Putting the Pieces Together: Stem Cells and the Quest to Heal a Broken Heart
But advances in modern medicine may find the Tin Man singing a different tune, especially when we consider innovations in investigation when it comes to the use of stem cells to reduce infarct size or remuscularize a diseased heart. Indeed, few areas in science and medicine have garnered the kind of attention we focus on stem cell research—attention that probably made our hearts pound faster with the recent awarding of the Nobel Prize in Physiology and Medicine to Shinya Yamanaka, MD, PhD, for his discovery of induced pluripotency.
In CV medicine, stem cells set hearts aflutter primarily for their potential role in cardiac repair for patients with acute and chronic MI. Inspired by successes in hematopoietic stem cell transplantation for blood diseases, CV scientists postulated early on that cardiomyocyte loss from myocardial injuries may benefit from transplantation of stem/progenitor cells derived from various autologous sources.
The ensuing stem cell transplantation studies centered on assessing the ability of stem cells to reduce myocardial damage and/or create new muscle cells in the infarcted area before fibrotic changes take root. We know that the fibrotic changes can occur despite reperfusion and pharmacological therapy, and that lost cardiomyocytes rarely regenerate, leading to abundant scar formation. These scars can impact long-term patient outcome due to their effects on remodeling of the heart and the induction of arrhythmias. So finding the right key to unlock the heart's healing potential becomes even more critical.
Do Go Changing to Try and Please Me
As Billy Joel crooned and we have all repeated to our soul mates and children, "Don't go changing, I love you just the way you are." When it comes to stem cells, the fact that they do change is at the heart of their appeal in CV and other medical specialties. In and of themselves, stem cells are unspecialized, but they offer the unique promise of giving rise to specialized cell types.
Although unspecialized, stem cells are not merely clones of each other, and thus research has tested the ability of different cells in the quest to mend broken hearts in a way no song can do.
One of the earliest stem cell transplant studies in the heart involved the use of skeletal myoblasts obtained from limb muscles and expanded ex vivo before transplantation into an animal model of MI. This initial study appeared highly promising: not only did these skeletal myoblasts engraft into the damaged heart, they expanded and differentiated into striated myofibrils in situ.1
Subsequently, clinical scientists looked to adopt this strategy into human clinical trials. Since skeletal myoblasts can be derived autologously, the need for post-transplantation immunosuppression is eliminated, which helped facilitate the regulatory approval of human studies.
Although a few promising early pilot studies showed injection of these cells into the heart to be feasible and potentially beneficial, they also uncovered a potential heart-stopper that almost severed the relationship: a significant increase in the frequency of malignant arrhythmia in patients receiving these cells. This may be due to the lack of integration of skeletal muscle bundle into existing myocardial fibers. Hence, all subsequent clinical studies required the study patients to be implanted with an internal cardioverter-defibrillator (ICD).
The largest clinical study to date with skeletal myoblast transplantation, the MAGIC trial, enrolled ~180 patients into one of three groups: control treatment, low-dose, or high-dose skeletal myoblast transplantation.2 The primary endpoint of the study was improved left ventricular ejection fraction (LVEF). While investigators found no statistically significant change in LVEF following injection of either low or high doses of skeletal myoblasts, some alterations seen in LV dimension suggest that favorable remodeling may have occurred in the transplanted heart. Given the requirement for ICD implantation and the apparent lack of definitive benefit, the CV stem cell community doesn't really "heart" the use of skeletal myoblast transplantation for myocardial repair.
The Poetry of Hematopoietic Cells
Concurrent with skeletal myoblast transplantation studies, other researchers were mending their broken hearts by stepping out with hematopoietic or bone marrow-derived stem cells. Work in animal models found that these cells may harbor greater developmental plasticity than previous studies suggested, leading to the hypothesis that transplantation of hematopoietic stem cells (HSCs) or bone marrow-derived mononuclear cells (BMNCs) may benefit patients undergoing MIs by their ability to transdifferentiate into cardiomyocytes.
Several meta-analyses have now analyzed the 30-plus randomized clinical studies of BMNC therapy in approximately 2,000 patients. BMNCs appear to provide modest positive effects, through reduction of infarct size, LV function improvement, and/or reduction in LV volumes. Furthermore, these studies show that BMNC transplantation is largely safe with no significant adverse event reported.
However, questions arose when it was shown that few (if any) transplanted cells had survived/been retained within the injured heart at 6 weeks after transplantation. Follow-up animal studies waxed more lyrical, suggesting that a non-cell autonomous effect (i.e., paracrine action) may be present to reconcile the observed lack of engraftment and the demonstrated functional benefit.
