Synthesis and bioactivity investigation of polymyxin B analogues with Thr2, Dab3, and Thr10 modifications

AIMS: To investigate the structure-activity (SAR) and structure-toxicity (STR) relationships of the three polar-face positions Thr2, Dab3, and Thr10 in polymyxin B (PMB) and identify substitutions retaining antibacterial potency while reducing HK-2 cytotoxicity. MATERIALS AND METHODS: Twenty-two PMB analogues were synthesized and evaluated for antibacterial activity and HK-2 cytotoxicity. New compound 22 was further assessed in a mouse systemic infection model. Its lipopolysaccharide (LPS) binding was examined by biolayer interferometry (BLI). Isothermal titration calorimetry (ITC) characterized peptide-membrane interactions. RESULTS: Ser substitution at Thr2 or Thr10 retained antibacterial activity with modestly reduced HK-2 cytotoxicity. Replacing Dab3 with 2,3-diaminopropionic acid (Dap) or ornithine (Orn) preserved activity and markedly reduced HK-2 cytotoxicity. Compound 7 showed a selectivity index (SI) range of >2000.0 to >8000.0. Combining Dap3 with reduced hydrophobicity at Leu7 maintained or improved activity and increased selectivity. Compound 22 showed an SI range of >2000.0 to >16666.7, efficacy comparable to PMB in a preliminary mouse infection model, LPS binding, and a smaller ITC response than PMB. CONCLUSIONS: At positions 2 and 10, hydroxyl-bearing residues best preserved antibacterial activity, position 10 showed greater substitution tolerance. Position 3 required cationic character and offered the clearest opportunity to improve selectivity.