Simultaneous determination of the enantiomers of tocainide in blood plasma using gas-liquid chromatography with electron-capture detection.

Tocainide is a new experimental antiarrhythmic agent used clinically as the racemic mixture of two enantiomers. Since the optical isomers may differ in their efficacy and toxicity, we have initiated studies on the stereoselective disposition of tocainide. For this purpose, an assay was developed for the simultaneous determination of the enantiomers of tocainide in blood plasma. Alkalinized 1-ml plasma samples containing tocainide and an internal standard, 2-amino-2',6'-acetoxylidide, are extracted with ethyl acetate. The organic extract is treated with the chiral reagent (S)-alpha-methoxy-alpha-trifluoromethylphenylacetyl chloride, and the resulting derivatives are resolved and quantified by gas-liquid chromatography with electron-capture detection. Calibration data were fitted by least-squares power curves of the form: drug enantiomer/internal standard peak area ratio = A X CB where A and B were constants and C was the concentration of tocainide enantiomer. The lower limit of sensitivity of the assay was 10 ng/ml of each enantiomer. Intra-assay coefficients of variation were 3.3 and 2.1% for (R)-(-)-tocainide at concentrations of 0.125 and 1.25 micrograms/ml, respectively, and 3.4 and 2.4% for the (S)-(+) enantiomer at the same concentrations. Diazepam may interfere with the determination of (R)-(-)-tocainide if concentrations smaller than 1 microgram/ml of this enantiomer are measured in the presence of higher-than-therapeutic (greater than 1.5 micrograms/ml) concentrations of diazepam.