EVIDENCE FOR CYP3A-MEDIATED N-DEETHYLATION OF AMIODARONE IN HUMAN LIVER MICROSOMAL FRACTIONS

Metabolism of amiodarone to its N-deethylated derivative was investiga ted on a bank of human hepatic microsomal fractions, two of them lacki ng the CYP2D6 isozyme. Michaelis-Menten constants for amiodarone N-dee thylation were 0.33 +/- 0.11 muM and 2.38 +/- 0.74 nmol/min/mg for K(M ) and V(max). The specific involvement of CYP3A gene subfamily in amio darone N-deethylation was provided by the following observations: 1) m etabolism of amiodarone is inhibited in a concentration-dependent mann er by ketoconazole, a specific CYP3A inhibitor, and by nifedipine, a s pecific substrate for CYP3A gene subfamily, with IC50 of 0.3 and 25 mu M, respectively; 2) nifedipine competitively inhibits amiodarone metab olism with a K(i) of 38 muM; 3) amiodarone N-deethylation is increased following incubation with hepatic microsomal fractions prepared from CYP3A-inducers such as rifampycin and triacetyloleandomycin, but also following the in vitro disruption of the ''cytochrome P-450-Fe-(II)-tr iacetyloleandomycin nitroso derivative'' complex; 4) antibodies raised against either rabbit or baboon monkey CYP3A gene subfamily inhibit a miodarone N-deethylation; and 5) microsomal fractions that specificall y express CYP3A4 biotransform amiodarone to its N-deethylated derivati ve. These studies indicate that CYP3A isozyme(a) mainly metabolize ami odarone to its N-deethylated derivative in human hepatic microsomal fr actions.