PAMAM dendrimers conjugated with an uncharged gadolinium(III) chelate with a fast water exchange: the influence of chelate charge on rotational dynamics.

A bifunctional ligand, DO3A-py(NO-C) (DO3A-py(NO-C) = 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid), was attached to different generations (G0, G1, G2, and G4) of ethylenediamine-cored PAMAM dendrimers (PAMAM = polyamidoamine). The gadolinium(III) complex of this ligand possesses one molecule of water in its first coordination sphere and has a unique combination of a short water residence lifetime (tau(M) = 34 ns), a neutral overall charge, and a possibility for rigid attachment to molecules bearing primary amino groups. These favorable properties predestine the ligand for constructions of highly efficient nanosized contrast agents for magnetic resonance imaging (MRI). The coupling reaction between the carboxylic group on the pyridine-N-oxide moiety of the protected ligand molecule and primary amines in the dendrimers was achieved by an active ester method under nonaqueous conditions using the coupling agent TBTU. This reaction afforded conjugates with high loadings (80-100% of the theoretically available primary amines) and of high purity. The gadolinium(III) complexes of the conjugates were studied by variable-temperature 17O NMR and 1H NMRD measurements. The water residence lifetime (tau(M) = 55 ns) found in the largest conjugate G4-[Gd(H2O)(do3a-py(NO-C))]57, though somewhat longer compared to the "monomeric" complex, is still short enough not to limit the relaxivity. Surprisingly, compared with analogous conjugates with negatively charged chelates, the prepared uncharged compounds displayed much faster global rotational correlation times (tau(g)) and lower relaxivities. This phenomenon can be explained on the basis of Coulomb interactions. The motion of the charged chelates is restrained due to interactions with their counterions and the chelates themselves, while the uncharged chelates are not affected. Comparison of the PAMAM-based conjugates bearing uncharged and (1-)-charged chelates based on relaxometric data, 1H DOSY spectra, and SAXS measurements reveals that tau(g) reflects the rotational motion of large segments (dendrons) of the conjugates rather than that of the whole macromolecule.