Identification of cytochrome P450 isoforms involved in citalopram N-demethylation by human liver microsomes

Studies to assess the enzyme kinetic behavior and to identify the cytochrome P450 (CYP) isoform(s) involved in the major metabolic pathway (N-demethylation) for citalopram (CIT), a selective serotonin reuptake inhibitor, were performed using human liver microsomes and cDNA-expressed human cytochrome P450 isoforms. The N-demethylation activities showed significant correlations with the alpha- and 4-hydroxylation activities of triazolam (r(s) = 0.818 and 0.851, respectively; P < .01) in 10 different human liver microsomes. Anti-CYP3A antibodies and ketoconazole strongly inhibited CIT N-demethylation. In addition, there was a significant correlation between CIT N-demethylation and (S)-mephenytoin 4'-hydroxylation (r(s) = 0.773, P < .05), although little inhibition was observed in the presence of anti-CYP2C antibodies or (S)-mephenytoin. cDNA-expressed CYP3A4 and CYP2C19 catalyzed CIT N-demethylation, whereas no appreciable activities were observed for CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6 and CYP2E1. The percentage contributions of CYP3A4 and CYP2C19 to the overall N-demethylation of CIT in human liver microsomes were estimated using a relative activity factor; respective values of 70% and 7% were calculated for microsomes obtained from livers from putative extensive metabolizers for (S)-mephenytoin 4'-hydroxylation. These results suggest that CYP3A4 is the major isoenzyme and CYP2C19 is the minor form involved in the major metabolic pathway for CIT in human liver microsomes.