Differential Time- and NADPH-Dependent Inhibition of CYP2C19 by Enantiomers of Fluoxetine

Fluoxetine [±- N -methyl-3-phenyl-3-[(α, α, (-trifluoro- p -tolyl)oxy]-propylamine)] a selective serotonin reuptake inhibitor, is widely used in treating depression and other serotonin-dependent disease conditions. Racemic, ( R )- and ( S )-fluoxetine are potent reversible inhibitors of CYP2D6, and the racemate has been shown to be a mechanism-based inhibitor of CYP3A4. Racemic fluoxetine also demonstrates time- and concentration-dependent inhibition of CYP2C19 catalytic activity in vitro. In this study, we compared fluoxetine, its ( R )- and ( S )-enantiomers, ticlopidine, and S -benzylnirvanol as potential time-dependent inhibitors of human liver microsomal CYP2C19. In a reversible inhibition protocol (30 min preincubation with liver microsomes without NADPH), we found ( R )-, ( S )- and racemic fluoxetine to be moderate inhibitors with IC 50 values of 21, 93, and 27 μM, respectively. However, when the preincubation was supplemented with NADPH, IC 50 values shifted to 4.0, 3.4, and 3.0 μM, respectively resulting in IC 50 shifts of 5.2-, 28-, and 9.3-fold. Ticlopidine showed a 1.8-fold shift in IC 50 value, and S -benzylnirvanol shifted right (0.41-fold shift). Follow-up K I and k inact determinations with fluoxetine confirmed time-dependent inhibition [ K I values of 6.5, 47, and 14 μM; k inact values of 0.023, 0.085, 0.030 min –1 for ( R )-, ( S )-, and racemate, respectively]. Although the ( S )-isomer exhibits a much lower affinity for CYP2C19 inactivation relative to the ( R )-enantiomer, it exhibits a more rapid rate of inactivation. Racemic norfluoxetine exhibited an 11-fold shift (18–1.5 μM) in IC 50 value, suggesting that conversion of fluoxetine to this metabolite represents a metabolic pathway leading to time-dependent inhibition. These data provide an improved understanding of the drug-interaction potential of fluoxetine.