Simultaneous quantitation of polymyxin B1, polymyxin B2 and polymyxin B1-1 in human plasma and treated human urine using solid phase extraction and liquid chromatography-tandem mass spectrometry.

Two liquid chromatographic-tandem mass spectrometric (LC-MS/MS) methods have been developed and validated for the quantitative determination of polymyxin B1, polymyxin B2 and polymyxin B1-1 concentrations in human plasma and treated urine. During method development, technical challenges such as the separation of structural isomers polymyxin B1and polymyxin B1-1 and nonspecific binding in urine samples were encountered and overcome. Two automated solid phase extraction methods were used to extract plasma samples (100μL) and urine samples (200μL) and the resulting extracts were analyzed using reversed phase LC-MS/MS with an electrospray (ESI) interface and selected reaction monitoring (SRM) in the positive ionization mode. Both methods were validated over a calibration curve range of 5.00-2000ng/mL with a linear regression and 1/x(2) weighting. The between-run relative standard deviation (%RSD) ranged from 4.5 to 9.5% for the plasma assay and from 1.1 to 7.1% for the urine assay. For the plasma assay, the between-run accuracy ranged from 100.5 to 115.2% of nominal at all QC concentrations including the LLOQ. For the urine assay, the between-run accuracy ranged from 92.0 to 106% of nominal at all QC concentrations including the LLOQ. The extraction recoveries for all polymyxins in both assays were between 54.0 and 64.2%. Long term matrix storage stability for all polymyxins was established at both -20°C and -70°C for up to 85 days in human plasma and for up to 55 days in treated human urine. Both assays were used for the measurement of polymyxin B1, polymyxin B2 and polymyxin B1-1 concentrations in human plasma and treated urine for the determination of bioequivalence and toxicokinetic parameters in clinical studies.