First Soluble M@C₆₀ Derivatives Provide Enhanced Access to Metallofullerenes and Permit in Vivo Evaluation of Gd@C₆₀[C(COOH)₂]₁₀ as a MRI Contrast Agent

M@C60 and related endohedral metallofullerenes comprise a significant portion of the metallofullerene yield in the traditional arc synthesis, but their chemistry and potential applications have been largely overlooked because of their sparse solubility. In this work, procedures are described to solublize Gd@C60 species for the first time by forming the derivative, Gd@C60[C(COOCH2CH3)2]10, and its hydrolyzed water-soluble form, Gd@C60[C(COOH)2]10. Imparting water solubility to Gd@C60 permits its evaluation as a magnetic resonance imaging (MRI) contrast agent. Relaxometry measurements for Gd@C60[C(COOH)2]10 reveal it to possess a relaxivity (4.6 mM-1 s-1 at 20 MHz and 40 °C) comparable to that of commercially available Gd(III) chelate-based MRI agents. An in vivo MRI biodistribution study in a rodent model reveals Gd@C60[C(COOH)2]10 to possess the first non-reticuloendothelial system (RES) localizing behavior for a water-soluble endohedral metallofullerene species, consistent with its lack of intermolecular aggregation in solution as determined by light-scattering measurements. This first derivatization and use of a M@C60 species suggests new potential for metallofullerene technologies by reducing reliance on the chromatographic purification procedures normally employed for the far less abundant M@C82 and related endohedrals. The recognition that water-soluble fullerene derivatives can be designed to avoid high levels of RES uptake is an important step toward fullerene-based pharmaceutical development.