Synthesis and Relaxivity Studies of a DOTA-Based Nanomolecular Chelator Assembly Supported by an Icosahedral Closo-B122？ -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).