Polymyxins are cationic lipopeptides (five cationic charges) and the last resort for the treatment of serious Gram-negative infections caused by multiresistant strains. NAB741 has a cyclic peptide portion identical to that of polymyxin B but carries in the linear peptide portion a threonyl-D-serinyl residue (no cationic charges) instead of the diaminobutyryl-threonyl-diaminobutyryl residue (two cationic charges). At the N terminus of the peptide, NAB741 carries an acetyl group instead of a mixture of methyl octanoyl and methyl heptanoyl residues. NAB741 sensitized Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae, and Acinetobacter baumannii to antibiotics against which the intact outer membrane is an effective permeability barrier. When tested by using Etest strips on plates containing increasing concentrations of NAB741, the fractional inhibition concentration index (FICI) of the combination of NAB741 with rifampin ranged from <or=0.111 to 0.158 and that with clarithromycin from <or=0.094 to 0.292. When tested by the checkerboard method, the corresponding FICI values against E. coli ATCC 25922 were <or=0.141 to <or=0.155 with rifampin and 0.094 with clarithromycin. In addition, at 4 microg/ml, NAB741 decreased the MICs of azithromycin, mupirocin, fusidic acid, and vancomycin for E. coli strains and E. cloacae by factors ranging from 8 to 200. A sister peptide, NAB752, carrying a threonyl-aminobutyryl residue as the linear peptide portion, was inactive. Furthermore, NAB741 sensitized E. coli to the bactericidal activity of fresh guinea pig serum. The renal clearance of NAB741 was approximately 400-fold, 16-fold, and 8-fold higher than those measured for colistin, NAB7061, and NAB739, respectively.