Imaging of matrix metalloproteinase activation and left ventricular remodeling

Molecular imaging of the cardiac extracellular matrix

In almost all cardiac diseases, an increase in extracellular matrix (ECM) deposition or fibrosis occurs, mostly consisting of collagen I. Whereas replacement fibrosis follows cardiomyocyte loss in myocardial infarction, reactive fibrosis is triggered by myocardial stress or inflammatory mediators and often results in ventricular stiffening, functional deterioration, and development of heart failure. Given the importance of ECM deposition in cardiac disease, ECM imaging could be a valuable clinical tool. Molecular imaging of ECM may help understand pathology, evaluate impact of novel therapy, and may eventually find a role in predicting the extent of ECM expansion and development of personalized treatment. In the current review, we provide an overview of ECM imaging including the assessment of ECM volume and molecular targeting of key players involved in ECM deposition and degradation. The targets comprise myofibroblasts, intracardiac renin-angiotensin axis, matrix metalloproteinases, and matricellular proteins.

Synthesis and evaluation of a novel hydroxamate based fluorescent photoprobe for imaging of matrix metalloproteinases

The assessment of matrix metalloproteinase (MMP) activity in vivo is highly desirable in various human diseases such as cancers. Hydroxamic acids based on CGS27023A or CGS25966 are nonpeptidyl lead structures that specifically target activated MMPs in vivo. The aim of this study was the modification and fluorescent labeling of these lead structures to develop a highly affine, nonpeptide MMP inhibitor (MMPI)-ligand for molecular optical imaging of activated MMPs. An 11 step synthesis was developed involving a PEGylated benzyl derivative as a spacer to minimize the interactions between the activated MMP and the dye of conjugate 11 with an azide as a protected amino function. After reducing the azide (Staudinger reaction) and labeling with Cy5.5, we obtained a CGS-based MMP inhibitor 11 with a fluorescent signaling flag. To evaluate the biological properties of this photoprobe, three human cancer cell lines (A-673, HT-1080 and BT-20) were characterized with respect to their MMP-2 and -9 (gelatinases) expression levels (real-time PCR) and protein levels (Western blotting). Initially, fluorogenic inhibition assays were used to assess the MMP inhibition potential. The PEGylated CGS 10 showed complete inhibition of MMP-2 and MMP-9 activities in vitro both for purified MMP-2/-9 (active and pro-forms) and MMP-2/-9 containing cell culture supernatants. To test the imaging potential in biological tissues, gelatinase activity was measured on tumor cryostat sections of the above-mentioned tumor cells using FITC-labeled dye-quenched gelatin. Gelatinase positive tumors revealed strong binding of CGS-Cy5.5 11, while gelatinase negative tumors were not targeted. In conclusion, this new CGS-based MMP photoprobe has a high affinity for MMP-2 and -9 and is thus a promising candidate for sensitive imaging of MMP activity in various diseases in patients.

PET and SPECT in cardiovascular molecular imaging

The current focus of cardiovascular medicine is on early detection and prevention of disease, to control the escalating costs of health care. To achieve this goal, novel imaging approaches that allow for early detection of disease and risk stratification are needed. Traditionally, the diagnosis, monitoring, and prognostication of cardiovascular disease were based on techniques that measured changes in metabolism, blood flow, and biological function. Molecular imaging is emerging as a new tool for the noninvasive detection of biological processes that can differentiate and characterize tissues before manifestation of gross anatomical features or physiological consequences. Leading the way are techniques involving high-sensitivity radiotracers that could revolutionize current diagnostic paradigms. This Review provides an overview of selected molecular-based single photon emission CT (SPECT) and PET imaging strategies for the evaluation of cardiovascular disease-including the evaluation of myocardial metabolism and neurohumoral activity of the heart-and potential future targeted methods of evaluating critical molecular processes, such as atherosclerosis, ventricular remodeling after myocardial infarction, and ischemia-associated angiogenesis.

Clinical cardiovascular molecular imaging

Molecular imaging holds the promise of becoming a key diagnostic modality in cardiovascular medicine by allowing visualization of specific targets and pathways that precede or underlie changes in morphology, physiology, and function. As such, molecular imaging aims at detecting precursors or early stages of cardiovascular disease and at monitoring and guiding novel, increasingly specific and versatile cardiovascular therapies. Imaging of myocardial metabolism and autonomic innervation are already used in current practice, and a wide variety of other targets and probes is on the horizon. This focused review provides an overview of the opportunities and challenges that molecular imaging faces to fulfill its promises in clinical cardiovascular medicine.