Acknowledgments
This work was supported in part by NIH grants P01 HL-78825 and UM1 HL-113530 (CCTRN).
Introduction
Cell-based therapy is an exciting new treatment modality that has the potential to revolutionize cardiovascular (CV) medicine. Research on the application of cell therapy for ischemic heart disease emerged in the late 1990s, when conventional wisdom regarded the heart as an organ devoid of capacity for endogenous repair. The dogma that the mammalian heart was a terminally differentiated organ incapable of regeneration and repair was predicated on the lack of significant tissue repair seen after acute myocardial infarction (MI) and during the ensuing period of left ventricular (LV) remodeling and heart failure (HF). However, this paradigm changed dramatically at the dawn of the new millennium, when evidence began to accumulate that formation of new myocytes in the adult heart is possible, and that cell therapy enhances cardiac function after MI. Over the past 15 years, the field of regenerative cardiology has grown exponentially, to the point that it is now generally accepted that cardiac myocytes can be regenerated. We believe that we are witnessing a veritable conceptual and therapeutic revolution that is likely to change CV medicine profoundly.
Various types of stem and/or progenitor cells have been tested in preclinical and clinical studies of cardiac repair or regeneration, mostly in the settings of MI and ischemic cardiomyopathy (ICM), but also in that of nonischemic cardiomyopathy (NICM). These cells include skeletal myoblasts, bone marrow mononuclear cells (BMMNCs), mesenchymal stromal cells (MSCs), proangiogenic progenitor cells, and cardiac progenitor cells (CPCs) ( Figure 22-1 ). Experimental and clinical work has advanced in parallel, leading to rapid translation of basic discoveries into clinical trials. Considering that the first preclinical study of cell therapy was published in 1998 and the first clinical application was used in 2001, the rapidity in which basic and clinical research has progressed in a relatively short time is truly remarkable. In the clinical arena, almost 100 trials of cell therapy for CV disease have been published, some of which have reported promising results. A large number of ongoing clinical trials have been registered with the National Institutes of Health (NIH); in April 2015, a search in the NIH-sponsored clinicaltrials.gov website for cell therapy trials in the context of MI or HF revealed 36 actively recruiting studies in adult populations, including phase III trials.
As mentioned previously, the two major clinical settings in which cell therapy has been studied are acute MI and chronic HF. In this chapter, we specifically review the application of stem and/or progenitor cells for the treatment of acute MI. Treatment of HF with stem and/or progenitor cells has been reviewed recently, and is beyond the scope of the present chapter.
Rationale for Cell Therapy in Acute Myocardial Infarction
The worldwide incidence of acute MI continues to increase at an accelerated pace (see Chapter 2 ). Acute MI leads to adverse LV remodeling (see Chapter 36 ), which causes further cardiomyocyte attrition, ventricular dilation, and HF (see Chapter 25 ), thereby initiating a downward spiral that can eventually culminate in death. Although it is now appreciated that the adult heart has some capacity for renewal, this capacity is limited and is overwhelmed by the ischemic insult and subsequent remodeling. The loss of cardiomyocytes associated with acute MI cannot be reversed with contemporary treatment modalities. Current therapies for LV remodeling and HF are predominantly palliative (see Chapter 36 ); they can improve symptoms and prolong life, but they do not address the underlying loss of contractile tissue. Consequently, the prognosis of patients with ischemic heart disease and HF remains bleak. Cell therapy offers a novel strategy that has the potential to reconstitute dead myocardium, and for the first time, reverse the fundamental cause of HF rather than merely delay its progression.
Cell Types Used for Cell Therapy in Myocardial Infarction
Embryonic Stem Cells
Pluripotent stem cells, that is, cells that have the ability to differentiate into tissues derived from all three germ layers (ectoderm, endoderm, and mesoderm), include embryonic stem cells (ESCs), which are harvested from the inner cell mass of preimplantation-stage blastocysts, and induced pluripotent stem cells (iPSCs), which are embryonic-like stem cells derived from adult cells. The functional components of the human heart (cardiomyocytes, endothelial cells, and smooth muscle cells) are of mesodermal origin.
