Determination of Quinidine and its major metabolites by high-performance liquid chromatography

Abstract

A specific and precise assay, capable of quantitating in human plasma simultaneously but separately quinidine, dihydroquinidine and the quinidine metabolites 2′-quinidinone, 3-OH-quinidine and a third metabolite found - tentatively identified as the product formed by rearrangement of quinidine-N-oxide - is reported. The assay uses a normal phase high-performance liquid chromatographic (HPLC) system with a variable-wavelength UV detector at 235 nm and has a limit of sensitivity at approximately 20 ng/ml. The mobile phase consists of hexanes-ethanol-ethanolamine (91.5:8.47:0.03). A 2-ml plasma sample is worked up by adding primaquine base as an internal standard and extracting with ether-dichloromethane-isopropanol (6:4:1). The organic extract is evaporated and the residue reconstituted in 100-600 μl of mobile phase and an aliquot injected onto the column. Comparison of this procedure with the Edgar and Sokolow (dichloroethane) extraction-fluorescence procedure and with the Cramer and Isaksson (benzene) double extraction-fluorescence assay indicates that both fluorescence procedures give quinidine concentrations up to 2.3 times those determined by HPLC. These discrepancies were shown to be due to carry-over of metabolites and some extraneous background fluorescence. © 1979.