The formation of R- and S-norfluoxetine was analyzed in vitro in human liver microsomes. Low apparent K(m) values for R-norfluoxetine formation of < or =8 microM and S-norfluoxetine of <0.2 microM were determined. R-Norfluoxetine formation rates in a characterized microsomal bank correlated with the catalytic activities for cytochrome P450 (CYP) 2D6, CYP2C9, and CYP2C8. Expressed CYP2C9, CYP2C19, and CYP2D6 formed R-norfluoxetine following incubation with 1 microM R-fluoxetine and exhibited apparent K(m) values of 9.7, 8.5, and 1.8 microM, respectively. Multivariate correlation analysis identified CYP2C9 and CYP2D6 as significant regressors with R-norfluoxetine formation. Antibodies to the CYP2C subfamily and CYP2D6 each exhibited moderate inhibition of R-norfluoxetine formation. Therefore, CYP2D6 and CYP2C9 contribute to this biotransformation. At pharmacological concentrations of S-fluoxetine, S-norfluoxetine formation rates in the bank of microsomes were found to correlate only with CYP2D6 catalytic activity and only expressed CYP2D6 was found to be capable of forming S-norfluoxetine. Thus, it would appear that both CYP2D6 and CYP2C9 contribute to the formation of R-norfluoxetine, whereas only CYP2D6 is responsible for the conversion to S-norfluoxetine. Since the enantiomers of fluoxetine and norfluoxetine are inhibitors of CYP2D6, upon chronic dosing, the CYP2D6-mediated metabolism of the fluoxetine enantiomers would likely be inhibited, resulting in R-norfluoxetine formation being mediated by CYP2C9 and S-norfluoxetine formation being mediated by multiple high K(m) enzymes.