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.
Induced Pluripotent Stem Cells
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.
Bone Marrow Mononuclear Cells
Most clinical trials of cell therapy in the setting of acute MI have been performed using BMMNCs ( Table 22-1 ). An advantage of these cells includes not needing to be cultured, and thus, they can be delivered quickly and relatively inexpensively. In acute MI, BMMNCs have usually been infused intracoronarily because of the concern that transendocardial injection into freshly infarcted myocardium might result in complications (e.g., arrhythmias or perforation of the LV wall). However, subsequent experience with transendocardial injection of BMMNCs at an average of 10 days after MI has not corroborated these concerns. The safety of this delivery modality has not been tested in the first week after MI.
| Study Name/First Author (year) | Design | Patient Number | Cell Type/Dose | Route of Injection | Imaging Modalities | Timing from MI to Cell Delivery | Follow-up/Results |
|---|---|---|---|---|---|---|---|
| BOOST Wollert (2004) Meyer (2006, 2009) | RCT | Treated: 30 Control: 30 | Nucleated BM cells 24.6 ± 0.94 × 10 8 | IC | CMR Echo | 4–6 days | 6 mos: Improvement in EF 18 mos: No improvement in EF, LV volumes, and RWM 5 yrs: No improvement in EF, LV volumes, infarct size, and RWM |
| Janssens (2006) | RDBCT | Treated: 33 Placebo: 34 | Nucleated BM cells 3 ± 1.28 × 10 8 containing 1.72 ± 0.72 × 10 8 BMMNCs | IC | CMR | 1–2 days | 4 mos: No improvement in EF; ↓infarct size, ↑RWM 12 mos: No improvement in EF, LV volumes, and infarct size; ↑ RWM |
| ASTAMI Lunde (2006, 2008) Beitnes (2009) | RCT | Treated: 47 Controls: 50 | BMMNCs 0.7 × 10 8 (0.54 × 10 8 to 1.3 × 10 8 ) | IC | SPECT Echo CMR | 4–8 days | 6 mos: No improvement in EF, infarct size, and LV volumes 12 mos: No improvement in EF, LV volumes, and RWM 3 yrs: No improvement in EF, LV volumes, LV mass, infarct size, and RWM |
| REPAIR-AMI Schächinger (2006) Assmus (2010, 2014) | RDBCT | Treated: 101 Placebo: 98 | BMMNCs 2.36 ± 1.74 × 10 8 | IC | LV angiography | 3–6 days | 4 mos: Improvement in EF, ESV and RWM 1 yr: ↓MACE, ↑RWM, 2 yrs: ↓MACE, ↑RWM, ↓infarct size, no improvement in EF and LV volumes 5 yrs: ↓MACE |
| FINCELL Huikiri (2008) | RDBCT | Treated: 40 Placebo: 40 | BMMNCs 4.02 ± 1.96 ×10 6 | IC | Echo LV angiography | 2–6 days | 6 mos: No improvement in EF; improved ΔEF |
| REGENT Tendera (2009) | RCT | Treated: 160 Controls: 40 | BMMNCs 1.78 × 10 8 CD34 + /CXCR4 + 1.9 × 10 6 | IC | CMR LV angiography | 3–12 days | 6 mos: No improvement in EF and LV volumes |
| Hare (2009) | RDBCT | Treated: 34 Placebo: 19 | Allogeneic BM MSCs 0.5, 1.6, 5 × 10 6 /kg | IV | Echo CMR | 1–10 days | 6 mos: No improvement EF |
| TIME Traverse (2012) | RDBCT | Treated: 80 Placebo: 40 | BMMNCs 1.5 × 10 8 | IC | CMR | 3–7 days | 6 mos: No improvement in EF, LV volumes, RWM, and infarct size |
| LateTIME Traverse (2011) | RDBCT | Treated: 58 Placebo: 29 | BMMNCs 1.5 × 10 8 | IC | CMR | 15–20 days | 6 mos: No improvement in EF, LV volumes, RWM, and infarct size |
| SWISS-AMI Surder (2013) | RCT | Treated early: 60 Treated late: 58 Controls: 49 | BMMNCs 1.4–1.6 × 10 8 | IC | CMR | 5–7 days 3–4 wks | 4 mos: No improvement in EF, LV volumes, and scar mass |
Clinical Trials
Since the initial report of cell therapy in patients with acute MI, many phase I and some phase II clinical trials have been performed using BM cells in patients with ST-elevation MI (STEMI). The preponderance of these investigations have tested unfractionated BMMNCs, with only a small number of studies using selected BM populations, such as CD34 + cells, CD133 + cells, and MSCs. The results have been inconsistent (see Table 22-1 ).
