Molecular and functional MRI of cardiac remodeling after myocardial infarction

Myocardial infarction is the leading cause of death world-wide. It is characterized by cardiomyocyte cell death resulting from local oxygen deprivation caused by obstructions in the coronary microcirculation. The heart has very limited potential to regenerate new myocardium, and instead the dead cardiomyocytes are replaced by a non-contractile fibrotic scar tissue. This is accompanied by a progressive cardiac remodeling process, resulting in left ventricular dilation and wall thinning of the infarcted myocardium and gradual infarct expansion into non-ischemic myocardium. Many patients, who survive the initial acute stage of cardiac infarction, however, ultimately develop heart failure because of the inadequate long-term response of the heart to the ischemic insult. Therefore, there is an urgent need for novel treatment strategies that can substantially improve the long-term prognosis after myocardial infarction. For successful clinical implementation of new therapies non-invasive readouts are crucial to 1) select patients for whom therapy is expected to be effective and 2) monitor a patient’s response to therapy. In this thesis, novel therapies and readouts were developed and their preclinical evaluation was performed in a mouse model of myocardial infarction. Magnetic resonance imaging (MRI) is an attractive non-invasive imaging technique that can be used to evaluate global and regional cardiac morphology and contractile function, but can also provide a detailed characterization of the myocardium at the cellular and even molecular level by using MRI contrast agents. Contrast agents can generate contrast between infarcted and remote myocardium either by differences in the local passive distribution pattern or by actively targeting them towards specific cells, proteins or enzymes involved in myocardial infarction. An attractive contrast agent platform are nanometer-sized particles. To stimulate reparative processes in the infarct, efficient delivery and retention of therapeutic agents is desired. This might be achieved by encapsulation of drugs in nanoparticles. To non-invasively evaluate the efficiency of nanoparticle trafficking to the infarct, paramagnetic contrast agents can be incorporated in these nanoparticles for in vivo visualization by MRI. Two types of paramagnetic and fluorescent lipid-based nanoparticles, namely micelles and liposomes, were applied in mice with acute and chronic myocardial infarction to determine their cardiac distribution pattern, and thus their possible utility as drug delivery vehicle to infarcted myocardium. In both acute and chronic infarcts, micelles permeated the entire necrotic myocardium, whereas liposomes displayed slower and more restricted extravasation from the vasculature. Therefore, paramagnetic micelles and liposomes are attractive nanocarriers for transporting distinct types of drugs to the infarct. Importantly, the successful in vivo MRI monitoring of the delivery and spatial distribution of paramagnetic nanocarriers is a promising tool for optimizing drug delivery to infarcted myocardium in preclinical pharmacological research. Moreover, paramagnetic lipid-based nanoparticles are also interesting constructs for molecular MR imaging of processes ongoing in the infarct at the cellular or molecular level to improve patient diagnosis and monitoring. For this purpose, nanoparticles must be functionalized with ligands that can bind to a specific cell receptor, protein or enzyme. We conjugated ICAM-1 specific antibodies to paramagnetic liposomes to explore their ability to visualize ICAM-1 upregulation on the endothelium of the vasculature in the infarcted myocardium and its border zones using in vivo MRI. ICAM-1 expression on blood vessels is critical for the recruitment of leukocytes, which can inflict additional damage to the myocardium. First, the in vitro binding behavior of ICAM-1 targeted liposomes to ICAM-1 expressing endothelial cells was evaluated. ICAM-1 targeted liposomes could differentiate between low and high levels of endothelial ICAM-1 expression with MRI. In addition, ICAM-1 binding was observed in the competing presence of leukocytes and when shear stress was applied to mimic blood flow. These promising in vitro results encouraged follow up in vivo studies. In healthy mice, the circulation half-life of ICAM-1 targeted liposomes was short compared to the half-life of non-specific liposomes, indicating binding to constitutively expressed ICAM-1. Indeed, massive binding of ICAM-1 targeted liposomes to ICAM-1 expressing lung endothelium was observed. In mice with myocardial infarction, ICAM-1 binding liposomes were mainly associated with the vasculature in the infarct periphery and borders, which are sites with highly increased ICAM-1 expression. This was deduced from ex vivo fluorescence microscopy. However, this targeting effect did not create specific in vivo signal enhancement in MR images of the infarcted heart, indicating that the sensitivity of in vivo MRI to detect ICAM-1 upregulation with this contrast agent formulation was not sufficient. A promising approach to improve the healing of the infarct is cell transplantation, which might generate new myocardium. This thesis describes a comparison of three distinct types of cell treatments – namely intra-infarct injection of rhythmically contractile cardiac progenitor cells, contractile myoblasts or non-contractile mesenchymal stem cells – to gain more insight in the requirements of the ideal cell type for myocardial regeneration. Conventional late gadolinium enhancement and cinematographic MRI techniques were used to monitor the effects of cell transplantation on infarct size and cardiac function. Cardiac progenitor cells were found to substantially improve the condition of the heart and were the only cell type leading to decreased infarct size. In addition, they reduced wall thinning of the infarct center and border zones and importantly they improved the contractile function in the infarct borders. Therefore, the ability of transplanted cells to adopt a contractile cardiomyocyte phenotype seems crucial to improve the local contractile function. Nevertheless, the survival of injected cells in the infarct center must be improved to further enhance the infarct contractility. To summarize, this thesis describes studies in mice on the development of novel diagnostic and therapeutic approaches for the treatment of myocardial infarction based on non-invasive MRI techniques. This might ultimately improve the monitoring and treatment of patients to decrease the mortality after myocardial infarction.