Improved isocratic liquid-chromatographic simultaneous measurement of phenytoin, phenobarbital, primidone, carbamazepine, ethosuximide, and N-desmethylmethsuximide in serum.

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Abstract
We describe an improved "high-pressure" liquid-chromatographic assay for simultaneous determination in serum of the five major antiepileptic drugs (ethosuximide, primidone, phenobarbital, phenytoin, and carbamazepine) and N-desmethylmethsuximide (the compound that must be quantitated for therapeutic drug monitoring of the antiepileptic drug methsuximide). Serum protein is precipitated with an acetone solution containing 5-ethyl-5-(p-methylphenyl)barbituric acid as the internal standard. The centrifuged supernate is injected onto the chromatographic column. Drugs and internal standard are eluted at 30 degrees C with mobile phase containing acetonitrile/methanol/phosphate buffer (17/28/55 by vol) at a flow rate of 0.7 mL/min, monitored at 195 nm. Analysis time is about 20 min. Quantitation is by measurement of peak areas. Analytical and absolute recoveries varied from 95 to 104%. Within-day coefficients of variation ranged from 1.6 to 5.4%, between-day CVs from 0.0 to 3.4% in subtherapeutic, therapeutic, and toxic samples. Resolution of therapeutic concentrations of all six drugs was complete. As yet, we have found no drug or drug metabolite that interferes.