Improvement of doxazosin determination in human plasma using high-performance liquid chromatography with fluorescence detection

A simple, sensitive, rapid, and reproducible high-performance liquid chromatographic method is developed and validated for the determination of doxazosin in human plasma without a solvent extraction procedure. This method involves plasma protein precipitation using methanol. The structurally related compound prazosin is used as an internal standard. Doxazosin is detected with high sensitivity using spectrofluorimetry. Over the concentration range 0.5-20 ng/mL, the absolute recovery values are all greater than 98%. The method has a quantitation limit of 0.5 ng/mL. The intra- and interday coefficient of variation and inaccuracy values are all less than 8% and 7%, respectively. Therefore, the method has been applied in pharmacokinetic studies of doxazosin.