Magnetic resonance imaging (MRI) has significantly enhanced clinical diagnosis of disease due to a number of advantages, such as high resolution, rich information content that can be derived from a single scan, its non-invasive nature, no limitation on tissue penetration depth and the lack of radiation burden that is often encountered when other imaging modalities are utilised. In recent years 19F MRI has gained renewed attention as an increasingly important MRI technique, mainly due to the commercial availability of high field scanners (up to 16.4 T in the preclinical setting). 19F has comparable nuclear magnetic resonance (NMR) properties as 1H, including large gyromagnetic ratio (40.03 MHz T-1, 94% relative to 1H), high sensitivity (83% relative to 1H) and 100% natural abundance. An essentially attractive feature of 19F MRI is the absence of confounding background signal because there is no endogenous 19F NMR-detectable fluorine in the body. Hence 19F MRI is a highly quantitative technique and naturally relies on the presence of 19F–containing imaging agents to generate an image. However, 19F MRI is not routinely used in clinical mainly because of the lack of suitable contrast agents.
In the past few years, partly-fluorinated polymers have been considered as excellent candidates for 19F MRI contrast agents due to the diverse architecture and functionality of polymers. However, understanding the relationship between molecular structure, NMR properties and imaging performance remains a challenging task. In addition, it is still a minimally explored field for the development of multifunctional agents, such as biologically-responsive 19F agents (“smart” agents) and 19F MRI-incorporating multimodal imaging agents. To tackle these challenges, this thesis aims to study the design of next-generation polymeric 19F MRI agents by developing a fundamental understanding of multifunctional polymers with different molecular architectures and components for selective 19F MRI, multimodal molecular imaging, improved 19F MRI, and theranostics.
The first approach adopted for achieving high 19F mobility was by distributing and separating the 19F segments in a hyperbranched structure. In Chapter 2, star-like polymers with a hyperbranched core and hydrophilic arms were synthesised using the arm-first approach by reversible addition─fragmentation chain transfer (RAFT) polymerisation. The core was composed of units of 2,2,2-trifluoroethyl acrylate (TFEA), 2-(dimethylamino)ethyl methacrylate (DMAMEA) and ethylene glycol dimethacrylate (EGDMA), while the arms consisted of brush-like homopolymers of poly(ethylene glycol) methyl ether methacrylate (PEGMA). The chemical structure and composition of the polymers were characterised to obtain detailed information about the molecular structure. The pH-responsiveness of particle size was also investigated by dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM). The 19F NMR properties, such as 19F signal intensity, spin-lattice (T1) relaxation time and spin-spin (T2) relaxation time were examined in solutions of different pH. Finally the imaging performance of the polymers at different pH was evaluated by in vitro 19F MRI.
As the T2 relaxation time is sensitive to the chemical environment of 19F nuclei, the influence of polymer architecture on imaging performance was then studied. Chapter 3 describes the development of core crosslinked star (CCS) polymers and these materials were compared to the star-like hyperbranched polymers described in Chapter 2. The CCS polymers were constructed of a biodegradable core and block copolymers as arms. In contrast to the polymers described in Chapter 2, the 19F units were positioned within the block copolymer arms by the RAFT copolymerisation of 2,2,2-trifluoroethyl methacrylate (TFEMA) and DMAEMA using PPEGMA as a macroCTA. The pH-responsiveness of 19F NMR properties was characterised, and the biodegradability were evaluated. The capability of the CCS polymers for selective 19F MRI was assessed in solutions of different pH. The change in imaging properties of different polymer architectures were also compared and discussed.
In order to expand the possible application of polymeric imaging agents, multifunctional hyperbranched polymers were synthesised for computed tomography (CT)/19F MRI bimodal molecular imaging. In Chapter 4, hyperbranched polymers containing units of 2-(2',3',5'-triiodobenzoyl)ethyl methacrylate (TIBMA) and PEGMA were synthesised by RAFT polymerisation and were chain extended with TFEA and PEGMA. The biodegradability was studied in the presence of reducing agents. Nanoparticles were formed and characterised by DLS and TEM techniques. The radio-opacity of these polymers was assessed by in vitro CT experiments, and the MRI performance was evaluated by 19F MRI. The material showed good imaging potential in both modalities pointing towards a new class of multimodal imaging agents.
In order to systematically study the relationship between molecular structure and imaging performance, segmented highly-branched polymers (SHBPs) were synthesised by self-condensing vinyl polymerisation via the RAFT process (RAFT SCVP). Chapter 5 describes the synthesis of a polymerisable chain transfer agent (CTA) that was used for the copolymerisation of fluoro monomers and PEG-based monomers. A series of SHBPs with different compositions and degrees of branching (DBs) were synthesised and thoroughly characterised. The 19F NMR properties were strongly affected by the sequence distribution of fluorinated units, type of polymer backbone and degree of branching. As a result, SHBPs consisting of statistical copolymer segments with acrylate backbones were excellent candidates for imaging due to a single 19F signal, long T2 relaxation times and high 19F contents. The SHBPs could be all imaged or selectively imaged by 19F MRI using different pulse sequences by taking advantage of the differences in relaxation times, demonstrating the tuneable and selective imaging performance through tailoring the structure and composition of the SHBPs.
Finally, Chapter 6 describes the synthesis of star polymers with a polyhedral oligomeric silsesquioxanes (POSS) core and eight partly-fluorinated arms. Here the aim was to investigate how to combine 19F MRI with drug delivery by designing a platform possessing 19F-containing polymers and POSS cages. A macroCTA having eight CTA molecules attached to a POSS core was synthesised and used for the synthesis of star polymers by the R-group approach. The arms were composed of statistical copolymers consisting of TFEA and PEGA. Star polymers having different arm lengths were prepared and characterised. The particle size and 19F NMR properties were studied by DLS and 19F NMR, respectively. Finally the imaging performance was preliminarily evaluated by calculating the theoretical 19F MRI intensities and comparing with the previous imaging results.
In summary, this thesis studies the development of multifunctional nanostructured polymers as contrast agents for 19F MRI-related molecular imaging and theranostics using the RAFT technique. The molecular level understanding gained in this work provides useful guidance for the future design of highly-efficient 19F MRI contrast agents.