N-demethylation of amitriptyline in vitro: role of cytochrome P-450 3A (CYP3A) isoforms and effect of metabolic inhibitors.

Biotransformation of amitriptyline (AMI) to its demethylated product nortriptyline (NT) was studied in vitro with human liver microsomes from four different donors, preselected to reflect a range of metabolic rates. Reaction velocity versus substrate concentration was consistent with a sigmoid Vmax model. Vmax varied from 0.42 to 3.42 nmol/mg/min, Km from 33 to 89 microM AMI. Ketoconazole was a highly potent inhibitor of N-demethylation, with a mean Ki value of 0.11 +/- 0.013 microM (+/- S.D.), whereas quinidine (up to 50 microM), a CYP2D6 inhibitor, and alpha-naphthoflavone (up to 5 microM), a CYP1A2 inhibitor only at low concentrations, showed no effect. All selective serotonin reuptake inhibitors (SSRIs) tested had an inhibitory effect on the formation of NT, with mean Ki values of 4.37 (+/- 3.38) microM for sertraline, 5.46 (+/- 1.95) microM for desmethylsertraline, 9.22 (+/- 3.69) microM for fluvoxamine, 12.26 (+/- 5.67) microM for norfluoxetine, 15.76 (+/- 5.05) microM for paroxetine, and 43.55 (+/- 18.28) microM for fluoxetine. A polyclonal rabbit antibody against rat liver CYP3A1, in antibody/microsomal protein ratios varying from 1:1 to 10:1, inhibited N-demethylation of AMI to an asymptotic maximum of 60%. These results are consistent with several case reports describing impairment of AMI metabolism by SSRIs. Inhibition of AMI demethylation by low concentrations of ketoconazole and by anti-3A antibody supports an important role for CYP3A isoforms in mediating this reaction.