In Vitro Characterization of Magnetic Resonance Imaging Contrast Agents for Molecular Imaging.

The aim of this work was to evaluate the biological properties of one citrate-coated and two different dextran-coated paramagnetic particles with comparable size (iron core 4–10 nm).

Endothelial cells from humans and mice as well as human macrophages were incubated for different time intervals with different particle suspensions. The cellular uptake was semi-quantitatively measured using the Prussian blue staining and, in addition, by cellular iron content. Furthermore the effect of known inhibitors of endocytosis was evaluated. In addition, it was evaluated whether linking of monoclonal antibodies to dextran-coated particles can make them bind specifically to certain cell surface structures.

The results showed that the bEnd.3 cell line, human umbilical vein endothelial cells (HUVECs) and THP-1/macrophage cell lines internalize paramagnetic particles. The ranking of cellular uptake was: VSOP > CMD-coated particles >> CLIO. The carboxydextran-coated SPIO uptake by endothelial cells is reduced by colchicine (50%). Conversely, cytochalasin B down-regulates the endocytosis of citrate-coated particle. Our data imply that the major mechanism of uptake would be pinocytosis for the VSOP and phagocytosis for the carboxydextran-coated particle CMD. The different surface coating can influence not only the quantity of the internalization, but also the pathway of internalization. CLIO linked to CD40 antibodies or to CD62E antibodies bound significantly better than IgG-linked CLIO. This was true especially for the anti-CD40-CLIO constructs where fluorescence increased two fold. Comparable results were observed with anti-CD62E-CLIO constructs; however, increase in fluorescence was higher than with CD40 binding; it increased on 3.9-fold (median) and 4.5-fold (mean).

In conclusions, the binding of antibody-conjugated CLIO to the antigen-expressing cells was specific, with an affinity similar to that of the free antibody. Thus, it seems feasible to use antibody linked SPIOs for molecular imaging.