A rapid and specific assay for the determination of lamotrigine in human plasma by normal-phase HPLC

A rapid and specific high-performance liquid chromatography assay of lamotrigine in human plasma is described. Lamotrigine is extracted with dichloromethane from buffered plasma to which an internal standard has been added. The solvent is directly injected into a 250 x 4.6-mm Spherisorb Silica column and the drug is eluted by using a mixture of methanol, n-heptane, dichloromethane, and 28-30% ammonium hydroxide (20:40:40:0.3 vol/vol) at a flow rate of 1 ml/min. The eluates are detected at 240 nm. The assay requires 250 microliters of sample, and concentrations as low as 0.4 microgram/ml can be measured accurately. The method is linear in the range of 0.4-16 micrograms/ml, with a mean coefficient of correlation (r) > or = 0.997. Within- and between-day relative standard deviations at three different concentration levels (1, 4, and 8 micrograms/ml) are < or = 8.6%.