1 Rapid Synthesis of PEGylated Ultrasmall Gadolinium Oxide 2 Nanoparticles for Cell Labeling and Tracking with MRI 3 Luc Faucher, †,‡ Me ́ lanie Tremblay, †,‡ Jean Lagueux, † Yves Gossuin, § and Marc-Andre ́ Fortin* ,†,‡ 4 † Axe me ́ tabolisme, sante ́ vasculaire et ré nale, Centre hospitalier universitaire de Que ́ bec (CRCHUQ-MSVR), 10 rue de l’Espinay, 5 Que ́ bec, G1L 3L5, Canada 6 ‡ Centre de recherche sur les maté riaux avance ́ s (CERMA) and De ́ partement de Ge ́ nie des Mines, de la Mé tallurgie et des Mate ́ riaux, 7 Universite ́ Laval, Que ́ bec, G1 V 0A6, Canada 8 § Service de Physique Expé rimentale et Biologique, Universite ́ de Mons, 24, avenue du Champs de Mars, 7000, Mons, Belgium 9 * S Supporting Information 10 ABSTRACT: Ultrasmall paramagnetic Gd 2 O 3 nanoparticles 11 have been developed as contrast agents for molecular and 12 cellular preclinical MRI procedures. These small particles 13 (mean diameter <5 nm) have the highest Gd density of all 14 paramagnetic contrast agents. They generate strong positive 15 contrast enhancement in T 1 -weighted MRI. Signal enhance- 16 ment is modulated by the interactions of water molecules with 17 Gd, and very small particles provide the optimal surface-to- 18 volume ratios necessary to reach high relaxivities. Conven- 19 tional Gd 2 O 3 nanocrystal synthesis techniques, and subsequent 20 polyethylene glycol (PEG) grafting procedures are usually 21 time-consuming and recovery losses are also limitative. The present study reports on a new, fast, and efficient one-pot Gd 2 O 3 22 synthesis technique that provides PEGylated nanoparticles of very small size (mean diameter = 1.3 nm). Readily coated with 23 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 24 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) 25 were labeled with the contrast agent and implanted in vivo (mice brains). The labeled cells appeared positively contrasted at least 26 48 h after implantation. Each one of the implanted animals developed a brain tumor. The performance of PEG-Gd 2 O 3 was also 27 compared with that of commercially available iron oxide nanoparticles. This study demonstrated that ultrasmall PEG-Gd 2 O 3 28 nanoparticles provide strong positive contrast enhancement effects in T 1 -weighted imaging, and allow the visualization of labeled 29 cells implanted in vivo. 30 KEYWORDS: magnetic resonance imaging MRI, contrast agents, gadolinium oxide, nanoparticles, polyethylene glycol, cell labeling, 31 cell tracking, glioblastoma multiforme 1. INTRODUCTION 32 “Positive” contrast agents (CA) are widely used to improve the 33 contrast between tissues using T 1 -weighted magnetic resonance 34 imaging (MRI). Nowadays, 30-50% of all clinical scans are 35 carried out by using systemic injections of CAs, almost all being 36 based on the element gadolinium (Gd). 1,2 This rare-earth 37 element has seven unpaired electrons on its valence orbitals, 38 leading to a high magnetic moment (7.94 μB). Also, the 39 electron spin relaxation time of Gd is long (1 × 10 -9 to 1 × 40 10 -8 s), maximizing dipole-dipole interactions of electrons and 41 hydrogen protons ( 1 H) in the vicinity of the CA. Such 42 interactions enhance the proton relaxation time and, in turn, 43 generate signal enhancement effects in MR images. CAs based 44 on the element Gd are classified as “positive”, by opposition to 45 “negative” CAs (e.g., iron oxide nanoparticles). 46 Cell transplantation and tracking in vivo is an area of intense 47 investigation in biomedical science and in medicine (e.g., 48 immune cell delivery, Langerhans islets and stem cell 49 implantation). MRI whole-body imaging allows the tracking 50 of cells at anatomical resolution (<100 μm). 3 Cell labeling and 51 tracking in MRI has so far been pursued mainly by using iron 52 oxide (IO) nanoparticles, which are high magnetic suscepti- 53 bility nanoparticles providing T 2 /T 2 * effects. In a typical 54 experiment, cells are labeled with nanoparticles in vitro, then 55 injected in vivo. The labeled cells appear as signal voids, or 56 areas of signal loss, in MR images. 4-6 However, the use of IO 57 particles has some drawbacks: (1) first, their large magnetic 58 susceptibility induces strong image artifacts affecting an area 59 that extends far beyond the volume of the labeled cells; this 60 susceptibility artifact is a limit to quantitative studies; (2) MRI 61 cannot distinguish voids generated by the agent from other 62 sources of signal loss (e.g., artifacts); 7 (3) finally, void detection Received: April 12, 2012 Accepted: July 26, 2012 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am3006466 | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX clp00 | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.2.i3 HF01:3823 | 2.0 alpha 39) 2012/07/02 15:37:27 | PROD-JCAVA | rq_1665307 | 8/01/2012 13:53:07 | 10