Simultaneous Quantitation of Tetrahydropalmatine and Protopine in Rabbit Plasma by HPLC–PAD, and Application to Pharmacokinetic Studies

A reversed-phase high-performance liquid chromatographic method coupled with photodiode-array detection (HPLC–PAD) has been developed and validated for simultaneous quantitation of dl-tetrahydropalmatine (dl-THP) and protopine (PTP), two active constituents of the herbal medicine Rhizoma Corydalis, in rabbit plasma. The pharmacokinetics of dl-THP and PTP were researched in rabbits after oral administration of extracts of Rhizoma Corydalis. Optimum separation was achieved on a reversed-phase column with methanol–phosphate buffer (pH 7.75, 65:35, v/v) as mobile phase. The experimental results showed the intra-day and inter-day precision and accuracy of the method were satisfactory for simultaneous determination of dl-THP and PTP at low, middle, and high concentrations. The lower limits of quantitation were 2.05 ng mL^?1 for dl-THP and 2.10 ng mL^?1 for PTP. Plasma concentration–time data for dl-THP were best fitted by the two-compartment linear pharmacokinetic model whereas data for PTP were best fitted by the one-compartment model.