Molecular Images of Neovascularization

Pharmacokinetics of contrast agents targeted to the tumor vasculature in molecular magnetic resonance imaging

Molecular magnetic resonance imaging (MRI) is increasingly used to investigate tumor angiogenic activity non-invasively. However, the pharmacokinetic behavior and tumor penetration of the often large contrast agent particles is thus far unknown. Here, pharmacokinetic analysis of cyclic asparagine-glycine-arginine (cNGR) labeled paramagnetic quantum dots (pQDs) was developed to quantify the contrast agent's homing efficacy to activated endothelial cells of angiogenic tumor vessels using dynamic contrast-enhanced (DCE) MRI. cNGR homes to CD13, an overexpressed aminopeptidase on angiogenic tumor endothelial cells. First, a two-compartment pharmacokinetic model, comprising the blood space and endothelial cell surface, was compared with a three-compartment model additionally including the extravascular-extracellular component. The resulting extravasation parameter was irrelevantly small and was therefore neglected. Next, the association constant K(a), the dissociation constant k(d) and the fractional plasma volume v(P) were determined from the time-series data using the two-compartment model. Magnitude and spatial distribution of the parameters were compared for cNGR-labeled and unlabeled pQDs. The tumor area with significant K(a) values was approximately twice as large for cNGR-pQDs compared with unlabeled pQDs (p < 0.05), indicating more contrast agent binding for cNGR-pQDs. Using cNGR-pQDs, a two-fold larger area with significant K(a) was also found for the angiogenic tumor rim compared with tumor core (p < 0.05). It was furthermore found that both contrast agents perfused the tumor at all depths, thereby providing unequivocal evidence that rim/core differences can indeed be ascribed to stronger angiogenic activity in the rim. Summarizing, molecular DCE-MRI with pharmacokinetic modeling provides unique information on contrast agent delivery and angiogenic activity in tumors.

Imaging of angiogenesis in cardiology

In the past decade, there have been major improvements in our understanding of angiogenesis at the genetic, molecular and cellular levels. Concentrated efforts in this area have led to new therapeutic approaches to ischaemic heart disease using angiogenic factors, gene therapy and progenitor cells. Despite very promising experimental results in animal studies, large clinical trials have failed to confirm the results in patients with coronary artery disease. Important questions such as selection of growth factors and donor cells, as well as the timing, dose and route of administration, have been raised and need to be answered. Molecular imaging approaches which may provide specific markers of the angiogenic process (e.g. integrin expression in endothelial cells) have been introduced and are expected to address some of these questions. Although few clinical imaging results are currently available, animal studies suggest the potential role of molecular imaging for characterisation of the angiogenetic process in vivo and for the monitoring of therapeutic effects.

Cardiovascular Molecular Imaging

The goal of this review is to highlight how molecular imaging will impact the management and improved understanding of the major cardiovascular diseases that have substantial clinical impact and research interest. These topics include atherosclerosis, myocardial ischemia, myocardial viability, heart failure, gene therapy, and stem cell transplantation. Traditional methods of evaluation for these diseases will be presented first, followed by methods that incorporate conventional and molecular imaging approaches.