Dextromethorphan pretreatment induces antipyrine clearance in the rat

Numerous agents that undergo extensive first-pass metabolism have been shown to inhibit oxidative drug metabolism. To examine whether this effect is related to the chemical structure or pharmacokinetic characteristics of the inhibiting agent, we determined the effect of dextromethorphan (a compound which exhibits pharmacokinetic similarities to, but is chemically dissimilar from, previously studied agents) on the disposition of antipyrine. A single oral dose of dextromethorphan hydrobromide, 100 mg/kg, 1 hr prior to antipyrine administration had no significant effect on the pharmacokinetics of this model substrate. The administration of dextromethorphan at the same dose twice daily for 3 days and an additional dose 1 hr prior to antipyrine administration resulted in a 33% increase in the clearance of antipyrine. These data indicate that dextromethorphan is capable of inducing hepatic microsomal enzymes. Studies are needed to determine if this effect also occurs upon chronic administration in humans. These data suggest that the pharmacokinetic characteristic of extensive first-pass metabolism is not necessarily associated with inhibition of drug metabolism.