In the BOOST trial, 60 patients with STEMI were randomized 4 to 6 days after MI to receive intracoronary BM cells (which included, but were not limited to, BMMNCs that were depleted of erythrocytes and platelets) or no treatment. At 6 months, there was a statistically significant improvement in global and regional LVEF and border zone wall motion (measured by cardiac magnetic resonance imaging [CMR]) in the treated group compared with the control group, although there was no improvement in LV volumes. However, despite this early benefit, the difference between the control and treated groups was no longer significant at 18 months and 5 years, primarily because of an improvement in the control group.
REPAIR-AMI
REPAIR-AMI, a double-blind, randomized controlled trial (RCT), is the largest study of cell therapy in acute MI performed so far. Two hundred four patients with STEMI were assigned to intracoronary infusion of BMMNCs or placebo 3 to 6 days after successful percutaneous coronary intervention (PCI). At 4 months, there was a statistically significant improvement in LVEF, as measured by contrast ventriculography, in the patients in the cell group compared with those in the placebo group. Furthermore, at 1 year, the number of major adverse cardiac events (MACE) (death and recurrent MI) was significantly reduced by cell treatment. However, the CMR substudy, which included 54 of the 204 patients, did not show any difference between the treated and placebo groups with regard to LVEF at 4 or 12 months. In another substudy of 58 patients, coronary flow reserve in the infarct-related artery became normal in the treated group at 4 months, suggesting that intracoronary BMMNC therapy may restore microvascular function.
All of the composite clinical endpoints remained significantly improved in the treated cohort at 2 years ; at the 5-year follow-up, only the composite of death, recurrent MI, and any revascularization remained significantly different between the treated and placebo groups, which was driven mainly by revascularization. Further analysis by CMR suggested that BMMNC therapy attenuated adverse LV remodeling, as demonstrated by a decrease in end-diastolic wall thickness in the infarcted and remote regions, and an increase in regional contractility. However, LV volumes were not significantly affected by cell therapy.
REPAIR-AMI was a landmark trial in cell therapy not only because of its size and design (randomized, double-blinded, and placebo-controlled with BM aspiration and sham cell infusion in control subjects), but also because it provided important insights that have had a profound influence on clinical research in this field. The REPAIR-AMI investigators made observations that were adopted in the design of most subsequent trials. In a prespecified subgroup analysis, they noted that the benefits of BMMNC therapy were confined to patients with LVEF at or below the median value of 48.9%. At 4 months, in the subset of patients with LVEF ≤48.9%, the increase in LVEF affected by BMMNC therapy was approximately 5 EF units (95% confidence interval [CI], 2.0 to 8.1) greater than that in the placebo group, whereas in the subset of patients with LVEF greater than 48.9%, the increase relative to placebo was only 0.3 unit (95% CI, −2.2 to 2.8). This dichotomy is not difficult to rationalize; if LV function is normal or near normal, it seems unlikely that cell therapy (or any therapy) could effect a significant improvement. The inverse relation between baseline LV function and magnitude of therapeutic effect reported in REPAIR-AMI formed the basis for the design of subsequent trials of cell therapy in acute MI, which have almost invariably excluded patients with normal or near-normal LV function. In these subjects, not only is the therapeutic effect likely to be small, but the need for cell therapy is less obvious. Conversely, in patients with large MIs (particularly those with anterior MIs), there is a clearer rationale for cell therapy, and the benefits are likely to be greater. Thus, in general, clinical trials performed after REPAIR-AMI have enrolled patients with baseline LVEF of ≤45%.
