A significant role of human cytochrome P4502C8 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 value s of amiodarone N-deethylation in microsomes from six human livers were 31. 6 +/- 7.5 mu M and 1.2 +/- 0.7 pmol/min/pmol of CYP, respectively. Ketocona zole and anti-CYP3A antibodies strongly inhibited amiodarone N-deethylase a ctivity in human liver microsomes at a substrate concentration of 50 mu M. Of 15 recombinant human CYP enzymes (19 preparations), CYP1A1, CYP3A4, CYP1 A2, CYP2D6, CYP2C8, and CYP2C19 catalyzed amiodarone N-deethylation. The am iodarone N-deethylase activity at a substrate concentration of 5 mu M was s ignificantly correlated with the paclitaxel 6 alpha-hydroxylase activity (r = 0.84, P < .05) in the human liver microsomes, whereas the amiodarone N-d eethylase activity at 100 mu M was significantly correlated with the testos terone 6 beta-hydroxylase activity (r = 0.94, P < .005). According to the c oncept of relative activity factor, it was clarified that CYP2C8 as well as CYP3A4 were significantly involved in amiodarone N-deethylation in human l ivers at clinically significant concentrations and that the contributions o f CYP1A2, CYP2C19, and CYP2D6 were relatively minor. However, there was a l arge interindividual variability in the contribution of each CYP isoform to amiodarone N-deethylase activity in human liver; the relevance of these en zymes would be dependent on the content of the respective isoforms and on t he amiodarone concentration in the liver.