Novel nitric oxide delivery systems for biomedical applications

Abstract

The aim of the research presented in this thesis is to investigate and develop novel nitric oxide (NO) delivery systems, specifically designed for application in medical areas. The initial work has focused on utilising metal organic frameworks (MOFs) as a delivery system for this radical gas, NO. Due to their high porosity, high thermal stability and the presence of coordinated unsaturated metal sites (CUSs) when fully activated, the CPO-27 (Coordination Polymer of Oslo) family of MOFs has been selected as a suitable host framework. CPO-27 (Ni), CPO-27 (Mg) and CPO-27 (Zn) have been prepared using reflux and room temperature processes without recourse to the use of any toxic or harmful solvents. The resulting products are characterised by powder XRD (X-ray diffraction) and SEM (Scanning electron microscopy), and their NO adsorption, storage and release properties are reported. The results indicate that the crystallinity, particle size and NO adsorption, storage and release performance are comparable to those of equivalent samples synthesised via traditional solvothermal methods, paving the way for a more easily scalable and environmentally friendly synthetic procedure for these types of MOF. Depending on which metal is employed; the NO uptake, storage and release varies, the more toxic nickel based framework shows enhanced performance in terms of concentration and duration of NO released against either the magnesium or zinc counterparts. In order therefore to reduce the risk of toxicity whilst retaining good performance, Ni (II) ions were doped into the 3D framework of CPO-27 (Mg) and CPO-27 (Zn) using novel water-based reflux and room temperature crystallization methods. Several characterization techniques strongly support the effective incorporation of Ni (II) ions into the 3D framework. Nitric oxide (NO) adsorption/release data, as well as in vitro tests demonstrate that NO dosage and biological response can be tuned via the Ni doping process allowing enhanced performance without the high toxicity of pure Ni MOFs. Such materials would be extremely advantageous and more applicable for use in medical fields. NONOates and other NO-complexes have also been investigated as alternative NO delivery systems. This study has focused on developing NO-drug complexes using a variety of different compounds commonly used by clinicians, namely the antiseptic (chlorhexidine, CHx), the antibiotic (ciprofloxacin) and diuretic (furosemide). A unique high pressure NO loading methodology has been developed to coordinate nitric oxide to these drug molecules and their NO release performance has been evaluated. The resulting NO-drug complexes are characterised using a series of spectroscopic techniques and the collected data highlights that the radical gas coordinates with the secondary amine groups present in the drug molecules. The interaction between the amine group and the gas is reversible; in fact the release of NO from these complexes can be triggered using water (11% RH) and/or UV-light. In addition, chlorhexidine has been incorporated into the pores of the CPO-27 framework. The amount of antiseptic incorporated was determined using a variety of characterisation techniques. The controlled release of significant concentrations of CHx from the CPO-27 materials are achieved by exposing each CHx loaded sample to a water solution, in doing so topical conditions are simulated. The CHx loaded samples have also been activated and NO loaded following the novel high pressure procedure specifically developed during this research. The resulting NO loaded material released the radical gas in the presence of water and/or UV-light. By incorporating the CHx into the MOF and NO loading this complex the duration and release of NO was greatly enhanced over that of either of the components alone. On formulating the CHx loaded 3D frameworks into pellets, or even into a polyurethane polymer film, their ability to release the antiseptic under simulated topical conditions was maintained. The NO-CHx-CPO-27 composite film that has been prepared has proven to be able to simultaneously store and release both NO and CHx. Each component of the complex has more than one function and the quantity and duration of release of NO is again higher and longer than either of the two moieties alone. The release of these two antibacterial agents from a MOF is novel and is very exciting as it opens up the possibility of engineering products with multiple actions to fight infection. Owing to their high stability and shape persistence properties, the CC3 cage series (CC3, RCC3, FT-RCC3 and AT-RCC3) was chosen as the basis of an investigation into the potential use of porous organic cages as delivery systems for nitric oxide gas. NO has been stored in these porous materials through coordination to amine groups forming Nnitrosamine groups. Release of NO from these types of compounds can be triggered by various mechanisms including water and UV-light, the amine group being regenerated after the release of NO. The release performance significantly increased when the materials were exposed to UV-light and/or suspended in water. As a result of this investigation, these covalent organic molecular cages can now be added to the existing list of NO-based therapies available to medical professionals.