SPECIFIC DETERMINATION OF QUINIDINE AND (3S)-3-HYDROXYQUINIDINE IN HUMAN-SERUM BY HIGH-PRESSURE LIQUID-CHROMATOGRAPHY

Abstract

A sensitive and selective HPLC [high-pressure liquid chromatography] assay was developed to measure quinidine and (3S)-3-hydroxyquinidine in serum of patients receiving quinidine. The method involves analysis of a benzene extract of alkalinized serum by a HPLC containing a reverse phase column and a fluorescence detector. The lower limit of sensitivity of this method for these compounds is 5 ng/ml. Coefficients of variation of the assay for quinidine and (3S)-3-hydroxyquinidine are 2% and 6%, respectively. Samples of serum from 10 cardiac patients apparently contained (3S)-3-hydroxyquinidine and quinidine in ratios ranging 0.18-1.0 (mean, 0.38 .+-. 0.28 [SD]). Quinidine serum levels in these patients determined by HPLC averaged 80% of those determined on the same samples by the routinely used fluorometric assay. The reason for the discrepancy is that this latter assay, involving a double extraction with quantitation in sulfuric acid by fluorometry, is nonspecific for quinidine. The (3S)-3-hydroxy metabolite having a fluorescence intensity similar to that of quinidine is 60% extracted under the fluorometric assay conditions.