Simultaneous determination of asperosaponin VI and its active metabolite hederagenin in rat plasma by liquid chromatography–tandem mass spectrometry with positive/negative ion-switching electrospray ionization and its application in pharmacokinetic study

A new liquid chromatography–tandem mass spectrometry (LC–MS/MS) method operated in the positive/negative electrospray ionization (ESI) switching mode has been developed and validated for the simultaneous determination of asperosaponin VI and its active metabolite hederagenin in rat plasma. After addition of internal standards diazepam (for asperosaponin VI) and glycyrrhetic acid (for hederagenin), the plasma sample was deproteinized with acetonitrile, and separated on a reversed phase C18 column with a mobile phase of methanol (solvent A)–0.05% glacial acetic acid containing 10 mM ammonium acetate and 30 μM sodium acetate (solvent B) using gradient elution. The detection of target compounds was done in multiple reaction monitoring (MRM) mode using a tandem mass spectrometry equipped with positive/negative ion-switching ESI source. At the first segment, the MRM detection was operated in the positive ESI mode using the transitions of m/z 951.5 ([M+Na]+) → 347.1 for asperosaponin VI and m/z 285.1 ([M+H]+) → 193.1 for diazepam for 4 min, then switched to the negative ESI mode using the transitions of m/z 471.3 ([M−H]−) → 471.3 for hederagenin and m/z 469.4 ([M−H]−) → 425.4 for glycyrrhetic acid, respectively. The sodiated molecular ion [M+Na]+ at m/z 951.5 was selected as the precursor ion for asperosaponin VI, since it provided better sensitivity compared to the deprotonated and protonated molecular ions. Sodium acetate was added to the mobile phase to make sure that abundant amount of the sodiated molecular ion of asperosaponin VI could be produced, and more stable and intensive mass response of the product ion could be obtained. For the detection of hederagenin, since all of the mass responses of the fragment ions were very weak, the deprotonated molecular ion [M−H]− m/z 471.3 was employed as both the precursor ion and the product ion. But the collision energy was still used for the MRM, in order to eliminate the influences induced by the interference substances from the rat plasma. The validated method was successfully applied to study the pharmacokinetics of asperosaponin VI and its active metabolite hederagenin in rat plasma after oral administration of asperosaponin VI at a dose of 90 mg/kg.