Loss of cardiomyocytes following myocardial infarction induces a contractile dysfunction of heart and the dead cardiac muscle cells are replaced by fibroblasts to form scar tissues. In most circumstances, chronic ischemia persists following infarction and leads to negative remodeling that can cause heart failure and death (Ambrose, 2006). Transplantation of fetal cardiomyocytes or skeletal myoblasts has been proposed as a future method for treatment of heart strokes (Soonpaa et al., 1994; Delcarpio and Claycomb, 1995; Leor et al., 1996; Murry et al., 1996; Taylor et al., 1998; Tomita et al., 1999). Nevertheless, this idea remains unfeasible because of the difficulty in obtaining donor cells and the percentage of failures associated with obtaining sufficient recovery of physiological function in transplanted hearts.
Several authors have demonstrated that intracoronary injection of mixed populations of bone marrow stem cells or of MSCs could represent a simple and successful approach to the treatment of heart diseases.
An interesting study on this topic was performed by Strauer et al. (2002). They enrolled 20 patients that had suffered from transmural infarction. After right and left catheterization, coronary angiography, and left ventriculography, patients underwent balloon angioplasty and stent implantation. Five to 9 days after acute infarction, bone marrow was aspirated from the ilium of 10 patients and mononuclear cells were isolated with classical Ficoll density separation. Cells were then transplanted into the infarcted zone with the use of a balloon catheter, which was placed within the infarct-related artery. Intracoronary transplantation consisted of six to seven fractional high-pressure infusions, each containing 1.5–4 × 106 cells. Ex vivo experiments demonstrated that these cells were able to generate mesenchymal cultures. Comparison of the two groups 3 months after cell or standard therapy showed several significant differences. In fact, the infarct region as a percentage of hypokinetic, akinetic, or dyskinetic segments of the circumference of the left ventricle decreased significantly in the cell-transplanted group. Ejection fraction increased in both groups. Perfusion defect was considerably decreased in the cell therapy group as detected by thallium scintigraphy (Strauer et al., 2002).
Another study on cell therapy for acute myocardial infarction treatment was carried out by Chen et al. (2004). Sixty-nine patients within 12 h of onset of infarction underwent emergency angiography or angioplasty. Patients were candidates for MSC treatment and were randomized to receive cell transplantation (n = 34) or saline treatment (n = 35) after percutaneous coronary intervention (PCI). Sixty milliliters of bone marrow from patients undergoing cell therapy was aspirated and mononuclear cells were cultured for 10 days to obtain MSCs. At the end of in vitro amplification, cells were collected and used for cell therapy. The infarct-related artery was occluded at the proximal edge of the previous angioplasty and 6 ml of MSC suspension (8–10 × 109 cells/ml) was injected into the target coronary artery. Control patients received standard saline injections. All patients underwent cardiac echocardiography once a month and positron emission tomography was performed 3 and 6 months after implantation. Electrocardiographic monitoring for 24 h was also recorded 3 months after the procedure. The percentage of hypokinetic, akinetic, and dyskinetic segments decreased significantly in the cell therapy group after 3 months compared to that at the beginning of treatment. This result was obtained to a lesser extent also in the control group. Wall movement velocity over the infarcted area increased significantly in cell therapy-treated patients but not in the control group. Also left ventricular ejection was higher in the cell therapy group compared to controls (Chen et al., 2004).
An interesting randomized trial (called BOOST trial) to assess the effectiveness of intracoronary transfer of autologous bone marrow cells for treatment of acute myocardial infarction was carried out by Wollert et al. (2004). They enrolled 60 patients suffering from acute heart infarction. After PCI, patients were randomly assigned to either a control group that received classical post-infarction treatment or to cell therapy. Bone marrow nucleated cells were collected from patients in the cell therapy group and 4–8 days post PCI were injected (about 24 × 108) into infarcted artery by a balloon catheter. Changes in left-ventricular end diastolic volumes (LVEDV) index, left-ventricular end systolic volumes (LVESV) index, and left-ventricular mass index did not differ significantly between the control group and bone marrow cell group. However, compared with the control group, patients in the cell therapy group had increased left-ventricular ejection fraction (LVEF) and systolic wall motion 6 months after transplantation. It is noteworthy that there were no differences between the two groups with respect to the number of premature ventricular complexes and the occurrence of non-sustained or sustained ventricular tachycardias by Holter monitoring follow-up at 6 weeks, 3 months, and 6 months. The authors suggested that autologous bone marrow cells can be used to enhance left ventricular functional recovery in patients that had acute heart infarction (Wollert et al., 2004).
