Cys-Arg-Glu-Lys-Ala-superparamagnetic iron oxide-Cy7 nanoparticles

Optical fluorescence imaging is increasingly being used to obtain images of biological functions of specific targets and in small animals (1, 2). Near-infrared (NIR) fluorescence (700–900 nm) detection avoids the background fluorescence interference of natural biomolecules, providing a high contrast between target and background tissues. NIR fluorescence imaging is becoming a non-invasive alternative to radionuclide imaging and in small animals. The superparamagnetic iron oxide (SPIO) structure is composed of ferric iron (Fe) and ferrous iron (Fe). The iron oxide particles are coated with a 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 magnetic resonance imaging (MRI), though it can shorten both T1 and T2/T2* relaxation processes. SPIO particle uptake into the reticuloendothelial system (RES) is by endocytosis or phagocytosis. SPIO particles are also 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 (3). A multimodal nanoparticle probe that consists of a contrast agent and a NIR fluorochrome may provide consistent imaging information. SPIO is composed of iron nanoparticles that are 4–6 nm diameter with a hydrodynamic diameter with dextran coating of 50 nm. SPIO nanoparticles can be internalized by RES cells and have long circulating times within an animal body. The accumulation of nanoparticles in cells causes a reduction in signal intensity with T2-weighted (T2*W) spin-echo pulse sequences. NIR fluorochromes (e.g., Cy5.5) provide an improved optical (NIR) signal from tissue. CLIO-Cy5.5 has been developed as a probe for multimodality imaging in small animals (4). A meshwork of clotted proteins that has been identified in tumor stroma and vessels is absent in normal tissues (5, 6). The tumor-homing peptide Cys-Arg-Glu-Lys-Ala (CREKA) was identified with phage display screening in tumor-bearing mice with minimal binding to normal vessels (7). CREKA was identified as a ligand that bound to the meshwork of clotted proteins in the tumor stroma and blood vessels. CREKA was conjugated to SPIO labeled with Cy7 (CREKA-SPIO-Cy7) to study biodistribution of the nanoparticles in tumor-bearing mice. CREKA-SPIO-Cy7 is a multimodal imaging agent that consists of SPIO nanoparticles (MRI) with attachment of CREKA and Cy7 (NIR).