Cell and gene therapies are exciting and ever developing strategies for regenerating/repairing the injured myocardium in HFrEF. A few studies have compared the effectiveness (e.g., cell retention rate) of the different routes of cell delivery, which have consistently demonstrated the superiority of IMI over intracoronary or intravenous routes1,2,3,4,5. Thus, it is not surprising that a large proportion of studies on translational models of stem cell therapy of the injured myocardium, deliver the injectate via IMI performed under direct view in an open chest procedure6,7. However, this approach has several limitations, including the invasive nature of the procedure, which carries the risk of peri-procedural mortality (often under-reported)8. In addition, an IMI under direct view does not eliminate the possibility for inadvertent injection into the ventricular cavity. In clinical practice an IMI during open chest surgery could be an appropriate method for therapeutic cell delivery, e.g., during coronary artery bypass graft surgery (CABG); however, this approach may not be appropriate for cell delivery in global cardiomyopathy of non-ischemic origin (e.g., HFrEF secondary to anthracycline-induced cardiomyopathy (AICM)).
There is no doubt that ischemic heart disease (IHD) is the most common cause of HFrEF (~ 66%)9,10; however, non-ischemic cardiomyopathy, including AICM, still affects a significant proportion of patients with HFrEF (33%)9. Indeed, recent advances in clinical oncology have resulted in more than 10 million survivors of cancer in the USA alone11, with estimates of a similar number in Europe, consistent with an overall trend towards improved survival of cancer patients12,13. Thus, exploring the benefits of novel therapies such as stem cell transplant for non-ischemic cardiomyopathy, as well as the trialing of an effective and minimally invasive route of stem cell delivery is of utmost importance, given the increasing number of patients affected by cardiotoxicity secondary to anticancer drugs.
Of note, hypothesis testing studies using stem cell therapy aiming to repair/regenerate the injured myocardium frequently involves the use of small rodents (e.g., mice and rats). These models often require expensive high frequency ultrasound systems for evaluation of myocardial function, usually equipped with linear array transducers which have some inherent associated limitations (e.g., reverberation)14. However, other models such as rabbits, representing a large preclinical model, have some advantages for hypothesis testing of stem cell therapies in HFrEF. Thus, in contrast to rats and mice, rabbits maintain a Ca+2 transport system and cellular electrophysiology that resembles that of humans and other large animals (e.g., dogs and pigs)15,16,17,18,19. Another advantage, is their amenability for cardiac ultrasound imaging using relatively inexpensive and widely available clinical echocardiography systems equipped with relatively high frequency phase array transducers, e.g., 12 MHz, such as those frequently used in neonatal and pediatric cardiology. These systems allow excellent echocardiographic imaging with state of the art technology, and they take advantage of the superiority of harmonic imaging20. Furthermore, extensive hypothesis testing of the potential of cardiac regenerative therapies (e.g., stem cell therapy), their safety, efficacy, cardiomyogenic potential, as well as evaluation of the fate of the injectate once delivered into the myocardium, is mandatory before they can be considered for human use, and they require the use of large preclinical animal models, such as the rabbit17,19. Here, we describe a minimally invasive technique for cell delivery via percutaneous contrast-echocardiography guided IMI using a clinical echocardiography system, which is aimed at stem cell transplant-based therapy for non-ischemic cardiomyopathy20. We also describe the benefits of India Ink (InI, also known as China Ink) as an ultrasound contrast agent and in situ tracer of the injectate in the rabbit heart.