According to the Centers for Disease Control, approximately one million heart attacks (Myocardial Infarction, or MI) occur per year in the U.S. . Loss of functioning heart muscle due to an MI can result in congestive heart failure which is the leading cause of death and disability inAmerica. It is estimated that about half of those with congestive heart failure die within five years of diagnosis . New therapies are desperately needed to treat and prevent the clinical complications that follow a heart attack. Unfortunately, the heart muscles (cardiomyocytes) do not ordinarily regenerate once damage has occurred by MI and thus many researchers are looking into stem cell therapies for heart failure and disease.
The widely accepted view that the heart was not capable of regeneration was established in 1925 by Karsner et al. who demonstrated that the heart grew larger due to cardiac hypertrophy (increase in cell size) as opposed to hyperplasia (increase in cell number) . However, in 2009, Bergmann et al. demonstrated that the human heart was capable of self-renewal by measuring the age of cardiomyocytes in individuals exposed to carbon-14 generated by nuclear bomb tests during the Cold War . The work of Hsieh et al. showed that adult heart regeneration was due to an actual subset of progenitor cells as opposed to increased proliferation of resident cardiomyocytes . This group used an α-myosin heavy chain Cre-Lox transgenic mouse model (Mer-CreMer-ZEG mouse), where 100% of the cardiomyocytes are b-galactosidase (b-gal) positive but activation of Cre recombinase by tamoxifen results in 80% EGFP (Enhanced Green Fluorescent Protein) positive cardiomyocytes and 20% b-gal positive cardiomyocytes. After myocardial infarction there was a 15% increase in b-gal positive cardiomyocytes indicating that cardiomyocytes were being generated from stem/ progenitor cells, which were never exposed to Cre recombinase. Thus, while there is insufficient capacity of mammalian adult heart tissue to undergo self-repair, injured myocardium can increase cardiomyocyte generation from non-myocyte precursors.
While protocols aimed to mobilize endogenous stem/ progenitor cells to sites of injury or disease would be greatly beneficial to cardiac patients, transplantation of autologous or allogeneic cells is also a promising alternative. Cell therapies may result in phenotypic replacement of heart cells, reduce scar formation, increase survival and function of endogenous cardiac cells by increasing in vivo bioavailability of growth factors, and exert immunomodulation by releasing soluble molecules and express immune-relevant receptors (chemokine receptors and CAMs (cellular adhesion markers)). The first clinical trial using a stem cell therapy for cardiac disease was performed over a decade ago using autologous bone marrow cells (BMCs) in patients after acute MI which reported improved heart function and myocardial perfusion . Since then, numerous adult stem cell therapies targeting cardiac disease have been launched with the majority of therapies involving BMCs .
Recently, a phase I clinical trial based on administration of autologous cardiosphere-derived cells (CDCs) to assess safety in patients with left ventricular dysfunction after MI was completed by Dr. Eduardo Marban atCedars-SinaiMedicalCenter. The formation of cardiospheres was first described by Messina’s group who showed that human and mouse heart explants generated a layer of fibroblast-like cells over which small, phase-bright cells migrated, and once these phase-bright cells are transferred to non-adherent plates, they generate three-dimensional spherical structures . Marban’s group modified Messina’s protocol by placing the cardiospheres in adherent plates where the cells begin to grow in monolayer, hence cardiosphere-derived cells, allowing for easier and faster expansion. Promising preclinical data which showed a reduction in infarct size and improved cardiac function after transplantation of CDCs in a porcine animal model prompted the phase I clinical trial .
This trial, known as CADUCEUS (CArdiosphere-Derived aUtologous stem CElls to reverse ventricUlar dySfunction), enrolled patients 2-4 weeks after MI (with left ventricular ejection of 25-45%) who received either standard of care (no intervention, usual medical management after MI), low cell dose (12.5 million) or high cell dose (25 million) of autologous CDCs (ClinicalTrials.gov NCT00893360). Percutaneous endomyocardial biopsies were used to obtain the heart tissue necessary for generation of the predetermined dose of autologous CDCs (usually within 36 days of sampling) and cells were delivered through an angioplasty catheter in the infarct-related artery. Results from the trial showed that the CDCs were relatively safe: no patients died, developed cardiac tumors or experienced a major adverse cardiac event (MACE). Preliminary efficacy suggested that patients treated with CDCs had reductions in scar mass and increases in viable heart mass as measured by magnetic resonance imaging (MRI), as well as increases in regional contractility and systolic wall thickening at 12 months compared to controls . However, the trial did not confirm whether there was true increased myocardial mass, whether the CDCs themselves could possibly distort the myocardial architecture and/or how the CDCs potentially could regenerate the injured heart.
Recently Marban’s group has shed light on the potential mechanism of how the CDC therapy reduces scarring and regenerates healthy tissue after an MI, published in EMBO Molecular Medicine . They applied lineage tracing systems using the Mer-CreMer-ZEG mouse with additional labeling tools to investigate the cellular origins of regeneration in an adult mouse after surgical induction of MI followed by intramyocardial injection of mouse CDCs. Impressive results revealed that after MI new cardiomyocytes arise from both pre-existing cardiomyocytes and undifferentiated progenitors and that transplanted CDCs further up-regulate host cardiomyocyte proliferation and recruitment of endogenous progenitors to the infarct site. The increase in cardiomyocyte proliferation and progenitor recruitment was accompanied by structural and functional changes in the infracted heart, specifically decreased scar size, increased infracted wall thickness and myocardium (cardiomyocyte hypertrophy was excluded), and increased cardiac function. Interestingly, these effects occurred despite robust CDC engraftment indicating that CDCs do not contribute to phenotypic replacement but promote heart regeneration by indirect means. These promising findings indicate that stem cell therapies may stimulate dormant surviving cells after injury and boost natural repair mechanisms.
However, there still remain many challenges for developing cell therapies for cardiac disease. First, the technology is still immature. With autologous approaches, patient-to-patient variability can arise such as purity and potency of the cells. Harvesting the source material, obtaining a sufficient quantity of cells, delivery of the end product and the optimal site of delivery will need to be established. In the future, Marban’s group plans on using allogeneic CDCs obtained from donor organs, which allows for an off-the-shelf-therapy, to treat patients following MI. A thorough cell biological characterization of the CDCs will be required to understand the molecular identity and mechanism(s) of action of these cells. Second, the underlying mechanism(s) of cardiac repair and/ or regeneration after MI remain elusive. Additionally, treating chronic or congestive heart failure with the aim of generating new contracting heart muscle is more complex. Third, it is difficult to make conclusive statements about the results obtained from many of the clinical trials since they have small patient size and therefore limited statistical power. It is unclear how the findings will generalize to a larger population of patients. Fourth, most of the clinical trials use surrogate endpoints which are measures of an effect of a certain treatment expected to predict clinical benefit (or harm, or lack of benefit or harm) such as measuring cardiac function. Even though surrogate endpoints may correlate with a real clinical endpoint, they do not have a guaranteed relationship. Therefore, clinically meaningful endpoints such as improvement in overall survival and prevention of future heart failure are needed. In the CADUCEUS trial there was only enhanced regional structure/ function but no significant effect on global functional endpoints such as ejection fraction. A phase II double-blinded placebo-controlled clinical trial will need to be performed to assess true efficacy. It will be several more years before we have a clear understanding of the true potential of cell therapy in cardiac disease. In the meantime, studies to understand the cellular sources and underlying cellular mechanisms involved in cardiac regeneration are still needed.
1. CDC. Heart Disease Facts. updated October 16, 2012; Available from: http://www.cdc.gov/heartdisease/facts.htm.
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