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 O-17 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 degrees 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) x 103 and (2.0 +/- 0.3) x 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).