I - INTRODUCTION
The idea of myocardial regeneration with stem cell transplantation after myocardial infarction receives tremendous interest. Preclinical and some small-scaled clinical trials have suggested feasibility and safety of cardiac stem cell therapy. However, there are still questions waiting for answers and results of large-scale randomized trials are needed for the appearance of myocardial cellular therapy as a clinical therapeutic option for patients with ischemic heart disease.
Acute myocardial infarction and resultant heart failure is a major cause of mortality and morbidity worldwide (1). Left ventricular remodeling after acute myocardial infarction (MI) is a complex pathological process producing alterations in topography of both the infarcted and non-infarcted regions of the ventricle leading to progressive dilatation and systolic dysfunction (2). Current developments in revascularization strategies and medical therapies such as beta-blockers and medications interfering with the action of renin-angiotensin aldosterone system have focused on remodeling. However these therapies are of limited value for restoration of cardiac functions after myocardial infarction. Cell therapy is currently emerging as a potential new treatment for post MI patients with the assumption that recolonization of the areas of scarred myocardium with exogenously supplied surrogates or precursors of cardiomyocytes can restore function and ultimately affect clinical outcomes.
Various cell types have been tested experimentally for cardiac repair so far, but only those of autologous origin have yet undergone clinical testing due to immune competency. These include bone marrow-derived cells, peripheral blood derived stem cells and skeletal myoblasts (3).
Several preliminary reports have demonstrated that local stem cell transplantation in patients with acute myocardial infarction is safe and may lead to improved myocardial function and perfusion. The bone morrow contains several stem cell types including hematopoetic stem cells, endothelial progenitor cells, mesenchymal (stromal) stem cells and multipotent adult progenitor cells. In experimental and clinical studies unselected mononuclear bone marrow cells as well as specific subpopulations have been used for transplantation. Although there is no definitive evidence, combinations of progenitor cells seem to be more benefical than specific stem cell type for cardiac repair (3). Interestingly, contradictory data exist for transformation of bone marrow derived progenitor cells (BMPC) into new cardiomyocytes and alternative mechanisms as enhanced neovascularization, enhanced scar tissue formation due to augmented inflammatory response and decreased apoptosis have been suggested for the benefical effects of these cells on myocardial function after myocardial infarction (4).
Repair of scar tissue constitutes a challenge for cardiac stem cell therapy due to lack of adequate nutrition and homing signals necessary for stem cells’ engraftment and survival. The resistant state of skeletal myoblasts to ischemia renders these cells suitable candidates for repair of chronically infracted and failing heart (5). However, in spite of their resistance to hypoxia only a small portion of cells survive when transplanted into an infarct scar (6). Injection of myoblasts into infarcted myocardium has been shown to augment systolic and diastolic functions in experimental animal studies (7). Myoblasts retain skeletal muscle properties and couple only sporadically to resident cardiomyocytes precluding synchronized contraction as mechanism of benefit (8). Moreover, it seems that paracrine action of skeletal myoblasts facilitate neighboring cardiomyocytes to maintain their replicative potential and/or stimulate differentiation of native cardiac stem cells (8,9). Stimulation of angiogenesis and stabilization of extracellular matrix have also been suggested to contribute to the benefit (9). However, lack of electromechanical coupling may create an arrhythmogenic substrate and is the main limitation of cellular therapy with myoblasts (10).
Modes of Cell Delivery
So far, progenitor cells for cardiac repair have been delivered via an intracoronary arterial route or by injection of the ventricular wall via percutaneous endocardial, percutaneous transcoronary venous or surgical epicardial approach.
Intracoronary application permits application of a maximum concentration of cells homogenously to the site of injury. Skeletal myoblasts are not suitable for this mode of delivery due to their large size and potential for distal obstruction of microcirculation. This mode of delivery has mostly been used for patients with acute myocardial infarction through the central lumen of an over-the-wire balloon catheter during transient balloon inflations to maximize the contact time of the cells with the microcirculation of the infarct related artery.
