Validation and application of a sensitive assay for butorphanol in human plasma by high-performance liquid chromatography with tandem mass spectrometry detection

A sensitive and convenient high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) assay for the opioid receptor agonist–antagonist butorphanol in human plasma is described. BC-2605, a cyclopropyl analogue of butorphanol, was employed as an internal standard. Butorphanol was recovered from plasma (84.4±10.9%) by liquid–liquid extraction. The mobile phase flow-rate was 0.3 ml/min and consisted of methanol–water–formic acid (90:10:0.1, v/v/v). The analytical column (4.6×100 mm) was packed with Partisil C8 (5 μm). The standard curve was linear from 13.7 to 1374 pg/ml (r2>0.99). The lower limit of quantitation was 13.7 pg/ml. The assay was specific, accurate (% deviation from nominal concentrations were <15%), precise and reproducible (within- and between-day coefficients of variation <7%). Butorphanol in plasma was stable over 3 freeze/thaw cycles and at room temperature for 1 day. The utility of the assay was demonstrated by following butorphanol plasma concentrations in two healthy subjects for 24 h following a 1 mg intranasal dose.