Discrete Nanomolecular Polyhedral Borane Scaffold Supporting Multiple Gadolinium(III) Complexes as a High Performance MRI Contrast Agent

An icosahedral closo-B12 2– scaffold supports 12 copies of Gd3+-chelate held in close proximity with each other by suitable linkers which employ azide–alkyne click chemistry. This design is the first member of a new class of polyfunctional MRI contrast agents carrying a high payload of Gd3+-chelate in a sterically constrained configuration. The resulting contrast agent shows higher relaxivity values at high magnetic fields. MRI contrast agents currently in use are not as effective in this regard, presumably due to a lack of steric constraint of gadolinium centers and lower water exchange rates. In vivo MRI studies in mice show excellent contrast enhancement even at one-seventh of the safe clinical dose (0.04 mmol Gd/kg) for up to a 1 h exposure.