Robust capsules for biomedical applications

Abstract

Metallosupramolecular capsules represent a facile route to three-dimensional structures that possess a central cavity. The synthetic accessibility is provided by self-assembly, which allows carefully designed components to form complex three-dimensional structures under thermodynamic control. The cavities allow the non-covalent binding of guests, changing their properties in a quick and reversible manner, thereby making capsules promising for catalytic, storage, extraction and biomedical applications. However, the requirement for thermodynamic control makes capsules susceptible to degradation by changes in conditions such as heat, dilution, pH, or the presence of nucleophiles, making them insufficiently robust for biomedical applications. This project constitutes the continuation of a project which sought to synthesise more robust capsules. This was achieved by exploiting the cobalt II/III redox couple, assembling the tetrahedra with labile cobalt (II) and then oxidatively “locking” them as inert cobalt (III). This led to the development of the tetrahedron C19, which proved stable over a range of temperatures, dilutions and pH values. C19 was shown to bind the common SPECT imaging agent precursor [99mTc]TcO4 -, suggesting radioimaging as a potential application of the cage. During this project, C19 was found to be degraded in biological environments by reducing agents such as glutathione. Therefore, a ligand with electron donating amine groups, L20, was developed to increase the strength of interaction with metal centres, and overall stability. A two-step assembly using C19 as a template and substituting L20 into this prearranged tetrahedron was required to prevent kinetically trapped byproducts. The resulting cage, C20, was found to have increased resistance to biological conditions relative to C19, and was able to significantly alter the uptake of [99mTc]TcO4 - in vivo, proving the potential of the system to act as a delivery agent. Further functionalisation of C20 was explored to allow bioconjugation and targeting. To this end, the ligand was functionalised with a terminal alkyne to allow versatile Huisgen “click” functionalisation. A methyl ether was trialled as an alternative to the amines of L20, and the resulting ligand, L38, was found to confer similar properties onto its assemblies. Incorporation of alkyne Raman labels into a cage allowed stimulated Raman scattering imaging of its interaction with cells. Overall, a robust cage system capable of retaining and altering the uptake of a significant diagnostic agent in vivo was developed. Progress was also made towards improving the properties, targeting and monitoring of the system.