A significant role of human cytochrome P450 2C8 in amiodarone N-deethylation: an approach to predict the contribution with relative activity factor.

Human cytochrome P450 (CYP) isoforms involved in amiodarone N-deethylation were identified, and the relative contributions of these CYP isoforms were evaluated in different human liver microsomes. The mean K(M) and V(max) values of amiodarone N-deethylation in microsomes from six human livers were 31.6 +/- 7.5 microM 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 microM. 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 microM was significantly correlated with the paclitaxel 6alpha-hydroxylase activity (r = 0.84, P <.05) in the human liver microsomes, whereas the amiodarone N-deethylase activity at 100 microM was significantly correlated with the testosterone 6beta-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.