Biologically-responsive polymers for MRI: measuring temperature and ionic strength

Abstract

The development of MRI imaging agents has been central to the rise of MRI as a leading medical diagnostic tool. While many advances have been made in the field of molecular imaging agents, the development of partially-fluorinated 19F MRI agents is hindered due to a lack of clear understanding of their properties. Therefore, it is very important to develop effective molecular imaging agents from both the fundamental and the applied perspectives, and to build a molecular-level understanding of the responsive behaviour of 19F MRI agents to external stimuli. To this end, this project aims to develop novel polymeric 19F MRI contrast agents for improved cancer diagnostics and therapy. In Chapter 2, fluorine-containing copolymers of oligo(ethylene glycol) methyl ether methacrylate (OEGMA) and 2,2,2-trifluoroethyl acrylate (TFEA) were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. The thermo- and ion-responsive properties were carefully studied by various characterisation techniques (1H and 19F NMR, DLS and UV-Vis) at different temperatures in the presence of salts. The influence of OEGMA side chain length, copolymer composition, and salt type on aggregation behaviour and dynamics was examined in detail. It was found that motion of the protons located in and near the hydrophobic main chain are more sensitive to temperature than protons in the hydrophilic OEGMA side chains. Molecular level understanding of the behaviour of responsive MRI contrast agents is very important for the design and application of the materials to the detection of important disease states in animals. Therefore, in Chapter 3, the combination of experimental techniques and molecular dynamics (MD) simulations was applied to the study of thermoresponsive copolymers of OEGMA and TFEA (poly(OEGMA-co-TFEA)). Below the LCST, a coil-like conformation is adopted and unimers are dominant in solution. As the temperature is further increased above the LCST, collapsed chains are observed. In the meantime, large particles are formed, due to the aggregation of polymer chains as supported by the DLS results and MD simulations of ensemble of three polymer chains. Correspondingly, the cross-peak intensities in the NOESY NMR experiments increased significantly as the temperature was increased above the LCST. The influence of dissolved salt on the responsive MRI contrast agents is a very important consideration for polymers intended for physiological applications. In order to consider this effect, in Chapter 4, molecular level studies of ion-responsive poly(OEGMA-co-TFEA) in the presence of NaCl were conducted through the combination of experimental techniques (DLS, 1H and 19F NMR) and MD simulations. The hydrodynamic size of the polymer was decreased with the addition of NaCl, indicating intramolecular aggregation of the polymer in the presence of NaCl. Oxygen atoms of some of the OEGMA side chains were directly bonded to Na+ leading to a partial dehydration effect on the polymer in the presence of NaCl. It was also observed that the 19F NMR T2 decreased with increasing NaCl concentration. Based on the much higher NaCl concentration in cancer cells compared with normal cells, the ion-responsive MRI contrast agent was applied for the detection of cancer cells by monitoring the changes of 19F NMR T2 in vitro. In many MRI examinations, contrast agents are required to improve the image quality and visualize specific body tissues. Therefore, in Chapter 5, perfluoropolyethers (PFPEs) were chosen as the source of fluorine for the synthesis of 19F MRI contrast agents with high fluorine content. The structural characteristics, 19F NMR relaxation times (T1 and T2), 19F MRI and 19F MRI signal-to-noise ratio (SNR) were studied in detail for PFPE end-functionalised homopolymer of oligo(ethylene glycol) methyl ether acrylate (poly(OEGA)-PFPEs). The 19F MRI SNR increased linearly as the fluorine content of poly(OEGA)-PFPEs was increased. A comparison of 19F MRI properties of poly(OEGA)-PFPEs and poly(OEGMA-co-TFEA) indicated that PFPEs is a promising fluorine monomer for the synthesis of highly sensitive 19F MRI contrast agents. The combination of experimental techniques and MD simulation studies opens a unique window on the molecular level understanding of the behaviour of responsive polymers. In Chapter 6, we apply this combination to investigate the thermal behaviour of dendronized polymers. Below the LCST, the interior segments of the dendrons, as opposed to those in the periphery, experienced a relative increase in mobility as the temperature was increased. As the temperature was increased above the LCST, the interior protons experienced a larger decrease in mobility than peripheral protons. The aggregation behaviour of the thermoresponsive dendronized polymers was also measured by diffusion ordered spectroscopy (DOSY) below and above the LCST, indicating the formation of aggregates with different sizes and mobilities. In summary, this thesis studies the development of novel thermo- and ionic responsive 19F MRI contrasted agents. The combination of experimental measurements and MD simulations provide a useful platform for the study of the behaviour of responsive materials.