55/52Mn2+ Complexes with a Bispidine-Phosphonate Ligand: High Kinetic Inertness for Imaging Applications

International audience; Bispidine (3,7-diazabicyclo[3.3.1]nonane) provides a rigid and preorganized scaffold that is particularly interesting for the stable and inert complexation of metal ions, especially for their application in medical imaging. In this study, we present the synthesis of two bispidine ligands with N-methanephosphonate (H4L1) and N-methanecarboxylate (H3L2) substituents as well as the physico-chemical properties of the corresponding Mn2+ and Zn2+ complexes. The two complexes [Mn(L-1)](2- )and [Mn(L-2)](-) have relatively moderate thermodynamic stability constants according to potentiometric titration data. However, they both display an exceptional kinetic inertness, as assessed by transmetallation experiments in the presence of 50 equiv excess of Zn2+, showing only similar to 40 and 20% of dissociation for [Mn(L-1)](2- ) and [Mn(L-2)](-), respectively, after 150 days at pH 6 and 37°C. Proton relaxivities amount to r(1 )= 4.31 mM(-1 )s(-1 )([Mn(L-1)](2-)) and 3.64 mM(-1) s(-1 )([Mn(L-2)](-)) at 20 MHz, 25°C, and are remarkable for Mn2+ complexes with one inner-sphere water molecule (q = 1); they are comparable to that of the commercial contrast agent [Gd(DOTA)(H2O)](-). The presence of one inner-sphere water molecule and an associative water exchange mechanism was confirmed by temperature-dependent transverse O-17 relaxation rate measurements, which yielded k(ex)(298) = 0.12 x 10(7) and 5.5 x 10(7) s(-1 )for the water exchange rate of the phosphonate and the carboxylate complex, respectively. In addition, radiolabeling experiments with Mn-52 were also performed with H-2(L-1)(2-) showing excellent radiolabeling properties and quantitative complexation at pH 7 in 15 min at room temperature as well as excellent stability of the complex in various biological media over 24 h.