Cardiac Cell Therapy Trials: Chronic Myocardial Infarctionand Congestive Heart Failure

Philippe Menasché

Received: 4 February 2008 /Accepted: 22 February 2008 /Published online: 14 March 2008# Springer Science + Business Media, LLC 2008

Abstract Although most cardiac cell therapy trials havefocused on patients with acute myocardial infarction,attempts at “regenerating” chronically failing hearts havealso been performed. These studies have entailed use ofskeletal myoblasts and bone marrow-derived cells. In thecase of skeletal myoblasts, the randomized placebo-con-trolled MAGIC trial has not achieved its primary end pointas 6-month ejection fractions did not significantly differbetween patients receiving cells or placebo, but the findingthat the highest dose of myoblasts resulted in a significantanti-remodeling effect (a prespecified secondary end point)compared with the placebo group provides an encouragingsignal. In the case of bone marrow cells, surgical injectionsof the mononuclear fraction combined with coronary arterybypass surgery have failed to show any substantial benefit.A catheter-based trial using a cross-over type of design hasreported more successful outcomes, but its results will thenhave to be confirmed. Indeed, the most positive results havebeen reported with intraoperative epicardial injections ofCD133 progenitors, which is probably explained by theangiogenic potential of these cells. There are three possiblereasons for these mixed results. The first is the markedheterogeneity of cell functionality (particularly in the caseof bone marrow), which would expectedly translate intovariable clinical outcomes. The second reason is the lowrate of sustained engraftment caused by early mechanical

leakage followed by biologically induced cell death. Thethird possible explanation is a mismatch between the choiceof end points and the presumed mechanism of action of thecells. The initial assumption that adult stem cells couldaffect myocardial tissue regeneration has led to the usualfocus on ejection fraction as the major surrogate end pointfor treatment efficacy. It is now increasingly recognized thatadult stem cells, in contrast to their embryonic counterparts,have little if any regenerative capacity and that theirpresumed beneficial effects more likely involve paracrinesignaling and/or limitation of remodeling, in which caseinfarct size, perfusion, or left ventricular volumes might bemore appropriate markers. Altogether, these observationsprovide a framework for future research the results ofwhich will then have to be integrated into the protocoldesign of second-generation clinical trials so as to maxi-mize their likelihood of yielding more successful results.

Keywords Stem Cells . Heart Failure . Transplantation

Although most cardiac cell therapy trials have focused onpatients with acute myocardial infarction, attempts at“regenerating” chronically failing hearts in patients havingsuffered extensive myocardial infarctions have also beenperformed. In practice, these studies have entailed the useof skeletal myoblasts and bone marrow-derived cells.

Skeletal Myoblast Trials

After almost a decade of experimental studies, clinical trialsof myoblast transplantation started in June, 2000, when weperformed the first human transplantation of autologous

J. of Cardiovasc. Trans. Res. (2008) 1:201–206DOI 10.1007/s12265-008-9017-1

P. Menasché (*)Assistance Publique-Hôpitaux de Paris,Hôpital Européen Georges Pompidou,Department of Cardiovascular Surgery,University Paris Descartes, Faculté de Médecine,INSERM U 633,Paris, Francee-mail: philippe.menasche@egp.aphp.fr

myoblasts in a patient with severe ischemic heart failure [1]This case initiated a 10-patient series in which an averageof 871 million cells, of which 87% were myoblasts, wereinjected in postinfarction scars [2]. Three other adjunct-to-coronary artery bypass grafting (CABG) were thenperformed [3–5]. Whereas the patient profile and techniqueof open-chest multiple injections were very similar to thoseused in our study, the number of transplanted myoblastswas highly variable (221×106 in the study of Gavira et al.[3] from 4±105 to 5±107 in the study of Siminiak et al. [4],1, 3, 10, 30×107 and 3×108 in the dose-escalating study ofDib et al. [5]). It is important that the protocol of these threestudies also differed from ours in that it systemicallyentailed a concomitant revascularization of the myoblast-injected areas.

Put together, these studies primarily demonstrated thefeasibility of the procedure (i.e., the possibility of growingseveral hundreds million cells from a 10-g muscular biopsyunder good manufacturing practice (GMP) conditions andwithin a 2- to 3-week time frame) as well as the safety ofmultiple needle punctures in the postinfarction scar andalong its borders. Likewise, none of the myoblast-injectedpatients has developed a cardiac tumor (our longestsurvivor was operated on in December 2000). Indeed, theonly safety concern has been an increased risk ofpostoperative sustained ventricular tachycardia [2, 4], andthis susceptibility to arrhythmias following myoblasttransplantation has been later confirmed in rat experiments[6]. Currently, the most commonly accepted mechanism ofthese arrhythmias is the electrical insulation of myoblastclusters from the surrounding cardiomyocytes [7] leading toa slowing of conduction and subsequent reentries [8]. Thishypothesis is primarily supported by data derived from co-culture experiments showing that myoblast transfectionwith connexin 43 decreases arrhythmogenicity [8]. How-ever, the origin of these posttransplantation arrhythmias maynot be univocal because of the possible involvement of theintrinsically arrhythmogenic myocardial substrate character-istic of heart failure and the role of needle-induced tissuedisruption and inflammation that may further contribute toconduction blocks [9]. The location of myoblast injectionscould also modulate the proarrhythmic potential of theprocedure as those lining the border zone of the scar seemless arrhythmogenic than those performed in its core [10].

