Facile Preparation of a New Gadofullerene-Based Magnetic Resonance Imaging Contrast Agent with High ¹H Relaxivity

A new magnetic resonance imaging (MRI) contrast agent based on the trimetallic nitride templated (TNT) metallofullerene Gd3N@C80 was synthesized by a facile method in high yield. The observed longitudinal and transverse relaxivities r1 and r2 for water hydrogens in the presence of the water-soluble gadofullerene 2 Gd3N@C80(OH)∼26(CH2CH2COOM)∼16 (M = Na or H) are 207 and 282 mM−1 s−1 (per C80 cage) at 2.4 T, respectively; these values are 50 times larger than those of Gd3+ poly(aminocarboxylate) complexes, such as commercial Omniscan and Magnevist. This high 1H relaxivity for this new hydroxylated and carboxylated gadofullerene derivative provides high signal enhancement at significantly lower Gd concentration as demonstrated by in vitro and in vivo MRI studies. Dynamic light scattering data reveal a unimodal size distribution with an average hydrodynamic radius of ca. 78 nm in pure water (pH = 7), which is significantly different from other hydroxylated or carboxylated fullerene and metallofullerene derivatives reported to date. Agarose gel infusion results indicate that the gadofullerene 2 displayed diffusion properties different from those of commercial Omniscan and those of PEG5000 modified Gd3N@C80. The reactive carboxyl functionality present on this highly efficient contrast agent may also serve as a precursor for biomarker tissue-targeting purposes.