Optimising the design of paramagnetic MRI contrast agents: influence of backbone substitution on the water exchange rate of Gd-DTPA derivatives

Among other factors influencing the residence time of the coordinated water (τ _M ) of paramagnetic contrast agents, the steric hindrance around the gadolinium ion seems to play a beneficial role. Such a crowding can be achieved by substituting the Gd-DTPA backbone on the C4 position. Several Gd-DTPA complexes carrying diverse groups at this position have thus been synthesised and characterised: Gd(S)-C₄-Me-DTPA, Gd(S)-C₄-n-Bu-DTPA, Gd(S)-C₄-iBu-DTPA, Gd(S)-C₄-iPr-DTPA, and Gd-C₄-diMe-DTPA. τ _M has been measured through the evolution of the water oxygen-17 transverse relaxation rate as a function of the temperature. The data show a reduction of τ _M of Gd(S)-C₄-Me-DTPA, Gd(S)-C₄-n-Bu-DTPA, Gd(S)-C₄-iBu-DTPA, Gd(S)-C₄-iPr-DTPA, and Gd-C₄-diMe-DTPA (τ _M ³¹⁰=91,82, 108,98, and 57 ns respectively, as compared to Gd-DTPA (τ _M ³¹⁰=143 ns)). At 310 K, the nuclear magnetic dispersion relaxation profiles of water protons are very similar for the five complexes which present longitudinal relaxivities slightly higher than those of Gd-DTPA. Regarding zinc transmetallation, C4-monosubstituted derivatives are more stable than Gd-DTPA. These results confirm that a judicious substitution of the DTPA skeleton allows for an acceleration of the coordinated water exchange rate. This observation can be useful for the design of vectorised contrast agents for molecular imaging.