Physical Properties of Eu(2+)-Containing Cryptates as Contrast Agents for Ultra-High Field Magnetic Resonance Imaging.

The kinetic stabilities and relaxivities of a series of Eu(2+)-containing cryptates have been investigated. Transmetallation studies that monitored the change in the longitudinal relaxation rate of water protons in the presence of Ca(2+), Mg(2+), and Zn(2+) demonstrated that cryptate structure influences stability, and two of the cryptates studied were inert to transmetallation in the presence of these endogenous ions. The efficacy of these cryptates was determined at different magnetic field strengths, temperatures, and pH values. Cryptate relaxivity was found to be higher at ultra-high field strengths (7 and 9.4 T) relative to clinically relevant field strengths (1.4 and 3 T), but the efficiency of these cryptates decreased as temperature increased. In addition, variation in pH did not yield significant changes in the efficacy of the cryptates. These studies establish a foundation of important properties that are necessary to develop effective positive contrast agents for magnetic resonance imaging from Eu(2+)-containing cryptates.