Modulation of the hydrophobic domain of polymyxin B nonapeptide: Effect on outer-membrane permeabilization and lipopolysaccharide neutralization

Polymyxin B nonapeptide (PMBN), a cationic cyclic peptide derived from the antibacterial peptide polymyxin B, is capable of specifically increasing the permeability of the outer membrane (OM) of Gram-negative bacteria toward hydrophobic antibiotics. In this study, we evaluated the contribution of the hydrophobic segment of PMBN (i.e., D-Phe(5)-Leu(6)) to this activity. Accordingly, we synthesized four analogs of PMBN by replacing D-Phe(5) with either with D-Trp or D-Tyr and Leu(6) with Phe or Ala and evaluated their ability to bind cell-free lipopolysaccharide (LPS) and increase bacterial OM permeability. Compared with PMBN, [D-Tyr(5)]PMBN and [Ala(6)]PMBN possessed reduced LPS affinity (IC50 = 2.5, 25, and 12 muM, respectively) and significantly reduced OM permeability and LPS neutralization activity. [Phe(6)]PMBN exhibited rather similar affinity to cell-free LPS (IC50 = 5 muM) and the same OM permeability capacity as PMBN. However, [D-Trp(5)]PMBN, despite its similar affinity to cell-free LPS (IC50 = 4 muM), had moderately reduced OM permeability capacity. These results demonstrate the significant role of the PMBN hydrophobic segment in promoting biological activity.