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 T/sub 1/ shortening agents an optimal water residence lifetime (/spl tau//sub 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 /spl tau//sub 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.
Gadolinium(III) Chelates as MRI Contrast Agents: Structure, Dynamics, and Applications.
A. Water Exchange 2326 B. Proton Exchange 2327 C. Electronic Relaxation 2327 D. Relaxivity 2331 E. Outerand Second-Sphere Relaxivity 2334 F. Methods of Improving Relaxivity 2336 V. Macromolecular Conjugates 2336 A. Introduction 2336 B. General Conjugation Methods 2336 C. Synthetic Linear Polymers 2336 D. Synthetic Dendrimer-Based Agents 2338 E. Naturally Occurring Polymers (Proteins, Polysaccharides, and Nucleic Acids) 2339
Studies of Regression in a Transplantable Tumor in Mice *
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