Although these initial studies were neither designed norpowered to provide efficacy data, the functional effects ofmyoblast injections were nevertheless assessed up to 4years [5] and even later (58 months in our trial) [11].Outcomes were found to range from stabilization of leftventrical (LV) ejection fraction and volumes [11] toimprovements in regional and global LV function frombaseline values [3, 4] and, occasionally, in metabolicviability of transplanted areas, as assessed by positron

emission tomography and magnetic resonance imaging(MRI) [3, 5]. It is clear, however, that the small size ofthese series, their open-label type of design and the lack ofcontrols made these data inconclusive.

To overcome these hurdles, we have implemented arandomized, double blind, placebo-controlled trial (MAGIC,an acronym for myoblast autologous grafting in ischemiccardiomyopathy), which involved 21 centers in Europeand included patients with severe left ventricular dys-function, a postinfarction nonviable scar, and an indica-tion for CABG. Muscular biopsies were cultured in twocore laboratories and 3 weeks later, either 400 or 800million cells or a placebo solution were injected inapproximately 30 sites in the core and the margins of theinfarct area during the bypass operation. Of note, animplantable cardioverter defibrillator (ICD) was implantedin every patient before hospital discharge and an indepen-dent blinded committee then adjudicated ventriculararrhythmias detected by the ICD read-outs. Out of 120randomized patients, 97 were effectively treated. At the 6-month study point, the proportion of patients who hadexperienced arrhythmias was not significantly differentbetween the myoblast-treated and the placebo-injectedgroups despite a trend toward a greater incidence of theseevents early after operation in the myoblast-treated groups.In terms of efficacy, the trial failed to meet its primary endpoint as neither regional nor global LV function, as assessedblindly by echocardiography in a core laboratory, weresignificantly improved by myoblast injections, regardless ofthe dose, compared with controls; however, the highestdose of cells resulted in a significant reversal of remodel-ing, evidenced by a decrease in LV end-diastolic andendystolic volumes (a prespecified secondary end point)compared with the placebo group [12]. Overall, thesemixed outcomes reflect a commonly observed discrepancybetween some signals supporting the proof of concept andthe lack of translation of these effects in a clinicallymeaningful improvement of LV function.

In parallel to these surgical trials, three phase-I catheter-based studies have been reported. One has entailedadministration of myoblasts through the coronary sinuswith a dedicated catheter, which allows direct cell injectionsinto the scar area under endovascular ultrasound guidance[13] and the trial has confirmed both the feasibility andsafety of this approach, although this route of cell transfermay be technically challenging, particularly in patients whohave previously undergone lead implantation for cardiacresynchronization therapy. In the other two percutaneoustrials, myoblasts have been injected through an endoven-tricular catheter under electromechanical guidance. One ofthe studies (10 patients) reported a 1-year improvement insystolic velocity of the cell-injected segments and anincrease in global ejection fraction during low-dose dobut-

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amine infusion [14]. The second study also demonstratedan improved function in six treated patients who werecompared with six case-matched controls [15]. However,the discrepancy in outcomes between the surgical phase Itrials and the MAGIC trial highlight how data collected insuch small-sized, uncontrolled, and open-label studies canbe misleading. To address these issues, a randomized studyhas then been performed, which has included 23 patientswith LV ejection fraction below 40% and old (>10 years)infarction allocated to endoventricular myoblast injectionsor optimal medical management alone [16]. The resultsreported orally at the 2007 Scientific Sessions of theAmerican Heart Association look encouraging, and a moredetailed publication of these data is now awaited.

