Design of potent, non-toxic antimicrobial agents based upon the naturally occurring frog skin peptides, ascaphin-8 and peptide XT-7

The frog skin peptides, ascaphin-8 (GFKDLLKGAAKALVKTVLF.NH₂) and XT-7 (GLLGPLLKIAAKVGSNLL.NH₂), show broad-spectrum antimicrobial activity but their therapeutic potential is limited by toxicity against mammalian cells. Circular dichroism spectra demonstrate that the peptides adopt an amphipathic α-helical conformation in a membrane-mimetic solvent. This study has investigated the cytolytic properties of analogs containing selected amino acid substitutions that increase cationicity while maintaining amphipathicity. Substitutions at Ala¹⁰, Val¹⁴, and Leu¹⁸ in ascaphin-8 by either l-Lys or d-Lys produced peptides that retained antimicrobial activity against the bacteria Escherichia coli and Staphylococcus aureus and the opportunistic yeast pathogen, Candida albicans but showed appreciably reduced toxicities (>10-fold) against human erythrocytes, HepG2 hepatoma-derived cells, and L929 fibroblasts. The improved therapeutic index of the l-Lys¹⁸ and d-Lys¹⁸ analogs correlated with a decrease in % helicity and in effective hydrophobicity. Substitution of Gly⁴ by l-Lys in XT-7 produced an analog with high potency against micro-organisms (MIC ≤ 25 mu;m) but low cytolytic activity against erythrocytes (LD₅₀ > 500 mu;m) and this increase in therapeutic index also correlated with decreased helicity and hydrophobicity. Analogs of XT-7 with increased cationicity, containing multiple substitutions by l-Lys, not only displayed increased antimicrobial potencies, particularly against Candida albicans (MIC ≤ 6 mu;m), but also increased hemolytic activities.