Author + information
- Kevin D. Costa, PhD∗ ()
- ↵∗Reprint requests and correspondence:
Dr. Kevin D. Costa, Cardiovascular Research Center, Icahn School of Medicine at Mount Sinai, One Gustave L. Levy Place, Box 1030, New York, New York 10029.
The past several decades have witnessed a steady decrease in mortality from myocardial infarction (MI) (1). However, adult myocardium is incapable of spontaneously regenerating the billions of cardiomyocytes lost during MI. Consequently, the increased number of survivors translates to a growing prevalence of patients experiencing heart failure (HF). Although current medical and surgical treatments can help alleviate the symptoms of HF, they do not restore contractile function to injured myocardium. Emerging cell transplantation therapies show promise for post-MI repair, but functional benefits appear modest in a recent meta-analysis of 16 randomized controlled clinical trials (2). Clearly, there is an urgent need to develop novel treatment strategies for post-MI repair of the heart.
One strategy has been to develop implantable biomaterials, with a living 3-dimensional patch of actively functional, human engineered myocardial tissue widely considered to be the Holy Grail for future surgical repair of infarcted myocardium. Meanwhile, an alternative approach involves using passive acellular hydrogels that can provide mechanical support, limit infarct expansion, and release therapeutic agents (3), while circumventing many challenges with delivering a living biological implant. In particular, Singelyn et al. (4) and Seif-Naraghi et al. (5) have developed an injectable myocardial matrix hydrogel, produced by decellularizing and partially digesting porcine ventricular myocardium, that has been shown to reduce adverse remodeling of the left ventricle (LV) and attenuate the loss in cardiac function post-MI in rats and pigs, demonstrating biocompatibility, hemocompatibility, and no evidence of arrhythmias; a related Phase I clinical trial is underway (A Study of VentriGel in Early and Late Post-myocardial Infarction Patients; NCT02305602).
In this issue of the Journal, Wassenaar et al. (6) examine the tissue-level mechanisms by which injection of their porcine myocardial matrix (PMM) hydrogel yields functional benefits when used to treat the heart post-MI. Using a rat ischemia–reperfusion model to represent acute MI after revascularization therapy, direct intramyocardial injection of the matrix hydrogel at 1 week post-injury abrogated the loss of left ventricular ejection fraction (LVEF) and improved LV hemodynamics versus saline control at 5 weeks post-treatment, extending their previous Journal study (4) to an earlier delivery time point post-MI. Using whole transcriptome RNA microarray analysis of infarct scar and border-zone tissue, principal component analysis and hierarchical clustering revealed a shift in global gene expression by 1 week post-injection of matrix compared with saline control, an effect not demonstrated with any other biologic-based therapies. Key modulated pathways included the inflammatory response; reductions of cardiomyocyte apoptosis, cardiac hypertrophy, and fibrosis; altered metabolic enzyme expression; and blood vessel and cardiac development.
Noteworthy were increases in cardiac oxidative metabolism and mitochondrial biogenesis predicted by Ingenuity Pathway Analysis and Gene Ontology analysis, and confirmed by quantitative polymerase chain reaction and significantly increased cardiomyocyte nuclear expression of PGC-1α, a key regulator of energy metabolism whose targets are known to be down-regulated in rodent models and patients with HF (7). The authors note that similar rescue of fatty acid metabolism has not been observed with various bone marrow cell therapies, nor with captopril, an angiotensin-converting enzyme inhibitor commonly used to treat HF, suggesting such cardioprotective modulation of metabolic enzymes may represent a unique benefit of PMM treatment post-MI.
Also notable was the PMM-induced inflammatory response, with predicted activation of cell migration and infiltration pathways, and increased transcription of macrophage-related genes such as CD68 and MMP12; however, no increase in macrophages was detected by immunohistochemistry, possibly indicating altered immune cell behavior. Instead, increased infiltration of tryptase+ mast cells, known to be involved in neovascularization and immunomodulation and to trigger events that limit cardiomyocyte apoptosis, was shown for the first time in response to implantation of a decellularized biomaterial. Because functional benefits of PMM have now been demonstrated in rats and pigs with similar safety and efficacy (5), it is unlikely these results reflect porcine xenograft–elicited inflammation.
