Introduction

Heart failure following myocardial infarction – ischemic cardiomyopathy – is a major cause of death and disability worldwide (1). This syndrome accounts for ~50% of heart failure with reduced ejection fraction (HFrEF) and ~25% of all heart failure (2). While existing pharmacological and device therapies improve functional status and reduce mortality in this syndrome, many patients progress to advanced heart failure and death (3). Cardiac transplantation can be an effective treatment but is limited by organ donor availability (4). To fill this therapeutic gap, stem cell therapy has recently emerged in the treatment of heart failure, and numerous preclinical and clinical studies using various types of stem cells have been shown to improve cardiac function and attenuate left ventricular (LV) remodeling. Induced pluripotent stem cells (iPSCs) have a proven capacity to derive functional cardiomyocytes, and their scalable production has made iPSCs a favorable cell source for cardiac regenerative medicine (5,6). In this article, we will discuss the current status in clinical trials of cardiac regenerative therapy using iPSCs worldwide.  

Induced pluripotent stem cell (iPSC)

In 2006, Dr. Yamanaka succeeded in reprograming adult somatic cells into pluripotent stem cells using four reprograming factors; octamer-binding transcription factor 4 (Oct4), Krüppel-like factor 4 (KLF4), sex-determining region Y-box 2 (SOX2), and c-Myc (7,8). The stem cells derived by Yamanaka are now known as iPSCs, which are reprogrammed somatic cells with the capacity to differentiate into cells of all three embryonic germ layers (9). The concept behind the development of iPSCs is that overexpression of the genes that maintain pluripotency are sufficient to reprogram a somatic cell into an embryonic stem cell-like state (7). Advantages of an iPSC-based approach include an unlimited source of replacement cells and avoidance of potential controversy concerning the use of fetal tissue (10).

Therefore, researchers have attempted to create cardiomyocytes from iPSCs as a better alternative to using ESCs. The different types of cardiomyocytes include the sinus node, atrial muscle, ventricular muscle, and Purkinje cells; they differ greatly in cell morphology, ion channels, contractile proteins, and electrophysiological and physiological characteristics. Ventricle-specific cardiomyocytes are required for the treatment of heart failure because they are transplanted into the left ventricular wall. At the time of transplantation, part of the cardiomyocyte phenotype remains in the fetal ventricular stage, although the transplanted ventricular muscle is a complete adult-type ventricular muscle. 

Current Clinical trials

iPSC-derived cardiomyocyte patch

A surgical group in Japan have explored a new strategy of myocardial regeneration therapy using iPSC-derived cardiomyocytes (iPSC-CMs) and cell-sheet technique. Cell-sheet technology involves coating culture dishes with poly(N-isopropylacrylamide) PNIPAAm, a thermo-responsive polymer, to release cells and produce cell-sheets upon changing temperature (11). This technique was used to fabricate a scaffoldless cardiomyocyte patch from iPSCs. Preclinical study with swine myocardial infarction models have demonstrated their therapeutic potential (12,13,14). The group has been conducting a clinical trial implanting a iPSC-CM patch in patients with end-stage ischemic cardiomyopathy (Trial ID: NCT04696328) and recently reported one-year outcomes (15). The inclusion criteria were a left ventricular ejection fraction (LVEF) of 35% or less and New York Heart Association (NYHA) class III or higher heart failure symptoms. Three iPSC-CM patches, with 3.3 × 107 cells per patch, about 3.5 cm in diameter, were transplanted on the LV anterior and lateral wall by left thoracotomy. They have recently reported the outcomes of the first 3 patients. There was no transplanted cell-related adverse events during the 1-year observation period. There were improvements in LV contractility by echocardiogram and  electrocardiogram-gated cardiac CT in two of the three patients. Also there was an improvement in myocardial blood flow by NH3-PET/CT in those two patients. No evidence of tumor formation was detected by whole-body FDG-PET scan or in blood testing of four tumor biomarkers after transplantation. The patients took immunosuppressive agents only for three months after transplantation of the allogeneic iPSC-CM patches without matching HLA typing because the transplanted cells were expected to be lost within the first three months. Paracrine effect is considered the primary mechanism of improvement in cardiac function.

