ATV, ortho-hydroxy-atorvastatin (o- ATV), and para-hydroxy-atorvastatin (p- ATV) in plasma and CSF were extracted with ethyl acetate. Cell samples were processed with acetonitrile for protein precipitation before LC-MS/MS. Then, the Waters ACQUITY UPLC I-Class system (Waters, USA) equipped with an Acquity UPLC BEH C18 column (1.7 μm, 100 × 2.1 mm, Waters, USA) was coupled online to a Waters Xevo TQD IVD triple quadrupole mass spectrometer (Waters, Ireland) with electrospray ionization and multiple reaction monitoring in positive ion mode. The mobile phase consisted of solvent A (acetonitrile with 0.01% formic acid) and solvent B (0.01% aqueous formic acid) with a gradient elution of 0.0 min, 80%B; 1.0 min, 60%B; 4 min, 10%B; 4.5 min, 90%B; 4.51 min, 80%B; and 5 min, 80%B. The flow rate and the injection volume were 0.4 mL/min and 10 μL, respectively. Data acquisition was performed by Masslynx (Version 4 1, Waters, USA).