Comprehensive evaluation of the physicochemical properties of Ln ^III complexes of aminoethyl-DO3A as pH-responsive T₁-MRI contrast agents

N-Substituted aminoethyl groups were attached to 1,4,7,10- tetraazacyclododecane-1,4,7-triacetic acid (DO3A) with the aim to design pH-responsive Ln^III complexes based on the pH-dependent on/off ligation of the amine nitrogen to the metal ion. The following ligands were synthesized: AE-DO3A (aminoethyl-DO3A), MAE-DO3A (N-methylaminoethyl-DO3A), DMAE-DO3A (N,N-dimethylaminoethyl-DO3A) and MEM-AE-DO3A (N-methoxyethyl-N- methylaminoethyl-DO3A). The physicochemical properties of the Ln^III complexes were investigated for the evaluation of their potential applicability as magnetic resonance imaging (MRI) contrast agents. In particular, a ¹H and ¹⁷ONMR relaxometric study was carried out for these Gd^III complexes at two different pH values: at basic pH (pendant amino group coordinated to the metal centre) and at acidic pH (protonated amine, not interacting with the metal ion). Eu^III complexes allow one to estimate the number of inner-sphere water molecules through luminescence lifetime measurements and obtain some structural information through variable-temperature (VT) high-resolution ¹HNMR studies. Equilibria between differently hydrated species were found for most of the complexes at both acidic and basic pH. The thermodynamic stability of Ca^II, Zn^II, Cu^II and Ln^III complexes and kinetics of formation and dissociation reactions of Ln^III complexes of AE-DO3A and DMAE-DO3A were investigated showing stabilities comparable to currently approved Gd^III-based CAs. In detail, higher total basicity (Σlog K_i^H) and higher stability constants of Ln ^III complexes were found for AE-DO3A with respect to DMAE-DO3A (i.e., log K_Gd-AE-DO3A=22.40 and log K_Gd-DMAE-DO3A=20.56). The transmetallation reactions of Gd^III complexes are very slow (Gd-AE-DO3A: t_1/2=2.7×10⁴h; Gd-DMAE-DO3A: 1.1×10⁵h at pH7.4 and 298K) and occur through proton-assisted dissociation. pH Switches: N-Substituted aminoethyl-1,4,7,10- tetraazacyclododecane-1,4,7-triacetic acid (DO3A)-based complexes were prepared to obtain pH-sensitive T₁-MRI contrast agents. A detailed evaluation of the physicochemical properties of Ln^III complexes was undertaken by different techniques, highlighting the structural variations triggered by the protonation of the pendant amine and the effect of the substituents on the corresponding nitrogen atom (see figure).