Kotaro Haga

BACKGROUND: The clinical application of human induced pluripotent stem cell–derived cardiomyocytes (CMs) for cardiac repair commenced with the epicardial delivery of engineered cardiac tissue; however, the feasibility of the direct delivery of human induced pluripotent stem cell–derived CMs into the cardiac muscle layer, which has reportedly induced electrical integration, is unclear because of concerns about poor engraftment of CMs and posttransplant arrhythmias. Thus, in this study, we prepared purified human induced pluripotent stem cell–derived cardiac spheroids (hiPSC-CSs) and investigated whether their direct injection could regenerate infarcted nonhuman primate hearts. METHODS: We performed 2 separate experiments to explore the appropriate number of human induced pluripotent stem cell–derived CMs. In the first experiment, 10 cynomolgus monkeys were subjected to myocardial infarction 2 weeks before transplantation and were designated as recipients of hiPSC-CSs containing 2×10 ⁷ CMs or the vehicle. The animals were euthanized 12 weeks after transplantation for histological analysis, and cardiac function and arrhythmia were monitored during the observational period. In the second study, we repeated the equivalent transplantation study using more CMs (6×10 ⁷ CMs). RESULTS: Recipients of hiPSC-CSs containing 2×10 ⁷ CMs showed limited CM grafts and transient increases in fractional shortening compared with those of the vehicle (fractional shortening at 4 weeks after transplantation: 26.2±2.1%; 19.3±1.8%; P <0.05), with a low incidence of posttransplant arrhythmia. Transplantation of increased dose of CMs resulted in significantly greater engraftment and long-term contractile benefits (fractional shortening at 12 weeks after transplantation: 22.5±1.0%; 16.6±1.1%; P <0.01, left ventricular ejection fraction at 12 weeks after transplantation: 49.0±1.4%; 36.3±2.9%; P <0.01). The incidence of posttransplant arrhythmia slightly increased in recipients of hiPSC-CSs containing 6×10 ⁷ CMs. CONCLUSIONS: We demonstrated that direct injection of hiPSC-CSs restores the contractile functions of injured primate hearts with an acceptable risk of posttransplant arrhythmia. Although the mechanism for the functional benefits is not fully elucidated, these findings provide a strong rationale for conducting clinical trials using the equivalent CM products.