead to their transdifferentiation into cardiac muscle, followed by a marked improvement in global cardiac function. There now appears general consensus that the controlled clinical studies thus far have provided little evidence of cardiac muscle regeneration.9,10 Numerous causes can influence progression of heart failure, including cell autonomous factors (contractility, cell survival, metabolic state, hypertrophy) and noncell autonomous factors (hemodynamic, scar formation and wound healing, chamber dilation, and wall stress). Therefore, the identification of the pathophysiological basis for any potential improvement in cardiac function may be critical in designing a trial with the optimal cell type, clinical surrogates, and patient subsets.
While the mechanisms that account for any improvement in cardiac function following bone-marrow precursor-cell therapy remain unclear, a number of experimental studies have begun to examine the physiological and molecular interactions between the transplanted cells and the neighboring native myocardial cells. Studies in genetically engineered mice have suggested that one of the central mechanisms may be a paracrine effect via the release of angiogenic cytokines from the transferred cells.11 Accordingly, it will become critical to examine angiogenesis-related endpoints in future clinical trials.
Examining the therapeutic effects of cells known to be highly angiogenic, such as fat cells, could provide interesting results. Experimental studies could be designed to directly compare the effects of cell-based therapies to recombinant protein therapies using the angiogenic cytokines themselves or other strategies to enhance targets and pathways that drive angiogenesis. In short, delivering on the promise of stem cell therapy for heart disease will require a systematic, rigorous, and coordinated effort from a team of scientists and clinicians working at the interface of stem cell biology, device technology, molecular imaging, and cardiac regeneration.
OPTIMIZING CELL TYPE
To date, a diverse group of muscle and nonmuscle cell types have been used in clinical studies, including myoblasts, mesenchymal cells, bone marrow precursors, and endothelial precursors (see Delivering on the Dream). The choice of these cell types has been based on the need of a sufficient number of cells to drive a sufficient level of muscle regeneration. But now that it is clear that none of these cell types is actually driving robust cardiac muscle regeneration, it would appear necessary to consider alternative cell types in the future.
Mesenchymal cells are currently being studied at both the clinical and experimental level because of their proposed allogeneic properties, which might allow their general use without necessitating immunosuppression.12 Recent studies suggest that the cells do not regenerate cardiac muscle following transplantation, so again any benefit might be ultimately related to paracrine effects. In this regard, the discovery of a number of resident cardiac progenitor cells appears to be worthy of further examination,13-15 particularly if the cells are capable of a sufficient level of self-renewal to allow the maintenance of potency, and triggered differentiation in the absence of significant karyotypic changes. The identification of specific renewal factors and their downstream pathways, which might allow the eventual cloning of postnatal progenitors from the heart, will become increasingly interesting. Also, it will become critical to determine the origin of these resident progenitor cells in the heart, as it has been found that hematopoietic stem cell lineages are localized in virtually every tissue, and several of these progenitors co-express hematopoietic markers.
One of the limitations in several of the clinical studies has been the relatively low efficiency of delivery and subsequent grafting of any of the cells into the heart following intracoronary delivery. This point is underscored by recent studies which have shown that the standard approach of intracoronary delivery leads to a rapid transit of the cells out of the heart, such that less than 2% of the cells remain in the heart after 24 hours. As a result, alternative strategies have been developed to deliver the cells by direct injection into cardiac muscle, either at the time of surgery or via a catheter-based approach. The development of novel device technology to enhance the efficiency of percutaneous catheter-based delivery systems would seem warranted, as would devising new ways via molecular imaging tools to track the cells and determine their ultimate fate (survival, death, differentiation, transformation). The homing and grafting of the cells into the neighboring myocardial syncytium is also a critical issue, and it would be particularly advantageous if this could be extended to the setting of nonischemic, dilated cardiomyopathy where there is clearly an unmet clinical need for additional functioning myocytes.
MINIMIZING ADVERSE EFFECTS
In the studies of myoblast cell therapy, there was a recorded incidence of ventricular arrhythmias, which eventually lead to the recommendation that defibrillators be implanted in the recipients of this cell-based therapy. Experimental studies had earlier suggested that while the myoblasts become differentiated into skeletal muscle in the heart following their transplantation, they do not become electrically coupled to the neighboring cardiac muscle syncytium, thereby setting up the potential for arrythmogenesis.16
The field faces several critical obstacles, including optimizing the cell type, the conditions for its scale-up, and the method for delivery.
It will become important to monitor this effect in all subsequent clinical trials of cell-based therapy for heart disease, particularly if the cell type that is utilized differentiates into muscle. At the same time, one of the major outcomes of several of these early trials of bone-marrow precursor cell therapy is that they have been notable for their absence of life-threatening side effects.
Nevertheless, there is a conceptual possibility that inflammatory pathways elicited by the hematopoietic cells might augment atherogenesis in the infarct-related artery, a point that has recently been raised in a small-scale study.17 Moreover, the delivery of cell types that are larger than hematopoietic precursors might also be associated with microembolic phenomena, which should be monitored in the future. Since most of these physiological endpoints cannot be adequately assessed in small-animal model systems, it will become increasingly important to move studies to larger animals where many of these adverse effects can be adequately monitored.
THE NEXT PLAY
Applying stem cell therapies for various forms of heart disease is part of a global effort to unlock the potential of regenerative medicine for a diverse set of human diseases. Bone marrow stem cell therapy for patients after myocardial infarction has been one of the first clinical plays, based on the view that the goal of regenerating functional myocardium was within striking distance. It now appears that we may still be quite far from that goal, based on early returns from the first round of clinical studies and reanalysis of earlier preclinical work. The field faces several critical obstacles, including the lack of an optimal cell type to drive authentic in vivo cardiomyocyte regeneration, conditions required for its scale-up that would maintain potency and karyotypic stability, and optimization of in vivo delivery.
The most prudent way forward may be to systematically attack each of these and other challenges by building a multifaceted team effort that focuses on the scientific underpinnings of each step in the critical path for the development of cardiac stem cell therapy: identification and purification of the optimal cell type, defining their renewal factors and pathways, maximizing in vivo grafting and coupling, defining potential adverse effects in the short and long term, and expanding studies into chronic heart-failure model systems. Significant strides may be made by increasing our commitment to integrate the most innovative technology and insights from developmental biology in general, and embryonic stem cell biology in particular, into the team.
Human embryonic stem (ES) cell technology is moving ahead rapidly, and it should be possible to develop tools that will eventually allow the clonal isolation of authentic cardiovascular progenitors. With further definition of the specific factors that drive the renewal of authentic cardiac progenitors, this technology could ultimately allow the cloning of rare postnatal progenitors from human heart tissue into clonal, stable, cardiac progenitor cell lines. It should also become possible to derive clonal lines of cardiac progenitors derived from human ES cells that have been genetically engineered to harbor disease genes of interest. This would allow higher-throughput genetic and chemical screens for pathway identification and drug discovery that have not yet been possible in murine-derived lines. And we may be able to optimize the tools for cell transplantation by first developing imaging tools to mark the cell fate and function of the transplanted cells in vivo in large animals.
Taking on the daunting task of moving the clinical target from postmyocardial infarction to end-stage heart failure, where there truly is a large unmet clinical need, appears warranted. For these patients, time is running out. Play on.
Kenneth Chien is the director of Massachusetts General Hospital Cardiovascular Research Center, and leads the Harvard Stem Cell Institute program in cardiovascular diseases. He is a professor in the Department of Cell Biology at Harvard Medical School and the Harvard Stem Cell Institute.
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Studies in mice and human stem cells demonstrate that the genome-editing technique CRISPR can correct disease-causing mutations.