High-performance liquid chromatography quantitation of plasma lamotrigine concentrations: application measuring trough concentrations in patients with epilepsy.

Lamotrigine is a phenyltriazine anticonvulsant recently approved for clinical use. A high-performance liquid chromatographic (HPLC) method was developed using a silica column (5 microm) with an aqueous methanol mobile phase consisting of 94% methanol, 5.92% water, and 0.08% NH4H2PO4 adjusted to a final apparent pH of 4.0 and pumped at a flow rate of 1.0 ml/minute. Ultraviolet detection was carried out at a wavelength of 280 nm, and plasma samples were prepared for HPLC analysis by extraction into ethyl acetate after basification. Retention times for lamotrigine and its internal standard (BWA725C) were 10.3 and 11.2 minutes, respectively, and there was no chromatographic interference from other commonly coadministered anticonvulsants. Calibration curves were linear over a concentration range of 0.5 to 30 mg/l, with intra-assay and interassay coefficients of variation less than 8%. Assessment of assay performance in an international quality assurance program showed an average bias of 0.3% compared with the consensus mean. A review of 52 patient specimens showed that, if patients were grouped according to coadministered anticonvulsants, a significant correlation between lamotrigine dosage and concentration was evident in those coadministered valproate (in the absence of metabolic inducers) and in those coadministered a combination of valproate and inducers, but not in patients coadministered inducers alone. Mean (SD) trough concentrations were 9.2 (5.2), 2.8 (1.3), and 3.8 (2.8) mg/l in the valproate, inducer, and combination groups, respectively.