Abstract

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 IC₅₀ values of 21, 93, and 27 μM, respectively. However, when the preincubation was supplemented with NADPH, IC₅₀ values shifted to 4.0, 3.4, and 3.0 μM, respectively resulting in IC₅₀ shifts of 5.2-, 28-, and 9.3-fold. Ticlopidine showed a 1.8-fold shift in IC₅₀ 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 IC₅₀ 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.