Bone Marrow Cells

A more limited number of studies have assessed theeffects of bone marrow cell transplantation in patientswith heart failure (reviewed in [17]). In the surgicalsetting, cells have been injected epicardially into the targetareas, except for one study in which they were alsoinfused directly into the coronary artery through thebypass graft [18]. Because small-sized uncontrolled andopen-label trials are of questionable relevance, two studiesdeserve further analysis. Mocini et al. [19] have studied 36patients with a recent (<6 months) myocardial infarctionand reasonably well-preserved LV function. These patientswere nonrandomly allocated to injections of autologousbone marrow mononuclear cells (MNC; mean number:292×106) in the border zone of the infarct area duringCABG or to a control group. There were no safetyconcerns except for an initially higher troponin I peakafter cell therapy. Three months after the procedure, bothregional and global LV function had significantly im-proved compared with baseline values. However, outcomemeasures did not differ between the two groups and,unexpectedly, ejection fraction did not improve followingbypass in control patients, which might have driven theresults in favor of the treated group. In the study ofHendrikx et al. [20], 20 patients with an ejection fractionin the range of 40% were randomized to receive in-scarinjections of autologous bone marrow mononuclear cells(mean number: 60×106) or saline in addition to CABG.Four months later, cell injections resulted in a significantlygreater regional function, as assessed by magnetic reso-nance imaging, but failed to improve ejection fraction orperfusion defects beyond values seen in control patients.Altogether, these results confirm previous experimentaldata showing that transplantation of unfractionated bonemarrow in chronically infarcted myocardium does notprovide a functional benefit [21]. Furthermore, although

these trials did not report safety problems, the recentexperimental finding that in-scar implanted mesenchymalstem cells (MSC), some of which are present in theunfractionated bone marrow cell injectate, may causeintramyocardial calcifications [22] raises a cautionary notethat requires further investigation.

In the more specific perspective of exploiting theangiogenic properties of some bone marrow cell popula-tions, other investigators have investigated the effects ofCD133 progenitors epicardially injected during CABG[23]. Although this trial was not strictly randomized, ithas been rigorously conducted and it shows the capacity ofthe CD133 population to improve LV function andperfusion at 6 months postoperatively, particularly inpatients with the poorest preoperative LV function. Asone would not expect these cells to transdifferentiate intonew cardiomyocytes, the prevailing assumption is that theirparacrine effects may have led to increased angiogenesis,rescue of reversibly injured native cardiomyocytes and,more hypothetically, recruitment of putative endogenouscardiac stem cells which, altogether, could account for theimproved LV function.

Catheter-based studies of bone marrow cells inpatients with heart failure are also limited. They havebeen pioneered by an open-label nonrandomized trial[24] in which 11 patients received endoventricularinjections of MNC and were reported to have an improvedexercise capacity and a reduced perfusion defect at the 1-year follow-up. In two other studies, MNC have beeninfused directly into the coronary arteries. The IACT study[25] has claimed myocardial “regeneration” on the basis ofimproved outcomes, but these results are highly question-able because of the multiplicity of potential flaws in thetrial design (such as the small sample size, the lack of truerandomization, and the huge heterogeneity in baseline LVfunction). Using a more elaborate cross-over type ofdesign in a 75-patient study, Assmus et al. [26] havesimilarly reported, at a follow-up of 3 months, the benefitsof intracoronary infusions of either circulating progenitorcells isolated from venous blood or of bone marrow-derived MNC on both global and regional LV function(the latter being more effective). Although still limited,these data open interesting perspectives for the catheter-based treatment of chronic heart failure by bone marrowcells although they currently remain weakened by the lackof robust preclinical models and of subsequent mechanis-tic insights.

Limitations and Remaining Hurdles

In a clinically oriented perspective, they can be stratifiedinto three main categories.

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Origin of Cells

So far, a strong argument favoring the use of skeletalmyoblasts or bone marrow-derived cells has been theirautologous origin. However, with accumulated clinicalexperience, the limitations of patient-specific products havebecome increasingly apparent. They include: (1) thenaturally occurring individual variability between patients,which makes it difficult to end up with a reproducible celltherapy product, particularly in patients with ischemiccardiomyopathy whose progenitor cells may be functionallyimpaired [27]; (2) the cost of customized quality controls,and (3) the logistical complexity related to back-and-forthshipments of the cellular products when their processing iscentralized in a core laboratory. These hurdles can beovercome by cell banks able to supply a readily available“off-the-shelf”, controlled, reproducible, and thoroughlycharacterized product. In turn, these allogeneic cells havethe disadvantage of immunogenicity, except, maybe, in thecase of mesenchymal stem cells, which are credited for animmune privilege [28]. Whether this issue can be addressedby appropriate donor-recipient immunomatching stillremains uncertain, but it is clear that autologous andallogeneic have to be investigated with regard to risk-benefit and cost-effectiveness ratios.

Transfer and Engraftment of Cells

A consistent finding of cell therapy studies is the very lowrate of sustained cell engraftment, which is usually in therange of 1% of the initial number of donor cells a fewweeks after transplantation [9]. Assuming that the cardio-myocyte deficit resulting from an infarction large enough tocause heart failure is on the range of one billion cells [29],one cannot reasonably expect a meaningful clinical benefitfrom such a tiny number of persisting donor cells,particularly in the case of skeletal myoblasts for which aclear dose–effect relationship has been documented [30].

