Lipid-based liquid crystals as drug delivery vehicles for antimicrobial peptides
The development of antimicrobial resistance is a great challenge within health sectors worldwide. Thus, demand for new, efficient treatments is urgent in order to treat various bacterial infections. Antimicrobial peptides (AMPs) are a group of antibiotics that have gained more and more attraction in the past decade. AMPs suffer from relatively low stability due to proteolytic and chemical degradation. As a consequence, carrier systems to protect the AMPs are highly needed to achieve efficient treatments in the clinic.
In this thesis, lipid-based liquid crystalline (LC) structures have been examined as carriers for AMPs. LC structures of polar lipids have potential to be used as carriers and delivery systems in various pharmaceutical applications. This is due to their ability to solubilize and encapsulate hydrophilic, hydrophobic and amphiphilic substances. An important feature of these LC systems is that they can coexist with an excess of water, which enables fragmentation of the highly viscous gels into LC nanoparticles (LCNPs), i.e. cubosomes and hexosomes, in the presence of a suitable stabilizer. Peptides and proteins can be incorporated into the lipid self-assembled structures, thereby protecting them from chemical and proteolytic degradation. Cubosomes and hexosomes were investigated as drug delivery vehicles for the three AMPs: i) AP114, an improved plectasin derivative originating from the fungus Pseudoplectania nigrella, ii) DPK-060, derived from the endogenous human protein kininogen and iii) LL-37, a human AMP found in the cathelicidin family. Phase behavior, different preparation methodologies of the LCNPs, antimicrobial effect and proteolytic protection of the AMPs were studied. Moreover, the interaction between AMP-loaded particles with bacteria and bacterial mimicking membranes was investigated. Formulations aimed for pulmonary and topical administration were also evaluated. Results showed that cubic LC phases were most sensitive to the incorporation of AMPs. Depending on the nature of the AMP, different changes in the curvature of the systems were observed. Cubosomes loaded with AMPs exhibited good antimicrobial activity and were found to protect the proteolytic sensitive LL-37 from enzymatic degradation. Data strongly suggested that the release of AMP from the particles cannot solely be explained by the antimicrobial effect. Cubosomes loaded with LL-37 are thought to adsorb onto bacterial membranes, resulting in cell death.
liquid crystalline nanoparticles