Clinical pharmacology and pharmacokinetics of oxcarbazepine

Oxcarbazepine (OCBZ) is a new antiepileptic drug (AED) structurally related to carbamazepine (CBZ) but differing in several important aspects, notably metabolism and induction of metabolic pathways. Consequently, OXCB has fewer drug-drug interactions compared with CBZ. Absorption of OCBZ is rapid and complete. In animals it is responsible for the pharmacological effect. In humans, however, the parent compound is rapidly and extensively metabolized to a monohydroxy derivative (MHD), which is responsible for the therapeutic effect. Exposure to the MHD increases dose proportionally, and steady state is achieved after only three or four doses in a twice-daily regimen. When given with food, systemic exposure to MHD increases by about 17%. MHD is eliminated with a half-life of about 8-10 h. About 27% of the dose is recovered in the urine as unchanged MHD and a further 49% as a glucuronide conjugate of MHD. Results suggest that the kinetics of OCBZ should not be affected by impaired liver function. Impaired kidney function does not affect the kinetics of MHD; the glucuronide conjugate will, however, accumulate in these patients. The conversion of OCBZ to MHD is catalyzed by reductase enzymes, which are not subject to induction. Furthermore, OCBZ itself does not appear to induce the cytochrome P-450 family in general, although it does induce the P-450 IIIA subfamily, which is responsible for the metabolism of estrogens and the dihydropyridine calcium-channel blockers (e.g., nifedipine, felodipine). In patients, linear and dose-proportional kinetics with no autoinduction of metabolism simplify OCBZ dosage adjustment.