This low rate of engraftment is initially caused by amechanical leakage of cells. A recent study has thus shown,in a porcine model of cardiopulmonary bypass, that only10% of intramyocardially injected microspheres approxi-mating the size of MSC are retained within the sites ofinjection after 30 min, regardless of whether the heart isarrested or beating [31]. Experimentally, it seems that bothsurgical epicardial and percutaneous transendocardial injec-tions result in equivalent engraftment rates [32], whereasstudies performed at the acute stage of myocardialinfarction have shown that only 2–5% of intracoronarilyinfused MNC are retained in the myocardium after a fewhours [33]. It is critical to address this issue of cell transfer,and different strategies are currently being investigated, whichinclude computer-driven injection devices, replacement of the

injection concept by cell sheets in the case of surgical celltherapy (see below) and techniques, which enhance myocar-dial homing if cells are delivered intravascularly.

The second event that decreases engraftment is the highpercentage of death of initially retained cells. This lossoccurs over the first weeks after transplantation and resultsfrom the interplay of three main factors: inflammation,ischemia caused by the poor vascularization of the injectedareas, and apoptosis subsequent to detachment of anchorage-dependent cells from their extracellular matrix (anoikis).The recognition of these contributing factors is nowleading to move from isolated cell delivery to morecomposite grafts that incorporate a scaffold and eventu-ally growth factors. In brief, whereas inflammation canbe blunted by a pulse of corticosteroids, the ischemiccomponent of cell death can be counteracted by a varietyof strategies including direct revascularization, wheneverfeasible, co-transplantation of angiogenic bone marrow-derived cells or cell engineering with genes encodingangiogenic growth factors [34]. Cell survival can beenhanced by an equally wide array of techniques like graftincorporation into biocompatible matrices, physical orpharmacological preconditioning of cells, or gene-basedboosting of survival pathways although, conceptually, themost appealing approach for preventing anoikis could be torespect cell-to-matrix and cell-to-cell connections, whichhave been successfully achieved with cell sheets prepared byculturing cells on temperature-sensitive or fibrin-coated filmsthat are then stacked and overlaid onto the infarct area [35].

Clearly, the development of these survival-enhancingstrategies should be paralleled by that of techniques of celltracking allowing a noninvasive and reliable assessment ofengraftment rates. A great deal of interest is currently paidto cell loading with iron superparamagnetic iron particlesfor detection by MRI, but this technique has its own caveats[36] and improvements in this area are eagerly awaited.

Functionality of Cells

A major conceptual question is to figure out what is theexpected mechanism of action of the transplanted cells andwhat their ultimate objective should be. If the premise isthat cells are going to act paracrinally by releasing factorsthat can stimulate angiogenesis, favorably affect thecomposition of the extracellular matrix, promote cellsurvival pathways, and even possibly recruit putativecardiac stem cells, then they do not necessarily need to bephenotypically matched to host cardiomyocytes as long asthey supply the appropriate mediators. In this view, adultcells like myoblasts or bone marrow cells appear reasonablecandidates, particularly the CD34 population, which isknown for its angiogenic capabilities [37]. Conversely, ifthe assigned goal is to restore contractility of akinetic

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myocardial areas characteristic of heart failure by newfunctional cells, it has become clear that neither skeletalmyoblasts [38] nor bone marrow cells [39] are the goodplayers. The reason is that none of these cell typesexpresses gap junction proteins allowing formation of asyncytium, which is the prerequisite for the graft to beat insynchrony with the recipient heart and consequently tocontribute to augment its pump function. In this case, it isnot unexpected that cells that best achieve this remuscula-rization are those which recapitulate the developmentalcardiomyogenic program. In this setting, the limitedavailability and poor scalability potential of fetal cardio-myocytes [40], along with the uncertainty about thepersistence of cardiac stem cells in adulthood, highlightthe potential interest of human embryonic stem cells (ESC),provided they have been appropriately specified in vitrotoward a cardiac lineage. Under these conditions, there iscompelling growing evidence that following engraftment inpostinfarction scars, these ESC-derived progenitors com-plete their differentiation in cardiomyocytes [41].

Because there is no animal model that can fully duplicatethe complex situation of patients with coronary arterydisease, we believe that it is legitimate to continue clinicaltrials provided that they are randomized, adequatelypowered, placebo-controlled and blinded. As it is likelypremature to launch large-scale mortality trials, surrogateend points and imaging modalities should be selected so asto confirm the proof of principle and help in providingmechanistic insights. In this perspective, the assessment ofcell-related morphological changes such as infarct size, LVremodeling, or regional wall thickness are probably asinformative as the commonly used measurements of globalLV function. However, it is likely that the outcomes ofthese trials will be optimized if their protocols integratelessons learned from the first wave of studies and thussuccessfully address issues related to transfer, survival, andfunctional integration.

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