Cytochrome P-450 (CYP) 2D6 is responsible for the biotransformation of over 35 pharmacologic agents. In the process of studying CYP2D6 we identified phenotype-genotype discordance in two individuals receiving terbinafine. This prompted evaluation of the potential for terbinafine to inhibit CYP2D6 in vitro. Human hepatic microsomes and heterologously expressed CYP2D6 were incubated with terbinafine or quinidine and the formation of dextrorphan from dextromethorphan was determined by HPLC. Additionally, preliminary conformational analyses were conducted to determine the fit of terbinafine into a previously described pharmacophore model for CYP2D6 inhibitors. The apparent Km and Vmax of dextrorphan formation from four human hepatic microsome samples ranged from 5.8 to 6.8 microM and from 172 to 300 pmol/min/mg protein, respectively. Values of Km and Vmax in the heterologously expressed CYP2D6 system averaged 6.5 +/- 2.1 microM and 1342 +/- 147 pmol/min/mg protein, respectively. Terbinafine inhibited dextromethorphan O-demethylation with an apparent Ki ranging from 28 to 44 nM in human hepatic microsomes and averaging 22.4 +/- 0.6 nM for the heterologously expressed enzymes. Results of quinidine in these systems produced values for Ki ranging from 18 to 43 nM. Such strong inhibition of CYP2D6 by terbinafine would not have been predicted by the previously proposed pharmacophore model of CYP2D6 inhibitors based on molecular structure. Terbinafine is a potent inhibitor of CYP2D6 with apparent Ki values well below plasma and tissue concentrations typically achieved during a therapeutic course. This agent needs to be evaluated in vivo to determine the impact of CYP2D6 inhibition by terbinafine on the metabolism of concomitantly administered CYP2D6 substrates.