Simultaneous measurement of quinine and quinidine in human plasma, whole blood, and erythrocytes by high-performance liquid chromatography with fluorescence detection.

A high-performance liquid chromatographic method with fluorescence detection is described for the simultaneous measurement of quinine and quinidine in plasma, whole blood, and erythrocytes. The compounds were separated on an Ultrasphere C18 reversed-phase column (25 cm x 4.6 mm inside diameter, 5 microns particle size) using a mobile phase of acetonitrile/water/triethylamine (11:88:1, vol/vol) at pH 2.5. The method, simple, accurate, and selective, requires only a single-step liquid-liquid extraction and uses the structurally similar alkaloid, cinchonine, as the internal standard. The commercial impurities, dihydroquinine and dihydroquinidine, and unknown metabolites were well resolved from the parent drugs. The assay is precise, with interassay coefficients of variation less than or equal to 7.0% and an accuracy of less than or equal to 7.3% over a concentration range of 0.125 to 4.0 micrograms/0.25 ml. The extraction recoveries of the two drugs were similar, averaging 82.9% for quinine and 79.3% for quinidine from the three biological fluids. The clinical application of the method for routine drug monitoring and for estimating the pharmacokinetics of quinine and quinidine in man are discussed.