Optimized high-performance liquid chromatographic method for determination of lamotrigine in serum with concomitant determination of phenytoin, carbamazepine, and carbamazepine epoxide.

Lamotrigine (LG), phenytoin (PY), carbamazepine (CM), and carbamazepine epoxide (CE) are measured with an optimized procedure that uses thin sorbent extraction disks and a highly selective, sterically protected bonded silica high-performance liquid chromatography (HPLC) column. Routinely, serum (200 microliters at pH 6.8 with cyheptamide as internal standard) is applied to an Empore octyl (C8) solid-phase extraction disk to isolate the drugs. a water wash removes interferences, and the retained drugs are eluted with a small volume of solvent. The eluate is directly injected onto a Zorbax Stable Bond cyanopropyl HPLC column with quantification at 214 nm. Evaporation-concentration steps are unnecessary. Overall, for all drugs, between-run precision coefficients of variation (n = 16 each) ranged from 2.1% to 4.9% at concentrations from 0.75 to 20.5 mg/l; extraction recoveries fell within a range of 96% to 110% at concentrations of 2, 10, and 30 mg/l tested for each drug; the lowest limit of detection was 0.15 to 0.35 mg/l. The analytical response was linear for each drug > 80 mg/l (LG) and > 50 mg/l (PY, CM, and CE). Optimization graphs are presented to illustrate the rationale for selection of test parameters for a robust method. In addition, a comparison study between two commercial laboratories demonstrates accuracy problems associated with LG testing.