Lanthanide(III) Complexes That Contain a Self-Immolative Arm: Potential Enzyme Responsive Contrast Agents for Magnetic Resonance Imaging
Abstract
Enzyme-responsive MRI contrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln(III) complex, with a concomitant response in its MRI-contrast-enhancing properties.
We synthesized and investigated a series of Gd(3+) and Yb(3+) complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for beta-galactosidase. All of the Gd(3+) complexes synthesized have a single inner-sphere water molecule.
The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mm(-1)s(-1) for complexes GdL1 and GdL2, respectively; 37 .deg.C, 60 MHz), which prevents application of this system as an enzyme-responsive T1 relaxation agent. Variable-temperature (17)O NMR spectroscopy and (1)H NMRD analysis were used to assess the parameters that determine proton relaxivity for the Gd(3+) complexes, including the water-exchange rate (kex(298), varies in the range 1.5-3.9x10(6) s(-1)).
Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiometric measurements (log KH=5.12 and 5.99 for GdL2 and GdL3, respectively). The Yb(3+) analogues show a PARACEST effect after enzymatic cleavage that can be exploited for the specific detection of enzymatic activity.
The proton-exchange rates were determined at various pH values for the amine derivatives by using the dependency of the CEST effect on concentration, saturation time, and saturation power. A concentration-independent analysis of the saturation-power-dependency data was also applied.
All these different methods showed that the exchange rate of the amine protons of the Yb(III) complexes decreases with increasing pH value (for YbL3, kex=1300 s(-1) at pH 8.4 vs. 6000 s(-1) at pH 6.4).