Antimicrobial Activity and Toxicity of the Major Lipopeptide Components of Polymyxin B and Colistin: Last-Line Antibiotics against Multidrug-Resistant Gram-Negative Bacteria

Polymyxin B and colistin are currently used as a “last-line” treatment for multidrug-resistant Gram-negative bacteria. However, very little is known about the pharmacological differences between polymyxin B 1, polymyxin B 2, colistin A, and colistin B, the major cyclic lipopeptide components present in polymyxin B and colistin products. Here, we report on the in vitro and in vivo antimicrobial activity and toxicity of these major lipopeptide components. All four lipopeptides had comparable minimum inhibitory concentrations (MICs) (<0.125–4 mg/L) against a panel of clinical Gram-negative isolates. They also had comparable in vivo antimicrobial activity (Δlog 10 colony-forming units (CFU)/mL > −3) and nephrotoxicity (mild to moderate histological damage) in mouse models. However, polymyxin B 1 and colistin A showed significantly higher (>3-fold) in vitro apoptotic effect on human kidney proximal tubular HK-2 cells than polymyxin B 2 and colistin B, respectively. Compared to the commercial polymyxin and colistin products, the individual lipopeptide components had slightly more in vivo antimicrobial activity. Our results highlight the need to reassess pharmacopoeial standards for polymyxin B and colistin and to standardize the composition of the different commercial products of polymyxin antibiotics.