Cardiac Spheroid

Another group in Japan has been conducting a phase I/II clinical study (LAPiS Study: jRCTa032200189) with a cardiac spheroid which is an aggregation of allogeneic iPSC-derived highly purified ventricular CMs. Highly purified ventricular CMs decrease the presence of arrythmia after transplantation of iPSC-CMs (16,17). Retention and survival rate of transplanted cells are improved by forming micro-tissue-like spheroids. One cardiac spheroid contains about 1,000 CMs and the diameter is about 200µm. This study is a phase I/II clinical study in patients with ischemic cardiomyopathy. The primary endpoint is safety at 26-weeks post-transplantation, and secondary efficacy endpoints include myocardial wall motion and LVEF. In this study, the cardiac spheroids are transplanted with a special administration needle into the LV myocardium during coronary artery bypass grafting (CABG) surgery. The expected mechanism is that the transplanted CMs electrically couple with the patient’s myocardium and improve cardiac function by remuscularization and secretion of angiogenic factors. They have recently reported 6-months outcomes of the first two patients in this study. They reported that LVEF improved and a decreased left ventricular end-diastolic volume (LVEDV) by echocardiogram and cardiac MRI in both patients. There was also improvement in the NYHA functional class and a decrease in NT-proBNP in both patients.

Biological Ventricular Assist Tissue (BioVAT)  

A Phase I/II trial (NCT04396899) being conducted in Germany evaluating the safety and efficacy of iPSC-derived Engineered Human Myocardium (EHM) as Biological Ventricular Assist Tissue (BioVAT) in end-stage heart failure. EHM are produced from an allograft iPS-cell line for off-the-shelf administration under immune suppression. EHMs can be produced on-stock and delivered as off-the-shelf products for individual to meet clinical dosing and size demands. EHM is administered by minimally invasive surgery onto the beating heart. Dr. Zimmermann recently reported the latest outcomes of the BioVAT; 1) Safe maximal dose of EHM grafts constructed from 800 million iPSC-CMs and stromal cells. 2) Evidence of vascularized remuscularization of the heart. 3) Proof of thickening of the heart wall by EHM grafts. 4) Proof for improvement of EF, NYHA and the Kansas City Cardiomyopathy Questionnaire (KCCQ).

Epicardial injection of iPSC-CMs

Two patients with ischemic cardiomyopathy underwent epicardial injection of allogeneic iPSC-CMs in combination with CABG in China under a clinical trial (NCT03763136) (18). This study is a dose-escalation, placebo-controlled, single-center phase I/IIa clinical trial (19). Patients with advanced heart failure are randomized either to receive epicardial injection of iPSC-CMs during CABG surgery or CABG surgery alone. The patients were followed by a 12-month investigation. The primary endpoint is to assess the safety of the epicardial injection of iPSC-CMs, including hemodynamically compromised ventricular arrhythmias and newly formed tumors during the initial six months. The secondary endpoint is to evaluate the efficacy of the combination of epicardial injection of iPSC-CMs and CABG surgery in comparison to isolated CABG surgery. 

Summary

Transplantation of iPSC-CMs represents an emerging therapeutic option for patients with end-stage heart failure. Of course, some challenges with the clinical translation of iPSC-CM therapy include: 1) Immune rejection, 2) Cell survival and retention, 3) Necessity of invasive surgery. In terms of immune rejection, solutions include major histocompatibility (MHC)-matching and the production of iPSC banks (20). Eventually, autologous transplantation of iPSC-CMs would be ideal, however the cost and time to produce autologous iPSC-CMs are hurdles for clinical application. To improve survival of transplanted iPSC-CMs, several approaches have been trialed including co-transplantation of iPSC-CMs with epicardial cells or ready-made microvessels (21,22). In terms of necessity of invasive surgery, minimally invasive approaches such as transcatheter implantation of iPSC-CMs have also been considered.

Although clinical translation of iPSC-CMs therapy still has several limitations, it has been shown that transplanted iPSC-CMs positively effects host cardiomyocytes and has potential to lead to functional recovery of the host heart. Transplantation of iPSC-CMs is one of the promising cardiac regenerative therapies and several clinical trials currently are in-progress worldwide.

References

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