Cy5.5-Gly-Pro-Leu-Gly-Val-Arg-Gly-Cys-gold nanoparticles

Extracellular matrix (ECM) adhesion molecules consist of a complex network of fibronectins, collagens, chondroitins, laminins, glycoproteins, heparin sulfate, tenascins, and proteoglycans that surround connective tissue cells, and they are mainly secreted by fibroblasts, chondroblasts, and osteoblasts (1). Cell substrate adhesion molecules are considered essential regulators of cell migration, differentiation, and tissue integrity and remodeling. These molecules play a role in inflammation and atherogenesis, but they also participate in the process of invasion and metastasis of malignant cells in the host tissue (2). Invasive tumor cells adhere to the ECM, which provides a matrix environment for permeation of tumor cells through the basal lamina and underlying interstitial stroma of the connective tissue. Overexpression of matrix metalloproteinases (MMPs) and other proteases by tumor cells allows intravasation of tumor cells into the circulatory system after degrading the basement membrane and ECM (3). Gold has not been used as an X-ray contrast agent . Gold has a higher atomic number and a higher absorption coefficient than iodine, providing 2.7-fold greater contrast/weight than iodine (4). Furthermore, imaging gold at 80–100 keV reduces interference from bone absorption and provides lower soft tissue absorption, which would reduce radiation to patients. Hainfeld et al. (4) used gold nanoparticles (AuNPs; 1.9 nm in diameter, ~50 kDa) as a computed tomography (CT) contrast agent in mice; these experiments showed enhanced CT contrast of the vasculature, kidneys, and tumor in mice. However, plasma proteins in blood adsorb onto the surface of bare AuNPs, which produces large aggregates (5) that may result in altered pharmacokinetics and biodistribution of AuNPs (6). Polyethylene glycol (PEG) has been found to minimize nonspecific adsorption of proteins onto NPs and to reduce their uptake by the liver (6). PEG-AuNPs have been being studied as cancer CT imaging and photothermal agents (7). Several families of MMPs are involved in atherogenesis, myocardial infarction, angiogenesis and tumor invasion and metastases (8-11). MMP expression in normal cells, such as trophoblasts, osteoclasts, neutrophils, and macrophages, is highly regulated. Elevated levels of MMPs have been found in tumors associated with a poor prognosis for cancer patients (12). The peptide Gly-Pro-Leu-Gly-Val-Arg-Gly-Cys-NH was found to be a MMP substrate and is cleaved between Leu and Gly residues. Lee et al. (13) used this sequence with a Cy5.5 NIR dye molecule to attach to AuNPs to form fluorescence-quenched nanoparticles (Cy5.5-Gly-Pro-Leu-Gly-Val-Arg-Gly-Cys-AuNPs (Cy5.5-MMP-AuNPs)). The Cy5.5 molecules are in close proximity, which results in fluorescence quenching because of efficient fluorescence resonance energy transfer to Au. The NIR fluorescence signal increases when the Leu-Gly bond is cleaved by MMPs, releasing Cy5.5-containing fragments. Cy5.5 is a NIR fluorescent dye with absorbance maximum at 675 nm and emission maximum at 694 nm with a high extinction coefficient of 250,000 Mcm. Cy5.5-MMP-AuNPs are being developed for NIR fluorescence imaging of MMPs expressed in tumors, atherosclerosis, myocardial infarction, and other diseases.