Stem Cell-Based and Gene Therapies in Heart Failure

Mechanisms of Action for Cardiac Cell Therapy

Cell therapy broadly aims to achieve two distinct but complementary goals: (1) direct cell replacement through implantation of cells or grafting of tissues into the injured myocardium and (2) stimulation of endogenous repair processes such as angiogenesis, inflammation, apoptosis, and fibrosis ( see Fig. 48.1 ). Direct cell or tissue replacement offers the most intuitive strategy to replace cardiac myocytes (“remuscularization”) and restore function following acute MI; however, it has also been the most elusive. Remuscularization may be the only effective strategy for irreversible fibrosis or remodeling associated with chronic injury, a larger albeit more complex indication than acute injury. However, replacement therapies remain exploratory at this stage in development. Whether adult stem cells are even capable of forming new cardiac muscle is an area of great debate ( see also Chapter 3 ). In contrast, replacement strategies based on pluripotent stem cells can clearly remuscularize infarcted myocardium in preclinical models, but have their own set of challenges that will need to be overcome before they are a viable clinical strategy.

The first cell type investigated for cardiac cell replacement therapy was adult skeletal myoblasts over two decades ago. Although initially thought to transdifferentiate into cardiomyocytes, this has been conclusively shown not to occur, and the cells themselves do not couple electrically with the host myocardium. Clinical trials of autologous skeletal myoblast transplantation in patients with heart failure did not improve regional or global LV function and caused ventricular arrhythmias, prompting abandonment of this cell type for therapy. More recently, efforts have shifted to other adult sources of cells purporting regenerative benefit through cell-cell and paracrine mechanisms, activating and stimulating endogenous regeneration and modulating repair mechanisms. Numerous autologous and allogeneic adult cell types have been investigated clinically, including adult stem cells of cardiac origin such as cardiac stem/progenitor cells (CSCs/CPCs) and noncardiac origin such as cardiosphere-derived cells (CDCs), as well as various bone marrow (BM)-derived cells (e.g., BM-derived mononuclear stem cells [BM-MNCs], and BM-derived mesenchymal stem cells [BM-MSCs]) ( Table 41.1 ). These so-called first-generation cell types have been further refined as “second-generation” cells composed of highly purified subpopulations or modified to potentiate their regenerative capacity. Development of these adult cell types has been accelerated to numerous phase II/III clinical trials within the past decade. With the realization that cell therapy has multiple effects in addition to regeneration, including cell-cell interaction and paracrine secretion of cardioactive cytokines and growth factors, investigators have expanded the indications for cell therapy from acute MI to ischemic and nonischemic cardiomyopathy to refractory angina, peripheral artery disease, and stroke. Recently, it has been recognize that cell-free preparations are capable of replicating the beneficial effects of cell therapy; these secretomes consist of extracellular vesicles/exosomes and growth factor-enriched myocardial matrixes. Whether they account for all of the benefits of cellular therapy and whether they are equally efficacious will need to await further clinical development.

