Valproic acid and several metabolites: quantitative determination in serum, urine, breast milk and tissues by gas chromatography-mass spectrometry using selected ion monitoring.

A method has been developed for the simultaneous quantitative determination of valproic acid (2-propylpentanoic acid) and its metabolites 2-propyl-2-pentenoic acid (trans), 2-propyl-3-pentenoic acid (trans), 2-propyl-4-pentenoic acid, 3-hydroxy-2-propylpentanoic acid, 4-hydroxy-2-propylpentanoic acid, 5-hydroxy-2-propylpentanoic acid, 3-oxo-2-propyl-pentanoic acid, and and 2-propylglutaric acid. All compounds were extracted at pH 5.0 with ethyl acetate. The concentrated extracts were trimethylsilylated and the resulting mixtures analyzed by a gas chromatography-mass spectrometry-computer system operated in the selected ion monitoring mode. Linear calibration curves were obtained in the concentration ranges studied (0.1-20 microgram/ml for metabolites, 0.1-150 microgram/ml for valproic acid. The recoveries of the drugs were between 92 and 97%. The relative standard deviations were between 3.9 and 8.1% (analysis of multiple 10-microliter samples of patient urine). The lower detection limits were found to be between 2.8 and 18 ng/ml using 200-microliter serum samples. The derivatized extracts were stable for at least one week. Applications of the method described include studies of placental transfer for valproic acid and metabolites in the human, the elimination of these substances by the neonate, their transfer via mother's milk, and their levels in mouse brain.