Optimising the relaxivities of Mn²⁺ complexes by targeting human serum albumin (HSA)

We report two novel macrocyclic ligands based on the 1,4-DO2AM platform (1,4-DO2AM = 2,2'-(1,4,7,10-tetraazacyclododecane-1,4-diyl)diacetamide) and containing two benzyl groups attached either to the nitrogen atoms of the macrocyclic unit (1,4-BzDO2AM) or to the amide pendant arms (1,4-DO2AMBz). The protonation constants of the ligands and the stability constants of their Mn²⁺ complexes were determined using pH potentiometry. The introduction of benzyl groups results in a slight decrease of the stability constants of the Mn²⁺ complexes and a slight increase of their acid-catalysed dissociation reactions. A detailed relaxometric characterisation of the complexes using nuclear magnetic dispersion relaxation (NMRD) and ¹⁷O NMR studies indicated that the increase in molecular weight associated with the presence of benzyl groups results in a remarkable increase of proton relaxivities r_1p, which take values of 3.8, 3.5 and 2.5 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺, [Mn(1,4-DO2AMBz)]²⁺ and [Mn(1,4-DO2AM)]²⁺ (at 25 °C and 20 MHz). The [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺ complexes form relatively strong adducts with Human Serum Albumin (HSA) with association constants of (3.9 ± 0.6) × 10³ and (2.0 ± 0.3) × 10³ M^-1, respectively. The interaction with the protein slows down the rotational tumbling of the complex in solution, which results in adducts endowed with remarkably high proton relaxivities (r_1p^b = 18.5 ± 0.7 and 27.4 ± 1.4 mM^-1 s^-1 for [Mn(1,4-BzDO2AM)]²⁺ and [Mn(1,4-DO2AMBz)]²⁺, respectively).