The collagen-binding adhesion protein 35-Oregon Green 488 (CNA35-OG488) is a fluorescently labeled molecular imaging agent developed for optical imaging of collagen in, amongst others, atherosclerosis (1-3). Oregon Green 488 (OG488) is a fluorescent dye with an excitation maximum of 496 nm, emission maximum (λmax emission) of 510–524 nm, and an extinction coefficient (emax) of 76,000 M–1cm–1 (1, 4).
Collagen has been found to play an important role in maintaining the integrity and stability of normal as well as atherosclerotic vessel wall (5-7). In atherosclerosis, collagen is essential for maintenance of plaque stability. Arterial lesions are rich in matrix, and collagen constitutes up to 60% of the total plaque protein (8). At least 19 different collagens encoded by 33 genes have been reported. Collagen types I, III, and IV are the most abundant fibrillar collagens in the vessel wall, and they are also associated with atherosclerosis and plaque progression (5, 8).
The ability to non-invasively image atherosclerosis is very appealing because in vivo imaging can potentially estimate the burden of atherosclerosis, measure the rate of atherosclerosis progression, the stability of specific plaques, and assess the efficacy of therapeutic interventions (9, 10). Development of a suitable molecular agent targeted to collagen in viable arteries may allow in vivo imaging and detection of atherosclerosis (1). The collagen adhesin (CNA) is a protein isolated from the cell surface of Staphylococcus aureus obtained from patients diagnosed with bacterial arthritis (11, 12). CNA contains a collagen-binding protein domain with inherent specificity for collagen. The collagen-binding part of this transmembrane protein is called CNA35 (11, 13). This binding part consists of two soluble domains, N1 and N2. The binding-apparent dissociation constants (Kd) range from 20 nM to 30 μM (2, 11-13). Krahn et al. (2) developed the CNA35-OG488– and OG448-labeled integrins (GST-α1I-OG448) for specific visualization of collagen in tissues and live cell cultures. The study reported that CNA35-OG488 was superior to GST-α1I-OG488 in visualizing the formation of collagen fibers around live human venous saphena (HVS) cells. Boerboom et al. (3) showed that CNA35-OG488 and Alexa Fluor 488–labeled CNA35 provided in vitro tissue images of collagen organization with improved detail when compared with the second harmonic generation multi-photon microscopy. Megens et al. (1) used CNA35-OG488 to successfully image atherosclerotic plaques in mice. The localization of CNA35-OG488 appears to be determined by collagen content and endothelial permeability. Thus, CNA35-OG488 may serve as an excellent in vivo molecular imaging agent for detecting atherosclerosis and studying plaque stability.