The other important insight garnered in REPAIR-AMI was the interaction between improvement in LVEF and time of cell infusion. Specifically, the beneficial effects of BMMNC infusion on the recovery of contractile function were observed only in patients who were treated ≥5 days after PCI, suggesting that early (within 4 days) administration of cell therapy may be ineffective, possibly because of the intense inflammatory response and the hostile environment that it creates for transplanted cells. However, this concept has not been supported by later studies, as detailed in the following.
Other Randomized Trials of Bone Marrow Mononuclear Cells
In contrast to REPAIR-AMI, two other contemporaneous RCTs reported no benefit from BMMNCs in acute MI. In the open-label ASTAMI trial, 100 patients with STEMI were randomized to BMMNCs or control (no treatment) at 4 to 8 days after MI. The two groups did not differ with respect to LVEF, LV end-diastolic volume, or infarct size at 6 months (as measured by single-photon emission computed tomography, echocardiography, and CMR) and 1 year (as measured by echocardiography). Long-term follow-up at 3 years also failed to demonstrate any benefit with respect to LVEF, LV volumes, infarct size, and wall motion, as evaluated both by CMR and echocardiography. Janssens and colleagues randomized 67 patients with STEMI to BMMNCs or placebo early (1 day) after PCI and measured outcome 4 months later using CMR. There was no improvement in LVEF, although infarct size and regional LV function improved in the cell therapy group. Similarly, at 1 year, LVEF and LV volumes did not differ between the two groups, although the favorable effects of cell therapy on selected LV remodeling parameters remained, such as wall motion in the infarct border zone and regional contraction in segments with transmural hyperenhancement (the early difference in infarct size was no longer significant at 1 year).
In addition, four other phase II RCTs examined the effect of BMMNCs in the context of acute MI and yielded negative results (see Table 22-1 ). In the multicenter open-label HEBE trial, 200 patients with a first large MI, baseline EF ≤45%, and successful PCI were assigned to intracoronary BMMNCs, peripheral blood mononuclear cells, or no treatment (control subjects) in addition to standard therapy 3 to 8 days after successful reperfusion. At 4 months, there was no difference among the groups with respect to LVEF (assessed by CMR), LV volumes, LV mass, infarct size, or clinical events. Long-term follow-up at 2 years showed no difference among the groups with regard to LVEF (evaluated by CMR); interestingly, the authors reported that the composite endpoint of death or recurrent MI was significantly more frequent in the peripheral blood mononuclear cell group at 5 years. In the National Heart, Lung, and Blood Institute Cardiovascular Cell Therapy Research Network (CCTRN) TIME trial, 120 patients with STEMI, LVEF ≤45%, and successful reperfusion, were randomized 2:1 to early intracoronary BMMNCs or placebo at either 3 or 7 days after acute MI. The purpose of this design was to verify the observation made earlier in REPAIR-AMI that BMMNC therapy was only helpful if delivered 5 days or more after MI. At the 6-month follow-up, there was no significant effect of cell therapy on recovery of global or regional LV function or LV volumes, as measured by CMR. Furthermore, no difference was observed between the patients treated at 3 and 7 days after MI. The CCTRN Late-TIME trial was designed to investigate the effects of BMMNCs administered 2 to 3 weeks after AMI; in this study, 87 patients with STEMI and LVEF ≤45% were assigned, in a 2:1 fashion, to intracoronary BMMNCs or placebo. The hypothesis was that the inflammation associated with acutely infarcted myocardium produces a hostile environment that is toxic to the cells, and that allowing the inflammation to subside may lead to greater cell survival and larger therapeutic effects. Again, the investigators found no improvement in global or regional LV function or LV volumes measured by CMR at 6 months. A combined analysis of all patients enrolled in TIME and Late-TIME was also neutral ; when this combined data set was examined for the effects of age, baseline LVEF, and time from PCI to infusion, only baseline LVEF was found to be significantly associated with changes in the LVEF (patients with lower LVEF exhibited greater increases in LVEF and vice versa). This relationship was observed regardless of treatment. Finally, in the SWISS-AMI trial, 200 patients with successfully reperfused STEMI were randomized 1:1:1 to an open-label control (no treatment) or two intracoronary BMMNC treatment groups (early, 5 to 7 days, and late, 3 to 4 weeks after MI). This trial was also designed to investigate the effects of early versus late administration of BMMNCs. Again, the treatment did not produce any improvement in LVEF, scar mass, or LV volumes at 4 months, as measured by CMR.
