Rapid Synthesis of PEGylated Ultrasmall Gadolinium Oxide Nanoparticles for Cell Labeling and Tracking with MRI

Ultrasmall paramagnetic Gd2O3 nanoparticles have been developed as contrast agents for molecular and cellular preclinical MRI procedures. These small particles (mean diameter <5 nm) have the highest Gd density of all paramagnetic contrast agents. They generate strong positive contrast enhancement in T-1-weighted MM. Signal enhancement is modulated by the interactions of water molecules with Gd, and very small particles provide the optimal surface-to-volume ratios necessary to reach high relaxivities. Conventional Gd2O3 nanocrystal synthesis techniques, and subsequent polyethylene glycol (PEG) grafting procedures are usually time-consuming and recovery losses are also limitative. The present study reports on a new, fast, and efficient one-pot Gd2O3 synthesis technique that provides PEGylated nanoparticles of very small size (mean diameter = 1.3 nm). Readily coated with PEG, the particles are colloidally stable in aqueous media and provide high longitudial relaxivities and small r(2)/r(1) ratios (r(1) = 14.2 mM(-1) s(-1) at 60 MHz; r(2)/r(1) = 1.20), ideal for T-1-weighted MRI. In this study, F98 brain cancer cells (glioblastoma multiforme) were labeled with the contrast agent and implanted in vivo (mice brains). The labeled cells appeared positively contrasted at least 48 h after implantation. Each one of the implanted animals developed a brain tumor. The performance of PEG-Gd2O3 was also compared with that of commercially available iron oxide nanoparticles. This study demonstrated that ultrasmall PEG-Gd2O3 nanoparticles provide strong positive contrast enhancement in T-1-weighted imaging, and allow the visualization of labeled cells implanted in vivo.