A different approach for treatment of myocardial infarction was presented by Katritsis et al. (2005). They acknowledged that intracoronary transplantation of autologous bone marrow-derived mononuclear cells has been shown to improve contractility of infarcted hearts. However, the authors stated that while administration of unpurified mononuclear cells avoids problems associated with cell culture expansion, it inevitably consists of a small percentage of pluripotent cells diluted among a huge amount of committed and differentiated cells. They hypothesized that a bone marrow population consisting of culture-expanded MSCs along with endothelial progenitors (EPCs) also present in marrow stroma would be capable of promoting both myogenesis (by MSCs) and angiogenesis (EPCs) at the infarcted area of the myocardium. The hypothesis relies on several studies suggesting that other cell populations besides hematopoietic stem cells also can give rise to ECs. In fact, adult bone marrow-derived stem/progenitor cells which are distinct from hematopoietic stem cells, have also been shown to differentiate to the endothelial lineage (Urbich and Dimmeler, 2004).
These authors enrolled patients with both recent and old anteroseptal myocardial infarction. All patients had been previously subjected to angioplasty and stent implantation of the left anterior descending artery. In a group of patients (n = 11), the day following PCI, bone marrow aspirates were collected and mononuclear cells were isolated by classical Ficoll separation. Cells were plated in cultures and on day 7, the adherent cells were washed, collected, and transferred to the operating room. The left coronary artery was catheterized for cell transplantation. Two cell suspensions (each containing 1–2 × 106 cells) were infused distally to the occluding balloon of the catheter. Both in the transplantation group and the controls, there was a trend for improvement in end-diastolic and end-systolic diameter, fraction shortening, ejection fraction, end-diastolic, and end-systolic volume. In 5 out 11 patients in the transplantation group, there was improvement of myocardial contractility in one or more previously non-viable myocardial segments. No one in the control group showed this improvement. Overall evaluation of obtained results indicated that the positive effect of cell therapy on myocardial contractility is mainly seen in patients with recent myocardial infarction (Katritsis et al., 2005).
All of the above-described results, along with similar studies (Assmus et al., 2002; Stamm et al., 2003), demonstrate that cell therapy with bone marrow-derived stem cells is feasible, safe, and may contribute to regeneration of myocardial tissue following infarction. Nevertheless, several issues are still controversial: which kind of stem cell is suitable for patients? When should cells be transplanted? How should the viability of transplanted cells be monitored? What is the ideal mechanism of action of stem cells: secretion of growth factors or cell-to-cell interactions?
A study performed by Perin et al. (2003) evaluated the hypothesis that transplants of bone marrow mononuclear cells in patients with end-stage ischemic heart disease may promote neovascularization and may prevent impairment of heart functionality which in turn can lead to myocardial infarction. They enrolled 21 patients, 14 were assigned to the cell therapy group and 7 to the control group. The inclusion criteria for patients were: (i) chronic coronary disease; (ii) left ventricular ejection fraction <40%; (iii) ineligibility for percutaneous or surgical revascularization. For patients undergoing cell therapy treatment, 50 ml of bone marrow was aspirated from the iliac crest and bone marrow mononuclear cells were separated using the Ficoll density procedure. Patients underwent heart catheterization on the left side and electromechanical mapping (EMM) of the left ventricle to target the specific treatment area by identifying viable myocardium. In this area, 15 injections of 2 ml were delivered for a total of ∼25 × 106 cells/patient. All patients underwent a complete non-invasive follow-up (clinical evaluation, ramp tread mill protocol, 2D Doppler echocardiogram, and single photon emission computed tomography analysis) 2 months later and an invasive follow-up (left ventricle angiograms and EMM) after 4 months. Two months after treatment, they observed a significant reduction in total reversible defect and improvement in global left ventricular function within the treatment group and between this and the control group. The 4-month follow-up revealed an improvement in ejection fraction and a reduction in end-systolic volume in the treated patients (Perin et al., 2003). This preliminary study demonstrated that cell therapy treatment could improve myocardial blood flow with associated enhancement of left ventricular functions in patients with severe ischemia and could reduce the risk of heart stroke.
