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-B₁₂²⁻ scaffold based nano-sized assembly capable of carrying a high payload of Gd³⁺-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-B₁₂²⁻ core via suitable linkers through click reaction. This nanomolecular structure supporting a high payload of Gd³⁺-chelate is a new member of the closomer MRI contrast agents that we are currently developing in our laboratory. The per Gd ion relaxivity (r₁) 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 (r₁ = 4.7 in PBS at 25 °C) compared to the clinically used MRI contrast agents Omniscan (r₁ = 4.2 in PBS at 25 °C) and ProHance (r₁ = 3.1 in PBS at 25 °C).