Simultaneous liquid-chromatographic determination of five antiarrhythmic drugs and their major active metabolites in serum.

We report an isocratic "high-performance" liquid chromatographic method for the simultaneous measurement of tocainide, lidocaine, procainamide, quinidine, disopyramide, and their major active metabolites in serum. The drugs are extracted from 200 microL of serum at pH 9.5 with 1.5 mL of 1,2-dichloromethane, concentrated by evaporation, and separated on a CN-bonded-phase column at 40 degrees C (flow rate 2 mL/min) with a pH 7.1 mobile phase of acetonitrile/methanol/phosphate buffer (60/7/33, by vol); the phosphate buffer contains 10 mmol of KH2PO4 and 0.5 mmol of triethylamine per liter. The antiarrhythmic drugs elute in the K' (capacity factor) range of 1.43 (lidocaine) to 5.7 (disopyramide). Results vary linearly with drug concentration to at least 30 mg/L; the detection limit is 0.1-0.2 mg/L. Within-run precision (CV) ranges from 0.9% to 5.0% and day-to-day precision from 2.1% to 6.2%, depending on the specific drug and its concentration in serum. Extraction efficiencies vary from 76% to 85% and analytical recoveries from 98.5% to 103%. At toxic serum concentrations, several basic drugs may interfere with the assay of some antiarrhythmics, but only hydroxyzine, verapamil, and certain local anesthetics interfere at therapeutic concentrations.