Application of a stereospecific high-performance liquid chromatography assay to a pharmacokinetic study of etodolac enantiomers in humans.

An HPLC assay suitable for pharmacokinetic analysis of enantiomers of etodolac [(+/-)-1,8-diethyl-1,3,4,9-tetrahydropyrano[3,4-b] indole-1-acetic acid] was developed. Following addition of internal standard (IS), (+/-)-2-(4-benzoylphenyl)butyric acid, the constituents were extracted from the specimen into a mixture of isooctane:isopropanol (95:5). The organic layer was evaporated and the drug and IS were sequentially derivatized with ethyl chloroformate and iota(-)-alpha-phenylethylamine. The diastereoisomers thus formed were extracted and chromatographed on a normal-phase column, with a mobile phase consisting of hexane:ethyl acetate:isopropanol (85:15:0.2) at a flow rate of 2 mL/min. The etodolac diastereoisomers were separated with a resolution factor of 6.4 and detected at a wavelength of 280 nm. Excellent linear relationships were found between the peak area ratios (etodolac:IS) and the plasma and urine concentrations (0.2-20 mg/L), with intra- and interday variations of less than 10.1%. The assay was applied to a preliminary pharmacokinetic study following seven repeated oral administrations of 200 mg/12 h of racemic etodolac to two healthy subjects. The plasma concentrations of the active S-(+)-enantiomer were considerably less than those of the inactive antipode (AUC S:R, 2.5:30.9 mg.L-1.h-1) due to a greater volume of distribution of the latter (S, 101 and 135 L versus R, 24 and 17 L). Considerable concentrations of conjugated enantiomers were also found in plasma (AUC conjugated: intact: S, 1.1; R, 0.23).