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H18/7 F(ab’)2 E-selectin monoclonal antibody conjugated to cross-linked iron oxide nanoparticles.


Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2007 Mar 26 [updated 2007 Apr 20].


Magnetic resonance imaging (MRI) maps information about tissues spatially and functionally. Protons (hydrogen nuclei) are widely used to create images because of their abundance in water molecules. Water comprises ~80% of most soft tissue. The contrast of proton MRI depends mainly on the density of nuclear proton spins, the relaxation times of the nuclear magnetization (T1, longitudinal and T2, transverse), the magnetic environment of the tissues, and the blood flow to the tissues. However, insufficient contrast between normal and diseased tissues requires development of contrast agents. Most contrast agents affect the T1 and T2 relaxation of the surrounding nuclei, mainly the protons of water. T2* is the spin-spin relaxation time composed of variations from molecular interactions and intrinsic magnetic heterogeneities of tissues in the magnetic field (1). The superparamagnetic iron oxide (SPIO) structure is composed of ferric iron (Fe3+) and ferrous iron (Fe2+). The iron oxide particles are coated with a protective layer of dextran or other polysaccharide. These particles have large combined magnetic moments or spins, which are randomly rotated in the absence of an applied magnetic field. SPIO is used mainly as a T2 contrast agent in MRI, though it can shorten both T1 and T2/T2* relaxation processes. SPIO particle uptake into the reticuloendothelial system (RES) occurs by endocytosis or phagocytosis. SPIO particles are taken up by phagocytic cells such as monocytes, macrophages, and oligodendroglial cells. A variety of cells can also be labeled with these particles for cell trafficking and tumor-specific imaging studies. SPIO agents are classified by their sizes with coating material (~20–3,500 nm in diameter) as large SPIO (LSPIO) nanoparticles, standard SPIO (SSPIO) nanoparticles, ultrasmall SPIO (USPIO) nanoparticles, and cross-linked iron oxide (CLIO) nanoparticles (1). USPIO nanoparticles are composed of iron particles of ~4–6 nm diameters and the hydrodynamic diameters are ~20–40 nm. The crystals are covered with a layer of dextran with significant T2 relaxation effects. USPIO nanoparticles have a long plasma half-life as a result of improved coating and small size. In humans, the blood pool half-life of plasma relaxation times is calculated to be >24 h (2) and 1-2 h in mice. Because of its long blood half-life, USPIO nanoparticles can be used as a blood pool agent during the early phase of intravenous administration (3). In the late phase, USPIO nanoparticles are suitable for the evaluation of RES in the body, particularly in lymph nodes (4). E-selectin is found on the cell surface of endothelial cells (5, 6). It binds to sialy-Lewisx (a carbohydrate moiety) on the cell-surface of leukocytes. Tumor necrosis factor α (TNFα) and interleukin-1 (IL-1), released from inflammatory stimuli, upregulated E-selectin and other adhesion molecule expression on the vascular endothelial cells, which leads to leukocyte adhesion to the activated endothelium. E-selectin and other selectins are involved in arresting leukocytes on the endothelium. An anti-human E-selectin monoclonal antibody fragment, H18/7 F(ab’)2, was conjugated to CLIO nanoparticles for noninvasive MRI of E-selectin expression in endothelial cells (7, 8).

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