HUMAN CYTOCHROMES MEDIATING N-DEMETHYLATION OF FLUOXETINE IN-VITRO

Biotransformation of the selective serotonin reuptake inhibitor antide pressant, fluoxetine, to its principal metabolite, norfluoxetine, was evaluated in human liver microsomes and in microsomes from transfected cell lines expressing pure human cytochromes. In human liver microsom es, formation of norfluoxetine from R,S-fluoxetine was consistent with Michaelis-Menten kinetics (mean K-m = 33 mu M), with evidence of subs trate inhibition at high substrate concentrations in a number of cases . The reaction was minimally inhibited by coincubation with chemical p robes inhibitory for P450-2D6 (quinidine), -1A2 (furafylline, cc-napht hoflavone), and -2E1 (diethyldithiocarbamate). Substantial inhibition was produced by coincubation with sulfaphenazole (K-i = 2.8 mu M), an inhibitory probe for P450-2C9, and by ketoconazole (K-i = 2.5 mu M) an d fluvoxamine (K-i = 5.2 mu M). However, ketoconazole, relatively spec ific for P450-3A isoforms only at low concentrations, reduced norfluox etine formation by only 20% at 1 mu M, and triacetyloleandomycin (grea ter than or equal to 5 mu M) reduced the velocity by only 20-25 %. Mic rosomes from cDNA-transfected human lymphoblastoid cells containing hu man P450-2C9 produced substantial quantities of norfluoxetine when inc ubated with 100 mu M fluoxetine. Smaller amounts of product were produ ced by P450-2C19 and -2D6, but no product was produced by P450-1A2, -2 E1, or 3A4. Cytochrome P450-2C9 appears to be the principal human cyto chrome mediating fluoxetine N-demethylation. P450-2C19 and -3A may mak e a further small contribution, but P450-2D6 is unlikely to make an im portant contribution.