Development of antimicrobial peptides as potential therapeutic agents

Antimicrobial peptides (AMPs) naturally exist in a wide range of organisms, varying from bacteria to humans. AMPs are expressed in organisms as a part of their defense system against invasion from the environment. The structure of AMPs is diverse, including α-helical, β-sheet and cyclic conformations, as well as those with negligible structure. However, they all exhibit two unique features: 1) a net positive charge of +2 or greater, owing to an excess of basic amino acids (Lys, Arg) over acidic amino acids (Asp, Glu) and 2) an amphipathic nature, which segregates basic and hydrophobic residues to opposite side of the molecule. AMPs have an unusually broad spectrum of activity, including antibacterial, antifungal, antiviral, antitumor and antiparasitic activity. Scientists are trying to develop antimicrobial peptides as therapeutic agents. It is generally accepted that antimicrobial peptides directly target the cell membrane, lyse the cells and cause cell death. Such peptides, exhibit a broad spectrum of activity and low rates of resistance. However, they also exhibit unwanted cytotoxicity to mammalian cells. The challenge of this development is to increase the antimicrobial activity while eliminating the cytotoxicity of these peptides.;We designed de novo series of α-helical antimicrobial peptides based on our starting peptides, V681 and V13K, derivatives of cecropin (1-8)-melittin (1-18) hybrid peptide (CEME), to study structure-activity relationship (SAR) by altering the peptide hydrophobicity/hydrophilicity, amphipathicity, net charge and charge distribution. The "specificity determinant(s)" design concept was developed in our laboratory and refers to positively charged residue(s) in the center of the non-polar face of synthetic amphipathic α-helical AMPs to create selectivity between eukaryotic and prokaryotic membranes, which maintained antimicrobial activity and decreased or eliminated cell toxicity to mammalian cells. Through changes on the polar face and non-polar face, we obtained an analog peptide D16, which resulted in a combined effect on hemolytic activity of a 746-fold decrease and unprecedented improvements in the therapeutic indices of 1,305-fold and 895-fold against A. baumannii (11 clinical isolates) and P. aeruginosa (6 clinical isolates), respectively, compared to peptide D-V681. We also evaluated the in vivo safety and efficacy of peptide D-V13K and D16 on Sprague dawley® rats.