Determination of lamotrigine, carbamazepine and carbamazepine epoxide in human serum by gas chromatography mass spectrometry.

A method for the identification and quantification of the antiepileptics lamotrigine, carbamazepine and carbamazepine epoxide (active metabolite) in human serum is described. In refractory epilepsy the combination of carbamazepine and lamotrigine was recently developed to a modern therapeutic concept. The goal of this paper is therapeutic drug monitoring (TDM) of these substances. Serum was extracted with a quick precipitation method using a modified commercial extraction-kit and analysed by gas chromatography mass spectrometry (GC-MS). A gas-chromatographic temperature-pressure programme was developed that allowed the determination of lamotrigine by gas chromatography mass spectrometry. A reference spectrum of pure lamotrigine is herewith published for the first time. A new mass spectra library was created to support the identification of the antiepileptics in human serum. In the Specified Ions Monitoring mode (SIM) a detection limit below the therapeutic range and recoveries above 90% were obtained. Comparison of results obtained by GC-MS or a commercially available high performance liquid chromatographic (HPLC) method (for lamotrigine) and a fluorescence polarisation immunoassay (FPIA) (for carbamazepine) from spiked serum samples showed disagreement of no more than 10% between the methods and demonstrated the accuracy of the new method. In addition, quantitative determinations of these antiepileptics in samples from patients under anticonvulsive therapy showed a strong linear correlation with r2 = 0.961 for carbamazepine and r2 = 0.964 for lamotrigine. Only two from a total of 46 results differed by more than 10%. Our method for quantifying lamotrigine in serum seems to be highly specific and capable of measuring lamotrigine in patients on single therapy, as well as on add-on therapy with carbamazepine or carbamazepine epoxide. No interference from other coadministered drugs was detected.