Investigation of non covalent interactions between some MRI contrast agents and HSA by NMR diffusometry

As compared to other imaging techniques, magnetic resonance imaging (MRI) has better spatial and temporal resolutions but its sensibility is relatively weak. In most cases, the use of contrast agents is required. Most of them are gadolinium complexes, which have the property of increasing the signal of the pathological zones, improving the image contrast. Very efficient contrast agents, i.e. contrast agents with a high proton relaxivity, are thus needed. This can be obtained for example with molecules interacting non-covalently with endogenous macromolecules, like human serum albumin (HSA). It is thus very important to have at one's disposal some techniques allowing the evaluation of this type of non-covalent interactions.
In this work, the NMR diffusometry1 technique was used to evaluate the interaction of six MRI contrast agents with HSA. The principle of this method is based on the variation of the ligand diffusion coefficient in the absence and in the presence of HSA. However, this high resolution NMR technique does not allow to use gadolinium complexes because of the signal broadening they produce. Europium complexes were thus used since this lanthanide has the advantage of shifting the ligand signals outside of the HSA background without inducing significant broadening of the peaks. The results show that this technique does not allow to distinguish weak and high affinity ligand binding. In fact, if the ligand interacts too strongly with HSA, the exchange between free and bound states is too slow and no significant evolution of the diffusion coefficient can be detected, as it would be the case for a ligand which has no affinity for HSA. Alternatively, competition experiments with competitors of known binding site on HSA (ibuprofen and salicylic acid), are very helpful and allow not only to specify the binding site of the contrast agents on HSA but also to assess the strength of their interaction with the protein.

1. Lucas, L.H., Larive, C.K., Conc. Magn. Res. 2004, Part A, 20A(1), 24-41