Synthesis and Relaxivity Studies of a DOTA-Based Nanomolecular Chelator Assembly Supported by an Icosahedral Closo-B(12)(2−) -Core for MRI: A Click Chemistry Approach

An icosahedral closo-B(12)(2−) scaffold based nano-sized assembly capable of carrying a high payload of Gd(3+)-chelates in a sterically crowded configuration is developed by employing the azide-alkyne click reaction. The twelve copies of DO3A-t-Bu-ester ligands were covalently attached to an icosahedral closo-B(12)(2−) core via suitable linkers through click reaction. This nanomolecular structure supporting a high payload of Gd(3+)-chelate is a new member of the closomer MRI contrast agents that we are currently developing in our laboratory. The per Gd ion relaxivity (r(1)) of the newly synthesized MRI contrast agent was obtained in PBS, 2% tween/PBS and bovine calf serum using a 7 Tesla micro MRI instrument and was found to be slightly higher (r(1) = 4.7 in PBS at 25 °C) compared to the clinically used MRI contrast agents Omniscan (r(1) = 4.2 in PBS at 25 °C) and ProHance (r(1) = 3.1 in PBS at 25 °C).