Paramagnetic water-soluble metallofullerenes having the highest relaxivity for MRI contrast agents

Water-soluble gadolinium (Gd) endohedral metallofullerenes have been synthesized as polyhydroxyl forms (Gd@C-82(OH)(n), Gd-fullerenols) and their paramagnetic properties were evaluated by in vivo as well as in vitro for the novel magnetic resonance imaging (MRI) contrast agents for next generation. The in vitro water proton relaxivity, R-1 (the effect on 1/T-1), of Gd-fullerenols is significantly higher (20-folds) than that of the commercial MRI contrast agent, Magnevist (gadolinium-diethylenetri-aminepentaacetic acid, Gd-DTPA) at 1.0 T close to the common field of clinical MRI. This unusually high proton relaxivity of Gd-fullerenols leads to the highest signal enhancement at extremely lower Gd concentration in MRI studies. The strong signal was confirmed in vivo MRI at lung, liver, spleen, and kidney of CDF1 mice after i.v. administration of Gd-fullerenols at a dose of 5 mu mol Gd/kg, which was 1/20 of the typical clinical dose (100 mu mol Gd/kg) of Gd-DTPA.