Reconciling Discordant Results from Clinical Trials
Multiple potential reasons have been proposed to explain the conflicting results of BMMNC trials in acute MI. A major problem with cell therapy is that the properties of the cell product may vary enormously from one study to another, and from one laboratory to another, depending on the specific protocol used to isolate and expand the cells. Even seemingly meaningless (and thus unrecognized) details can have a profound effect on the viability and potency of the cell product. For example, although all trials used density gradient centrifugation to separate mononuclear cells from the BM aspirate, there were significant differences in the protocols that could have resulted in differences in the composition and quality of the cell product (e.g., the percent of progenitor cells such as CD34 + and CD133 + cells or the functional competence of the cells). A recent retrospective analysis of TIME suggested that the content of CD31 + cells in the BM injectate was an important determinant of LV functional recovery. Along these lines, the discrepancy between REPAIR-AMI and ASTAMI has been attributed to the efficiency of the cell isolation protocol, which resulted in different degrees of cell viability. By comparing the two protocols, the REPAIR-AMI investigators concluded that the protocol used in ASTAMI resulted in lower cell recovery, a lower number of hematopoietic, endothelial, and mesenchymal colony-forming units (CFUs), significantly reduced migration capacity of the cells in response to the chemotactic stromal-derived factor-1 (SDF-1), and abolished capacity of the cells to promote neovascularization in an experimental hind limb ischemia model. However, the investigators in the negative HEBE trial validated the cell isolation protocol against the one used in REPAIR-AMI before enrolling the first patient and demonstrated that the quantity and quality of the cells were similar in both studies. Similarly, the CCTRN investigators argued that such concerns were not warranted. Going forward, it is crucial that trials of cell therapy incorporate appropriate assays to verify the viability and functional competence of the cell product under investigation.
Another potential explanation for the discrepancies is erythrocyte contamination of the BM cell product. In an exploratory analysis of the injectate in the REPAIR-AMI cohort, the number of red blood cells (RBCs) contaminating the final cell product was significantly correlated with reduced recovery of LVEF 4 months after BMMNC therapy. Higher numbers of RBCs in the BMMNC preparation were associated with reduced cell viability, CFU capacity, and migratory capacity in vitro; furthermore, neovascularization capacity was significantly impaired in vivo in a murine model of hind limb ischemia after infusion of BMMNCs contaminated with RBCs compared with BMMNCs alone. An additional variable that has been proposed to contribute to the discrepancies is the use of heparin. The REPAIR-AMI investigators found that heparin profoundly and dose-dependently inhibits SDF-1–induced BMMNC migration in vitro, and that pretreatment of BMMNCs with heparin significantly reduces the homing of the injected cells in a murine ear-wound model. However, it is doubtful that the RBC and heparin issues account for the inconsistent results of clinical trials. For example, in the CCTRN TIME trial, the cell product was devoid of significant RBC contamination and contained only minuscule amounts of heparin, and in the SWISS-AMI trial, no heparin was used in the final product. Nonetheless, both trials were null.
