Polymeric Nanoparticles as Carriers for Antimicrobial Peptides : Factors Affecting Peptide and Membrane Interactions

Abstract: As resistance towards conventional antibiotics is becoming more pronounced, cationic antimicrobial peptides (AMPs) have received considerable attention as possible therapeutic alternatives. Thousands of potent AMPs occur in humans, animals, plants and fungi as a natural part of the immune system. However, there are several challenges with AMP therapeutics related to formulation and delivery. Examples include proteolytic sensitivity and serum protein binding, resulting in quick degradation, loss of activity and clearance. Therefore, it is important to find a suitable drug delivery system to meet these protection and delivery challenges. Micro-/nanogels are loosely crosslinked polymer colloids with high water content that can be made to trigger at a wide range of stimuli. They have shown promise as delivery systems for AMPs, as the aqueous environment they create allows the peptides to maintain their natural conformation, while their gel networks offer protection and triggered release. This thesis aims towards expanding the knowledge about degradable and non-degradable pH-responsive micro-/nanogels as carriers for AMPs.The results in this thesis show that factors relating to the drug delivery system (degradability, charge and crosslinker density), the surrounding media (pH and ionic strength) and the peptide properties (length, charge, PEGylation) all affect the peptide loading to, protection, release from and effect of AMP-loaded gels. Studies of the interaction of AMP-loaded microgels with bacteria-modelling liposomes and lipid bilayers have verified peptide effect after gel incorporation, as further demonstrated by in vitro studies on several bacterial strains. Neutron reflectometry provided detailed mechanistic information on the interaction between AMP-loaded gels and bacteria-modelling lipid bilayers, showing that the antimicrobial unit is the released peptide. All gels showed low, promising hemolysis and some gels could offer protection against proteolytic degradation of AMPs.In summary, non-degradable and degradable micro-/nanogels are versatile and interesting candidates as AMP carriers. Small changes in the gel composition or the AMP used can dramatically change the peptide loading, release and effect. It is therefore necessary to carefully consider and evaluate the optimal carrier for every AMP and the application at hand.