Enantioselective analysis of N-hydroxymexiletine glucuronide in human plasma for pharmacokinetic studies

Enzymatic hydrolysis with beta-glucuronidase/sulfatase was used for the enantioselective determination of N-hydroxymexiletine glucuronide in plasma for pharmacokinetic studies. N-Hydroxymexiletine glucuronide was determined as the quantity of mexiletine released by hydrolysis (difference between the enantiomeric concentrations of mexiletine obtained with and without hydrolysis). Plasma samples (100 microliters) were treated at pH 5.0 with 10 mg of the enzyme (Limpet Acetone Powder type I) for 16 hr at 37 degrees C and extracted at pH 10.4 with diisopropyl ether. Chiral mexiletine discrimination was obtained by reaction with o-phthalaldehyde/N-acetyl-L-cysteine, separation of the resulting diastereomers on a C-18 reversed-phase column with a mobile phase of methanol-0.05 N acetate buffer, pH 5.5 (6.5:3.5, v/v), and fluorescence detection (lambda ex 350 nm, lambda em 455 nm). The performance characteristics for the enantioselective analysis of mexiletine preceded by enzymatic hydrolysis were recovery approximately 90%, quantification limit 1 ng/ml, and linearity up to 1000 ng/ml plasma for both enantiomers. The coefficients of variation obtained in the study of intra- and inter-day precision were respectively 5% and 7% for both enantiomers. The assay was shown to be suitable for a pharmacokinetic study performed in a patient with the arrhythmic form of chronic Chagas' heart disease treated with 200 mg t.i.d. of racemic mexiletine hydrochloride. The high sensitivity of the method allows analysis of only 100 microliters plasma.