Antimicrobial spectrum against wound pathogens and cytotoxicity of star-arranged poly-l-lysine-based antimicrobial peptide polymers

<b>Introduction.</b> As growing numbers of patients are at higher risk of infection, novel topical broad-spectrum antimicrobials are urgently required for wound infection management. Robust pre-clinical studies should support the development of such novel antimicrobials.<b>Gap statement.</b> To date, evidence of robust investigation of the cytotoxicity and antimicrobial spectrum of activity of antimicrobial peptides (AMP)s is lacking in published literature. Using a more clinical lens, we address this gap in experimental approach, building on our experience with poly-l-lysine (PLL)-based AMP polymers.<b>Aim.</b> To evaluate the <i>in vitro</i> bactericidal activity and cytotoxicity of a PLL-based 16-armed star AMP polymer, designated 16-PLL₁₀, as a novel candidate antimicrobial.<b>Methods.</b> Antimicrobial susceptibilities of clinical isolates and reference strains of ESKAPE (<i>Enterococcus</i> spp., <i>Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa</i>, <i>Enterobacter</i> spp.) pathogens, to 16-PLL₁₀ were investigated. Human erythrocyte haemolysis and keratinocyte viability assays were used to assess toxicity. Modifications were made to 16-PLL₁₀ and re-evaluated for improvement.<b>Results.</b> Minimum bactericidal concentration of 16-PLL₁₀ ranged from 1.25 µM to ≥25 µM. At 2.5 µM, 16-PLL₁₀ was broadly bactericidal against ESKAPE strains/wound isolates. Log-reduction in colony forming units (c.f.u.) per millilitre after 1 h, ranged from 0.3 (<i>E. cloacae</i>) to 5.6 (<i>K. pneumoniae</i>). At bactericidal concentrations, 16-PLL₁₀ was toxic to human keratinocyte and erythrocytes. Conjugates of 16-PLL₁₀, Trifluoroacetylated (TFA)-16-PLL₁₀, and Poly-ethylene glycol (PEG)ylated 16-PLL₁₀, synthesised to address toxicity, only moderately reduced cytotoxicity and haemolysis.<b>Conclusions.</b> Due to poor selectivity indices, further development of 16-PLL₁₀ is unlikely warranted. However, considering the unmet need for novel topical antimicrobials, the ease of AMP polymer synthesises/modification is attractive. To support more rational development, prioritising clinically relevant pathogens and human cells, to establish selective toxicity profiles <i>in vitro</i>, is critical. Further characterisation and discovery utilising artificial intelligence and computational screening approaches can accelerate future AMP nanomaterial development.