Inhibitory effects of amiodarone and its N-deethylated metabolite on human cytochrome P450 activities: prediction of in vivo drug interactions

Abstract

Aims: To predict the drug interactions of amiodarone and other drugs, the inhibitory effects and inactivation potential for human cytochrome P450 (CYP) enzymes by amiodarone and its N-dealkylated metabolite, desethylamiodarone were examined.

Methods: The inhibition or inactivation potency of amiodarone and desethylamiodarone for human CYP activities were investigated using microsomes from B-lymphoblastoid cell lines expressing CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. The in vivo drug interactions of amiodarone and desethylamiodarone were predicted in vitro using the 1+Iu/Ki values.

Results: Amiodarone weakly inhibited CYP2C9, CYP2D6, and CYP3A4-mediated activities with Ki values of 45.1-271.6 microm. Desethylamiodarone competitively inhibited the catalytic activities of CYP2D6 (Ki=4.5 microm ) and noncompetitively inhibited CYP2A6 (Ki=13.5 microm ), CYP2B6 (Ki=5.4 microm ), and CYP3A4 (Ki=12.1 microm ). The catalytic activities of CYP1A1 (Ki=1.5 microm, alpha=5.7), CYP1A2 (Ki=18.8 microm, alpha=2.6), CYP2C9 (Ki=2.3 microm, alpha=5.9), and CYP2C19 (Ki=15.7 microm, alpha=4.5) were inhibited by desethylamiodarone with mixed type. The 1+Iu/Ki values of desethylamiodarone were higher than those of amiodarone. Amiodarone inactivated CYP3A4, while desethylamiodarone inactivated CYP1A1, CYP1A2, CYP2B6, and CYP2D6.

Conclusions: The interactions between amiodarone and other drugs might occur via the inhibition of CYP activities by its N-dealkylated metabolite, desethylamiodarone, rather than by amiodarone itself. In addition, the inactivation of CYPs by desethylamiodarone as well as by amiodarone would also contribute to the drug interactions.

Figure 1

Chemical structures of amiodarone and desethylamiodarone.

Figure 2

Inhibitory effects of amiodarone (○) and desethylamiodarone (•) on human CYP activities. (a) EROD by recombinant CYP1A1 was determined at a 7-ethoxyresorufin concentration of 2 μm. The control activity was 10.8 pmol min⁻¹ pmol⁻¹ CYP. (b) POD by recombinant CYP1A2 was determined at a phenacetin concentration of 10 μm. The control activity was 0.5 pmol min⁻¹ pmol⁻¹ CYP. (c) COH by recombinant CYP2A6 was determined at a coumarin concentration of 50 μm. The control activity was 6.7 pmol min⁻¹ pmol⁻¹ CYP. (d) BROD by recombinant CYP2B6 was determined at a 7-benzyloxyresorufin concentration of 2 μm. The control activity was 0.4 pmol min⁻¹ pmol⁻¹ CYP. (e) S-WFOH by recombinant CYP2C9 was determined at a S-warfarin concentration of 10 μm. The control activity was 0.05 pmol min⁻¹ pmol⁻¹ CYP. (f) S-MPOH by recombinant CYP2C19 was determined at a S-mephenytoin concentration of 100 μm. The control activity was 1.3 pmol min⁻¹ pmol⁻¹ CYP. (g) BFOH by recombinant CYP2D6 was determined at a bufuralol concentration of 1 μm. The control activity was 1.3 pmol min⁻¹ pmol⁻¹ CYP. (h) CZXOH by recombinant CYP2E1 was determined at a chlorzoxazone concentration of 50 μm. The control activity was 2.0 pmol min⁻¹ pmol⁻¹ CYP. (I) TESOH by recombinant CYP3A4 was determined at a testosterone concentration of 100 μm. The control activity was 13.5 pmol min⁻¹ pmol⁻¹ CYP. Each data point represents the mean of duplicate determinations. The IC₅₀ values of amiodarone (AMD) and desethylamiodarone (DEA) are shown as μm.

Figure 3

Mechanism-based inactivation of CYP2D6 by desethylamiodarone. The recombinant CYP2D6 was preincubated with 0 μm (○), 0.5 μm (•), 1 μm (□), and 2 μm (▪) desethylamiodarone for 0, 5, 10, 15, and 20 min at 37 °C in the presence of an NADPH-generating system. After preincubation, bufuralol was added to the reaction mixture and BFOH was determined.