Simultaneous determination of thioridazine and its S-oxidized and N-demethylated metabolites using high-performance liquid-chromatography on radially compressed silica.

A method for simultaneously quantifying thioridazine, northioridazine, thioridazine-2-sulfoxide, thioridazine-2-sulfone and thioridazine-5-oxide in serum and plasma is described. Following solvent extraction these compounds were separated by high-performance liquid chromatography on radially compressed silica gel and detected by UV absorbance at 254 nm. Chromatography time is less than 7 min. The relative retention of these compounds as a function of the methanol and methylamine content of the mobile phase is discussed. Practical limits of detection, based upon on assayed plasma or serum volume of 1 ml, were 20 ng/ml for thioridazine-5-oxide and 10 ng/ml for the other compounds. The coefficient of variation for all compounds was less than 13%. The method is compared with more conventional high-performance liqiud chromatographic and gas chromatographic methodology.