Determination of casopitant and its three major metabolites in dog and rat plasma by positive ion liquid chromatography/tandem mass spectrometry

A sensitive, selective and quantitative method for the simultaneous determination of casopitant, a potent and selective antagonist of the human Neurokinin 1 (NK-1) receptor, and its three major metabolites M12, M13 and M31 was developed and validated in dog and rat plasma. Acetonitrile containing stable labeled internal standards for the four analytes was used to precipitate proteins in plasma. Chromatographic separation was obtained using a reversed phase column with multiple reaction monitoring turboionspray positive ion detection. The lower and upper limits of quantification for casopitant and its metabolites were 15 and 15,000 ng/mL, using a 50 μL of dog or rat plasma aliquot, respectively. The inter-day precision (relative standard deviation) and accuracy (relative error) in dog plasma, derived from the analysis of validation samples at 5 concentrations, ranged from 4.1% to 10.0% and −10.8% to 8.7%, respectively, for casopitant and its 3 major metabolites. The intra-day precision (relative standard deviation) and accuracy (relative error) in rat plasma, derived from the analysis of validation samples at 5 concentrations, ranged from 3.9% to 6.6% and −9.6% to 8.3%, respectively, for casopitant and its three metabolites. All analytes were found to be stable in analytical solutions for at least 43 days at 4 °C, in dog and rat plasma at room temperature for at least 24 h, at the storage temperature of −20 °C for at least 6 months, and following the action of three freeze–thaw cycles from −20 °C to room temperature. All analytes were also found to be stable in processed extracts at 4 °C for at least 72 h. This assay proved to be accurate, precise, fast and was used to support long-term toxicology studies in dog and rat.