Effective tracking of bone mesenchymal stem cells in vivo by magnetic resonance imaging using melanin-based gadolinium nanoparticles.

Tracking transplanted stem cells is necessary to clarify cellular properties and improve transplantation success. In this study, we designed and synthesized melanin-based gadolinium (Gd )-chelate nanoparticles (MNP-Gd ) of ∼7 nm for stem cell tracking in vivo. MNP-Gd possesses many beneficial properties, such as its high stability and sensitivity, shorter T1 relaxation time, higher cell labeling efficiency, and lower cytotoxicity compared with commercial imaging agents. We found that the T1 relaxivity (r ) of MNP-Gd was significantly higher than that of Gd-DTPA; the nanoparticles were taken up by bone mesenchymal stem cells (BMSCs) via endocytosis and were broadly distributed in the cytoplasm. Based on an in vitro MTT assay, no cytotoxicity of labeled stem cells was observed for MNP-Gd concentrations of less than 800 µg/mL. Furthermore, we tracked MNP-Gd -labeled BMSCs in vivo using 3.0T MRI equipment. After intramuscular injection, MNP-Gd -labeled BMSCs were detected, even after four weeks, by 3T MRI. We concluded that MNP-Gd nanoparticles at appropriate concentrations can be used to effectively monitor and track BMSCs in vivo. MNP-Gd nanoparticles have potential as a new positive MRI contrast agent in clinical applications. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 131-137, 2017.