Early stage autoinduction of carbamazepine metabolism in humans.

Six healthy young adult male volunteers were given two 600 mg (2540 mu moles) oral doses of carbamazepine (CBZ) 5 days apart. Serial concentrations of CBZ and its 10,11-epoxy (CBZ-epoxide) and 10,11-dihydro-10,11-trans-dihydroxy (CBZ-diol) metabolites in plasma, and daily excretions of these substances and the 2-hydroxy (2-OH-CBZ), 3-hydroxy (3-OH-CBZ) and 9-hydroxymethyl-10-carbamoylacridan (acridan) metabolites in urine were followed for 5 days after each dose. Pharmacokinetic analysis showed that autoinduction of CBZ metabolism was present within 6-10 days of the initial drug dose. The mean oral clearance of CBZ increased from 1.48 to 1.74 l.h-1 (difference 0.26 l.h-1, 95% confidence interval 0.11 to 0.41 l.h-1) and the mean percentage urinary recovery of the amount of CBZ eliminated increased from 41.8% to 44.6% (difference 2.8%, 95% confidence interval 0.5 to 5%) between the two studies 5 days apart. The data for daily clearance to metabolite and the time-courses of the plasma CBZ-epoxide to CBZ and CBZ-diol to CBZ concentration ratios suggested that autoinduction had begun by the second day after CBZ intake, and involved not only the epoxide-diol pathway but, to a lesser extent, the oxidations to phenolic derivatives.