Determination of doxazosin enantiomers in rat plasma and investigation of their chiral inversion

The study is to establish an HPLC method using fluorescence detector for the determination of doxazosin enantiomers and investigate their chiral inversion in vitro and in vivo.  Ultron ES-OVM was taken as the chiral chromatographic column, and the column temperature was 30 ℃.  Isocratic elution using a mobile phase of phosphate buffer−acetonitrile (85∶15, v/v) at a flow rate of 0.8 mL·min⁻¹ was done.  The fluorescence detection was set at λ_Ex = 255 nm and λ_Em = 385 nm.  Prazosin was used as the internal standard.  (−) Doxazosin or (+) doxazosin added into rat plasma in vitro was determined after incubating in 37 ℃ water bath for 2, 5 and 10 days.  (−) Doxazosin or (+) doxazosin was administered orally to the rats for one months.  Plasma samples were taken at 8 h after the last administration.  A good linear relationship was achieved when the concentration of doxazosin enantiomers was within the range of 4 − 2 000 ng·mL⁻¹.  The average recovery for (−) doxazosin was 99.5% with RSD 3.6%, and for (+) doxazosin was 99.3% with RSD 4.3%.  Chiral inversion was observed neither in vitro nor in vivo studies.  The method is selective, accurate and reproducible, which is suitable for the detection of doxazosin enantiomers in rat plasma.  The in vitro and in vivo studies indicate that chiral inversion occurs uneasily between (−) doxazosin and (+) doxazosin in the rat.