The same research team further evaluated the effectiveness of their cell therapy protocol by evaluating patients with severe ischemia 6 and 12 months after transendocardial injection of autologous bone marrow cells. They showed that total reversible defect, detected by SPECT perfusion scanning, was reduced in the cell therapy group compared with controls. Moreover, at 12 months, exercise capacity was significantly improved in cell therapy-treated patients (Perin et al., 2004). These data further support the effectiveness of autologous bone marrow infusion for ischemic cardiomyopathy treatment.
Arterial (re)stenosis is a pathophysiological phenomenon that can follow angioplasty, arteriotomy, or by-pass creation in humans and experimental vascular injury in animal models, causing an occlusion of the arterial lumen of variable extension that often requires a new revascularization procedure. Vascular injury, with cell loss in the intima and media tunicae, elastic lamina fragmentation and damage to tissue architecture, leads to excessive pathological repair and remodeling that involves vascular smooth muscle cell (SMC) migration and proliferation, resulting in neointimal hyperplasia (Forte et al., 2001; Xu et al., 2004). Recent evidence has shown that vascular function depends not only on cells within the vessels, but is also significantly modulated by circulating cells derived from the bone marrow. Stem cells hold a great potential for the regeneration of damaged tissues in cardiovascular diseases. In particular, in the past, it was believed that the regeneration of injured endothelium and media in arteries was due to migration and proliferation of neighboring ECs and SMCs. Recent studies clearly indicated that different stem cell populations, derived from bone marrow and characterized by different markers and with different behaviors, contribute to vascular remodeling after injury (Tanaka et al., 2003). On this basis, it has been hypothesized that the restenosis process could be prevented through stem cell-mediated early injury repair.
One interesting study was carried out using EPCs. The study is part of the HEALING-FIM (Healthy Endothelial Accelerated Lining Inhibits Neointimal Growth-First In Man) Registry. HEALING I was a single-center, prospective, non-randomized registry trial. It was conducted by Aoki by applying in patients the Genous™ Bio-engineered R stent (OrbusNeich Company), the first stent designed to accelerate the natural healing response by capturing a patient's own EPCs from the blood stream (Aoki et al., 2005). Once captured, EPCs rapidly form a protective endothelial layer over the stent, providing protection against thrombus and minimizing restenosis. This stainless steel stent is coated with a murine monoclonal antibody against human CD34.The first published results of this clinical study were obtained on 16 patients and revealed that this coated stent was safe and feasible. On this basis, the group started with the HEALING II study, which included 63 patients at 10 centers in Europe. Whole blood samples were analyzed to quantify the number of EPCs in each patient. Data showed that the EPC titer directly correlated with angiographic outcomes. There were no target lesion revascularizations in patients with normal numbers of circulating EPCs, while patients with low EPCs were affected by restenotic and cardiac events. It should be mentioned that the large majority of patients with normal EPC levels were on statin therapy, while most in the low EPC group were not. Previous studies revealed that statin injection is effective in EPC mobilization (Walter et al., 2002). On the basis of these encouraging results, the HEALING III study has been designed to verify and substantiate these findings and will be conducted in 2006 (Silber, 2006). The HEALING III study will also assess the effect of combining statin therapy and EPC capturing stents.