Design, Synthesis and Applications Of Novel Multi-Functional Imaging Agents

This thesis is concerned with designing and synthesising novel chemosensors for use as multi-functional imaging agents for both MRI and optical imaging towards Zn2+ sensing. The divalent zinc ion (Zn2+) is an essential biological element for living organisms and partakes in a number of biological mechanisms. As a result, any Zn2+ change in its homeostasis can lead to implications to a broad range of pathologies, both physical and neurological. This has made imaging Zn2+ ions crucial towards the elucidation of its distribution and function in cells and tissues. For this project, we have been designing novel multi - modal molecular imaging agents that will target Zn2+ in cells, and hopefully in the presence of possible competing cations. It is the synergistic combination of MRI and optical imaging that has led to the design and synthesis of a first generation of compounds. The aims have been to synthesise chemical probes based on MRI agents with the ability to sense Zn2+ levels via optical - MRI methods. In order to synthesise these probes, both a Gd3+ based MRI contrast agent and chromophore have been used for MRI and fluorescence visualization respectively. A Zn2+ sensor has also been attached, which upon binding should cause a molecular or electronic change which would be detected primarily via the chromophore. Three probes were designed and their synthesis towards them explained throughout the thesis. The first section of the thesis describes the preparation of a quinoline-based DO3A derivative for eventual complexation to a Gd3+ centre. The synthesis and purification methods are discussed, followed by some preliminary studies of the probe‟s behaviour towards Zn2+ ions. The second part of the thesis explains the various building blocks required to subsequently synthesise the final two probes, by discussing various synthetic approaches, in particular efforts to derivatise new fluorophores and Zn2+ sensing groups. The final part of the thesis discusses approaches towards a 1,7 - bi-substituted DOTA - based Gd3+ complex and a DOTA - based tetraamide Gd3+ complex. The successful probes that were developed were fully characterised and have undergone preliminary MRI and biological tests. They have been found to be MRI active with the ability to enter and be non - toxic towards cells with an ability to sense Zn2+ ions selectively. Fluorescence tests have also exhibited a change in fluorescence by the probe when perturbed by Zn2+ ions

Towards understanding the design of dual-modal MR/fluorescent probes to sense zinc ions

A series of gadolinium complexes were synthesised in order to test the design of dual-modal probes that display a change in fluorescence or relaxivity response upon binding of zinc. A dansyl-DO3ATA gadolinium complex [GdL1] displayed an increase and a slight blue-shift in fluorescence in the presence of zinc; however, a decrease in relaxation rate was observed. Consequently, the ability of the well-known zinc chelator, BPEN, was assessed for relaxivity response when conjugated to the gadolinium chelate. The success of this probe [GdL2], lead to the inclusion of the same zinc-probing moiety alongside a longer wavelength emitting fluorophore, rhodamine [GdL3], to arrive at the final iteration of these first generation dual-modal zinc-sensing probes. The compounds give insight into the design protocols required for the successful imaging of zinc ions

Dual-Modal Magnetic Resonance/Fluorescent Zinc Probes for Pancreatic β-Cell Mass Imaging

Despite the contribution of changes in pancreatic β-cell mass to the development of all forms of diabetes mellitus, few robust approaches currently exist to monitor these changes prospectively in vivo. Although magnetic-resonance imaging (MRI) provides a potentially useful technique, targeting MRI-active probes to the β cell has proved challenging. Zinc ions are highly concentrated in the secretory granule, but they are relatively less abundant in the exocrine pancreas and in other tissues. We have therefore developed functional dual-modal probes based on transition-metal chelates capable of binding zinc. The first of these, Gd⋅1, binds ZnII directly by means of an amidoquinoline moiety (AQA), thus causing a large ratiometric Stokes shift in the fluorescence from λem=410 to 500 nm with an increase in relaxivity from r1=4.2 up to 4.9 mm−1 s−1. The probe is efficiently accumulated into secretory granules in β-cell-derived lines and isolated islets, but more poorly by non-endocrine cells, and leads to a reduction in T1 in human islets. In vivo murine studies of Gd⋅1 have shown accumulation of the probe in the pancreas with increased signal intensity over 140 minutes