AR2p/R1pRatiometric Procedure to Assess Matrix Metalloproteinase-2 Activity by Magnetic Resonance Imaging

Enzyme-Responsive LipoCEST Agents: Assessment of MMP-2 Activity by Measuring the Intra-liposomal Water 1 H NMR Shift

Mobile proton-containing solutes can be detected by MRI by the chemical exchange saturation transfer (CEST) method. CEST sensitivity is dramatically enhanced by using, as exchanging protons, the water molecules confined inside liposomes, shifted by a paramagnetic shift reagent. The chemical shift of the intraliposomal water resonance (δ^IL ) is affected by the overall shape of the supramolecular system. δ^IL of a spherical LipoCEST acts as a sensitive reporter of the distribution of streptavidin proteins anchored at the liposome surface by biotinylated phospholipids. This finding prompted the design of a MMP-2 responsive LipoCEST agent as the streptavidin moieties can be released from the liposome surfaces when a properly tailored enzyme-cleavable peptide is inserted on the phospholipids before the terminal biotin residues. δ^IL reports on the overall changes in the supramolecular architecture associated to the cleavage carried out by MMP-2.

Enzymatically Shifting Nitroxides for EPR Spectroscopy and Overhauser-Enhanced Magnetic Resonance Imaging

In vivo investigations of enzymatic processes using non-invasive approaches are a long-lasting challenge. Recently, we showed that Overhauser-enhanced MRI is suitable to such a purpose. A β-phosphorylated nitroxide substrate prototype exhibiting keto-enol equilibrium upon enzymatic activity has been prepared. Upon enzymatic hydrolysis, a large variation of the phosphorus hyperfine coupling constant (Δa(P)=4 G) was observed. The enzymatic activities of several enzymes were conveniently monitored by electronic paramagnetic resonance (EPR). Using a 0.2 T MRI machine, in vitro and in vivo OMRI experiments were successfully performed, affording a 1200% enhanced MRI signal in vitro, and a 600% enhanced signal in vivo. These results highlight the enhanced imaging potential of these nitroxides upon specific enzymatic substrate-to-product conversion.

Ratiometric MRI sensors based on core-shell nanoparticles for quantitative pH imaging

Ratiometric MRI sensors consist of paramagnetic cores and pH-sensitive polymer shells. The core-shell nanostructure enables the coexistence of two incompatible NMR relaxation properties in one particle. The sensors show pH sensitivity in transverse relaxivity (r2 ), but not in longitudinal relaxivity (r1 ). Quantitative pH imaging is achieved by measuring the r2 /r1 value with a clinical 3 T MRI scanner.