Synthesis and relaxivity studies of a tetranuclear gadolinium(III) complex of DO3A as a contrast-enhancing agent for MRI.

A tetranuclear gadolinium(III) complex, [Gd4(H2O)8], of DO3A appended onto the pentaerythrityl framework was synthesized to improve the water proton relaxivity for MRI application. The longitudinal relaxivity of [Gd4(H2O)8] is 28.13 mM-1 s-1 (24 MHz, 35+/-0.1 degrees C, pH 5.6) which is 5.86 times higher than that of [Gd(DO3A)(H2O)2]. The relaxivity is based on "molecular" relaxivity of the tetramer and the r1p value is "7 per Gd". The high relaxivity of the tetramer is the result of the decrease in the rotational correlation (tauR) and the presence of eight inner-sphere water molecules (q=8). The complex exhibits pH-dependent longitudinal relaxivity, and the high relaxivity both at low and high pH (r1p=28.13 mM-1 s-1 at pH 5.6 and 16.52 mM-1 s-1 at pH 9.5) indicates that it could be used as a pH-responsive MRI contrast agent. The transverse relaxivity of the tetramer is 129.97 mM-1 s-1 (24 MHz, 35+/-0.1 degrees C, pH 5.6), and the r2p/r1p ratio of 4.6 shows that it could be used as a T2-weighted contrast agent.