Felbamate Levels in Patients with Epilepsy

The usefulness of felbamate (FBM) levels in managing epilepsy patients has not been determined. The purpose of the present study was to determine if FBM levels obtained at routine office visits correlated with side effects reported by patients. We determined FBM levels by high-pressure liquid chromatography (HPLC) of 46 epilepsy patient plasma specimens (41 patients) and assessed medication toxicity and seizure frequency by a questionnaire. Thirty-six patients were treated with other antiepileptic drugs (AEDs); concomitant AED levels not in ranges believed to cause toxicity. FBM levels ranged from 9 to 134 microgram/ml, and were divided into three groups for analysis, resulting in low-range (9-36 microgram/ml), midrange (37-54 microgram/ml), and high-level (44-134 microgram/ml) groups. Anorexia and complaints of severe side effects were reported significantly more often in the high-level group as compared with the low- and midrange groups. Significantly more patients in the high-level group (10/13) reported decreased seizure frequency, as compared with 12 of 30 patients in the low- and midrange groups combined. FBM levels correlated linearly with doses overall, but most closely in FBM monotherapy patients.