Quantitative metabolic profiling of valproic acid in humans using automated gas chromatographic/mass spectrometric techniques

An automated gas chromatographic/mass spectrometric assay is described for the antiepileptic drug valproic acid (VPA) and 14 of its metabolites in plasma or urine. Quantitative analysis of the parent drug and its biotransformation products was carried out with the aid of trimethylsilyl derivatives, and was performed by selected ion monitoring gas chromatography/mass spectrometry (normally of [M – CH3]+ species) using an HP 5790 mass selective detector (MSD) quadrupole mass spectrometer. The analysis was fully automated, in that simple injection, data acquisition, integration, quantification and report functions were carried out during unattended operation by an HP 59970C ChemStation™ computer system. The method exhibits good accuracy and high precision, with correlation coefficients greater than 0.990 for all standard curves. Replicate analyses of pooled plasma samples over a 4 month period exhibited an inter‐day variation of less than 15% for the parent drug and ten of its metabolites. Moreover, the high dynamic range of the MSD instrument permitted quantification of VPA and minor metabolites thereof (e.g. the hepatotoxic terminal olefin, Δ4‐VPA) at levels as disparate as 260 μg ml−1 (VPA) and 14 ng ml−1 (Δ4‐VPA) in a single analysis. The high stability and sensitivity of the assay, combined with the fully automated features of the instrumentation, make the method ideally suited to expanded clinical studies and for the routine monitoring of potentially high‐risk patients on VPA therapy.