Formulation approaches to pulmonary delivery of colistin combination antibiotic therapy
thesisposted on 31.01.2017, 04:50 by Wallace, Stephanie
Abstract Advanced formulations such as liposomes are increasingly being employed to improve the therapeutic profile of existing drugs, and to facilitate appropriate delivery of combination drug products. This is particularly the case for antibiotic therapies, due to the development of bacterial resistance and consequent need for combination drug approaches. Multi-drug resistant (MDR) Gram-negative bacteria have become a major medical problem worldwide over the last decade, with rapidly diminishing therapeutic options and very few new agents in the drug development pipeline. This situation has forced the reintroduction of an old antibiotic, colistin, back into clinical use. Colistin inhalation therapy is being increasingly used as a salvage therapy for the treatment of MDR Gram-negative pulmonary infections; however, there is a dearth of information on its pharmacokinetics and pharmacodynamics. Importantly, current inhalable formulations have not been optimised. Combination therapy may be an important approach towards optimising the use of colistin in the treatment of MDR pulmonary infections. Inhalable formulations providing sustained concentrations of the combination of colistin and a second antibiotic within the lungs would offer a more effective inhalation therapy against MDR Gram-negative pulmonary infections. The research undertaken in this thesis investigates the use of colloidal-based drug delivery systems for the co-formulation of colistin and model poorly-water soluble co-drug, azithromycin. The surfactant-like structure of colistin and its prodrug, colistin methanesulphonate (CMS), has been suggested to impact on their solution behaviour and stability, however their self-assembly in solution has not been well studied. In this thesis dynamic light scattering studies have confirmed the formation of colistin and CMS association colloids at critical micelle concentrations of 2.1 and 6.1 mg/mL, respectively. The self- assembly of CMS impacted the rate of conversion of CMS to colistin in solution confirming that the concentration-dependent stability of CMS is attributable to micelle formation. Micelle formation by colistin and CMS in solution enabled solubilization of poorly-water soluble drugs, however the limited capacity of the micelles was deemed insufficient to use micellar solubilization solely as a means to co-formulate a poorly- water soluble antibiotic with either colistin or CMS. Consequently, liposomes were investigated as a drug delivery system with potential to co-formulate colistin and CMS with other antibiotics. Colistin was successfully incorporated into liposomes, and the incorporation of cholesterol into the liposome bilayer enhancing associations between colistin and the bilayer. However, it was found that CMS-loaded liposomes exhibited poor colloidal stability, resulting in particle size growth and changes in particle surface charge over time, and eventual phase separation. The formulation of CMS within a liposomal carrier also accelerated the conversion of the prodrug to colistin. The CMS-loaded liposome approach was therefore abandoned and further studies concentrated on colistin-loaded liposomes. Co-formulation of azithromycin and colistin in liposomes was consequently investigated. Remote-loading of liposomes using the pH gradient method enabled greater solubilization of azithromycin (azithromycin to lipid ratio 0.20:1) compared to passive loading (azithromycin to lipid ratio 0.16:1). In vitro release studies demonstrated that upon dilution, colistin rapidly re-established equilibrium associations with the liposome bilayer. Incorporation of colistin into the remotely-loaded azithromycin formulation accelerated the rate of azithromycin release from ‘slow release’ liposomes in a concentration-dependent manner, indicating that the association of colistin with liposomes modified bilayer fluidity. However, the rate of release of azithromycin from liposomes was also reduced by increasing the cholesterol composition in the lipid bilayer. Liposomes were investigated for their potential to provide co-localisation of colistin and azithromycin within the lungs following pulmonary delivery in a rat model. The temporal aspects of availability of drug in the lungs were investigated under the assumption that local drug availability was reflected by the rate of pulmonary absorption. Pulmonary absorptive drug clearance in turn was assumed to be reflected in plasma drug concentrations. Thus changes in plasma concentration profiles with changes in formulation were used as an indicator of drug availability in the lungs as a consequence of release of drug from the formulation. Liposomal encapsulation resulted in a 47% decrease in maximum plasma concentrations of colistin and a 26% reduction in systemic exposure, compared to unencapsulated colistin. The rate of pulmonary absorption of azithromycin, however, was not impeded by liposomal encapsulation. This thesis is the first study to investigate the potential of advanced drug delivery systems to provide co-localisation of colistin and azithromycin within the lungs following pulmonary delivery.