Evaluating Labelling Efficiency of Commercial SPIONs in Mesenchymal Stem/Stromal Cells for Magnetic Particle Imaging Applications

Magnetic Particle Imaging (MPI) is a state-of-the-art, highly sensitive modality for non-invasive cell tracking. This study evaluated labelling efficiency, biocompatibility, intracellular localization, and MPI detection sensitivity of four commercial superparamagnetic iron oxide nanoparticles (SPIONs)—ProMag, VivoTrax, SynoMag-D, and Ferumoxytol—in mouse mesenchymal stem/stromal cells.

SPION labelling efficiency and cytotoxicity was assessed at varying concentrations and incubation times using Prussian blue staining and ATP-based viability assays, respectively. MPI characterization and transmission electron microscopy (TEM) evaluations were performed for cells labelled for two-hour with ProMag or VivoTrax.

For >90% labelling efficiency, ProMag required 20□µg Fe/mL across all time points. VivoTrax, however, required ≥240□µg Fe/mL, reducing cell viability by >20% necessitating a reduction to 120 µg/mL for further analyses. Transfection agents improved SynoMag-D and Ferumoxytol labelling but compromised viability. MPI analysis revealed linear dependence of signal intensity on labelled cell numbers for ProMag and VivoTrax (r ² =0.99). ProMag yielded higher signal intensity due to greater iron uptake, although VivoTrax exhibited higher signal per unit iron. TEM confirmed intracellular SPION localization, with ProMag present as individual particles and VivoTrax as aggregates within endocytic vesicles. Low-temperature assays confirmed energy-dependent endocytosis as the primary uptake mechanism. Despite ProMag’s stronger MPI signals and lower detection threshold (12,500 cells), VivoTrax’s superior magnetization per iron suggests its potential following further optimization of cell uptake.

Overall, ProMag and VivoTrax emerged as optimal candidates for MPI-based stem cell tracking. These findings underscore the importance of optimizing both nanoparticle selection and labelling protocols to maximize MPI performance and inform future in vivo applications.