The role of water exchange in attaining maximum relaxivities for dendrimeric MRI contrast agents

Macrocyclic Gd-III complexes attached to dendrimers represent a new class of potential MRI contrast agents. They have an extended lifetime in the blood pool, which is indispensable For their application in magnetic resonance angiography, and high relaxivities, which reduce the dose required to produce quality images. We performed a variable-temperature and -pressure O-17 NMR study in aqueous solution and at 14.1, 9.4, and 1.4 T on the water exchange and rotational dynamics of three macrocyclic Gd-III complexes based on polyamidoamine dendrimers, as well as on the Gd-III complex of the monomer unit with the linker group. The water exchange rates k(ex)(298) for generation 5 [G5(N{CS}N-bz-Gd-{DO3A}{H2O})(52)], generation 4 [G4(N{CS}N-bz-Gd{DO3A}{H2O})(30)], generation 3 [G3(N{CS}N-bz-Gd{DO3A}{H2O})(23)], and the monomer [Gd(DO3A-bz-NO2)(H2O)] complexes are 1.5+/-0.1, 1.3+/-0.1, 1.0+/-0.1, and 1.6+/-0.1 x 10(6) s(-1), respectively, and the activation volumes Delta V double dagger of water exchange on the latter two compounds are +3.1+/-0.2 and +7.7+/-0.5 cm(3) mol(-1), indicating dissociatively activated exchange reactions ({CS}N-bz-{DO3A} = 1-(4-isothiocyanatobenzyl)amido-4,7,10-tri(acetic acid)tetraazacyclododecane). The rotational correlation times for the dendrimers are 4 to 8 times longer than for monomeric or dimeric Gd-III poly(amino carboxylates). As a consequence of the slow rotation, the proton relaxivities of these dendrimer complexes are considerably higher than those of smaller complexes. However, the low water exchange rates prevent the dendrimer proton relaxivities from attaining the values expected from the increase in the rotational correlation times. Modifications of the chelating ligand may result in a faster water exchange and thus allow the full benefit of slow rotation to be achieved.