Abstract

Human cytochrome P450 (CYP) isoforms involved in amiodaroneN-deethylation were identified, and the relative contributions of these CYP isoforms were evaluated in different human liver microsomes. The mean K_M andV_max values of amiodaroneN-deethylation in microsomes from six human livers were 31.6 ± 7.5 μM and 1.2 ± 0.7 pmol/min/pmol of CYP, respectively. Ketoconazole and anti-CYP3A antibodies strongly inhibited amiodarone N-deethylase activity in human liver microsomes at a substrate concentration of 50 μM. Of 15 recombinant human CYP enzymes (19 preparations), CYP1A1, CYP3A4, CYP1A2, CYP2D6, CYP2C8, and CYP2C19 catalyzed amiodarone N-deethylation. The amiodarone N-deethylase activity at a substrate concentration of 5 μM was significantly correlated with the paclitaxel 6α-hydroxylase activity (r = 0.84,P < .05) in the human liver microsomes, whereas the amiodarone N-deethylase activity at 100 μM was significantly correlated with the testosterone 6β-hydroxylase activity (r = 0.94, P < .005). According to the concept of relative activity factor, it was clarified that CYP2C8 as well as CYP3A4 were significantly involved in amiodarone N-deethylation in human livers at clinically significant concentrations and that the contributions of CYP1A2, CYP2C19, and CYP2D6 were relatively minor. However, there was a large interindividual variability in the contribution of each CYP isoform to amiodarone N-deethylase activity in human liver; the relevance of these enzymes would be dependent on the content of the respective isoforms and on the amiodarone concentration in the liver.