Abstract:
This thesis explores the development of responsive MRI contrast agents (CA) targeting glutamate (Glu) and γ-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters (NT). Chapter 1 outlines key concepts in MRI physics and discusses both non-specific and stimulus-responsive MRI CA, including small-molecule and nanoscale dendrimer-based systems. It also covers fundamental aspects of chemical neurotransmission and highlights challenges in designing CA that respond to zwitterionic amino acid neurotransmitters. Additionally, it reviews current progress in MR-based imaging of Glu and GABA. The examined bismacrocyclic CA ligate amino acids (AA) via predominant Gd-Ocarboxylate interaction and multiple H-bonds between ammonium cation and the 18C6 moiety. Thus, the focus of Chapter 2 is on chemical modification of binding sites with emphasis on the 18-crown-6 (18C6) fragment to selectively improve the affinity towards zwitterionic NT. This approach implied the synthesis of a family of Gd-complexes with varying natures of both binding sites and revealing structure-reactivity relationships using various analytical techniques, including relaxometric profiling and DFT calculations. In Chapter 2 it is revealed that recognition of AA transmitters by cyclen-18C6 chemosensors is governed by the nature of the pending arm on the 18C6 motif. Based on these findings, Chapter 3 reports a novel design of a ditopic molecular host with improved affinity to Glu and GABA transmitters suitable for T2/T1w ratiometric MR imaging. The application of a bSSFP pulse sequence on phantoms demonstrated signal enhancement for cooperatively bound Glu and GABA over hydrogen carbonate, wherein an MR signal of hydrogen carbonate remained unchanged. After successful preparation of a synthetic host with higher selectivity for AA over hydrogen carbonate, the focus has turned on biocompatibility improvements. Thus, Chapter 4 describes a synthetic approach for the preparation of the first dendrimer-GBCA conjugate responsive to AA. The previously characterized small-molecule Gd-chelate in Chapter 2 was grafted onto the end amines of the G4 PAMAM dendrimer in order to assess the impact of conjugation on binding AA. To tune solubility at physiological pH, the conjugate was PEGylated with hydrophilic polyethylene glycol. Efficacy of the probe was evaluated in relaxometric titration with AA and competitive metabolites, and the potential for ratiometric T2/T1w imaging was estimated. Chapter 5 describes the development of a novel approach intended for the 13C chemical shift imaging (CSI) of Glu induced by the lanthanide chelate. To test the effects of lanthanide ions, a single bismacrocyclic chelator with the highest affinity to Glu and GABA (Chapter 2) was metalated with various Ln3+ ions, other than Gd3+, to produce a series of ditopic paramagnetic receptors. The 13C NMR titration of Ln-chelates performed with Gly-13C2 showed a stronger impact than that observed for Glu-13C5. The resonances of carboxylic carbons of ternary host-guest adducts exhibited the greatest signal change, presumably due to the shortest Ln-13Ccarboxylic distance.
contrast agents