In vivo MRI assessment of a novel GdIII-based contrast agent designed for high magnetic field applications

Gd3L is a trinuclear Gd3R complex of intermediate size, designed for contrast agent applications in high field magnetic resonance imaging (H12L is based on a trimethylbenzene core bearing three methylene-diethylenetriamine-N,N,N00,N00-tetraacetate moieties). Thanks to its appropriate size, the presence of two inner sphere water molecules and a fast water exchange, Gd3L has remarkable proton relaxivities at high magnetic field (r1¼10.2 vs 3.0mM S1 sS1 for GdDOTA at 9.4 T, 37-C, in H2O). Here we report an in vivo MRI feasibility study, complemented with dynamic g scintigraphic imaging and biodistribution experiments using the 153Sm-enriched analog. MRI experiments were performed at 9.4 T in mice with Gd3L and the commercial contrast agent gadolinium(III)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (GdDOTA). Gd3L was well tolerated by the animals at the dose of 8mmol Gd kgS1 body weight. Dynamic contrast enhanced (DCE) images showed considerably higher signal enhancement in the kidney medulla and cortex after Gd3L injection than after GdDOTA injection at an identical dose. The relaxation rates, DR1, were calculated from the IR TrueFISP data. During the excretory phase, the DR1 for various tissues was similar for Gd3L and GdDOTA, when the latter was injected at a three-fold higher dose (24 vs 8mmol Gd kgS1 body weight). These results point to an approximately three times higher in vivo relaxivity (per Gd) for Gd3L relative to GdDOTA, thus the ratio of the relaxivities of the two compounds determined in vitro is retained under in vivo conditions. They also indicate that the two inner sphere water molecules per Gd in Gd3L are not substantially replaced by endogenous anions or other donor groups under physiological conditions. Gd3L has a pharmacokinetics typical of small, hydrophilic complexes, involving fast renal clearance and no retention in the blood pool. The dynamic g scintigraphic studies and the biodistribution experiments performed in Wistar rats with 153Sm-enriched *Sm3L are also indicative of a fast elimination via the kidneys.