A rapid cost-effective high-performance liquid chromatographic (HPLC) assay of serum lamotrigine after liquid-liquid extraction and using HPLC conditions routinely used for analysis of barbiturates

Lamotrigine (lamictal) is a new anticonvulsant drug approved by the FDA for clinical use. Therapeutic monitoring of lamotrigine is useful for patient management and avoidance of toxicity. The suggested therapeutic range is 1 to 4 mu g/ml. The authors describe a simple high-performance liquid chromatographic (HPLC) method for analysis of lamotrigine from serum. Serum (0.5 ml) was alkalinized with berate buffer (pH 9.8). Lamotrigine and the internal standard thiopental were extracted with 10 ml of chloroform. After evaporation of the extract, the residue was reconstituted in the mobile phase (prepared by mixing 750 ml of potassium dihydrogen phosphate, 550 ml of deionized water, 430 ml of methanal, and 100 mu l of triethylamine as an ion pairing reagent) and injected into an LC-18 column (15 cm x 4.6 mm). The authors use this HPLC system routinely in their laboratory for the analysis of barbiturates. They demonstrated that the same system can be used for the analysis of lamotrigine. The within-run and between-run precisions of the lamotrigine assay were 1.63% (X = 3.05, SD = 0.05 mu g/ml, n = 6) and 3.7% (X = 2.97 mu g/ml, SD = 0.11, n = 8). The assay was linear for serum lamotrigine concentrations of 0.5 mu g/ml to 20 mu g/ml with a detection limit of 0.5 mu g/ml. The authors observed excellent correlation between serum lamotrigine concentrations measured by their assay and a reference laboratory in six patients receiving lamotrigine. Their assay is free from interferences from common tricyclic antidepressants, benzodiazepines, other common anticonvulsants, salicylate, and acetaminophen.