Carbamazepine-danazol drug interaction: its mechanism examined by a stable isotope technique.

Carbamazepine (CBZ) labeled with 15N was used to investigate the mechanism of its pharmacokinetic interaction with the antiestrogenic steroid danazol during treatment of a patient with epilepsy. Danazol led to a pronounced inhibition of CBZ metabolism. During danazol coadministration, CBZ elimination half-life increased from a pretreatment value of 11 to 24.3 h. Carbamazepine plasma clearance decreased from 57.7 to 23.2 ml/h/kg. Kinetic analysis of the plasma concentration-time curves and urinary excretion of [15N]trans-CBZ-diol revealed that danazol inhibited the epoxide-trans-diol pathway of carbamazepine metabolism. Observations in five other female patients confirm that the steady-state plasma concentrations of UCBZ increase between 50 and 100% during coadministration of danazol.