Activation of cardiac development pathways was also predicted, with quantitative polymerase chain reaction evidence of a concurrent increase of 6 key markers of cardiac development (GATA4, Nkx2.5, MEF2d, myocardin, Tbx5, and Tbx20), and infiltration of cKit+/tryptase− cells, some of which were also positive for Nkx2.5, suggesting an increase of putative cardiac progenitors cells (CPCs) at 1 week after matrix injection. However, lineage tracing studies would be required to demonstrate cardiac progenitor activation or de novo myogenesis. Indeed, the role of c-Kit+ CPCs in post-MI regeneration is highly controversial, with recent studies suggesting c-Kit+ cells in the murine heart are actually endothelial cells and not CPCs (8). Nevertheless, the increase of several cardiac-specific transcription factors, together with a decrease in cardiac hypertrophy and fibrosis, is a novel finding supporting the reparative potential of PMM treatment.
Advantages of an Injectable Biomatrix Approach
Compared with other engraftment strategies for treating MI, one advantage of an injectable biomatrix hydrogel relates to the method of delivery. Therapeutic cells used to treat MI are most commonly suspended in solution for percutaneous delivery by intracoronary catheter, or via image-guided intramyocardial catheter injection, aiming for a minimally invasive procedure that could translate to the clinical setting. However, cell retention at the injection site is notoriously inefficient, with some estimates as low as 10% of the injected cell population. Suspending the cells in an injectable hydrogel can enhance retention, but may impede integration of grafted cells with the host tissue, and does not overcome issues with the risk, cost, and timing of autologous cell harvesting and scale-up, with significant functional improvement or regeneration of de novo myocardium remaining an elusive goal. To provide functional heart muscle, bioengineers have created beating 3-dimensional cardiac tissue using cardiomyocytes derived from human pluripotent stem cells; although useful for in vitro screening and disease modeling applications, challenges for surgical implantation include vascularization of a thick tissue graft, open-chest procedures for surgical implantation, electrical discontinuity across the host–graft interface, and phenotypic immaturity of the engineered construct. In some early engineered tissue implantation studies, acellular control scaffolds showed benefits comparable to their cell-seeded counterparts, supporting the notion that cell-free scaffolds may provide an alternative strategy for MI repair.
But what would be the ideal source for such a scaffold? Synthetic biomaterials have been designed to mimic various biophysical and biochemical attributes of natural tissue, and to perform functions such as time-controlled release of encapsulated substances and nontoxic biodegradation in vivo. An alternative is to remove the cellular fraction of a native tissue, rendering the acellular matrix immunocompatible while retaining essential structural and compositional features of the native scaffold, that even allows recellularization and cell-specific organization as a step toward creating functional whole-heart grafts in vitro (9). Wassenaar et al. (6) have focused on a more immediate application of decellularized heart matrix by performing additional digestion, freezing, and lyophylization steps that yield a powder that can be stored and rehydrated in distilled water as needed. The liquid myocardial matrix is readily delivered to the heart through an intramyocardial catheter, where it conforms to the interstitial space and polymerizes into a hydrogel, enabling ingrowth of cells from the host tissue with no evidence of arrhythmogenesis, thus overcoming important challenges facing the tissue engineered cardiac patch.
Challenges with a Biomaterials Approach to Cardiac Repair
For clinical applications, human myocardial matrix rather than PMM would seem like the preferred choice. Although decellularization protocols have been optimized for human cardiac tissue samples, restricted availability of human samples is a reality that would preclude widespread use. Furthermore, proteomics analysis of decellularized human heart matrix has recently demonstrated significant patient-to-patient variability, with the advanced age of typical human donors further limiting the quality of the matrix and the successful processing into hydrogel material (10). Because the PMM is harvested from young healthy pigs, it may be more consistent and biochemically potent than elderly human matrix, and the decellularization process has been optimized to eliminate immunogenic components of the tissue. Assessing the safety of PMM in human patients is a primary endpoint of the ongoing Phase I trial.
