Roles of polymorphic enzymes CYP2D6 and CYP2C19 for in vitro metabolism of amitriptyline at therapeutic and toxic levels

The metabolism of the tricyclic antidepressant amitriptyline was studied in vitro in the presence of the main metabolite nortriptyline to simulate the steady state with amitriptyline and nortriptyline present in the ratio 1:1. The metabolism of the active metabolite nortriptyline in the presence of amitriptyline was also studied. The contribution of cytochrome P450 (CYP) 2D6, CYP2C19, and CYP3A4 was assessed by in vitro experiments at therapeutic (5 μM) and toxic (50 μM) concentrations for amitriptyline or nortriptyline with human liver microsomes (HLM). The results indicated that nortriptyline acted as a competitive inhibitor at the steady-state concentrations, lowering the amitriptyline metabolism, and thus changed the importance of different metabolic pathways. When the concentration of substrate amitriptyline was increased from therapeutic level to toxic level (in the steady state), the contribution of CYP2D6 significantly decreased for its demethylation and hydroxylation pathways. However, the contribution of CYP3A4 increased according to the increase in substrate concentration. Similar results could be obtained when nortriptyline was used as substrate in the presence and absence of the precursor amitriptyline. For CYP2C19 in the average HLM, its contribution to metabolism of both amitriptyline and nortriptyline was found to be small. However, it was also found that CYP2C19, in a “high activity” CYP2C19 HLM, was active for N-demethylation of both amitriptyline and nortriptyline and likewise the importance of CYP2C19 decreased when the concentration of substrate was increased. The above results indicate that CYP2D6 or CYP2C19 are generally not likely to be critical enzymes related to accidental intoxication under treatment with amitriptyline, but other contributing factors would be important; for example, low levels of various CYP enzymes and/or drug-drug interactions resulting in inhibition of several CYP enzymes may cause amitriptyline poisoning.