Potent inhibition of cytochrome P450IID6 (debrisoquin 4-hydroxylase) by flecainide in vitro and in vivo.

Flecainide acetate, a class Ic antiarrhythmic agent, is eliminated to a larger extent by renal excretion and to a minor extent by the liver. In patients with impaired renal function or with elevated urinary pH, however, its elimination is dominated by hepatic metabolism. Recent evidence suggests that the in vivo metabolism of flecainide is controlled by the genetic polymorphism of the debrisoquin/sparteine type; i.e., it is a substrate of cytochrome P450IID6. We investigated the inhibitory effect of flecainide on bufuralol 1'-hydroxylation in human liver microsomes in vitro and on the metabolic dextromethorphan urinary ratio in eight healthy male volunteers. Both bufuralol and dextromethorphan are well-known substrates of cytochrome P450IID6. Microsomal bufuralol 1-hydroxylation was competitively inhibited by flecainide with an apparent Ki of 0.954 mumol/L. Moreover, a statistically significant increase in the urinary metabolic ratio (MR) of dextromethorphan/dextrorphan after 1 week of administration of oral flecainide was observed (p = 0.013) in all subjects. One individual increased the urinary MR to a value consistent with the poor metabolizer phenotype. We conclude that flecainide is a potent inhibitor of cytochrome P450IID6 in vitro and in vivo and that careful drug monitoring is required with respect to renal function, debrisoquine phenotype, and concomitant drug administration.