Validated LC–MS–MS method for determination of m-nisoldipine polymorphs in rat plasma and its application to pharmacokinetic studies

A sensitive and specific liquid chromatography–tandem mass spectrometric (LC–MS–MS) method has been developed to determine m-nisoldipine in rat plasma. Sample was pretreated by a single-step protein precipitation with acetonitrile, in contrast to the liquid–liquid procedure frequently used for the extraction of 1,4-dihydropyridines from biologic samples. Separation of analyte and internal standard (I.S.) was performed on a Symmetry RP-C18 analytic column (50 mm × 4.6 mm, 3.5 μm) with a mobile phase consisting of acetonitrile–water (80:20, v/v) at a flow rate of 0.5 ml/min. The API 4000 triple quadrupole mass spectrometer was operated in multiple reaction monitoring (MRM) scan mode using TurboIonSpray ionization (ESI) source. The method was sensitive with a lower limit of quantification (LLOQ) of 0.2 ng/mL, with good linearity (r ≥ 0.9982) over the linear range of 0.2–20 ng/mL. All the validation data, such as accuracy, precision, and inter-day repeatability, were within the required limits. The method was successfully applied to pharmacokinetic and relative bioavailability studies of m-nisoldipine polymorphs in rats.