Substitution of a proline residue in the frog skin host-defense peptide, figainin-2PL by D-lysine generates an analog with potent activity against antibiotic-resistant ESKAPE pathogens

Figainin-2PL (FLGTVLKLGKAIAKTVVPMLTNAMQPKQ.NH₂) is active against a range of antibiotic-resistant bacteria as well as human tumor-derived cells. The study aimed to determine the effects of a proline-substitution on the biological potency of the peptide. Secondary structure predictions indicate that figainin-2PL can adopt a helix-turn-helix conformation in membrane-mimetic environments. Circular dichroism spectra are consistent with these predictions. Replacement of the Pro¹⁸ residue by either L-Ala, L-Lys or D-Lys increased the extent and stability of the helical domains. All substitutions increased cytotoxic activity against a range of human tumor-derived cells (between 1.5- and 3-fold) but major (between 4- and 10-fold) increases in hemolytic activity against mouse erythrocytes were also observed. While the substitutions Pro¹⁸ → L-Ala and Pro¹⁸ → L-Ala produced only minor and inconsistent changes in antibacterial activity, the [P18k] analog displayed a 2- to 4-fold greater (MIC and MBC in the range 1–8 µM) against the ESKAPE pathogens Enterococcus faecalis, Enterococcus faecium, Klebsiella pneumoniae and Pseudomonas aeruginosa compared with figainin-2PL and the peptide retained high potency against Acinetobacter baumannii, Escherichia coli, Staphylococcus aureus and Clostridium difficile (MIC and MBC = 2 µM). Consequently, the [P18k] peptide shows therapeutic potential for development into a broad-spectrum, topical antimicrobial agent.