Protein binding to lanthanide(III) complexes can reduce the water exchange rate at the lanthanide

The Gd-III-based magnetic resonance imaging contrast agent MS-325 targets the blood protein serum albumin, resulting in an increased efficacy (relaxivity) as a relaxation agent. MS-325 showed different relaxivities when bound to serum albumin from different species, e.g., r(1) = 30.5 mM(-1) s(-1) (rabbit) vs 46.3 mM(-1) s(-1) (human) at 35 degrees C and 0.47 T. To investigate the mechanism for this difference, surrogate complexes were prepared where the Gd-III ion was replaced by other Ln(III) ions. Fluorescence lifetime measurements of the Eu-III analogue indicated that the hydration number was q = 1 and did not change when bound to either human, rat, rabbit, pig, or dog serum albumin. The Yb-III analogue, YbL1, was prepared and characterized by H-1 NMR. Line-shape analysis of the paramagnetic-shifted H-1 NMR resonances in the presence of increasing amounts of human (HSA) or rabbit (RSA) serum albumin allowed estimation of the transverse relaxation rate, R-2, of these resonances for the protein-bound YbL1. The rotational correlation time of YbL1 was calculated from R-2, and the Yb-H distance and was tau(R) = 8 +/- 1 ns when bound to HSA and 13 +/- 2 ns when bound to RSA. The water exchange rate at the Dy-III analogue, DyL1, was determined from variable-temperature R-2 measurements at 9.4 T when DyL1 was bound to either HSA or RSA. At 37 degrees C, water exchange at DyL1 was (31 +/- 5) x 10(6) s(-1) when bound to HSA but (3.8 +/- 0.2) x 10(6) s(-1) when bound to RSA. Slower water exchange upon RSA binding explains the differences in relaxivity observed. The approach of using surrogate lanthanides to identify specific molecular parameters influencing relaxivity is applicable to other protein-targeted Gd-III contrast agents.