Liquid chromatography-tandem mass spectrometry method for determination of the pyridinium aldoxime 4-PAO in brain, liver, lung, and kidney

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was validated and applied to the in vitro determination of 4-[(hydroxyimino)methyl]-1-octylpyridinium cation (4-PAO), which can penetrate the blood–brain barrier and reactivate acetylcholinesterase (AChE) inhibited by alkylphosphonate in the brain, liver, lung, and kidney. The limit of detection (LOD) was 0.235 μg cation/g wet weight, and the quantification range and linearity of the calibration curve extended over a range of 0.470–941 μg cation/g wet weight. For the proof of applicability, when 4-PAO was administrated intravenously via the rat tail vein at 10% LD50, we were able to quantify the 4-PAO concentration in the tissues: brain 7.60 ± 1.32 μg cation/g wet weight (mean ± SD, n = 5), liver 26.8 ± 2.82 μg cation/g, lung 76.4 ± 24.9 μg cation/g, and kidney 638 ± 266 μg cation/g. In addition, the methods for 4-[(hydroxyimino)methyl]-1-decylpyridinium bromide (4-PAD) and 4-[(hydroxyimino)methyl]-1-(2-phenylethyl) pyridinium bromide (4-PAPE) were partly validated referring to the findings of the 4-PAO full validation. Thus, the LC-MS/MS method described in this study can be useful for quantification of pyridinium aldoxime methiodide (PAM)-type oximes in biological samples.