High-performance liquid chromatographic determination of the enantiomers of flecainide in human plasma and urine

A stereospecific high-performance liquid chromatographic method for the determination of (R,S)-flecainide acetate [(R,S)-N-(2-piperidylmethyl)-2,5-bis-(2,2,2-trifluoroethoxy)benzam ide acetate] in human plasma and urine is described. After addition of the internal standard [IS; (R,S)-N-(2-piperidylmethyl)-2,3-bis(2,2,2-trifluoroethoxy)- benzamide hydrochloride], a single-step extraction of alkalinized samples was performed with distilled diethyl ether. The organic layer was evaporated and the drug and IS were derivatized with 1-[(4-nitrophenyl)sulfonyl]-L-propyl chloride at 80 degrees C for 2 h. The diastereomeric derivatives of flecainide and IS were chromatographed on a C18 reversed-phase column with a mobile phase consisting of acetonitrile: water:triethylamine (45:55:0.2) at a flow rate of 1 mL/min. Flecainide diastereomers were separated with a resolution factor of 1.25 and detected by UV spectroscopy at a wavelength of 280 nm. An excellent linearity was observed between the peak area ratios (flecainide derivatives:IS) and plasma concentrations, and the intra- and interday coefficients of variation were always less than 9.8%. The lowest quantifiable concentration was set at 50 ng/mL for each enantiomer (CV of 4.9 and 4.4%), while the lowest limit of detection (signal:noise, 3:1) was on the order of a few nanograms. The assay was used to study the pharmacokinetics of flecainide enantiomers in a patient receiving (R,S)-flecainide therapy. The steady-state plasma time courses for the enantiomers were found to be parallel, but the difference between (R)- and (S)-flecainide concentrations was significant. The urinary excretion data were consistent with the plasma results. The method is suitable for therapeutic monitoring of flecainide enantiomers and for stereoselective pharmacokinetic studies in humans.