Analogues of the frog skin peptide alyteserin-2a with enhanced antimicrobial activities against Gram-negative bacteria

The emergence of strains of multidrug-resistant Gram-negative bacteria mandates a search for new types of antimicrobial agents. Alyteserin-2a (ILGKLLSTAAGLLSNL.NH ₂) is a cationic, α-helical peptide, first isolated from skin secretions of the midwife toad, Alytes obstetricans, which displays relatively weak antimicrobial and haemolytic activities. Increasing the cationicity of alyteserin-2a while maintaining amphipathicity by the substitution Gly ¹¹→Lys enhanced the potency against both Gram-negative and Gram-positive bacteria by between fourfold and 16-fold but concomitantly increased cytotoxic activity against human erythrocytes by sixfold (mean concentration of peptide producing 50% cell death; LC ₅₀=24μm). Antimicrobial potency was increased further by the additional substitution Ser ⁷→Lys, but the resulting analogue remained cytotoxic to erythrocytes (LC ₅₀=38μm). However, the peptide containing d-lysine at positions 7 and 11 showed high potency against a range of Gram-negative bacteria, including multidrug-resistant strains of Acinetobacter baumannii and Stenotrophomonas maltophilia (minimum inhibitory concentration=8μm) but appreciably lower haemolytic activity (LC ₅₀=185μm) and cytotoxicity against A549 human alveolar basal epithelial cells (LC ₅₀=65μm). The analogue shows potential for treatment of nosocomial pulmonary infections caused by bacteria that have developed resistance to commonly used antibiotics.