Cy5.5-c(Lys-Ala-His-Trp-Gly-Phe-Thr-Leu-Asp)NH

Optical fluorescence imaging is increasingly being used to monitor biological functions of specific targets in small animals (1-3). However, the intrinsic fluorescence of biomolecules poses a problem when fluorophores that absorb visible light (350–700 nm) are used. Near-infrared (NIR) fluorescence (700–1,000 nm) detection avoids the natural background fluorescence interference of biomolecules, providing a high contrast between target and background tissues in small animals. NIR fluorophores have a wider dynamic range and minimal background fluorescence as a result of reduced scattering compared with visible fluorescence detection. NIR fluorophores also have high sensitivity, attributable to low background fluorescence, and high extinction coefficients, which provide high quantum yields. The NIR region is also compatible with solid-state optical components, such as diode lasers and silicon detectors. NIR fluorescence imaging is a non-invasive alternative to radionuclide imaging in small animals. 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 (4). 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 (5). 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 (6). Several families of proteases are involved in atherogenesis, myocardial infarction, angiogenesis, and tumor invasion and metastases (7-10). The gelatinase family is a subgroup of MMPs consisting of gelatinase A (MMP-2) and gelatinase B (MMP-9) (11). Gelatinase expression in normal cells, such as trophoblasts, osteoclasts, neutrophils, and macrophages, is highly regulated. Elevated levels of gelatinases have been found in tumors associated with a poor prognosis for cancer patients (12). A cyclic peptide, c(CTTHWGFTLC)NH (C1), was identified by phage screening to be potent inhibitor of MMP-2 and MMP-9 (13); however, C1 is readily degraded . A synthetic peptide, c(KAHWGFTLD)NH (C6), was linked with a Cy5.5 NIR dye molecule the α-NH of Lys to form Cy5.5-C6 for imaging of MMP-2/-9 activity (14). Cy5.5-C6 is being developed for NIR fluorescence imaging of MMP-2/-9 proteolytic activity in tumors.