Lipid-based liquid crystals as drug delivery vehicles for antimicrobial peptides

Abstract: The development of antimicrobial resistance is a great challenge within the health sector around the world. The demand for new efficient treatments is alarming in order to treat various bacterial infections in the near future. Antimicrobial peptides (AMPs) are a group of novel antibiotics that have gain more and more attraction the past decade. However, AMPs suffers from relatively low stability due to proteolytic and chemical degradation. As a consequence, carrier systems protecting the AMPs are highly needed for achieving efficient treatments.

In this thesis, lyotropic liquid crystalline (LC) structures consisting of cubic glycerol monooleate/water and hexagonal glycerol monooleate/oleic acid/water have been examined as carriers for three AMPs (AP114, DPK-060 and LL-37). Both bulk gels and discrete dispersed structures, i.e. cubosomes and hexosomes have been studied. Moreover, two different peptide loading approaches for the cubosomes were tested and compared; pre- and post-loading. Characterization of the LC structures was performed using small-angle x-ray scattering (SAXS), dynamic light scattering, ζ-potential, and cryogenic transmission electron microscopy (Cryo-TEM) and peptide loading efficacy by liquid chromatography. The antimicrobial effect of the AMP loaded LC nanoparticles (LCNPs) was studied in vitro using minimum inhibitory concentration (MIC) and time-kill assays. Proteolytic protection was investigated by incubating the formulations with two elastases and the antimicrobial effect after proteolysis was studied using radial diffusion assay (RDA).

Results showed that the most hydrophobic peptide (AP114) was prone to induce an increase in negative curvature of the bulk cubic LC gel, hence pushing the system towards a hexagonal structure. The most polar peptide (DPK-060) induced a decrease in negative curvature while LL-37 did not change the LC phase at all. The hexagonal LC phase was not affected by any of the AMPs. The cubic pre- and post-loaded LCNPs displayed promising antimicrobial activity, and sometimes could a synergetic effect be observed, resulting in a slightly better activity than the unformulated AMP. The hexagonal LCNPs were found to be very efficient in encapsulating the AMPs, but did not display any antimicrobial effect, indicating insufficient delivery of peptide to the bacteria. Moreover, cubosomes post-loaded with LL-37 was found to protect the peptide from proteolytic degradation, resulting in a significant better bactericidal effect after proteolysis.