A factor that may have contributed to the negative results of the CCTRN TIME and Late-TIME trials is that the enrollment of patients was based upon the LVEF measured in the qualifying echocardiograms, which were obtained earlier after PCI than the baseline CMRs. In the TIME trial, LVEF averaged approximately 37% in the qualifying echocardiograms, but they increased to approximately 45% in the baseline CMR; in the Late-TIME trial, these values were approximately 36% and approximately 48%, respectively. In patients with baseline LVEF of approximately 45% to 48%, the benefits of cell therapy would be expected to be attenuated. However, SWISS-AMI did not experience this limitation (the baseline LVEF averaged 37.4% at an average of 6 days after MI) and was still negative. Finally, the possibility that the preceding trials had insufficient statistical power cannot be excluded. For example, the HEBE and SWISS-AMI trials were powered to detect a 6.0- and 3.5-unit improvement in LVEF, respectively ; a smaller effect would likely have been missed.
Meta-Analyses of Bone Marrow Cell Therapy
Many of the individual trials in acute MI were limited by their size and/or design. Therefore, in the absence of phase III trials, no definitive conclusions can be made regarding the effects of cell therapy in this setting. At present, the best available evidence comes from meta-analyses. In a collaborative meta-analysis, Delewi and colleagues pooled the data from 16 RCTs that had enrolled more than 30 patients in the BM arm, reaching a total of 1494 patients. At 3 to 6 months, LVEF improved by 2.55 units among BM cell–treated patients compared with control subjects ( P <.001) ( Figure 22-2 ). Moreover, cell therapy significantly reduced end-systolic and end-diastolic LV volumes. In subgroup analyses, the benefit of treatment on LVEF was more pronounced in younger patients compared with older ones, and in patients with baseline LVEF of less than 40% compared with those with LVEF ≥40%. There was also a trend in favor of patients treated ≥7 days after primary PCI compared with patients treated at less than 7 days and in patients who received more than 10 8 BMMNCs compared with ≥10 8 cells (see Figure 22-2 ).
Another meta-analysis that combined the results of 22 RCTs involving 1513 patients arrived at similar conclusions; the overall improvement in LVEF at 6 months and at 6 to 18 months after cell therapy was 2.10 and 3.04 units, respectively ( P = .004 and P = .0008, respectively) ( Fig. 22-e1 ). In addition, this study pooled the clinical endpoints and reported no beneficial effects after a median follow-up of 6 months. Contrary to the meta-analysis by Delewi and colleagues, however, the subsets of patients who were treated less than 8 days after PCI benefited more than the patients treated at ≥8 days ( P = .009). However, both of these studies failed to detect a beneficial effect of cell therapy on LVEF when the analysis was restricted to trials that used CMR for outcome assessment, an important finding that has been echoed by other meta-analyses. Of note, all of the major trials that used CMR as the method for LVEF quantification have been negative.
In contrast to these group-based meta-analyses, an individual patient data (IPD)–based meta-analysis provided sobering conclusions. ACCRUE is an ongoing collaborative database that includes IPDs from randomized and cohort studies of cell therapy in patients with ischemic heart disease. The investigators compiled 12 studies involving 1085 patients, including 2 studies that used cell types other than BMMNCs, although most patients received BMMNCs. Importantly, the availability of IPDs enabled them to perform an intention-to-treat analysis. Using this approach, at a median of 6 months after MI (range, 3 to12 months), no effect of cell therapy was identified with the combined endpoint of all-cause death, recurrent MI, stroke, and target vessel revascularization (14.0% in cell-treated patients vs. 16.3% in control subjects; hazard ratio, 0.86; 95% CI, 0.63 to 1.18), on death (1.4% vs. 2.1%), or on the composite endpoint of death, recurrent MI, and stroke (2.9% vs. 4.7%), nor were any difference in LVEF observed in comparison with control subjects (mean difference between treated and control subjects: 0.96%, 95% CI, −0.2 to 2.1) ( Figure 22-3 ). Subgroup analyses did not reveal any interaction between improvement in EF on one hand and baseline LVEF, use of CMR for EF quantification, and time from reperfusion to cell therapy on the other hand.
