In Vitro Biotransformation of Sildenafil (Viagra): Identification of Human Cytochromes and Potential Drug Interactions

The in vitro biotransformation of sildenafil to its major circulating metabolite, UK-103,320, was studied in human liver microsomes and in microsomes containing heterologously expressed human cytochromes. In human liver microsomes, the mean K(m) (+/-S.E. ) was 14.4 +/- 2.0 microM. A screen of the chemical inhibitors omeprazole (10 microM), quinidine (10 microM), sulfaphenazole (10 microM), and ketoconazole (2.5 microM) only revealed detectable inhibition with ketoconazole. Sildenafil biotransformation (36 microM) was inhibited by increasing concentrations of ketoconazole and ritonavir (IC(50) values less than 0.02 microM), which are established cytochrome P450 (CYP) 3A4 inhibitors. Using microsomes containing cDNA-expressed cytochromes, UK-103,320 formation was found to be mediated by four cytochromes: CYP3A4, -2C9, -2C19, and -2D6. Estimated relative contributions to net intrinsic clearance were 79% for CYP3A4 and 20% for CYP2C9; for CYP2C19 and -2D6, estimated contributions were less than 2%. These results demonstrate that CYP3A4 is the primary cytochrome mediating UK-103,320 formation and that drugs that inhibit CYP3A4 are likely to impair sildenafil biotransformation.