Self-Assembly of Antimicrobial Peptoids Impacts Their Biological Effects on ESKAPE Bacterial Pathogens

Abstract

Antimicrobial peptides (AMPs) are promising pharmaceutical candidates for the prevention and treatment of infections caused by multidrug-resistant ESKAPE pathogens, which are responsible for the majority of hospital-acquired infections. Clinical translation of AMPs has been limited, in part by apparent toxicity on systemic dosing and by instability arising from susceptibility to proteolysis. Peptoids (sequence-specific oligo-N-substituted glycines) resist proteolytic digestion and thus are of value as AMP mimics. Only a few natural AMPs such as LL-37 and polymyxin self-assemble in solution; whether antimicrobial peptoids mimic these properties has been unknown. Here, we examine the antibacterial efficacy and dynamic self-assembly in aqueous media of eight peptoid mimics of cationic AMPs designed to self-assemble and two nonassembling controls. These amphipathic peptoids self-assembled in different ways, as determined by small-angle X-ray scattering; some adopt helical bundles, while others form core–shell ellipsoidal or worm-like micelles. Interestingly, many of these peptoid assemblies show promising antibacterial, antibiofilm activity in vitro in media, under host-mimicking conditions and antiabscess activity in vivo. While self-assembly correlated overall with antibacterial efficacy, this correlation was imperfect. Certain self-assembled morphologies seem better-suited for antibacterial activity. In particular, a peptoid exhibiting a high fraction of long, worm-like micelles showed reduced antibacterial, antibiofilm, and antiabscess activity against ESKAPE pathogens compared with peptoids that form ellipsoidal or bundled assemblies. This is the first report of self-assembling peptoid antibacterials with activity against in vivo biofilm-like infections relevant to clinical medicine.

Efficacy of Cathelicidin-Mimetic Antimicrobial Peptoids against Staphylococcus aureus

“…Prior studies with the 12mer Peptoid 1 have shown great promise against planktonic S. aureus and biofilm-forming bacteria ( 18 , 21 , 23 , 24 , 27 ). The design of this peptoid was originally inspired by that of magainin-2, an antimicrobial peptide found in Xenopus laevis , and consists of four lysine-like monomers ( N Lys) and eight phenylalanine-like monomers ( N -1-S-phenylethyl, N spe) patterned to give the peptoid helical, water-soluble, and amphipathic properties ( 24 ).…”

“…Peptoid 1-C13 4mer is five residues long total, with its amino-terminal peptoid side chain being a 13-carbon-long N -alkyl tail, which has shown good activity against biofilms ( 23 , 24 ). The long alkyl tail of Peptoid 1-C13 4mer promotes its self-assembly into ellipsoidal micelles, rather than helical bundles as for Peptoid 1 ( 18 ). The chemical structures of the peptoids used in this study are depicted in Fig.…”

“…In addition, they are easier and less expensive to synthesize than regular α-peptides. Similar to LL-37, Peptoid 1 has been shown to self-assemble in solution forming helical bundles ( 18 ). The mechanism of antibacterial and antifungal action for various AMP-mimetic peptoids has been investigated ( 15 , 19 , 20 ), and involves a combination of membrane disruption and profound intracellular effects, including DNA- and RNA-binding and ribosome flocculation, similar to what has been observed for LL-37 ( 15 ).…”

“…Peptoid 1 has previously been shown to have potent activity against a broad spectrum of bacteria, including resistant strains ( 18 , 21 – 24 ), fungi ( 18 ), and viruses ( 25 ) while showing minimal cytotoxicity profiles against a number of mammalian cell lines ( 21 , 25 ), and almost no toxicity to primary human cells grown at the air-liquid interface ( 25 ). Interestingly, Peptoid 1 has been shown to be extremely effective in preventing biofilm formation of Pseudomonas aeruginosa compared to standard antibiotics ( 23 , 24 ), activity that is particularly relevant because of the tendency for wound infections to involve the growth of a strong biofilm.…”

“…Peptoids also recently have been demonstrated to reduce bacterial counts in a murine model of an invasive S. aureus infection ( 26 , 27 ). Remarkably, Peptoid 1 and several analogues have been proven to retain their anti-biofilm activity even in the context of polymicrobial infections comprised by clinical isolates of P. aeruginosa and S. aureus in host-mimicking conditions in vitro ( 18 ). In a murine abscess model, where Peptoid 1 was injected into the abscess, Peptoid 1 reduced both P. aeruginosa and S. aureus abscess size and bacterial load ( 18 ).…”