Structural Permutation of Potent Cytotoxin, Polytheonamide B: Discovery of Cytotoxic Peptide with Altered Activity

Polytheonamide B (<b>1</b>) is an ion-channel forming natural peptide with a d,l-alternating 48 amino acid sequence, which is an exceedingly potent cytotoxin. We recently designed and synthesized a simplified dansylated polytheonamide mimic <b>2</b>, in which six amino acid residues were modified from <b>1</b>, and demonstrated that <b>2</b> emulated the functions of <b>1</b>. Here we report a comprehensive structure–activity relationship study of substructures of <b>2</b>. A unified synthetic strategy was developed for highly automated syntheses of 13 peptide sequences of 27 to 39 amino acid residues, and the artificial 37-mer peptide <b>6</b> was discovered to be significantly more toxic than the other 12 compounds toward P388 mouse leukemia cells (IC₅₀ = 3.7 nM). Ion exchange activity experiments of <b>6</b> using the liposome and P388 cells both demonstrated that <b>6</b> did not possess ion-channel activity, strongly suggesting that <b>6</b> exerted its potent cytoxicity through a distinct mode of action from <b>1</b> and <b>2