Cardiologists battle our nation’s number one killer: heart disease. Bone marrow transplant and cancer have their own challenges and carry grave connotations, but cardiovascular (CV) disease causes most of the deaths and disabilities in the US, claiming nearly one million lives each year.1 The cost of cardiac care is estimated to approach $400 billion per year and, with an aging US population and epidemics in diabetes, metabolic syndrome, and obesity, the incidence of CV diseases will increase.2 Moreover, with improved survival rates after acute ischemic syndromes, the number of patients reaching the later stages of ischemic heart disease (e.g. left ventricular dysfunction, heart failure, arrhythmias, and premature death) will also increase.
Important strides have been made in optimizing medical management with antiplatelet agents, beta-blockers, statins, renin–angiotensin inhibitors, aldosterone inhibitors, blood-pressure reduction, and smoking cessation. However, additional pharmacotherapeutic manipulations are likely to bring only modest increases in benefit, as reflected by the pharmaceutical industry’s decreasing emphasis on this area. These considerations, plus the well-recognized limitations in mechanical hearts, heart transplantation, implantable defibrillators, restriction devices, and mitral valve repair, have spurred interest in novel approaches for left ventricular dysfunction.
Pre-clinical Evidence of Cell Therapy for Cardiovascular Disease
Numerous animal studies have demonstrated that transplantation of bone-marrow-derived cells improves cardiac function in settings of acute myocardial infarction (MI) and ischemic cardiomyopathy.3 Whole bone marrow (autologous and allogeneic), CD34+ cells, CD133+ cells, mesenchymal stem cells (MSC) (autologous and allogeneic), and endothelial progenitor cells represent several transplanted marrow elements. Moreover, various marrow-to-myocardium transplant techniques have been used, including intracoronary infusions, intraventricular myocardial injections, and extraventricular myocardial injections, via a surgical approach. Each technique has resulted in benefit to some degree.
Where the situation gets controversial is mechanism. How do marrowderived cells assist in myocardial regeneration? Initially, there was intense interest in direct transdifferentiation of hematopoietic cells into myocardium. Can adult marrow cells turn into heart muscle? Studies by Orlic et al.4 and Yoon et al.5 found evidence of bone marrow plasticity in rodent hearts, but further testing failed to show transdifferentiation of adult marrow cells into cardiomyocytes.6–8 A second possibility is paracrine regulation. Do marrow cells secrete cytokines that promote angiogenesis, quell myocardial inflammation, and/or prevent myocyte apoptosis? Certain elements of the marrow, namely MSCs, are most likely to work by this mechanism.5,9–11 Alternatively, do marrow cells lead to better myocardial perfusion by providing vasculogenic precursors? Indeed, in animal studies hematopoietic cell implantation into the heart contributes to myocardial neovascularization, which is not surprising given the hemangioblast activity of adult hematopoietic stem/progenitor cells.12–15
Together, successful pre-clinical studies and the need for novel approaches to improve left ventricular dysfunction have paved the way for early clinical trials.
Cell Mobilization Trials for Cardiovascular Disease
A straightforward method of increasing concentrations of circulating marrow stem and progenitor cells is to administer leukocyte-mobilizing factors. Bone marrow transplant physicians routinely use granulocyte colony-stimulating factor (G-CSF) to mobilize hematopoietic cell precursors. Moreover, G-CSF is thought to have direct cardioprotective effects in infarcted hearts via STAT3 and PI3K/AKT pathways.16 Thus, G-CSF was used in early CV cell therapy trials.
Two of the largest trials to date include Front-Integrated Revascularization and STem cell Liberation IN Evolving Acute Myocardial Infarction (FIRSTLINE-AMI) and REgenerate VItal myocardium by Vigorous Activation of bone marrow stem cells (REVIVAL-2). In FIRSTLINE-AMI, 50 acute MI patients were randomized in an open-label fashion to receive standard care plus or minus G-CSF 10mcg/kg subcutaneously for six days after percutaneous coronary intervention (PCI) stenting.17 In the G-CSF-treated group, CD34+ cells increased 15-fold compared with controls at day six. Regarding safety, there was no evidence of increased blood viscosity, increased inflammatory reactions, or an accelerated rate of in-stent restenosis compared with controls. At four-month follow-up, the treatment group demonstrated improved left ventricular function based on resting echocardiography, dobutamine stress test, and fluorodeoxyglucose positron emission tomography imaging.
In contrast, REVIVAL-2 demonstrated no improvement with G-CSF.18 In REVIVAL-2, 114 acute MI patients were randomized to receive standard care plus or minus G-CSF 10mcg/kg or placebo subcutaneously for five days after PCI stenting. CD34 cell mobilization and safety end-points were similar to those in the FIRSTLINE-AMI trial. However, in REVIVAL-2, after four to six months of follow-up, no improvements in left ventricular function were evident based on angiography, cardiac magnetic resonance imaging (MRI), and technetium-99 sestamibi scan. Differences between the trials may be the result of trial design (open-label versus placebo-controlled), number of patients, and/or imaging techniques used in follow-up (sensitivity and variability).
Based on results from the Myocardial regeneration and Angiogenesis in myocardial infarction with G-csf and Intra-Coronary stem cell infusion (MAGIC) trial, concerns were raised about the safety of delivering G-CSF to patients with acute MI.19 In the MAGIC trial, in-stent restenosis rates were much higher in patients treated with G-CSF only, prompting early closure of the trial. However, a recent meta-analysis of 106 acute MI patients receiving G-CSF found no evidence of accelerated restenosis rates.20 Possible explanations for the restenoses seen in the MAGIC trial may include the low number of patients (n=10) and trial design. (G-CSF given four days prior to PCI may have exacerbated intracoronary inflammation.)
