Formation of active metabolites of anticonvulsant drugs. A review of their pharmacokinetic and therapeutic significance.

All of the commonly used anticonvulsants drugs, except possibly primidone, are cleared from the human body mainly by metabolism. The metabolites of phenytoin, phenobarbital and ethosuximide have so far not been shown to possess significant pharmacological activity. However, carbamazepine-10,11-epoxide, derived from carbamazepine, has anticonvulsant activity comparable with that of its progenitor, while oxcarbazepine, a new anticonvulsant congener of carbamazepine, is essentially a prodrug for its 10-hydroxy derivative. Valproic acid forms numerous metabolites through a variety of pathways; 2-en valproic acid, a beta-oxidation derivative, probably contributes to its anticonvulsant action, though the extent of the contribution is uncertain. Another metabolite, 4-en-valproic acid, has been considered a possible hepatotoxin and teratogen. N-Methyl-phenobarbital and primidone, though both anticonvulsants in their own right, are metabolised to phenobarbital, which probably mediates much of their antiseizure effect. Primidone also yields the weaker anticonvulsant phenylethylmalonamide. The various benzodiazepine anticonvulsants form numerous metabolites, some of which possess both antiseizure and other forms of pharmacological activity. As yet, there is little understanding of how best to interpret simultaneous plasma concentration measurements of anticonvulsant drug and its active metabolite (or metabolites) in the clinical situation, and the possible roles of anticonvulsant metabolites in the idiosyncratic toxicity of these drugs remain largely unexplored.