Stereospecific determination of amisulpride, a new benzamide derivative, in human plasma and urine by automated solid-phase extraction and liquid chromatography on a chiral column. application to pharmacokinetics.

Amisulpride, a drug belonging to the benzamide series, demonstrates antischizophrenic and antidepressant (antidysthymic) properties in man. For the pharmacokinetic studies of the racemic drug in man, a method of determination based on solid-phase extraction (SPE) from plasma and HPLC on a stereoselective column was developed. For this aim, one millilitre of plasma, after the addition of the internal standard, tiapride or metoclopramide, is diluted with a borate buffer at pH 9, then automatically loaded onto a SPE C18 100-mg column. The column is washed with different solvents, then eluted with 0.5 ml of methanol. After evaporation of the eluted fraction, the residue is reconstituted in 0.25 ml of eluent mixture. An aliquot is injected onto the HPLC column, a Chiralpak AS, equilibrated with an eluent mixture constituted by n-hexane-ethanol, (67:33, v/v) containing 0.2% (v/v) of diethylamine (DEA) or n-heptane-ethanol, (70:29.8, v/v) containing 0.2% of DEA and connected to a UV detector set at 280 nm or to a fluorimetric detector set at lambda ex = 280 nm and lambda cm = 370 nm. The limit of quantitation (LOQ) in human plasma is 2.5 ng ml-1 for both S-(-)- and R-(+)-amisulpride isomers with both detection methods. The method has been demonstrated to be linear in the range 2.5-320 ng ml-1 for both R-(+)- and S-(-)-amisulpride in human plasma with both UV and fluorescence detection. Absolute recovery of S-(-)- and R-(+)-amisulpride enantiomers from human plasma, as well as selectivity, precision and accuracy have been demonstrated to be satisfactory for pharmacokinetics in man and equivalent for both the proposed methods that have been cross-validated on real dosed human plasma samples. The methods have been used for clinical pharmacokinetic studies allowing pharmacokinetic parameters for amisulpride enantiomers in agreement with those obtained for the racemate to be obtained. After dilution with water, urinary samples from subjects treated with amisulpride racemate can be analysed according to the method used for plasma.