Conjugation of a water-soluble gadolinium endohedral fulleride with an antibody as a magnetic resonance imaging contrast agent

Water-soluble gadofullerides exhibited high efficiency as magnetic resonance imaging (MRI) contrast agents. In this paper, we report the conjugation of the newly synthesized gadofulleride, Gd@C82O6(OH)(16)(-)(NHCH2CH2COOH)(8), with the antibody of green fluorescence protein (anti-GFP), as a model for tumor targeted imaging agents based on endohedral metallofullerenes. In this model system, the activity of the anti-GFP conjugate can be conveniently detected by green fluorescence protein (GFP), leading to in vitro experiments more direct and facile than those of tumor antibodies. Objective-type total internal reflection fluorescence microscopy revealed that each gadofulleride aggregate conjugated on average five anti-GFPs, and the activity of anti-GFPs was preserved after conjugation. In addition, the gadofulleride/antibody conjugate exhibited higher water proton relaxivity (12.0 mM(-1) s(-1)) than the parent gadofulleride aggregate (8.1 mM(-1) s(-1)) in phosphate buffered saline at 0.35 T, as also confirmed by T-1-weighted images of phantoms. These observations clearly indicate that the synthesized gadofulleride/antibody conjugate not only has targeting potential, but also exhibits higher efficiency as an MRI contrast agent.