Rapid Gas Chromatographic Procedure for the Determination of Topiramate in Serum

A rapid gas chromatographic method for the routine determination in serum of the new anticonvulsant drug topiramate (Topamax) (TOP) is described. The method involves extracting 0.50 mL of sample, previously adjusted to pH 9.5 with saturated borate buffer with ethyl acetate. One-microliter aliquots of the extract were injected into a 10-m x 0.53-mm i.d. x 0.5-microm 100% methyl silicone megabore capillary column connected to a nitrogen-phosphorus detector. The column temperature was initially at 170 degrees C for 0.1 min, then programmed at 10 degrees C/min to 240 degrees C, then 20 degrees C/min to 280 degrees C for 0.5 min. Under these conditions of the assay, the retention times of TOP and mepivicaine, internal standard, were 4.0 and 3.4 min, respectively. Quantitative determinations were performed with peak-height ratios of TOP to the internal standard. Calibration curves were linear from 2.5 to 150 mg/L TOP. The assay had a limit of quantitation of 2.5 mg/L. The overall within-run precision of the method yielded coefficients of variation (CV) of 3.9% at 10 mg/L (n = 10) and 3.1% at 100 mg/L (n = 10). The overall between-run precision calculated by three determinations on a single day for a week yielded CVs of 7.3% at 23 mg/L (n = 12) and 7.8% at 85 mg/L (n = 12). Common anticonvulsant and basic/neutral extractable drugs were found not to interfere with the assay. At present, no correlation has been demonstrated between trough plasma TOP concentrations and clinical efficacy. However, TOP values observed in our laboratory in serums from patients receiving adjunctive treatment for seizure disorders ranged from 2.5 to 35 mg/L.