Magnetic Nanoparticles in Macrophages and Cancer Cells Exhibit Different Signal Behavior on Magnetic Particle Imaging

Cell labeling with magnetic nanoparticles (MNPs) is a promising method of cell tracking. In particular, a novel quantitative tomography method called magnetic particle imaging (MPI) has the potential to estimate the number of successfully transplanted MNP-labeled cells, thereby helping predict clinical outcomes. However, the biological factors that shape the MPI signals of MNPs during cell labeling are not well understood. To better understand these factors, the MPI signals of MNPs in various extracellular and intracellular conditions were assessed. Firstly, carboxydextran-coated MNPs (Resovist®) in the presence or absence of the transfection agents heparin and/or protamine were subjected to dynamic light scattering analysis and magnetic particle spectroscopy. Secondly, RAW264 macrophages and Colon26 carcinoma cells were labeled with Resovist® by using their intrinsic phagocytic activity or with the assistance of the transfection agents, respectively, after which the cells were visualized by our MPI scanner and transmission electron microscopy, and their absolute amounts of intracellular iron were measured by thiocyanate colorimetry. The MPI pixel values were normalized to intracellular iron concentrations. Finally, the effect of cell lysis on the MPI signal was assessed with magnetic particle spectroscopy. The presence of protamine, but not heparin, increased the hydrodynamic diameter of the MNPs and inhibited their MPI signals. Cell uptake drastically decreased the normalized MPI pixel values. This was particularly marked in the colon cancer cells. The transfection agents did not further alter the MPI signal of the MNP-labeled colon cancer cells. Transmission electron microscopy showed that there was much more MNP aggregation in colon cancer cells than in macrophages. After the MNP-labeled cells were lysed, the MPI signal recovered partially. In conclusion, MPI pixel values can be influenced by the cell-labeling process and cellular uptake. The MPI signals from intracellular magnetic nanoparticles may also differ depending on the cell type.