Metabolism of carbamazepine and co-administered anticonvulsants during pregnancy

Urinary excretions of carbamazepine, carbamazepine-10,11-epoxide, carbamazepine-10,11-trans-diol, 9-hydroxyacridan and 2- and 3-hydroxycarbamazepine were measured at various stages of pregnancy, and in the post-natal period, in ten epileptic women, six of whom took no other enzyme-inducing anticonvulsant and four of whom took such co-medication. Mean plasma carbamazepine apparent clearance was increased in pregnancy, but only by virtue of the increased clearance in the anticonvulsant co-medicated women. Alterations in the proportions of the carbamazepine dose cleared via the various excretion pathways studied were quantitatively minor, but there was evidence consistent with impaired conversion of carbamazepine-10,11-epoxide to carbamazepine-10,11-trans-diol during all pregnancies studied. Clearances of carbamazepine to the various excretory products studied were consistent with there being (i) increased urinary excretion of unmetabolised drug in pregnancy, possibly related to the increased glomerular filtration rate, (ii) increased formation of oxidative metabolites of the drug, particularly in women co-medicated with enzyme-inducing anticonvulsants, this effect being offset, in full (in non-co-medicated women) or in part (in co-medicated women) by (iii) inhibition of the epoxide-diol pathway in pregnancy, an inhibition to which folate intake may have contributed.