Intradose and circadian variation in circulating carbamazepine and its epoxide in epileptic patients: a consequence of autoinduction of metabolism.

Total and free carbamazepine (CBZ), and CBZ 10,11 epoxide (CBZ-E) concentrations were measured over 24 h in 19 patients receiving CBZ 400 mg b.i.d. either as monotherapy (n = 13) or combined with another anticonvulsant (n = 6). Differences in CBZ and CBZ-E disposition between day and night dosing were minor. Mean plasma CBZ concentrations were higher and CBZ-E/CBZ ratios were lower in the monotherapy patients. Variations in total and free plasma CBZ levels were comparable in the monotherapy and polypharmacy groups. Peak free and total CBZ concentrations coincided at approximately 4 h postdose. Free CBZ levels correlated significantly with total in each patient. The extent of variation in total plasma CBZ concentration during 24 h correlated significantly with antipyrine clearance in the monotherapy group. Circadian rhythms are unlikely to influence CBZ disposition to a clinically relevant extent. Measurement of peak and trough CBZ concentrations should improve the value of therapeutic drug monitoring. The diurnal variation in CBZ concentration appears related to the degree of autoinduction of metabolism and is substantial enough to warrant the development of a slow-release preparation of the drug.