Ultrasmall superparamagnetic iron oxide-Leu-Ile-Lys-Lys-Pro-Phe

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, which comprise >80% of most soft tissues. The contrast of proton MRI images depends mainly on the density of nuclear proton spins, the relaxation times of the nuclear magnetization (T1, longitudinal; T2, transverse), the magnetic environment of the tissues, and the blood flow to the tissues. However, insufficient contrast between normal and diseased tissues requires the use 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). Cross-linked iron oxide (CLIO) and other iron oxide formulations affect T2 primarily and lead to a decreased signal. On the other hand, the paramagnetic T1 agents, such as gadolinium (Gd) and manganese (Mn), accelerate T1 relaxation and lead to brighter contrast images. Apoptosis (programmed cell death) plays an important role in the pathophysiology of many diseases, such as cancer, neurodegenerative disorders, vascular disorders, and chronic hepatitis, as well as in the biology of normal cells like epithelial cells and immune cells (2). During apoptosis, there is rapid redistribution of phosphatidylserine (PS) from the inner membrane leaflet to the outer membrane leaflet, exposing the anionic head group of PS. PS is also accessible for annexin V binding in apoptosis and necrosis because of disruption of the plasma membrane. Annexin V binds to PS with high affinity (dissociation constant = 7 nM). Annexin V has been radiolabeled with I, I, and Tc for use with single-photon emission computed tomography and positron emission tomography imaging (PET) (3-5). Recently, annexin V was successfully labeled with F (6, 7), and F-labeled annexin V is being developed as a PET agent for imaging apoptosis as well as necrosis. Cy5.5-annexin V and quantum dot (QD)-annexin V (8) have been evaluated as optical probes for near-infrared (NIR) imaging. Various bimodal annexin V–conjugated nanoparticles have been developed for imaging apoptosis, such as annexin V-CLIO-Cy5.5 and annexin V-QD-gadolinium for use with NIR and MRI imaging (9, 10). A cyclic peptide, Cys-Leu-Ile-Lys-Lys-Pro-Phe-Cys, was identified with phage screening against PS (11). A linear peptide, Leu-Ile-Lys-Lys-Pro-Phe (P826), was synthesized and conjugated to the 8-amino-3,6-dioxaoctanoyl linker to form g-R826, which was conjugated with 2-(4-isothiocyanatobenzyl)-diethylenetriamine pentaacetic acid (-SCN-Bn-DTPA) to form DTPA-g-R826 for labeling with Gd (Gd-DTPA-g-R826) for MRI of PS in apoptosis. Gd-DTPA-g-R826 was validated on apolipoprotein E–deficient (apoE−−) mice with MRI for characterization of atherosclerotic plaque. However, Gd-DTPA-g-R826 exhibited low MRI efficacy (low and relaxivity values). Burtea et al. (12) prepared ultrasmall superparamagnetic iron oxide-R826 (USPIO-R826) for use with MRI of PS in apoptosis for characterization of atherosclerotic plaques.