Abstract
Aims The study investigated the mechanism of the interaction between valproate and carbamazepine which causes raised plasma carbamazepine-10,11-epoxide concentrations with unchanged plasma carbamazepine concentrations. This interaction has usually been attributed to valproate inhibiting epoxide hydrolase, the enzyme that catalyses the biotransformation of carbamazepine-10,11-epoxide to carbamazepine-10,11-trans-diol.
Methods Clearances of plasma carbamazepine, carbamazepine-epoxide and carbamazepine-diol to relevant carbamazepine metabolites present in urine were measured under steady-state conditions in 17 adults receiving carbamazepine as anticonvulsant monotherapy, and in 10 adults taking the drug together with valproate.
Results Plasma carbamazepine-epoxide concentrations were higher, relative to carbamazepine dose, in the co-medicated patients. Plasma apparent clearances of carbamazepine, relative to drug dose, were similar whether or not valproate was taken. Formation clearances of carbamazepine-10,11-trans-diol conjugate, and probably of carbamazepine-10,11-trans-diol, were lower in subjects co-medicated with valproate, and a higher proportion of the carbamazepine dose was excreted in urine as carbamazepine-10,11-epoxide.
Conclusions Valproate appears to inhibit the glucuronidation of carbamazepine-10,11-trans-diol, and probably also inhibits the conversion of carbamazepine-10,11-epoxide to this trans-diol derivative, rather than simply inhibiting the latter reaction only.
Keywords: carbamazepine-valproate interaction, carbamazepine-trans-diol, carbamazepine-epoxide, epoxide hydrolase, glucuronidation
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