For investigators in Germany, the small but significant benefit from the early BMNC studies justified jumping into the relationship in a big way with a phase 3 study with a planned enrollment of 1000+ patients that is currently underway. Some of the important issues that remain to be addressed include:
- the optimal patient population to benefit from BMNC treatment
- the most effective dose to achieve functional benefit after transplantation
- the most efficient delivery route that gives rise to the highest degree of cell retention
- the best timing for the introduction of BMNC post-MI
With regards to timing—always a delicate issue in poetry and relationships—the most recent study from the NHLBI Cardiovascular Cell Therapy Research Network-sponsored trial (TIME) showed no benefit with intracoronary BMNC infusion compared with placebo control when given in a randomized, double-blinded fashion at either 3 or 7 days after PCI in patients with LV dysfunction (LVEF <45%) after an anterior ST-segment elevation MI (STEMI).3 However, a subpopulation of patients from the SWISS-AMI study receiving BMNC at 4.5 hours after acute STEMI appears to benefit from treatment. The next refrain will depend on whether the results from the phase 3 trial will produce different outcomes from the TIME trial and whether certain subgroups of patients with LV dysfunction will see greater benefit with BMNC infusion.
An interesting dilemma has emerged: improvements in preventing prevent acute MIs leads to dwindling numbers of patients eligible for these trials. Instead, in our next wave of clinical trials, we may want to redirect stem cell therapy efforts towards treatment of patients with systolic heart failure from chronic ischemia/infarction.
The Heart Is the Only Hunter
As CV stem cell science continues to progress, the newest heartthrob in clinical trials is the autologous cardiac-derived stem cell. Early studies in mice showed the presence of a rare but expandable c-kit+ cell population from the adult heart that can differentiate into cardiomyocytes in vitro and improve cardiac function in vivo following transplantation.4
Concurrently, cardiac-derived cells that express the marker for stemness in hematopoietic cells such as stem cell antigen (Sca-1) or ABCG-2 have been reported as well. Heterogenous in phenotype, these distinct cell populations have variable capacity to differentiate into cardiomyocytes in vitro and in vivo. When transplanted into animal models of myocardial injury, these cells demonstrated histological appearance of cardiomocyte differentiation by immunofluorescence staining. Improvement in cardiac function by echocardiographic or magnetic resonance imaging (MRI) was reported as well.
Encouraged by these findings in animal studies, two recent clinical trials looked to play matchmaker between patients with a prior history of MI and cardiac stem cell transplantation. The phase 1 SCIPIO study employs c-kit+ cells expanded from right ventricular (RV) biopsy samples that were introduced into the peri-infarct region of patients undergoing coronary artery bypass grafting (CABG).5
The pairing appears to work: The published data show that beyond demonstrated safety, the trial's primary objective, LVEF improved by ~8% over controls. What's more, treated hearts showed better regional wall motion scores and in those who underwent cardiac MRI, scar size had shrunk. But before we plunge too far into this relationship, remember the study was not designed to evaluate LV functional improvement as a primary endpoint, and that the study was performed in an open-label and not blinded fashion.
An important take-away, according to first author Roberto Bolli, MD, University of Louisville, Kentucky, is that the therapy can be applied nearly universally to almost every HF patient, because cells can be harvested with a percutaneous biopsy on an outpatient basis. At AHA.12, he also noted that these undifferentiated cardiac stem cells are capable of undergoing multiple divisions, "so, in principle, these cells can generate millions or even billions of cells once transplanted into the heart," he added.
"Double-dating" with the SCIPIO trial, the CADUCEUS trial examined the effect of transplanting cardiosphere-derived cells (CDCs) into the peri-infarct region of patients with MI.6 Also obtained by biopsy of RV myocardium, these cells were expanded in culture and reported to express c-kit in a minor fraction; they differentiated into cardiomyocytes in animal models of MI.
Unlike SCIPIO, CADUCEUS showed no change in LVEF by MRI assessment in patients receiving cell transplantation; however, patients demonstrated some improvement in exercise capacity (6-minute walk distance) and a remarkable reduction in scar size that was interpreted as evidence for either infarct protection or remuscularization by the transplanted CDCs. In the Lancet article, the authors noted that the CADUCEUS population's baseline LVEF was only slightly impaired, so they had little room for improvement; nonetheless, they found the increases in regional function seen in treated patients to be "reassuring" in support of the functional importance of tissue changes.
What's Your Type?
Other autologous cells have been racking up dates as well. For example, the recently reported POSEIDON compared transendocardial delivery of allogenic versus autologous mesenchymal stem cells in acute MI.7 While there appears to be no significant difference in benefit or toxicity when autologous or allogenic cells are transplanted, at AHA.12 Joshua M. Hare, MD, University of Miami, Miller School of Medicine, Florida, underscored the viability of using either of these sets of cells in that sphericity of patients' hearts normalized and scar tissue was reduced, which is at the heart of the investigators' belief as to what will make the therapy work.
"Even in patients who had heart attacks several decades before treatment, both donor and recipient stem cells reduced the amount of scarring substantially, by one-third," Dr. Hare noted.
In addition, adipose tissue-derived regenerative cells (ADRCs) are being prospectively studied in the ADVANCE study, a multicenter, randomized, prospective, placebo-controlled phase 2b/3 study in 375 patients with recent acute STEMI. ADRCs have been touted to improve LV function and myocardial perfusion after MI with anti-apoptotic, immunomodulatory, and pro-angiogenic potential due to paracrine effects.
Reasons to Break Up?
