Antibody-magnetite nanoparticles: in vitro characterization of a potential tumor-specific contrast agent for magnetic resonance imaging.

Target-specific superparamagnetic contrast agents may allow the localization of specific tissues such as tumors by magnetic resonance imaging (MRI). In this report the preparation and in vitro characterization of tumor-specific superparamagnetic particles (SMP) are described. Particles of uniform size (9.6 +/- 0.8 nm) were prepared from an alkaline solution of ferric and ferrous ions and isolated by differential centrifugation. The resulting nanoparticle suspension is stabilized in buffer using a polypeptide coat to which a monoclonal antibody, specific to carcinoembryonic antigen (CEA), was covalently attached at the hinge region. The resulting anti-CEA SMP have a hydrodynamic radius of less than 50 nm, and specifically bind to CEA in vitro. The visualization of epitopes, present on a cell surface in very low density as expected for tumor antigens or receptors, may be achieved due to the high R2 relaxivity of 300 L mmol-1s-1 of the contrast agent described here. Furthermore, the polypeptide coat chosen provides an ideal platform for the attachment of biological modifiers needed for the reduction of the antigenicity and blood clearance rate of anti-CEA SMP.