However, for the unperfused regions of the myocardium this mode of delivery may not be efficient.
Direct injection is the preferred route for chronic ischemic heart failure patients with considerable scar tissues. Cell homing signals as ‘vascular endothelial growth factor (VEGF)’ and ‘stromal cell derived factor-1 (SDF-1)’ are expressed at low levels late in disease process precluding successful engraftment of cells applied via intracoronary route (11). Direct injection techniques are especially suited for the application of large cells as myoblasts and mesenchymal stem cells. Direct delivery of progenitor cells into scar tissue or areas of hibernating myocardium by catheter-based endocardial injection or direct injection during open-heart surgery techniques are not limited by cell uptake from the circulation or by embolic risk. Endocardial electromechanical mapping techniques are often used as an adjunct to endocardial injection to identify viable but hibernating myocardial regions, which are preferred areas for stem cell injection (12). Transepicardial cell injection has been performed as an adjunct to coronary artery bypass grafting (CABG). A novel technique for direct myocardial cell delivery is via transvenous approach using coronary veins with a special catheter system under fluoroscopic and intravascular ultrasound guidance (13).
II - CLINICAL TRIALS OF CARDIAC STEM CELL THERAPY
1. Acute Myocardial Infarction
The first small clinical trial justifying the safety and feasibility of the intracoronary application of autologous BM-SCs in patients after recent myocardial infarction (MI) was published in 2002. Strauer et al. (14) applied bone marrow mononuclear cells after AMI via the intracoronary route five to nine days after acute myocardial infarction in 10 patients. After 3 months of follow up the investigators found that the infarct region significantly decreased and cardiac functions were significantly improved in the treatment group without any significant adverse effects. Afterwards, results of small clinical trials as TOPCARE-AMI (Transplantation of Progenitor Cells and Regeneration Enhancement in Acute Myocardial Infarction) and BOOST (Bone Marrow Transfer to Enhance ST-Elevation Infarct Regeneration) studies were reported (15-18). Overall, these trials showed marginal improvement of cardiac functions and myocardial perfusion after 3 to 6 months, without significant adverse effects. The beneficial effects of progenitor cell transplantation persisted during a 1-year follow-up period without significant adverse events in TOPCARE-AMI trial (19). However, no significant differences between patient and control groups regarding improvement of cardiac functions were reported at 18 months’ follow-up of BOOST trial (20).
Observations from initial clinical studies encouraged investigators to perform larger scale randomized trials. Recently published randomized, double-blind, placebo controlled REPAIR-AMI trial randomly assigned 204 patients with acute myocardial infarction to receive an intracoronary infusion of progenitor cells derived from bone marrow or placebo into the infarct artery 3 to 7 days after successful reperfusion therapy. At 4 months, the absolute improvement in the global left ventricular ejection fraction (LVEF) was significantly greater in the BMPC group than in the placebo group (21). At 1 year, intracoronary infusion of BMPC was associated with a reduction in the prespecified combined clinical end point of death, recurrence of myocardial infarction, and any revascularization procedure (22). Contradictory to these results, another recently published randomized study; ASTAMI trial showed no effects of intracoronary injection of autologous mononuclear BMPC on global left ventricular function could be observed during 6-month follow-up period (23). One hundred patients with a reperfused anterior myocardial infarction patients were randomly assigned to stem cell implant or not, 5-8 days after an acute event in this study. The reasons for the conflict between these two studies are not clear. However, different cell isolation protocols used in these studies may have contributed to this discrepancy (24).
