The metabolism of carbamazepine in humans: steric course of the enzymatic hydrolysis of the 10,11-epoxide

Carbamazepine 10,11-oxide (1a,10b-dihydro-6H-dibenzo[b,f]oxireno[d]azepine-6-carboxamide), a key intermediate in carbamazepine metabolism, was found to be unusually resistant to enzymatic hydrolysis when incubated with microsomal and cytosolic fractions from rabbit, rat, and guinea pig livers. However, its hydrolysis product, trans-10,11-dihydro-10,11-dihydroxy-5H-dibenzo[b,f]azepine-5-carboxamide , was excreted, as previously reported, both in the free and in conjugated forms, as the main metabolite in the urine of humans under carbamazepine treatment. The free diol and that obtained after treatment with beta-glucuronidase/arylsulfatase were both found by Mosher's method to be formed in an enantiomeric excess of 80%, the prevalent enantiomer having the (-)-10S,11S absolute configuration, as determined by applying the CD exciton coupling method to its bis[p-(dimethylamino)benzoyl] ester. This finding confirms the pronounced enantioselectivity of the microsomal Epoxide Hydrolase toward meso and racemic substrates, but is in contrast with the prevalent formation of (R,R)-diols in most other known cases of enzymatic hydrolysis of epoxides. Preparatively useful syntheses of the racemic trans-10,11-dihydro-10,11-diol and of 9-(hydroxymethyl)-10-carbamoylacridan, another carbamazepine metabolite, are reported for the first time.