17 (link) Briefly, Oasis MAX μElution Plate (Waters) was used to pretreat 250 μL of plasma by solid phase extraction. The extract was analyzed by ultra‐high‐performance liquid chromatography coupled to tandem mass spectrometry using the Nexera X2 LC system coupled to LCMS−8040 Liquid Chromatograph Mass Spectrometer (Shimadzu) equipped with electrospray ionization. The 15N4‐CP‐I and CMPF‐d5 were used as internal standards for CP‐I and CMPF, respectively. CP‐I was measured with (M + H)+ signal in positive ion mode, and CMPF was measured with (M – H)− signal in negative ion mode. The tandem mass spectrometry transitions monitored were mass‐to‐charge ratio (m/z) 655.4 → m/z 596.3 for CP‐I, m/z 659.3 → m/z 600.3 for 15N4‐CP‐I, m/z 239.0 → m/z 195.2 for CMPF, and m/z 244.2 → m/z 200.2 for CMPF‐d5. The assays were validated in accordance with the US Food and Drug Administration Guidance for Bioanalytical Method Validation.
20 The lower limit of quantification was 0.1 ng/mL for CP‐I and 50 ng/mL for CMPF. The within‐batch accuracy of the assay ranged from 92.1% to 110.2% for CP‐I, and from 99.1% to 109.3% for CMPF. The within‐batch precision was less than 7.6% for CP‐I and 3.4% for CMPF.