Functional magnetic resonance imaging (fMRI), which is based on blood-oxygen-level dependent (BOLD) contrast mechanism, is the main tool for noninvasive study of the brain in neural and cognitive sciences. However, BOLD fMRI measures neural activity only indirectly, by means of hemodynamics and neurovascular coupling. As such, it has unavoidable physiological limitations that are associated with the very vascular origin of the signal, which reduces spatiotemporal resolution and often complicates functional interpretation of activations. Despite recent advances in understanding of the neurophysiological basis of fMRI signals, the relationship between the measured BOLD signal and the underlying neural activity remains elusive.
A step forward in the fMRI field is performing neuroimaging with molecular probes (contrast agents) sensitive to aspects of neurophysiological changes. These bioactivated, responsive or “smart” contrast agents (SCA) are able to produce signals that are directly linked to neuronal processing. To this end, several possible markers that are responsive to the concentration of certain ions or neurotransmitters are envisaged.
Our research is focused on the design and synthesis of molecules based on paramagnetic metal complexes, which are sensitive to changes in calcium flux, pH, or the concentration of neurotransmitters. We also prepare contrast agents which localize specifically in the target tissue, thus reporting its function. Finally, we develop methods for in vitro, ex vivo and in vivo validation of these agents by means of NMR spectroscopy and MR imaging.
Calcium is an essential metal ion for neural signaling. The ability to track calcium fluctuations by means of MRI would be of paramount importance for biomedical research. We therefore develop MRI sensors, molecules with integrated calcium chelators and MR reporting moieties. Upon selective interaction with calcium ions, these SCA alternate their magnetic properties and thus the MR contrast. We take advantage of different contrast mechanisms and record the MR signals at frequencies of different nuclei to yield the maximal signal upon calcium-concentration change. We use:
a) paramagnetic gadolinium complexes (so-called T1 agents) which change their hydration number and thus their relaxivity / MR contrast when interacting with calcium;
b) paramagnetic metal complexes other than gadolinium that exhibit alternation in chemical exchange saturation transfer (CEST) effect;
c) a wide selection of lanthanide complexes which contain fluorine atoms, making these SCA suitable for 19F MRI employing the paramagnetic relaxation enhancement effect (PRE).
The scope of SCA application in MR neuroimaging can be tremendously expanded by utilizing neurotransmitter-sensitive contrast agents. These MR sensor molecules interact with selected amino acid neurotransmitters, subsequently producing an MR signal change. However, the development of neurotransmitter-sensitive SCA is very challenging. We follow the common approaches from host-guest chemistry to achieve effective molecular recognition between the host SCA and the guest neurotransmitter molecules. To this end, we employ crown-ethers as receptor molecules which interact with desired amino acids. The MR signal changes are induced following the same contrast mechanisms defined for the above class of SCA.
Target-specific agents distribute specifically in the target tissue, allowing its improved three-dimensional localization. Unlike SCA, they do not alternate the MR signal depending on changes in their microenvironment but accumulate in target tissue, reporting its function by labeling specific receptors.
We develop molecules with ligands which specifically interact with the targeting receptors (e.g. avidin/biotin as a receptor/ligand pair), and allow multimodal readout by MR or optical imaging methods. We amplify the signal by linking MR reporters to carrier molecules such as dendrimers. The building block principle we employ yields target-specific, multimodal contrast agents that display a much stronger MR signal and allow MRI with high spatial resolution.