Sensitive quantification of roflumilast and roflumilast N-oxide in human plasma by LC–MS/MS employing parallel chromatography and electrospray ionisation

A high throughput bioanalytical method based on semi-automated liquid extraction and liquid chromatography–tandem mass spectrometry (LC–MS/MS) has been developed for the sensitive quantification of roflumilast and its metabolite roflumilast N-oxide, a phosphodiesterase (PDE) inhibitor in human plasma and serum. The sample work-up procedure comprised liquid extraction using penta-deuterated analogues of both analytes as internal standards. Chromatography was performed on C18 revered phase analytical columns at a flow rate of 0.5 mL/min in the dual column mode employing a column switching technique and a linear gradient from 18% to 54% acetonitrile in 0.005 M aqueous ammonium acetate containing 0.006% formic acid. Mass spectrometry was performed on an API 4000 instrument in the positive ion SRM-mode (selected reaction monitoring) with the Turbo-V® ionspray interface. The method showed linear detector responses over the entire calibration range between 0.1 ng/mL (lower limit of quantification (LLOQ)) and 50 ng/mL (upper limit of quantification (ULOQ)) for both analytes. Linear regression analysis with concentration-squared weighting (1/x2 for roflumilast and 1/x for roflumilast N-oxide) yielded inaccuracy and precision values <15% and coefficients of correlation (r) for the calibration curves >0.99 for both analytes.