Metabolites and DNA-binding of carbamazepine and oxcarbazepine in vitro by rat liver microsomes

DNA-binding of carbamazepine (CBZ) and oxcarbazepine (OCBZ) catalysed by non-induced, phenobarbital-induced or methylcholanthrene-induced rat liver microsomes in vitro was studied. C-14-CBZ 200 nmol incubated with DNA, liver microsomes and cofactors led to the formation of a significant amount of CBZ-epoxide, which has been suspected as the cause of teratogenesis and other side-effects of CBZ,(1,2) but has not been reactive in any test systems for genotoxicity, including the Ames test.(3) No enzyme-dependent DNA-binding of CBZ was found. Using the same conditions, however, OCBZ was bound to DNA. This binding was dependent on the presence of NADPH. 10-hydroxy-10,11-dihydro-carbamazepine, which is known to be the major metabolite of OCBZ, and an unknown peak were demonstrated by HPLC. These results are the first indication of a higher level of covalent DNA binding of OCBZ than of CBZ. The nature of the unknown metabolite and the pathway leading to covalent binding remain to be studied.