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.
Kobayashi
K
K
Department of Clinical Pharmacy, School of Pharmaceutical Sciences, Showa University, Tokyo, Japan.
Chiba
K
K
Yagi
T
T
Shimada
N
N
Taniguchi
T
T
Horie
T
T
Tani
M
M
Yamamoto
T
T
Ishizaki
T
T
Kuroiwa
Y
Y
D001189:Aryl Hydrocarbon Hydroxylases; D015283:Citalopram / Q000378:metabolism*; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D018076:DNA, Complementary; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity
D018076:DNA, Complementary; D007527:Isoenzymes; D011994:Recombinant Proteins; D015283:Citalopram; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; C104464:CYP3A protein, human; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human