Molecular imaging: the key to advancing cardiac stem cell therapy

Molecular Imaging in Drug Discovery and Development

Noninvasive imaging has played an increasing role in the process of cardiovascular drug development. This review focuses specifically on the use of molecular imaging, which has been increasingly applied to improve and accelerate certain preclinical steps in drug development, including the identification of appropriate therapeutic targets, evaluation of on-target and off-target effects of candidate therapies, assessment of dose response, and the evaluation of drug or biological biodistribution and pharmacodynamics. Unlike the case in cancer medicine, in cardiovascular medicine, molecular imaging has not been used as a primary surrogate clinical end point for drug approval. However, molecular imaging has been applied in early clinical trials, particularly in phase 0 studies, to demonstrate proof-of-concept or to explain variation in treatment effect. Many of these applications where molecular imaging has been used in drug development have involved the retasking of technologies that were originally intended as clinical diagnostics. With greater experience and recognition of the rich information provided by in vivo molecular imaging, it is anticipated that it will increasingly be used to address the enormous time and costs associated with bringing a new drug to clinical launch.

Multimodality molecular imaging in cardiac regenerative therapy

Stem cell therapy holds great promise for the repair and regeneration of damaged myocardium. Disappointing results from recent large-scale randomized trials using adult stem cells, however, have led some to question the efficacy of this new therapeutic. Because most clinical stem cell trials have not incorporated molecular imaging to track cell fate, it may be premature to abandon this approach. Herein, we will review how multimodality imaging can be incorporated into cardiac regenerative therapy to facilitate the translation of stem cell therapy.

New imaging probes to track cell fate: reporter genes in stem cell research

Cell fate is a concept used to describe the differentiation and development of a cell in its organismal context over time. It is important in the field of regenerative medicine, where stem cell therapy holds much promise but is limited by our ability to assess its efficacy, which is mainly due to the inability to monitor what happens to the cells upon engraftment to the damaged tissue. Currently, several imaging modalities can be used to track cells in the clinical setting; however, they do not satisfy many of the criteria necessary to accurately assess several aspects of cell fate. In recent years, reporter genes have become a popular option for tracking transplanted cells, via various imaging modalities in small mammalian animal models. This review article examines the reporter gene strategies used in imaging modalities such as MRI, SPECT/PET, Optoacoustic and Bioluminescence Imaging. Strengths and limitations of the use of reporter genes in each modality are discussed.

Molecular imaging of stem cells for the treatment of acute myocardial infarction

Stem cell therapy has a unique potential and promises hope for the treatment of acute myocardial infarction. Preclinical studies have identified barriers to clinical translation, one of which involves the monitoring of transplanted cells and the elucidation of their fates in vivo. Molecular imaging may help the solutions for these challenges. In this review, we illustrate the mechanisms by which molecular imaging enables insights into and the development of stem cell therapy.

Molecular imaging in cardiovascular disease: Which methods, which diseases?

Techniques for in vivo assessment of disease-related molecular changes are being developed for all forms of non-invasive cardiovascular imaging. The ability to evaluate tissue molecular or cellular phenotype in patients has the potential to not only improve diagnostic capabilities but to enhance clinical care either by detecting disease at an earlier stage when it is more amenable to therapy, or by guiding most appropriate therapies. These new techniques also can be used in research programs in order to characterize pathophysiology and as a surrogate endpoint for therapeutic efficacy. The most common approach for molecular imaging involves the creation of novel-targeted contrast agents that are designed so that their kinetic properties are different in disease tissues. The main focus of this review is not to describe all the different molecular imaging approaches that have been developed, but rather to describe the status of the field and highlight some of the clinical and research applications that molecular imaging will likely provide meaningful benefit. Specific target areas include assessment of atherosclerotic disease, tissue ischemia, and ventricular and vascular remodeling.

Assessment of three techniques for delivering stem cells to the heart using PET and MR imaging

BACKGROUND

Stem cell therapy has a promising potential for the curing of various degenerative diseases, including congestive heart failure (CHF). In this study, we determined the efficacy of different delivery methods for stem cell administration to the heart for the treatment of CHF. Both positron emission tomography (PET) and magnetic resonance imaging (MRI) were utilized to assess the distribution of delivered stem cells.

METHODS

Adipose-derived stem cells of male rats were labeled with super-paramagnetic iron oxide (SPIO) and 18 F-fluorodeoxyglucose (FDG). The left anterior descending coronary artery (LAD) of the female rats was occluded to induce acute ischemic myocardial injury. Immediately after the LAD occlusion, the double-labeled stem cells were injected into the ischemic myocardium (n = 5), left ventricle (n = 5), or tail vein (n = 4). In another group of animals (n = 3), the stem cells were injected directly into the infarct rim 1 week after the LAD occlusion. Whole-body PET images and MR images were acquired to determine biodistribution of the stem cells. After the imaging, the animals were euthanized and retention of the stem cells in the vital organs was determined by measuring the cDNA specific to the Y chromosome.

RESULTS

PET images showed that retention of the stem cells in the ischemic myocardium was dependent on the cell delivery method. The tail vein injection resulted in the least cell retention in the heart (1.2% ± 0.6% of total injected cells). Left ventricle injection led to 3.5% ± 0.9% cell retention and direct myocardial injection resulted in the highest rate of cell retention (14% ± 4%) in the heart. In the animals treated 1 week after the LAD occlusion, rate of cell retention in the heart was only 4.5% ±1.1%, suggesting that tissue injury has a negative impact on cell homing. In addition, there was a good agreement between the results obtained through PET-MR imaging and histochemical measurements.

CONCLUSION

PET-MR imaging is a reliable technique for noninvasive tracking of implanted stem cells in vivo. Direct injection of stem cells into the myocardium is the most effective way for cell transplantation to the heart in heart failure models.

Marching towards regenerative cardiac therapy with human pluripotent stem cells

Damage in cardiac tissues from ischemia or other pathological conditions leads to heart failure; and cell loss or dysfunction in pacemaker tissues due to congenital heart defects, aging, and acquired diseases can cause severe arrhythmias. The promise of successful therapies with stem cells to treat these conditions has remained elusive to the scientific community. However, recent advances in this field have opened new opportunities for regenerative cardiac therapy. Transplantation of cardiomyocytes derived from human pluripotent stem cells has the potential to alleviate heart disease. Since the initial derivation of human embryonic stem cells, significant progress has been made in the generation and characterization of enriched cardiomyocytes and the demonstration of the ability of these cardiomyocytes to survive, integrate, and function in animal models. The scope of therapeutic potential from pluripotent stem cell-derived cardiomyocytes has been further expanded with the invention of induced pluripotent stem cells, which can be induced to generate functional cardiomyocytes for regenerative cardiac therapy in a patient specific manner. The reprogramming technology has also inspired the recent discovery of direct conversion of fibroblasts into cardiomyocyte-like cells, which may allow endogenous cardiac repair. Regenerative cardiac therapy with human pluripotent stem cells is now moving closer to clinic testing.