Due to technical difficulties and the invasive nature of harvesting stem cells from bone marrow, investigators also tried peripheral blood derived progenitor cells (PBPC’s) for cardiac repair after myocardial infarction. These cells were equally effective as BMPC’s for improving cardiac functions in TOPCARE-AMI trial (15). The major concern is the insufficient number of these cells in circulation and need for ex vivo proliferation or utilization of stem cell mobilization factors as ‘stem cell factor (SCF)’ or ‘granulocyte colony-stimulating factor (G-CSF)’. Both of these factors have been proposed to stimulate myogenesis and angiogenesis in the infracted area and to improve cardiac function after AMI in experimental studies (25,26). Granulocyte colony stimulating factor (G-CSF) is frequently used to mobilize marrow stem cells into circulation but is associated with mobilization of other immune cells, which leads to nonspecific inflammation. In the first clinical investigation, the MAGIC cell randomized clinical trial, Kang et al. (27) prospectively randomised 27 patients with myocardial infarction who underwent coronary stenting for the culprit lesion of infarction into three groups; cell infusion group in which G-CSF mobilized peripheral blood-derived progenitor cells were used (n=10), G-CSF alone group (n=10) and control group (n=7). Changes in left ventricular systolic function and perfusion were assessed after 6 months. G-CSF injection and intracoronary infusion of the mobilised PBSC did not aggravate inflammation and ischaemia during the periprocedural period. Exercise capacity, myocardial perfusion and systolic function improved significantly in patients who received cell infusion. However, an unexpectedly high rate of in-stent restenosis was observed at culprit lesion in patients who received G-CSF, which prompted a premature termination of the study. Local augmentation of inflammation by G-CSF was suggested to contribute to this observation but no such effect was observed in other clinical trials integrating G-CSF therapy into primary percutaneous coronary intervention (PCI) management of patients with AMI (28,29). The FIRSTLINE-AMI (Front-Integrated Revascularization and Stem Cell Liberation in Evolving Acute Myocardial Infarction by Granulocyte Colony-Stimulating Factor) trial reported benefical effect of G-CSF therapy given 85 ± 30 min after primary PCI. At 12 months, treatment with G-CSF resulted in a significant improvement of EF of %8 while EF of control subjects decreased by %5 (28). No adverse events as enhanced inflammation, thrombosis, electrical instability or accelerated restenosis were observed throughout the study. However, results of another randomized, prospective, double-blinded, placebo controlled phase II trial indicate no benefit of G-CSF therapy when PCI is performed late (30).
Two-year follow-up results of MAGIC Cell trial indicate better results with intracoronary cell infusion with mobilized PBSC’s by G-CSF than G-CSF therapy alone, which may indicate inability to achieve sufficient homing and integration of stem cells in the infarct territory unless stem cells are infused locally (31). In a recently published trial Zhan-Quan et al. (32) reported significant improvement of global left ventricular function and significant decrease of ventricular volumes at 6th month with intracoronary autologous PBSCs transplantation pre-mobilized by G-CSF therapy.
2. Chronic Ischemic Heart Disease
In the first clinical trial concerning autologous skeletal myoblast transplantation in patients with severe ischemic cardiomyopathy, Menasche et al. (10) transplanted skeletal myoblasts to 10 patients at the time of CABG surgery. At an average follow-up of 10.9 months, the functional capacity of patients was significantly improved and ejection fractions were significantly increased (%24 ± 1 to %32± 1, p<0.02). Four out of ten patients experienced delayed episodes of sustained ventricular tachycardia and were implanted with an internal defibrillator. Simultaneous effect of revascularization obscured the analysis about the efficiency of myoblasts. One of the patients died 17 months after transplantation because of a non-cardiac cause and detailed histological examination of the heart revealed engrafted skeletal muscle cells demonstrating the survival ability of these cells in hostile environment (33). The improvement of both clinical status and EF persisted during a median of 56 months of follow up period with a strikingly low incidence of hospitalizations for heart failure. Arrhythmic risk of patients could be controlled by medical therapy and/or ICD implantation (34). Regardless of the injection route, several phase I studies have demonstrated the survival, feasibility, and safety of autologous myoblast transplantation and have suggested that this modality offers a potential therapeutic treatment for end-stage heart disease with arrhythmogenic potential (35-36). In a multicenter, randomized, double blind, placebo-controlled phase II trial 97 patients were assigned to low-dose myoblast, high-dose myoblast or placebo transplantation groups during CABG. There were no improvements in contractility of cell-transplanted segments or in global left ventricular function. However, there were no statistical increases in arrhythmic risk and there was reversal of remodeling and improved scintigraphic LVEF in the high cell dose group (37). These results justify further exploration of this strategy with larger scale controlled trials.
