Simultaneous stereoselective analysis of tramadol and its primary phase I metabolites in plasma by liquid chromatography: Application to a pharmacokinetic study in humans

This paper describes a bioanalytical method involving a simple liquid–liquid extraction for the simultaneous HPLC determination of the enantiomers of tramadol, the active metabolite O-desmethyltramadol (M1), and the other main metabolite N-desmethyltramadol (M2) in biological samples. Chromatography was performed at 5 °C on a Chiracel OD-R column containing cellulose tris(3,5-dimethylphenylcarbamate) as chiral selector, preceded by a achiral end-capped C8 column (LiChrospher 60-RP-selected B 5 μm, 250 mm×4 mm). The mobile phase was a mixture of phosphate buffer containing sodium perchlorate (1 M) adjusted to pH 2.5–acetonitrile–N,N-dimethyloctylamine (74.8:25:0.2). The flow rate was 0.5 ml/min. Fluorescence detection (λex 200 nm/λem 301 nm) was used. Fluconazol was selected as internal standard. The limit of quantitation of each enantiomer of tramadol and their metabolites was 0.5 ng/ml (sample size = 0.5 ml). The chiral conditions and the LC optimisation were investigated in order to select the most appropriate operating conditions. The method developed has also been validated. Mean recoveries above of 95% for each enantiomer were obtained. Calibration curves for tramadol enantiomers (range 1–500 ng/ml), M1 enantiomers (range 0.5–100 ng/ml), and M2 enantiomers (range 0.5–250 ng/ml) were linear with coefficients of correlation better than 0.996. Within-day variation determined on four different concentrations showed acceptable values. The relative standard deviation (R.S.D.) was determined to be less than 10%. This method was successfully used to investigate plasma concentration of enantiomers of tramadol, O-desmethyltramadol and N-desmethyltramadol in a pharmacokinetic study.