Stem Cell Therapy in Patients with Myocardial Infarction

Preclinical Studies

ESC-derived cardiomyocytes (hESC-CMs) display adult cardiomyocyte morphology with properly organized sarcomeric proteins, spontaneous beating activity, and characteristic atrial, ventricular, and nodal action potentials. The ability of ESCs to generate bona fide cardiomyocytes has spurred interest in the use of ESCs or hESC-CMs for cardiac regeneration after MI. A number of preclinical reports have described engraftment of hESC-CMs and/or differentiation of ESCs into adult cardiomyocytes with subsequent attenuation of LV remodeling and improvement in left ventricular ejection fraction (LVEF) in rodent models. In one study, investigators transplanted murine cardiac-committed ESCs into the infarcted myocardium of sheep 2 weeks after MI. One month after transplantation, the ESCs had differentiated into cardiomyocytes, engrafted to the host heart, and improved LV function. Shiba and colleagues demonstrated, in a guinea pig model of MI, that implanted hESC-CMs partially integrated into the host myocardium and contracted synchronously with the host muscle. The engrafted hearts showed improved mechanical function and a reduced incidence of both spontaneous and induced ventricular tachycardia. Chong and colleagues transplanted one billion hESC-CMs into a nonhuman primate model of reperfused MI under intense immunosuppression. They reported extensive remuscularization of the infarcted area, which averaged 2.1% of the LV and 40% of the infarct volume, as well as formation of electromechanical junctions between the graft and host cardiomyocytes. Study of calcium transients indicated electrical activation of the cardiomyocyte grafts and electromechanical coupling. However, this study raised significant concerns in the scientific community, primarily because the observations were anecdotal (1 to 2 monkeys were assessed at each time point), the infarcts were small (7% to 10% of the LV), the infarct size was not reduced, the evidence for remuscularization of the infarcted tissue was inadequate, cardiac function was not assessed, and importantly, malignant ventricular arrhythmias were observed in all monkeys that received the transplants.

Barriers to Clinical Development

Despite the obvious promise of hESCs and hESC-CMs, the use of these cells faces formidable hurdles and is unlikely to become a therapy for CV disease. The allogeneic nature of ESCs necessitates lifelong use of immunosuppressive therapy, with its attendant risks and morbidity, which could be worse than the disease being treated. The recent finding that hESC-CMs are arrhythmogenic constitutes another problem. Even more concerning is the risk of teratoma formation, which is inherent in the embryonic nature of the cells; despite attempts to minimize it, the occurrence of this serious consequence cannot be completely eliminated. These risks and problems are all the more unacceptable because potentially safer alternatives are available, including iPSCs and adult stem cells; the latter have already been tested in numerous clinical trials, with an excellent safety profile (vide infra). In view of these considerations, it is difficult to rationalize using ESCs for therapeutic purposes in patients with CV disease. Not surprisingly, no clinical trial of ESC-based therapy for CV disease has been published despite the fact that these cells have been studied for two decades; meanwhile, over this time period, thousands of patients have been safely treated with adult stem and/or progenitor cells with results that have been sufficiently encouraging to warrant phase III trials.

Preclinical Studies

In 2006, Takahashi and Yamanaka reported generation of iPSCs by transducing adult mouse fibroblasts with a cocktail of transcription factors, including Oct3/4, Sox2, c-Myc, and Klf4, which are the so-called Yamanaka factors. The embryonic-like cells expressed ESC marker genes and exhibited morphology and growth properties similar to those of ESCs. It was subsequently demonstrated that iPSCs possess a cardiogenic potential comparable to that of ESCs, and more importantly, functional properties typical of cardiac cells, such as cardiac ion channel expression, spontaneous beating, and contractility. iPSCs have a capacity equivalent to ESCs to differentiate into nodal-, atrial-, and ventricular-like cardiomyocyte phenotypes, based on action potential characteristics. iPSC-derived cardiomyocytes (iPSC-CMs) exhibit typical sarcomeric organization and respond to β-adrenergic stimulation, with an increase in the spontaneous rate and a decrease in action potential duration. Initial studies in small animal models have reported improvement in cardiac function as a result of iPSC administration. In a porcine model of MI, intramyocardial transplantation of human iPSC-CMs, endothelial cells, and smooth muscle cells, in combination with a three-dimensional fibrin patch loaded with insulin growth factor–encapsulated microspheres, resulted in human iPSC-CMs being integrated into the host myocardium and generating organized sarcomeric structures ; moreover, endothelial and smooth muscle cells were also incorporated into the host vasculature, although their contribution was minimal. At 4 weeks, LV function was significantly improved compared with untreated animals, along with a trend toward a reduction in infarct size. In this study, cell treatment was delivered after reperfusion, and the animals were immunosuppressed.

Barriers to Clinical Development

Despite these promising results, the iPSC technology is associated with a number of safety concerns, including the potential for genetic and epigenetic abnormalities related to the origin and manipulation of the cells and for tumorigenicity related to retroviral transgene activation, insertional mutagenesis, and contamination with undifferentiated pluripotent stem cells and/or differentiation-resistant cells. Despite the initial hope that these autologous cells would not require immunosuppression, issues of immunogenicity of transplanted cells have emerged. Some of these safety concerns have been addressed (e.g., virus-free induction of iPSCs ), but other concerns persist. Furthermore, clinical translation is hindered by major practical hurdles related to cell procurement, including efficient induction of cardiomyocyte lineages from iPSCs, selective expansion and/or survival of iPSC-CM lineages, and purification of differentiated iPSC-CMs by elimination of residual undifferentiated iPSCs. Although major progress has been made in differentiation protocols, significant problems remain to be resolved. Additional limiting factors are the cost and effort required to generate autologous iPSCs in every patient who is treated. Although iPSC technology is evolving rapidly, and these challenges may be overcome, at present it seems unlikely that these cells will be used in clinical trials in the near future.

A new strategy that has recently emerged for cardiac regeneration is in vivo direct reprogramming of nonmyocyte cardiac cells, which make up more than 50% of the cells in the heart. This direct reprogramming strategy entails direct transdifferentation of one cell type (i.e., cardiac fibroblast) to another (i.e., cardiomyocyte), bypassing the need for dedifferentiation to an earlier embryonic state before redifferentiation toward a cardiomyocyte fate. This approach is still in its nascent phase, and it is unclear whether clinical translation will ever be feasible.