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The relaxivity of Gd(HP-DO3A) was studied as a function of pH and buffer composition in order to identify the main factors of the observed relaxation enhancement due to the exchange of the coordinated hydroxyl proton. It was established that the paramagnetic relaxation time, T 1M , of the coordinated hydroxyl proton is about 50% shorter than that of the protons in the coordinated water molecule. The control of the p K of the coordinated alcoholic -OH moiety in the ligand is fundamental to utilize the proton exchange enhanced relaxivity under physio/pathologic conditions. A new derivative of Gd(HP-DO3A) was synthesized by replacing the -CH 3 group with a -CF 3 moiety. In this complex, the -OH group becomes more acidic. Consequently, the maximum contribution of the proton exchange to the relaxivity is shifted to a lower pH region with the fluorinated ligand.

Novel contrast agent based systems, which selectively visualize specific cells, e.g., neurons in the brain, would be of substantial importance for the fast developing field of molecular magnetic resonance imaging (MRI). We report here the synthesis and in vitro validation of a Gd(III)-based contrast agent designed to act as an MRI responsive probe for imaging the activity of the enzyme glutamic acid decarboxylase (GAD) present in neurons. Upon the action of the enzyme, the Gd(III) complex increases its hydration sphere and takes on a residual positive charge that promotes its binding to endogenous macromolecules. Both effects contribute in a synergic way to generate a marked relaxation enhancement, which directly reports enzyme activity and will allow activity detection of GAD positive cells in vitro and in vivo selectively.

Four novel MRI Gd(III)-based probes have been synthesized and evaluated for their labeling properties on cultured cell lines K562, C6, and B16. The labeling strategy relies upon the fact that cells display a large number of reactive exofacial protein thiols (EPTs) that can be exploited as anchorage points for suitably activated MRI probes. The probes are composed of a Gd(III) chelate (based on either DO3A or DTPA) connected through a flexible linker to the 2-pyridyldithio chemical function for binding to EPTs. GdDO3A-based chelates could efficiently label cells (up to a level of 1.2 x 10(10) Gd(III) atoms/cell), whereas GdDTPA-based chelates showed poor or no cell labeling ability at all. Among the GdDO3A based compounds, that having the longest spacer (compound GdL1A) showed the best labeling efficacy. The mechanism of EPT mediated cell labeling by GdL1A involves probe internalization without sequestration of the Gd(III) chelate within subcellular structures such as endosomes.

The in vitro validation of a new pH mapping MRI method based on a supramolecular poly-beta-cyclodextrin-19F-Gd adduct is reported.

Cells display on the outer surface of the plasma membrane a large number of protein thiols that can be reversibly labelled with suitably designed Gd(III)-based contrast agents for cell tracking by MRI.