Specific determination of quinidine and (3S)-3-hydroxyquinidine in human serum by high-pressure liquid chromatography.

A sensitive and selective HPLC assay was developed to measure quinidine and (3S)-3-hydroxyquinidine in the serum of patients receiving quinidine. The method involves analysis of a benzene extract of alkalinized serum by a high-pressure liquid chromatograph containing a reverse phase column and a fluorescence detector. The lower limit of sensitivity of this method for these compounds is 5 ng/ml. The coefficients of variation of the assay for quinidine and (3S)-3-hydroxyquinidine are 2% and 6%, respectively. Samples of serum from 10 cardiac patients were found to contain (3S)-3-hydroxyquinidine and quinidine in ratios ranging from 0.18 to 1.0 (mean, 0.38 +/- 0.28 [S.D.]). 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.