Abstract

Many drugs are used as racemates, and the enantiomers may differ in terms of pharmacological properties and disposition. Stereoselective disposition of the enantiomers can arise from metabolism of the enantiomers via different routes catalyzed by different enzymes. In contrast, the enantiomers may be metabolized by the same enzyme at different rates. In the latter case, the enantiomers can compete for this metabolic step, giving rise to the possibility of an enantiomer/enantiomer interaction. We have chosen the antiarrhythmic propafenone, for which in vivo data indicated an interaction between (S)- and (R)-propafenone, as a model substance to study the mechanism underlying that interaction in human liver microsomes. We used the cytochrome P450IID6-mediated 5-hydroxylation of propafenone as a model pathway, because this metabolic step constitutes the major route of biotransformation of propafenone. The Michaelis-Menten kinetics for 5-hydroxylation were determined after incubation of (R)- and (S)-propafenone and a pseudoracemate consisting of (S)-[2H4]propafenone and (R)-propafenone. Inhibition experiments were performed using (S)-[2H4]propafenone as an inhibitor of the 5-hydroxylation of (R)-propafenone, and vice versa. The kinetic model of mixed alternative substrates was used to simulate inhibition experiments. Experimental data were compared with those predicted by this model. We observed a substantial stereoselectivity after incubation of the individual enantiomers [(S)-propafenone: Vmax, 10.2 pmol/micrograms/hr, and Km, 5.3 microM; (R)-propafenone: Vmax, 5.5 pmol/micrograms/hr, and Km, 3.0 microM]. In contrast, no substrate stereoselectivity was observed after incubation of the pseudoracemate [3.1 pmol/micrograms/hr for (S)-[2H4]propafenone and 3.3 pmol/micrograms/hr for (R)-propafenone]. Application of the model revealed Ki values of 2.9 and 5.2 microM for the inhibition of 5-hydroxylation of (S)-[2H4]-propafenone by (R)-propafenone and for inhibition of 5-hydroxylation of (R)-propafenone by (S)-[2H4]-propafenone, respectively. The predicted and the experimental data were in good agreement, and both indicated the mode of inhibition to be competitive. In conclusion, the enantiomers of propafenone interact with respect to 5-hydroxylation, with (R)-propafenone being a more potent inhibitor than the S-enantiomer with respect to cytochrome P450IID6-mediated 5-hydroxylation. Because beta-blocking properties of propafenone reside in the S-enantiomer, inhibition of metabolism of this enantiomer by (R)-propafenone may have therapeutic consequences.