Determination of the R(-) and S(+)-enantiomers of vigabatrin in human plasma by ultra-high-performance liquid chromatography and tandem mass-spectrometry.

An analytical method was developed for the quantification in plasma of the R and S enantiomers of vigabatrin (VGB), a drug used for the treatment of some refractory pediatric epileptic syndromes. After adding 50µL of the internal standard, which consisted of a 15mg/L solution of deuterated racemic VGB, and 100µL of water to 100µL of plasma samples, a protein precipitation was performed by adding 600µL of methanol. The supernatant was evaporated to dryness under a stream of nitrogen and the dry residue was reconstituted with 500µL of water. Then, 100µL of 0.01M o-phthaldialdehyde and 0.01M N-acetyl-l-cysteine in borate buffer (0.1M, pH=9.5) were added for pre-column derivatization of the enantiomers as diastereomeric isoindoles. One microliter of the resulting mixture was injected in the chromatographic system. The chromatographic separation was performed in gradient elution mode at a flow rate of 400µL/min using a phenomenex EVO C-18 column with a mobile phase composed of 5mM ammonium acetate and a methanol:acetonitrile (63:37v/v) mixture. Detection was performed by mass spectrometry in selected reaction monitoring mode using heated electrospray ionization in positive mode as the ion source. Intra- and inter-day precision and accuracy were lower than 15% over the calibration range (0.2-50mg/L for each enantiomer) and the method was successfully used to assess plasma concentrations of VGB in epileptic children.