Review: Structure-Activity Relationship of Antimicrobial Peptoids

Due to their broad-spectrum activity against Gram-negative and Gram-positive bacteria, natural antimicrobial peptides (AMPs) and their synthetic analogs have emerged as prospective therapies for treating illnesses brought on by multi-drug resistant pathogens. To overcome the limitations of AMPs, such as protease degradation, oligo-N-substituted glycines (peptoids) are a promising alternative. Despite having the same backbone atom sequence as natural peptides, peptoid structures are more stable because, unlike AMP, their functional side chains are attached to the backbone nitrogen (N)-atom rather than the alpha carbon atom. As a result, peptoid structures are less susceptible to proteolysis and enzymatic degradation. The advantages of AMPs, such as hydrophobicity, cationic character, and amphipathicity, are mimicked by peptoids. Furthermore, structure-activity relationship studies (SAR) have shown that tuning the structure of peptoids is a crucial step in developing effective antimicrobials.

Antibacterial mechanisms of GN-2 derived peptides and peptoids against Escherichia coli

Escherichia coli is the main etiological agent of urinary trait infections, able to form biofilms in indwelling devices, resulting in chronic infections which are refractory to antibiotics treatment. In this study, we investigated the antimicrobial and anti-biofilm properties exerted against E. coli ATCC 25922, by a set of peptoids and peptides modeled upon the peptide GN-2, previously reported as a valid antimicrobial agent. The putative antimicrobials were designed to evaluate the effect of cationicity, hydrophobicity and their partitioning on the overall properties against planktonic cells and biofilms as well as on LPS binding, permeabilization of Gram-negative bacteria membranes and hemolysis. The data demonstrated that peptides are stronger antimicrobials than the analogue peptoids which in return have superior anti-biofilm properties. In this study, we present evidence that peptides antimicrobial activity correlates with enhanced LPS binding and hydrophobicity but is not affected by partitioning. The data demonstrated that the enhanced anti-biofilm properties of the peptoids are associated with decreased hydrophobicity and increased penetration of the inner membrane, compared to that of their peptide counterpart, suggesting that the characteristic flexibility of peptoids or their lack of H-bonding donors in their backbone, would play a role in their ability to penetrate bacterial membranes.

In Vitro ADME Properties of Two Novel Antimicrobial Peptoid-Based Compounds as Potential Agents against Canine Pyoderma

Antimicrobial peptides (AMPs) hold promise as the next generation of antimicrobial agents, but often suffer from rapid degradation in vivo. Modifying AMPs with non-proteinogenic residues such as peptoids (oligomers of N-alkylglycines) provides the potential to improve stability. We have identified two novel peptoid-based compounds, B1 and D2, which are effective against the canine skin pathogen Staphylococcus pseudintermedius, the main cause of antibiotic use in companion animals. We report on their potential to treat infections topically by characterizing their release from formulation and in vitro ADME properties. In vitro ADME assays included skin penetration profiles, stability to proteases and liver microsomes, and plasma protein binding. Both B1 and D2 were resistant to proteases and >98% bound to plasma proteins. While half-lives in liver microsomes for both were >2 h, peptoid D2 showed higher stability to plasma proteases than the peptide-peptoid hybrid B1 (>2 versus 0.5 h). Both compounds were suitable for administration in an oil-in-water cream formulation (50% release in 8 h), and displayed no skin permeation, in the absence or presence of skin permeability modifiers. Our results indicate that these peptoid-based drugs may be suitable as antimicrobials for local treatment of canine superficial pyoderma and that they can overcome the inherent limitations of stability encountered in peptides.