Identification of Cytochrome P450 Involved in the Metabolism of Donepezil and In Vitro Drug Interaction Study in Human Liver Microsomes

Donepezil hydrochloride is a reversible cholinesterase inhibitor with high specificity for centrally active cholinesterases, and is currently used for the treatment of Alzheimer's disease as Aricept® in many countries. In human liver microsomes donepezil is metabolized mainly to M4 by N-dealkylation, and to a lesser extent to M1 and M2 by O-dealkylation. In this study, cytochrome P450 involved in the metabolism of donepezil was identified and may serve to predict the potential for interaction with other drugs. The results showed that M4 was formed mainly by CYP3A4 and to a lesser extent by CYP2C9. The formations of M1 and M2 were both mediated mainly by CYP2D6. To predict possible drug interactions with donepezil in vivo, the effects of known inhibitors of drug metabolism, cimetidine, theophylline, erythromycin, quinidine and ketoconazole, on the formation of M1, M2 and M4 were studied by using human liver microsomes. The results suggested that cimetidine, theophylline and erythromycin have little or no effect on the metabolism of donepezil in vivo. On the other hand, quinidine and ketoconazole are predicted to inhibit donepezil metabolism. Quinidine significantly inhibited the formation of M1 and M2 mediated principally by CYP2D6. The Ki values of quinidine for M1 and M2 formation were 0.526 and 1.372 μM, respectively. In the presence of 0.7, μM of quinidine (the effective unbound plasma concentration of quinidine in clinical use) the hepatic intrinsic clearance of donepezil (a total of Vmax/Km for M1, M2 and M4 formation) was assessed to be reduced to 82.0%, and the elevation of the plasma concentration of donepezil by the co-administration of quinidine will be at most 1.2 times. Ketoconazole was a high affinity inhibitor of M1, M2 and M4 formation and the Ki values were 0.117, 0.563, and 0.102, μM, respectively. The hepatic intrinsic clearance of donepezil will be reduced to 57.9% in the presence of 0.08 μM (the effective unbound plasma concentration of ketoconazole in clinical use) of ketoconazole, and the plasma concentration of donepezil was predicted to increase at most 1.7 times. By the present study, the inhibitory profiles of quinidine and ketoconazol on metabolism of donepezil were confirmed. A drug interaction may occur by co-administration of substrate or inhibitor for CYP2D6 as well as CYP3A4 with donepezil, although the contribution of CYP2D6 to hepatic intrinsic clearance for donepezil is lower than that of CYP3A4.