The discussions surrounding the use of stem cells for disease treatment involve not only issues such as safety, efficacy, and feasibility but deeply felt ethical concerns regarding stem cells derived from human embryos. Those raging debates were not helped when the entire field was embarrassed due to the high-profile retraction of a human therapeutic cloning paper in Science by Woo-suk Hwang, MD, from South Korea, generating skepticism that this field is more about hype than substance, like a bogus online profile. Some also suggest clinical trials may be moving too quickly, like speed dating, and that the application is getting ahead of the science.
Given the repeated lukewarm trial results, some may be ready to call it quits to this relationship. Others remain hopeful that wedding bells will ring eventually.
But consider that while many of the studies suggest marginal benefit, as measured in terms of EF improvement and/or ventricular remodeling, no hard clinical endpoint such as improved survival and reduced hospitalizations long-term has been achieved thus far. Others are concerned the rate at which some researchers are embarking on clinical studies in hopes of a home run seem more like shotgun weddings than long, careful engagements due to the lack of sufficient supportive preclinical data. But, like the promise of a forever love, the lure of the next big breakthrough fuels the passion for continued clinical trials, especially given the growing number of patients with ischemic cardiomyopathy and the lack of a definitive cure besides cardiac transplantation.
Visions of Pluripotent Stem Cells
As we consider the future of stem cell therapy for CV disease, one emerging area of research blooming like young love involves the use of pluripotent stem cells such as embryonic stem cells (ESC) or induced pluripotent stem cells (iPSC), both of which have unquestioned ability to generate beating cardiomyocytes in vitro.
However, prior to Dr. Yamanaka discovering iPSCs, many considered the use of human pluripotent stem cell-derived cardiomyocytes for cardiac therapy ethically problematic, a relationship road not travelled. Despite this, several proof-of-principle studies in animal models have shown the feasibility of transplanting cardiomyocytes derived from large-scale in vitro differentiation and purification from human ESCs.8
Thus far, we've seen no evidence of intracardiac teratoma formation when differentiated cells that are highly enriched for cardiomyocytes have been transplanted. However, since all of these studies have been executed as xenograft models with immunosuppression either genetically or pharmacologically, it remains to be seen if the problem of teratoma formation resurfaces in human studies.
Currently, one biopharmaceutical company is actively enrolling patients in United Kingdom for human ESC-derived retinal epithelial cell therapy for Stargardt's disease and for aged-related macular degeneration. According to the initial report, the first two patients transplanted show no evidence for teratoma formation,9 a finding mirrored preliminarily in the first three patients enrolled in the now defunct Geron human ESC-derived oligodendrocyte trial.
Might human ESCs/iPSCs provide a more robust prospect for remuscularization in an infarcted heart? To pop that question, the California Institute for Regenerative Medicine recently awarded a $20 million grant to investigators at Stanford University, University of California San Francisco, Cedars-Sinai Medical Center, and the City of Hope Hospital to perform the necessary IND-enabling studies for patients with ischemic HF to gain FDA approval for clinical trials in the next 4 years. According to Joseph C. Wu, MD, PhD, a cardiologist at Stanford University, these studies "will provide the necessary data to put Stanford and partner institutions in California in a great position to perform the first-in-human studies for human ESC-derived cardiomyocyte therapy for heart failure."
Where is this relationship heading? Many signs point to stem cell therapy going from being a BFF to a soul mate as a strategy to combat cardiac disease. Despite several thousand patients treated with a variety of stem cells over the last decade, we've witnessed no catastrophic events, an encouraging sign even though benefit in some cases appears modest.
Getting serious about the relationship will mean taking the leap to larger, multicenter, double-blinded, and randomized studies once we identify the most effective cell type and delivery route, and appropriate patient selection and timing of the intervention. Importantly, the availability of standardized strategies to isolate and prepare cells in a GLP/GMP-compliant fashion will be imperative. With all this in place, we look forward to the day cell therapy becomes the standard of care for a defined cardiac patient population. Then mending broken hearts won't rely on crooners, chocolate, or even the wizardry of Oz, but cells and science.—by Sean M. Wu, MD, PhD,10 and Jagmeet Singh, MD, DPhil11
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2. Menasché P, Alfieri O, Janssens S, et al. Circulation. 2008;117:1189-200.
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5. Bolli R, Chugh AR, D'Amario D, et al. Lancet. 2011;378:1847-57.
6. Makkar RR, Smith RR, Cheng K, et al. Lancet. 2012; 379:895-904.
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8. Laflamme MA, Chen KY, Naumova AV, et al. Nat Biotechnol. 2007;25:1015-24.
9. Schwartz SD, Hubschman JP, Heilwell G, et al. Lancet. 2012; 379:713-20.
10. Division of Cardiovascular Medicine, Department of Medicine; Stanford Cardiovascular Institute; Institute for Stem Cell Biology and Regenerative Medicine; Stanford University School of Medicine, Stanford, California
11. Cardiac Resynchronization Therapy Program, Division of Cardiology, Department of Medicine; Massachusetts General Hospital; Harvard Medical School, Boston, Massachusetts
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