Engineering lanthanide complexes with controlled water exchange rates for magnetic resonance imaging

The control of the rate of water exchange in lanthanide complexes is critical to their future development as contrast agents in magnetic resonance imaging. In traditional T₁ shortening agents an optimal water residence lifetime (τ_M) of around 25 ns is required if the highest relaxivities are to be achieved. In contrast the newer PARACEST agents require that water exchange is much slower with τ_M on the order of microseconds. It is now possible to rationally design complexes that exhibit water exchange rates that are ideal for their intended use. The selection of appropriate ligating groups allow a broad control mechanism. Anionic ligating groups such as carboxylates favour faster exchange whereas neutral ligands such as amides favour slower exchange. By altering the steric hindrance around the water coordination site water exchange rates can be fine tuned. The steric hindrance can be altered by controlling the coordination environment of the lanthanide ion and optimal water exchanges rates so achieved.