Biotransformation and pharmacokinetic studies of four alkaloids from Uncaria rhynchophylla in rat plasma by ultra-performance liquid chromatography with tandem mass spectrometry

Uncaria rhynchophylla (Miq.) Miq. ex Havil (U. rhynchophylla), a traditional Chinese medicine that has been officially included in the Chinese Pharmacopeia, is used to treat cardiovascular and central nervous system diseases. The major alkaloids isolated from U. rhynchophylla are two pairs of epimer including rhynchophylline (RIN) and isorhynchophylline (IRN), along with corynoxeine (CN) and isocorynoxeine (ICN). An ultra-performance liquid chromatography with tandem mass spectrometry method (UHPLC-MS/MS), which was highly accurate, stable and sensitive, was established and validated for the simultaneous determination of four alkaloid compounds (RIN, IRN, CN, ICN) in rat plasma samples after oral administration of RIN, IRN, CN, ICN and U. rhynchophylla extract. In this study, the biotransformation and pharmacokinetics of RIN, IRN, CN and ICN were determined for the first time. An ACQUITY UPLC®HSS T3 column (1.8 µm, 2.1 mm × 100 mm) was used to complete the chromatographic separation within 3 min. The isocratic mobile phase was composed of 0.1 % formic acid water (v/v) and acetonitrile, and the flow rate was 0.2 mL/min. The multireaction monitoring mode was adopted, and the tandem mass spectrometry in the positive ion mode was detected by the electrospray ionization source. The method was fully verified and linear at a wide concentration (r > 0.9913), and the linear concentration range was 0.1552-124.5 ng/mL. The intraday and interday precisions of the four analytes were lower than 8.20 % and 13.42 %, respectively. The accuracy range was - 2.64 % and 13.63 %. The extraction recoveries of the analytes exceeded 83.74 %, and the matrix effect range was 98.43-103.7 % in the plasma samples. The four alkaloids can be quickly absorbed into the blood (Tmax, 0.22-3.83 h) and cleared comparatively slowly (T1/2, 7.67-12.13 h). This method has been successfully applied to the biotransformation and pharmacokinetic studies of SD rat plasma after oral administration of U. rhynchophylla extracts. This proved that RIN with IRN and CN with ICN can transform into each other in vivo. The results are of great significance for determining the mechanism of action and guiding the clinical application of U. rhynchophylla extracts.