An LC–MS/MS method for determination of bioactive components of licorice and Semen Strychni in rat plasma: application to a pharmacokinetics study

Semen Strychni is known for its treatment of rheumatic arthritis with a low therapeutic index. Licorice contributes a lot in herb detoxification according to the traditional Chinese medicine theory. A simple, rapid and sensitive liquid chromatography–mass spectrometric method was developed and validated for simultaneous determination of main bioactive ingredients in licorice and Semen Strychni in rat plasma. Using moclobemide and cyproterone acetate as the internal standards, the analytes were pretreated via protein precipitation with methanol. An Ultimate AQ-C18 column (3.0 μm, 3.0×100 mm) was employed for chromatographic separation, combining with gradient elution. The mobile phase consisted of 0.07% formic acid and 0.12% ammonium acetate in aqueous phase (A) and acetonitrile in organic phase (B). The elution program was as follows: 0–0.5 min, 20% B; 0.5–1 min, 20–60% B; 1–7 min, 60–85% B; and 7–7.5 min, returned to 20% B, then continued to 12 min. Selected reaction monitoring was performed in both positive and negative ESI. Positive mode was adopted for detection of strychnine, brucine and moclobemide, while negative mode was used for glycyrrhizic acid, glycyrrhetinic acid, liquiritigenin, isoliquiritigenin, liquiritin and cyproterone acetate. The method was validated for specificity, linearity, matrix effect, recovery, precision, accuracy and stability. The results show that this method is sensitive, accurate and robust for biological matrix analysis. Moreover, the proposed method was applied to a pharmacokinetic study in Sprague-Dawley rats for investigating the mechanism of which liquorice detoxifies Semen Strychni.