Magainin

Magainin is a family of broad-spectrum antimicrobial peptides isolated from the skin of the African clawed frog Xenopus laevis, first characterized by Michael Zasloff in 1987. Magainin-2 is the primary studied isoform — a 23-amino acid cationic peptide with activity against gram-positive and gram-negative bacteria, fungi, and protozoa. It was the first frog-derived AMP to enter clinical development.

In aqueous solution, magainin-2 is largely unstructured. Upon contact with negatively charged bacterial membranes, it folds into an amphipathic alpha-helix and inserts into the bilayer via a 'carpet model' or toroidal pore mechanism. At sufficient concentrations, magainin causes membrane thinning, transient pore formation, and eventually membrane disintegration and leakage of cytoplasmic contents. Selectivity for prokaryotic over eukaryotic membranes arises from the abundance of anionic phospholipids in bacterial outer leaflets versus zwitterionic phosphatidylcholine-dominated mammalian membranes.

Magainin research established foundational principles of AMP membrane disruption, influencing drug design of hundreds of synthetic analogs. A magainin analog (pexiganan/MSI-78) reached Phase III trials for diabetic foot ulcer infections but failed to achieve superiority versus standard antibiotic comparators (1999), partly due to regulatory endpoint requirements. Research continues on magainin structural analogs with improved potency and reduced toxicity. Magainin-2 remains a gold-standard positive control in AMP research.