| Drug interactions involving antiepileptics are relatively common, and often are the result of an interference in the metabolism of an antiepileptic drug by the interacting drug. The objectives of this project were to further characterize the metabolism of several antiepileptics, and to determine the mechanisms of selected drug interactions involving antiepileptics. Hepatocytes from human and porcine liver sources were prepared by a two-step perfusion process. Liver microsomes from porcine (n = 2) and human (n = 8) sources were prepared and characterized for protein content and CYP450 activity. Felbamate (1mM) was incubated with hepatocytes and liver microsomes from both sources, and the formation of metabolites was determined by HPLC-UV. The monocarbamate metabolite of felbamate was the only metabolite formed in porcine and human hepatocyte reactions. The 2-OH metabolite was the most abundant felbamate metabolite formed by porcine liver microsomes (n = 1). The p-OH metabolite was the most abundant felbamate metabolite formed by human liver microsomes (n = 1). A GC-MS assay was developed to quantitate the formation of HPPH, the major phenytoin metabolite in humans, in liver microsomal reactions. The effect of felbamate, sulfaphenazole and several selective serotonin reuptake inhibitors (SSRIs) on HPPH formation was determined. In porcine and human liver microsomes, felbamate did not have a measurable effect on BPPH formation. In human liver microsomes, sulfaphenazole was a potent inhibitor; however, in porcine liver microsomes sulfaphenazole did not inhibit HPPH formation at concentrations up to 500 μM. Fluoxetine and its active metabolite, norfluoxetine, inhibited HPPH formation in every human liver microsome sample (n = 7 (fluoxetine); n = 3 (norfluoxetine)), with average Ki values of 38.1 μM (±14.6) and 51.1 μM (±9.4), respectively. Sertraline and paroxetine were also inhibitors of HPPH formation in HLM5–7, with average Ki values of 52.2 μM (±21.5) and 80.0 μM (±7.2), respectively. The effect of felbamate and lamotrigine on microsomal epoxide hydrolase (HYL1) was studied in human liver microsomes (HLM1–4 (felbamate); HLM2 & 3 (lamotrigine)) by measurement of carbamazepine-10,11-epoxide conversion to the corresponding trans-diol metabolite. Felbamate was, at best, a weak inhibitor of HYL1 (average Ki 1922 μM (±1357)). Lamotrigine did not inhibit HYL1 activity. |