What about structural considerations? The heart has evolved with a specialized multilayered helical alignment of cardiac muscle fibers that confers anisotropic mechanical and electrical properties critical to cardiac pump performance. Injectable hydrogels lack the mesoscale structure of the myocardial tissue from which they originated, which likely influences the organization and interconnection of infiltrating cells. Although PMM eventually gets degraded with time after implantation, detailed analysis of the resulting fiber organization in the repaired tissue must be examined.
Evaluating the efficacy of a biomaterial intervention on cardiac pump performance can also be challenging. For example, Wall et al. (11) used finite element analysis of LV mechanics to study the effects of passive material injected into an apical infarct, which caused the endocardium to bulge into the LV cavity and increased local wall thickness, restoring LV wall stress. Although an increased LVEF was observed, this reflected the decrease in end-diastolic volume displaced by the inwardly bulging endocardium rather than a beneficial increase in stroke volume. Indeed, there was no effect of the injected material on the Starling curve relating stroke volume and end-diastolic pressure. Unfortunately, interventions to vary diastolic pressure are challenging in HF patients. Therefore, conclusions about biomaterial efficacy based on LVEF must be interpreted with caution.
Future of Injectable Biomaterials for the Failing Heart
When considering the future of injectable acellular scaffolds for cardiac repair, their ultimate success or failure will depend on the ability to provide clinically significant improvements in cardiac pump function and quality of life for HF patients. Preliminary results from the AUGMENT-HF Phase 2 clinical trial (A Randomized, Controlled Study to Evaluate Algisyl-LVR™ as a Method of Left Ventricular Augmentation for Heart Failure; NCT01311791) testing intramyocardial injection of a calcium-alginate hydrogel in 9 patients with dilated cardiomyopathy undergoing coronary artery bypass grafting or mitral valve repair, indicated a significant and sustained improvement in mean LVEF from 27.1% at screening to 34.8% at 24 months post-discharge, with a decrease in ventricular tachycardia, improvement in Kansas City Cardiomyopathy Questionnaire quality of life scores, improvement in New York Heart Association HF classification, and no adverse events attributed to the treatment (12). However, the modest increase in EF of about 8%, which is comparable to findings in many stem cell trials, is confounded by the concomitant coronary artery bypass grafting or mitral valve repair surgery.
Functional benefits may be enhanced by augmenting the scaffold material with cardioactive agents such as epicardial FSTL1 protein, which has recently been shown to induce regeneration and restore cardiac function in mice and swine models of MI when delivered using an FSTL1-treated engineered collagen patch sutured to the epicardium (3). Could the calcium-alginate hydrogel similarly be loaded with FSTL1 before injection to augment myocardial repair?
Alternatively, as Christman and coworkers (4,5) have previously shown, the extracellular matrix of native myocardium can be processed into an injectable hydrogel equipped with a cocktail naturally formulated to support cardiac function. They have demonstrated significant benefits of PMM treatment for preserving cardiac structure and contractile performance in small and large animals post-MI, and the current study by Wassenaar et al. (6) is one of the first to provide biological insights into some of the unique tissue-level mechanisms underlying the PMM performance. It will be of great interest to learn whether the findings of their ongoing clinical trial will support the translation of this technology to a HF patient cohort with LVEF of 25% to 45% secondary to MI.
Indeed, one of the most difficult challenges in translating such animal studies to humans involves selecting the appropriate injury model and therapeutic time window to best represent the target patient population, which is rarely a healthy individual with an induced cardiac injury. Ongoing development of species-specific in vitro model systems, such as human engineered cardiac tissues (13), may help to bridge this longstanding gap in the therapeutic translation process.
Taken together, these studies reflect an increasing impact of technology on cardiology research and practice, and illustrate the advantages of a quantitative engineering approach for the rational design of novel cardiac repair strategies. Continued active collaboration between cardiologists and biomedical engineers will help to realize the exciting potential of injectable scaffolds for improving the lives of patients with heart failure.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Costa has reported that he has no relationships relevant to the contents of this paper to disclose.
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