Besides the skeletal myoblasts, BMPCs were used for cellular therapy of chronic ischemic cardiomyopathy as well. In the first pilot trial Stamm et al (38) injected CD133+ bone marrow stem cells into the infarct border zones of 12 patients during the CABG operation. Scintigraphic imaging demonstrated significantly improved local perfusion in the stem cell-treated infarct area. LV dimensions and LV ejection fraction (%39.7 ± 9 vs %48.7 ± 6, p=0.007) improved without any significant adverse events as new ventricular arrhythmia or neoplasia. After this pilot trial, results of a randomized controlled trial were reported (Regeneration of Human Infarcted Heart Muscle by Intracoronary Autologous Bone Marrow Transplantation in Chronic Coronary Artery Disease: the IACT study) (39). In this study autologous bone marrow mononuclear cells were transplanted via intracoronary route to 18 patients with chronic MI (5 months to 8.5 years old). After three months, infarct size was reduced by 30% and global left ventricular ejection fraction and infarction wall movement velocity increased significantly in the transplantation group. However, no significant changes were observed in infarct size, left ventricular ejection fraction, or wall movement velocity of infarcted area in the control group. No significant adverse events as significant arrhythmias and restenosis of the culprit artery were observed during follow-up. After bone marrow cell transplantation, there was an improvement of maximum oxygen uptake and of regional 18F-fluoro-desoxy-glucose uptake into infarct tissue. These results demonstrated the possibility of functional and metabolic regeneration of infarcted and chronically avital tissue by bone marrow mononuclear cell transplantation in humans. Some phase I studies also investigated the feasibility and efficacy of percutaneous endocardial transplantation of BMPCs guided by electromechanical mapping (40-42). Improvements in symptoms, myocardial function and function at the ischemic regions were demonstrated without significant adverse events as arrhythmias, evidence of myocardial inflammation or increased scar formation.
III - UNRESOLVED ISSUES IN CARDIAC STEM CELL THERAPY
Experience with preclinical studies have raised important questions that will hopefully be resolved by larger scale clinical studies (43,44). These trials will further clarify the effects of stem cell therapy on surrogate markers of cardiac functions as LVEF, myocardial perfusion, or exercise capacity and long-term safety of the procedure.
Although a marginal benefit was observed in many preclinical studies with stem cell therapy, the mechanisms of benefit are still poorly understood. Optimal cell type and cell dosage to be used for different situations of ischemic heart disease and timing of cell transfer are also important issues that need to be further clarified. Theoretically, timing of cell infusion may be unsuitable - either too early or too late, since early administration causes implanted cells to be killed by an inflammatory environment and late administration can lead to disconnection between implanted cells and cardiomyocytes by fibrosis scar. Long-term experimental data regarding the fate of transplanted cells are still missing. Transdifferentiation and homing mechanisms are poorly understood. Cell labeling and imaging techniques need to be developed to track stem cell fate in patients and correlate cell retention and engraftment with functional outcomes. Pharmacological and genetic strategies seem to offer enhancement of stem cell retention, engraftment, differentiation and paracrine capability, which deserves further exploration (45).
The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.
Cardiac stem cell therapy appears as a promising mode of therapy for ischemic heart disease. Although data from preclinical experiments and small clinical studies point to the potential for restoring cardiac functions following myocardial infarction; there are still uncertainties about the technique, safety and long-term clinical outcome. As such, stem cell therapy can not be regarded as a valid therapeutic option for patients with cardiovascular disease in the present era of evidenced-based medicine. Data from large, randomized controlled trials are needed to clarify the short and long term effects of myocardial cellular therapy and suitability for clinical application.