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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+I u /K i values. Results Amiodarone weakly inhibited CYP2C9, CYP2D6, and CYP3A4-mediated activities with K i values of 45.1-271.6 mm. Desethylamiodarone competitively inhibited the catalytic activities of CYP2D6 (K i =4.5 mm) and noncompetitively inhibited CYP2A6 (K i =13.5 mm), CYP2B6 (K i =5.4 mm), and CYP3A4 (K i = 12.1 mm). The catalytic activities of CYP1A1 (K i =1.5 mm, a=5.7), CYP1A2 (K i =18.8 mm, a=2.6), CYP2C9 (K i =2.3 mm, a=5.9), and CYP2C19 (K i = 15.7 mm, a=4.5) were inhibited by desethylamiodarone with mixed type. The 1+I u /K i 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.

Small minipigs (Bland name, Micromini Pig; registered as a novel variety of pig in the Japanese Ministry of Agriculture, Forestry and Fisheries) were developed with the aim of non-clinical pharmacological/toxicological use. They were principally mated with<10 kg body weight at 7 months-old resulting in good handling. Cytochrome P450 (P450)-and flavin-containing monooxygenases (FMO)-dependent drug oxidation activity of liver microsomes prepared from male Microminipigs (8 months-old) was compared with that for pooled dogs, monkeys, and humans. High P450 2D-dependent bufuralol 1'-hydroxylation and FMO-dependent benzydamine N-oxygenation activity was observed in liver microsomes from Microminipigs. Typical P450 1A, 2B, 2C, 2E, and 3A-dependent drug oxidation activity was also seen in Microminipigs. However, occasional differences might give undetected low P450 2A-dependent coumarin 7-hydroxylation in Microminipigs at 8-months-old, in contrast to liver microsomes from one 10-days-old Microminipis and commercially available pooled minipigs which had low but detectable coumarin 7-hydroxylation activity. The present results suggest that there is some overlap in Microminipig and human P450 substrate specificity. These findings should provide important information for greater understanding of drug metabolism in Microminipigs, as an experimental animal model for non-clinical use.

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1.  The purpose of this study was to investigate the inhibitory effects of diclofenac on human cytochrome P450 1A2-, 2C19- and 3A4-mediated drug oxidations and to evaluate the drug interaction potential of diclofenac and 4'-hydroxydiclofenac. 2.  Diclofenac was converted to 4'-hydroxydiclofenac by recombinantly expressed human P450 1A2 with Km and Vmax values of 33 µM and 0.20 min(-1), respectively. Diclofenac and 4'-hydroxydiclofenac suppressed flurbiprofen 4'-hydroxylation by P450 2C9 strongly and moderately, respectively; however, they did not affect P450 2C19-dependent S-mephenytoin hydroxylation or P450 3A4-dependent midazolam hydroxylation. 3.  Although the caffeine 3-N-demethylation activity of liver microsomal P450 1A2 was inhibited by simultaneous incubation with diclofenac, the riluzole N-hydroxylation activities of recombinant P450 1A2 and human liver microsomes were inhibited after preincubation with diclofenac or 4'-hydroxydiclofenac for 20 min in the presence of NADPH. Using the inhibition constant (37 µM) of diclofenac on caffeine 3-N-demethylation and the reported 95th percentiles of maximum plasma concentration (10.5 µM) after an oral dose of diclofenac, the in vivo estimated increase in area under the plasma concentration-time curve was 29%. 4.  These results suggest that diclofenac could inhibit drug clearance to a clinically important degree that depends on P450 1A2. Clinically relevant drug interactions in vivo with diclofenac are likely to be invoked via human P450 1A2 function in addition to those caused by the effect of diclofenac on P450 2C9.