Given their proven potential to remuscularize infarcted tissue, there is great interest in pluripotent stem cells such as human embryonic stem cells (ESCs) as a renewable source of differentiated cardiomyocytes. First isolated in 1998, human ESCs are isolated from the inner cell mass of the blastocyst in the early stages of embryogenesis. These cells retain the potential to differentiate into any somatic cell type given the appropriate stimulation. Initially, there was hope that the heart milieu itself could provide either critical cell-cell cues or growth factors to guide ESCs to a cardiac phenotype and integrate into host myocardium. This notion was quickly dispelled as injected ESCs into mouse myocardium formed teratomas rather than mature cardiomyocytes, in addition to eliciting immunogenicity and graft rejection. However, cardiomyocytes derived from ESCs can be transplanted and survive in normal rodent hearts and electrically couple with existing cardiomyocytes in porcine models. When transplanted into recipient rodent models after MI, there was a reproducible and durable improvement in LV function and electrical coupling with the host myocardium. Efficient methods for high-purity, clinical-grade cardiomyocyte production from ESCs now allow extension of replacement cell strategies into preclinical large animal studies. Upward of one billion human ESC-derived human cardiomyocytes (hESC-CMs), approximating the cell loss during anterior MI, have been successfully implanted in postinfarction, nonhuman primates. In this seminal study by Chong et al. and Murry et al., hESC-CMs were engrafted into infarcted primate hearts via epicardial injection, resulting in significant remuscularization. The human graft became vascularized and electromechanically coupled with the host myocardium within 2 weeks posttransplant and remained durable up to 2 months, albeit with significant immunosuppression to prevent xenograft rejection. A more recent example demonstrated the effectiveness of allogenic transplantation of 400 million primate induced pluripotent stem cell (iPSC)-derived cardiomyocytes into major histocompatibility complex (MHC) haplotype homozygous matched primates. As expected with significant engraftment of ectopic ventricular tissue, ventricular arrhythmias were observed in both studies, albeit transient and hemodynamically tolerated. Arrhythmogenicity of such grafts appears be proportional to graft size and the size of the host heart (no ventricular arrhythmias were noted in preceding small-model studies) and can be expected to pose a challenge in large animal studies. Although these preclinical proof-of-concept studies show promise and are grounded in more than three decades of basic science scholarship transitioning from in vitro and small animal models to more relevant large animal studies, significant challenges to clinical translation remain. Efficient and reproducible cell production and processing, avoidance or suppression of ventricular arrhythmias, graft survival without prohibitive immunosuppression, and currently surgical epicardial delivery must be addressed prior to broad clinical acceptance. To circumvent many of these issues, an alternative strategy using a surgically placed epicardial patch seeded with ESC-derived CPCs is already enrolling a first-in-human trial, despite recent evidence suggesting that cardiac progenitors do not durably engraft and any benefit is mediated through transient paracrine mechanisms.

The discovery of an alternative pluripotent stem cell, iPSCs, by Takahashi and Yamanaka et al. has markedly accelerated pluripotent stem cell research and translation into potential therapies. Overexpression of four genes (c-Myc, Oct3/4, SOX2, and Klf4) known to maintain pluripotency in stem cells reprogramed somatic cells back to a state of pluripotency. The process has been validated using a full spectrum of somatic cells, including cells isolated from a single hair follicle or a sample of blood. iPSCs offer benefits over ESCs such as autologous source, allowing patient-specific human leukocyte antigen (HLA) compatibility, potentially obviating the need for immunosuppression, and avoiding the societal issues surrounding blastocyst and embryo research. However, the reprograming process has been reported to result in genomic abnormalities and incomplete reprograming, leaving residual epigenetic marks that are of uncertain clinical significance. However, cardiomyocytes generated from iPSCs appear functionally indistinguishable from cardiomyocytes derived from ESCs and native cardiomyocytes, although they are phenotypically immature. Engineered sheets of iPSC-derived cardiomyocytes have been ectopically transplanted onto failing postinfarction hearts in pig with benefit, although the clinical relevance of such an approach is unclear given that such grafts failed to couple electromechanically or vascularize with the host myocardium.

Clinical Trials of Cardiac Cell Therapy

The first cardiac cell therapy trials were performed more than 20 years ago, with intracoronary delivery of BM-MNCs. There have now been more than 100 clinical trials of cell therapy for MI, more than 90 for chronic ischemic cardiomyopathy, and 25 for nonischemic cardiomyopathy. Trials to date have generally used heterogeneous populations of adult cell types and have consistently shown safety regardless of the specific investigational cell product, delivery approach, dosing protocol, or patient characteristics. Individual trials initially suggested efficacy, but these early trials were small without randomization, standardized enrollment criteria, or end points. More recent trials with larger cohorts and superior study design have generally failed to convincingly show benefit over guideline-directed medical therapy (see Table 41.1 ). A recent Cochrane meta-analysis of 38 randomized control trials capturing 1907 post-MI patients concluded the current body of evidence for cell therapy to be low quality and lacking evidence for benefit by composite end point of mortality, nonfatal MI, and/or heart failure readmission. Long-term mortality greater than 12 months and incidence of nonfatal MI were individually reduced with cell therapy but confounded by relatively low event rates, small study cohorts, and nonstandardized trial designs and adjudication. Numerous challenges remain for the clinical translation of cardiac cell therapy ( Fig. 41.2 ). Several representative trials of cell therapy are presented here to highlight these challenges facing cardiac cell therapies.