Metabolism of amiodarone (Part III): identification of rabbit cytochrome P450 isoforms involved in the hydroxylation of mono-N-desethylamiodarone

Abstract

1. Amiodarone (AMI) is a potent anti-arrhythmic drug and mono-N-desethylamiodarone (MDEA) is its only known metabolite. It was found recently that in rabbit liver microsomes MDEA was biotransformed to n-3-hydroxybutyl-MDEA (3OH-MDEA). 2. In liver microsomes isolated from the untreated rabbit, the formation of 3OHMDEA obeyed Michaelis-Menten enzyme kinetics with K_m = 6.39 ± 1.07 μm and V_max = 0.56 ± 0.21 nmol min⁻¹ mg⁻¹ protein. 3. Furthermore, (1) among chemicals usually used as inhibitors of cytochrome P450, only midazolam (MDZ), cyclosporin A and ketoconazole inhibited the MDEA hydroxylase activity significantly (> 60% inhibition), (2) MDZ, a substrate of CYP3A, inhibited the 3OH-MDEA formation competitively (K_i = 10 ± 5 μm), (3) the formation rates of 3OH-MDEA correlated positively with those of 1′OH-MDZ (r = 0.81; n = 6), and (4) MDEA hydroxylase activity of microsomes isolated from rabbit rifampicininduced cultured hepatocytes was 4-fold more active than the control. 4. Since CYP3A6 is mainly induced by rifampicin in rabbit-cultured hepatocytes, the data suggest that this isoform is involved in the biotransformation of MDEA to 3OHMDEA. 5. Since α-naphthoflavone, cimetidine and quinidine also partially inhibited the MDEA hydroxylase activity, it is possible that other CYPs, such as 1A, 2C and 2D, may also be active in the metabolism of amiodarone.