Design, Synthesis and Biological Evaluation of Antibacterial Peptides Derived from Lactoferricin and Porphyrin

Abstract

Pseudomonas aeruginosa is a commonly occurring opportunistic pathogen with the capacity to form biofilms. Pathogenic bacteria such as Pseudomonas aeruginosa have been known to develop bacterial resistance. Therefore new compounds are required to address this problem; antimicrobial peptides have been identified as a potential solution. The bioactive peptide lactoferricin has known activity against Pseudomonas aeruginosa but suffers from a number of drawbacks including metabolic instability and possible development of resistance. To address these issues, this thesis describes four strategies of peptide-design; namely incorporation of non-protein amino acids, truncation with novel C-terminal modifications, tetrabranching and conjugation to porphyrin. It was a necessity that the synthesized compounds incorporating these modifications were able to exhibit sufficient antimicrobial activity. Porphyrin-peptide conjugates were synthesized for their potentially novel antibacterial mechanism of action via intrinsic haem-uptake pathways. Both solid and solution phase synthetic strategies were explored for the porphyrin-peptide conjugates, with the solid phase strategy giving superior results. All successful analogues, were biologically evaluated by agar diffusion and MIC assays. The results of this have demonstrated that the incorporation of the non-protein amino acid, 3-aminobenzoic acid, resulted in loss of antibacterial activity of the original sequence; rendering any potential improvements on metabolic stability futile. Conversely, all other structural modifications demonstrated promising MIC values against Pseudomonas aeruginosa verifying these compounds as potent antimicrobials. Those showing promising antimicrobial activity render themselves as future candidates for evaluation of their enhanced therapeutic potential including analysis of metabolic stability. A potentially novel finding was the observation of antimicrobial activity of porphyrin-peptide conjugates in the absence of applied light. Porphyrin compounds which have shown antibacterial activity in the literature act exclusively through photodynamic mechanisms which have a requirement of an applied source of irradiation. The porphyrin-peptides in this study show retention of antimicrobial activity which is either due solely to antimicrobial peptide sequence by itself or possibly due to a mechanism related to the manipulation of haem-uptake pathways. At present, no explicit declaration of such mechanisms of action can be made. The promising antibacterial activity displayed by the series of novel compounds proposes further evaluation with potential to address the universal issue of bacterial resistance development.