Background: Polymyxin B is an old antibiotic with increasing clinical relevance in the treatment of multidrug-resistant Gram-negative bacterial infections. However, current dosing regimens are largely empiric as clinical pharmacological characterization of the drug has been hindered by the lack of assays to measure polymyxin B in human plasma.
Methods: A high-performance liquid chromatography–mass spectrometry assay was developed to quantify polymyxin B1 and B2 in human plasma using pure calibrators. After purification with a solid-phase extraction column, polymyxin B1 and B2 were separated on a C18 column by gradient chromatography with an overall runtime of 12 minutes. Polymyxin B1 and B2 were ionized by positive electrospray ionization, and the resulting ions specific to polymyxin B1 and B2 were monitored (selected ion recording).
Results: The dominant ions produced were (M + 2H)2+ at m/z 602.6 and 595.5 for polymyxin B1 and polymyxin B2, respectively. The assay was linear between concentrations of 100 and 2500 ng/mL, with interday precision of 5.9% and 3.4% at 100 ng/mL and 5.3% and 4.0% at 2000 ng/mL for polymyxin B1 and polymyxin B2, respectively. Accuracy was 80.2% and 82.2% at 100 ng/mL and 99.9% and 109.6% at 2000 ng/mL for polymyxin B1 and polymyxin B2, respectively. No interference from other drugs commonly administered with polymyxin B was detected. The performance of the assay is affected by gross hemolysis and hyperlipemia. The method was successfully applied to patient samples. Interestingly, in a single patient the ratio of B1 and B2 did not change over a period of 12 hours after administration of the drug.
Conclusions: A simple method for the simultaneous measurement of polymyxin B1 and polymyxin B2 in human plasma is described, which has the potential to optimize clinical use of this valuable antibiotic by permitting pharmacokinetic studies and therapeutic drug monitoring.