Carbamazepine 10, 11 epoxide concentrations in epileptics on carbamazepine alone and in combination with other anticonvulsants.

Steady state carbamazepine (CBZ) plasma concentrations were similar in 15 epileptics receiving monotherapy and in 24 patients taking CBZ in combination with one other anticonvulsant. The ratio of CBZ 10, 11-epoxide (CBZ-E) to the parent drug was significantly higher (P less than 0.01) in those patients taking concomitant phenytoin (n = 9), phenobarbitone or primidone (n = 9), and valproic acid (n = 6) than in the patients receiving CBZ alone. In the monotherapy group, there was a significant correlation between CBZ-E/CBZ ratio and the concentration of the parent drug (P less than 0.05). If CBZ-E has equipotent anticonvulsant properties to CBZ in man as is the case in animal models, routine CBZ-E concentrations may provide further refinement in the therapeutic drug monitoring of CBZ.