Impact of surface coating and particle size on the uptake of small and ultrasmall superparamagnetic iron oxide nanoparticles by macrophages

Shigeyoshi Saito,¹ Mana Tsugeno,¹ Daichi Koto,¹ Yuki Mori,² Yoshichika Yoshioka,² Satoshi Nohara,³ Kenya Murase<sup>1

1</sup>Department of Medical Physics and Engineering, Division of Medical Technology and Science, Faculty of Health Science, Graduate School of Medicine, ²Biofunctional Imaging Lab, Immunology Frontier Research Center (WPI-IFReC), Osaka University, Osaka, Japan; ³The Nagoya Research Laboratory, Meito Sangyo Co, Ltd, Kiyosu, Aichi, Japan

Purpose: Magnetic resonance imaging (MRI) using contrast agents like superparamagnetic iron oxide (SPIO) is an extremely versatile technique to diagnose diseases and to monitor treatment. This study tested the relative importance of particle size and surface coating for the optimization of MRI contrast and labeling efficiency of macrophages migrating to remote inflammation sites.
Materials and methods: We tested four SPIO and ultrasmall superparamagnetic iron oxide (USPIO), alkali-treated dextran magnetite (ATDM) with particle sizes of 28 and 74 nm, and carboxymethyl dextran magnetite (CMDM) with particle sizes of 28 and 72 nm. Mouse macrophage RAW264 cells were incubated with SPIOs and USPIOs, and the labeling efficiency of the cells was determined by the percentage of Berlin blue-stained cells and by measuring T₂ relaxation times with 11.7-T MRI. We used trypan blue staining to measure cell viability.
Results: Analysis of the properties of the nanoparticles revealed that ATDM-coated 74 nm particles have a lower T₂ relaxation time than the others, translating into a higher ability of MRI negative contrast agent. Among the other three candidates, CMDM-coated particles showed the highest T₂ relaxation time once internalized by macrophages. Regarding labeling efficiency, ATDM coating resulted in a cellular uptake higher than CMDM coating, independent of nanoparticle size. None of these particle formulations affected macrophage viability.
Conclusion: This study suggests that coating is more critical than size to optimize the SPIO labeling of macrophages. Among the formulations tested in this study, the best MRI contrast and labeling efficiency are expected with ATDM-coated 74 nm nanoparticles.

Keywords: ultrasmall superparamagnetic iron oxide, cultured mouse macrophage cells, surface coating, particle size, MRI
 

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