Marrow Cell Injection Trials for Cardiovascular Disease
Numerous cohort studies and randomized clinical trials favor marrow cell injections for the treatment of ischemic heart disease.21 Although in aggregate a benefit is apparent, the individual trials demonstrate modest improvements. Specifically, major randomized clinical trials using intracoronary delivery of bone marrow mononuclear cells (BMMNCs) after acute MI have demonstrated improvements in left ventricular ejection fraction of only 6–8%.22–25 Moreover, heart function improvements compared with control patients may be transient, peaking at early followup (six months) and then disappearing later (18 months).26 When scrutinizing the results, however, the disappearance of improvement appears to be the result of late improvements in the control patients, in the end suggesting that cell therapy may bring about quicker improvement in left ventricular function post-MI.
In chronic ischemic heart disease, injection of marrow-derived cells also results in modest improvements in left ventricular function.21 For chronic ischemic heart disease, intramyocardial injections have been performed using electromechanical mapping via catheter-based delivery27 and a surgical approach during coronary artery bypass grafting.28 Interestingly, patients with the most severe left ventricular dysfunction seem to have the most improvement with marrow cell therapy.29 The most ill seem to benefit the most.
In the nascent field of regenerative medicine for CV disease, many questions remain about issues such as cell type, cell dose, timing of injections and which patients most benefit.
Regarding cell type, Osiris Therapeutics, Inc. is currently conducting a phase I trial of ex vivo expanded adult allogeneic mesenchymal stem cells (MSCs) in the treatment of acute MI. MSCs are injected intravenously within 10 days of a heart attack. Preliminary results on short-term follow-up indicate significant improvements in left ventricular ejection fraction. This trial is closed to accrual and in longitudinal follow-up (ClinicalTrials.gov identifier NCT00114452).
Based on earlier cell dose-escalation studies demonstrating a reduction of chest pain after intramyocardial injections of autologous CD34+ cells,30 Baxter Healthcare Corporation is currently conducting a randomized, double-blind, placebo-controlled, phase II study to determine the tolerability, efficacy, safety, and dose range of intramyocardial injections of G-CSF-mobilized autologous CD34+ cells for the reduction of angina episodes in patients with refractory chronic myocardial ischemia (ACT34- CMI). G-CSF is administered at 10mcg/kg subcutaneously for five days. Apheresis is performed on day five and CD34+ cell selection is performed using Baxter’s Isolex™ unit. This trial is currently recruiting subjects (ClinicalTrials.gov identifier NCT00300053).
National Institutes of Health National Heart, Lung, and Blood Institute Cardiovascular Cell Therapy Research Network
Recently, the National Institutes of Health (NIH) National Heart, Lung, and Blood Institute (NHLBI) established a network of sites actively engaged in cell therapy to rapidly translate CV cell therapy into the clinical arena (http://www.cctrn.org). The primary goal of the network is to conduct multiple collaborative proof-of-concept cell therapy clinical protocols to improve treatment of CV diseases. To achieve this goal, five centers were selected: the University of Florida, Texas Heart Institute, the University of Minnesota, Cleveland Clinic, and Vanderbilt University Medical Center. The Cardiovascular Cell Therapy Research Unit (CCTRN) has initiated three protocols: TIME, Late TIME, and FOCUS. Initiation of trials is planned for February 2008.
The primary hypothesis of the TIME trials is that the administration of cell therapy will improve global and regional left ventricular function after acute MI, and that the benefit of cell therapy will depend on the timing of cell delivery. These trials are randomized, double-blind, two-by-two factorial, placebo-controlled clinical trials that will assess the effect of cell therapy and the timing of that delivery on global and regional left ventricular function determined by cardiac MRI. The target population includes subjects who have moderate to large anterior infarctions, no prior history of coronary artery bypass graft or MI that resulted in left ventricular dysfunction, and initial ejection fraction <45%.
The TIME trials will use autologous BMMNCs harvested via aspiration. BMMNCs will be separated using a Sepax™ unit. The TIME trial will test the difference of intra-coronary marrow cell injections at three days post-MI versus seven days post-MI versus placebo. The Late TIME trial will test the difference of intra-coronary marrow cell injections at two to three weeks post-MI versus placebo. Primary end-points include global ejection fraction and regional left ventricular function as measured by cardiac MRI.
The hypothesis of FOCUS is that BMMNCs injected directly into reversibly injured areas of myocardium in patients with chronic, severe ischemic left ventricular dysfunction (LVEF 45%), and either limiting angina (class II to IV) or heart failure (New York Heart Association class II to III), will improve in terms of myocardial perfusion, regional ventricular function, and clinical symptoms.
FOCUS is a randomized, double-blind, placebo-controlled study evaluating the safety and efficacy of autologous bone-marrow-derived mononuclear cell injection patients with coronary heart disease and chronic ischemic left ventricular dysfunction and/or symptomatic angina, heart failure, or both who are not eligible for other revascularization procedures. FOCUS will use autologous BMMNCs harvested via aspiration. BMMNCs will be separated using a Sepax™ unit. Cells will be delivered by a percutaneously inserted catheter using direct endomyocardial injections with electromechanical mapping. Primary end-points at six months include exercise capacity evaluated by maximal oxygen consumption, left ventricular end systolic volume assessed by echocardiography with contrast, and myocardial perfusion assessed by single-photon-emission computed tomography.
Importantly, the immediate goals of the CCTRN are to successfully define optimal timing of injections and best patient populations for BMMNC-based cell therapy clinical trials. Future trials will build on these early experiences by administering enhancing agents to promote BMMNC paracrine regulation and neovasculogenesis. In addition, it is expected that the next generation of CCTRN trials will rapidly test novel cell types such as gene-modified cells. ■