Relaxometric, Thermodynamic and Kinetic Studies of Lanthanide(III) Complexes of DO3A‐Based Propylphosphonates

Two DO3A‐based ligands appended with the propylphosphonate side arm and their Ln 3+ complexes were investigated. Proton relaxometric in vitro studies at 20 MHz and 60 MHz and 37 °C of the Gd 3+ complex containing free acid exhibited relative changes of up to 56 % in r 1 relaxivity when the pH of the medium was changed from 4 to 7. This change is explained by the decrease in the number of coordinated water molecules from two to one. Temperature‐dependent relaxivity and NMRD profiles of Gd 3+ complexes showed a fast water exchange and a slightly increased rotational correlation time, which is characteristic of phosphonate‐containing compounds. Thermodynamic and kinetic studies of the Gd 3+ and Eu 3+ complexes were performed by means of potentiometry and luminescence spectroscopy. The results indicate that the thermodynamic stability and kinetic inertness of these complexes are sufficient for their in vivo application. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)