Development and validation of a high-performance liquid chromatography tandem mass spectrometry assay for atorvastatin, ortho-hydroxy atorvastatin, and para-hydroxy atorvastatin in human, dog, and rat plasma

A liquid chromatographic/mass spectrometric method to quantitate atorvastatin (AT) and its active metabolites ortho-hydroxy (o-AT) and para-hydroxy (p-AT) atorvastatin in human, dog, and rat plasma was validated. The method consisted of washing plasma samples at high pH with diethyl ether and subsequently extracting the analytes and two internal standards, [d5]-atorvastatin ([d5]-AT) and [d5]-ortho-hydroxy atorvastatin ([d5]-o-AT), from acidified plasma by using diethyl ether. The ether layer was evaporated to dryness and the residue reconstituted in ammonium acetate (20 mM, pH 4.0)-acetonitrile-isopropanol (60:40:1, v/v/v). Chromatographic separation of analytes was achieved by using a YMC J'Sphere H80 (C-18) 150 × 2 mm, 4 µm particle size, column with a mobile phase consisting of acetonitrile-0.1% acetic acid, (70:30, v/v). Analytes were detected by using MS/MS. Sample introduction and ionization was by electrospray ionization in the positive ion mode. The method proved suitable for routine quantitation of AT, o-AT, and p-AT over the concentration range of 0.250 to 25.0 ng/mL. Approximate retention time ranges of p-AT, o-AT, [d5]-o-AT, AT, and [d5]-AT were 2.27 ± 0.21, 3.36 ± 0.23, 3.54 ± 0.46, 4.12 ± 0.61, and 4.65 ± 0.65 min, respectively. No peaks interfering with quantitation were observed throughout the validation processes. Mean recoveries of AT, o-AT, and p-AT from plasma ranged 100%–107%, 70.6%–104%, and 47.6%–85.6%, respectively. Mean recoveries of the [d5]-AT and [d5]-o-AT internal standards ranged 98.0%–99.9% and 97.3%–97.9%, respectively. Interassay precision, based on the percent relative deviation for replicate quality controls for AT, o-AT, and p-AT, was ≤7.19%, 8.28%, and 12.7%, respectively. Interassay accuracy for AT, o-AT, and p-AT was ±10.6%, 5.86%, and 15.8%, respectively. AT, o-AT, and p-AT in human, dog, and rat plasma quality controls were stable to three freeze-thaw cycles. AT, o-AT, and p-AT were stable frozen for 127, 30 and 270 days in human, dog, and rat plasma quality control samples, respectively. Human plasma quality control samples containing AT, o-AT, and p-AT were stable for at least 4 days at ambient room temperature and 37 °C. The lower limit of quantitation for all analytes was 0.250 ng/mL for a 1.0-mL sample aliquot.