Specific and sensitive analysis of nefopam and its main metabolite desmethyl-nefopam in human plasma by liquid chromatography-ion trap tandem mass spectrometry

A specific and sensitive liquid chromatography-tandem mass spectrometric (LC-MS-MS) method using an ion trap spectrometer was developed for quantitation of nefopam and desmethyl-nefopam, in human plasma. Nefopam, desmethyl-nefopam and the internal standard (ethyl loflazepate) were extracted in a single step with diethyl ether from 1 mL of alkalinized plasma. The mobile phase consisted of acetonitrile with 0.1% formic acid (50:50, v:v). It was delivered at a flow-rate of 0.3 mL/min. The effluent was monitored by MS-MS in positive-ion mode. Ionisation was performed using an electrospray ion source operating at 200 degrees C. Nefopam and desmethyl-nefopam were identified and quantified in full scan MS-MS mode using a homemade MS-MS library. Calibration curves were linear over the concentration range of 0.78-100 ng/mL with determination coefficients > 0.996. This method was fast (total run time < 6 min), accurate (bias < 12.5%), and reproducible (intra- and inter-assay precision < 17.5%) with a quantitation limit of 0.78 ng/mL. The high specificity and sensitivity achieved by this method allowed the determination of nefopam and desmethylnefopam plasma levels in patients following either intermittent or continuous intravenous administration of nefopam. (c) 2006 Elsevier B.V. All rights reserved.