LC–MS/MS determination and comparative pharmacokinetics of strychnine, brucine and their metabolites in rat plasma after intragastric administration of each monomer and the total alkaloids from Semen Strychni

• We developed a novel LC–MS/MS method for determination of strychnine, brucine, strychnine N-oxide and brucine N-oxide in plasma.
• The method was validated to be rapid, specific, accurate and sensitive.
• The pharmacokinetics of strychnine, brucine, strychnine N-oxide and brucine N-oxide in the total alkaloids from Semen Strychni was investigated in rats.
• Other strychnos alkaloids can affect the pharmacokinetics of strychnine and brucine in the total alkaloids from Semen Strychni.

A rapid, specific and sensitive liquid chromatography–tandem mass spectrometric (LC–MS/MS) method was developed and validated for the simultaneous determination of strychnine, brucine, strychnine N-oxide and brucine N-oxide in rat plasma. Plasma samples were pretreated via simple protein precipitation with methanol and ephedrine hydrochloride was used as internal standard. Chromatographic separation was carried out on an ZORBAX Eclipse XDB-C18 column (2.1 × 150 mm, 3.5 μm) by gradient elution with methanol and 10 mM ammonium acetate (adjusted to pH 4.0 with formic acid). The quantification of the analytes was performed by mass spectrometry with TurboIonSpray ionization (ESI) inlet in the positive ion multiple reaction monitoring (MRM) mode. The results showed that the calibration curve was linear in the concentration range of 0.510∼306.3 ng mL−1 for strychnine, brucine and 0.102∼306.0 ng mL−1 for strychnine N-oxide and brucine N-oxide, respectively. The intra- and inter-day precisions were less than 14.9%, and the accuracy ranged from 89.4 to 113% at three QC levels for the 4 analytes. The validated method was successfully applied to the pharmacokinetic study of strychnine, brucine, strychnine N-oxide and brucine N-oxide in rat plasma after oral administration of each monomer and the total alkaloids from Semen Strychni. After single oral administration of the total alkaloids from Semen Strychni at 4 dose levels, Cmax, AUC0−t of strychnine and brucine increased and were proportional to the oral doses. In comparative pharmacokinetics studies, no significant difference was found between each monomer and the total strychnos alkaloids on the pharmacokinetic parameters such as Cmax and AUC. Mean Cmax and AUC of strychnine and brucine were slight increased in the monomer groups in comparison to the total strychnos alkaloids groups, which suggested that some other alkaloids in the Semen Strychni may decrease the absorption of strychnine and brucine in body.